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here are a number of well-established (see ASTM E 112,for example) procedures thatare used to decorate the P γ GBs

during a heat-treatment cycle, e.g., theMcQuaid-Ehn carburizing test (Fig. 1)and the oxidation test. In some medium-carbon steels, at a specic cooling rate,proeutectoid ferrite will precipitate at thePγ GBs, while in high-carbon steels (gen-erally hypereutectoid tool steels), proeu-tectoid cementite will precipitate on thePγ GBs upon slow cooling from elevatedtemperatures. These conditions are oftenseen in as-cast or as-rolled steels, as shown

in Figures 2 and 3. But these methodscannot be applied to determine the prior-austenite grain size of a steel part or com-ponent that has already been heat treated,as these methods will produce a differentgrain size. For this problem – and this isa common situation in failure analysis –one can only use an etching technique toreveal the P γ GBs.

History of Prior-Austenite GrainBoundary Etch DevelopmentOne of the earliest etchants to have somesuccess at revealing P γ GBs in some steelswas Vilella’s reagent, published in 1938. [1]

This etch has had limited success, mainlywith tool steels. Subsequently, Schradermodied Vilella’s reagent, but it also haslimited value. In 1949, Miller and Day [2]

published a 5% aqueous ferric chloridereagent for low-carbon martensitic steels.Aqueous ferric chloride and HCl solutionshave also been suggested. Nital, gener-ally in concentrations of 2-10% (do notstore more than 3% HNO 3 in ethanol ina tightly closed bottle, as it can explode),will reveal the grain boundaries in only afew steels – highly alloyed tool steels inthe as-quenched or lightly tempered con-dition, such as D2 and high-speed steels.

T

George F. Vander Voort – Consultant, Struers Inc.; Westlake, Ohio

The mechanical properties of heat-treated alloy steels are strongly inuenced bythe grain size of the parent austenite phase before quenching. But revealing theprior-austenite grain boundaries (P γ GBs or PAGBs) can be quite difcult dependingupon the alloy and its microstructure.

Fig. 1. Microstructure of 9310 alloy steelafter the McQuaid-Ehn test using alkalinesodium picrate (90°C – 45 sec.) to darkenthe cementite precipitated in the prior-

austenite grain boundaries (magnicationbar is 100 µm long)

Fig. 2. Proeutectoid ferrite precipitated inthe prior-austenite grain boundaries in anas-cast Fe - 0.38%C - 0.26%Si - 0.79% Mnsteel (2% nital)

Fig. 3. Proeutectoid cementite precipitatedon the prior austenite grain boundariesduring cooling from hot rolling in this Fe -1.31% C water-hardenable tool steel. The

cementite has been darkened using alka-line sodium picrate (90°C - 60 sec.), 500X.

RevealingRevealing

Prior Austenite GrainPrior-Austenite GrainBoundaries inBoundaries inHeat Treated SteelsHeat-Treated Steels

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The rst reasonably successful etchantfor Pγ GBs was published in 1955 by Bechetand Beaujard [3] using a saturated aqueouspicric-acid solution (as had been used in

studies of temper embrittlement) con-taining 0.5% of a wetting agent, “Teepol”(sodium alkylsulfonate), at room tempera-ture. This etchant has been the founda-tion of many subsequent modications toimprove its effectiveness. The writer tried this etch [4] on speci-mens of 8620, 4140 and 5160 in the as-quenched condition and after temperingat 400, 800 and 1200°F using sodiumtridecylbenzene sulfonate as the wettingagent. It did not reveal grain boundaries

on any of the 8620 specimens. It did re-veal the P γ GBs on as-quenched and tem-pered (400 and 800°F) specimens of 4140and 5160 but did not reveal them on anyspecimens tempered at 1200°F. Temperedmartensite and tempered bainites bothrespond to this etch but only for mediumto high-carbon steels and only when tem-pered below ~1050°F.

It is well known that saturated aque-ous picric acid with a wetting agent (usedat room temperature) reveals P γ GBsif phosphorus is present in the grainboundaries, and this is easier if the speci-men has been heated in the temper em-brittlement range. [5] Segregation of Sn orSb to the P γ GBs, which also causes tem-per embrittlement, does not help revealthe P γ GBs using this etch in steels freeof phosphorus. [6,7] Preece and Carter [8]

showed using TEM that there was a cleardifference in appearance between grainboundaries that were temper embrittleddue to a high local phosphorous concen-tration compared to a non-embrittledspecimen with a lower local phosphorouscontent, even though saturated aqueouspicric acid reveals the P γ GBs in bothcases and the boundaries looked similarby light microscopy.

