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CHAPTER 1

Process Selection Guide

SURFACE HARDENING, a process that • Thermochemical diffusion methods, which includes a wide variety of techniques (Table 1), modify the chemical composition of the sur- is used to improve the wear resistance of parts face with hardening species such as carbon, without affecting the more soft, tough interior of nitrogen, and boron. Diffusion methods allow the part. This combination of hard surface and effective hardening of the entire surface of a resistance to breakage on impact is useful in part and are generally used when a large num- parts such as a cam or ring gear that must have ber of parts are to be surface hardened. a very hard surface to resist wear, along with a • Applied energy or thermal methods, which tough interior to resist the impact that occurs do not modify the chemical composition of during operation. Further, the surface hardening the surface but rather improve properties by of steel has an advantage over through harden- altering the surface metallurgy; that is, they ing, because less expensive low- and medium- produce a hard quenched surface without carbon steels can be surface hardened without additional alloying species. the problems of distortion and cracking associ- • Surface coating or surface-modification ated with the through hardening of thick sec- methods, which involve the intentional tions. buildup of a new layer on the steel substrate There are three distinctly different ap- or, in the case of ion implantation, alter the proaches to the various methods for surface subsurface chemical composition hardening (Table 1): Each of these approaches for surface hardening is briefly reviewed in this chapter, with emphasis placed on process comparisons to Table 1 Engineering methods for surface facilitate process selection. More detailed infor- hardening of steels mation on the various methods described can be found in subsequent chapters. Diffusion methods Carburizing Nitriding Carbonitriding Diffusion Methods Nitrocarburizing Boriding of Surface Hardening Thermal diffusion process Applied energy methods As previously mentioned, surface hardening Flame hardening by diffusion involves the chemical modification Induction hardening of a surface. The basic process used is thermo- Laser beam hardening Electron beam hardening chemical, because some heat is needed to enhance the diffusion of hardening species into the Coating and surface modification surface and subsurface regions of a part. The Hard chromium plating Electroless nickel plating depth of diffusion exhibits a time-temperature Thermal spraying dependence such that: Weld hardfacing Chemical vapor deposition Case depth K Time (Eq 1) Physical vapor deposition Ion implantation where the diffusivity constant, K, depends on Laser surface processing temperature, the chemical composition of the © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) 2 / Surface Hardening of Steels

