Etching Isothermally Treated Stee Ls
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ETCHING ISOTHERMALLY TREATED STEELS Relative advantages of he microstructure stituent identification in the isother- of iron-based al- mally transformed microstructures of using nital, picral, and loys is very com- low-alloy steels containing 0.2, 0.4, and 10% sodium metabisulfite, plicated, being in- 0.6% carbon (SAE 8620, 4140, and fluenced by 5160, respectively). an underutilized tint composition, ho- SMB deserves to be more widely etchant, to explore the mogeneity, pro- used. It behaves much like nital but is cessing, and sec- more uniform in its action. And un- microstructures of three Ttion size. Microstructures of like picral, it will reveal the structure low-alloy steels. coarse-grained steels are much easier of as-quenched martensite. It also is to observe than those of fine-grained excellent for revealing the diffusion- steels. Of course, steels are normally controlled products ferrite, pearlite, made with a fine grain size to optimize and bainite, as well as the diffusion- by George F. Vander Voort* their mechanical properties. less product, martensite. In addition, Buehler Ltd. In general, it is easiest to identify 10% SMB often gives the best contrast, Lake Bluff, Ill. heat treated structures after transfor- and is safer to use than either nital or mation and before tempering. How- picral. ever, in most applications, hardened steels must be tempered and are usu- Specimen preparation basics ally examined in this condition. If a To observe the microstructure of fer- mixed microstructure of bainite and rous metals, they must be properly pre- martensite is formed during quench- pared. Many view this task as a trivial ing, these constituents will become exercise, yet its proper execution is crit- more difficult to identify reliably as ical to successful interpretation. the tempering temperature increases Sampling:The first step is to select toward the lower critical temperature the test locations to be sampled. This (Ac1). is critical if the interpretation is to be These factors make it more difficult valid for the part or lot being evalu- to identify phases and constituents in ated. Specimens must be representa- steels. Further, while ferrous metallo- tive of the lot. Note that the plane of graphers tend to use nital almost ex- polish may be oriented in different di- clusively for etching, nital is not al- rections relative to the piece being ways the best reagent for properly sampled. revealing all microstructures. It is un- Sectioning is almost always re- fortunate that some companies pro- quired to obtain a piece of the proper hibit use of picral for the reason that size and orientation for subsequent picric acid can be made to detonate metallographic examination. The abra- under certain conditions. Picral is an sive cut-off saw is the most commonly excellent etchant for revealing certain used sectioning device. It produces microstructural constituents in steel good surfaces with minimal damage, and, frankly, accidents have been less when the proper blade is used with common with it than with nital. adequate coolant. More aggressive This article compares the advan- sectioning methods are often used in tages of using nital, picral, and 10% production operations. These tend to aqueous sodium metabisulfite (SMB), cause greater damage, which must be *Fellow of ASM International a tint etchant, as aids to phase/con- removed if the true structure is to be HEAT TREATING PROGRESS • APRIL/MAY 2001 examined. grinding, can be done using several croetching Metals and Alloys.” Mounting:The specimen is then products. The traditional silicon car- Nital, usually 2%, is the most com- mounted in a polymeric material to bide paper is always satisfactory, but monly used etchant for steel speci- facilitate handling, simplify prepara- has a short life. Aluminum oxide mens. It is excellent for revealing the tion, enhance edge retention, and/or paper may also be used. Always start structure of martensite. Nital is also make it easy to identify the specimen. with the finest possible abrasive that very good for revealing ferrite in a Mounting may be done in a press can remove the damage from cutting martensitic matrix and for bringing using a thermosetting or thermoplastic and get all of the specimens in the up ferrite grain boundaries in low- resin, or with a castable resin that does holder co-planar in a reasonable time. carbon steels. Recommended nital not require external heat and pressure Note that continuing to grind after the composition: 1 to 5 mL nitric acid for polymerization. paper has lost its cutting efficiency will (HNO3), 100 mL pure ethanol. (Avoid Grinding and polishing:The use generate heat and damage. using methanol as it is a cumulative of automation in specimen prepara- Resin- or metal-bonded grinding poison.) tion has grown enormously over the disks are excellent for obtaining flat- Picral is better for revealing the ce- past 25 years. Automatic grinder/pol- ness and edge retention and yield high mentite in ferritic alloys and the struc- isher machines produce better results stock removal rates. Their surface is ture of the ferrite-cementite constit- than can be achieved manually. They covered by small, diamond-containing uents pearlite and bainite. Recom- yield more consistent results, better pads. Diamond-free regions sur- mended picral composition: 4 g picric flatness, and better edge retention, rounding the pads reduce surface ten- acid (trinitrophenol, (NO2)3C6H2OH), with greater productivity. sion and increase cutting efficiency. 