IMAGING PHASES IN STEELS

Transcription

1 Hints for IMAGING PHASES IN STEELS Some techniques lerned from experience cn help metllogrphers identify certin phses in steels. G. F. Vnder Voort* Buehler Ltd., Lke Bluff, Illinois E. P. Mnilov** Polzunov Centrl Boiler nd Turine Institute, St. Petersurg, Russi Fig. 1 Low-cron sheet steels contin crides (cementite) in ferritic mtrix. Etching with ) 2% nitl rings up the ferrite grin oundries s well s the cementite. To est see the mount, shpe, nd distriution of the cementite, etch with ) 4% picrl. The microstructure of metls nd lloys cn e quite complex, prticulrly in certin lloy systems such s iron-sed lloys. A good understnding of physicl metllurgy, the role of composition nd processing on structure formtion nd modifiction, nd n ide of the pproximte expected hrdness of different phses or constituents will help nrrow the rnge of possile phses tht might e present. However, this knowledge is sometimes not enough to enle even skilled metllogrphers to revel nd identify importnt phses. For exmple, the polishing response of certin constituents is n indictor of the identity of vrious phses. Illumintion techniques other thn the stndrd right field illumintion cn lso e quite helpful. Finlly, selective etchnts offer nother very useful tool. These procedures, lthough not especilly rcne or difficult, re not well known nd re not commonly tught. This rticle discusses methods lerned from experience, nd presents numer of hints nd tricks to mke the metllogrpher s jo esier nd more precise. It covers wys to improve the visiility nd hence the identifiction of so-clled white-etching phses in steels (ferrite nd delt ferrite, cementite, nd retined ustenite) etched with stndrd regents where the mtrix cn vry considerly. Etching is criticl Metllogrphers generlly rely upon the light * Fellow of ASM Interntionl **Memer of ASM Interntionl microscope to chrcterize microstructures, nd success depends on correct specimen preprtion. However, in mny cses, preprtion involves more thn sectioning, mounting, grinding, nd polishing. Unless the specimen is properly etched, ll the prep time is for nught. In other words, the key to seeing the true microstructure is to choose the etchnt tht est revels the phses or constituents in the mteril. Becuse the phses depend on composition nd processing conditions, knowing wht phses to look for is not simple tsk. Furthermore, the numer nd vriety of etchnts ville grows every dy, nd the pplicility of the etchnts my not e well documented. The tle shows the composition of the etchnts discussed here. In well-chrcterized lloy systems, the metllogrpher cn exmine the microstructure nd compre it to pulished imges to identify phses nd constituents. However, in less well-studied systems, phse identifiction is more chllenging. Etching for ferrite Ferrite is very low hrdness ( soft ) phse in steels. Low-cron sheet steels re nerly 100% ferrite. Becuse ferrite is soft, polishing scrtches cn e difficult to remove. Identifiction cn e chllenging ecuse vrious etchnts hve different effects on ferrite. For exmple: Nitl nd picrl: Nitl ttcks ferrite t rte tht vries with the crystl orienttion of ech grin reltive to the plne of polish, while picrl is insensitive to crystl orienttion. Consequently, nitl revels the ferrite grin oundries while picrl does not. However, ecuse nitl is orienttion- 32 ADVANCED MATERIALS & PROCESSES/FEBRUARY 2005

2 Etchnts descried in the text Numer Composition Comments ml ethnol Nitl, the most common etchnt for steels. Do not stock nitl with >3% nitric cid in ethnol ml HNO 3 Use y immersion or swing (light pressure) ml ethnol Picrl, etter thn nitl for nneled microstructures. Does not revel ferrite grin oundries. 4 g picric cid Etch y immersion or swing ml ethnol Vilell s regent, good for higher lloyed steels, tool steels, nd mrtensitic stinless steels. 5 ml HCl Etch y immersion or swing. 1 g picric cid 4 85 ml ethnol Etch for duplex stinless steels developed y Crpenter Technology. Immerse specimens 15 ml HCl min (time is not criticl) to revel the grin nd phse oundries in duplex stinless steels ml stock solution* Klemm s I tint etch. It colors ferrite strongly; lso colors mrtensite nd inite, ut not 1 g K 2 S 2 O 5 crides or retined ustenite. Use y immersion only until the surfce is colored. * wter sturted with N 2 S 2 O ml wter Alkline sodium picrte, used ºC y immersion only. Colors cementite (Fe 3 C) 25 g NOH nd M 6 C crides. 