By George F. Vander Voort
The mechanical properties of heat-treated alloy steels are strongly influenced by the grain size of the parent austenite phase before quenching. But revealing the prior-austenite grain boundaries (PgGBs or PAGBs) can be quite difficult depending upon the alloy and its microstructure.
There are a number of well-established (see ASTM E 112, for example) procedures that are used to decorate the PgGBs during a heat-treatment cycle, e.g., the McQuaid-Ehn carburizing test (Fig. 1) and the oxidation test. In some medium-carbon steels, at a specific cooling rate, proeutectoid ferrite will precipitate at the PgGBs, while in high-carbon steels (generally hypereutectoid tool steels), proeutectoid cementite will precipitate on the PgGBs upon slow cooling from elevated temperatures. These conditions are often seen in as-cast or as-rolled steels, as shown in Figures 2 and 3. But these methods cannot be applied to determine the prior-austenite grain size of a steel part or component that has already been heat treated, as these methods will produce a different grain size. For this problem – and this is a common situation in failure analysis – one can only use an etching technique to reveal the PgGBs.
History of Prior-Austenite Grain Boundary Etch Development
One of the earliest etchants to have some success at revealing PgGBs in some steels was Vilella’s reagent, published in 1938. This etch has had limited success, mainly with tool steels. Subsequently, Schrader modified Vilella’s reagent, but it also has limited value. In 1949, Miller and Day published a 5% aqueous ferric chloride reagent for low-carbon martensitic steels. Aqueous ferric chloride and HCl solutions have also been suggested. Nital, generally in concentrations of 2-10% (do not store more than 3% HNO3 in ethanol in a tightly closed bottle, as it can explode), will reveal the grain boundaries in only a few steels – highly alloyed tool steels in the as-quenched or lightly tempered condition, such as D2 and high-speed steels.
The first reasonably successful etchant for PgGBs was published in 1955 by Bechet and Beaujard using a saturated aqueous picric-acid solution (as had been used in studies of temper embrittlement) containing 0.5% of a wetting agent, “Teepol” (sodium alkylsulfonate), at room temperature. This etchant has been the foundation of many subsequent modifications to improve its effectiveness.
The writer tried this etch on specimens of 8620, 4140 and 5160 in the as-quenched condition and after tempering at 400, 800 and 1200°F using sodium tridecylbenzene sulfonate as the wetting agent. It did not reveal grain boundaries on any of the 8620 specimens. It did reveal the PgGBs on as-quenched and tempered (400 and 800°F) specimens of 4140 and 5160 but did not reveal them on any specimens tempered at 1200°F. Tempered martensite and tempered bainites both respond to this etch but only for medium to high-carbon steels and only when tempered below ~1050°F.
It is well known that saturated aqueous picric acid with a wetting agent (used at room temperature) reveals PgGBs if phosphorus is present in the grain boundaries, and this is easier if the specimen has been heated in the temper embrittlement range. Segregation of Sn or Sb to the PgGBs, which also causes temper embrittlement, does not help reveal the PgGBs using this etch in steels free of phosphorus.[6,7] Preece and Carter showed using TEM that there was a clear difference in appearance between grain boundaries that were temper embrittled due to a high local phosphorous concentration compared to a non-embrittled specimen with a lower local phosphorous content, even though saturated aqueous picric acid reveals the PgGBs in both cases and the boundaries looked similar by light microscopy.
Studies conducted in this time period examined the effect of a variety of wetting agents on the etch response. Nelson conducted the most extensive comparison using five wetting agents, including the most popular one, sodium tridecylbenze sulfonate, with several different etchants including the saturated aqueous picric-acid solution. Without the wetting agent added, saturated aqueous picric acid was an excellent general-purpose etchant for steels, but PgGBs were not revealed. When this wetting agent was added, general-structure etching was suppressed and PgGBs were revealed. None of the other wetting agents tried were as effective. A number of studies on the use of wetting agents in etchants have been reviewed. The original tridecyl version of this wetting agent has branched molecular chains, which are difficult to manufacture and have poor biodegradability. More recent versions have linear chains and are biodegradable. Kilpatrick evaluated the dodecyl version of this wetting agent, which is more easily made, readily biodegradable and works as well. Consequently, this wetting agent is the most commonly used today for revealing PgGBs.
Fig. 4. Intergranular SCC cracks in 4340 alloy steel revealed by etching with saturated aqueous picric acid, plus HCl and Nacconol 90G wetting agent (80°C – 60 sec.), magnification bars are 20 µm long.
