ADR Journals 2014. All Rights Reserved.Development of a New Etchant for Austenitic Manganese Steel Shankha Pathak*, Abhimanyu Maharshi** Abstract Austenitegrainsizeisanimportantparameterfortheductilityofsteel.The ductility of Austenitic Manganese Steel is directly dependent on the grain size. Therefore, it is most important to measure the grain size and to investigate the microstructureofthesteeltopredictthemechanicalpropertiesofthesteel. Torevealtheprioraustenitegrainboundariescommonlyusedetchantsare picricacidand2%Nital.Themaindrawbacksoftheseetchantsarethe lightlyetchedgrainboundariesanduncertaintytorevealthetwins.Oneof themainobjectivesofthe present work wasto find outaviableetchantthat can efficiently reveal the grain boundaries along with the twins. The results of the presented etchant examinations serve as an important basis for revealing a clear picture of the austenitic grain boundaries. Keywords: High Manganese Austenitic Steel, Grain boundary, etchant, twins. Introduction Use of Microstructure Evolution Thetermmicrostructureisusedtodescribethe appearanceofthematerialonthenm-cmlength scale.It isdefinedasthestructureofaprepared surfaceorthinfoilofmaterialasrevealedbya microscope above 25 magnification.Themicrostructureofamaterialcanstrongly influencephysicalpropertiessuchasstrength, toughness,ductility,hardness,corrosion resistance,high/lowtemperaturebehavior,wear resistancewhichgoverntheapplicationofthese materialsinindustrialpractice.The microstructuresofnaturalmineralsprovide informationabouttheircomplexgeological history.Microstructuresofamaterialcanbe tailoredtogiveitspecificpropertiesanditis becauseoftheseabilitiesthatmicrostructureisa fundamentalpartofallmaterialsandminerals science.Hence,itisclearthatsincemicrostructures facilitatethedeterminationofthephysical properties of a material, so, its determination is of greatimportanceasfarasthehistoryandthe properties of the material is concerned. High Manganese Austenitic Steel Theoriginalausteniticmanganesesteel, containingabout1.2%Cand12%Mn,was inventedbySirRobertHadfieldin1882andwas firstproducedbytheUSin1892.Thebasic conditionforthechemicalcompositionof Austenite Manganese Steel is the ratio Mn/C=>10. Theupperborderlineisnormally15%Mn,butin practice this ratio can be increased to 20 % mostly in thick walled castings. The main reason for using highcontentofMnistoimprovethe hardenability.Thesteelwasuniquesinceit exhibitedhightoughness,highductility,high workhardeningabilityandexcellentwear resistance.Becauseofitsproperties,itrapidly gainedacceptanceasaveryusefulengineering material.Hadfield`sausteniticmanganesesteelis still used extensively, with minor modifications in compositionandheattreatment.Austeniticsteels withhighermanganesecontents(>15%)have recently been developed for applications requiring lowmagneticpermeability,lowtemperature (cryogenic)strengthandlowtemperature toughness.Itisavailableunderthetradename Manganalasplates,hot-rolledbarsandspecial shapes.Ataround10%manganesecontent,the steelretainsinitsaustenite formatroom * Deputy Manager, Texmaco Rail & Engg. Ltd. **BITS Pilani, K.K. Birla Goa Campus Correspondence to: Mr. Shankha Pathak, Deputy Manager, Texmaco Rail &Engg. Ltd. E- mail: dgpkid@yahoo.com Pathak S et al. 2 ISSN: 2393-8315 J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9. temperature if cooled correctly. Both hardness and ductilityreachtheirhighestpointsaround12% manganesedependingonotheralloyingelements in steel. Properties Themechanicalpropertiesofaustenitic manganesesteelvarywithbothcarbonand manganesecontent.Ascarbonisincreased,it