steel, and the concentration gradient of a given of the absence of oxygen in the furnace at- hardening species. In terms of temperature, the mosphere. Salt bath and pack carburizing have diffusivity constant increases exponentially as a little commercial importance but are still done function of absolute temperature. Concentration occasionally. gradients depend on the surface kinetics and Gas carburizing can be run as a batch or a reactions of a particular process. continuous process. Furnace atmospheres con- Methods of hardening by diffusion include sist of a carrier gas and an enriching gas. The several variations of hardening species (such as carrier gas is supplied at a high flow rate to carbon, nitrogen, or boron) and of the process ensure a positive furnace pressure, minimizing method used to handle and transport the harden- air entry into the furnace. The type of carrier gas ing species to the surface of the part. Process affects the rate of carburization. Carburization methods for exposure involve the handling of by methane is slower than by the decomposition hardening species in forms such as gas, liquid, of carbon monoxide (CO). The enriching gas or ions. These process variations naturally pro- provides the source of carbon and is supplied at duce differences in typical case depth and hard- a rate necessary to satisfy the carbon demand of ness (Table 2). Factors influencing the suitabil- the work load. ity of a particular diffusion method include the Most gas carburizing is done under condi- type of steel, the desired case hardness, and the tions of controlled carbon potential by measure- case depth. ment of the CO and carbon dioxide (CO2) con- It is also important to distinguish between tent. The objective of the control is to maintain total case depth and effective case depth. The a constant carbon potential by matching the loss effective case depth is typically approximately in carbon to the workpiece with the supply of two-thirds to three-fourths the total case depth. enriching gas. The carburization process is The required effective depth must be specified complex, and a comprehensive model of car- so that the heat treater can process the parts for burization requires algorithms that describe the the correct time at the proper temperature. various steps in the process, including carbon diffusion, kinetics of the surface reaction, kinetics of the reaction between the endogas and Carburizing enriching gas, purging (for batch processes), Carburizing is the addition of carbon to the and the atmospheric control system. surface of low-carbon steels at temperatures Vacuum carburizing is a nonequilibrium, (generally between 850 and 950 °C, or 1560 and boost-diffusion-type carburizing process in 1740 °F) at which austenite, with its high solu- which austenitizing takes place in a rough vac- bility for carbon, is the stable crystal structure. uum, followed by carburization in a partial pres- Hardening of the component is accomplished sure of hydrocarbon gas, diffusion in a rough by removing the part and quenching or allowing vacuum, and then quenching in either oil or gas. the part to slowly cool and then reheating to the Vacuum carburizing offers the advantages of austenitizing temperature to maintain the very excellent uniformity and reproducibility be- hard surface property. On quenching, a good cause of the improved process control with vac- wear- and fatigue-resistant high-carbon marten- uum furnaces, improved mechanical properties sitic case is superimposed on a tough, low- due to the lack of intergranular oxidation, and carbon steel core. Carburized steels used in case reduced cycle time. The disadvantages of vac- hardening usually have base carbon contents uum carburizing are predominantly related to of approximately 0.2 wt%, with the carbon con- equipment costs and throughput. tent of the carburized layer being fixed between Plasma (ion) carburizing is basically a 0.8 and 1.0 wt%. Carburizing methods include vacuum process using glow-discharge technol- gas carburizing, vacuum carburizing, plasma ogy to introduce carbon-bearing ions to the steel (ion) carburizing, salt bath carburizing, and surface for subsequent diffusion. This process is pack carburizing. These methods introduce car- effective in increasing carburization rates, bon by use of an atmosphere (atmospheric gas, because the process bypasses several dissocia- plasma, and vacuum), liquids (salt bath), or tion steps that produce active soluble carbon. solid compounds (pack). The vast majority of For example, because of the ionizing effect of carburized parts are processed by gas carburiz- the plasmas, active carbon for adsorption can be ing, using natural gas, propane, or butane. Vac- formed directly from methane (CH4) gas. High uum and plasma carburizing are useful because temperatures can be used in plasma carburizing, © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) Process Selection Guide / 3 m (1 µ distortion), slightly harder case than carburizing, gas control critical high equipment costs, close case control mil), no white layer, most are proprietary processes quenching not required, low distortion, process is slow, usually a batch than gas carburizing, high equipment costs applied over hardened tool steels, high process temperature can cause distortion salt disposal problem, baths require frequent maintenance applied over hardened tool steels, high process temperature can cause distortion continuous operation, good gas controls required, can be dangerous case depth accurately carbon steel, most processes are proprietary batch process, salt disposal problems Lower temperature than carburizing (less Faster than gas nitriding, no white layer, Usually used for thin hard cases <25 Hardest cases from nitriding steels, Excellent process control, bright parts, faster Produces a hard compound layer, mostly Faster than pack and gas processes, can pose Produces a hard compound layer, mostly Good control of case depth, suitable for Low equipment costs, difficult to control metals Process characteristics Typical base carbon alloy steels, stainless steel steels, stainless steels including cast irons steels, stainless steels carbon alloy steels medium-carbon steels carbon alloy steels cobalt and nickel alloys carbon alloy steels carbon alloy steels HRC 50–65(a) Low-carbon steels, low- 50–70 Alloy steels, nitriding 50–70 Most ferrous metals 50–70 Alloy steels, nitriding 50–63(a) Low-carbon steels, low- >70 Tool steels, alloy 50–65(a) Low-carbon steels, low- 40–>70 Alloy steels, tool 50–63(a) Low-carbon steels, low- 40–60(a) Low-carbon steels Low-distortion process for thin case on low- 50–63(a) Low-carbon steels, low- 50–65(a) Low-carbon steels Good for thin cases on noncritical parts, Case hardness, m m m µ µ µ m m–0.75 mm m–1.5 mm µ depth m–0.75 mm m–0.75 mm m–0.75 mm m–1.5 mm m–1.5 mm m–1.5 mm µ µ µ µ µ µ µ µ (0.1–5 mils) (3–30 mils) (3–30 mils) (0.1–30 mils) (5–30 mils) (3–60 mils) (0.08–0.8 mil) (0.5–2 mils) (3–60 mils) (0.1–1 mil) (5–60 mils) Typical case 2.5–125 75 75 2.5 125 75 2–20 50 (2–60 mils) 12.5–50 75 2.5–25 125 Process (1400–1600) (1400–1600) (650–1050) (950–1050) (900–1100) (1500–2000) (1475–2285 °F) (1500–1800) (750–2100) (1500–1800) (1050–1250) (1500–2000) 760–870 760–870 340–565 510–565 480–590 800–1250°C 815–980 400–1150 565–675 temperature, °C (°F) and nitrogen and nitrogen nitrogen compounds nitrogen compounds nitrogen compounds via salt bath processing possibly nitrogen boron compounds and nitrogen Diffused carbide layers Diffused carbon process nitrocarburizing Table 2 Typical characteristics of diffusion treatments Liquid (cyaniding) Diffused carbon Carbonitriding Gas Diffused carbon Ion Diffused nitrogen, Salt Diffused nitrogen, Nitriding Gas Diffused nitrogen, Vacuum Diffused carbon 815–1090 Thermal diffusion Liquid Diffused carbon and Gas Diffused carbon 815–980 Other Boriding Diffused boron, ProcessCarburizing Pack Nature of case Diffused carbon 815–1090 (a) Requires quench from austenitizing temperature Ferritic © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) 4 / Surface Hardening of Steels