100 mL pure ethanol. There is no one correct procedure These disks have a long life. Use Nital and picral both dissolve fer- for successfully preparing ferrous metal-bonded disks for harder ferrous rite, but nital’s dissolution rate is a metal specimens. Many procedures alloys and resin-bonded disks for the function of crystal orientation while and many different products can be softest steels. picral’s is uniform. Other reagents used, although some methods may Rigid grinding discs (RGDs) can be have their uses, especially when favor certain types of specimens or be used for the second step, and even for dealing with higher alloy grades or better suited to address specific prob- the first (planar) grinding step with when trying to selectively reveal cer- lems. The procedures given in the coarse diamond (30 to 45 µm). They tain constituents or prior-austenite table (last page of article) were used also yield very flat surfaces and are grain boundaries. to prepare the specimens discussed recommended when edge retention is Tint etchant: Because some labora- here, and are appropriate for most critical. These disks do not contain em- tories are prohibited from using picral, steel specimens. They give consistent bedded abrasive; rather, diamond is the specimens prepared for this article results with good edge retention. added periodically to the surface, usu- were also etched with 10% aqueous Other variations are possible de- ally as a suspension. “Universal” sodium metabisulfite (abbreviated pending upon needs and specimens. RGDs suitable for preparing most SMB, and also called sodium pyro- The first step, often called planar steels, and ones designed for the sulfite), which has many of the virtues softest grades are available. of both nital and picral. It is prepared Fig. 1 — Isothermal transformation (IT) or There are cloth alternatives for the by adding 10 g sodium metabisulfite time-temperature-transformation (TTT) di- second step that work well, but they (Na2S2O5) to 100 mL of water. Com- agrams for three low-alloy steels: (a) SAE don’t last as long as an RGD. pared with other tint etchants, it is rel- 8620, (b) SAE 4140, and (c) SAE 5160. A f Etching:Etchants for steels are atively simple to use. Etching of these and A are equivalent to the upper and lower s listed in many standard textbooks1 specimens was by immersion for 5 to critical temperatures Ac3and Ac1, respec- and handbooks2, and in ASTM E tively. A = austenite, F = ferrite, C = cemen- 15 seconds. Never swab. tite or carbide. (Ref. 5) 407–99, “Standard Practice for Mi- SMB will etch a wide range of com- (a) SAE 8620 (b) SAE 4140 HEAT TREATING PROGRESS • APRIL/MAY 2001 positions and does produce coloration, lected for this work: SAE 8620, 4140, holding times were short enough so which is not strong, but can be en- and 5160. They were austenitized at that only ferrite formed isothermally hanced using partially crossed polar- the recommended temperatures and before the specimens were water izing filters with a sensitive-tint plate. then held isothermally at different sub- quenched (which transformed the re- It will lightly color ferrite (like nital or critical (below Ac1) temperatures to maining austenite to martensite). picral) and reveal ferrite grain bound- convert some or all of the austenite to However, proeutectoid ferrite is much aries (like nital) but with greater uni- other phases and constituents. The harder to form in 5160, due to its high formity. It will also reveal pearlite and TTT diagrams for these steels are re- carbon content. Note that there is no bainite as well as picral, and will re- produced in Fig. 1. separate region on the 5160 TTT dia- veal as-quenched martensite. Terminology:There is confusion gram (Fig. 1) indicating formation of The etchant is quite safe to use. Of about and misuse of certain mi- proeutectoid ferrite. So, for the 5160 course, it should not be ingested and crostructural terms. Sorbite and troos- specimen, we observe a few tiny direct contact should be avoided. tite, for example, were dropped from patches of proeutectoid ferrite (much the metallographic lexicon in 1937 be- less than 1% by volume), pearlite, and Isothermal transformations cause they referred to microstructural as-quenched martensite. Microstructures, even in steels, were constituents inaccurately. However, SAE 8620: Figure 2 shows that nital not well understood and heat treat- they are still occasionally used.