Metallographic Techniques for Tool Steels

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ASM Handbook, Volume 9: Metallography and Microstructures Copyright © 2004 ASM International® G.F. Vander Voort, editor, p644–669 All rights reserved. DOI: 10.1361/asmhba0003766 www.asminternational.org

George F. Vander Voort, Buehler Ltd.

TOOL STEELS can be prepared for macro- hot working axis) are preferred. However, lon- disks cut from failed components; however, scopic and microscopic examination using the gitudinal disks are better suited to studying de- room-temperature macroetching, which is also same basic procedures used for carbon and al- formation fiber or segregation. Transverse disks quite common, often uses a 10% aqueous nitric loys steels. However, because many tool steels are preferred for general-quality evaluations. If acid (HNO3) solution. Smooth-ground speci- are highly alloyed and are generally heat treated very hard, the disks should be tempered before mens are immersed for up to a few minutes in to much higher hardness than most carbon and etching. For routine-quality studies, a saw-cut this solution to reveal such surface conditions as alloy steels, specific aspects of their preparation surface is adequate. If better resolution of detail decarburization, carburization, nitriding, hard- differ slightly. The reasons for these differences is required or if photography is to be conducted, ened layers, and grinding damage. Internal qual- and the required procedural modifications are surface grinding is performed after cutting. ity problems, such as segregation, are revealed discussed in the following sections. Also cov- Macroetching. The macroetchant most but usually less effectively than by hot-acid etch- ered are the effects of hot working, composition, widely used to evaluate macrostructural quality ing. After etching, the surface is washed, austenitizing, and tempering on microstructure. of tool steels is the standard immersion solution scrubbed to remove etching smut, and dried. of equal parts of hydrochloric acid (HCl) and Etching of small polished sections in 2 to 5% Macroexamination water at 70 to 80 C (160 to 180 F) for 15 to 45 nital will also bring out surface conditions that min. Such etching will reveal segregation, cannot be as clearly revealed by the cold 10% cracks, porosity, inclusions (manganese sulfides, aqueous HNO solution. However, nital is less Specimen selection is generally based on the 3 original ingot locations. Sampling is usually per- for example), flow lines, surface decarburization effective on a smooth, ground surface. Figure 2 formed after primary hot working to billet or or carburization, and hardness variations. Inter- shows the carbide distribution on longitudinal bloom shapes. Disks 12 to 25 mm (0.5 to 1.0 in.) pretation of results is aided by referring to stan- planes of M2 and T1 high-speed steels revealed thick are most often cut from billet or bloom dard charts (Ref 1Ð4). Figure 1 illustrates the use by macroetching discs of varying diameter with locations corresponding to the top and bottom of of the 50% hot HCl macroetch to reveal the case 10% nital. Figures 3 and 4 show the carbide seg- the ingot. They are sometimes cut from the mid- depth in different sized brine-quenched and tem- regation by light microscopy examination in dle location. The hardness of these products can pered specimens of W1 tool steel with three lev- three of the sizes for the M2 and T1 discs. be rather high, unless the billet or bloom was els of hardenability. Note that there is excellent The sulfur print test (Ref 5, 6), another prev- subjected to a full annealing cycle. Thus, sec- contrast between the hardened case and the non- alent technique, is used to evaluate the distri- tioning of such specimens may be more difficult hardened core. bution of manganese sulfides. Fracturing of than for carbon or alloy steels. For most work, This etching procedure can be used to evaluate hardened transverse etch disks is also performed transversely oriented disks (perpendicular to the disks cut from sections smaller than billets or to detect oxide inclusion stringers or graphite on

Fig. 1 AISI W1 tool steel austenitized at 800 C (1475 F), brine quenched, and tempered2hat150C (300 F). Black rings are hardened zones in 75, 50, and 25 mm (3, 2, and 1 in.) diameter bars. Core hardness decreases with increasing bar diameter (all one-half actual size). (a) Shallow-hardening grade. Case, 65 HRC; core, 34 to 43 HRC. (b) Medium-hardening grade. Case, 64.5 HRC; core, 36 to 41 HRC. (c) Deep-hardening grade. Case, 65 HRC; core, 36.5 to 45 HRC. Hot 50% HCl Metallographic Techniques for Tool Steels / 645 the longitudinally oriented fracture. The fracture Microexamination hacksaws. However, such operations produce a surface is often heated to produce a blue temper substantial zone of deformation beneath the cut color, because the uncolored oxides exhibit Sectioning. Relatively soft specimens (less and rather rough surfaces. Thus, the initial rough strong contrast against the dark fracture. than 35 HRC) can be cut using band saws or grinding with a coarse abrasive (80- to 120-grit silicon carbide, for example) must remove this damage. However, such coarse abrasives generate large amounts of damage. Sectioning with an abrasive wheel developed for metallography is recommended, because it produces less damage and yields a better surface finish, so that damaging, coarse abrasive grinding can be mini- mized or eliminated. Then, a somewhat finer abrasive, which generates less damage, can be used for the first step (often called planar grinding with automated, multispecimen preparation systems). This procedure facilitates generation of a correctly prepared surface so that the true microstructure can be observed. Higher-hardness specimens must be cut using water-cooled abrasive cutoff wheels. The blade should have a “soft” bond (that is, it breaks down readily, exposing fresh abrasive to the cut) for effective cutting and avoidance of burning. Ef- fective water cooling limits heat generation but is inadequate by itself if the wrong blade is used, that is, a blade that does not break down at the proper rate. Dull abrasives generate excessive heat and damage in cutting. Cutting as-quenched specimens is very difficult without introducing some damaging heat that may cause burning or cracking. Cutting quenched and lightly tempered tool steels is much easier but still requires use of a wheel designed for such steels. Heat generated by improper technique can produce a highly tempered appearance in the martensite and, if heat- ing is excessive, can re-austenitize the surface. In extreme cases, melting can occur at the surface, with an extensive heat-affected zone below this area. Subsequent grinding steps cannot easily remove such damage without introducing more damage. Fig. 2 Carbide distribution on longitudinal planes of high-speed steels revealed by macroetching discs of varying When working with as-quenched high-alloy -tool steels, it may be helpful to fracture the spec ןdiameter with 10% nital. (a) M2 tool steel. (b) T1 tool steel. Both 1

1 5 1 Fig. 3 AISI M2 round bars. Carbide segregation at the center of round bars of different diameters. (a) 27 mm (1 ⁄16 in.) diam. (b) 67 mm (2 ⁄8 in.) diam. (c) 105 mm (4 ⁄8 in.) diam. ןnital. 100 10% 646 / Metallography and Microstructures of Ferrous Alloys

