Main damages on upper die in industrial hot forging Glavne ... - RMZ

RMZ – Materials and Geoenvironment, Vol. 57, No. 4, pp. 453–464, 2010453<strong>Main</strong> <strong>damages</strong> on upper die in industrial hot forgingGlavne poškodbe na zgornjem orodju za vroče kovanjev industrijiLejla Lavtar 1, * , Milan Terčelj 1 , Matevž Fazarinc 1 , Goran Kugler 11University of Ljubljana, Faculty of Material Science and Engineering, Department ofMaterials and Metallurgy, Aškerčeva cesta 12, SI-1000 Ljubljana, Slovenia*Corresponding author. E-mail: lavtar@steel.siReceived: July 8, 2010 Accepted: July 21, 2010Abstract: An analysis of main <strong>damages</strong> on industrial hot-forging die,made of Dievar hot-working die steel is presented. Die was previouslygas nitrided, sectioned and examined after its service life. Themeasurements of microhardness depth profiles and optical microscopyof nitrided layers were applied in analysis of wear, cracks andfailures on the most loaded sections of tool. In order to prolong dieservice life some improvements were suggested.Izvleček: V tem prispevku so prikazane glavne poškodbe na orodju zavroče kovanje, narejenem iz orodnega jekla Dievar. Orodje je bilopredhodno plinsko nitridirano ter preiskovano po izteku njegovetrajnostne dobe. Za analizo poškodb, tj. obrabe, razpok, zlomov,na najbolj obremenjenih delih orodja, so bile uporabljene optičnamikroskopija in meritve profila mikrotrdote. Za izboljšanje trajnostnedobe orodja so bile predlagane izboljšave.Key words: hot forging, die, gas nitriding, <strong>damages</strong>Kjučne besede: vroče kovanje, orodje, plinsko nitridiranje, poškodbeOriginal scientific paper

454 Lavtar, L., Terčelj, M., Fazarinc, M., Kugler, G.IntroductionHot-forging dies are during the processof hot forging subjected to a complexloading, i.e. to simultaneous thermal,mechanical, chemical and tribologicalloads. Various parts of hot-forgingdies are subjected to mentioned loadsthat results in different types of <strong>damages</strong>,like erosive, abrasive and adhezivewear, [1–4] plastic deformation, [1,8] [1, 7]thermal and mechanical cracking,gross cracking, etc. [1, 5, 6] Die life (servicetime) plays an important role ineconomy of hot forged products, thusgoal of every technologist in practice isprolongation of die service life as muchas possible on one hand and accurateprediction of die service time on theother hand. [1]Damages and their propagation arealso additionaly influenced by applieddie steel, die shape, and applied technologicalprocessing parameters, i.e.schedule of forging sequences, way ofmanufacturing the die, operating forgingparameters, used forging press, aswell as forging stock properties, i.e.temperature, scaling, local adhesionbetween die and the workpiece, etc.Further, die steel should be produced inan appropriate way in order to achieveoptimal microstructure, i.e. grain sizeas well as distribution, type, size andshape of carbides. Finally, manufacturingof die should be performed withoutnegative influences on the die surfacequality, and also shapes of die shouldbe optimized with sequential forgingsteps in order to avoid areas of essentiallyhigher loads in regard to other areasof die. [9,10]In order to increase die service time,wear and fatigue resistance, the die surfaceis improved by coating, diffusionprocesess like nitriding, etc. [11] Duringnitriding the die steels, two differenttypes of microstructure are formed onthe die surface, i.e. microstructure with“compound layer” on top of the surfaceand diffusion layer below, and inthe second case diffusion layer alone.It is advisable for hot forging dies toproduce nitrided microstructure withoutcompound layer since it consists ofbrittle iron nitrides, i.e. ε phase (ε-Fe 2-N), γ’ phase (γ’-Fe N), or mixed phases3 4(ε+γ’). At slightly higher contact pressures(about 20 MPa) [12, 13] compoundlayer usually spalls of the die surface atthe very beginning of forging processand that usually essentially acceleratesprocess of wear. In each application nitridedmicrostrucure as well as die designshould correspond to actual loadsthat prevail on die in the selected formingprocess.In order to elucidate doubtless relationshipsbetween occurrence of die <strong>damages</strong>,loads, properties of applied steel,die shape, etc. there are still not suffi-RMZ-M&G 2010, 57

