Defects and Distortion in Heat-Treated Parts

Home , Annealing (metallurgy), Austempering, Carburizing, Coining (metalworking), Galling, Hardening (metallurgy)

Anil Kumar Sinha, Bohn Piston Division

MOST OF THE PROBLEMS in heat- ature and effectiveness of the proprietary treated parts are attributed to faulty heat- grain inoculants applied to the mold surface), treatment practices (such as overheating and in heavily ground parts, and in affected zones burning, and nonuniform heating and quench- of welds (Ref 4). The usual practice is to ing), deficiency in the grade of steels used, reject the overheated products as being un- part defect, improper grinding, and/or poor suitable for service. part design. This article discusses overheat- It has now been established that over- ing and burning, residual stresses, quench heating is essentially a reversible process cracking, and distortion in some detail and caused by the solution of MnS particles in offers some suggestions to combat them. austenite during heating or reheating at high Most of these conditions result in a charac- temperatures; the amount increases with teristic appearance of the treated parts that temperature, and its subsequent reprecipi- can be easily recognized by simple inspec- tation during cooling occurs at intermediate tion. Some of these factors do not produce rates as very fine (-0.5 i~m) arrays of any distinguishing features in the semifin- a-MnS particles on the austenite grain ished or finished part. In particular, some of boundaries. On subsequent heat treatment the visual evidence does not recognize the the intergranular network of sulfides may presence of overheating and burning and the provide a preferential, lower-energy frac- 166,6 ~rn development of residual stresses leading to ture path in contrast to a normal transgran- distortion, quench cracking, and eventual ular fracture path. As a result, when impact failure of the heat-treated parts; metallurgical loaded, a ductile intergranular fracture de- laboratory examination is needed to establish velops due to decohesion of the MnS/matrix these problems that contribute significantly to interface and progress of microvoid coales- the service performance of the part. Tool cence. Figures 1 (a) and (b) show the usual designers must also be aware of the problems appearance of the fracture surface at differ- and difficulties in manufacture, heat treat- ent magnifications (Ref 1). ment, and use. When the low-alloy steel is preheated prior to hot working at too high a tempera- Overheatin 8 and Burning of ture (normally > 1400 °C, or 2550 °F), local melting occurs at the austenite grain bound- Low-Alloy Steels aries as a result of the segregation of phos- When low-alloy steels are preheated to phorus, sulfur, and carbon (Ref 5). During high temperature (usually > 1200 °C, or 2200 cooling, initially dendritic sulfides (proba- °F), prior to hot mechanical working (such as bly type II-MnS) form within the phospho- forging) for a long period, a deterioration in rus-rich austenite grain boundary, which the room-temperature mechanical properties then transforms to ferrite. This results in I I (particularly tensile ductility and impact excessively weak boundaries. Subsequent 12.5 p,rn strength or toughness) can be obtained after heat treatment provides a very poor impact Fracture surface of an impact loaded speci- the steel has been given a final heat treatment strength and almost completely intergranu- Fig 1 men. (a) Appearance of intergranular fracture (comprising reaustenitizing, quenching, and lar fracture surface after impact failure. of 4.25Ni-Cr-Mo steel containing 0.34% Mn and tempering) (Ref 1-3). Linked with the im- This phenomenon is termed burning. Burn- 0.008% S, in fully heat-treated condition but after paired mechanical properties is the appear- cooling from 1400 °C (2550 °F) at 10 °C/min (20 °F/rain). ing thus occurs at a higher temperature than (b) Same specimen as in (a) but at higher magnifica- ance of intergranular matte facets on the overheating. If this occurs during forging, tion, showing ductile dimples nucleated by MnS par- normal ductile fracture surface of an impact the forging will often break during cooling ticles precipitated at austenite grain boundaries. specimen. This phenomenon is known as or subsequent heat treatment (Ref 4). Courtesy of The Institute of Metals overheating and has been a matter of con- cern, especially in the case of steel forgings. Detection of Overheating heating, namely, fracture testing and metal- Overheating has also been noticed in steel There are two basic methods for the lography (or etch testing). Overheating may castings (due to variation in pouring temper- determination of the occurrence of over- also be detected by a decrease in mechani- 602 / Process and Quality Control Considerations

Table 1 Etching characteristics of overheated and burned steels Reagent Method Action on overheated steel Action on burned steel 2.5% nitric acid in ethyl Swab surface for 30 s May produce grain contrast, but White boundaries outlining alcohol not indicative of overheating preexisting austenite grains Saturated aqueous solution Electrolytic, specimen anode, White boundaries outlining Black boundaries outlining of ammonium nitrate current density 1.0 A cm -2 preexisting grains preexisting austenite grains (6.5 A in. -2) Aqueous 10% nitric acid + Etch for 30 s, swab surface; Black boundaries outlining White boundaries outlining 10% sulfuric acid repeat three times, then preexisting austenite grains preexisting austenite grains repolish lightly 85% orthophosphoric acid Electrolytic, specimen anode, Does not differentiate between Attacks inclusions at grain (Fine's reagent) current density 0.15 A cm -2 overheated and nonoverheated boundaries (1.0 A in.-2), etching time 15 steel min Oberhoffer's reagent Swab surface for 30 s Does not differentiate between Shows phosphorus segregation at overheated and nonoverheated grain boundaries steel Source: Ref 13 cal properties. But such changes are not ed with a lowering of impact strength (Ref acid and ammonium nitrate solution) used very marked unless overheating tempera- 12). for overheating can be successfully em- ture is high or overheating is too prolonged Metallography (or Etch Testing). The most ployed for detecting burning. When applied or severe; in some instances the mechanical widely used etchant technique uses to burned steels, these etchants react in a properties do not change, even after the Austin's reagent (aqueous solution of 10% manner opposite to that of overheated observation of extensive faceting. Usually nitric and 10% sulfuric acids), ammonium steels. Preece and Nutting (Ref 13) found the two methods mentioned above should persulfate, molten zinc chloride, saturated ammonium nitrate solution to be the ideal be used in conjunction with some measure solution of picric acid at 60 °C (140 °F), and reagent to detect this phenomenon. Other of toughness by impact or other testing in an electrolytic etch based on saturated reagents are Stead's and Oberhoffer's re- order to get a clear understanding of the aqueous ammonium nitrate. Table 1 shows agents, which may also be used to check the degree and severity of overheating (Ref 2). the etching characteristics of overheated burning effect. However, these etchants are Fracture Testing. The direction of fracture and burned steels (Ref 13). The etchant unable to differentiate between overheated testing is important in steels manufactured procedure with Austin's etchant is as fol- and nonoverheated steels. by conventional methods. It has been ob- lows: The sectioned specimen is etched for served by some workers (Ref 6) that the 30 s in the etchant, removed, washed off, Factors Affecting Overheating longitudinal fracture test specimens parallel and repeated three times. If the steel has The occurrence and severity of overheat- to the rolling direction do not exhibit face- been overheated, the original austenite ing depend principally on important factors, ring until the corresponding transverse frac- grain boundaries will be preferentially at- notably steel composition, temperature, tures display extensive faceting. However, tacked, and a black network of etch pits will cooling rate, and method of manufacture. the testing direction in electroslag-refined be observed under the microscope (Ref 14). Composition. Sulfur is the constituent (ESR) steels has been found to be insignif- According to Preece and Nutting (Ref 13), that greatly influences overheating. For icant (Ref 7). the best results are obtained when ammoni- steels with less than 0.002 wt% sulfur, over- The scanning electron microscope is con- um nitrate etch is applied on the sectioned heating does not occur; this is because of sidered to be the best and most convenient steel specimen in the fully heat-treated con- the very low volume fraction of sulfides tool to detect the facets on the overheated dition where this etchant preferentially at- formed. However, the commercial produc- fracture surfaces. These facets are charac- tacks the matrix (original austenite grains), tion of such very-low-sulfur steels (for ex- terized by small, well-defined, ductile dim- leaving the grain boundary unaffected ample, ESR steels) is expensive. Above this ples; each dimple is usually nucleated, pre- (which appears as a white network). level of sulfur, the overheating onset tem- sumably by fine arrays of inclusion Bodimeade (Ref 15) concluded that all these perature rises with the increasing amount of particles: a-MnS particles (Fig 1) in Mn- etchants did not cope with mildly overheat- sulfur. It has now been explained that steels bearing steels (Ref 8, 9) or chromium sul- ed low-sulfur steels. Table 2 is a summary with low sulfur content (0.01 to 0.02%) are fides in Mn-free steels (Ref 10, 11). of the results of potentiostatic etching tech- more prone to this defect than those with It is now well recognized that the fracture niques carried out by McLeod (Ref 12) high sulfur content (>0.3%) because the test specimen should always be tested in the using nitric-sulfuric, saturated aqueous pic- transgranular strength is high, and therefore toughest possible state (for example, ric acid (at 60 °C, or 140 °F), and ammonium a small amount of grain-boundary sulfide quenched and highly tempered [in the range nitrate etchants. He considered that when precipitation is enough to induce intergran- 600 to 650 °C, or I 110 to 1200 °F] steels after the suitable etching conditions were estab- ular failure (Ref 16). The phosphorus con- high-temperature austenitization) because lished, the potentiostatic etching method tent has been regarded with the most con- this condition is most prone to overheating rendered more reliable and reproducible cern in connection with burning. At effects. Baker and Johnson (Ref 5) have results as compared with the conventional constant phosphorus level, there is an in- suggested that an increased proportion of etching techniques. However, the same crease in the overheating temperature with facets in the fracture specimens with in- problem with mildly overheated low-sulfur the increase of sulfur content, whereas the creasing tempering temperature is attribut- steels still persisted. Hence, the use of etch burning onset temperature decreases. Burn- ed to the corresponding increase of the tests for low-sulfur low-alloy steels is not ing temperature is reduced with the increase plastic zone size. In this case a slight recommended for the detection of mild in phosphorus content. At low sulfur con- amount of weakening will be sufficient to overheating. tents, a wide gap between overheating and impart faceting because the grain boundary burning temperatures exists. For example, strength becomes lower (Ref 2). It should be Detection and Effects of Burning in the case of vacuum remelted steels, the noted that the existence of facets in the Burning is not commonly encountered. temperature gap between the onset of over- fractured specimens is not always associat- The two etchants (namely, nitric-sulfuric heating and burning is -300 to 400 °C (-570 Defects and Distortion in Heat-Treated Parts / 603