3 1 1 Fig. 4 AISI T1 round bars. Carbide segregation at the center of round bars of different diameters. (a) 35 mm (1 ⁄8 in.) diam. (b) 64 mm (2 ⁄2 in.) diam. (c) 83 mm (3 ⁄4 in.) diam. 10% ןnital. 100 imen. This will produce a flat, damage-free sur- However, some mounts have poor resistance to ture, transparent methyl methacrylate compres- face due to the extreme brittleness of such steels. solvents such as alcohol, and most polymeric sion-mounting thermoplastic material can be The fractured surface can then be carefully mounts are badly degraded if heated etchants are used. Somewhat better results can be obtained ground, with adequate cooling, and polished for required. The compression-mounting epoxies using cast, “cold”-mounting epoxy resins. examination. prevent these problems. If a transparent mount Casting epoxies are the only materials that For high-hardness, high-alloy steels, section- is required to control grinding to a specific fea- produce true adhesive bonding to the sample. ing with a low-speed saw or with a linear-pre- cision saw at higher rotational speeds, with alumina wafering blades, diamond, or cubic boron Table 1 Five-step preparation practice for tool steels nitride non-consumable blades, can provide Load high-quality surfaces with minimum cutting-in- Speed, Surface Abrasive/size N lbf rpm/direction Time, min duced damage. Although the cutting rate is Waterproof grinding 120/P120- to 240/P280-grit SiC, 22Ð27 5Ð6 Comp Until plane slower than with an abrasive cutoff saw, such paper (or equivalent) water cooled surfaces are smooth, damage is minimal, and Silk cloth or rigid 9 lm diamond (with lubricant) 22Ð27 5Ð6 Comp 5 grinding can begin with rather fine grits (320- or grinding disk 400-grit silicon carbide, for example). Section- Woven (napless) or 3 lm diamond (with lubricant) 22Ð27 5Ð6 Comp 3 pressed cloths ing is a violent process, and excessive damage Woven or pressed cloths 1 lm diamond (with lubricant) 22Ð27 5Ð6 Comp 2 introduced in sectioning rarely is removed fully Medium-nap cloth 0.05 lm colloidal silica or sol- 22Ð27 5Ð6 Contra 1.5Ð2 by the preparation method. gel-type alumina suspensions Mounting. Bulk samples frequently can be polished without mounting. Coding of un- mounted specimens is generally limited to a few Table 2 Four-step preparation practice for tool steels stamp marks (if the steel is soft enough to Load stamp), such as a job number. Vibratory scribers Speed, Surface Abrasive/size N lbf rpm/direction Time, min can be used, but subsequent corrosion may make Waterproof grinding 120/P120- to 240/P280-grit SiC, 27 6 240Ð300 Until plane these marks hard to read. Most modern auto- paper (or equivalent) water cooled Comp mated grinding/polishing devices can handle un- Rigid grinding disk (or 9 lm diamond suspension (with 27 6 120Ð150 5 mounted specimens. If edge retention is impor- woven cloth) lubricant) Comp tant, mounting is recommended. Plating the Woven (napless) or 3 lm diamond (with lubricant) 27 6 120Ð150 3 pressed cloths Comp surface prior to mounting (Ref 5) produces ex- Medium-nap cloth 0.05 lm colloidal silica or sol- 27 6 120Ð150 2 cellent results but is not always necessary. Com- gel-type alumina suspensions Contra pression-mounting epoxy resins with fillers provide excellent edge retention, even with nonplated specimens. Automated grinding/pol- Table 3 Three-step preparation practice for tool steels ishing devices, rather than hand polishing, yield much better edge retention. Modern practices us- Load Speed, ing napless polishing cloths provide excellent Surface Abrasive/size N lbf rpm/direction Time, min edge retention. Rigid grinding disks produce su- Waterproof paper (or 120/P120- to 320/P400-grit SiC, 27 6 240Ð300 Until plane equivalent) water cooled Comp perb edge retention. Hard or soft rigid 3 lm diamond suspension 27 6 120Ð150 5 For small or oddly shaped specimens, mount- grinding disks Comp ing is preferred. If the edge is not of particular Medium-nap cloth 0.05 lm colloidal silica or sol- 27 6 120Ð150 5 interest, most mounting media are satisfactory. gel-type alumina suspensions Contra Metallographic Techniques for Tool Steels / 647

When used correctly, they produce the lowest Polishing is most commonly performed using work, polishing with 9 and 3 lm diamond abra- heat during polymerization and are useful when one or more diamond abrasive stages, followed sives is generally adequate. The diamond abra- the specimen cannot tolerate the higher heat by one or more final abrasive stages. For routine sive may be applied to the polishing cloth in used in compression mounting. However, con- Table 4 Microstructural etchants for tool steels siderable heat can be generated during polymerization of epoxy resins if the process is not Etchant Comments controlled properly. Cast acrylic resins generate 1Ð10 mL HNO3 and 99Ð90 mL Nital. Most commonly used etchant. Do not store solutions with more than 3% HNO3 alcohol in ethanol. Reveals ferrite grain boundaries and ferrite-carbide interfaces in substantial heat, because they polymerize in less annealed sample. Preferred etchant for martensite. Reveals prior-austenite grain than 10 min. When edge retention is not re- boundaries in as-quenched and lightly tempered high-alloy tool steels. 2Ð3% nital quired and heat degradation is not anticipated, most common; 5Ð10% nital used for high-alloy grades. Use by immersion. low-cost phenolic compression-molding mate- 4 g picric acid and 100 mL ethanol Picral. Recommended for annealed structures or those containing pearlite or bainite. Does not reveal ferrite grain boundaries in annealed specimens. Etching response rials can be used, although they are badly de- improved by adding 10Ð20 drops zephiran chloride. For high-alloy grades, add 1Ð5 graded by boiling etchants. Identification marks mL HCl to improve etching response. Use by immersion. are easier to add on the back of a mounted spec- 1 g picric acid, 5 mL HCl, and 100 Vilella’s reagent. Used in the same manner as picral or picral plus HCl mL ethanol imen than on a nonmounted specimen. Non- 10 g picric acid and 100 mL ethanol Superpicral. Must be heated to dissolve picric acid. Use by immersion, up to 1 min or mounted specimens degrade polishing cloths more. A few drops of HCl may be added to increase etch rate. faster than mounted specimens. 2 g picric acid, 25 g NaOH, and 100 Alkaline sodium picrate. Immerse sample in boiling solution for 1Ð15 min or use Grinding is performed in the same manner mL H2O electrolytically at 6 V dc, 20 C (68 F), 30Ð120 s, stainless steel cathode. Colors cementite and Fe4W2C as for carbon and alloy steels. Various manual 10gK3Fe(CN)6 (potassium Murakami’s reagent. Use by immersion, fresh solution, hot or cold, up to 10 min. or automated devices may be used. Water- ferricyanide), 10 g KOH or NaOH, Cold darkens chromium carbides and tungstides; cementite not attacked. Hot attacks and 100 mL H2O cementite. cooled silicon carbide paper (200 to 300 mm, or 8 to 12 in., diameter) is used, although alu- 1 g CrO3 and 100 mL H2O Electrolytic etch, 2Ð3 V dc, 20 C (68 F), 30 s, stainless steel cathode. MC and M7C3 darkened; Mo2C outlined mina-coated paper is preferred for ferrous alloys 10 mL H2O2 (30%) and 20 mL 10% Immerse 10 s at 20 C (68 F). Fe2MoC, Mo2C, and M6C outlined (latter also colored) (but is not as readily available). The initial grit aqueous NaOH size selected depends on the technique used to 4 g KMnO4 (potassium Groesbeck’s reagent. Immerse at 20 C (68 F). Fe2MoC and M4C outlined and permanganate), 4 g NaOH, and colored (blue and brown, respectively), Mo2C colored brown, (Fe,Cr)23C6 attacked generate the cut surface. Historically, the usual 100 mL H2O but (Fe,Mo)23Q not attacked grit sequence is 120-, 240-, 320-, 400-, and 600- 4 g NaOH and 100 mL saturated Immerse at 20 C (68 F). Mo2C and M7C3 attacked; M6C outlined and colored brown grit with the American National Standards In- aqueous KMnO4 Saturated aqueous picric acid plus Prior-austenite grain-boundary etch for hardened steels. Many wetting agents can be stitute/Coated Abrasives Manufacturers Institute small amount of wetting agent used; sodium tridecylbenzene sulfonate most commonly used. Use at 20 to 100 C scale (P120, P280, P400, P600, and P1200 for (68 to 212 F) by immersion for 2 to 60 min. Addition of approximately 1% HCl the Federation of European Producers of Abra- useful for higher-alloy grades. Room-temperature etching most common. Etching sives scale). Modern preparation procedures with solution in a beaker in an ultrasonic cleaner works well. Lightly backpolish to remove surface smut. (Tables 1Ð3) use a single SiC (or alternative sur- 50 mL cold saturated aqueous Klemm’s I (tint etch) reagent. Immerse (never swab) at 20 C (68 F) for 40Ð100 s to face) grinding step. Grinding pressure should be Na2S2O3 (sodium thiosulfate) and 1 color ferrite (blue or red) and martensite (brown). Cementite and austenite gK2S2O5 (potassium metabisulfite) unaffected moderate to heavy, and grinding times of 1 to • 2 min are typical to remove the scratches and 1gNa2MoO4 (sodium molybdate) Beraha’s tint etch for cementite. Add 0.2Ð0.3 g NH4F HF (ammonium bifluoride). and 100 mL H2O Add HNO3 to produce a pH of 2.5Ð3.0. Preetch sample with picral. Colors Fe3C deformation from the previous step. Fresh paper yellow-orange. Immerse up to 60 s; never swab. should be used; worn or loaded paper will pro- 3gK2S2O5, 10 g anhydrous Beraha’s tint etch. Immerse (never swab) until surface is colored red-violet. Colors duce deformation. Wheel speeds in grinding are Na2S2O3, and 100 mL H2O ferrite, martensite, bainite, and pearlite. Cementite unaffected generally in the range of 240 to 300 rpm. Note: When water is specified, use distilled water.