because the process takes place in an oxygen- Liquid nitriding (nitriding in a molten salt free vacuum, thus producing a greater carbur- bath) uses temperatures similar to those used in ized case depth than both atmospheric gas and gas nitriding and a case-hardening medium of vacuum carburizing. molten, nitrogen-bearing, fused salt bath con- Salt bath or liquid carburizing is a taining either cyanides or cyanates. Similar to method of case hardening steel in a molten salt salt bath carburizing, liquid nitriding has the bath that contains the chemicals required to pro- advantage of processing finished parts because duce a case comparable with one resulting from dimensional stability can be maintained due to gas or pack carburizing. Carburizing in liquid the subcritical temperatures used in the process. salt baths provides a convenient method of case Furthermore, at the lower nitriding tempera- hardening, with low distortion and considerable tures, liquid nitriding adds more nitrogen and flexibility and uniformity of control of the case. less carbon to ferrous materials than that ob- However, the expense and environmental prob- tained with high-temperature treatments be- lems associated with disposing of salt baths, cause ferrite has a much greater solubility for particularly those containing cyanide, have lim- nitrogen (0.4% max) than carbon (0.02% max). ited the use of this process, although non- Plasma (ion) nitriding is a method of sur- cyanide-containing salts have been developed. face hardening using glow-discharge technol- Pack carburizing is the oldest carburizing ogy to introduce nascent (elemental) nitrogen to process. In this case-hardening method, parts the surface of a metal part for subsequent diffu- are packed in a blend of coke and charcoal with sion into the material. The process is similar to “activators” and then heated in a closed con- plasma carburizing in that a plasma is formed in tainer. Although a labor-intensive process, pack a vacuum using high-voltage electrical energy, carburizing is still practiced in some tool rooms, and the nitrogen ions are accelerated toward the because facility requirements are minimal. workpiece. The ion bombardment heats the part, cleans the surface, and provides active Nitriding nitrogen. The process provides better control of Nitriding is a process similar to carburizing, case chemistry, case uniformity, and lower part in which nitrogen is diffused into the surface of distortion than gas nitriding. a ferrous product to produce a hard case. Unlike Carbonitriding and carburizing, nitrogen is introduced between 500 and 550 °C (930 and 1020 °F), which is below Ferritic Nitrocarburizing the austenite formation temperature (Ac1) for Carbonitriding introduces both carbon and ferritic steels, and quenching is not required. As nitrogen into the austenite of the steel. The a result of not austenitizing and quenching to process is similar to carburizing in that the form martensite, nitriding results in minimum austenite composition is enhanced and the high distortion and excellent control. The various surface hardness is produced by quenching to nitriding processes (Table 2) include gas nitrid- form martensite. This process is a modified ing, liquid nitriding, and plasma (ion) nitriding. form of gas carburizing in which ammonia is Gas nitriding is a case-hardening process introduced into the gas-carburizing atmosphere. that takes place in the presence of ammonia gas. As in gas nitriding, elemental nitrogen forms at Either a single-stage or a double-stage process the work-piece surface and diffuses along with can be used when nitriding with anhydrous carbon into the steel. Typically, carbonitriding ammonia. The single-stage process, in which a takes place at a lower temperature and a shorter temperature of 495 to 525 °C (925 to 975 °F) is time than gas carburizing, producing a shal- used, produces the brittle nitrogen-rich com- lower case. Steels with carbon contents up to pound zone known as the white nitride layer at 0.2% are commonly carbonitrided. the surface of the nitrided case. The double- Ferritic nitrocarburizing is a subcritical stage process, or Floe process, has the advan- heat treatment process, carried out by liquid, tage of reducing the white nitrided layer thick- gaseous, or plasma techniques, and involves the ness. After the first stage, a second stage is diffusion of carbon and nitrogen into the ferritic added either by continuing at the first-stage phase. The process results in the formation of a temperature or increasing the temperature to thin white layer or compound layer with an 550 to 565 °C (1025 to 1050 °F). The use of the underlying diffusion zone of dissolved nitrogen higher-temperature second stage lowers the in iron, or alloy nitrides. The white layer case hardness and increases the case depth. improves surface resistance to wear, and the dif- © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) Process Selection Guide / 5

fusion zone increases the fatigue endurance bonded carbide or nitride coating at the sub- limit, especially in carbon and low-alloy steels. strate surface. Alloy steels, cast irons, and some stainless The hard alloy carbide, nitride, and carboni- steels can be treated. The process is used to pro- tride coatings in the TD method can be applied to duce a thin, hard skin, usually less than 25 µm steels by means of salt bath processing or flu- (1 mil) thick, on low-carbon steels in the form of idized beds. The salt bath method uses molten sheet metal parts, powder metallurgy parts, borax with additions of carbide-forming ele- small shaft sprockets, and so forth. ments, such as vanadium, niobium, titanium, or chromium, which combine with carbon from the Boriding substrate steel to produce alloy carbide layers. Because the growth of the layers is dependent on Boriding, or boronizing, is a thermochemical carbon diffusion, the process requires a relatively surface-hardening process that can be applied to high temperature, from 800 to 1250 °C (1470 to a wide variety of ferrous, nonferrous, and cer- 2280 °F), to maintain adequate coating rates. met materials. The boronizing pack process is Carbide coating thicknesses of 4 to 7 µm are pro- similar to pack carburizing, with the parts to be duced in 10 min to 8 h, depending on bath temper- coated being packed with a boron-containing ature and type of steel. The coated steels may be compound such as boron powder or ferroboron. cooled and reheated for hardening, or the bath Activators such as chlorine and fluorine com- temperature may be selected to correspond to the pounds are added to enhance the production of steel austenitizing temperature, permitting the the boron-rich gas at the part surface. Process- steel to be quenched directly after coating. ing of high-speed tool steels that were previously quench hardened is accomplished at 540 °C (1000 °F). Boronizing at higher temperatures up to 1090 °C (2000 °F) causes diffu- Surface Hardening by Applied Energy sion rates to increase, thus reducing the process time. The boron case does not have to be The surface methods described in this section quenched to obtain its high hardness, but tool include conventional thermal treatments, such steels processed in the austenitizing temperature as flame and induction hardening, and technolo- range need to be quenched from the coating gies that incorporate high-energy laser or elec- temperature to harden the substrate. tron beams. All of these methods may be classi- Boronizing is most often applied to tool steels fied as simply a thermal treatment without or other substrates that are already hardened by chemistry changes. They can be used to harden heat treatment. The thin (12 to 15 µm, or 0.48 to the entire surface or localized areas. When 0.6 mil) boride compound surfaces provide even localized heating is carried out, the term selec- greater hardness, improving wear service life. tive surface hardening is used to describe these Distortion from the high processing tempera- methods. tures is a major problem for boronized coatings. Flame hardening consists of austenitizing Finished parts that are able to tolerate a few thou- the surface of steel by heating with an oxy- sandths of an inch (75 µm) distortion are better acetylene or oxyhydrogen torch and immedi- suited for this process sequence, because the thin ately quenching with water. After quenching, coating cannot be finish ground. the microstructure of the surface layer consists of hard martensite over a lower-strength interior core of other steel morphologies, such as ferrite Thermal Diffusion Process and pearlite. A prerequisite for proper flame The thermal diffusion (TD) process is a hardening is that the steel must have adequate method of coating steels with a hard, wear- carbon and other alloy additions to produce the resistant layer of carbides, nitrides, or carboni- desired hardness, because there is no change in trides. In the TD process, the carbon and nitro- composition. Flame-hardening equipment uses gen in the steel substrate diffuse into a deposited direct impingement of a high-temperature flame layer with a carbide-forming or nitride-forming or high-velocity combustion product gases to element, such as vanadium, niobium, tantalum, austenitize the component surface and quickly chromium, molybdenum, or tungsten. The dif- cool the surface faster than the critical cooling fused carbon or nitrogen reacts with the carbide- rate to produce martensite in the steel. This is and nitride-forming elements in the deposited necessary because the hardenability of the com- coating to form a dense and metallurgically ponent is fixed by the original composition of © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) 6 / Surface Hardening of Steels