becomesincreasinglydifficulttoretainallofthe carboninsolidsolution,whichmayaccountfor reductionsintensilestrengthandductility. Manganalthrivesonseverewearconditions.The surfacehardnessincreaseswithmoreimpactand hammering.Thesteelcanachieveuptothree timesitssurfacehardness duringconditionsof impactwithoutanyincreaseinbrittleness, which isusuallyassociatedwithhardness,whichallows the steel to retain its toughness. This characteristic, knownaswork-hardening,plusthefactthatit remainsductileunderneath,makesitthemost effectivesteelincombatingimpactandabrasion. It has very high strength, ductility, toughness, and excellentwearresistanceintheroughest applications.Ithasalowcoefficientoffriction whichisveryimportanttowearresistancewhich isconsideredtobeanessentialcharacteristicin metal-to-metalapplications.Itisnon-magnetic substantially.Continuoushightemperaturecan embrittleit.Itcanbeflamecutbyoxy-acetylene torch, cut by abrasive cut- off machine and electric welded. The high hardness of face layers increases theabrasivewearresistancebutsincethemiddle partkeepsgoodtoughness,thesteelcomponent supportshighimpactstress.Itisapparently immunetohydrogenembrittlement.Thereisa gradualdecreaseinimpactstrengthwith decreasingtemperature.Resistancetocrack propagationishighandisassociatedwithvery sluggish progressive failures. Because of this, any fatigue cracks that develop might be detected, and theaffectedpartorpartsremovedfromservice before complete failure occurs. Uses AusteniticManganeseSteelisprimarilyusedin thefieldsofearthmoving,mining,quarrying,oil welldrilling,steelmaking,railroadingandinthe manufacture of cement and clay products. It is also used in equipments for handling and processing in a low magnetic permeability, low temperature and lowtemperaturetoughnessconditions.Other applicationsareinthefieldofdevelopmentof superconductingtechnologiesusedin transportationsystems,nuclearfusionresearch andstructuralmaterialstostoreandtransport liquefiedgases.Mnalloyhasbeenusedinthe cementmixers, rockcrushers,railwayswitches and crossings, crawler treads for tractors and other highimpactandabrasiveenvironments.Itisalso usedinhighimpactenvironmentslikeinsidea shotpeeningmachine,andearthenmaterials. Otherapplicationsincludefragmentizerhammers andgratesforautomobilerecyclingandmilitary applications such as tank track pads ColorofDifferentPhasesasunder Optical Microscope Inopticalmicroscope,differentphases/ constituents are identified by the color and texture.Asthedifferentphasesofsteelwithspecific crystalstructurecorrodeatdifferentrates,the amount of light reflected from the polished etched surfaceisdifferentandtheangleofreflection from different phases is different too. This, in turn, createsdifferentcolorsandtexture,andtherefore thecolorandtextureofdifferentphasesare different.Typicalcolors/textureofdifferent phasesrevealedby2%Nitalhasbeentabulated below: Table 1.Color and Texture of Different Phases after Etching with 2% Nital Phase / CompoundColor / Texture FerriteWhite AusteniteWhite / Yellow PearliteBlack , Lamellar MartensiteBlack and white , Acicular CementiteWhite 3 Pathak S et al. J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9.ISSN: 2393-8315 Microstructure Preparation Process Sawing the section to be examined Mounting in resins (if sample is too small) Coarse grinding Check Point: Grinding on progressively finer emery paper in perpendicular to the previous direction of polishing Check Point: Polishing using alumina powder or diamond paste on rotating wheel Etching in suitable etchant reagent Washing in alcohol and drying Theentiremicrostructurepreparationprocesscan be