Fig. 5 AISI W2 (1.05% C), spheroidize annealed. (a) Etched with 4% picral to outline only cementite (uniform dissolution of the ferrite matrix). (b) Etched with 2% nital, which reveals ferrite grain boundaries and outlines cementite. Note that the ferrite in some grains is weakly attacked, and the carbides within these grains are barely visible. (c) ןEtched lightly with 4% picral, then tint-etched with Klemm’s I reagent to color the ferrite (blue and red). 1000 648 / Metallography and Microstructures of Ferrous Alloys

paste, slurry, or aerosol form. Charging a new abrasives, generally 0.3 lm alumina (A12O3) a rigid grinding disk. All methods produce ex- cloth with paste, however, is most effective, be- and 0.05 lm c-Al2O3, have been widely em- cellent results. cause the removal rate is high as soon as polish- ployed with medium-nap cloths for final polish- In these methods, the cloth is first charged ing starts. For the diamond abrasives, low-nap ing. Colloidal silica (SiO2, with a particle size with diamond paste of the specified size, and or napless cloths are preferred. A lubricant, or range of 0.02 to 0.06 lm) is also very effective then the appropriate lubricant is added. During extender, compatible with the diamond abrasive (Ref 5). Wheel speeds, pressure, and times are the polishing cycle, diamond of the same size is should be added to moisten the cloth and mini- the same as for rough polishing with diamond added periodically in suspension form to keep mize drag. Wheel speeds of 100 to 150 rpm and abrasives. In general, tool steels are relatively the cutting rate high. Comp stands for comple- moderate pressure should be used. Polishing easy to polish to scratch-free and artifact-free mentary and means that the head (specimen times depend on the number of steps in the pro- condition due to their relatively high hardnesses. holder) and base (platen) are both rotating in the cedure and the nature of the alloy (composition Table 1 lists a contemporary five-step practice same direction, counterclockwise (usually). and heat treatment condition). for preparing tool steels; Table 2 lists a contem- Contra means that the head is rotating clockwise Final polishing can be conducted manually or porary four-step practice using a rigid grinding while the base is rotating counterclockwise. automatically, using various devices. Alumina disk; and Table 3 lists a three-step practice using Contra is more aggressive than complementary.

Fig. 6 AISI W2 (1.05% C), spheroidize annealed. (a) Etched with boiling alkaline sodium picrate for 60 s to color the cementite brown. (b) Etched lightly with 4% picral and tint etched with Beraha’s Na2S2O3/K2S2O5 reagent to color the ferrite (wide range of colors). (c) Etched lightly with 4% picral and tint etched with Beraha’s Na2MoO4 reagent to ןcolor the cementite dark orange. 1000

Fig. 7 AISI D2 austenitized at 1040 C (1900 F), air quenched, and tempered at 200 C (400 F). Inﬂuence of etchant on revealing martensite. (a) 10% nital etch reveals grain boundaries, carbides, and martensite (light). (b) 4% picral plus HCl etch reveals carbides and martensite (light). (c) Heat tinted at 540 C (1000 F) for 5 min after 10% nital ןetch to produce greater contrast and reveal the retained austenite. (d) Superpicral etch reveals retained austenite as white, but carbide also appears white. 1000 Metallographic Techniques for Tool Steels / 649

If the head rotates at less than 100 rpm, the slur- interfaces between carbide and ferrite. Nital also heat treated specimen of W1 (1% C) water-hard- ries stay on the surface reasonably well. How- reveals the ferrite grain boundaries that generally ening tool steel. ever, if the head rotational speed is greater than obscure the carbide shape. Also, because nital is Pepperhoff interference film technique 100 rpm, the abrasive will be thrown off the orientation sensitive, carbides within some of the also improves contrast among constituents. A platen onto the user and the walls. In comple- ferrite grains will be poorly delineated, making thin layer of a dielectric compound, such as zinc mentary mode, the centrifugal force throws the spheroidization ratings more difficult. Figures selenide (ZnSe), is vapor deposited onto the sur- abrasive off the platen surface and down the 5(a) and (b) illustrate the difference in etching face of the sample in a bell jar (Ref 5). As the drain. Therefore, the amount of abrasive that response between nital and picral with spheroid- thickness of this layer increases above 400 nm, must be added during a cycle depends on the ize-annealed W2 tool steel. Note that within colors are observed (Ref 12). First-order red to surface being used, the head speed, and the ro- some grains, nital did not clearly reveal the ce- violet produces the best results. An example of tational directions. When grinding, especially mentite. this method, and a comparison to a standard with cast polymeric mounts, complementary ro- A 2% nital solution is usually preferred. etchant, is given in Fig. 9. tation may produce chatter and vibration, which Stronger concentrations increase the speed of Prior-Austenite Grain Size. Many tool steels is eliminated when using contra rotation. In cer- etching, making it more difficult to control. Etch- can be etched with nital to reveal the prior-aus- tain specimens that have very hard or very soft ing of martensitic high-alloy tool steels, such as tenite grain boundaries. The high-speed steels (relative to the matrix) particles that are poorly the high-speed steels, may require a 5 or 10% and the D-type cold work tool steels (Fig. 7) can bonded to the matrix, one may see excessive re- concentration solution. Mix this solution fresh be handled in this way as long as the tempering lief around these particles after the last step. If using ethanol; do not store the solution in a temperature used is not too high. Etching tech- this happens, and it is specimen-specific and not tightly stoppered bottle. Etching with nital or pi- niques that reveal the prior-austenite grain frequent, simply repeat the last step in comple- cral is usually performed by immersion. If swab- boundaries are employed but can be difficult to mentary rotation, and this problem will be elim- bing is used, pressures should be light. Etching implement successfully; therefore, a fracture inated. times are difficult to generalize because of the grain size method is widely used. The Shepherd Microetching. The etchant most widely used wide range of tool steel compositions and be- fracture grain size technique is simple, quick, for tool steels is 2 to 5% nital. Stock solutions cause heat treatment can markedly alter etch re- and accurate, as long as the sample is marten- exceeding 3% HNO3 in ethanol should not be sponse. Trial and error will determine the degree sitic—retained austenite may be present in sub- stored in pressure-tight bottles. If higher concen- of surface dulling necessary to obtain the correct stantial amounts—and not tempered to such an trations are desired as a stock reagent, a bottle degree of etching. extent that reasonably flat, brittle (macroscopic) with a pressure-relief valve should be used. Other etchants, although less frequently used, fractures cannot be obtained (Ref 5). Methanol also may be substituted for ethanol, can be of great value. Table 4 lists compositions although methanol poses health risks and thus of a number of specialized reagents for achieving may not be recommended. (See the article “Lab- selective etching or enhancing contrast among Microstructures of Tool Steels oratory Safety in Metallography” in this vol- microconstituents. As examples of the use of ume.) various etchants, Fig. 5(c) and 6 show the same A wide range of microstructures is observed Nital is generally used for tool steels regard- W2 specimen as in Fig. 5(a) and (b) but etched in tool steels because of variations in composi- less of the anticipated microstructural constitu- with four alternate reagents to evaluate the tion and heat treatment. The mill metallurgist is ents. Although nital is superior to picral (4% pi- spheroidized cementite. Figure 7 shows the mi- generally most concerned with annealed micro- cric acid in ethanol) for etching martensitic crostructure of D2 tool steel, in the quenched and structures and undesired surface decarburization. structures, picral produces better results for ex- tempered condition, revealed using three differ- The failure analyst sees a broad spectrum of mi- amining annealed samples. When examining ent etchants and by heat tinting, while Fig. 8 crostructures, both normal and abnormal. spheroidize-annealed tool steels (the most com- demonstrates the effect of four different etchants Hot-Worked Microstructures. The structure mon annealed condition), picral reveals only the used to examine the structure of an improperly produced after hot working can have a marked