Studies conducted in this time periodexamined the effect of a variety of wettingagents on the etch response. Nelson [9]

conducted the most extensive comparisonusing ve wetting agents, including the

most popular one, sodium tridecylbenzesulfonate, with several different etchants

including the saturated aqueous picric-acid solution. Without the wetting agentadded, saturated aqueous picric acid wasan excellent general-purpose etchantfor steels, but P γ GBs were not revealed.When this wetting agent was added, gen-eral-structure etching was suppressed andPγ GBs were revealed. None of the otherwetting agents tried were as effective. Anumber of studies on the use of wettingagents in etchants have been reviewed. [10]

The original tridecyl version of this wet-ting agent has branched molecular chains,which are difcult to manufacture andhave poor biodegradability. More recentversions have linear chains and are biode-gradable. Kilpatrick [14] evaluated the do-decyl version of this wetting agent, whichis more easily made, readily biodegradableand works as well. Consequently, this wet-

ting agent is the most commonly used to-day for revealing P γ GBs.

Barraclough [11] reviewed etchants triedby 10 different authors to reveal P γ GBs.He concluded that it was necessary totemper embrittle specimens to obtain ad-equate grain-boundary delineation to per-mit measurements to be made of the grainsize. His work conrmed that picric acidwas the most suitable agent for revealingPγ GBs and the solvent used was critical.Alcohols did not work, but water or ethergave good results. Petroleum ether is lessdangerous than ethyl ether, but both areexplosive when heated above 100°C, andstatic electricity can cause explosions.Several wetting agents were tried, allwere suitable, but he preferred “Teepol”(Teepol is a registered trademark of theShell Chemical Co. of Houston, Texas).He found that the aqueous solution couldbe used at temperatures up to 85°C, but

he did not indicate if higher tempera-tures produced any benet or detriment.

Fig. 4. Intergranular SCC cracks in 4340 alloy steel revealed by etching with saturated aque-ous picric acid, plus HCl and Nacconol 90G wetting agent (80°C – 60 sec.), magnication barsare 20 µm long).

Fig. 5. Etching with 2% ni tal (a) reveals packets of lath martensite; etching with aqueoussaturated picric acid solution with HCl and a wetting agent at 20°C (b) faintly revealedthe prior-austenite grain boundaries in SAE 723, Grade 3, Class 3 pressure vessel steel (Fe

- 0.33%C - 0.25%Mn - 0.13%Si - 3.55%Ni - 1.66%Cr - 0.48%Mo - 0.12%V). Using aqueoussaturated picric acid plus HCl and a wetting agent at 90°C, however, revealed the prior-aus-tenite grain boundaries: (c) bright-eld illumination and (d) dark-eld illumination.

a) b)

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Barraclough used swabbing and lightlyback-polished his specimens to reduce theetch details of the martensite within thegrains, which is now an excellent common

practice.Brownrigg et al. [12] followed up on this

study with a slight modication that theystated allowed them to bring out P γ GBs foras-quenched steels from 0.03-0.8% C withbainitic structures. They used a solution of100 mL saturated aqueous picric acid, plus2 mL “Teepol” plus 6 drops of HCl. Aftermixing, they ltered out the excess picricacid, which they stated reduces stainingof the specimen surface. They immersedspecimens at room temperature for 4-10

minutes. They demonstrated that P γ GBscould be revealed in low-carbon (0.04%),as-rolled bainitic structures that were notrecrystallized after hot rolling. Bodnar et al. [13] studied developmentof Pγ GBs in CrMoV rotor steels using 13different etchants. The saturated, aque-ous picric-acid etchant produced betterresults than most but was still inadequate.Tempering specimens in the embrittle-ment range did not help because the phos-phorous content was too low. Addition ofa small amount (3-5 drops per 50 mL ofetchant) of HCl to the etchant producedmarkedly better results. They etched for5-8 minutes with the beaker placed in anultrasonic cleaner for agitation (the waterlevel in the ultrasonic cleaner must belower than the etchant level in the beaker,

or the beaker will ip over). This was fol-lowed by light repolishing to remove some

of the etch detail within the grains. Otheretchants for revealing P γ GBs have beendeveloped; reference [4] lists 28 reagentspublished prior to 1984 for this purpose.

Experimental ProcedureBefore specimens can be etched, they mustbe properly prepared to a very high qual-ity level. The rst and most critical step issectioning, which must be conducted toinduce minimal damage. Use abrasive cut-off machines (avoid torch cutting, shear-ing, band saws or power hack saws as theyinduce far too much damage) with a blade/wheel designed for metallography and forsteels of the hardness level being prepared.