broadly divided into three categories: Rough preparation. Polishing. Etching. Rough Preparation Ifthesampleisnotflat,itisnecessarytoremove somematerialonthelatheorgrindingmachine first. After it, the sample should be moved forward and backward on the paper until the whole surface iscoveredwithunidirectionalscratches.Itisthen washedwithrunningwatertoremovedebris associatedwiththegrade ofpaperused.Itisthen groundonthenextfinerpapersuchthatthe scratchesproducedareatrightanglestothose formed by the previous paper. This helps to easily decidewhenthescratchesfromthecoarserpaper havebeencompletelyremoved.Thisprocedureis repeatedthrougharangeofpapers.Whenthe specimen has been ground on the final paper, it is generallyworthwhilerotatingitthroughand grinding again with less pressure than before. This techniquecandecreasethetimerequiredforthe next stage, which is polishing. Polishing Due to the very small depth of field obtained from anopticalmicroscope,itisessentialthatthe surfaceisopticallyflat,actingasaperfectmirror whichiswhythespecimenhastobepolished. This is done using rotatingwheels covered with a clothimpregnatedwithaveryfineabrasive compound.Thecommoncompoundsusedare diamond and alumina. Etching Afterexaminingthespecimeninaspolished conditionfortheoptimumsurfacequality,the specimenshouldbeetchedwithsuitableetchants todevelopadditionalcontrasttorevealthe microstructure. Theory of Etching Metallographicetchingistheuseofchemical solutionsknownasetchantstocreatecontrast between the elements of the metal's microstructure byselectivelycorrodingsomeofthoseelements, which show up as darker regions, used to highlight featuresofmetalsatmicroscopiclevels.Thisis possiblebecausedifferencesinthecomposition, structureorphaseofametalwillcreate electrochemicalpotentialsthataltertherelative rates of corrosion when exposed to an etchant. Thefeatureshighlightedbyetchingincludegrain boundaries, phase differences and inclusions.Exposegrainboundaries:Theboundariesof metallic grains can corrode very quickly when the appropriate etchant is used, highlighting the edges ofthegrainsandmakingitpossibletodetermine their size and shape. Highlightmetallicphases:Differentphaseswill havedifferentshadingandtextureafteretching, allowingforphasefractionestimationand identification of undesirable phases.Identifyinclusions:Nonmetallicinclusionsoften havecharacteristicshapesandcolorationsafter appropriateetching,whichallowsforquicksize measurement and number density estimation. Bystudyingthecharacter,quantityand distributionofthesedifferentfeatures, metallurgistsarecapableofpredictingand explainingphysicalpropertiesandperformance failures of a given sample of metal. Threemainetchingprocessesusedin metallographic sample preparation are: Chemical etching Wet etching Dry etching Electrolytic etching Heat tinting Pathak S et al. 4 ISSN: 2393-8315 J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9. Chemical Etching Thistypicallyinvolvesimmersingthesamplein anetchantorswabbingthesurfacewithan etchant.Theetchantselectivelycorrodes microstructuralfeatures.Immersiontimeor etchingtimeishighlydependentonthesystem. Theselectionoftheoptimumetchantisalsovery important in sample production. Deeper etches are preferredforlowmagnificationexaminations, whileshallowetchesarepreferredforhigher magnificationetches.Basedonthereagentsused andthetechniquesused,chemicaletchingis divided into wet etching and dry etching. Wet Etching Wetetchingistheuseofchemicalsolutionto dissolve a material when immersed in it. A wet etchingprocessinvolvesmultiplechemical reactionsthatconsumetheoriginalreactants andproducenewreactants.Thewetetch processcanbedescribedbythreebasicsteps. (1)Diffusionoftheliquidetchanttothe structure that is to be removed. (2) The reaction betweentheliquidetchantandthematerial beingetchedaway.Areduction-oxidation (redox)reactionusuallyoccurs.Thisreaction entailstheoxidationofthematerialthen dissolvingtheoxidizedmaterial.