Fig. 8 AISI W1 (1% C) overaustenitized at 925 C (1700 F) and water quenched, producing martensite, retained austenite, and small patches of pearlite. Influence of etchant on revealing quenched martensite. (a) 2% nital etch reveals martensite and pearlite (black). (b) 4% picral etch reveals pearlite but only faintly reveals martensite. (c) 5% aqueous sodium metabisulfite etch produces a strong contrast between the martensite and retained austenite (white). (d) Beraha’s Na2S2O3/K2S2O5 reagent produces similar results to (c), but ןpearlite is more visible. 500 650 / Metallography and Microstructures of Ferrous Alloys influence on the distribution and morphology of high-carbon, water-hardening grades may de- nealed microstructures is discussed in the article carbides after the subsequent spheroidize anneal. velop grain-boundary carbide networks after “Introduction to Heat Treating of Tool Steels” in The micrographs in Fig. 10 to 17 illustrate the cooling from the hot rolling temperature, as Heat Treating, Volume 4 of Metals Handbook, complex microstructures that are often present shown in Fig. 19. Stringers of complex alloy car- 9th ed. (1981). For a given grade, the hardness in the as-hot-worked condition prior to annealing bides may be observed in certain grades, and decreases as the degree of spheroidization in- and the influence of this structure on the an- these are often harmful. Figure 20 shows an ex- creases. Once spheroidization has been obtained, nealed microstructure. In general, the cooling af- ample in H13 tool steel. growth of the carbides, which produces fewer ter hot working must be controlled to produce as Annealed Microstructures. Because most carbides per unit volume and a greater apparent uniform a carbon distribution as possible, so that tool steels are relatively hard, even when an- spacing, further reduces hardness. However, if the annealed carbide distribution does not vary. nealed, it is usually necessary to control carbide the carbide structure is too coarse, dissolving the Some tool steels, such as the 5% Cr hot work morphology during annealing to maximize required amount of carbon during austenitization grades and the 12% Cr plastic molding steels, machinability and formability. For most tool will be more difficult. In addition, many tool tend to form carbide networks at the prior-aus- steels, a spheroidal carbide shape is the desired steels require a fine, uniform distribution of un- tenite grain boundaries present at the end of the condition, although for a few of the low-alloy dissolved carbides to resist grain growth during hot working operation. Such networks can be tool steels, certain machining operations are im- austenitization. difficult to remove during annealing and can de- proved when the structure is partially pearlitic. Control of the spheroidization annealing pro- grade tensile ductility and toughness, even if Most tool steels and all high-alloy grades are cess is important for good machinability and they are semicontinuous. Figure 18 illustrates spheroidize annealed at the mill. Control of an- good formability, such as in hobbing. The more grain-boundary carbide networks in type 420 stainless steel used for plastic molding. The

Fig. 10 AISI W1 (1.3% C), as-rolled, containing pearl- Fig. 11 AISI L6, as-rolled, containing bainite and mar- ןtensite (white). 2% nital. 500 ןite and acicular cementite. 4% picral. 500

Fig. 9 AISI D2, quenched and tempered. Use of vapor- deposited zinc selenide to accentuate carbide detection and retained austenite. Samples were etched first with 4% picral plus HCl (a) to outline the carbides, then Fig. 13 AISI S4, as-rolled, containing bainite and mar- coated with a thin layer of zinc selenide (b) to reveal the tensite (featureless patches). This bar was carbides (dark violet), retained austenite (white), and mar- Fig. 12 AISI S4, as-rolled, containing ferrite (white) and cooled at a faster rate after rolling than the one in Fig. 12. ןpicral. 500 4% ןpearlite. 4% picral. 500 ןtensite (dark). 1000 Metallographic Techniques for Tool Steels / 651 uniform the starting microstructure (the as-rolled oidize annealing, unless it was normalized be- matrix phase, chiefly martensite, and the amount microstructure), the more uniform the spheroid- fore annealing. Figures 22 to 36 illustrate the and type of carbides remaining undissolved in ization and the softer the annealed hardness, all wide range of annealed microstructures that can the matrix. other conditions being the same. The as-rolled be observed in a variety of tool steel grades. Cementite (M3C) is an iron-rich carbide with microstructures shown in Fig. 10 to 17 demon- Carbides in Tool Steels. The amount and an orthorhombic crystal structure. In annealed strate varying degrees of non-uniformity and type of carbides present depend on the bulk car- tool steels, it will be very low in tungsten, mo- variability in the starting microstructure. Figure bon content and the quantity of carbide-forming lybdenum, or vanadium content and relatively 21 shows the influence of the starting micro- elements (chromium, molybdenum, vanadium, low in chromium content, while the manganese structure on the spheroidization of W1 tool steel. and tungsten, for example) in the grade. Ce- content can be high. Cementite is found in all The as-rolled pearlite (Fig. 21a) is coarser in in- mentite is present in the carbon and low-alloy carbon tool steels and in alloy tool steels terlamellar spacing than the normalized speci- grades; more complex carbide types are found in quenched and tempered below 538 C (1000 F). men (Fig. 21c), and the annealed structure, al- the highly alloyed grades. The hardness of car- M7C3 is a chromium-rich carbide with a hexag- though similar, is lower in hardness, with larger, bides varies with their composition, from ap- onal crystal structure that is observed in steels more widely spaced carbides for the normalized proximately 800 HV for pure Fe3C to approxi- with medium-to-high chromium contents and and annealed specimen than for the as-rolled and mately 1400 HV when other elements, such as only moderate amounts of other carbide-forming annealed specimen. Of course, if the specimen chromium, are substituted for a portion of the alloy elements. It dissolves very slowly in aus- contained grain-boundary carbide networks after iron. The high wear resistance of heat treated tenite during the hardening process. This carbide rolling, they would still be present after spher- tool steels is attributable to the hardness of the is present in the D-type cold work grades in high- speed steels in the annealed or highly tempered conditions. M23C6 is a chromium-rich carbide with a face-centered cubic (fcc) crystal structure with a high solubility for iron, but much less for other strong carbide formers, such as molybdenum or tungsten. It is observed in type 420 stainless steel and in annealed high-speed steels. Fig- ure 37 shows a portion of the isothermal section of the Fe-Cr-C system at 870 C (1600 F), illustrating the location of the various chromium- rich carbides observed in several chromium- bearing tool steels. M6C is either a molybdenum- or a tungsten- rich carbide with a fcc crystal structure, such as Fe4Mo2C-Fe3Mo3C. A fair amount of chromium or vanadium can be present in the carbide. It is commonly seen in high-speed steels and dissolves slowly during austenization. Figure 38 shows an isothermal section in the Fe-W-C system at 1200 C (2190 F), illustrating the location of M6C carbides and the composition of several hot work die steels. M2C is a molybdenum- or tungsten-rich carbide with a hexagonal crystal Fig. 15 AISI O1, as-rolled, containing bainite and mar- Fig. 14 AISI S5, as-rolled, containing bainite and martensite (white patches). The dark patches are structure. It is not frequently seen in tool steels, -except for certain grades in the annealed condi ןpearlite. 4% picral. 500 ןtensite (white). 2% nital. 500

Fig. 16 AISI L1, as-rolled, containing pearlite and a grain-boundary cementite net- Fig. 17 AISI A2, as-rolled, containing plate martensite (black) and retained austenite ןwhite). 2% nital. 500) ןwork. Boiling alkaline sodium picrate. 100 652 / Metallography and Microstructures of Ferrous Alloys