Table 2 Summaryof potentiostatic etching experiments produces cracking and distortion of the Best etching conditions parts (Ref 2). Anodic loop Solution voltage, mV Observed effect Voltage, mV Observed effect Comments Reclamation of Overheated Steel Saturated aqueous -400 Slight general 2200 (for 2 Classic white Operates best in the Severely overheated steels can often be ammonium etching min) boundaries on a transpassive region at nitrate dark background >+1500 mV; time at completely restored by any of the following any potential is heat treatments: important Underetching: random • Repeated normalizing (as many as six) array of black pits Overetching: uniform starting at temperatures 50 to 100 °C (90 black surface film to 180 °F) higher than usual, followed by Aqueous 10% nitric 200 Vigorous None Most aggressive etchant a standard normalizing treatment (Ref 2) acid + 10% dissolution of the three examined • Repeated oil-hardening and tempering sulfuric acid of specimen; treatments after prolonged soaking at 950 formation of flaky black to 1150 °C (1740 to 2100 °F) in carburizing film atmosphere. Rehardening more than -250 Milder attack; About -250 Discontinuous Polish lightly after etching three times is not advisable large black (for 30 s) array of to eliminate matrix • Soaking at 900 to 1150 °C (1650 to 2100 pits in grain-boundary etching effects mildly pits and some °F) for several hours. This causes growth etched random pits of MnS particles by the Ostwald ripening matrix within grains process and results in an excessive scale Saturated aqueous 100 No real, None Anodic loop very weak, formation and a loss of dimensional accu- picfic acid at positive necessitating long 60°C (140 °F) indication of etching times because racy of the forgings overheating current density is very low; Teepol additions gave no improvement Residual Stresses Source: Ref 12 Heat treatment often causes stress- and strain-related problems such as residual stress, quench cracks, and deformation and/ to 750 °F) and there is a remote possibility between a temperature low enough for the or distortion. The residual stress may be of burning occurring within the forging metal to be safe and high enough to be defined as the self-equilibrating internal or range, unless the overheating is severe (Ref sufficiently plastic. The better the tempera- locked-in stress remaining within a body with 2). However, at high sulfur content the gap ture control, the better the compromise. no applied (external) force, external con- becomes narrow. Severe overheating can be reduced to straint, or temperature gradient (Ref 18, 19). Temperature. To avoid overheating, care mild overheating by soaking the steel at There are two types of residual stresses: must be exercised in choosing a correct 1200 °C (2200 °F); with care, it may be • Macro- or long-range residual stress is a removed completely. Hot working through heating temperature so that uneven heating, first-order stress that represents an aver- the overheating range to a low finish tem- flame impingement, and so forth, do not age of body stresses over all the phases in perature is also reported to remove the occur (Ref 3). polyphase materials. Macroresidual effects of overheating. Cooling Rates. The cooling rate through stresses act over large regions as com- The alloying additions with a greater sul- the overheating range affects the size and pared to the grain size of the material. dispersion of intergranular et-MnS particles. fide-forming tendency, such as calcium, zir- Traditionally, engineers consider only conium, cerium (-0.3% of the melt), or The intermediate cooling rate generally em- this type Of residual stress when design- mixed rare earth metals (in the form of ployed, 10 to 200 °C/min (20 to 360 °F/min), ing mechanical parts misch metal containing 52% Ce, 25% La, gives rise to maximum faceting as well as to • Microresidual stress, also termed tesse- the greatest loss in impact strength. How- and 12% Nd), have been shown to increase lated stress or short-range stress is a significantly both the overheating tempera- ever, slow and rapid cooling rates will sup- second-order or texture stress, which is ture and mechanical properties of the steel press overheating. At very slow cooling associated with lattice defects (such as (for example, ductility and toughness). Pro- rates, the sulfide particles become large, vacancies, dislocations, and pile-up of vided that a high Ce/S ratio (>2) existed, a small in number, and more widely dis- dislocations) and fine precipitates (for ex- complete change in sulfide morphology oc- persed, and they have no more deleterious ample, martensite) (Ref 20-22). Microre- curred in low-alloy steels where the elon- effects than the other inclusions already sidual is the average stress across one gated MnS inclusion occurring in the un- present. At rapid rates, the sulfide inclu- grain or part of the grain of the material. sions are too fine to produce any damaging treated steel was totally replaced by small This information is indispensable in globular type-I rare earth sulfides and ox- effect (Ref 17). studying the essential behavior of materi- ysulfides of high thermal stability even after Methods of Manufacture. Electroslag- al deformation remelted steels are less susceptible than austenitizing at 1400 °C (2550 °F) (Ref 2). vacuum-remelted steels, presumably due to This treatment does not show intergranular These two types of residual stresses may the difference in oxygen level. Similarly, faceting. Burning can also be avoided in the also be classified further as a tensile or nickel steels are more prone to overheating. same way by treating with calcium, zirconi- compressive stress located near the surface Vacuum-remelted steels have a lower over- um, cerium, or mixed rare earth addition to or in the body of a material. This section heating temperature than some comparable form refractory, less-soluble sulfides. focuses on the effects, development, con- air-melted steels. Control of Cooling Rates. Control of cool- trol, and measurement of long-range resid- ing rates is not a practical method for large ual stresses. Prevention of Overheating and forgings because extremely slow cooling is Burning prohibitively time consuming and causes Effects of Residual Stress For preventing overheating of steels, a excessive scaling and decarburization, and The major effects of residual stress in- properly selected temperature should lie rapid quenching from high temperatures clude dimensional changes and resistance to 604 / Process and Quality Control Considerations

Surface residual stress (root of notch), ks• and nonferrous alloys, and surface harden- -200 -160 -120 -80 -40 0 40 ing treatments are widely used to produce 1100 8645 notch cold rolled I 8645 notch warm rolled 160 residual compressive stresses at the compo- 0.25 notch radius/~- I 0.25 notch radius nent surface. ~i~ J / 18645 I Residual tensile stresses at the surface of a 1045 j I-"~.~/ I shot peenedI untempered a~-,,,,L// • j14B35 part are usually undesirable because they 825 I 1045- ~,, " temper~-- 120 can effectively increase the stress levels; ~. tempered ..... may cause unpredicted stress-corrosion cracking (due to the combined effect of te,~,ered•\~~,\ temperecl .~ stress and environment), fatigue failure, ._E I 8630-N ~\ .E_ quench cracking, and grinding checks at -~= 550 I I tempered~ 8630 80 N°~ low external stresses; and tend to reduce Specimen X~/oil quenched fatigue life and strength of a part. In this case the extent of residual stresses may be 6.75 ~ ~'~1 8660 oil uenched "¢' i 8645 --"~ ~. / closer or even larger than the strength of the 275 -I I tempered [~'~-~_~ 40 material. Residual tensile stresses in the interior of L_ 1.750 in. L1.550 in. diam ~ 8645 oil quenched a component also may be damaging because 60° V-notch 1 diam 0.025 root radiu Compression ~--~-Tension of the existence and consequence of defects 0 i i I I 0 that serve as stress raisers in the interior -1375 -1100 -825 -550 -275 0 275 part. The uncommon phenomenon of de- Surface residual stress (root of notch), MPa layed cracking, in the absence of adverse Effect of surface residual stress on the endurance limit of selected steel. All samples were water environments and large applied stresses, Fig 2 quenched except as shown, and all specimen dimensions are given in inches. Source: Ref 23, 24 has now been attributed to the action of residual stresses on minute defects in the material (Ref 26). For example, a 17.5 cm crack initiation. Dimensional changes occur with the surface residual compressive stress (6.9 in.) diam × 125 cm (49.2 in.) long steel when the residual stress (or a portion of it) developed by specific heat treatment and shaft exploded into several pieces while in a body is eliminated. In terms of crack surface processing. It is also apparent that, lying free of any applied loads, on a labora- initiation, residual stresses can be either in the presence of high compressive stress, tory floor. Under normal loading, it would beneficial or detrimental, depending on a poor microstructure in steel samples has a have required a tensile strength larger than whether the stress is tensile or compressive. small influence on good endurance limit 150 MPa (22 ks•) to rupture the shaft. Compressive Residual Stress. Because re- fatigue strength (Ref 23-25). These fatigue Hence, the understanding of residual stress sidual stresses are algebraically summed improvements are of great significance in formation is very important, and this must with applied stresses, residual compressive components, particularly where stress rais- be given due consideration in the manufac- stresses in the surface layers are generally ers, such as notches, keyways, oil holes, ture and performance analysis of processed helpful because the built-in compressive and so forth, are highly desirable in the parts (Ref 26). stresses can reduce the effects of imposed design of components (for example, crank- tensile stresses that may produce cracking shafts, half-shafts, and so on) (Ref 26). Development of Residual Stress in or failure. Compressive stresses therefore Many fabrication methods have been devel- Processed Parts contribute to the improvement of fatigue oped to exploit this phenomenon. Pre- Variations in stresses, temperature, and strength and resistance to stress-corrosion stressed parts (including shrink-fits, pre- chemical species within the body during cracking in a part and an increase in the stressed concrete, interference fits, bolted processing cause the production of mac- bending strength of brittle ceramics and parts, coined holes, wire-wound concrete roresidual stresses. Various manufacturing glass (Ref 22). pipe), mechanical surface working pro- processes such as forming, machining, heat Figure 2 shows that the endurance limit cesses (such as shot peening, surface roil- treatment, shot peening, casting, welding, fatigue strength of selected steels increases ing, lapping, and so on) of hardened ferrous flame cutting, and plating render their characteristic residual stress pattern to processed parts. Table 3 lists a summary of Table 3 Summary of compressive and tensile residual stresses at the surface of the parts compressive and tensile residual stresses at created by the common manufacturing processes the surface of parts fabricated by common Compression at the surface Tension at the surface manufacturing processes. In heat-treated parts, residual stresses Surface working: shot peening, surface rolling, Rod or wire drawing with deep penetration may be classified as those caused by a lapping, and so on Rolling with deep penetration thermal gradient alone, and a thermal gra- Rod or wire drawing with shallow penetration(a) Swaging with deep penetration Rolling with shallow penetration(a) Tube sinking of the outer surface dient in combination with a structural Swaging with shallow penetration(a) Plastic bending of the shortened side change (phase transformation). When a Tube sinking of the inner surface Grinding: normal practice and abusive conditions steel part is quenched from the austenitizing Coining around holes Direct-hardening steel (through-hardened)(b) temperature to room temperature, a residu- Plastic bending of the stretched side Decarburization of steel surface Grinding under gentle conditions Weldment (last portion to reach room temperature) al stress pattern is established due to a Hammer peening Machining: turning, milling combination of thermal gradient and local Quenching without phase transformation Built-up surface of shaft transformation-induced volume expansion. Direct-hardening steel (not through-hardened) Electrical discharge machining Thermal contraction develops nonuni- Case-hardening steel Flame cutting Induction and flame hardening form thermal (or quenching) stress due to Prestressing different rates of cooling experienced by Ion exchange the surface and interior of the steel part. (a) Shallow penetration refers to ~<1% reduction in area or thickness; deep penetration refers to ~1%. (b) Depends on the efficiency of Transformational volume expansion in- quenching medium. Source: Ref 22 duces transformation stress arising from Defects and Distortion in Heat-Treated Parts / 605

Table 4 Changes in volume during the Table 5 Relevant physical properties in the development of thermal stresses transformation of austenite into different Coefficient of phases Modulus of elasticity expansion Thermal conductivity

