The Effect Of Heat Treatment Technics On The Thermal Fatigue Property Of 3Cr2W8V Steel

Thermal fatigue property of 3Cr2W8V processed by conventional heat treatment isquite poor, which leads to damage. This work adopted isothermal spheroidizing annealing,quenching hot & high tempering, and utilized the orthogonal experiment method tooptimize the parameters of the treatment. The results shows that, quenching temperature,spheroidizing annealing temperature& temper temperature obtained by orthogonalexperiments has obvious influences on the thermal fatigue property of 3Cr2W8V, and theoptimized parameters are: spheroidizing annealing temperature is 830℃, holding time is 2hour; isothermal temperature is 730℃, holding time is 4 hour; quenching temperature is1150℃, holding time is 30min; temper temperature is 630℃, holding time is 2 hour, andtwice a time.The influences of treatments like spheroidizing annealing, quenching and tempering,on the microstructures of 3Cr2W8V is studied by utilizing the OM, and other moderntechnologies like SEM, TEM, etc. The results show that, isothermal spheroidtzingannealing should be chosen as the pretreatment prior to conventional annealingtemperature, which makes the pearlites much more spheroidized and uniform. Thequenching hot is in favor of improving the alloying degree in martensites, and delay thereversion and recrystallization process in quenched steel, which improves the temperingstability in steel, but too high quenching temperature will lead to over-dissolution ofcarbides, and grow-up of grains. The tempering stability is quite well in materialsquenched at high temperature, as a result, it is suitable to use high-tempering to makemartensite decomposed to small & fine carbides distributing uniformly in the matrix.Tempered acicular martensite will disappear with the increase of tempering temperature,and transformed to tempered sorbite.Thermal fatigue experiments are based on Uddeholm self-controlled fatigueexperiment methods, during which the generation processing of fatigue crack is observed.The thermal fatigue properties is evaluated by calculation of surface densification factor,and referred to standard atlas. The results show that, crinkles and oxidation cankers appearfirst on the surfaces of the materials, then the canker grooves emerge, which is thegermination of fatigue crack. With the processing of thermal fatigue cycle, the cankergrooves develop on the axes & landscape orientation, which form the fatigue crack. Thefatigue cracks can be observed in the low W area from SEM.Theαphase which existed inthe surface of the sample recovered and recrystallized during the developing of fatigue cracks, and the original martensite transferred to the equixed ferritic sub- grains. The finecarbide turned out to be coarse and gathered which changed to M₂₃C₆ at last.The optimized scheme obtained by the comparing experiments is as same as the oneobtained by orthogonal experiment methods. The thermal properties of the materialsobtained by this scheme is improved by one time compared to materials obtainedconventional treatments, which has reference meanings to the heat work processing on the3Cr2W8V in practice situations.