Fig. 18 Grain-boundary carbide networks in type 420 martensitic stainless steel (Fe-0.35%C-0.4%Mn-13%Cr) with two different etchants. (a) Vilella’s reagent. (b) Beraha’s sulfamic ןacid tint etch. Heat treatment: 1038 C (1900 F). Air quench: 177 C (350 F) temper. 500 tion. MC is most frequently a vanadium-rich car- graphitization are not controlled. Processing pro- carbon hobbing steels, such as AISI P2, must be bide with a fcc NaCl-type crystal structure. Other cedures can also affect graphitization; therefore, carburized after hobbing and before hardening. strong carbide-forming elements, such as tita- in carbon tool steels with more than approxi- These low-carbon grades are rather soft as-an- nium, niobium, tantalum, and zirconium, can mately 1.1% C, processing must also be care- nealed and are designed to minimize work hard- form MC carbides, but these elements are not fully controlled. ening during forming. Medium-carbon AISI S5 common in tool steels. MC is observed in steels Sulfur is added to several tool steel grades to must be very carefully spheroidize annealed to with moderate-to-high vanadium contents, improve machinability. Generally, the amount of as low a hardness as possible to enhance hobb- chiefly high-speed tool steels. It has very strong sulfur added to tool steels is less than that added ability. Achieving a low annealed hardness with bonds and is the hardest carbide. It resists dis- to free-machining carbon steels. Because man- this grade is difficult, however, because its high solution during austenization. ganese sulfide inclusions degrade toughness, silicon content substantially strengthens the fer- The amount of carbides present in tool steels certain tool steel grades are made with very low rite. is greater in the annealed condition than after sulfur contents, often less than 0.003%, for criti- Heat Treated Microstructures. Tool steel austenitizing and quenching, because the car- cal applications. Although this practice enhances compositions range from carbon tool steels with bides supply the austenite with the carbon nec- mechanical properties, these tool steels can be no alloy additions to high-speed steels contain- essary to achieve high hardness levels. The type more difficult to machine. ing 20% or more alloying elements. Conse- of carbides obtained also varies as a function of Hobbing steels must be quite soft to permit quently, hardenabilities vary widely, producing composition. As an example, Fig. 39 shows the optimal cold workability and maximum life of quenching requirements that vary from brine to composition and amount of carbides observed in the master hob. Such steels are very low in car- air. Each tool steel grade has been studied to de- different high-speed steels in the annealed and bon, although some medium-carbon alloy termine the proper quench media, as a function in the hardened conditions. grades, such as AISI S5, are hobbed. The low- Effect of Composition on Microstructure. Because tool steels are somewhat more difficult to machine than carbon and alloy steels, the compositions of some grades are adjusted by increasing the silicon content to retain a certain amount of the carbon present as graphite. When viewed on a transverse plane (Fig. 40), the graphite appears as small, globular particles, but they are not nodular in shape as in ductile cast iron. On the longitudinal plane (Fig. 41), the graphite particles are shown to be elongated, although their aspect ratios are not excessively high. The most commonly used graphitic tool steel is AISI O6, which typically contains approximately 0.3 to 0.5% of the total carbon content as graphite. The amount of carbon as graphite must be controlled carefully to ensure uniform hardening response. The presence of graphite improves machining Fig. 19 Grain-boundary carbide networks after cooling and wear characteristics. from the hot rolling temperature of high-carbon, water-hardening grade (Fe-1.31%C-0.35%Mn- Fig. 20 Carbides in light-etching segregation band of Undesired graphitization can occur in high- 0.25%Si, as-rolled). Alkaline sodium picrate etch: 90 C AISI H13 hot work die steel (Fe-0.40%C- ן0.8%Si-5.25%Cr-1%V-1.35%Mo). 2% nital. 500 ןcarbon tool steels if those elements that promote (195 F), 60 s. 500 Metallographic Techniques for Tool Steels / 653 of section size, to permit hardening to marten- greater) to obtain good wear resistance. Excep- ground on one of the fracture faces and tested site. Although the carbon tool steels are usually tions are the hot work tool steels that are for hardness. As the austenitizing temperature not through hardened, most of the other grades quenched and tempered to hardnesses from ap- increases, hardness will increase, level off, then are. Another exception is the low-carbon hob- proximately 42 to 55 HRC and prehardened plas- decrease. The fracture grain size will remain bing grades that are carburized and surface hard- tic molding steels, such as AISI P20, that are relatively constant, usually up to the austenitiz- ened. Figure 42 shows the surface of a poorly sold in the heat treated condition at approxi- ing temperature where the as-quenched hardness carburized specimen of P5 tool steel with an ex- mately 32 HRC. The prehardened tool steels are levels out, then will decrease due to grain tensive intergranular carbide network and a car- machined and used in this condition without sub- growth. The optimal austenitizing temperature is bide film on the surface. This mold failed pre- sequent heat treatment except, perhaps, for a that temperature or range where the hardness is maturely in service as a result. A few grades are stress-relief temper. highest and the grain size is finest. Dilatometry also nitrided or carbonitrided for special appli- The correct austenitizing temperature for each is generally conducted before such tests to es- cations. Figure 43 shows the microstructure of grade has been determined experimentally by us- tablish the optimal temperature range. gas nitrided H13 tool steel, while Fig. 44 shows ing an austenitizing series. Samples are heated To illustrate the effect of varying the austeni- an ion-nitrided H13 specimen. Note the (brittle) to various temperatures and quenched at a rate tizing temperature on the microstructure of tool white-etching iron nitride layer at the extreme consistent with the anticipated hardenability. steels, it is necessary to examine grades with surface. Each as-quenched sample is fractured to rate the varying carbon content and carbide types. Figure Most tool steels are hardened and tempered to prior-austenite grain size by the Shepherd com- 45 shows the microstructure of air-hardened S7 rather high hardnesses (generally, 58 HRC or parison method. Next, the samples are carefully tool steel with a carbon content of approximately

Fig. 21 AISI W1 (1.05% C). Influence of starting structure on spheroidization. (a) As-rolled; contains coarse and fine pearlite. (b) After spheroidization (heat to 760 C, or 1400 F; cool at a rate of 11 C/h, or 20 F/h, to 595 C, or 1100 F; air cool). (c) Austenitized at 870 C (1600 F) and oil quenched to produce fine pearlite. (d) Austenitized as in ןc); annealed as in (b). Note the more uniform spherical carbide shape compared to (b). 4% picral. 500)

Fig. 22 AISI W4 water-hardening tool steel (0.98C- Fig. 23 AISI W4 water-hardening tool steel (0.96C- Fig. 24 AISI W1 water-hardening tool steel (0.94C- 0.74Mn-0.14Cr-0.19Ni), as-received (mill an- 0.66Mn-0.23Cr), as-received (full annealed). 0.21Mn), as-received (mill annealed). 170 HB. nealed). 187 HB. Spheroidal cementite in a matrix of fer- 170 HB. Structure consists of spheroidal cementite in a fer- Structure: mixture of lamellar pearlite and spheroidal ce- rite; a considerable amount of lamellar pearlite is also rite matrix; no lamellar constituent is present. Compare mentite in a matrix of ferrite, with a few large, globular ןcarbide particles. 3% nital. 1000 ןwith Fig. 22. 4% picral. 1000 ןpresent. 4% picral. 1000 654 / Metallography and Microstructures of Ferrous Alloys

0.5%. The preferred austenitizing temperature ing retained austenite. A more dramatic effect on starting point for establishing the correct austen- for S7 is 940 C (1725 F), and the grade is rather the microstructure is observed when a high-car- itizing temperature. For these steels, maximum sensitive to under- and overaustenitization. Un- bon, high-alloy tool steel, such as type D2, is hardness results when approximately 0.60 to deraustenitization does not dissolve enough car- examined in the overaustenitized condition. Fig- 0.65% C is put into solution. Therefore, the as- bide to get full hardness. Overaustenitization re- ure 47 shows D2 after air quenching from vari- quenched structure will consist of martensite and sults in grain growth, because all of the carbide ous temperatures, from 1010 to 1230 C (1850 residual cementite, that is, the carbides not put is put in solution and it is not aluminum-killed, to 2250 F). The recommended austenitizing into solution. A small amount of retained aus- so there is no AlN to inhibit grain growth. Figure temperature is 1010 C (1850 F). Note that re- tenite will also be present, but it will not be de- 46 shows the microstructure of O1 tool steel, an tained austenite, which is present even at the cor- tectable with a light microscope if the proper oil-hardening grade with approximately 0.9% C. rect austenitizing temperature, cannot be ob- austenitizing temperature is used. Figure 48 The preferred austenitizing temperature is 800 served by light microscopy until a substantial shows the microstructure of W1 tool steel in the C (1475 F), so this series compares the correct amount is present; certainly, more than 10% is hardened case, the transition zone, and the un- quenched microstructure to overaustenitized mi- required before it can be seen. hardened core after brine quenching a 19 mm crostructures. It demonstrates how increasing the Carbon tool steels are hypereutectoid and con- (0.75 in.) diameter bar. The case is high-carbon carbon content of the austenite depresses the tain only cementite, which is easily dissolved. martensite, and the core is very ﬁne pearlite. Not martensite start temperature, resulting in increas- The iron-carbon equilibrium diagram is a good all of the cementite should be put into solution,

Fig. 25 AISI L1, spheroidize annealed. Note the very- well-formed spheroidal carbides. 4% picral. Fig. 26 AISI S2, spheroidize annealed. 4% picral. Fig. 27 AISI S5, spheroidize annealed. 4% picral. ן500 ן1000 ן500