Change in volume, %, as Metal GPa psi x 106 10-6/K 10-6pF W m -1 k -l Btu in./ft 2 • h • °F a function of carbon Transformation content (% C) Pure iron (ferrite) 206 30 12 7 80 555 Typical austenitic steel 200 29 18 10 15 100 Spheroidized pearlite -4.64 + 2.21 × (% C) Aluminum 71 10 23 13 201 1400 ---, austenite Copper 117 17 17 9 385 2670 Austenite ~ 4.64 - 0.53 x (% C) Titanium 125 18 9 5 23 160 martensite Spheroidized pearlite 1.68 x (% C) Source: Ref 29 martensite Austenite ~ lower 4.64 - 1.43 × (% C) bainite Spheroidized pearlite 0.78 x (% C) ture gradients in good thermal conductors illustration of the distribution of residual lower bainite (for example, copper and aluminum), but it stress over the diameter of a quenched bar Austenite ~ upper 4.64 - 2.21 x (% C) is much more likely in steel and titanium due solely to thermal contraction in the bainite (Ref 29). Another term involving thermal longitudinal, tangential, and radial direc- Spheroidized pearlite 0 upper bainite conductivity, called thermal diffusivity tions (Ref 19). (Dth), is sometimes used in context with The maximum residual stress attained on Source: Ref 4 temperature gradient. It is defined as Dth = quenching increases as the quenching tem- k/pc, where k is the thermal conductivity, p perature and quenching power of the cool- is the density, and c is the specific heat. It is ant are increased. Tempered glass is made the transformation of austenite into mar- clear that low Oth (or k) promotes large by utilizing quenching techniques in which tensite or other transformation products temperature gradient or thermal contrac- glass is heated uniformly to the annealing (Ref 27). Table 4 lists the changes in vol- tion. It should be emphasized that large size temperature and then surface cooled rapidly ume during the transformation of austenite of the part and high heating or cooling rates by cold air blasts. This produces compres- into different structural constituents (Ref (severity) of quenching medium also aug- sive surface stresses to counteract any ten- 28). ment temperature gradients leading to large sile bending stress, if developed during Thermal Contraction. The relation be- thermal contraction. loading of the glass, thereby increasing its tween the thermal stress ~th during cooling Table 5 lists some of the relevant material load-carrying capacity (Ref 31). and the corresponding temperature gradient properties that affect thermal and residual Residual Stress Pattern Due to Thermal in the component is given by: stresses (Ref 29). and Transformational Volume Changes (Ref Residual Stress Pattern Due to Thermal 32). During quench hardening of a steel (or tYth = E- AT" ct (Eq 1)Contraction. Residual stress is developed other hardenable alloy) part, hard martens- where E is the modulus of elasticity, and tx during quenching of a hot solid part that ite forms at the surface layers, associated is the thermal coefficient of expansion of involves thermal volume changes without with the volume expansion, whereas the the material. It is thus apparent that thermal solid-state phase transformation. This situ- remainder of the part is still hot and ductile stresses are greatest for materials with high ation also exists when a steel part is cooled austenite. Later, the remainder austenite elastic modulus and coefficient of thermal from a tempering temperature below the A t. transforms to martensite, but its volumetric expansion. Temperature gradient is also a Figure 3 shows the development of longitu- expansion is restricted by the hardened function of thermal conductivity. Hence, it dinal thermal and residual stresses in a 100 surface layer. This restraint causes the cen- is quite unlikely to develop high-tempera- mm (4 in.) diam steel bar on water quench- tral portion to be under compression with ing from the austenitizing temperature, 850 the outer surface under tension. Figure 4(c) °C (1560 °F) (Ref 30). At the start of cooling, illustrates the residual stress distribution Water quenched the surface temperature S falls drastically as over the diameter of a quenched bar show- 100 mm (4 in.) ing volume expansion associated with phase specimen compared to the center temperature C (top 1000 left sketch of Fig 3). At time w, the temper- transformation in the longitudinal, tangen- ? c w 1700 ou- ature difference between the surface and tial, and radial directions (Ref 19). At the core is at a maximum of about 550 °C (1020 same time during the final cooling of the 1100 ~ interior, its contraction is hindered by the ~ 500 °F), corresponding to a thermal stress of ~. u 1200 MPa (80 tons/in. E) due to linear differ- hardened surface layers. This restraint in 600 E ential contraction of about 0.6%, if relax- contraction produces tensile stresses in the ~- 0 100 ~- ation does not take place. Under these interior and compressive stresses at the 1 10 103 Time, s conditions, tensile stresses are developed in outer surface. However, the situation as the case with a maximum value of a (lower shown in Fig 4(c) prevails, provided that the diagram), corresponding to time w in the net volumetric expansion in the interior, upper diagram, and the core will contract, after the surface has hardened, is larger producing compressive stresses with a max- than the remaining thermal contraction. In imum of b. The combined effect of tensile some particular conditions, these volumet- and compressive stresses on the surface and ric changes can produce sufficiently large • _ core, respectively, will result in residual residual stresses that can cause plastic de- e~ e~ stresses as indicated by curve C, where a formation on cooling, leading to warping or E E o o complete neutralization of stress will occur distortion of the steel part. While plastic at some lower temperature u. Further de- deformation appears to reduce the severity Development of thermal and residual stresses crease in temperature, therefore, produces of quenching stresses, in most severe Fig 3 in the longitudinal direction in a 100 mm (4 in.) longitudinal, compressive residual stresses quenching the quenching stresses are so diameter steel bar on water quenching from the aus- at the surface and the tensile stresses at the high that they do not get sufficiently re- tenitizing temperature, 850 °C (1560 °F). Transforma- tion stresses are not taken into consideration. Source: core, as shown in the lower right-hand leased by plastic deformation. Consequent- Ref 30 diagram of Fig 3. Figure 4(a) is a schematic ly, the large residual stress remaining may 606 / Process and Quality Control Considerations

I surface transforms before, the stress reversal (Fig 4c and bottom of Fig 5), whereas compressive surface residual stress takes +~ place when the core transforms before, and the surface transforms after, the stress reversal (top of Fig 5). His analysis is capable of explaining complex stress patterns for various combinations of part sizes, quenching rate, and steel hardenability (Ref 21). I i However, the residual stress pattern in the Longitudinal Longitudinal hardened steels can be modified either with different transformation characteristics or I during the tempering and finish-machining .~_ Tj (after hardening) operations. I Residual Stress Pattern after Surface Hard- ening. In general, thermochemical and thermal surface-hardening treatments produce beneficial compressive residual stresses at the surface. Carburized and Quenched Steels. When low-carbon steels are carburiZed and

i quenched, first the core transforms at high I L = longitudinal I temperature (600 to 700 °C, or ll00 to 1300 Tangential T= tangential Tang?ntial °F) to ferrite and pearlite with the attendant I R = radial I relaxation of any transformation stresses. Later, the high-carbon case transforms to martensite at much lower temperature (less than 300 °C, or 570 °F), accompanied by volume expansion and under conditions of I t-'--->' no (or minimum) stress relaxation. As a Radial Rad al result, residual compressive stress is devel- (a) (b) (el oped in the case with a maximum at the Schematic illustration of the distribution of residual stress over the diameter of a quenched bar in the surface. Fig 4 longitudinal, tangential, and radial directions due to (a) thermal contraction and (c) both thermal and Large differences in carbon level between transformational volume changes. (b) Schematic illustration of orientation of directions. Source: Ref 19 the case and the core determine the se- quence of phase transformation on cooling after carburizing and the resultant develop- reach or even exceed fracture stress of During the rapid quenching of the medium- ment of compressive residual stress in the steel. This localized rupture or fracture is size (30 mm, or 1.2 in.) bar diameter, the case. Likewise, compressive residual stress called quench cracking (Ref 32, 33). start of bainite transformation at the center in the case increases as the core carbon It should be emphasized again that for a coincides approximately with the transfor- content decreases. Increasing case depth given grade of steel, both large size of the mation of martensite on the surface. This reduces the contribution from the low-car- part and higher quenching speed contribute results in compressive stresses at both the bon core in the development of compressive to the larger value of thermal contraction, surface and center, with tensile stresses in stress in the case, thereby adversely affect- as compared to the volumetric expansion, the intermediate region (middle of Fig 5). ing the fatigue properties (Ref 36). of martensite. In contrast, when the parts When the smaller-diameter (10 mm, or 0.4 In actual practice, a maximum compres- are thin and the quenching rate is not high, in.) bar is drastically quenched (for exam- sive stress develops at some distance away thermal contraction of the part subsequent ple, in brine), the entire bar transforms to from the surface (Fig 6 and 7). This effect to the hardening of the surface will be martensite. This is associated with very occurs because of the presence of retained smaller than the volumetric expansion of little temperature variation between the sur- austenite, the extent of which depends on martensite. Similarly, for a given quenching face and the center of the part. In this steel composition, carbon content of the rate, the temperature gradients decrease situation, tensile residual stress is devel- case, quenching temperature, and severity with decreasing section thickness, and con- oped at the surface and compressive stress of quench. According to Koistinen (Ref 38) sequently the thermal component of the at the center of the bar (bottom, Fig 5) (Ref and Salonen (Ref 39) the peak compressive residual stress is also decreased (Ref 24). 34, 35). stress takes place at 50 to 60% of the total Figure 5(a) shows the continuous cooling Although the shallower hardening steels case depth corresponding to about 0.5 to transformation diagram of DIN 22CrMo44 exhibit higher surface compressive stresses, 0.6% carbon level, which produces a low low-alloy steel exhibiting austenitic decom- deep hardening steels may develop moder- retained austenite content and martensite position with the superimposed cooling ately high surface compressive stresses hardness around the maximum. Another curves of the surface and center in round with severe water quenching. When these factor that might influence this compressive bars of varying dimensions. If the large- deep hardening steels are through-hardened residual stress profile is that the martensite diameter (100 mm, or 4 in.) bar is water in a less efficient quenchant, they may formed in the lower-carbon regions of the quenched (that is, for slack quenching), exhibit surface tensile stresses (Ref 24, 31). case is of the lath type, which also affects martensitic transformation occurs at the Rose has pointed out the importance of the retained austenite content (Ref 20). The surface, and pearlitic + bainitic transforma- transformations of core and surface before reversal sign of residual stress takes place at tions occur at the center, resulting in a and after the stress reversal. According to or near the case/core interface. Later, when residual stress pattern (top of Fig 5) similar him the tensile surface residual stress oc- Koistinen's theory was applied to the mea- to that due solely to thermal stress (Fig 4a). curs when the core transforms after, and the sured data, it appeared that the position of Defects and Distortion in Heat-Treated Parts / 607

1.0 1000 1830 Distribution of 800 ,~ 1470 residual stresses c" +20 +3 o 0.5 600 ~ ~ 1110 8 400 \ ~._ 750

200 "~ ~ 390 Surface Center -o , c.-~_ 0 -20 -3 1000 1830 ~ o

800 ~ ~ 1470 m -40 -6

~ Center Surface ~ •~ +20 +3 "~ Tensile 400 - 750 X E E •"o .-"o m "~ o o 200 390 er 30 mm 0 Surfacel -20 diam -3 .9_ tt- 1000 1830

800 ( 1470 Center Surface +20 [ +3 Compressive 600 ~.._.... 1110 Distance from the surface / 0 0 400 " ' ~-~,...... _. 750 Relationship between carbon content, re- I~/0 mm Fig 6 tained austenite, and residual stress pattern. It -20 ~¢ diam -3 shows the development of peak compressive stress some distance away from the surface. Source: Ref 20 Center Surface nter 0 + = Tensile stresses 1 10 100 103 - = Compressive stresses strength, which permits the application of Time, s significantly higher stresses than could normally be possible in fatigue loading. As in (a) (b) the carburizing practice, the surface com- (a) Continuous cooling transformation diagrams of DIN 22CrMo44 steel showing austenitic decompo- pressive residual stresses are usually found Fig 5 sition with the superimposed cooling curves of the surface and center during water quenching of round to increase, with depth below the surface bars of varying dimensions. (b) The corresponding residual stress pattern developed because of thermal and transformational volume changes. Source: Ref 34, 35 (Ref 45) (Fig 9, Ref 44). A fairly sharp transition to a tensile state takes place near the hardness drop-off between the case and maximum compressive stress depends on trocarburizing, a (macro-) compressive re- unhardened surrounding material. With an severity of quenching, total case depth, sidual stress is produced in the compound increase in distance from the steep transi- steel hardenability, and so forth (Ref 21, layer and gamma prime phase (Ref 41). tion, the tensile condition gradually fades 40). Figure 7 shows the details of generation When nitrocarburized parts are rapidly away toward zero stress (Ref 44). In induc- of axial stress distribution of a carburized quenched, the above properties are further tion hardening, an increase in hardenability gear (made from deeper hardening steel) enhanced (Ref 42). changes the depth at which transition from during quenching. In the early stages, the In borided steel processed at 900 °C (1650 compressive to tensile stress occurs. The contour lines of equal stress were largely °F), a high compressive residual stress is increase in the rate of heating produces an unaffected by the surface profile. Later a developed at the surface layers (Fig 8), increase in the maximum compressive and zone of high compressive stress distribution which consists of FeB and Fe2B phases (Ref tensile residual stresses without affecting occurred in the central portion of the teeth, 43); this is attributed to the lower thermal the mode of stress distribution (Ref 46). which remained until the end of the quench expansion coefficient and the larger specific Residual Stress in Other Processing Steps. (Ref 37). volume in a borided layer compared to that As welding progresses, the temperature dis- In nitriding, like carburizing, a compres- in a ferrite matrix (Ref 18, 43). tribution in the weldment becomes nonuni- sive residual stress is set up in the surface In an induction-hardened steel part, a form and varying as a result of localized layers. High-temperature nitriding produces compressive surface residual stress is pro- heating of the weldment by the welding heat a little relaxation of stresses, whereas low- duced when wear-resistant hard martensite source. During the welding cycle, compris- temperature nitriding imparts a maximum (with slightly lower density) is formed on ing heating and cooling, complex strains residual stress. In nitrocarburizing, im- the surface of a section concurrently with develop in the weld metal and adjacent provement in residual surface compressive volume expansion while nonhardened core areas. As a result, appreciable residual stress and fatigue strength depends on the remains essentially unchanged (Fig 9) (Ref stresses remain after the completion of hardness and depth of diffusion zone. These 44, 45). The magnitude of the compressive welding. Since the weld metal and heat- properties, in turn, decrease with increasing stress, which is affected by both thermal affected zone contract on cooling (Fig 10a), carbon and alloy content (that is, increased contraction and martensite formation, may they are restrained by the cool adjacent hardenability). During quenching, after ni- be a considerable fraction of the yield part. This produces tensile residual stress in 608 / Process and Quality Control Considerations

