高温复杂加载条件下应变强化奥氏体不锈钢的疲劳行为和寿命预测研究

Cold stretching technique can improve the yield strength of austenitic stainless steel, which is an effective way to achieve lightweight during the manufacturing of austenitic stainless steel pressure vessels. However, plastic loss and deformation-induced martensitic transformation occurs when a certain type of austenitic stainless steel is stretched, and thus the risk increment may be caused during service, particularly under extreme conditions. The domestic S30408 austenitic stainless steel will be taken as study object in this program and the variation law of fatigue behavior and life for cold stretched steel under complex loading at high temperature will be basically investigated. Firstly, fatigue test for solution annealed and cold stetched S30408 steel under one-step at high temperature will be conducted, and variation law of the fatigue propreties for the two steels will be theoretically analyzed. Then, fatigue test for solution annealed and cold stetched S30408 steel under two-step and multi-step at high temperature will be carried out, meanwhile the mathematical model between the amount of stretch and fatigue life will be established. Finally, based on the nonliner fatigue evolution model, a life prediction model will be proposed under complex load coupling the amount of cold stretching for S30408 steel at high temperature. Throuth the research of this program, the theoretical and technical foudation of anti-fatigue design for cold stretched austenitic stainless steel pressure vessels will be laid.

应变强化能有效提高奥氏体不锈钢的屈服强度，采用该技术来制造奥氏体不锈钢压力容器是实现压力容器轻量化的有效途径。然而，应变强化在提高材料强度的同时，也会导致材料塑性损失及发生形变诱发马氏体相变，从而造成应变强化容器服役过程（特别是极端条件下）可能的风险增量。本项目以国产S30408奥氏体不锈钢为研究对象，对应变强化后材料在复杂加载条件下的高温疲劳行为和疲劳寿命变化规律进行基础科学研究。首先开展应变强化前后S30408钢在单级加载下的高温疲劳试验，掌握应变强化前后材料疲劳性能变化规律并理论分析内在原因；然后研究应变强化前后S30408钢在复杂加载条件下的高温疲劳行为和疲劳寿命以及应变强化量与材料疲劳寿命间的数学模型；最后基于材料非线性损伤模型，建立复杂加载下耦合应变强化量的S30408钢高温疲劳寿命预测模型。通过本项目的研究，可以为应变强化奥氏体不锈钢压力容器的抗疲劳设计奠定理论和技术基础。

应变强化是实现奥氏体不锈钢制压力容器轻量化的有效途径。但是，应变强化在提高材料强度的同时，也会导致材料内部损伤，从而造成应变强化容器服役过程的风险增量。对固溶态和应变强化预处理态奥氏体不锈钢在550℃、应力控制模式下的疲劳行为开展研究。研究结果表明：应变强化预处理前后材料均表现出持续的循环硬化响应，但经应变强化预处理后材料的循环应变幅和平均应变响应均小于固溶态材料；室温下不超过10%的应变强化预变形处理能延长材料的高温疲劳寿命，从而为应变强化奥氏体不锈钢制容器在高温环境下的可靠运行提供保证。基于Hull-Rimmer空洞长大理论，建立适用于应力控制下的材料疲劳寿命预测模型，并对固溶态和应变强化态奥氏体不锈钢进行寿命预测，预测结果与试验值吻合较好。在此基础上，结合不同应变强化量下材料的疲劳寿命变化趋势，进一步建立耦合应变强化预处理量的材料疲劳寿命预测模型，与实测寿命相比，预测寿命位于±1.5倍误差带之内，预测效果良好。建立的高温应力控制下的材料疲劳寿命预测模型形式简洁且具有清晰明确的物理意义，可用于应力加载下金属材料的高温疲劳寿命预测。本项目的研究可以为应变强化奥氏体不锈钢压力容器的抗疲劳设计奠定理论和技术基础。