304不锈钢棘轮变形过程中应变诱发马氏体相变行为研究

STUDY ON STRAIN-INDUCED MARTENSITE TRANSFORMATION OF 304 STAINLESS STEEL DURING RATCHETING DEFORMATION

利用OM,SEM和XRD对单轴非对称应力循环下304不锈钢棘轮变形过程中的微观组织变化进行了实验观察.结果表明:304不锈钢棘轮变形过程中,当棘轮应变达到一定值后会产生应变诱发马氏体相变,形成板条状马氏体,并且随循环周次的增加,形成的应变诱发马氏体相对量逐渐增加.因马氏体相变而诱发的塑性变形对总的棘轮变形量产生一定的影响,材料的棘轮应变应由两部分组成,即应力引起的塑性应变和相变诱发的塑性应变.

Ratcheting behavior of the materials has been extensively investigated and simulated in the last two decades, since it is important in the design and assessment of engineering structure components subjected to asymmetrical stress controlled cyclic loading. However, most of the referable literature concerned only the macroscopic phenomenon of ratcheting and its phenomenological models. Therefore, the microstructure evolution of SS304 stainless steel during uniaxial stress-controlled cyclic loading with the ratcheting deformation was observed by using OM, SEM and XRD methods. Several specimens with the same applied stress level were first tested under the stress-controlled cyclic loading with different numbers of cycles maeroseopically, and then the thin-filmed samples obtained from the specimens subjected to different numbers of cycles were microscopically observed. The results show that when the ratcheting strain reaches a certain value high enough, the strainAnduced martensite transformation occurs in the process of rateheting deformation. The resultant phase is the lath a- martensite and the amount of the induced martensite increases gradually with the number of cycles, which is proved by the quantitative analysis of XRD. It is also concluded that the plastic deformation caused by the martensite transformation has a un negligible contribution to the total value of ratcheting strain. The total ratcheting deformation should be considered as a superimposition of two parts, i.e., the one caused by applied cyclic stress and the other caused by the strain-induced martensite transformation.