the steel. Thus, equipment design is critical to possible, and there can be greater accessibility the success of the operation. Flame-heating to hard-to-get areas with the flexibility of opti- equipment may be a single torch with a spe- cal manipulation of light energy. cially designed head or an elaborate apparatus Electron Beam Hardening. In electron that automatically indexes, heats, and quenches beam hardening, the surface of the hardenable parts. With improvements in gas-mixing equip- steel is heated rapidly to the austenitizing tem- ment, infrared temperature measurement and perature, usually with a defocused electron beam control, and burner rig design, flame hardening to prevent melting. The mass of the workpiece has been accepted as a reliable heat treating conducts the heat away from the treated surface process that is adaptable to general or localized at a rate that is rapid enough to produce harden- surface hardening for small or medium-to-high ing. Materials for application of electron beam production requirements. hardening must contain sufficient carbon and Induction heating is an extremely versatile alloy content to produce martensite. With the heating method that can perform uniform surface rapid heating associated with this process, the hardening, localized surface hardening, through carbon and alloy content should be in a form that hardening, and tempering of hardened pieces. quickly allows complete solid solution in Heating is accomplished by placing a steel part in the austenite at the temperatures produced by the magnetic field generated by high-frequency the electron beam. In addition, the mass of the alternating current passing through an inductor, workpiece should be sufficient to allow proper usually a water-cooled copper coil. The depth of quenching; for example, the part thickness must heating produced by induction is related to the be at least ten times the depth of hardening, and frequency of the alternating current: the higher hardened areas must be properly spaced to pre- the frequency is, the thinner or more shallow vent tempering of previously hardened areas. the heating. Therefore, deeper case depths and To produce an electron beam, a high vacuum even through hardening are produced by using of 10–3 Pa (10–5 torr) is required in the region lower frequencies. The electrical considerations where the electrons are emitted and accelerated. involve the phenomena of hysteresis and eddy This vacuum environment protects the emitter currents. Because secondary and radiant heat are eliminated, the process is suited for production line areas. Table 3 compares the flame- and Table 3 Comparison of flame- and induction-hardening processes. induction-hardening processes Laser surface heat treatment is widely Characteristics Flame Induction used to harden localized areas of steel and cast Equipment Oxyfuel torch, Power supply, iron machine components. The heat generated special head inductor, by the absorption of the laser light is controlled quench system quench system to prevent melting and is therefore used in the Applicable Ferrous alloys, Same material carbon steels, selective austenitization of local surface regions, alloy steels, which transform to martensite as a result of rapid cast irons Speed of heating Few seconds to few 1–10 s cooling (self-quenching) by the conduction of minutes heat into the bulk of the workpiece. This process Depth of 1.2–6.2 mm 0.4–1.5 mm is sometimes referred to as laser transformation hardening (0.050–0.250 in.) (0.015–0.060 in.); 0.1 mm hardening to differentiate it from laser surface (0.004 in.) for melting phenomena. There is no chemistry impulse change produced by laser transformation hard- Processing One part at a time Same Part size No limit Must fit in coil ening, and the process, similar to induction and Tempering Required Same flame hardening, provides an effective technique Can be automated Yes Yes Operator skills Significant skill Little skill to harden ferrous materials selectively. required required The process produces typical case depths for after setup steel ranging from 0.75 to 1.3 mm (0.030 to Control of process Attention required Very precise Operator comfort Hot, eye protection Can be done in 0.050 in.), depending on the laser power range, required suit and hardness values as high as 60 HRC. Laser Cost processing has advantages over electron beam Equipment Low High Per piece Best for large Best for small hardening in that laser hardening does not work work require a vacuum, wider hardening profiles are © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) Process Selection Guide / 7