Generally, mounting is performed butmay not be necessary if the structure at theedges of the sample is not important (as in

a specimen cut from the interior of a part).Commence grinding with SiC paper, us-

ing as ne a grit size as possible. As a rule,start grinding with 120-grit SiC for steels≥60 HRC; start with 180-grit SiC for steelsbetween 35 and 60 HRC; and start with220- to 240-grit SiC for steels <35 HRC. Next, polish the specimens using at,low-resilience cloths, such as DP/MD-Planor DP/MD-Pan with 9-µm diamond, usinga load of 25-30 N per specimen, 150 rpm,for at least 5 minutes. Next, polish withDP/MD-DAC, DP/MD-DUR or DP/MD-SAT cloths with 3-µm diamond, same loadand rpm, for 5 minutes. For martensiticand bainitic steels that are to be etched toreveal prior-austenite grain boundaries, a1-µm step is not necessary. The nal stepwould be to use either a synthetic neo-prene cloth (such as DP/MD-Chem) or anapped or ocked cloth (such as DP/MD-

Fig. 6. Prior-austenite grain boundaries are not revealed in martensitic A-350 (LF3) alloysteel (Fe - 0.07%C - 0.74%Mn - 3.66%Ni - 0.2%Cr - 0.07%Mo (1350°F temper) using nital (a)but are revealed using aqueous saturated picric acid plus HCl and a wetting agent at 90°C -2 minutes (b, as etched).

Fig. 7. Prior-austenite grain boundaries re-vealed in fully martensitic Modied 4330Valloy steel with only 0.005%P (Fe - 0.29%C- 0.39%Mn - 3.54%Ni - 1.69%Cr - 0.54%Mo- 0.11%V (1110°F temper) using aqueous

saturated picric acid plus HCl and a wettingagent at 90°C for 60 seconds.

Fig. 8. Prior-austenite grain boundaries revealed in 8620 alloy steel using aqueous saturat-ed picric acid plus HCl and a wetting agent at 80-90°C, 60 seconds; specimen as-quenched(a) and after tempering at 400°F (b). Prior-austenite grain boundaries revealed in 8620

alloy steel using aqueous saturated picric acid plus HCl and a wetting agent at 80-90ºC, 60seconds; after an 800°F (c) and a 1200°F (d) temper.

a) b)

a) b)

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Floc or DP/MD-Nap) using either colloidalsilica (such as OP-S) or a neutral aluminasuspension (such as OP-AN). Polishing isusually conducted at 120-150 rpm, same

load, from 1-3 minutes. A good practice is to lightly etch thespecimens after the last step with a gen-eral-purpose reagent, such as 2% nital, tosee what the structure actually is and howwell prepared the specimens are beforeproceeding to use the saturated aqueouspicric-acid etch. After examination, re-peat the last step for at least 1 minute toremove this etch. Cleaning after each stepis important to prevent contamination ofthe next step and poor results.

The writer has been using saturatedaqueous picric acid plus a wetting agentand a small HCl addition (when steelshave more than about 1% Cr) for sometime but formerly at room temperature.The specimen would be placed polishedface vertical in a beaker with at least 100mL of the etchant in an ultrasonic cleaner(the water level in the ultrasonic cleanershould not be higher than the etchantlevel in the beaker or it will ip over).The timer would be set for 7 minutes withetching at room temperature. Results with 8620, 4140 and 5160 weredescribed above (without the HCl addi-tion). Light back-polishing was alwaysdone to try and improve the visibility ofthe grain boundaries. As sodium tride-cylbenzene sulfonate became difcult toobtain, the writer switched to the dodecylversion with no apparent difference.

Results

Several broken, heat-treated, 4340-alloy-steel nut inserts from the riser of an oilrig were examined. To determine if thecrack patterns were intergranular, speci-mens were etched in the saturated aque-ous picric-acid ltered solution with HClusing Nacconol 90G as the wetting agent(Nacconal is a registered trademark of theStepan Company of Northeld, Ill.). Thisis described as sodium alkyl benzene sul-fonate. On the MSDS sheet, the compo-sition is given as 90-93% sodium dodecyl-

benzen sulfonate, 5% sodium sulfate, 1%sodium chloride and 1.5% water. HCl was

added in the amount of 6 drops per 100 mLof the saturated aqueous picric-acid solu-tion (1-500 mL). After this was mixed, theexcess picric acid was removed by ltering.