Fig. 28 AISI S7, spheroidize annealed. 4% picral. Fig. 29 AISI A6, spheroidize annealed. 4% picral. Fig. 30 AISI A6, partially spheroidized. Note lamellar ןpearlite. 4% picral. 1000 ן1000 ן1000 Metallographic Techniques for Tool Steels / 655 so residual cementite should be present at all lo- In general, if the amount of retained austenite is less common. Brine- and water-quenching cations. Quenched and tempered microstructures is high enough to be observed with a light mi- grades are also susceptible to formation of soft of a variety of other low-alloy grades with me- croscope, the steel has been overaustenitized. spots within the case, caused by localized slow dium levels of carbon are shown in Fig. 49 to Tempering will not convert high levels of re- cooling from tongs holding the sample or by in- 54, while the microstructure of higher-alloy tool tained austenite to martensite or bainite unless adequate agitation (failure to break up vapor steels are shown in Fig. 55 to 66. the tempering temperature is rather high. More- pockets due to localized boiling of the quench If a sufﬁciently high austenitizing temperature over, the retained austenite will not be stable media). is used, the carbon content of the austenite will enough to withstand shock-induced transforma- After quenching, tool steels must be immedi- be raised to such an extent that the martensite tion to martensite during service. When such ately tempered to reduce the very high transfor- ﬁnish temperature is well below room tempera- transformation occurs, the high-hardness matrix mation stresses, or quench cracking may occur. ture. Because retained austenite is relatively soft lacks enough ductility to accommodate the trans- Quench cracking is more likely to occur as the compared to martensite, the bulk hardness will formation stresses, and cracking results. quench rate increases and if the geometry of the be lower. The microstructure will exhibit coarse Low heat treated hardnesses may also result if specimen exhibits stress raisers. Air-hardenable plate (acicular) martensite and retained austenite. the section size and quench rate are selected in- steels are less susceptible to quench cracking, As the austenitizing temperature exceeds the op- correctly for a particular grade. When the har- because the slow rate of cooling helps to relieve timal temperature, the amount of residual ce- denability is inadequate to permit full hardening, some of the transformation stresses. mentite decreases, the amount of retained aus- pearlite or bainite will be observed in the micro- Many tool steels are often tempered at 175 to tenite increases, the hardness decreases, and the structure. Ferrite may also be visible in the few 230 C (350 to 450 F). These low tempering grain size increases. tool steels that are hypoeutectoid, although this temperatures do little to the structure except re-

Fig. 33 AISI A7 tool steel, box annealed at 900 C (1650 F) for 1 h per 25 mm (1.0 in.) of con- tainer thickness and cooled at no more than 28 C/h (50 Fig. 31 AISI H13 chromium hot-worked tool steel, Fig. 32 AISI M2 molybdenum high-speed tool steel, F/h). Massive alloy carbide and spheroidal carbide in a ןferrite matrix. 4% nital. 1000 ןspheroidize annealed. 4% picral. 1000 ןspheroidize annealed. 4% picral. 1000

Fig. 35 AISI H23 tool steel, annealed by austenitizing at 870 C (1600 F) for 2 h and cooling at 28 Fig. 36 AISI H26 tool steel, annealed by austenitizing Fig. 34 AISI A10 tool steel, as-received (mill annealed). C/h (50 F/h) to 540 C (1000 F), then air cooling. 98 HRB. at 900 C (1650 F), cooling at 8.5 C/h (15 F/ Section transverse to rolling direction. At the Structure consists of tiny spheroidal and some larger alloy h) to 650 C (1200 F), then air cooling. 22 to 23 HRC. magniﬁcation used, the structure is poorly resolved. Nital. carbide particles in a matrix of ferrite. Kalling’s reagent. Structure consists of a dispersion of ﬁne particles of alloy ןcarbide in a matrix of ferrite. Picral with HCl, 10 s. 500 ן500 ן100 656 / Metallography and Microstructures of Ferrous Alloys

Fig. 38 Isothermal section of Fe-W-C diagram at 1200 C (2190 F). Nominal compositions of tungsten, carbon, and iron for some tungsten hot work steels are plotted. Source: Ref 8

cause secondary hardening and to change the nature of the carbides present. These changes, however, can be detected only by methods more so- Fig. 37 Portion of the 870 C (1600 F) isothermal section of the Fe-Cr-C system with approximate compositions of phisticated than light microscopy. In general, the AISI H13, A2, D2, and type 420 steels indicated. Source: Ref 7 lowest-alloy tool steels receive a single temper, typically 2 h for every 25 mm (1 in.) of thick- lieve quenching stresses. Hot work and high- tent, these grades can resist softening during ness. Higher-alloy tool steels are usually tem- speed steels are usually tempered at relatively tempering to rather high levels. These higher- pered twice; high-speed steels may be tempered high temperatures. Due to their high alloy con- alloy steels are often tempered hot enough to three times. Double and triple tempering is required to condition and stabilize the microstructure. When a substantial amount of strong carbide- forming elements are present, tempering at high temperatures, where secondary hardening occurs, results in a change in carbide composition and type. To illustrate this, Fig. 67 shows the effect of time and temperature on the type of carbide present in a 12% Cr martensitic steel, more highly alloyed than type 420. At low tempering temperatures, M3C will be present. How- ever, with higher temperatures and longer times, M3C is replaced by M7C3 and M23C6, only the latter being present at still higher temperatures and longer times. This steel contained a somewhat high nitrogen content, and M2N nitrides

Fig. 39 Carbides in various high-speed steels, both in annealed conditions and after being heated to normal austen- Fig. 40 AISI O6, spheroidize annealed, transverse sec- itizing temperatures. Open bars represent quantities in annealed steels. Solid bars indicate amounts after tion. Note the globular appearance of the ןaustenitizing at hardening temperatures indicated. graphite (black). 4% picral. 500 Metallographic Techniques for Tool Steels / 657 were observed as well. Figure 68 shows the se- carbide segregation and poor ingot-to-finished properties, tempers, designations, and applica- quence of carbide formation in two tungsten- product yields. Consequently, the powder met- tions of tool steels can be found in the articles type high-speed steels as a function of tempering allurgy process now makes many of these grades “Wrought Tool Steels” and “P/M Tool Steels” in temperature. Both exhibit strong secondary routinely. Indeed, there are some grades that Properties and Selection: Irons, Steels, and hardening at approximately 600 C (1110 F). have been developed that can only be made by High-Performance Alloys, Volume 1 of ASM For many tool steels, tempering to approxi- powder metallurgy, and these grades exhibit out- Handbook (1990) and in Ref 11. mately 540 C (1000 F) produces only subtle standing resistance to the high temperatures ex- differences in the microstructure viewed opti- perienced by cutting tools. Figures 79 to 82 il- cally, as shown in Fig. 69. The speed at which lustrate T15 high-speed steel made by powder etching occurs and the darkness of the matrix metallurgy, showing the microstructure after hot will change with tempering. Tempering just be- isostatic pressing, after it was spheroidize an- low the lower critical temperature is required to nealed, and after it was hardened. The carbides produce pronounced microstructural changes, are fine and evenly distributed, without any ap- but such high tempers have no practical appli- parent segregation. cations for tools and dies, except as an anneal. Compositions for the tool steels illustrated in Heat treated tool steel microstructures are this article are listed in Table 5. Information on similar in appearance when the grades are properly heat treated. The primary differences are the amount and type of the residual, undissolved carbides that will influence the coarseness of the plate martensite. In many tool steels, the martensite phase is so fine that little detail is observed. This is not the case for grades that exhibit very little residual carbides, for example, AISI S1, S5, and S7. These shock-resisting tool steels have lower carbon contents, and nearly all of the carbon is dissolved in the austenite. Consequently, the martensite is coarser, with more detail ob- servable by light microscopy. In some cases, quenching does not produce a fully martensitic structure. This occurs mainly when the hardenability is inadequate for through hardening for the section size and quench medium chosen. The continuous cool- Fig. 42 Poorly carburized AISI P5 plastic-mold tool ing transformation (CCT) diagram is a good steel with an extensive intergranular carbide Fig. 41 AISI O6, spheroidize annealed, longitudinal network and a carbide film on the surface. Three steps with tool for the metallurgist when evaluating heat section. Note that the graphite is elongated in a pressed synthetic chemical-textile pad; 10 min on step 3. ןNital. 200 ןtreatment problems. Unfortunately, diagrams do the rolling direction. 4% picral. 500 not exist for all standard grades. Figure 70 shows a CCT diagram for S7 tool steel. Figures 71 and 72 illustrate the microstructural variations in S7 produced by varying the quench rate from 2780 to 28 C/h (5000 to 50 F/h). Iso- thermal transformation (IT) diagrams (also called time-temperature-transformation or S- curves) have often been used when diagnosing heat treatment problems, but they cannot be used with most tool steels, because the transformations depicted by the IT diagram are much different than those shown in the corresponding CCT diagram. As an example, compare the CCT diagram for S7 shown in Fig. 70 with the IT diagram for S7 shown in Fig. 73. There is some similarity here, but with higher-alloyed tool steels, the similarity is less. The IT diagrams are very useful for developing annealing cycles and for isothermal treatments such as ausforming. Figure 74 shows the microstructure of S7 held at 704 C (1300 F) for 30 min (only a small amount of transformation before quenching) and for 4 h (almost complete transformation to pearlite). A wide range of isothermally formed microstructures is shown in Fig. 75 to 78 for S5 tool steel. Powder Metallurgy Products. Highly alloyed grades, such as the high-speed steels, are difficult to manufacture by ingot-making tech- ןb) 1000) .ןnology, because they are plagued by alloy and Fig. 43 Gas nitrided AISI H13 tool steel. Four steps with a rigid grinding disk. Nital. (a) 200 658 / Metallography and Microstructures of Ferrous Alloys

