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

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