from oxidizing and avoids scattering of the elec- • Low coefficient of friction trons while they are still traveling at a relatively • Thick layers possible low velocity. Electron beam hardening in hard Disadvantages: vacuum units requires that the part be placed in a chamber that is sufficiently large to manipu- • Poor thickness uniformity on complex com- late the electron beam gun or the workpiece. ponents Out-of-vacuum units usually involve shrouding • Hydrogen embrittlement the workpiece; a partial vacuum (13 Pa, or 10–2 • Environmental problems associated with torr), is obtained in the work area by mechanical plating bath disposal. Chromium replacement pumps. coatings, such as electroless nickel and thermal spray coatings, are being used increas- ingly. Surface Hardening by Coating or Surface Modification Electroless Nickel Coating Process Description. The coating is Plating or coating treatments deposit hard deposited by an autocatalytic chemical reduc- surface layers of completely different chem- tion of nickel ions by hydrophosphite, amino- istry, structure, and properties on steel sub- borane, or borohydride compounds. Currently, strates and are applied by well-established tech- hot acid hypophosphite-reduced baths are most nologies such as electrodeposition, electroless frequently chosen to coat steel. Heat-treated deposition, thermal spraying, and weld hardfac- deposit hardness exceeds 1000 HV. ing. In more recent years, coating or surface- Applications. Electroless nickel coatings modification methods long used in the electron- have good resistance to corrosion and wear and ics industry to fabricate thin films and devices are used to protect machinery found in the have been used to treat steels. These include petroleum, chemicals, plastics, optics, printing, vapor deposition techniques and ion implanta- mining, aerospace, nuclear, automotive, elec- tion. Laser surface processing (melting, alloy- tronics, computers, textiles, paper, and food ing, and cladding) has also been carried out industries. on steels. These various surface-engineering treatments can deposit very thin films (e.g., 1 Advantages: to 10 µm for physical vapor deposition) or thick • Low-temperature treatment (<100 °C, or coatings (e.g., 3 to 10 mm for weld hardfacing). 212 °F) • More corrosion resistance than electroplated Hard Chromium Plating chromium Process Description. Hard chromium plat- • Ability to coat complex shapes uniformly ing is produced by electrodeposition from a • Incorporation of hard particles to increase hardness solution containing chromic acid (CrO3) and a catalytic anion in proper proportion. The metal • Good solderability and brazeability so produced is extremely hard (850 to 1000 HV) Disadvantages: and corrosion resistant. Plating thickness ranges • Higher costs than electroplating from 2.5 to 500 µm (0.1 to 20 mils). • Poor welding characteristics Applications. Hard chromium plating is • Slower plating rate, as compared to rates for used for products such as piston rings, shock electrolytic methods absorbers, struts, brake pistons, engine valve • Heat treatment needed to develop optimal stems, cylinder liners, and hydraulic rods. Other properties applications are for aircraft landing gears, tex- tile and gravure rolls, plastic rolls, and dies and molds. The rebuilding of mismachined or worn Thermal Spraying parts comprises large segments of the industry. Process Description. Thermal spraying is a Advantages: generic term for a group of processes in which a metallic, ceramic, cermet, and some polymeric • Low-temperature treatment (60 °C, or 140 °F) materials in the form of powder, wire, or rod are • High hardness and wear resistance fed to a torch or gun with which they are heated to © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) 8 / Surface Hardening of Steels

or slightly above their melting point. The result- Advantages: ing molten or nearly molten droplets of material are accelerated in a gas stream and projected • Most metals, ceramics, and some polymers against the substrate to form a coating. Com- can be sprayed. monly employed methods of deposited thermal • Significant substrate heating does not occur spray coatings can be classified as wire flame with most thermal spray processes. spray, powder flame spray, electric arc, plasma • Worn or damaged coatings can be stripped spray, and high-velocity oxyfuel (HVOF) spray. without changing the properties or dimen- Process characteristics are compared in Table 4. sions of the part. • Localized treatments are possible. Applications. Thermal spray coatings are used for prevention against wear, corrosion, or Disadvantages: oxidation. Table 19 in Chapter 11 lists a wide variety of wear-resistant applications for ther- • Line-of-sight process employed. mal spraying. Thermal spray coatings, particu- • Most sprayed coatings contain some porosity. larly HVOF coatings, are being used increas- • The adhesion of sprayed coatings is generally ingly to replace chromium electrodeposits. poor, compared to other processes.

Table 4 Comparison of major thermal spray coating processes

Process

Property Wire ﬂame Powder ﬂame High-velocity or characteristic Coating type spray spray Electric arc Plasma spray oxyfuel (HVOF)

Bond strength, Ferrous metals 14 (2) 28 (4) 41 (6) 34+ (5+) 62 (9) MPa (103 psi) Nonferrous metals 21 (3) 21 (3) 41+ (6+) 34+ (5+) 70 (10.2) Self-fluxing alloys . . . 69+ (10+)(a) ...... 62 (9)(b) Ceramics . . . 14–34 (2–5) . . . 21+ (3+) . . . Carbides . . . 34–48 (5–7) . . . 55–69 (8–10) 83+ (12+) Density, % that of equivalent Ferrous metals 90 90 90 95 98+ wrought material Nonferrous metals 90 90 90 95 98+ Self-fluxing alloys . . . 100(a) ...... 100(a) Ceramics . . . 95 . . . 95+ . . . Carbides . . . 90 . . . 95+ 98+ Hardness Ferrous metals 84 HRB– 80 HRB– 95 HRB– 80 HRB– 90 HRB– 35 HRC 35 HRC 40 HRC 40 HRC 50 HRC Nonferrous metals 95 HRH– 30 HRH– 40 HRH– 40 HRH– 100 HRH– 40 HRC 20 HRC 80 HRB 40 HRC 55 HRC Self-fluxing alloys . . . 30–60 HRC ...... 50–60 HRC Ceramics . . . 50–65 HRC . . . 50–70 HRC . . . Carbides . . . 50–60 HRC . . . 50–60 HRC 55–65 HRC Permeability Ferrous metals Medium Medium High Low Negligible Nonferrous metals Medium Medium High Low Negligible Self-fluxing alloys . . . None(a) ...... None(a) Ceramics . . . Medium . . . Low . . . Carbides . . . Low . . . Low Negligible Coating-thickness limitation, Ferrous metals 1.25–2.5 1.25–2.5 1.25–2.5 1.25–2.5 1.25–2.5 mm (in.) (0.05–0.1) (0.05–0.1) (0.05–0.1) |(0.05–0.1) (0.05–0.1) Nonferrous metals 1.25–5 1.25–5 1.25–5 1.25–5 2.5–5 (0.05–0.2) (0.05–0.2) (0.05–0.2) (0.05–0.2) (0.1–0.2) Self-fluxing alloys . . . 0.4–2.5 ...... 1.25 (0.015–0.1) (0.05) Ceramics . . . 0.4 . . . 0.4 . . . (0.015) (0.015) max Carbides . . .0.4 . . . 0.4 0.6 (0.015) (0.015) max (0.025)