Fig. 44 Ion nitrided AISI H13 tool steel with a brittle white-etching iron nitride layer at the extreme surface. (a) Mounted with silica-ﬁlled epoxy. (b) Nickel plated and mounted ןwith silica-ﬁlled epoxy. Vilella’s reagent. Note that in (b) the iron nitride layer may be easily missed due to the similar color of it on the plating. 1000

Fig. 45 AISI S7 (0.5% C). Inﬂuence of austenitizing temperature. (a) Austenitized at 915 C (1675 F) 1 h for every 25 mm (1.0 in.) of thickness and air quenched. Sample is underaustenitized. (b) Austenitized at 925 C (1700 F). Slightly underaustenitized. (c) Austenitized at the preferred temperature of 940 C (1725 F). (d) Austenitized at 955 ןC (1750 F). Slightly overaustenitized; note coarsening, no visible carbide. 4% picral. 500

Fig. 46 AISI O1. Inﬂuence of austenitizing temperature on microstructure. (a) Austenitized at 800 C (1475 F) 1 h for every 25 mm (1.0 in.) of thickness. 65 HRC, grain size 9.5. Specimen properly austenitized. (b) Austenitized at 870 C (1600 F). 65 HRC, grain size 9. Overaustenitized. (c) Austenitized at 980 C (1800 F). 64 HRC, grain size 7. ןVery overaustenitized; all carbide dissolved. (d) Austenitized at 1100 C (2010 F). 64 HRC, grain size 3. Severely overaustenitized; note retained austenite (white). 4% picral. 500 Metallographic Techniques for Tool Steels / 659

-after air quenching from various austenitizing tem (ןFig. 47 AISI D2 tool steel microstructure (Vilella’s etch, 1000 peratures. Note that retained austenite, which is present even at the correct austenitizing temperature (1010 C, or 1850 F), cannot be observed by light microscopy until a substantial amount is present. (a) 1010 C (1850 F) air quenched. (b) 1065 C (1950 F) air quenched. (c) 1120 C (2050 F) air quenched. (d) 1175 C (2150 F) air quenched. (e) 1230 C (2250 F) air quenched 660 / Metallography and Microstructures of Ferrous Alloys

Fig. 48 AISI W1 (1.05% C), 19 mm (0.75 in.) diam bars, brine quenched. (a) Hardened case microstructure. 64 HRC. Case contains as-quenched martensite and undissolved carbides. 4% picral. (b) 2% nital etch reveals martensite as dark rather than light. (c) Transition zone. 55 HRC. Martensite is light and undissolved, carbide is outlined, and pearlite is dark. 4% picral. (d) Core microstructure. 42 to 44 HRC. 4% picral etch reveals ﬁne pearlite matrix (black) containing some patches of martensite (white) and undissolved ןcarbides (outlined white particles). 1000

Fig. 49 AISI S2, heated to 845 C (1550 F), water Fig. 50 AISI L6, heated to 840 C (1550 F), oil Fig. 51 AISI O2, heated to 850 C (1560 F), oil quenched, and tempered at 150 C (300 F). quenched, and tempered at 150 C (300 F). 61 quenched, and tempered at 175 C (350 F). 61 59.5 HRC. Structure consists of martensite and some very HRC. Martensite and undissolved carbides are revealed. HRC. Martensite and a small amount of undissolved car- ןbide are revealed. 2% nital. 1000 ןnital. 1000 2% ןﬁne undissolved carbide. 2% nital. 1000 Metallographic Techniques for Tool Steels / 661

AISI S5, heated to 870 C (1600 F), oil Fig. 53 Fig. 52 AISI S1, heated to 955 C (1750 F), oil quenched, and tempered at 175 C (350 F). 60 Fig. 54 AISI P20, heated to 900 C (1650 F), water quenched, and tempered at 525 C (975 F). 32 ןquenched, and tempered at 150 C (300 F). 58 HRC. Only martensite is visible. 2% nital. 1000 HRC. Matrix is martensite. Dark particles are manganese ןto 59 HRC. Only martensite is visible. 2% nital. 500 sulﬁdes. Contrast process orthochromatic ﬁlm. 2% nital. ן500

Fig. 55 AISI S7, heated to 940 C (1725 F), air Fig. 56 AISI S7, heated to 940 C (1725 F), air Fig. 57 AISI A6, heated to 840 C (1550 F), air quenched, and tempered at 200 C (400 F). 58 quenched, and tempered at 495 C (925 F). 52 quenched, and tempered at 150 C (300 F). HRC. Martensite and a small amount of undissolved car- HRC. Martensite and a small amount of undissolved car- 61.5 HRC. Martensite plus a small amount of undissolved ןcarbide are observed. 2% nital. 1000 ןbide are observed. Vilella’s reagent. 1000 ןbides are observed. Vilella’s reagent. 1000

Fig. 58 AISI H11, heated to 1010 C (1850 F), air quenched, and double tempered at 510 C (950 F). 52 HRC. Martensite plus a small amount of very ןﬁne carbide are visible. Vilella’s reagent. 1000 662 / Metallography and Microstructures of Ferrous Alloys

Fig. 59 AISI H13, heated to 1025 C (1875 F), air quenched, and double tempered at 595 C (1100 F). 42 HRC. All martensite plus a small amount of Fig. 63 AISI M1, heated to 1175 C (2150 F), oil ןvery ﬁne undissolved carbide. 2% nital. 1000 quenched, and triple tempered at 480 C (900 Fig. 61 AISI D2, heated to 1010 C (1850 F), air F). 62 HRC. Martensite plus undissolved carbide are re- ןquenched, and tempered at 200 C (400 F). vealed. 2% nital. 1000 59.5 HRC. Martensite plus substantial undissolvedcarbide; ןnote the prior-austenite grain boundaries. 2% nital. 1000

Fig. 60 AISI H21, heated to 1200 C (2200 F), oil Fig. 62 AISI D3, heated to 980 C (1800 F), oil Fig. 64 AISI M2, heated to 1120 C (2050 F), oil quenched, and tempered at 595 C (1100 F). quenched, and tempered at 200 C (400 F). quenched, and double tempered at 480 C 53.5 HRC. Martensite and undissolved carbide are ob- 60.5 HRC. Martensite plus substantial undissolved carbide (900 F). 62 HRC. Martensite plus undissolved carbide are ןrevealed. Vilella’s reagent. 1000 ןare visible. 2% nital/Vilella’s reagent. 1000 ןserved. 2% nital/Vilella’s reagent. 1000 Metallographic Techniques for Tool Steels / 663

Fig. 65 AISI M4, heated to 1220 C (2225 F), oil Fig. 66 AISI M42, heated to 1175 C (2150 F), oil quenched, and double tempered at 480 C quenched, and triple tempered at 565 C (1050 (900 F). 62 HRC. Martensite plus undissolved carbide are F). 65 HRC. Martensite plus undissolved carbide are ob- ןserved. Vilella’s reagent. 1000 ןrevealed. Vilella’s reagent. 1000