Carburized SNC815 Distance from surface, in. 300 0 0.002 0.004 0.006 0300 -900 500 -900 -600 -600 60~j -- 50

3OO 0 /xA z, /x A 0 Gz = 200MPa -600 0 z~ ~ 0 #_ - -50 100 ~ -300 -500 - -100

( ~100 -1000 ~ • -150 ~ i ~ o --200 "~ '~ -1500 • (b © FeB - -250 rr 300 300 • • Fe2B -2000 0 0 /x Ferrite - -300 0 l -2500 600 0 0.05 0.10 0.15 Distance from surface, mm

Residual stress distribution of FeB and Fe2B Fig 8 layers in borided steel processed at 900 °C t= 3 s 30 s 60 s (1650 °F). Source: Ref 18, 43

Fig 7 Axial stress distribution (given in MPa) in carburized gear during quenching process. Source: Ref 37 alloys, and so on, a significant amount of thermal stress is generated during quench- the weldment region and compressive resid- gentle grinding method is expensive from ing prior to precipitation hardening. The ual stress in the surrounding base metal the viewpoint of operating time and wear of quenching process in this condition does region (Fig 10b). the wheel. not invariably involve a phase change; rath- In general, a steep residual stress gradient As a result of temperature gradient during er, this is confined to the postquenching is developed because of the steep tendency cooling, castings develop compressive stress- aging treatment. In other nonferrous alloys of the thermal gradient. This may, in turn, es at the surface and tensile stresses in the such as uranium and titanium alloys, the lead to hot cracking (between columnar interior (Ref 22). However, transient temper- final structural condition is not obtained by grains) or severe center line cracking in the ature gradient and phase transformation oc- a slow cool. weld area (Ref 48). Catastrophic failures of curring during the early stages of solidifica- When high-strength titanium alloy is welded bridges and all-welded ships are tion and cooling of continuous steel castings quenched from a solution annealing temper- mostly attributed to the existence of large in the mold may give rise to the development ature of 850 to 1000 °C (1560 to 1830 °F), it and dangerous tensile residual stress in of harmful residual stresses leading to the develops large residual stress caused by them (Ref 49). formation of cracks (Ref 51). poor thermal conductivity of titanium lead- The grinding step in manufacturing is Chemical processes such as electroplat- ing to high-temperature gradient. This prob- important, since it is always utilized to ing, scale formation, and corrosion of met- lem can, however, be avoided by stress- produce the finished surface. It has been als can produce residual stresses due to relief annealing at 650 to 700 °C (1200 to shown that gentle surface grinding, using a coherency strains arising from the matching 1290 °F), which produces a slight reduction soft sharp wheel and slow downfeed, pro- tendency of crystal structures of the outer in mechanical properties. When a high- duces compressive residual stress at the surface product with the crystal structure of strength aluminum age-hardening alloy is surface, whereas conventional (normal the adjacent layer (Ref 22). Residual stress- rapidly quenched from the solution temper- practice) and abrasive grinding result in es are also introduced when heat-treated surface tensile stresses of very high magni- parts are subjected to successive heating tude (Fig l l) (Ref 22, 50). However, the and cooling cycles during service condi- Depth below surface, mil tions. 3.15 6.3 9.45 12.6 Residual Stress in the Heat-Treated Non- 800 120 Distance from surface, in. ferrous Alloys. In nonferrous alloys, notably 0.08 0.16 0.24 0.32 0.40 /\. - 9O 400 (58) 8O age-hardenable aluminum alloys, copper- 600 500 gm Knoop test beryllium alloys, certain nickel-base super- A A 3 mm case I o g_g r~ - 60 L~ I I Is ss 60 -r- ~: 400 200 (29) -- Abrasive #_ Residual stress % Compression Tension \ "~ 200 30 0 40 ~ \ Conventional r,~ o 0 -% -200 (-29) 20 ,~ i i i i i i i i i i I i i ~ - Gentle ,-, .... tr / Hardness u,l ~. -200 - -30 Yl E -400 (-58) 0 o -400 -60 2 4 6 8 10 12 (a) (b) 0 80 160 240 320 Distance from surface, mm Fig 10 (a) The transverse shrinkage occurring in Depth below surface, pm Fig 9 A typical hardriess and residual stress profile butt weldments. (b) Longitudinal residual in induction-hardened (to 3 ram, or 0.12 in., stress patterns in the weldment and surrounding re- Residual stress distribution after gentle, con- case depth) and tempered (at 260 °C, or 500 °F) 1045 gions. This also shows longitudinal shrinkage in a butt Fig 11 ventional, and abrasive grinding of hard- steel. Source: Ref 44 weld. Source: Ref 47 ened 4340 steel. Source: Ref 22 Defects and Distortion in Heat-Treated Parts / 609

Table 6 A compiled summary of the maximum residual stresses in surface heat-treated related to the residual stress. The above steels quantities are usually dependent on the stress Residual stress (longitudinal) and material parameters (such as metallurgical textures), which are difficult to quantify Steel Heat treatment MPa ksi (Ref 54, 56). 832M13 (type) Carburized at 970 °C (1780 °F) to 1 mm (0.04 in.) case with The x-ray diffraction method is the well- 0.8% surface carbon Direct-quenched 280 40.5 established technique for measuring both Direct-quenched, -80 °C (- 110 °F) subzero treatment 340 49.0 macro- and microresidual stress nondestruc- Direct-quenched, -90 °C (-130 °F) subzero treatment, 200 29.0 tively. In most instances, the x-ray diffraction tempered method has been employed to provide quan- 805A20 Carburized and quenched 240-340(a) 35.0--49.0 805A20 Carburized to 1.1-1.5 mm (0.043-0.06 in.) case at 920 °C 190-230 27.5-33.5 titative values for residual stress profiles in (1690 °F), direct oil quench, no temper surface or fully hardened components (Ref 805A ! 7 400 58 57). This technique depends on the determi- 805A17 Carburized to 1.1-1.5 mm (0.043-0.06 in.) case at 920 °C 150-200 22-29 nation of lattice strains and the stress-induced (1690 °F), direct oil quench, tempered 150 °C (300 °F) 897M39 Nitrided to case depth of about 0.5 mm (0.02 in.) 400--600 58.0-87.0 differences in the lattice spacing. Macroresid- 905M39 800-1000 116.0-145.0 ual strain is measured from the shift of dif- Cold-rolled steel Induction hardened, untempered 1000 145.0 fraction lines in the peak position using the Induction hardened, tempered 200 °C (390 °F) 650 94.0 so-called nonlinear SinZC method from which Induction hardened, tempered 300 °C (570 °F) 350 51 Induction hardened, tempered 400 °C (750 °F) 170 24.5 residual stress is calculated (Ref 57). For the measurement of microstrain the Voigt single- (a) Immediately subsurface, that is. 0.05 mm (0.002 in.). Source: Ref 29 line method is applied (Ref 58). Precision in lattice strain measurement of the order of 0.2% is possible. ature, high thermal and residual stresses are called the dissection method, and the non- Portable x-ray diffraction equipment is induced due to high coefficient of expansion destructive methods comprising mainly now commercially available in various of aluminum. Uphill quenching from liquid x-ray diffraction, neutron diffraction, ultra- forms that allow stress measurement to be nitrogen temperature (- 196 °C, or - 320 °F) sonic, and magnetic methods. made very quickly (ranging from 4 to 30 s). in a steam blast alleviates this problem. This Destructive (or Dissection) Method. This The main drawbacks are that it cannot be induces stresses opposite in sign to those method is old but reasonably accurate, applied to noncrystalline materials such as developed on water quenching from the practically nondestructive, uses well-estab- plastics, and it is only capable of measuring solutionizing and cancels out their effect. lished methods, and can be employed in residual stresses of materials very close to This is followed by aging of the alloy in the confined situations at site (Ref 53). Howev- the surface under examination. That is, the conventional manner (Ref 29). er, it is tedious, time consuming, and expen- measurement is purely surface related (a Fast polyalkylene glycol (PAG) quench- sive (Ref 54). The other drawbacks are the depth of 0.01 mm, or 0.4 mil, is commonly ing of solution-treated aluminum alloys destructive, or at best semidestructive na- quoted) (Ref 59). tends to reduce residual stress levels be- ture of the method, and its ability to mea- Neutron radiography or diffraction, used cause of its more uniform heat extraction sure only the macroresidual stresses. The for polycrystalline materials, has a much rate (thermal shock is smaller, and thereby hole-drilling method is used extensively for deeper penetration than x-rays, but has machining is less likely to produce further measuring residual stresses, which depends major safety problems and the disadvantage distortion), thereby helping solve major and on the dissection approach. It consists of of being nonportable. long-standing distortion problems among the mounting of strain gages or a three- Ultrasonic method for evaluating residual aluminum workpieces (Ref 52). element strain-gage rosette on the surface stress involves ultrasonic stress birefrin- and measurement of strains. Then a rigidly gence or sonoelasticity; this depends upon Control of Residual Stresses in guided milling cutter is used to drill a small, the linear variation of the velocities of Heat-Treated Parts straight, circular, perpendicular, and fiat- sound in a body (that is, ultrasonic waves) Table 6 lists some typical values of max- bottomed hole not exceeding 3.2 mm (0.125 with the stress. This method has the poten- imum residual stresses developed in the in.) at the center of the rosette and into the tial for greater capability, versatility, and surface-hardened steels that have been re- surface of the component being analyzed. usefulness in the future (Ref 53, 56). How- ported in the literature (Ref 29). It is worth Strain redistribution occurring at the sur- ever, this has the disadvantage, in common noting that there is a marked influence of face in the surrounding area of the hole with the magnetic methods, that it requires tempering on the residual stress level. Tem- (resulting from the residual stress relief) is transducers shaped to match the surface pering must be accomplished at about 150 then measured with the previously installed being inspected (Ref 60). °C (300 °F) to maintain 50 to 60% retention strain gages. The residual stress is calculat- The magnetic method is based on the of the residual stress level obtained after ed at a large number of points in a surface stress dependence of the Barkhausen noise quenching because a higher tempering tem- from the strain measurements using the amplitude. Each time an alternating mag- perature greatly reduces surface compres- well-established method (Ref 22, 28). To netic field induced in a ferromagnetic mate- sive stresses. However, a higher stress- minimize the introduction of spurious rial is reversed, it generates a burst of relief temperature (-600 °C, or 1110 °F) is strains by the grinding operation, the rate of Barkhausen noise. The peak amplitude of used for mechanically deformed compo- metal removal should be less than 3.125 x the burst, as determined with an inductive nents (for example, hot-rolled bars) or com- 10 -4 m/s (1.23 × 10 -2 in./s), and readings coil near the surface of the component ponents with tensile surface residual stress- are recorded after 15 min of the end of the material, varies with the surface stress lev- es. Alternatively, serious residual tensile grinding process to ensure that any heat el. Since Barkhausen noise depends on stresses may be avoided effectively by gen- generated has been dissipated (Ref 55). composition, texture, and work hardening, tle grinding of the surface. Nondestructive Methods. The main diffi- it is necessary in each application to use culty with the nondestructive methods is that calibrated standard (reference) samples Measurement of Residual Stresses measurements of crystallographic lattice pa- with the same processing history and com- There are two methods of measuring re- rameters, ultrasonic velocities, or magnetiza- position as the component being analyzed. sidual stresses: the destructive method, also tion changes are made that are indirectly This method is used to measure residual 610 / Process and Quality Control Considerations stresses well below the yield strength of the A decrease in carbon content from 0.72 to ferromagnetic materials. This method is 0.61% has been shown to slightly increase rapid, and the measurements are made with the thermal crack resistance of rim- the commercially available portable equip- quenched railroad wheels (Ref 62). ment. However, this method is limited to Because of segregation of carbon and only ferromagnetic materials (Ref 56). alloying elements, some steels are more Thermal evaluation for residual stress anal- prone than others to quench cracking. ysis (TERSA) is a new nondestructive meth- Among these steels, 4140H, 4145H, 4150H, od that is in an experimental stage. It has the and 1345H appear to be the worst. A good advantage that it is completely independent, option is to replace the 4100 series with the remote, and noncontacting. It consists of 8600 series. An additional disadvantage merely directing a controlled amount of ener- with the use of 1345H steel is the manga- gy from a laser energy source into the volume nese floating effect, which leads to very of the material being inspected and then mak- high manganese content in the steel rolled ing a precise determination of changes in the from the last ingot in the same heat. Simi- resulting temperature rise by infrared radiom- larly, dirty steels (that is, steels with more etry. However, the working instrument will than 0.05% S, for example, AISI 1141 and ( also require some form of display to enable 1144) are more susceptible to cracking than visual examination to be made of any high- the low-sulfur grades. The reasons for this Typical appearance of thumbnail check as stressed regions (Ref 60). are that they are more segregated in alloying Fig 1 2 soft spot on chipping chisel. Source: Ref 64 elements, the surface of this hot-rolled high- Quench Cracking sulfur steel has a greater tendency to form seams, which act as stress raisers during are most likely to occur in the water-hard- Anything that produces excessive quenching, and they are usually coarse ening steels, especially where the tool is quenching stress is the basic cause of crack- grained (for better machinability), which grabbed with tongs for quenching. Normal- ing. Quench cracking is mostly intergranu- increases brittleness and therefore pro- ly the cleaned surface shows adequate hard- lar, and its formation may be related to motes cracking. If these high-sulfur grades ening and the scaled surface insufficient some of the same factors that cause inter- are replaced by calcium-treated steels or hardening, which can be examined with a granular fracture in overheated and burned cold-finished leaded steels, this problem file. Soft spots may occur from the use of steels. The main reasons for cracking in can be obviated (Ref 61). fresh water, or water contaminated with oil heat treatment are: part design, steel Part Defects. Surface defect or weakness or soap. Most large tools emerging from grades, part defects, heat-treating practice, in the material may also cause cracking, for hardening operations contain some soft and tempering practice (Ref 61). example, deep surface seams or nonmetallic spots. However, accidental soft spots in the Part Design. Features such as sharp cor- stringers in both hot-rolled and cold-fin- wrong place should be investigated, and ners, the number, location, and size of holes, ished bars. Other defects are inclusions, steps must be taken to eliminate them. deep keyways, splines, and abrupt changes in stamp marks, and so forth. For large-seam Figure 12 shows the typical appearance of section thickness within a part (that is, badly depths, it is advisable to use turned bars or a thumbnail check as soft spot on chipping unbalanced section) enhance the crack for- even magnetic particle inspection. The forg- chisels, which occurs on the bit near the mation because while the one (thin) area is ing defects in small forgings, such as seams, cutting edge. The cracks enclosing the soft cooling quickly in the quenchant, the other laps, flash line, or shearing crack, as well as spots should be avoided by switching to (thick) area immediately adjacent to it is cool- in heavy forgings, such as hydrogen flakes brine quench (Ref 64). ing very slowly. One solution to this problem and internal ruptures, aggravate cracking. Air-Hardening Steel. Similarly, when air is to change the material so that a less drastic Similarly, some casting defects, for exam- hardening steels are improperly handled, quenchant (for example, oil) can be em- ple, in water-cooled castings, promote they are likely to crack. For example, ployed. An alternate solution is to prequench, cracking (Ref 50). avoidance of tempering treatment or use of that is, to cool it prior to the rest of the part. Heat-Treating Practice. Higher austenitiz- oil quenching in air-hardening steel can lead This will produce an interior of the hole or ing temperatures increase the tendency to cracking. However, the common practice keyway that is residually stressed in compres- toward quench cracking. Similarly, steels in the treatment of air-hardening steels is sion, which is always desirable for better with coarser grain size are more prone to initially to quench in oil until "black" fatigue properties (Ref 61). The third solution cracks than fine-grain steels because the (about 540 °C, or 1000 °F), followed by air is a design change, and the fourth is to use a latter possess more grain-boundary area to cooling to 65 °C (150 °F) prior to tempering. milder quenchant. stop the movements of cracks, and grain As compared to air cooling right from the Steel Grades. Sometimes this can be boundaries help to absorb and redistribute quenching temperature, this practice is to- checked by means of a spark test, whereas at residual stresses. An outstanding contribu- tally safe and minimizes the formation of other times a chemical analysis must be tor to severe cracking is improper heat- scale. made. In general, the carbon content of steel treating practice, for example, nonuniform Polymer quenchants have found well-es- should not exceed the required level; other- heating and nonuniform cooling of the com- tablished use in the quenching of solution- wise, the risk of cracking will increase. The ponent involved in the heat-treatment cy- treated aluminum alloys, hardening of plain suggested average carbon contents for water, cle. It is a good heat-treating practice to carbon steels with less than 0.6% C, spring brine, and caustic quenching are given below: anneal alloy steels prior to the hardening steels, boron steels, hardenable stainless treatment (or any other high-temperature steels, and all carburizing and alloy steels Method Shape Carbon, % treatment, for example, forging, welding, with section thickness greater than about 50 mm (2 in.), through-hardening and carbur- Induction hardening Complex 0.33 and so forth) because this produces grain- Simple 0.50 refined microstructure and relieves stresses izing steel parts, and induction and flame- Furnace hardening Complex 0.30 (Ref 63). hardening treatments because of their nu- Simple 0.35 Water-Hardening Steel. The water-hard- merous beneficial effects, including Very simple, such elimination of soft spots, distortion, and as bar 0.40 ening steels are most susceptible to cracks if they are not handled properly. Soft spots cracking problems associated with trace Defects and Distortion in Heat-Treated Parts / 611