<strong>Main</strong> <strong>damages</strong> on upper die in industrial hot forging455cient data on failures of industrial dies,especially on dies that were previouslynitrided, coated or duplex treated. Thusmore data on die damage analysis fromindustrial practice are needed. Analysisof main <strong>damages</strong> that occurred on a gasnitrided industrial die for hot forgingthat has failed have been presented inthis paper with aim to prolong its servicelife.ExperimentalDescription of hot-forging die andshape of forged productThe main <strong>damages</strong> of die (Figure 1)with two impressions for penultimateforging operation and two impressionsfor the last forging operation were analysedafter its service life. However,analysis was predominately focused onthe impression used for the last forgingoperation since it gave final shape anddimensions of the product (see Figure2). This shape was achieved in five sequentialforging operations. Upper andlower parts of die had similar designof impressions. Essential differencebetween the upper and the lower diewas height of mandrels. With regard toreference, [1] it could be supposed thatthe die surface temperature was in therange of 600–700 °C and contact pressureexceeded the value of 1000 MPa.Lubricant that was used was water suspensionof graphite.Die life is determined by surface qualityof forged product as well as by itsshape and dimensions. In our case customerhad special demands for forgedproduct in respect of the quality ofimpression surface, and the shape anddimensions at the base of mandrels.These areas were considered as criticalareas of the hot-forging die (see Figure1). The die with higher mandrelsfailed after 45 766 forging strokes dueto change of shape of forged product inthe depression around high mandrels.Initial billet made of C35E steel forforging the steering mechanism had diameterof Ø = 32 mm and weight of930 g, and was heated to temperatureinterval of 1230–1280 °C. Geometry offorged product is adapted for steeringmechanism in car that is a relativelysimple 3D-geometry (Figure 2).Die material, applied methodsDievar, a high performance chromium-molybdenum-vanadiumalloyedhot-working die steel (see Table 1),developed by Uddeholm, was used inmanufacturing industrial die; steel ischaracterized by excellent toughness,ductility in all the directions, good temperingresistance and high-temperaturestrength, excellent hardenability, andgood dimensional stability throughoutthe heat treatment procedure, as well asduring coating operations. Consequently,this die steel has a very good resist-RMZ-M&G 2010, 57

456 Lavtar, L., Terčelj, M., Fazarinc, M., Kugler, G.Figure 1. Upper die after 45 766 strokes; two impressions for penultimateforging operation and two impressions for the last forging operation withmandrelsance to thermal fatigue, gross cracking,hot wear and plastic deformation. It isused for dies in die casting, hot forging,and hot extrusion. [14]Figure 2. Shape of steering mechanism afterthe last hot forging operationDie was gas nitrided and taken outof industrial process after its servicetime. Die was sectioned and examinedwith light microscopy (OlympusGX51) and by Vickers microhardnessmeasurements (Leitz Miniload 2). Diesurface was examinated using macroand micrograph techniques to revealsurface <strong>damages</strong> of nitrided layer. Additionally,metallographic examinationsof samples taken from certaincross-sections were performed. Nitaletchant for metallographic preparationof nitrided samples was used.RMZ-M&G 2010, 57

<strong>Main</strong> <strong>damages</strong> on upper die in industrial hot forging457Table 1. Chemical composition of applied die steel (Dievar) in mass fractions, w/%C Si Mn Cr Mo V0.37 0.26 0.5 5.0 2.36 0.55Figure 3. Microstructures and microhardness profile on cross-section of the dieedge: nitrided diffusion layer with compound layer (a-b), microhardness profile (c)Results and discussionInitial nitrided microstructuresDiffusion layer can be well seen oncross-sections of the surface edge (Figure3a). The depth of diffusion layer isnot homogenous and it is thick from73 μm to 122 μm (see Figure 3a). Thisnonhomogeneity could be atributed tounappropriate preparation of die surfacebefore the nitriding process thatresulted in different adsorption abilitiesof nitrogen into the die surface layer.Thickness of compound layer was inthe range of about 0–3.5 μm (Figure3b). Further, presence of nitrides wasvisible on grain boundaries. It wasalso remarked that microstructure wasslightly overnitrided (nitrides on grainboundaries were in that area pependicularto diffusion front and parallel todie surface) that could result in prematurespalling of die material from thedie surface layer. Figure 3c gives microhardnessprofile; max. values wereabove HV = 1200.RMZ-M&G 2010, 57