(a) Fused coating. (b) Unfused coating © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) Process Selection Guide / 9

• High-quality coatings on reentrant surfaces Chemical Vapor Deposition (CVD) produced with difficulty. Process Description. Chemical vapor deposition involves the formation of a coating on a Weld Hardfacing heated surface by a chemical reaction from the Process Description. Welding is a solidifi- vapor or gas phase. Deposition temperatures are cation method for applying coatings with corro- generally in the range of 800 to 1000 °C (1470 sion, wear, and erosion resistance. Weld-overlay to 1830 °F). The most widely deposited wear- coatings, sometimes referred to as hardfacing, resistant coatings are titanium carbide (TiC), offer unique advantages over other coating sys- titanium nitride (TiN), chromium carbide, and tems in that the overlay/substrate weld provides alumina. Thicknesses are restricted to approxi- a metallurgical bond that is not susceptible to mately 10 µm due to thermal expansion mis- spallation and can easily be applied free of match stresses that develop on cooling. porosity or other defects. Welded deposits of Applications. The use of the CVD process surface alloys can be applied in thicknesses for steels has been largely limited to the coating greater than most other techniques, typically in of tool steels for wear resistance. the range of 3 to 10 mm. Most welding processes Advantages: are used for application of surface coatings, and • High coating hardness; for example, TiN on-site deposition can be more easily carried out, coatings have a hardness of 2500 HV. particularly for repair purposes. • Good adhesion (provided the coating is not Hardfacing applications for Applications. too thick) wear control vary widely, ranging from very • Good throwing power (i.e., uniformity of severe abrasive wear service, such as rock coating) crushing and pulverizing, to applications to minimize metal-to-metal wear, such as control Disadvantages: valves where a few thousandths of an inch of • High-temperature process (distortion a prob- wear is intolerable. Hardfacing is used for con- lem) trolling abrasive wear, such as encountered by • Shard edge coating is difficult due to thermal mill hammers, digging tools, extrusion screws, expansion mismatch stresses. cutting shears, parts of earthmoving equipment, • Limited range of materials can be coated. ball mills, and crusher parts. It is also used to • Environmental concerns about process gases control the wear of unlubricated or poorly lubri- cated metal-to-metal sliding contacts, such as Physical Vapor Deposition (PVD) control valves, undercarriage parts of tractors Process Description. Physical vapor depo- and shovels, and high-performance bearings. sition processes involve the formation of a coat- Hardfacing also is used to control combinations ing on a substrate by physical deposition of of wear and corrosion. atoms, ions, or molecules of the coating species. Advantages: There are three main techniques for applying • Inexpensive PVD coatings: thermal evaporation, sputtering, • Applicable to large components and ion plating. Thermal evaporation involves • Localized coating possible heating of the material until it forms a vapor that • Excellent coating/substrate adhesion condenses on a substrate to form a coating. • High deposition rates possible Sputtering involves the electrical generation of a plasma between the coating species and the Disadvantages: substrate. Ion plating is essentially a combina- • Residual stresses and distortion can cause tion of these two processes. A comparison of the serious problems. process characteristics of PVD, CVD, and ion • Weld defects can lead to joint failure. implantation is provided in Table 5. • Minimum thickness limits (it is impractical to Applications. Similar to CVD, the PVD produce layers less than 2 to 3 mm thick) process is used to increase the wear resistance of • Limited number of coating materials avail- tool steels by the deposition of thin TiN or TiC able, compared to thermal spraying coatings at temperatures ranging from 200 to © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) 10 / Surface Hardening of Steels

550 °C (400 to 1025 °F). This temperature range implanted tool steels used for forming and cut- is much more suitable for the coating of tool ting tools. Titanium plus carbon implantation steels than the temperatures required for CVD. has also proved beneﬁcial for tool steels. Advantages: Advantages: • Excellent process control • Produces surface alloys independent of ther- • Low deposition temperature modynamic criteria • Dense, adherent coatings • No delamination concerns • Elemental, alloy, and compound coatings • No signiﬁcant dimensional changes possible • Ambient-temperature processing possible Disadvantages: • Enhance surface properties while retaining • Vacuum process with high capital cost bulk properties • Limited component size treatable • High degree of control and reproducibility • Relatively low coating rates Disadvantages: • Poor throwing power without manipulation of components • Very thin treated layer (1 µm or less) • High-vacuum process Ion Implantation • Line-of-sight process Process Description. Ion implantation • Alloy concentrations dependent on sputtering involves the bombardment of a solid material • Relatively costly process; intensive training with medium- to high-energy ionized atoms and required, compared to other surface- offers the ability to alloy virtually any elemen- treatment processes tal species into the near-surface region of any • Limited commercial treatment facilities substrate. The advantage of such a process is available that it produces improved surface properties without the limitations of dimensional changes or delamination found in conventional coatings. Laser Surface Processing Applications. For steels, the most common Process Description. Laser surface pro- application of ion implantation is nitrogen- cessing involves the melting of a surface with a

