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

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