New Technology Of Grain Refinement Of Superalloy PCX And The Effect On The Fatigue Property

As the heat treatment can′t improve the unsuited microstructure effectively thatcaused the poor fatigue properties,to improve the microstructure of the superalloy andinhence the fatigue properties, CrN was choosed as refiner for the refinement of thewhole superalloy. By vacum induction melting,fine-grain samples were getted,and themicrostructure of conventional and fine-grain samples were examined and analyzed.Atlast,conventional and fine-grain samples for fatigue best bars were tested.The mainresults are as follows:(1)After heat treatment,the gains、the primary dendrite axis length and the seconddendrite arm distance of the superalloy were little changed,but the spaces of the seconddendrite arm was reduced.Also carbide、eutectic and microporous that distribute alongthe the space of grains and grain boundaries were got little change. After heat treatment,bulk carbides began melt,at the same time,the second carbide separated out in the large γ′of eutectic.Theγ′phase of dendrite arm in the heat treatment samples compared withconventional samples were more regular,smaller. And the macro-segregation,after heattreatment,was lighter.(2)CrN as refiner was choosed for the new kind of superalloy,and the instructionand mechanism of the refinement were gived.By the ball milling,CrN particles,whichthe average sizes were0.5μm,were getted.After the preparation of equipment、ceramicmold shell、crucible、master alloy and refiner,the establishment of remelt process,thesuperalloy was remelt in the vacuum induction melting furnace,according to theprocess of1450℃*8min and poured at1390℃that the refiner was plused.Finally thefine-grain samples were getted.(3)The macro-seris of grain of cast fine-grain samples,compared withconventional samples,was increased from-3.78to-1.42,the average grain size wasreduced from1.189023mm to0.523747mm;the primary dendrite axis length wasreduced from656.67μm to352.23μm,but the second dendrite axis space was not changed due to the same cooling speed. After refinement,the number、size anddistribution of the carbide and microporosity compared with conventional samples gotsmller and more uniform;the sharp of carbide was changed from Chinses-form andstrip-form to granule and shor rod-like. The cubic γ′phase of dendrite arm in fine-grainsample,compared with regular sample, was more uniform. The distribution of eutecticwas not changed,but the size,after refinement,was largely reduced.(4) After fatigue testing,the high-cycle fatigue property of fine-grained samplescompared with conventional fatigue samples got signifincantly improved.By grainrefining,the size of micropoperties and carbide were largely reduced,inclusions gotdispersed, which largely reduced the cranny number of fatigure fracture.At the sametime, due to the more grain boundaries and the refined phase, crack grow slowlywhich largely inhenced the property of high-cycle fatigue of the superalloy.