Table 5 Comparison of PVD, CVD, and ion implantation process characteristics

Processing Throwing Coating applications Process temperature, °C power Coating materials and special features

Vacuum evaporation RT–700, usually <200 Line of sight Chieﬂy metal, especially Al Electronic, optical, decorative, (a few simple alloys/a few simple masking simple compounds) Ion implantation 200–400, best <250 Line of sight Usually N (B, C) Wear resistance for tools, dies, for N etc. Effect much deeper than original implantation depth. Precise area treatment, excellent process control

Ion plating, ARE RT–0.7 Tm of coating. Moderate Ion plating: Al, other metals Electronic, optical, decorative. Best at elevated to good (few alloys). ARE: TiN Corrosion and wear temperatures and other compounds resistance. Dry lubricants. Thicker engineering coatings

Sputtering RT–0.7 Tm of metal Line of sight Metals, alloys, glasses, Electronic, optical, wear coatings. Best >200 oxides. TiN and other resistance. Architectural for nonmetals compounds(a) (decorative). Generally thin coatings. Excellent process control CVD 300–2000, usually Very good Metals, especially refractory Thin, wear-resistant ﬁlms on 600–1200 TiN and other compounds(a), metal and carbide dies, tools, pyrolytic BN etc. Free-standing bodies of refractory metals and pyrolytic C or BN

PVD, physical vapor deposition; CVD, chemical vapor deposition; RT, room temperature; ARE, activated reactive evaporation; Tm, absolute melting temperature. (a) Compounds: oxides, nitrides, carbides, silicides, and borides of Al, B, Cr, Hf, Mo, Nb, Ni, Re, Si, Ta, Ti, V, W, and Zr © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) Process Selection Guide / 11

laser, with or without surface additions. With example, in nitrided and carburized compo- laser surface melting, melting and controlled nents, while in others there is an abrupt change, cooling are combined to refine the microstruc- as, for example, for parts where a coating of ture or to produce an amorphous (or nearly vapor-deposited titanium nitride has been de- amorphous) structure. No external material is posited on steel. Such interface characteristics added during this process. The composition and may significantly influence the performance of properties of the surface can also be modified by a surface-modified system. adding external material via powder injection or The performance requirements of surface- wire feed. External material can also be placed engineered systems may vary widely. For on the surface by powder deposition, electro- example, heavily loaded systems, such as bear- plating, vapor deposition, or thermal spray, then ings and gears, require deep cases to resist incorporated by laser scanning. The nature of rolling contact and bending stresses that result incorporating material in the modified surface in fatigue damage. Other applications may varies depending on laser processing parame- require only very thin surface films to resist ters, such as energy density and traverse speed. near-surface abrasion or scuffing or to reduce Alloy, clad, and composite surface layers may friction between moving surfaces. Many of be formed in this way. these requirements are based on complex inter- Applications. Although laser surface pro- actions between applied static and cyclic stress cessing has not reached commercial signifi- states and gradients in structures and properties cance for steels, various carbon and low-alloy of the surface-modified systems (see, for exam- steels, tool steels, and stainless steels have been ple, the discussion of bending fatigue strength laser processed to varying degrees of success. of carburized steels in Chapter 2). See Chapter 11 for application examples. Design Constraints. The component design constraints include consideration of the size and Advantages: shape of the component, because they may • Rapid rates of processing produce novel affect surface-treatment process capabilities. structures in the surface region not possible Will the component fit in the coating equip- with conventional processing. ment? Can a line-of-sight process be used? Do • For laser cladding, low weld-metal dilution small holes or channels require a process with rates and distortion, compared with compet- high throwing power? What kind of masking ing arc welding methods will be required to prevent coating unwanted areas? Is the temperature required by the surface Disadvantages: treatment compatible with the temperature limitations of the component material? What kind • High capital equipment costs of postcoating treatment, including heat treat- • Some substrate materials are not compatible ment and finishing, will be required? with the laser thermal conduction require- Economic Analysis. The economic issue of ments. fundamental importance is a cost/benefit analysis. This analysis should be based on the full life-cycle costs of the surface treatment, including the process costs (preparation, application, Important Considerations finishing, quality control, waste disposal), uti- for Process Selection lization costs and benefits (productivity of coated components), and added value of any Performance Requirements. The key to products produced or treated. Other economic proper selection of surface-hardening tech- factors that must be considered include the niques is in the identification of the performance availability of the process, number of compo- requirements for a given surface-modified nents to be treated, quality-control require- material system in a given application. Not only ments, delivery schedules, and so on. Note that must the properties of the surface be considered the analysis should not be based on just the ini- but also the properties of the substrate and the tial cost of the surface treatment. Life-cycle interface between the surface and substrate. In costs are as important when comparing the cost some systems there is a gradual change in prop- of various surface treatments as in comparing a erties between the surface and interior, as, for surface treatment to an untreated surface. © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) 12 / Surface Hardening of Steels