Fig. 67 Time-temperature diagram of carbides present in two 12% Cr martensitic stainless steels after tempering. Solid lines (for alloy A) and broken lines (for alloy B) show when ﬁne precipitates are ﬁrst observed. Al- loy A composition: 0.21% C, 13.2% Cr, and 0.24% N. Al- loy B composition: 0.18% C, 0.58% Mn, 0.31% Si, 0.18% Ni, 11.7% Cr, 0.49% Mo, 0.01% Al, 0.38% V, 0.20% Nb, Fig. 68 Sequence of alloy carbide formation in two and 0.033% N. This diagram is based on observations of tungsten-type high-speed steels as a function of specimens subjected to more than 30 different tempering tempering temperature. Steel A contains 0.8% C, 18% W, treatments in the temperature interval of 500 to 700 C (930 4% Cr, 2% V, and 10% Co; steel B contains 0.8% C, 9% to 1290 F). Source: Ref 9 W, 3% Cr, and 3% Co. Source: Ref 10 664 / Metallography and Microstructures of Ferrous Alloys

All specimens austenitized at 800 C (1475 F), oil quenched, and tempered at .(ןFig. 69 Inﬂuence of tempering temperature on microstructure of AISI O1 (all: 4% picral, 500 different temperatures. (a) 200 C (400 F). 60 HRC. (b) 315 C (600 F). 55 HRC. (c) 425 C (800 F). 49 HRC. (d) 540 C (1000 F). 43 HRC

Fig. 70 Continuous coding transformation diagram for AISI S7 tool steel. Ms, martensite start; Mf, martensite finish; Bs, bainite start; Bf, bainite finish; Ps, pearlite start; Pf, pearlite finish. Source: Atlas of Time-Temperature Diagrams, ASM International Metallographic Techniques for Tool Steels / 665

Fig. 71 AISI S7 continuous cooling transformations. Some very ﬁne undissolved carbide is present in all specimens in this series. (a) Austenitized at 940 C (1725 F) and cooled at 2780 C/h (5000 F/h). 62 HRC. Structure is martensite plus a small amount of bainite. (b) Cooled at 1390 C/h (2500 F/h) to produce a greater amount of bainite. 61.5 ןHRC. (c) Cooled at 830 C/h (1500 F/h). 56.5 HRC. Structure is mostly bainite plus some martensite (light). 4% picral. 500

Fig. 72 AISI S7 continuous cooling transformations. Some very fine carbide is present in all specimens in this series. (a) Austenitized at 940 C (1725 F) and cooled at 445 C/h (800 F/h). 51.5 HRC. Structure is nearly all bainite with some small patches of martensite (white). (b) Cooled at 220 C/h (400 F/h). 45 HRC. Structure is mostly bainite with fine pearlite at the prior-austenite grain boundaries. (c) Cooled at 28 C/h (50 F/h) to 620 C (1150 F), then water quenched. Austenite present at 620 C (1150 F) was transformed ןto martensite. Structure is mostly fine pearlite with patches of martensite (white). See also Fig. 71. 4% picral. 500 666 / Metallography and Microstructures of Ferrous Alloys

Fig. 73 Isothermal transformation diagram for AISI S7 tool steel. Composition: 0.50% C, 0.71% Mn, 0.30% Si, 3.20% Cr, and 1.32% Mo. Austenitized at 940 C (1725 F). Ms, martensite start; Bs bainite start; Bf, bainite ﬁnish; Ps, pearlite start; Pf, pearlite ﬁnish. Source: Atlas of Time-Temperature Diagrams, ASM International

of AISI S7 tool steels with isothermal heat treatments. (a) Held at 704 C (1300 F) for 30 min (only a small amount of transformation (ןFig. 74 Microstructure (picral etch, 500 before quenching). (b) Held at 704 C (1300 F) for 4 h (almost complete transformation to pearlite) Metallographic Techniques for Tool Steels / 667

AISI S5 austenitized and isothermally trans- Fig. 76 AISI S5 austenitized, isothermally transformed AISI S5 austenitized, isothermally transformed Fig. 75 Fig. 77 formed at 650 C (1200 F) for 4 h (air cooled) at 595 C (1100 F) for 8 h, and air cooled to (partially) at 540 C (1000 F) for 8 h, and water ן to form ferrite and coarse pearlite. 23 to 24 HRC. 4% picral. form ferrite and ﬁne pearlite. 36 HRC. 4% picral. 1000 quenched to form upper bainite (dark); balance of austenite ןformed martensite. 4% picral/2% nital. 1000 ן1000

Fig. 78 AISI S5 austenitized, isothermally transformed at 400 C (750 F) for 1 h, and air cooled to Fig. 79 AISI T15, powder-made. Sample was slow Fig. 80 AISI T15, powder-made. Sample was hot isos- form lower bainite. 37 to 38 HRC. 4% picral/2% nital. cooled after hot isostatic pressing. 28 HRC. tatically pressed, forged, and annealed. 24 ןHRC. Structure is fully annealed. 3% nital. 1000 ןStructure is partially annealed. 3% nital. 1000 ן1000 668 / Metallography and Microstructures of Ferrous Alloys

Fig. 81 AISI T15, powder-made. Processed as in Fig. 80, then hardened; heated to 1230 C (2250 F) Fig. 82 AISI T15, powder-made. Same sample as in for 5 min in salt, oil quenched, triple tempered 2 h each at Fig. 81 but etched in 100 mL H2O, 1 mL HCl, ן1gK2S2O5, and 1 g NH4•HF. 1000 ןC (1000 F). 65 HRC. 3% nital. 1000 540

Table 5 Nominal compositions of illustrated tool steel grades

Composition, % AISI type C Mn(a) Si(b) Cr Ni V W Mo Co Ti W1 0.6Ð1.4 ...... 0.25 ...... W2 0.6Ð1.4 ...... S1 0.5 . . . 0.75 1.5 . . . 0.2(c) 2.5 ...... S2 0.5 0.4 1.0 ...... 0.5 ...... S4 0.55 0.8 2.0 ...... S5 0.55 0.8 1.9 0.25(c) . . . 0.2(c) . . . 0.4 ...... S7 0.5 0.7 . . . 3.25 ...... 1.40 ...... O1 0.9 1.0 . . . 0.5 . . . 0.2(c) 0.5 ...... O2 0.9 1.6 ...... O6 1.45 0.8 1.1 ...... 0.25 ...... A2 1.0 0.7 . . . 5.25 . . . 0.2(c) . . . 1.1 ...... A6 0.7 2.0 . . . 1.0 ...... 1.35 ...... A7 2.00Ð2.85 0.8 0.5 5.0Ð5.75 0.3 3.9Ð5.15 0.5Ð1.5 0.90Ð1.40 ...... A10 1.25Ð1.50 1.6Ð2.1 1.0Ð1.5 . . . 1.55Ð2.05 ...... 1.25Ð1.75 ...... D2 1.5 0.5 . . . 12.0 . . . 0.2Ð0.9(c) . . . 0.8 ...... D3 2.1 ...... 12.0 0.5(c) ...... H11 0.35 . . . 0.9 5.0 . . . 0.4 . . . 1.5 ...... H13 0.35 . . . 1.0 5.25 . . . 1.0 . . . 1.3 ...... H21 0.35 ...... 3.5 . . . 0.4(c) 9.0 ...... H23 0.25Ð0.35 0.15Ð0.40 0.15Ð0.60 11.0Ð12.75 0.3 0.75Ð1.25 11.0Ð12.75 ...... H26 0.45Ð0.55 0.15Ð0.40 0.15Ð0.40 3.75Ð4.5 0.3 0.75Ð1.25 17.25Ð19.0 ...... T1 0.7 ...... 4.0 . . . 1.0 18.0 ...... T15 1.5 ...... 4.0 . . . 5.0 12.0 . . . 5.0 . . . M1 0.8 ...... 4.0 . . . 1.1 1.5 8.5 ...... M2 0.85 ...... 4.0 . . . 2.0 6.0 5.0 ...... M4 1.3 ...... 4.5 . . . 4.0 5.5 4.5 ...... M42 1.1 ...... 3.75 . . . 1.15 1.5 9.5 8.0 . . . L1 1.0 ...... 1.4 ...... L6 0.75 0.75 . . . 0.9 1.75 ...... 0.35 ...... F2 1.25 0.75 ...... 0.35 ...... P5 0.1 ...... 2.25 ...... P20 0.35 ...... 1.25 ...... 0.4 ...... AHT 1.0 ...... 3.0 . . . 0.25 1.05 1.1 . . . 1.0 (a) All tool steels contain some manganese, generally 0.2Ð0.4% when not listed. (b) Tool steels usually contain 0.2Ð0.35% Si unless listed otherwise. (c) Optional addition at discretion of manufacturer Metallographic Techniques for Tool Steels / 669

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