Another cause of microcracking is the increased carbon content of martensite (that is, increased hardenability), which is a function of austenitizing temperature and/or time (Ref 67). This finding was established for 8620H steel, which has a higher austenitizing temperature prior to quenching where there is a greater tendency to micro- crack (Ref 69). This problem can be avoided by selecting a steel with less hardenability (that is, with less austenitizing temperature). Another solution is to change the heat-treating cycle to carburizing, slow cooling to black temperature, reheating to, for example, 815 or 845 °C (1500 or 1550 °F), and quenching (Ref 61). Microcracking in case-hardened surfaces may be aggravated by the existence of hydrogen, which tends to absorb during carburizing. However, this hydrogen-enhanced microcracking can be eliminated by tempering the carburized parts at 150 °C (300 °F) immediately after quenching. Tempering exhibits an additional beneficial effect in that it has the ability to heal the microcracks due to the volume changes and associated plastic flow that develop during the first stage of tempering (Ref 70). No adverse report on the influence of microcracks on the mechanical properties has been noted; however, the controlling factors should be varied so as to keep 4.7 ~m the incidence of microcracks to a minimum (Ref 67). Fig 13 Microcracking in a Ni-Cr steel. Source: Ref 67 Tip cracking refers to the cracking that appears in the teeth of carburized and quenched gears and runs partly or fully to water contamination in quenching oils (Ref tion must be avoided, especially on all deep- the ends of the teeth in a direction parallel 65). hardening steels, either by providing some to the axis of the part. Many heat treaters Agitation is an important parameter in type of protective atmosphere during the have solved this problem to a great extent polymer quenching applications both to en- heating operation, stock removal by grind- by decreasing the carbon content and case sure a uniform polymer film around the ing, or carbon restoration process. In addi- depth to the minimum acceptable design quench part and to provide a uniform heat tion to protective atmosphere, salt baths, level or by copper plating the outer diame- extraction from the hot part to the adjacent inert packs, or vacuum furnaces may be ter of the gear blank prior to hobbing (Ref area of quenchant by preventing a buildup used to obtain the desired surface chemistry 66). of heat in the quench region. on the tools or dies. The fact that the better Nitrided Steels. The nitrided cases are Salt bath cooling of induction-hardened and more consistent performance of the very brittle. Consequently, cracking may complex-shaped cast iron parts reduces tools is observed after regrinding reveals occur in service prior to realizing any im- danger of cracking, which is usually expe- the existence of partial decarburization re- proved wear and galling resistance. This rienced when air cooling followed by hot- maining. can be avoided by a proper tool design, for water quenching is used (Ref 66). Carburized Alloy Steel. Two types of example, incorporating all section .changes Decarburized Steel. Decarburization usu- peculiar cracking phenomena prevail in the with a minimum radius of 3 mm (0.125 in.). ally arises from insufficient protection as a carburized and hardened case of the car- Tempering Practice. The longer the time result of plant failure (for example, defec- burized alloy steels: microcracking and tip the steel is kept at a temperature between tive furnace or container seals, defective cracking. Microcracking of quenched room temperature and 100 °C (212 °F) after valves), poor process control (for example, steels are small cracks appearing across or the complete transformation of martensite insufficient atmosphere-monitoring equip- alongside martensite plate (Fig 13) (Ref 67) in the core, the more likely the occurrence ment, poor supervision), or the existence of and the prior austenite grain boundaries of quench cracking. This arises from the decarburizing agents in the furnace atmo- (Ref 68). They form mostly on those volumetric expansion caused by isothermal sphere (for example, CO2, water vapor, and quenched steel parts that contain chromi- transformation of retained austenite into Hz in the Endogas (Ref 61, 67). um and/or molybdenum as the major alloy- martensite. A partially decarburized surface on the ing elements with or without nickel con- There are two tempering practices that part occurring during tool hardening also tent and where the hardening is done by lead to cracking problems: tempering too contributes to cracking because martensite direct quenching. soon after quenching, that is, before the transformation is completed therein well Microcracks are observed mostly in steel parts have transformed to martensite before the formation of martensite in the coarse-grained structures, such as large in hardening, and skin tempering, usually core. Decarburized surface on the tools has martensite plates. This is presumably be- observed in heavy sections (=>50 mm, or 2 reduced hardness, which will lead to prema- cause of more impingements of the larger in., thick in plates and >75 mm, or 3 in., in ture wear and scuffing. Partial decarburiza- plates of martensite by other large plates. diameter in round bars). 612 / Process and Quality Control Considerations

