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

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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
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