(3)Diffusion ofthebyproductsinthereactionfromthe reactedsurface.Thisisasimpletechnology whichrequiresonlyacontainerwithaliquid solutionthatwilldissolvethematerialin question.Itwillgivegoodresultsifa combinationofetchantandmaskmaterialcan befoundtosuitourapplication.But unfortunately,therearecomplicationssince usuallyasuitablemaskthatdoesnotdissolve or at least etches much slower than the material tobepatterned,needstobefoundout.Wet etchingworksverywellforetchingthinfilms onsubstrates,andcanalsobeusedtoetchthe substrateitself.Theproblemwithsubstrate etchingisthatisotropicprocesseswillcause undercuttingofthemasklayerbythesame distanceastheetchdepth.Anisotropic processesallowtheetchingtostoponcertain crystalplanesinthesubstrate,butstillresults in a loss of space, since these planes cannot be verticaltothesurfacewhenetchingholesor cavities.Togetridofthislimitation,dry etching must be considered.Dry Etching Indryetching,plasmasoretchantgases remove the substrate material. The reaction that takesplacecanbedoneutilizinghighkinetic energyofparticlebeams,chemicalreactionor a combination of both. Based on this, there are three main types of dry etching: I.Physical Dry EtchingPhysicaldryetchingrequireshighenergy kinetic energy (ion, electron, or photon) beams toetchoffthesubstrateatoms.Whenthehigh energyparticlesknockouttheatomsfromthe substratesurface,thematerialevaporatesafter leavingthesubstrate.Thereisnochemical reactiontakingplaceandthereforeonlythe material that is unmasked will be removed. II.Chemical Dry Etching Chemicaldryetchingorvaporphaseetching does not use liquid chemicals or etchants. This processinvolvesachemicalreactionbetween etchantgasestoattackthesubstratesurface. Thechemicaldryetchingprocessisusually isotropicandexhibitshighsensitivity. Anisotropicdryetchinghastheabilitytoetch withfinerresolutionandhigheraspectratio thanisotropicetching.Duetothedirectional natureofdryetching,undercuttingcanbe avoided. III.Reactive Ion Etching Reactiveionetching(RIE)usesbothphysical andchemicalmechanismstoachievehigh levelsofresolution.Theprocessisoneofthe mostdiverseandmostwidelyusedinindustry andresearch.Sincetheprocesscombinesboth physical and chemical interactions, the process ismuchfaster.Thehighenergycollisionfrom theionizationhelpstodissociatetheetchant moleculesintomorereactivespecies.Inthe RIE-process,cationsareproducedfrom reactivegaseswhichareacceleratedwithhigh energytothesubstrateandchemicallyreact with it. Electrolytic Etching Electrolytic etching involves the use of the anodic compartment as the specimen to be etched and the cathodiccompartmentasamaterialthatis conductivebuthighlyinsolubleintheelectrolyte used.Typicalexamplesofthematerialsthatare generallyusedasthecathodeareplatinumand graphite.Mostelectrolyticetchingprocessesuse directcurrentelectrolysis.Since,oxidationoccurs at the anode, therefore, specimen to be etched gets oxidized in the process. 