It is the normal practice to temper imme- changes in the metallurgical structure (for respectively; the volume increases involved diately after the quenching operations. In example, phase changes). These dimension- in the transformation of austenite to pearlite this case, some restraint must be exercised, al changes sometimes can be corrected by in the same steels are 2.4 and 1.33%, re- especially for large sections (>75 mm, or 3 mechanical processing to remove extra and spectively. Such volume increases are less in.) in deep-hardening alloy steels. The rea- unwanted material or to redistribute resid- in alloy steels and least in 2C-12Cr and A10 son is that the core has not yet completed its ual stresses or by heat treatment (annealing, tool steels. It should be noted that plastic transformation to martensite with the ex- tempering, or cold treatment). deformation (or strain) occurs during such pansion, whereas the surface and/or projec- When heat-treated parts suffer from dis- transformations at stresses that are lower tions, such as flanges, begin to temper with tortion beyond the permissible limits, it may than the yield stress for the phases present shrinkage. This simultaneous volume lead to scrapping of the article, rendering it (Ref 75). The occurrence of this plastic change produces radial cracks. This prob- useless for the service for which it was deformation, called the transformation plas- lem can become severe if rapid heating intended, or it may require necessary cor- ticity effect, influences the development of practice (for example, induction, flame, rection. Allowable distortion limits vary to stresses during the hardening of steel parts lead, or molten salt bath) is used for tem- a large extent, depending on service appli- (Ref 76). During quenching from the austen- pering. Therefore, very large and very intri- cations; in cases where very little distortion ite range, the steel contracts until the M~ cate tool steel parts should be removed can be tolerated, specially desired tool temperature is reached, then expands dur- from the quenching medium, and tempering steels are used. These steels possess metal- ing martensitic transformation; finally, ther- should be started while they are slightly lurgical characteristics that minimize distor- mal contraction occurs on further cooling to warm to hold comfortably in the bare hands tion. room temperature. As the hardening tem- (-60 °C, or 140 °F). perature increases, a greater amount of car- Skin tempering occurs in heavy section Types of Distortion bide goes into solution; consequently, both parts when the final hardness is >360 HB. Distortion is a general term that involves the grain size and the amount of retained This is due to insufficient tempering time all irreversible dimensional change pro- austenite are increased. This also increases and is usually determined when the surface duced during heat-treatment operations. the hardenability of steel. hardness falls by 5 or more HRC points This can be classified into two categories: More trouble with distortion comes from from the core hardness. This cracking often size distortion, which is the net change in the quenching or hardening operation than occurs several hours after the component specific volume between the parent and during heating for hardening, in which the has cooled from the tempering temperature transformation product produced by phase faster the cooling rate (that is, the more and often runs through the entire cross transformation without a change in geomet- severe the quenching), the greater the dan- section. This problem can be removed by rical form, and shape distortion or warpage, ger of distortion. When the milder quen- retempering for 3 h at the original tempering which is a change in geometrical form or chants are used, the extent of distortion is temperature, which is associated with a shape and is revealed by changes of curva- lessened. The severity of quenching thus change in hardness of 2 HRC points maxi- ture or curving, bending, twisting, and/or influences the distortion of components. mum (Ref 61). nonsymmetrical dimensional change with- The dependence of volume increase, par- out any volume change (Ref 72, 73). Usually ticularly in tools of different dimensions, on Distortion in Heat Treatment both types of distortion occur during a grain size (or hardenability) is another im- heat-treatment cycle. portant factor. Variations in volume during Distortion can be defined as an irrevers- Dimensional Changes Caused by Changes quenching of a fine-grained shallow-harden- ible and usually unpredictable dimensional in Metallurgical Structure during Heat Treat- ing steel in all but small sections is less than change in the component during processing ment. Various dimensional changes pro- a coarse-grained deep-hardening steel of the from heat treatment and from temperature duced by a change in metallurgical structure same composition. variations and loading in service. The term during the heat-treatment cycle of tool Tempering. There is a certain correlation dimensional change is used to denote steels are described below (Ref 74). between the tempering temperature and changes in both size and shape (Ref71). The Heating (Austenitizing). When annealed volume change. Tempering reduces the vol- heat-treatment distortion is therefore a term steel is heated from room temperature, ther- ume of martensite but not adequately often used by engineers to describe an un- mal expansion occurs continuously up to enough to equalize completely the prior controlled movement that has occurred in a Ac~, where the steel contracts as it trans- volume increase as a result of martensitic component as a result of heat-treatment forms from body-centered cubic (bcc) fer- transformation unless the components are operation (Ref 72). Although it is recog- rite to face-centered cubic (fcc) austenite. completely softened. In low-alloy and plain nized as one of the most difficult and trou- The extent of decrease in volumetric con- (medium- and high-) carbon steels, during blesome problems confronting the heat traction is related to the increased carbon the first and third stages of tempering, a treater and the heat-treatment industries on content in the steel composition (Table 4). decrease in volume occurs that is associated a daily basis, it is only in the simplest Further heating expands the newly formed with the decomposition of: high-carbon thermal heat-treatment methods that the austenite. martensite into low-carbon martensite plus mechanism of distortion is understood. Hardening. When austenite is cooled ~-carbide in the former stage, and aggregate Changes in size and shape of tool-steel parts quickly, martensite forms; at intermediate of low-carbon martensite and t-carbide into may be either reversible or irreversible. cooling rates, bainite forms; and at slow ferrite plus cementite in the latter stage. In Reversible changes, which are produced by cooling rates, pearlite precipitates. In all the second stage, however, an increase in applying stress in the elastic range or by these transformation sequences, the magni- volume takes place (due to the decomposi- temperature variation, neither induce tude of expansion increases with the de- tion of retained austenite into bainite) that stresses above the elastic limit nor cause crease in carbon content in the austenite tends to compensate for the early volume changes in the metallurgical structure. In (Table 4). The volume increase is maximum reduction. As the tempering temperature is this situation, the initial dimensional values when austenite transforms to martensite, increased further toward the A~, more pro- can be restored to their original state of intermediate with lower bainite, and is least nounced volume reduction occurs. In some stress or temperature. with upper bainite and pearlite (Table 4). highly alloyed tool-steel compositions, the Irreversible changes in size and shape of The volume increases associated with the volume changes during martensite forma- tool-steel parts are those that are caused by transformation of austenite to martensite in tion are less striking because of the large stresses in excess of the elastic limit or by 1 and 1.5% carbon steels are 4.1 and 3.84%, proportion of retained austenite and the Defects and Distortion in Heat-Treated Parts / 613

Table 7 Typical volume percentages of microconstituents existing in four different tool nents occur during heat treatment as a steels after their standard hardening treatments result of improper support of components Retained Undissolved or warped hearth in the hardening fur- As-quenched Martensite, austenite, carbides, nace. Hence, large, long, and complex- Steel Hardening treatment hardness, HRC vol% vol% vol% shaped parts must be properly supported W1 790 °C (1450 °F), 30 rain; WQ 67.0 88.5 9 2.5 at critical positions to avoid sagging or L3 845 °C (1550 °F), 30 min; OQ 66.5 90 7 3.0 preferably are hung with the long axis on M2 1225 °C (2235 °F), 6 rain; OQ 64 71.5 20 8.5 D2 1040 °C (1900 °F), 30 rain; AC 62 45 40 15 the vertical • Nonuniform agitation/quenching or non- Note: WQ, water quenched; OQ, oil quenched; AC, air cooled. uniform circulation of quenchant around a part results in an assortment of cooling rates that creates shape distortion (Ref resistance to tempering of alloy-rich mar- lesser extent than carbon (Ref 77). This 79). Uneven hardening, with the forma- tensite. These hardened steels show sharp table provides comparative data on size tion of soft spots, increases warpage. increases both in hardness and volume be- distortion in a variety of steels; however, Similarly, an increase in case depth, par- tween 500 and 600 °C (930 and 1110 °F) this information cannot be used alone to ticularly uneven case depths in case-hard- owing to the precipitation of very finely predict shape distortion factor. ening steels, increases warpage on dispersed alloy carbides from the retained Shape Distortion or Warpage. This is quenching (Ref 80) austenite. This produces a depleted matrix sometimes called straightness or angularity • Tight (that is, thin and highly adherent) in alloy content, raising the M~ temperature change. It is found particularly in nonsym- scale and decarburization, at least in cer- of retained austenite. During cooling down metrical components during heat treatment. tain areas. Tight scale is usually a prob- from the tempering temperature, further From the practical viewpoints, warpage in lem encountered in forgings hardened transformation of retained austenite into water- or oil-hardening steels is normally of from direct-fired gas furnaces having martensite will occur with an additional greater magnitude than is size distortion and high-pressure burners. Quenching in ar- increase in volume. is more of a problem because it is usually eas with tight scale is extremely retarded Size Distortion. Table 7 shows the typical not predictable. This is caused by the sum compared to the areas where the scale volume percentages of microconstituents effect of more than one of these factors: comes off. This produces soft spots, and, present in four different tool steels after in some cases, severe unpredicted distor- their standard hardening treatments. Typi- • Rapid heating (or overheating), drastic tion. Some heat treaters coat the compo- cal dimensional changes during hardening (or careless) quenching, or nonuniform nents with a scale-loosening chemical pri- and tempering of several tool steels are heating and cooling causes severe shape or to their entry into the furnace (Ref 79). given in Table 8. It is apparent here that distortion. Slow heating as well as pre- Similarly, the areas beneath the decarbur- some steels such as M3 and M41 high-speed heating of the parts prior to heating to ized surface do not harden as completely steels show appreciable increase in size of the austenitizing temperature yields the as the areas below the nondecarburized about 0.2% after hardening and tempering most satisfactory result. Rapid quench- surface. The decarburized layer also var- between 540 and 595 °C (1000 and 1100 °F) ing produces thermal and mechanical ies in depth and produces an inconsistent to produce complete secondary hardening. stresses associated with the martensitic softer region as compared to the region Other types, such as A10, expand very little transformation. In the case of low- and with full carbon. All these factors can when hardened and tempered over the en- high-hardenability steels, respectively, cause a condition of unbalanced stresses tire temperature range up to 595 °C (1100 this problem becomes severe or very with resultant distortion (Ref 79) °F). Excessive size changes in oil-hardening small • Long parts with small cross sections (>L nonshrinkable tool steel is usually caused • Residual stresses present in the compo- = 5d for water quenching, >L = 8d for oil by lack of stress relief (when necessary), nent before heat treating. These arise quenching, and >L = 10d for austemper- and hardening and/or tempering at the in- from machining, grinding, straightening, ing, where L is the length of the part, and correct temperature. The golden rule is to welding, casting, spinning, forging, and d is its diameter or thickness) learn to be suspicious of tools that are rolling operations, which will also furnish • Thin parts with larger areas (>A = 50t, seriously off size in only one dimension. It a marked contribution to the shape where A is the area of the part, and t is its is further noted that alloying addition in change (Ref 78) thickness) steels brings about a change in the specific • Applied stress causing plastic deforma- • Unevenness of, or greater variation in, volume of many microconstituents, but to a tion. Sagging and creep of the compo- section

Table 8 Typical dimensional changes during hardening and tempering of several tool steels Hardening treatment Total change in Total change in linear dimensions~ %, after tempering at Tool Temperature Quenching linear dimensions 150 *C 205 *C 260 *C 315 *C 370 *C 425 *C 480 *C 510 *C 540 *C 565 *C 595 *C steel ~U- 1¢ medium after quenching, % 300 *F 400 *F 500 *F 600 *F 700 *F 800 *F 900 *F 950 oF 1000 oF 1050 *F 1100 *F Ol 815 1500 Oil 0.22 0.17 0.16 0.18 • • • OI 790 1450 Oil 0.18 0.09 0.12 0.13 • - • 06 790 1450 Oil 0.12 0.07 0.10 0.14 0.10 0.00 -0.05 -0.06 -0.07 A2 955 1750 Air 0.09 0.06 0.06 0.08 0.07 • • • 0.05 0.04 0.06 A10 790 1450 Air 0.04 0.00 0.00 0.08 0.08 0.01 0.01 0.02 0.01 0.02 D2 11)10 1850 Air 0.06 0.03 0.03 0.02 0.00 .... 0.01 -0.02 0.06 D3 955 1750 Oil 0.07 0.04 0.02 0.01 -0.02 D4 1040 1900 Air 0.07 0.03 0.01 -0.01 -0.03 • • -0.4 -0.03 0.05 D5 1010 1850 Air 0.07 0.03 0.02 0.01 0.00 • ' 0.3 0.03 0.05

HII 1010 1850 Air 0.11 0.06 0.07 0.08 0.08 • • 0.3 0.01 0.12 HI3 1010 1850 Air -0.01 ..... 0.00 0.06 M2 1210 2210 Oil -0.02 -0.06 0.10 0.14 0.16 M41 1210 2210 Oil -0.16 -0.17 0.08 0,21 0.23 614 / Process and Quality Control Considerations

