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

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