458 Lavtar, L., Terčelj, M., Fazarinc, M., Kugler, G.Damages on die surfaceThe main <strong>damages</strong> of the die on macrolevel are shown in Figure 4. The relativelysharp edges of the upper die impressionaround mandrels show wearusually combined with plastic deformationand spalling (see Figures 4c, 4e,4f). On these edges high contact pressuresas well as large sliding lengthsbetween deformed material and diesurface took place. These conditionsled to higher heating and consequentlyto higher tempering of the die surfacelayer that favoured wear, spalling offragments of nitrided layer and plasticdeformation. Spalling of relative largefragments is seen on the upper edge ofmandrels (Figures 4c, 4e) for penultimateforging impressions. Mentionedspalling of fragments was, next tohigher contact pressures that prevailedon these surfaces, also consequence ofpresence of nitrides on grain boundariesof the diffusion layer (see Figure3b) that decreased toughness of diesteel. But spalling on mandrels tookplace only on one mandrel (Figure 4e)while on the other one only emphasizedwear was observed (Figure 4b).Further, also spalling on top of mandrel(see Figure 4i) was detected. Thisoccurrence of spalling could be atributedto thermal fatigue in combinationwith high contact pressure. On impressionradii shown in Figures 4g and 4d1plastic deformation was not observedbut only furrows as consequence ofabrasion were visible. Namely contactpressures on this surfaces wererelatively lower since radius of mandrelwas greater. The base of mandrelspreviously indicated as critical areasin the hot-forging die (see Figure 1) isalso critical for service life of dies andthe quality of forged products. In theseareas, due to pressure of forged materialon both depression surfaces andconsequently due to bending load onbottom of die depression, stress concentrationstook place. These repeatingmechanical loads resulted in formationof cracks at the base of mandrel (seeFigure 4h) that caused surface, geometryand shape inadequacies of forgedproducts. For comparison, such damagewas not observed on similar area ofdie impression for penultimate forgingsequence (see Figure 4d2). This indicatedthat the last impression was subjectedto comparatively higher bendingloads, and that required changesof impression design at the mentionedfive sequential forging steps. In Figures4a and 4f abrasion wear close to outerarea of die impression as consequenceof higher sliding length was observed.For further explanation and analysisof surface <strong>damages</strong> the cross-sectionalmicrostructures especially of criticalareas, i.e. on top, on upper and bottomedge as well as in the middle area ofmandrel, will be presented.RMZ-M&G 2010, 57

<strong>Main</strong> <strong>damages</strong> on upper die in industrial hot forging459Figure 4. Damages of the die after 45 766 strokes; plastic deformation (a, f), mechanical deformation (b, c, d, e, g),spalling (d, e), cracks (d, h), wear (i).RMZ-M&G 2010, 57

460 Lavtar, L., Terčelj, M., Fazarinc, M., Kugler, G.Damages on mandrel cross-sectionsCross-sections of mandrel for thelast forging operation revealed compoundlayer on the mandrel top (Figure5) and in its middle area (Figure7), while compound layer on the upper(Figure 6) and the bottom edge(Figure 8) was removed during theforging process; furthermore, formationof cracks in the diffusionlayer was observed (see Figures 5and 6). Removal of compound layercould be atributed to higher contactpressures and sliding lengths thatpreavailed in these areas and theyled first to its cracking and thenFigure 5. Hardness profile and cross-sectional microstructure on the topof mandrelFigure 6. Hardness profile and cross-sectional microstructure on the upperedge of mandrelRMZ-M&G 2010, 57

<strong>Main</strong> <strong>damages</strong> on upper die in industrial hot forging461to its spalling; compound layer isnamely very brittle in comparisonto the diffusion layer. Moreover,as mentioned above, nitrides werepresent on grain boundaries of nitridedmicrostructure that decreasedtoughness and accelerated spallingof fragments.As already mentioned, cracks werealso found in the diffusion layer onthe upper edge having lengths ofabout 96 μm (Figure 6) and on thetop of the mandrel with lengths ofabout 69 μm (Figure 5). Compoundlayer on the top of the mandrel alsoexhibited cracking as consequenceFigure 7. Hardness profile and cross-sectional microstructure in the middlepart of mandrelFigure 8. Hardness profile and cross-sectional microstructure on the bottomedge of mandrel.RMZ-M&G 2010, 57

