Thermomechanical Treatment of High-alloyed Martensitic Stainless Steels for Complex Parts

除了耐腐蚀性外，高合金马氏体不锈钢还具有高硬度和高强度的特点。由这些钢制成的零部件的制造主要通过在软退火条件下对钢进行冷成型来进行。只有在成型后，材料才进行淬火和回火，以调整所需的最终机械性能。然而，使用冷成形，可实现的最大成形程度通常不足以制造具有复杂几何形状的零件。这种限制可以通过热机械处理（TMT）来克服，即具有成形和温度状态的精确控制的热成形工艺。TMT对用户提出了技术和材料科学挑战，因为各种物理冶金现象（例如相变、再结晶或碳化铬的形成）被激活，这对成形性和由此产生的机械性能（硬度、强度、耐腐蚀性）有很大影响。在本研究中，我们将系统地研究TMT工艺参数（奥氏体化温度、保温时间、变形程度、变形温度和应变速率）对马氏体不锈钢成形性能、力学性能和腐蚀性能的影响。目的之一是确定稳定的TMT流程的基本要求。此外，物理冶金现象和微观组织的变化，激活TMT过程中，以及它们对动力学的沉淀和相变的影响，必须加以确定。在工作计划中，考虑到热预应变（DCCT）和奥氏体化温度（TTA）影响的连续冷却转变（CCT）图将通过金相、金相和仪器硬度测试来确定。此外，所得的微观结构将通过光学显微镜和电子显微镜表征。实验程序是由热力学计算补充。成形性将通过热流动曲线、杯突和深冲试验的性能以及流动极限图（FLD）的测定进行评估。将进行硬度测量和拉伸试验，以表征所得性能。腐蚀行为将在动电位极化测试的基础上进行评估。借助实验的统计设计，将定义实验程序，并确定工艺参数与最终性能之间的关系。在此基础上，将创建现象学模型，用于多变量优化，以定义TMT的工艺窗口，以开发特定的机械和防腐蚀性能。该研究项目的框架旨在建立稳定的TMT工艺，从而增强马氏体不锈钢的适用性。

In addition to their corrosion resistance, high alloy martensitic stainless steels are characterized by a high hardness and strength. The manufacturing of component parts made of these steels is mainly performed by cold forming of the steel in soft-annealed condition. Only after forming, the material is quenched and tempered to adjust the desired final mechanical properties. However, using cold forming, the maximum achievable degree of forming is often not high enough to manufacture parts with complex geometries. This limitation can be overcome by a thermomechanical treatment (TMT), i.e. a hot forming process with an accurate control of forming and temperature regime. The TMT poses technological and material-scientific challenges for the user, since various physical metallurgical phenomena (e.g. phase transformation, recrystallization or formation of chromium carbides) are activated, which have a strong impact on the formability and the resulting mechanical properties (hardness, strength, corrosion resistance). Yet these effects for the martensitic stainless steels have not been investigated sufficiently.In this project, the effect of TMT processing parameters (austenitizing temperature, holding time, degree and temperature of deformation and strain rate) on the formability as well as mechanical and corrosive properties of these steels will be investigated systematically. One aim is to determine basic requirements for a stable TMT process. Furthermore, physical metallurgical phenomena and the microstructural changes activated during TMT, as well as their influence on the kinetics of precipitation and phase transformation, have to be identified. In the work plan, Continuous Cooling Transformation (CCT) diagrams considering the influence of hot prestrains (DCCT) and austenitizing temperature (TTA) will be determined by dilatometry, metallography and instrumented hardness testing. Moreover, the resulting microstructures will be characterized by optical as well as electron microscopy. The experimental program is complemented by thermodynamic calculations. Formability will be evaluated by hot flow curves, the performance of cupping and deep drawing tests as well as the determination of Flow Limit Diagrams (FLD). Hardness measurements and tensile tests will be carried out to characterize the resulting properties. The corrosion behavior will be evaluated on the basis of potentiodynamic polarization testing. With the aid of statistical design of experiments, the experimental program will be defined and the relationships between process parameters and final properties will be identified. On this basis, phenomenological models will be created to be used in a multivariable optimization to define process windows for the TMT in order to develop specific mechanical and anticorrosive properties. The framework of this research project aims on the establishment of stable TMT processes that lead to an enhanced applicability of martensitic stainless steels.