Examples of Distortion cessive, and the files can no longer be used surface on which the axle rests in the hous- in service. One practical solution is to give ing has to be given a high burnishing polish Ring Die. Quenching of ring die through the files a reverse camber prior to quench- employing a circular pressure tool that is the bore produces the reduction in bore ing. The dead fiat files could, however, be made of !.2C-1.5Cr steel. For satisfactory diameter as a result of formation of martens- made possible, and the judgment with re- results, the hardness of the tool surface ire, associated with the increased volume. gard to the actual camber needed depends should be about 60 HRC. It has been found In other words, metal in the bore is upset by upon the length and the slenderness of the that the tool usually cracks before its with- shrinkage of the surrounding metal and is recur files (Ref 82). drawal from the cold-water quenching bath. short when it cools (Ref 24). However, Similarly, when a long slender shear knife This problem may, however, be avoided by allover quenching causes the outside diam- is heat treated, it tends to curve like a dog's quenching the tool in water for 10 s prior to eter to increase and the bore diameter to tail, unless special precautions are taken. transferring it to an oil bath for finish increase or decrease, depending upon pre- Hardening of Chisels (Ref 63). Chisels quenching. Time quenching can be judi- cise dimensions of the part. When the out- about 460 mm (18 in.) long and made from ciously applied for many heat treatment side diameter of the steel part is induction- 13 mm (0.5 in.) AISI 6150 bar steel are problems of distortion or cracking. Stress- or flame-hardened (with water quench), it austenitized at 900 °C (1650 °F) for 1.5 h and relieving treatment after the use of the tool causes the part to shrink in outer diameter quenched in oil at 180 °C (360 °F) by stand- for some time may also enhance its perfor- (Ref 63). These are the examples of the ing in the vertical position with chisel point mance life. As indicated above, martemper- effect of mode of quenching on distortion down in special baskets that allow stacking ing is also one of the solutions for this (Ref 81). of two 13 mm (0.5 in.) round chisels per 650 problem (Ref 81). Thin die (with respect to wall thickness) is mm 2 (1 infl) hole. Subsequently, hardened Hardening of Case-Carburized Mild Steel. likely to increase in bore diameter, decrease chisels are tempered between 205 and 215 If oil-hardening steels are not available for in outside diameter, and decrease in thick- °C (400 and 420 °F) for 1.5 h. These heat- making a component, mild steel parts are ness when the faces are hardened. If the die treated parts show 55 to 57 HRC hardness carburized and water quenched to obtain has a very small hole, insufficient quench- but are warped. The reasons for this distor- the desired hardness, possibly resulting in ing of the bore may enlarge the hole diam- tion are: excessive distortion, which is very difficult to straighten without cracking. eter because the body of die moves with the • The portion of the bar that touches the outside hardened portion. Hardening of Carburized Low-Carbon basket cools slowly, producing uneven Bore of Finished Gear. Similarly, the bore Steel Rollers. The best course of quenching contraction and thermal stress of a finished gear might turn oval or change carburized En32 steel rollers (25 mm diam • The martensite formation is delayed on to such an extent that the shaft cannot be × _->600 mm long, or I in. diam × ->2 ft the inner or abutting side of the bar, fitted by the allowances that have been long), employed in textile printing, is to roll causing unequal expansion during trans- provided. Even a simple shape such as a them down skids into water-quenching formation. This distortion can be elimi- diaphragm or orifice plate may, after heat tanks because this produces less warpage nated or minimized by loading the parts in treatment, lose its flatness in such a way than when quenched slowly with the bar the screen-basket in such a way that that it may become unusable. either in vertical, horizontal, or inclined stacking arrangement permits sufficient Production of Long Pins. In the case of the positions. These are the procedures adopt- space between each part and by slightly production of long pins (250 mm long x 6 ed for hardening of cylinders with length decreasing the austenitizing temperature mm diameter, or 10 × V4 in.) made from considerably greater than the diameter. (Ref 62). Distortion can also be mini- medium-alloy steel, it was found, after con- Hardening of Helix Gears. The distortion mized by austempering the part, provided ventional hardening, that when mounted of the helical gears made of IS 20MnCrl that the carbon content is on the high side between centers, the maximum swing was grade steel (similar to AISI 5120) used as of specification to produce the lower bai- over 5 mm (0.20 in.). However, the camber the third speed gear in the gear box of Tara nitic structure of 55 to 57 HRC. If higher could be reduced to within acceptable limits trucks is an unavoidable natural conse- yield stress is not warranted, only chisel by martempering, intense or press quench- quence of the hardening process after car- ends need hardening and subsequent tem- ing. burizing. This type of distortion is linked pering (Ref 63) Hardening and Annealing of Long Bar. with increased length and decreased diam- When a 1% carbon steel bar, 300 mm long Hardening of a Two-Pounder Shot. The eter and occasionally increased helical an- (or more) × 25 mm diameter (12 in. long, or hardness of a two-pounder shot was specified gle (Ref 83). If the extent of distortion can more, × 1 in. diameter), is water quenched at 60 HRC on the nose and 35 HRC at the be controlled, a constant correction to the vertically from 780 °C (1435 °F), the bar base. A differential hardening technique was helix angle can be imparted in the soft-stage increases both in diameter and volume but performed on the shot made of a Ni-Cr-Mo manufacturing (machining) prior to heat decreases in length. When such bars are steel. This technique consisted of quenching treatment so that this correction can com- annealed or austenitized, they will sag badly the shot in the ice-cold water by its immersion pensate for the distorted angle and may between the widely spaced supports. in a tank up to the shoulder, followed by result in a gear with desired helix angle. Hence, they should be supported along drawing out the water from the tank at a Thus a constant magnitude of distortion their entire length in order to avoid distor- stipulated rate until the water line reached the without minimization is assured in every job tion. base of the nose. The final step involved of every batch of production in commercial Hardening of Half-Round Files. Files are withdrawing the shot from the tank when manufacturing. However, the residual stress usually made from hypereutectoid steel completely cold. The back end was then system and metallurgical properties such as containing 0.5% chromium. Files are heated softened by heating in a lead bath after initial core strength, case depth, surface hardness, to 760 °C (1400 °F) in an electric furnace tempering. The first few shots hardened in proper microhardness in the surface regions, after being surface coated with powdered this way were observed to split vertically and so forth, are assured (Ref 84). Similarly, wheat, charcoal, and ferrocyanide to pre- across the nose. The failure was, however, when heavy-duty tooth gear is gas carbur- vent decarburization. They are then avoided by withdrawal of the shot before ized and quenched to harden the surface quenched vertically in a water tank. On attaining ice-cold temperature and its subse- layer, the diameter and tooth span increase their removal from the tank, the files appear quent immersion in warm water (Ref 82). and tapering and bending also occur. like the proverbial dog's tail. The flat side Hardening of a Burnishing Wheel. In the Nitriding of Screw. A rolling mill screw, has curved down, the camber becomes ex- manufacture of railway axles, the gearing after liquid nitriding, may also show a small Defects and Distortion in Heat-Treated Parts / 615 decrease in length, which causes pitch er- agitation during hardening must be provid- out-of-roundness close to tolerance limits. rors in the screws (Ref 83). ed When hardening large hollows, either re- Induction and Flame Hardening of Spur straining bands on the outside during tem- Gears. Spur gears, after induction and flame Methods of Preventing Distortion pering or articulated fillers serve the same hardening, exhibit increased circular pitch, (Ref 82, 87) purpose. the error being maximum for the tooth Straightening is one method to remove or Quenching Fixtures. When water quench- groove quenched first. Similarly, in line- minimize distortion. Since straightening (af- ing or oil quenching is essential, distortion heating process, the thin plate undergoes ter hardening) can largely relieve the desir- can be minimal by employing properly de- convex bending and the thick plate concave able residual compressive stresses (in plain- signed quenching fixtures that forcibly pre- bending (Ref 83). carbon and low-alloy steels) that may cause vent the steel from distorting (Ref 88). Fig- breakage, it would be better to accomplish ure 14 shows a typical impingement-type this before the steel cools below the Ms quenching fixture. The requirements essen- Precautions temperature, that is, when the steel is in the tial for the better design of this type of Inadequate support during the heat-treat- metastable austenitic state (Ref 35). This fixture are as follows (Ref 79): temperature is above 260 °C (500 °F) for ment cycle, poorly designed jigs and quench- • There must be an accurate positioning of most tool steels and is preferably about 400 ing fixtures, or incorrect loading of the parts the part in the fixture. Whenever possi- °C (750 °F) for long shear knives, which are may cause distortion (Ref 73). In general, ble, round bars should be rotated during usually made of 2C-12Cr steel. Warping on plain-carbon and low-alloy steels have such a quenching to level out variations in jet parts such as shafts and spindles can be low yield strength at the hardening tempera- pressure around the part ture that the parts are capable of distorting corrected by straightening during or after hardening, followed by grinding to size (Ref • There should be an unhindered flow of under their own weight. Every care, there- quenchant through the sufficiently large fore, must be taken to ensure that parts are 84). Mostly high-alloy steels are straightened after hardening due to the higher per- holes (3.3 to 6.4 mm, or 0.13 to 0.25 in. in carefully supported or suspended during heat- diameter). Jets as large as 12.25 mm (0.50 centage of retained austenite and their com- ing; long parts are preferably heated in a in.) in diameter may be employed with paratively low yield stress. Straightening vertical furnace or with the length in the furnace-heated heavy sections (for exam- also can be accomplished during the tem- vertical plane (Ref 85). They should be ple, plates). A large portion of the excess pering process (Ref 35). However, straight- quenched in the vertical position with vertical quenchant with these large jets is for the ening of hardened parts with higher strength agitation of the quenchants. Also, it must be removal of scale (Ref 89) will cause a loss of fatigue properties and remembered that many tool steels are spoiled • Spacing between the holes should be rea- possibly initiation of cracks at the surface. by failure to provide enough support when sonably wide (for example, 4d, where d is Hence, straightening after the hardening they are taken out from the furnace for the hole diameter) treatment must be very carefully controlled quenching. Thus, every precaution is taken to • For oil-quenching fixtures, the facility to and should be followed by a low-tempera- ensure that parts are adequately supported submerge the part is required to reduce ture tempering treatment. during entire heat treatment by employing fumes and flashing The case-hardened (for example, nitrided, well-designed jigs, fixtures, and so on. • There must be the provision for efficient carburized) parts can be straightened to a Other precautions to minimize distortion cleaning of the holes very large extent as a result of their lower include: • A facility must be available to drain out core hardness. Nitrided parts may be straight- the hot quenchant for effective quenching • Tool steels should be heated to hardening ened at 400 °C (750 °F) (Ref 35). performance with cold quenchant temperature slowly, or in steps, and uni- Support and Restraint Fixtures. Fixtures formly. Hot salt baths are used to render for holding finished parts or assemblies dur- Pressure quenching is the most efficient fast, uniform heat input ing heat treatment may be either support or method of cooling parts from elevated tem- • It is best to heat small sections to the restraint type. For alloys that are subjected perature by using a combination of high lower region of the recommended hard- to very rapid cooling from the solution- pressure (such as 5 MPa, or 5 atm) and ening temperature range and to heat large treatment temperature, it is common prac- turbulent gas flow throughout the entire sections at the higher temperature range. tice to use minimum fixturing during solu- surface area of the workload (Ref 90). This Overheating by employing too high a tem- tion treatment and to control dimensional is economical and fast, provides even cool- perature or too long a heating time must relations by using restraining fixture during ing, offers a unique design and minimum be avoided aging. Support fixtures are used when re- distortion and improved metallurgical qual- • It is a good practice to protect the surface straint type is not needed or when the part ities. As a result of these beneficial effects of the component from decarburization itself renders adequate self restraint. Long this is suited to quench large-diameter tool- (by packing it in cast iron chips or using a narrow parts are very easily fixtured by ing for the aluminum extrusion industry; vacuum furnace, for example). If a sepa- hanging vertically. Asymmetrical parts may quench larger-diameter carburized gear, rate preheating furnace is not available, be supported by placing on a tray of sand or larger fasteners, and precision gears to be the part can be put in a cold furnace, after a ceramic casting formed to the shape of the jigged vertically; harden high-speed steel which the temperature is raised to proper part (Ref 64). Restraint fixtures may require tools (such as saw blades, dies, and other preheating temperature and kept at that machined grooves, plugs, or clamps. Some parts with edge configuration) and 718 jet temperature to attain uniform heating straightening of parts can be accomplished engine compressor blades (Ref 90). This is throughout, prior to proceeding to the in aging fixtures by forcing and clamping also employed to quench (vacuum pro- hardening temperature (Ref 86) slightly distorted parts into the fixture. The cessed) large sections of titanium alloy cast- • With the slower cooling rate, which is threaded fasteners for clamping should not ings for aircraft applications (Ref 91). Fig- consistent with good hardening practice, be used because they are difficult to remove ure 15 is a pressure-quench module that a lower thermal gradient will be devel- after heat treatment. It is preferable to use a may be attached to vacuum-sealed oped, thereby producing less distortion slotted bar held in place by a wedge (Ref quenched and continuous-vacuum furnace • Thus rapid heating and cooling rates of 64). The bore of a hub, the most important as a replacement for the oil-quench section. irregularly shaped parts must be avoided dimension in the hardening of thin spur Press quenching is widely employed in • Proper selection of quenchant with desir- gears, can be mechanically plugged to pre- preventing and controlling quench distor- able quenching properties and adequate vent the reduction of the bore and keep the tion in components where the geometry 616 / Process and Quality Control Considerations