2 g picric cid ml wter Electrolytic etch for stinless steels. Use t 3 V dc, 10 s to color ferrite (usully tn or light lue) 20 g NOH nd sigm (ornge), ut not ustenite. Mix fresh ml wter Murkmi s regent. Used to color ferrite nd sigm ( ºC for up to 3 min) in 10 g NOH (or KOH) stinless steels. At room temperture, it will not color ferrite ut will color certin crides. 10 g K 3 Fe(CN) 6 At high temperture it colors ferrite, sigm, nd crides, ut not ustenite ml wter Berh s sulfmic cid regent no. 4. Colors phses in highly lloyed tool steels nd 3 g K 2 S 2 O 5 mrtensitic stinless steels. Use y immersion only, s, usully. Mix fresh. 2 g sulfmic cid Best to use plstic eker nd plstic tongs g NH 4 F HF ml wter Berh-type etch for duplex stinless steels (similr to BI). Mix fresh. Use y immersion 15 ml HCl until the surfce is colored. Colors ferrite ut not ustenite. 1 g K 2 S 2 O 5 sensitive, not ll of the grin edges re visile. If cementite is present, nd this is quite common in sheet steels, it cn e hrd to see using nitl, s mny prticles will e in the grin oundries. However, picrl does not revel the ferrite grin oundries, mking it esy to oserve cementite prticles. Neither etch drkens either ferrite or cementite, nd oth pper white in right field (BF) illumintion. Figure 1 shows the microstructure of sheet steel etched with nitl nd with picrl to illustrte this difference in visiility. Thus, if you wnt to see where the crides re locted, use picrl. The eye nd rin cn see the cementite well, s nothing else is visile, other thn inclusions nd possily nitrides. Neither etchnt provides positive identifiction of either the cementite prticles or the ferrite mtrix. However, severl etchnts do preferentilly color ferrite nd cementite. Klemm s I colors ferrite strongly. It lso colors mrtensite nd inite, ut not cementite or retined ustenite. Figure 2 shows low-cron steel weld etched with 2% nitl nd with Klemm s I. Note the vivid nd distinct rendering of the ferrite grins compred to nitl, long with positive identifiction. Sometimes ferrite is oserved in the mrtensitic mtrix of het-treted lloy steels, either due to under-ustenitiztion or lck of hrdenility (ferrite cn lso e seen t decrurized surfce). Figure 3 shows the microstructure of under- Fig. 2 This cron steel weld ws etched with ) 2% nitl nd with ) Klemm s I in order to study the grin structure of the weldments. The weld metl is t the left nd the se metl is t the right. Note the very cler demrction from the fine-grined se metl to the columnr structure. The Ac 1 ws reched t the interfce etween the fine-grined se nd the columnr zone of the het-ffected zone. The Ac 3 ws reched t the left end of the columnr zone, which is less distinct oundry. The fusion line etween the weld nd the het-ffected zone is clerly visile with either etchnt, ut is certinly more vivid in color. ADVANCED MATERIALS & PROCESSES/FEBRUARY

3 Fig. 3 Microstructure of under-ustenitized 4140 lloy steel fter quenching nd tempering reveling white-etching phse in mrtensitic mtrix. Etching with ) 2% nitl revels the white constituent (un-etched) while etching with ) Klemm s I tint etch revels tht the white phse is colored nd is ferrite. Mgnifiction r is 10 µm long. Fig. 4 Microstructure from turine disk mde of EP428 mrtensitic stinless steel (similr to type 422) fter 140,000 h of service t out 350 ºC reveling delt ferrite nd crides when etched with ) Vilell s regent nd with ) Murkmi s regent (90 ºC for 60 s). Murkmi s reveled the prolem re etter (the crides were not esily oserved with Vilell s) nd lso gve positive identifiction. (Mgnifiction rs re 10 µm long). Fig. 5 Delt ferrite in 312 stinless steel weld metl cn e seen y etching with ) Vilell s regent. However, it is reveled more clerly, nd proven to e delt ferrite, y selective etching with ) 20% NOH, 3 V dc, 10 s (mgnifiction r is 20 µm). ustenitized 4140 lloy steel where ferrite is present in mrtensitic mtrix. Etching with nitl revels the structure, ut re the white-etching grins ferrite, retined ustenite, or possily cementite? Etching with Klemm s I revels coloring of these grins nd proves tht they re indeed ferrite. Finding delt ferrite Mrtensitic nd precipittion-hrdenle stinless steels, s well s high-chromium tool steels, cn