Etching was conducted at room tem-perature for 7 minutes using the ultrasoniccleaner for agitation. However, the resultswere marginal. Hence, the writer heatedthe solution to 80-90°C (below the boilingpoint to retard evaporation). Specimenswere swab etched for 2 minutes and thenback-polished. Results were exceptionallygood, as shown in Figure 4, revealing analmost fully intergranular crack path fromstress-corrosion cracking (bulk hardnesswas well above the safe limit for high-strength steel in salt water). In a study of SAE 723 Grade-3 Class-3pressure-vessel steel forgings, it was nec-essary to reveal the prior-austenite grainstructure. A previous investigator hadetched the specimens with 2% nital andclaimed it revealed a very coarse prior-austenite grain size. As nital will onlyreveal the lath martensite and give a con-

trast etch to the lath packets, the writerused the saturated aqueous picric-acid

etchant, plus HCl and Nacconol 90G asthe wetting agent. Figure 5a shows an ex-ample of the structure etched with nital,revealing a coarse lath packet size. First,the saturated picric-acid etch was used atroom temperature, but the results (Figure5b) were inadequate. The specimen wasre-polished and etched at 90°C, and theresults were much better (Figure 5c) asthere was much less structure etching.Dark-eld illumination may be used ef-fectively to reveal the boundaries withstrong contrast (Figure 5d). The value of back-polishing after etch-ing is illustrated in Figure 6, which showsa 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 carboncontent, which usually makes the task im-possible. Also, the phosphorous contentwas only 0.008%, and it was tempered at1350°F. These three factors, very low carbon andphosphorus and very high tempering tem-perature, would normally make it impos-

sible to reveal the P γ GBs. But, as shown inFigure 6, it was possible with the etchant

Fig. 9. Prior-austenite grain boundaries revealed in fully martensitic 4140 alloy steel usingaqueous saturated picric acid plus HCl and a wetting agent at 80-90°C; specimen in theas-quenched condition (a) and after a 400°F (b) temper. Prior-austenite grain boundariesrevealed in fully martensitic 4140 alloy steel using aqueous saturated picric acid plus HCland a wetting agent at 80-90°C; after an 800°F (c) and a 1200°F (d) temper.

a)

c)

b)

d)

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heated. This gure also demonstrates thebenet of careful light back-polishing afteretching to remove extraneous etch detailwithin the grains and make the boundar-

ies more visible. A specimen from a forg-ing of modied 4330V with only 0.005%phosphorus that was tempered at 1110°Fwas also successfully etched (Fig. 7). Withthis low P content and high temperingtemperature, other etchant variationswould not reveal the P γ GBs. For this study, the writer re-prepared thepreviously mentioned 8620 and 4140 spec-imens that were in the as-quenched andquenched-and-tempered (400, 800 and1200°F) conditions. They had been etched

with the saturated aqueous picric-acid re-agent without the HCl addition at roomtemperature. This time, HCl was addedand etching was done at 80-90°C for 60-120 seconds. Figure 8 shows the four fullymartensitic 8620-alloy-steel specimens inthe as-quenched and quenched-and-tem-pered conditions, while Figure 9 shows thefour fully martensitic 4140-alloy-steelspecimens in the same heat-treated condi-tions. In all cases, the prior-austenite grainboundaries are visible. Four specimens of 4340-alloy steel wereisothermally transformed to lower bainiteand then tempered at 300, 500, 700 and900°F. They were etched with the heatedsaturated aqueous picric-acid reagent con-taining HCl and Nacconol 90G wettingagent. Figure 10 shows the specimens tem-

pered at 300 and 500°F that were carefully

back-polished after etching. Results weresimilar for the two tempered at 700 and900°F.

ConclusionsRevealing prior-austenite grain boundarieshas been one of the most difcult and frus-trating tasks assigned to the metallography/materialography laboratory. The most suc-cessful etch had been saturated aqueouspicric acid containing a wetting agent, usu-ally sodium dodecylbenzene sulfonate, atroom temperature for periods of 4-20 min-utes. However, this etch was unable to re-veal P γ GBs in martensitic or bainitic steelswith carbon contents below ~0.3% or withphosphorus contents below ~0.010%, evenwhen subjected to step-embrittlement cy-cles 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 onone specimen), these limitations are over-come. Filtering the solution before usedoes help reduce staining/pitting attack.Careful, low-pressure back-polishing ona stationary cloth using OP-AN aluminaslurry is very effective at reducing extra-neous etch detail within the grains andenhancing grain-boundary visibility.

AcknowledgementThe writer is grateful to Michael He andthe staff at Scot Forge (Spring Grove, Ill.),for the use of their equipment and for sup-plying some of the forged specimens testedin this program. IH

References (available online)

Figure 10: Prior-austenite grain boundaries in 4340 alloy steel isothermally transformed tolower bainite using aqueous saturated picric acid plus HCl and a wetting agent after tem-pering at 300°F (a) and 500°F (b). These specimens were back polished after etching.

a) b)

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