Oil level treatment just before the final machining operation, which decreases the distortion to an appreciable extent. This is of special importance for intricate parts with closed dimensional tolerances (Ref 80). Stress re- "//'r/// duction is necessary to avoid distortion f during hardening and to avoid cracking re- J . # . ,// // sulting from the combination of residual In-hi III/ stress to the thermal stress produced during l//x heating to the hardening temperature. In the

"///" ""....i / / .,, ,11, event that stress relieving is not performed after heat treatment, large distortions of the E ¢ / / I / / / / ~--7i//., i part can be removed by heavy grinding. i//., ,//I However, the drawbacks of this operation are: possible elimination of most, if not all, of the hardened case of the carburized and hardened part; and danger of burning and crack formation on the surface layers. • ~'h,/z Hence, it is customary to stress relieve Plan view Section A- A plain carbon or low-alloy steel parts at a temperature of 550 to 650 °C (1020 to 1200 Fig 14 A typical impingement-type quenching fixture. Source: Ref 80 °F) (for I to 2 h), hot-worked and high-speed steels at 600 to 750 °C (1110 to 1380 °F), and the heavily machined or large parts at 650 renders them particularly prone to distor- l °C (1200 °F) (for 4 h) prior to final machining TIR = K- (Eq2) tion (Ref 92). For example, flat circular d and heat-treatment operations. Subresonant diaphragms of spring steel used in the con- stress relieving may also be employed to trol or measurement of pressure are press where TIR is the total indicator reading of neutralize thermally induced stress without quenched between two copper blocks, straightness, l is the length (in.), d is the changing the mechanical properties or the which cannot be accomplished by direct diameter (in.), and K is the constant = shape of the component. These components quenching (Ref 80). 10-4" include: large workpieces, premachined or Rolling Die Quenching. A rolling die For minimum yield strength requirements finish-machined structural or tubular, non- quench machine can provide uniform wa- of 310 MPa (45 ksi), air-hardened or normal- ferrous, hardened, nonsymmetrical or vary- ter quenching with minimal distortion for ized parts with negligible distortion can be ing section thickness, stationary, or assem- large-production runs. When a heated part produced (Ref 79). bled. However, this does not work on is placed on the rollers, the die closes and Stress Relieving. The presence of residual copper-rich alloys and the edges of burned the rolls turn. This removes any distortion stresses in the parts caused by cold work- plates (Ref 93). incurred during heating. According to ing, drawing, extrusion, forging, welding, manufacturers of rolling die quench ma- machining, or heading operations greatly Control of Distortion chines, symmetrical parts with the follow- increases the tendency of distortion. How- In order to remove or minimize distor- ing straightness can be achieved in produc- ever, these residual stresses can be relieved tion, the modern trend is to shift from tion: by subcriticai annealing or normalizing water-quenching practice to milder quenching, for example, oil quenching, polymer quenching, martempering, austempering, or Water Heating Mobile Pressure Heavy-duty Pressure jacket chamber insulating lock finned cooling lock even air-hardening practice. Milder quen- ~ barrier / chants produce slower and more uniform cooling of the parts, which drastically reduces the potential distortion. Other strate- gies of controlling distortion for age-hardening aluminum, beryllium, and other alloys include: alloy and temper selection, fixturing, age-hardening temperatures, proper machining, and stamping operations (Ref F-'.w°rkin . "~ / hR::r~T L/~ Id r .Work'in 1 '-~ "" 94). The fewer the number of reheats applied to components in case-hardening steels following carburizing, the smaller is the distortion on the finished part. When top priority is given to minimum distortion, it is desirable to make the parts from oil- Water Sonic velocity / ~ L .... / .. hardening steels with a controlled grain size jacket and to harden them by martempering direct from carburizing. Presently polyalkylene glycol-base quenchants, such as UCON Impeller drive quenchants HT and HT-NN, are variously used for direct quenching from the forging treatment, continuous cast quenching, and Fig 15 vacuumPressure'quenchfurnaces,modulesource: fOrRef attachment90 into standard vacuum-sealed quenched and continuous usual hardening of forged and cast steels and cast iron. In this case boiling does not Defects and Distortion in Heat-Treated Parts / 617 take place at the component surface but when specification for the case depth is to 25 mm (1 in.), and 17 to 22% for larger than rather at the external surface of the depos- be applied. 25 mm (1 in.) section thicknesses in casting ited polymer film. More uniform cooling Distortion may also occur after heat treat- alloys) with sufficient agitation, lower bath occurs, and thermal stresses are released. ment, with time, owing to the completion of temperature, proper fixture (throughout so- Because of the lower boiling point and high any unfinished transformation or the effect lutionizing, quenching, and age-hardening thermal conductivity, UCON quenchants of increased temperature during grinding. treatments), and straightening (in the as- act through the martensite zone more rap- For example, fully hardened components quenched state after taking out from the idly than oil (Ref 95). such as blade shears may be damaged by fixture) procedure. The initial cost of these Distortion during ferritic nitrocarburizing characteristic crazing pattern because of polymer solutions as a replacement to the is minimal because of low treatment tem- heavy and careless grinding. Local over- conventional hot-water quenching method perature and the absence of subsequent heating results in the transformation of un- is easily compensated for by other advan- phase transformations (Ref 66). There are decomposed austenite, and the accompany- tages such as reduced scrap, reduced ma- many methods of reducing distortion in ing changes in volume produce sufficient chining (compared to two machining opera- induction-hardened components; these stresses to cause cracking and developing of tions required--one before and another methods are usually found by experience a crazing pattern. after heat treatment--in the conventional with variables such as the hardening tem- Dimensional Stability. To achieve dimen- water-quenching method), and increased fa- perature and the type and temperature of sional stabilization or stability (that is, re- tigue life as a result of reduced convective quenching medium employed. Methods of tention of their exact size and shape) over heat transfer or film coefficient between the reducing distortion in induction-hardened long periods, which is a vital requirement part and the quenchant, more uniform parts include: the hardening of small spin- for gages and test blocks, the amount of quench, precise control of quench rates, dles held vertically in jigs; the plug-quench- retained austenite in heat-treated parts must and improved heat-transfer qualities from ing of gears to prevent the bores from be reduced because retained austenite slow- the deposition of liquid organic polymer on closing in; the flattening of cams by clamp- ly transforms and produces distortion when the surface of the part being quenched (Ref ing them together during tempering; and the the material is kept at room temperature, 97-99). This method costs less, therefore selective hardening of complex shapes (Ref heated, or subjected to stress. Dimensional saves time and allows easy shaping, bend- 96). stabilization also reduces internal (residual) ing, and twisting of the parts without estab- As a replacement of medium- or slow- stress, which causes distortion in service. lishing residual stresses. Such parts as lead- quenching oils, UCON quenchants E and Stabilization can be obtained by multiple ing edge wing skins, spars, and bulkheads E-NN can be readily used in induction- and tempering (with prolonged tempering are used in the aerospace industries (Ref flame-hardening operations, both in spray times); the first tempering reduces internal 96). and immersion types, for high-carbon and stress and facilitates its transformation to most alloy steels and traditional hardening martensite on cooling. The second and third Importance of Design of cast iron and cast or forged steels of retempering reduce the internal stress pro- complex geometry with better distortion- duced during the transformation of retained The wrong design of the tool material reduction properties. Agitation of quen- austenite. may result in the establishment of nonuni- chant should be carried out by motor-driven It is the usual practice to carry out a form heating and cooling of the compo- stirrers to move the medium with respect to single or repeated cold treatment after the nents, which produces overload and/or in- the part being quenched or by pumps that initial tempering treatment. In cold treat- ternal stresses leading to distortion and force the medium through the appropriate ment, the part is cooled below the Mf, failure during or after hardening. Correct orifice. Alternatively, the parts are moved which will cause the retained austenite to consideration at the design stage plays an through the medium, and for some applica- transform to martensite; the extent of trans- important role in lessening the distortion tions, spray quenchant is recommended. formation depends on whether the tool part and danger of cracking. The basic principle Water additives are sometimes employed in is untempered or first tempered. Cold treat- of successful design is to plan shapes that salt baths to increase heat extraction (Ref ment is normally accomplished in a refrig- will minimize the temperature gradient 64). erator at a temperature of -70 to -95 °C through the part during quenching. Funda- Ultrasonic quenching is also effective in (-100 to -140 °F). Tools must be retem- mental rules such as maintaining a simple, controlling distortion, which involves the pered immediately after return to room tem- uniform, regular, and symmetrical section introduction of ultrasonic energy (waves perature following cold treatment in order with comparatively few shape changes, en- with a frequency of 25 kHz) in the quench- to reduce internal stress and increase the suring small and smooth cross-sectional ing bath. This breaks down the vapor film toughness of the fresh martensite. Finally, size changes, and using large radii are still that surrounds the part in the initial stages they are ground to size. It may be pointed too frequently overlooked at the design of water or oil quenching (Ref 86). out that vibratory techniques are being used stage. Thus, successful heat treatment de- more frequently to achieve dimensional sta- mands a rational design that avoids sharp bility but do not offer any metallurgical corners as well as sudden and undue Distortion after Heat Treatment benefits (Ref 80), changes of section. Straightening. When every possible case It is often possible for tool designers to has been employed to minimize distortion, Distortion and Its Control in compensate for size distortion. For exam- it may still be essential to straighten after Heat-Treated Aluminum Alloys ple, in preparing precision hobs for gear heat treatment, which has already been The high levels of residual stress and cutting, dimensional accuracy must be kept discussed. distortion that are produced in the water- within very close tolerances. On linear lon- Grinding after Heat Treatment. In the quenched aluminum extrusion and forgings gitudinal growth, it is the general practice to case of carburized or nitrided parts, the (such as 2000, 6000, and 7000 series) and go out-of-round in the following high-speed metallurgist and designer, together with the aluminum castings can be reduced 60 to steel bars as much as 0.3% in M1 type, 0.2% production engineer, must collaborate re- 100% by using proper selection of polyalky- in M2 type, and 0.15% in T1 type during garding the amount to be removed by grind- lene glycol quenchant or polyvinyl pyrroli- heat treatment. These data will alter slightly ing after heat treatment. This grinding al- done 90 concentration (for example, 25% with changes in design of the hobs, but lowance must be taken into account when solutions for wrought alloys, 20 to 30% essentially the growth in tungsten-base determining the initial dimensions and also UCON quenchant A for thicknesses up to high-speed steel is lower than that of the 618 / Process and Quality Control Considerations molybdenum-base high-speed steel (M1 and • If sharp corners are unavoidable, provide ceedings of Hardenability Concepts M2). This does not require any difficulty if relief notches in place of sharp edges with Applications to Steel, D.V. the growth is compensated for and if the • The insertion of identification marks on Doane and J.S. Kirkaidy, Ed., TMS- steel is consistent in its growth (Ref 87). the hardened component is recommend- AIME, 1978, p 579-606 The distortion produced in the surface ed, preferably after hardening with tools 22. E.B. Evans, in Encyclopaedia of Ma- hardening of long shafts by the scanning having well-rounded edges and minimum terials Science and Engineering, Per- method can be a great problem if the equip- deformation (shallow penetration depth), gamon Press, 1986, p 4183-4188 ment is not in very good condition. Due and at positions far away from the high- 23. R.F. Kern and M.E. Suess, Steel Se- consideration must be given so that locating stress concentration zones (reentrant an- lection, Wiley-Interscience, 1979 centers run concentrically, in line and at the gles, bends, and so on) (Ref 101) 24. R.F. Kern, Selecting Steels and De- appropriate speed; the coil must be accurate- • Large intricate dies should be made up in signing Parts for Heat Treatment, ly aligned, and the quench must be correctly sections, which frequently simplifies heat American Society for Metals, 1969 designed with sufficient number of holes of treatment (Ref 64) 25. R.B. Liss, C.G. Massieon, and A.S. suitable size and angle. 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