5 Pathak S et al. J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9.ISSN: 2393-8315 Heat Tinting Heattinting,sometimescalledthermaletching,is theprocessofoxidizingasampleinafurnace. Duetodifferenceinoxidationratesofdifferent partsofthesamplesurface,variousstructuresare revealed. Scope of Present Innovation Torevealtheaustenitegrainboundaries, commonly used etchants are picric acid and 2% Nital.Austenitegrainsizeisanimportant parameterfortheductilityofsteel.The ductilityofAusteniticManganeseSteelis directly dependent on the grain size. Therefore, itismostimportanttomeasurethegrainsize andtoinvestigatethemicrostructureofthe steel to predict its mechanical properties.Themaindrawbacksoftheseetchantsarethe lightly etched grain boundaries and uncertainty to reveal the twins.One of the main objectives ofthepresentworkwastofindoutaviable etchantthatcanefficientlyrevealthegrain boundaries along with the twins. The results of thepresentedetchantexaminationsserveasan importantbasisforrevealingaclearpictureof the austenitic grain boundaries. Experimental Procedure & ResultsForcarryingoutthisexperiment,following equipmentsareused:opticalmicroscope(Carl Zeiss,Germany)with1000Xmagnification, polishingmachine,imageanalysissoftware. Samplewascollectedfromcast,heattreated highmanganeseausteniticrailwaycrossing. Thechemistryofthesteelisasfollows: %Carbon-1.15,%Manganese-12.45,%Silicon-0.38, %Sulphur-0.008, %Phosphorus-0.045. 1.Reagent:3gmAmmoniumpersulfate+1g silver nitrate+ 3-4 ml H3PO4 + 50ml H2O H3PO4 (P+5)isagoodoxidizingagent.Sinceits oxidizing character is less as compared to sulfuric acidorhydrochloricacid,sotheoxidizingtime willbehigh.Tofastenthereaction,catalystlike silvernitrateisused.H2Oactsasthewetting agent. Etching time: 10-15 sec. Figure 1.Magnification: 100XFigure 2.Magnification: 500X Lightly etched grain boundaries observed in fig.1 and fig.2 2.Reagent:Ammoniumpersulfate5gm+5ml HNO3+ 50ml H2O+Heat 50-70oC. Ammoniumpersulfateandnitricacidbothare verystrongoxidizingagents.Ammonium persulfateissolubleinnitricacidandactsasa stabilizerfornitricacidandalsoincreasesits oxidizing character. Since ammonium persulfate is mostsolubleinH2O,so,H2Oactsasthewetting agent.Heatisprovidedinordertofastentherate of the reaction. Etching time: 5-10 sec. Pathak S et al. 6 ISSN: 2393-8315 J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9.

Figure 3.Magnification: 100X Figure 4.Magnification: 500X Moderately etched grain boundaries without twins are observed in both the magnifications 3. Reagent: Ammonium persulfate 5gm + 1-2 gm KMnO4 + 50 ml H2O. KMnO4+Fe+2Mn+2 + Fe+3. Hence,wecanseethatKMnO4(Mn+7)isagood oxidizing agent. So, KMnO4 as well as ammonium persulfateactasoxidizingagents.H2Oactsasa wetting agent.Etching time: 5 sec.

Figure 5.Magnification: 100X Figure 6.Magnification: 500X Moderately etched grain boundaries without twins are observed in both the magnifications 4. Reagent: 5gm Ammonium persulfate + 1-2 gm KMnO4+5ml HCl+50ml H2O+Heat 50-70oC 2KMnO4+16HCl2KCl+2MnCl2+8H2O+5Cl2 From the above reaction, it can be inferred that Cl2 isreleasedduringthecourseofthereaction. Chlorinehasthehighest electronaffinity andthe fourthhighest electronegativity ofallthereactive elements.Forthisreason,chlorineisa strong agent. Ammonium persulfate also acts as an oxidizing agent. H2O acts as a wetting agent. Heat is applied in order to speed up the reaction. Etching time: 3-5 sec.7 Pathak S et al. J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9.ISSN: 2393-8315

Figure 7.Magnification: 100XFigure 8.Magnification: 500X Deeply etched grain boundaries with twins are observed 5. Reagent: 5gm Ammonium persulfate + 1-2 gm KMnO4+ 3-4ml conc. H2SO4 + 50ml H2O. 6 KMnO4 + 9 conc.H2SO4 6 MnSO4 + 3 K2SO4 + 9 H2O + 5 O3

Ozoneisanexcellentoxidizingagent.Conc. sulfuricacidshouldnotbeaddedtoomuchasit mayleadtotheproductionofMn2O7whichis explosive.Hence,bothammoniumpersulfateand ozoneactasoxidizingagents.H2Oactsasa wetting agent. Etching time: 5 sec.