462 Lavtar, L., Terčelj, M., Fazarinc, M., Kugler, G.of thermal fatigue and presence ofhigher contact pressures (compareFigure 5 with Figure 4i). Relativesliding between the die surface andthe deformed material was reducedin these areas thus suface materialwas removed by spalling. Effectivenitriding depths on all the cross-sectionswere in range of about 150–160μm. Microhardness measurementsshowed the highest drop of hardnessvalues from HV 0.1= 1292 on the edgeof die after the nitriding process, toHV 0.1= 1044 (see Figure 6) on theupper edge of the high mandrel afterdie failure.Furthermore, crack with length ofabout 1.5 mm at the base of mandrelwas detected (Figure 8, comparedwith Figure 4h). In this figure alsomore intensive spalling of diffusionlayer on the surface at the beginningof crack was observed. In fact,this surface damage limited die lifesince the surface quality and shapeof forging did not fulfil demands ofcustomer.In Figures 5–8, increase of microhardnessvalues was found at thedepths of about 60 μm and less. Thiscould be atributed to diffusion of nitrogenfrom the die surface layer intointerior or eventual increase of carboncontent since lubricant based ongraphite was used for lubrication andcooling of die impressions.ConclusionsIn the presented work occurrence of<strong>damages</strong> of hot-forging die, made ofDievar, hot-working die steel that wasused for hot forging of steering mechanismhas been analyzed. Gas nitrideddie was sectioned and examined afterits service time, i.e. after 45 766 strokesusing light microscopy and Vickers microhardnessmeasurements. The essentialconclusions are:• Nitrided microstructure on crosssectionsrevealed presence ofcompound layer and nitrides ongrain boundaries. Non-homogenousdepth of diffusion layer thatresulted in different adsorptionabilities of nitrogen was observedand could be atributed to impropersurface preparation priorto nitriding. In order to improvesurface activation proper surfacepreparation prior to nitridingshould be performed too.• Proper selection of nitriding parametersis needed in order toavoid formation of nitrides ongrain boundaries.• Metallographical examinationrevealed that three main types of<strong>damages</strong> could be seen on die surfaces,i.e. cracks as consequenceof mechanical and thermal fatigue,spalling of nitrided layer asconsequence of thermal fatigueand mechanical loads, and furrowsas consequence of abrasion.RMZ-M&G 2010, 57

<strong>Main</strong> <strong>damages</strong> on upper die in industrial hot forging463• Mechanical fatique cracking occurredat the base of mandrelsand it was decisive mechanismthat was influencing die servicelife. Crack with length of about1.5 mm was observed on the bottomedge of mandrel. This damagecould be avoided by reducingthe pressure of deformed materialon die surface in the depression,i.e. by optimizing deformationsequences (steps) in forging.References[1]Hansen, P. H. (1990): Analysis of weardistribution in forging dies. Ph.D.Thesis, Pub. No. MM. 91.06,Technical University of Danmark.[2]Rooks, B. W. (1974):The effect of dietemperature on metal flow and diewear during high speed hot forging.Proceedings of the 15 th InternationalMTDR Conference, Birmingham,UK, Macmillan, London,487–494.[3]Singh, A. K., Rooks, B. W. & Tobias,S. A. (1973): Factors affecting diewear, Vol. 25, No. 2, 271–279.[4]Silva, T. M. & Dean, T. A. (1974): Wearin drop forging dies. Proceedingsof the 15 th International MTDRConference, Birmingham, UK,Macmillan, London, 479–486.[5]Sharma, R. & Arrowsmith, D. J.(1982): The wear of forging inthe first five forging blows. Wear1981–1982, Vol. 74, 1–10.[6]Siedel, R. & Luig, H. (1992): Frictionand wear processes in hot dieforging. New Materials ProcessesExperiences for Dieing, Proceedingsof the International EuropeanConference on Dieing Materials,Switzerland, 7.–9. September1992, 467–480.[7]Doege, E., Groche, P. & Bobke, Th.(1990): Application of adhesiontheory to friction and wear processesin hot forging. Proceedingsof the 3 rd International Conferenceon Technology on Plasticity, Vol.1, 27–32.[8]Baar, C. (1993): SchmelzmetallurgischeMassnahmen zur Standmengenerhöhungvon Schmiedewerkzeugen.Proceedings of the Seminaron Werkzeug- und Formenbau,Univerität Hannover, September1993, Hannover, 2/1–2/13.[9]Schaefer, F. & Behrens, B. A. (2005):Prediction of wear in hot forgingdies by means of finite-elementanalysis.Journal of MaterialsProcessing Technology; Vol. 167,309–315.[10]Ebara, R. & Kubota, K. (2008): Failureanalysis of hot forging diesfor automotive components. EngineeringFailure Analysis 2008;Vol. 15, 881–893.[11]Pye, D. (2003): Practical Nitriding andFerritic Nitrocarburizing, ASM International,Materials Park, Ohio,2003.[12]Terčelj, M., Smolej, A., Fajfar, P. &Turk, R. (2007): Tribol. Int. 2007;Vol. 40, 374–384.[13]Castro, G., Fernández-Vicente, A. &Cid, J. Wear (2007): 263, 1375–1385. [14]RMZ-M&G 2010, 57