Process Comparisons ing (boriding), and chromizing. The hardest metal coating is chromium plate, although hard- Hardness versus Wear Resistance. The ened electroless nickel plate can attain values wear processes that are usually mitigated by the just under that of chromium. The surfaces that use of hard surfaces are low-stress abrasion, exceed the hardness of chromium are the cer- wear in systems involving relative sliding of mets or ceramics or surfaces that are modified conforming solids, fretting wear, galling, and, so that they are cermets or ceramics. These to some extent, solid-particle erosion. Unfortu- include nitrides, carbides, borides, and similar nately, there are many caveats to this statement, compounds. The popular solid ceramics used and substrate/coating selection should be care- for wear applications—aluminum oxide, silicon fully studied, with proper tests carried out if carbide, and silicon nitride—generally have necessary. Coating suppliers should also be hardnesses in the range of 2000 to 3000 consulted. Chapter 3 provides additional infor- kg/mm2. As shown in Fig. 1, when materials mation on wear processes and the means to pre- such as aluminum oxide are applied by plasma vent specific types of wear. spraying or other thermal spray process, they Figure 1 shows typical ranges in hardness for have hardnesses that are less than the same many of the surface-engineering processes used material in solid pressed-and-sintered form. to control wear. All of the treatments shown in This is because the sprayed materials contain this figure have hardness values greater than porosity and oxides that are not contained in the ordinary constructional steel or low-carbon sintered solid form. steel. The surface-hardening processes that rely Cost must be weighed against the perform- on martensitic transformations all have compa- ance required for the surface-treatment system. rable hardness, and the diffusion treatments that A low-cost surface treatment that fails to per- produce harder surfaces are nitriding, boroniz- form its function is a wasted expense. Unfortu-

Fig. 1 Range of hardness levels for various materials and surface treatments. EB, electron beam; HSLA, high-strength, low-alloy © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) Process Selection Guide / 13

nately, it is nearly impossible to give absolute • Labor costs comparative costs for different surface- • Environmentally related costs, for example, engineering options. Often, a range of prices is disposal of spent plating solutions offered for a particular job from different, • Expected service life of the coating equally competent candidate suppliers. Proba- bly the most important factor that relates to costs Because of these various factors, it is difficult of producing a wear-resistant surface on a part is to compare costs with a high degree of accu- part quantity. Treating many parts usually racy. Figure 2 provides some general guidelines allows economies in treatment and finishing. for cost comparisons. Another consideration when assessing sur- Distortion or Size Change Tendencies. face-treatment costs is part size. There are Figure 3 shows the surface temperatures that some critical sizes for each surface-treatment are encountered in various surface-engineering process above which the cost of obtaining the processes. As indicated in the figure, the treatment may be high. A number of surface processes are categorized into two groups: one treatments require that the part fit into the work group produces negligible part distortion, and zone of a vacuum chamber. The cost of vacuum the other group contains processes that have equipment goes up exponentially with chamber varying potential for causing distortion. Obvi- volume. ously, if a part could benefit from a surface Other factors to be considered are: treatment but distortion cannot be tolerated, processes that require minimal heating should • The time required for a given surface treat- be considered. ment Coating Thickness Attainable. Figure 4 • Fixturing, masking, and inspection costs shows the typical thickness/penetration capabil- • Final finishing costs ities of various coating and surface treatments. • Material costs As indicated in the figure, some surface-engi- • Energy costs neering treatments penetrate into the surface,

Fig. 3 Maximum surface temperatures that can be anticipated for various surface-engineering processes. The dashed vertical line at 540 °C (1000 °F) represents the temperature limit for distortion for ferrous metals. Obviously, a temperature of 540 °C (1000 °F) would melt a number of nonferrous metals, and it would cause distortion on metals such as aluminum or magnesium. How- ever, this process temperature information can be used to compare the heating that will be required for a particular process. EB, electron beam

Fig. 4 Typical coating thickness/depth of penetration for various coating and surface-hardening processes. SAW, submerged arc welding; FCAW, ﬂux cored arc welding; GMAW, gas metal arc welding; PAW, plasma arc welding; GTAW, gas tungsten arc welding; EB, electron beam; OAW, oxyacetylene welding; FLSP, ﬂame spraying; PSP, plasma spraying © 2002 ASM International. All Rights Reserved. www.asminternational.org Surface Hardening of Steels: Understanding the Basics (#06952G) Process Selection Guide / 15

and there is no intentional buildup on the sur- SELECTED REFERENCES face. Other surface treatments coat or intention- ally build up the surface. This is a selection fac- • K.G. Budinski, Surface Engineering for tor. Can a part tolerate a buildup on the surface? Wear Resistance, Prentice-Hall, 1988 If not, the selection process is narrowed to the • J.R. Davis, Ed., Surface Engineering for treatments that penetrate into the surface. Other Corrosion and Wear Resistance, ASM factors affecting the thickness of a given surface International and IOM Communications, treatment include dimensional requirements, 2001 the service conditions, the anticipated/allowable • G. Krauss, Advanced Surface Modiﬁca- corrosion or wear depth, and anticipated loads tion of Steels, J. Heat Treat., Vol 9 (No. on the surface. Questions or concerns related to 2), 1992, p 81–89 coating thickness should be discussed with the • S. Lampman, Introduction to Surface contractor. Available speciﬁcations should also Hardening of Steels, Heat Treating, Vol be reviewed. 4, ASM Handbook, 1991, p 259–267 ASM International is the society for materials engineers and scientists, a worldwide network dedicated to advancing industry, technology, and applications of metals and materials.

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