Barraclough reviewed etchants tried by 10 different authors to reveal PgGBs. He concluded that it was necessary to temper embrittle specimens to obtain adequate grain-boundary delineation to permit measurements to be made of the grain size. His work confirmed that picric acid was the most suitable agent for revealing PgGBs and the solvent used was critical. Alcohols did not work, but water or ether gave good results. Petroleum ether is less dangerous than ethyl ether, but both are explosive when heated above 100°C, and static electricity can cause explosions. Several wetting agents were tried, all were suitable, but he preferred “Teepol” (Teepol is a registered trademark of the Shell Chemical Co. of Houston, Texas). He found that the aqueous solution could be used at temperatures up to 85°C, but he did not indicate if higher temperatures produced any benefit or detriment. Barraclough used swabbing and lightly back-polished his specimens to reduce the etch details of the martensite within the grains, which is now an excellent common practice.
Brownrigg et al. followed up on this study with a slight modification that they stated allowed them to bring out PgGBs for as-quenched steels from 0.03-0.8% C with bainitic structures. They used a solution of 100 mL saturated aqueous picric acid, plus 2 mL “Teepol” plus 6 drops of HCl. After mixing, they filtered out the excess picric acid, which they stated reduces staining of the specimen surface. They immersed specimens at room temperature for 4-10 minutes. They demonstrated that PgGBs could be revealed in low-carbon (0.04%), as-rolled bainitic structures that were not recrystallized after hot rolling.
Bodnar et al. studied development of PgGBs in CrMoV rotor steels using 13 different etchants. The saturated, aqueous picric-acid etchant produced better results than most but was still inadequate. Tempering specimens in the embrittlement range did not help because the phosphorous content was too low. Addition of a small amount (3-5 drops per 50 mL of etchant) of HCl to the etchant produced markedly better results. They etched for 5-8 minutes with the beaker placed in an ultrasonic cleaner for agitation (the water level in the ultrasonic cleaner must be lower than the etchant level in the beaker, or the beaker will flip over). This was followed by light repolishing to remove some of the etch detail within the grains. Other etchants for revealing PgGBs have been developed; reference lists 28 reagents published prior to 1984 for this purpose.
Before specimens can be etched, they must be properly prepared to a very high quality level. The first and most critical step is sectioning, which must be conducted to induce minimal damage. Use abrasive cut-off machines (avoid torch cutting, shearing, band saws or power hack saws as they induce far too much damage) with a blade/wheel designed for metallography and for steels of the hardness level being prepared.
Generally, mounting is performed but may not be necessary if the structure at the edges of the sample is not important (as in a specimen cut from the interior of a part). Commence grinding with SiC paper, using as fine a grit size as possible. As a rule, start grinding with 120-grit SiC for steels ≥60 HRC; start with 180-grit SiC for steels between 35 and 60 HRC; and start with 220- to 240-grit SiC for steels <35 HRC.
Next, polish the specimens using flat, low-resilience cloths, such as DP/MD-Plan or DP/MD-Pan with 9-µm diamond, using a load of 25-30 N per specimen, 150 rpm, for at least 5 minutes. Next, polish with DP/MD-DAC, DP/MD-DUR or DP/MD-SAT cloths with 3-µm diamond, same load and rpm, for 5 minutes. For martensitic and bainitic steels that are to be etched to reveal prior-austenite grain boundaries, a 1-µm step is not necessary. The final step would be to use either a synthetic neoprene cloth (such as DP/MD-Chem) or a napped or flocked cloth (such as DP/MD-Floc or DP/MD-Nap) using either colloidal silica (such as OP-S) or a neutral alumina suspension (such as OP-AN). Polishing is usually conducted at 120-150 rpm, same load, from 1-3 minutes.
A good practice is to lightly etch the specimens after the last step with a general-purpose reagent, such as 2% nital, to see what the structure actually is and how well prepared the specimens are before proceeding to use the saturated aqueous picric-acid etch. After examination, repeat the last step for at least 1 minute to remove this etch. Cleaning after each step is important to prevent contamination of the next step and poor results.
The writer has been using saturated aqueous picric acid plus a wetting agent and a small HCl addition (when steels have more than about 1% Cr) for some time but formerly at room temperature. The specimen would be placed polished face vertical in a beaker with at least 100 mL of the etchant in an ultrasonic cleaner (the water level in the ultrasonic cleaner should not be higher than the etchant level in the beaker or it will flip over). The timer would be set for 7 minutes with etching at room temperature.
Results with 8620, 4140 and 5160 were described above (without the HCl addition). Light back-polishing was always done to try and improve the visibility of the grain boundaries. As sodium tridecylbenzene sulfonate became difficult to obtain, the writer switched to the dodecyl version with no apparent difference.
Several broken, heat-treated, 4340-alloy-steel nut inserts from the riser of an oil rig were examined. To determine if the crack patterns were intergranular, specimens were etched in the saturated aqueous picric-acid filtered solution with HCl using Nacconol 90G as the wetting agent (Nacconal is a registered trademark of the Stepan Company of Northfield, Ill.). This is described as sodium alkyl benzene sulfonate. On the MSDS sheet, the composition is given as 90-93% sodium dodecylbenzen sulfonate, 5% sodium sulfate, 1% sodium chloride and 1.5% water. HCl was added in the amount of 6 drops per 100 mL of the saturated aqueous picric-acid solution (1-500 mL). After this was mixed, the excess picric acid was removed by filtering.