exhiit the high-temperture form of ferrite known s delt ferrite. Ferrite my lso e seen in len ustenitic stinless steels nd is deliertely stilized t room temperture in duplex stinless steels. Weld compositions in ustenitic stinless steels re deliertely djusted to precipitte t lest 5% ferrite to prevent hot crcking. As result, mny metllogrphers encounter delt ferrite in their work. However, in some lloys it my e difficult to differentite delt ferrite from the other phses present. Severl etchnts color delt ferrite under specific conditions. Electrolytic queous 20% NOH (3 V dc, 10 s) colors delt ferrite very well. Murkmi s regent nd severl of its modifictions color delt ferrite (nd certin crides) when used etween 80 nd 100ºC, ut not t room temperture. Figure 4 shows the microstructure of Russin mrtensitic stinless steel (similr to type 422). It is used in lnd-sed gs turine disks nd ldes for electric power genertion in Russi. The specimen hd 140,000 hours of service t nominl temperture of out 350ºC (660 F). Etching with Vilell s regent, stndrd procedure for mrtensitic stinless steels, reveled wht looked like ptches of delt ferrite. However, the specimen ws re-polished nd etched with Murkmi s regent (90ºC, 60 s), which colored the delt nd more clerly reveled crides tht precipitted during service t the delt ferrite-mrtensite interfce. These crides were not esily oserved with Vilell s regent. Figure 5 shows 312 stinless steel weld (se metl not shown) tht contins ferrite nd ustenite. Vilell s regent revels the ferrite, lthough not s crisply s we d like. On the other hnd, etching with oiling Murkmi s regent produces etter imge of the delt ferrite nd gives positive identifiction. Figure 6 shows the microstructure of plte of 7 Mo Plus duplex stinless steel etched with 15% HCl in ethnol (immersion for 15 minutes), which clerly revels the grin nd phse oundries. But which res re ustenite nd which re ferrite? Severl etchnts cn help nswer tht question, including the previously mentioned electrolytic NOH nd Murkmi s. Figure 6 shows Berh-type etchnt, similr to his BI regent, tht colors ferrite ut not ustenite. In ddition, certin tint etches color the ustenite preferentilly. See the rticle on Color Metllogrphy in the new edition of the ASM Hndook, Volume 9, Metllogrphy nd Microstructures, for more detils. Seeing cementite Sometimes given lloy contins smll mount of white-etching phse, nd this cn e quite common in cron nd lloy steels with >0.5% cron. However, this cn e confusing, ecuse the phse could e ferrite, undissolved cementite, or possily retined ustenite if it ws hrdened. Cementite, or Fe 3 C, is cride nd is much hrder thn ferrite. The hrdness of cementite cn vry etween out 800 nd 1500 HV, with hrdness incresing s other elements such s mngnese nd chromium sustitute for iron. Compre this with the hrdness of ferrite, which my e s low s 100 HV, lthough it cn e strengthened sustntilly y elements such s silicon nd nickel. If the cron content nd het tretment detils 34 ADVANCED MATERIALS & PROCESSES/FEBRUARY 2005

4 re known, the metllogrpher cn ssess the likelihood tht the white phse is ferrite, cementite, or retined ustenite. If the lloy is hypereutectoid, the white phse could e cementite or retined ustenite, unless it is locted t decrurized surfce, where it proly is ferrite. Furthermore, ecuse of the hrdness difference, polishing scrtches re fr more likely to e present in ferrite thn in cementite. Figure 7 shows the microstructure of n s-hotrolled r of 1.31% cron wter-hrdening tool steel tht hs oth intergrnulr nd intrgrnulr cementite in very fine perlitic mtrix. Etching with either nitl or picrl is indequte to see the intergrnulr cementite, lthough the intrgrnulr cementite is more esily oserved. However, etching with lkline sodium picrte ner the oiling point drkens the cementite very well. This etch will lso color M 6 C cride, ut such cride is oserved only in lloys with sustntil molydenum nd tungsten, such s high speed steels. Retined ustenite is never oserved in such morphologies, ut only s smll, ngulr ptches etween mrtensite pltelets (or needles, lthough this term is inccurte). Figure 8 shows the microstructure of white cst iron, in which primry cementite is formed during the eutectic rection nd perlite is formed y the lower-temperture eutectoid rection. Etching with picrl