Figure 9.Magnification: 100XFigure 10.Magnification: 500X Moderately etched grain boundaries without twins observed 6.Reagent:5gmAmmoniumpersulfate+10ml H2O2+ 5ml HCl +50ml H2O H2O2 (aq.) + HCl (aq.)H2O + HOCl (aq.) HOCl (aq.) + HCl (aq.)Cl2 + H2O Someoxygenisalsoproducedfromtheordinary decompositionofhydrogenperoxide:2H2O2 2H2O + O2. So, as a whole, both oxygen and chlorine which are very good oxidizing agents forironarereleasedduringthecourseofthe reaction. H2O acts as a wetting agent. Etching time: 5-10 sec. Pathak S et al. 8 ISSN: 2393-8315 J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9.

Figure 11.Magnification: 100XFigure 12.Magnification: 500X Austenitic grain boundaries revealed but very lightly etched 7.Reagent: 5gmAmmoniumpersulfate+50ml H2O +1-2 gm KMnO4+ 10ml H2O2+ 5ml HCl. Inthisreagent,thesequencefollowedisH2O followedbyKMnO4 followedbyH2O2.After mixing,HClisaddedandfinallyammonium persulfate is added. 2KMnO4+H2O22KOH+MnO2+O2(good oxidizing agent) After HCl is added, KOH+HCIKCL+H2OMnO2+4HClMnCl2+2H2O+Cl2(Good oxidizing agent) So,bothoxygenandchlorinewhicharegood oxidizing agents are released. The unused KMnO4 cancontributeasanextraoxidizingagent. Ammoniumpersulfateactsasanoxidizingagent. H2O act as a wetting agent. Etching time: 3-5 sec. Figure 13.Magnification: 100X Figure 14.Magnification: 500X Lightly etched Austenitic grain boundary observed 8.ConventionalReagent:2mlNitricAcid+98 ml Ethyl Alcohol. Herepolishedsurfaceisetchedwiththesolution for a few seconds to a few minutes. Etching time: 20-35 sec. 9 Pathak S et al. J. Adv. Res. Manu. Mat. Sci. Metall. Engi. 2015; 2(1): 1- 9.ISSN: 2393-8315 Figure 15.Magnification: 100XFigure 16.Magnification: 500X.Lightly etched Austenitic grain boundary without twins observedConclusion By studying and comparing all the micrographs, it isevidentthatreagent3andreagent4revealed highmanganeseausteniticsteelmicrostructure most prominently. However, with reagent 4, grain boundariesalongwithTWINShavebeen observedwithconsiderableclarity.Therefore, reagent4whichisasolutionofammonium persulfate,KMnO4,HClandH2Oisfoundtobe the most efficient etchant among all the reagents.ConventionalNew Etchant Low clarityBetter Clarity Very uncertain for High Manganese austenitic steel Almost Nil Lightly etched grain boundaryDeeply etched grain boundary Twins may be / may not be detected Clear twins can be detected High etching timeLess etching time Less rate of corrosionVery high rate of corrosion Table 2.A tabular comparison of conventional etchant vs. new etchant References 1.NayakAP,LogeeswaranVJ,IslamMS. University of California, Davis. California. 2. VanderVoortGF.ASMHandbook MetallographyandMicrostructure.Volume 9:493512. 3.GrajcarA,KrukiewiczW,KoodziejS. Corrosionbehaviourofplasticallydeformed high-Mnausteniticsteels.JAMMENov 2010; 43(1): 228- 35. 4.WalkerP,TarnWH.HandbookofMetal Etchants. 5.ReiterJ,BernhardC,PresslingerH. Austenite grain size in the continuous casting process:Metallographicmethodsand evaluation.MaterialCharacterizationJun 2008; 59(6): 737- 46. 6.MukhopadhyaySM.Samplepreparationfor Microscopicandspectroscopic characterizationofsolidsurfaceandfilms. DepartmentofMechanicalandMaterials Engineering,WrightStateUniversity, Dayton, Ohio.