Etching was conducted at room temperature for 7 minutes using the ultrasonic cleaner for agitation. However, the results were marginal. Hence, the writer heated the solution to 80-90°C (below the boiling point to retard evaporation). Specimens were swab etched for 2 minutes and then back-polished. Results were exceptionally good, as shown in Figure 4, revealing an almost fully intergranular crack path from stress-corrosion cracking (bulk hardness was well above the safe limit for high-strength steel in salt water).
In a study of SAE 723 Grade-3 Class-3 pressure-vessel steel forgings, it was necessary to reveal the prior-austenite grain structure. A previous investigator had etched the specimens with 2% nital and claimed it revealed a very coarse prior-austenite grain size. As nital will only reveal the lath martensite and give a contrast etch to the lath packets, the writer used the saturated aqueous picric-acid etchant, plus HCl and Nacconol 90G as the wetting agent. Figure 5a shows an example of the structure etched with nital, revealing a coarse lath packet size. First, the saturated picric-acid etch was used at room temperature, but the results (Figure 5b) were inadequate. The specimen was re-polished and etched at 90°C, and the results were much better (Figure 5c) as there was much less structure etching. Dark-field illumination may be used effectively to reveal the boundaries with strong contrast (Figure 5d).
The value of back-polishing after etching is illustrated in Figure 6, which shows a specimen of A-350 (LF3) high-alloy steel (Fe-0.07%C-0.74%Mn-3.66%Ni-0.2%Cr-0.07%Mo). Note the very low carbon content, which usually makes the task impossible. Also, the phosphorous content was only 0.008%, and it was tempered at 1350°F.
These three factors, very low carbon and phosphorus and very high tempering temperature, would normally make it impossible to reveal the PgGBs. But, as shown in Figure 6, it was possible with the etchant heated. This figure also demonstrates the benefit of careful light back-polishing after etching to remove extraneous etch detail within the grains and make the boundaries more visible. A specimen from a forging of modified 4330V with only 0.005% phosphorus that was tempered at 1110°F was also successfully etched (Fig. 7). With this low P content and high tempering temperature, other etchant variations would not reveal the PgGBs.
For this study, the writer re-prepared the previously mentioned 8620 and 4140 specimens that were in the as-quenched and quenched-and-tempered (400, 800 and 1200°F) conditions. They had been etched with the saturated aqueous picric-acid reagent without the HCl addition at room temperature. This time, HCl was added and etching was done at 80-90°C for 60-120 seconds. Figure 8 shows the four fully martensitic 8620-alloy-steel specimens in the as-quenched and quenched-and-tempered conditions, while Figure 9 shows the four fully martensitic 4140-alloy-steel specimens in the same heat-treated conditions. In all cases, the prior-austenite grain boundaries are visible.
Four specimens of 4340-alloy steel were isothermally transformed to lower bainite and then tempered at 300, 500, 700 and 900°F. They were etched with the heated saturated aqueous picric-acid reagent containing HCl and Nacconol 90G wetting agent. Figure 10 shows the specimens tempered at 300 and 500°F that were carefully back-polished after etching. Results were similar for the two tempered at 700 and 900°F.
Revealing prior-austenite grain boundaries has been one of the most difficult and frustrating tasks assigned to the metallography/materialography laboratory. The most successful etch had been saturated aqueous picric acid containing a wetting agent, usually sodium dodecylbenzene sulfonate, at room temperature for periods of 4-20 minutes. However, this etch was unable to reveal PgGBs in martensitic or bainitic steels with carbon contents below ~0.3% or with phosphorus contents below ~0.010%, even when subjected to step-embrittlement cycles or for steels tempered above ~1050°F.
However, if a small amount of HCl is added and the etchant is used at ~80-90°C (results were good at 70°C when tried on one specimen), these limitations are overcome. Filtering the solution before use does help reduce staining/pitting attack. Careful, low-pressure back-polishing on a stationary cloth using OP-AN alumina slurry is very effective at reducing extraneous etch detail within the grains and enhancing grain-boundary visibility.
The writer is grateful to Michael He and the staff at Scot Forge (Spring Grove, Ill.), for the use of their equipment and for supplying some of the forged specimens tested in this program.
George F. Vander Voort, Consultant, Struers Inc.; Westlake, Ohio
George Vander Voort has a background in physical, process and mechanical metallurgy and has been performing metallographic studies for 43 years. He is a long-time member of ASTM Committee E-4 on metallography and has published extensively in metallography and failure analysis. He regularly teaches MEI courses for ASM International and is now doing webinars. He is a consultant for Struers Inc. and will be teaching courses soon for them. He can be reached at 1-847-623-7648, firstname.lastname@example.org and through his web site: www.georgevandervoort.com
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