【作者】 蒋良；

【导师】 颜银标；

【作者基本信息】 南京理工大学 ， 材料学， 2007， 硕士

【摘要】 基于目前3Cr2W8V钢常规热处理工艺后热疲劳性能较差，容易失效的现状，本文采用等温球化退火、高淬高回的热处理工艺，引入正交实验设计优化热处理工艺参数，根据正交分析计算得出淬火温度、球化退火等温温度、回火温度显著影响3Cr2W8V钢的热疲劳性能，并得出可能最佳方案为：830℃×2h+730℃×4h球化退火，1150℃×30min淬火，630℃×2h回火2次。通过进一步对比性实验，采用金相显微镜、扫描电镜、透射电镜等手段观察3Cr2W8V钢的微观组织，研究球化退火、淬火和回火工艺对组织的影响。结果表明：预处理工艺选择等温球化退火所获得的球化珠光体组织比常规球化退火更完全、均匀；高温淬火有利于提高马氏体合金化程度，延缓淬火钢的回复和再结晶过程，从而提高钢的回火稳定性，但过高的淬火温度使碳化物溶解过多，导致晶粒粗大；高温淬火后材料回火稳定性较好，需用较高温度回火使马氏体分解，析出的碳化物呈细小弥散分布。采用Uddeholm自约束热疲劳试验法进行热疲劳试验，观察疲劳裂纹的萌生过程，通过计算表面损伤因子，参照标准图谱比较不同工艺参数试样的热疲劳性能。研究表明热疲劳循环过程中的疲劳裂纹形成过程为：试样表面先是出现起皱和氧化腐蚀斑，然后从起皱和氧化腐蚀斑区域衍生出氧化腐蚀沟，这是热疲劳裂纹的萌芽；随着热疲劳循环次数的增加，这些腐蚀沟进入扩展阶段，沿轴向、横向延伸，形成热疲劳裂纹。电镜观察与分析表明：热疲劳裂纹起源于钨含量较低的区域，热疲劳裂纹扩展过程中，试样表层组织中α相回复、再结晶，原先板条马氏体转变为等轴铁素体亚晶，细小碳化物出现粗化和聚集现象，转变为M₂₃C₆型碳化物。由对比性实验验证了正交实验设计得出的最佳热处理工艺方案，经相同热疲劳试验次数后，用优化工艺处理的3Cr2W8V钢热疲劳裂纹级别为常规工艺的二分之一，热疲劳性能提高，为3Cr2W8V钢合理热处理工艺制定提供了参考数据。更多还原

【Abstract】 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.更多还原