revels the perlite nicely nd outlines the cementite. Etching with lkline sodium picrte, ner oiling, drkens the cementite, which is seen to e the continuous phse. Mny other etchnts will color, outline, or ttck vrious other cride types in lloy steels, ut spce prohiits complete listing of these etchnts nd their ehvior. Anlyzing ustenite Retined ustenite is nother white-etching phse tht cn e encountered in steels. However, it is only oserved in s-hot-rolled steels, scrurized steels, nd steels tht hve een het treted in n effort to form mrtensite. Furthermore, retined ustenite is only oserved with the light microscope when the cron content is high, generlly > 0.6%. Fig. 6 Microstructure of 7 Mo Plus duplex stinless steel in the hot-rolled nd nneled condition (longitudinl plne) etched with ) 15% HCl in ethnol, which revels the grin oundries, nd with ) Berh s regent (15 ml HCl, 85 ml wter, 1 g K 2 S 2 O 5 ) which colors the ferrite preferentilly. (Mgnifiction rs re 50 µm long). Fig. 8 The microstructure of white cst iron is commonly reveled y etching with nitl or with ) 4% picrl. This rings out the perlite etween the cementite tht forms during the eutectic rection. The cementite cn e reveled selectively y etching with ) oiling lkline sodium picrte. Note tht the cementite is continuous phse. High-lloy steels with cron contents down to 0.4% cn form smll mounts of retined ustenite, ut it cn e oserved only with drk field illumintion of thin foils with the TEM. Wter-hrdened tool steels, such s AISI W1 or W2, re spheroidize-nneled fter hot rolling, nd contin gloulr cementite in ferritic mtrix. The reltionship is well-known etween the cron dissolved in ustenite (efore quenching) nd the s-quenched hrdness. As the cron in the ustenite increses, the hrdness of squenched mrtensite increses. However, ove out 0.5% cron in ustenite, the s-quenched hrdness increses only slightly, nd if greter mounts of cron (from the crides eing dissolved) re put into solution, the hrdness will drop. This is cused y the reduction in the tem- c Fig. 7 This high cron (1.31%) wter-hrdening tool steel specimen is in the s-rolled condition nd hs n lmost complete grin oundry network of cementite, s well s some intrgrnulr Widmnstätten cementite. Etching with ) 2% nitl or ) 4% picrl is reltively ineffective in reveling the hrmful intergrnulr cementite, ut is cceptle for the intrgrnulr cementite. However, etching with c) oiling lkline sodium picrte clerly nd vividly revels the intergrnulr nd intrgrnulr cementite s it colors cementite rown. The fine cementite in the perlite is lso colored ut, ecuse it is very thin, it is not s visile ( nd t 1000X, c t 500X mgnifiction). ADVANCED MATERIALS & PROCESSES/FEBRUARY

5 c Fig. 9 Microstructure of the surfce of crurized specimens of 8720 lloy steel where the retined ustenite t the surfce (to depth of out 1 mm) ws nlyzed y X-ry diffrction. These specimens were etched with 2% nitl nd the imge nlysis mesurements of the retined ustenite contents were much lower, () 25.4% retined ustenite y X- ry diffrction, () 19.7% retined ustenite y X-ry diffrction, (c) 16.2% retined ustenite y X-ry diffrction. pertures t which mrtensite strts to form (M s ) nd the temperture t which formtion is complete (M f ). High-tech tools for phse identifiction X-rys, trnsmission electron microscopy, energy-dispersive spectroscopy, electron-energy loss spectroscopy, nd electron ckscttered diffrction re the high-tech (nd expensive) tools for phse identifiction. X-rys cn help with phse identifiction in new lloys, or unfmilir lloy systems. X-ry diffrction (XRD) cn e pplied directly to ulk specimens. Or, the mtrix cn e dissolved nd XRD cn e pplied to the second-phse precipittes. Trnsmission electron microscopy (TEM) is effective for nlyzing either thin foils or extrction replics. After TEM, the compound my e identified y electron diffrction, perhps ided y energy-dispersive spectroscopy (EDS) nd/or electron energy loss spectroscopy (EELS) for elementl detection nd quntifiction. Recently, ckscttered electron diffrction (EBSD) hs een introduced s scnning electron microscope (SEM) lterntive diffrction procedure for phse identifiction. These methods involve very expensive devices nd highly qulified personnel to properly operte them, otin good dt, nd interpret the dt. However, only universities nd lrge reserch orgniztions cn fford to purchse nd operte these devices. Most metllogrphers do not hve such equipment t their disposl, or they hve very limited ccess to one or more of these devices. Although cron hs the strongest influence on these tempertures, dissolving greter mounts of lloying elements in ustenite will lso suppress these tempertures. If the M f is elow room temperture, ustenite will e retined fter quenching, unless quenching is continued to tempertures elow mient. Excessive mounts of unstle retined ustenite re considered to e very detrimentl to the performnce of tool steels, ut sustntil mounts re eneficil to the performnce of crurized gers. This difference rises from the nture nd mgnitude of the stresses involved in these pplictions, nd the toughness or rittleness of the overll component. Retined ustenite cnnot e oserved with the light microscope when there is less thn out 10% present. Very low levels cn e oserved under TEM exmintion, ut drk field illumintion is required. X-ry diffrction cn detect mounts s low s 0.5%, nd perhps even less under idel conditions, lthough 2% is often listed s the minimum detectle mount. When mounts re >15%, retined ustenite cn e seen in the light microscope, ut imge nlysis mesurements of retined ustenite will e sustntilly lower thn the true level determined y XRD. Figure 9 shows light microscope BF imges of crurized specimens of 8720 lloy steel tht were studied first y XRD to determine the retined ustenite content, which turned out to e 25.4, 19.7, nd 16.2%. Imge nlysis results re signif- c Fig. 10 Chnge in microstructure of 440C mrtensitic stinless steel with progressively higher over-ustenitizing tempertures showing in () mostly mrtensite with some retined ustenite; more retined ustenite in () nd lmost ll ustenite in (c). The white prticles re crides, M 7 C 3 type (Berh sulfmic cid no. 4 etch). ()1150 ºC, Oil Quench (mgnifiction r is 10 µm long); () 1204 ºC, Oil Quench (mgnifiction r is 10 µm long); (c) 1260 ºC, Oil Quench (mgnifiction r is 50 µm long). 36 ADVANCED MATERIALS & PROCESSES/FEBRUARY 2005

6 icntly lower thn the true vlues, nd 2% nitl gve the poorest results. Addition of wetting gent, enzlkonium chloride, to nitl gve much etter results, ut the mesurements (13.3, 8.5, nd 1.2%, respectively) re more thn 10% elow the XRD vlues. This is due to the fineness of the retined ustenite when in low concentrtions. Unfortuntely, no simple mens hs een developed to preferentilly color retined ustenite, prticulrly when in smll mounts in lloy steels. If retined ustenite is in lrger mounts, or is the mjor phse, it cn e colored y complex tint etch, s shown in Fig. 10. Here, 440C mrtensitic stinless steel ws over-ustenitized t progressively higher tempertures, creting nerly fully ustenitic structure t the highest temperture (note the nneling twins). Berh s sulfmic cid regent (No. 4) colored oth mrtensite nd retined ustenite, ut not the M 7 C 3 crides. The new ASM Hndook, Volume 9, Metllogrphy nd Microstructures, is ville this month. This uthorittive work replces our 1985 edition with updted, new coverge on metllogrphic techniques nd improved emphsis on the interprettion of microstructurl imges. View the contents pge nd smple rticle t To order, cll Customer Service t 440/ Cost is $152 for memers, $189 for nonmemers. The list goes on These exmples hve een chosen to help the metllogrpher etter revel nd identify white-etching phses in steels ferrite, cementite, nd retined ustenite. Specimens must e properly prepred if they re to revel the true structure. However, selection of the est etchnt to clerly identify the constituents is not simple. In mny cses, the stndrd nitl etchnt is dequte to revel the structure, ut other etchnts cn lmost lwys e found tht re fr etter. Mny etchnts produce selective ttck or coloring of specific phses, nd these etchnts re extremely vlule s n id to phse identifiction. Unfortuntely, only limited numer of exmples cn e shown here. The ASM Hndook, Volume 9, Metllogrphy nd Microstructures, which is now ville, is n excellent source for this type of informtion. For more informtion: George Vnder Voort is Director of Reserch & Technology, Buehler Ltd., 41 Wukegn Rod, Lke Bluff, IL ; tel: 847/ ; fx: 847/ ; e-mil: george. vndervoort@uehler.com; We site: ADVANCED MATERIALS & PROCESSES/FEBRUARY 2005 Circle 12 or visit 37