Mechanism-based modelling of tempering phenomena in steels

为了提高淬火部件的操作能力，马氏体淬火工艺通常由随后的回火工艺补充。马氏体硬化后，材料在淬火状态下处于极端的非平衡状态，缺乏韧性。此外，还可能产生破坏性残余应力。在回火过程中，材料通过碳的扩散、碳化物的析出、残余奥氏体的溶解以及残余应力的松弛向平衡方向发展。作为这些机制的结果，材料获得了许多应用所需的延展性，并且热处理部件的寿命将会增加。虽然热处理的建模和模拟在许多应用领域都很成熟，但很少有人考虑回火，尽管回火对淬火钢件的性能特性具有特殊的重要性。该项目的目的是研究回火和之前的马氏体硬化过程以及它们之间的相互作用，从而开发一个基于物理的模型来描述发生机制。建模的特点是所产生的显微组织、硬度、机械性能和残余应力以及零件的变形。不同的工艺将在不同的钢和热处理下进行研究，在每种情况下，某些工艺占主导地位。基于高合金钢X40CrMoV5-1的穿透硬化，RWTH Aachen的IWM重点研究了碳化物的形成和力学性能。以IWT不来梅100Cr6轴承钢为研究对象，分析了淬火(自动回火)过程中已经发生的马氏体形成、奥氏体稳定化和回火过程之间的相互作用及其对后续回火的影响。采用42CrMo4钢在IAM-WK试验台上对感应表面淬火过程中的短时奥氏体化和短时回火引起的组织不均匀进行了研究。最后，基于这些协作，能够进行包含回火期间的所有单一效果的建模。

To improve the operational capability of hardened components the process of martensitic hardening is classically supplemented by a subsequent tempering process. After the martensitic hardening, the material in its as-quenched condition has an extreme non-equilibrium state and lacks of toughness. In addition, destructive residual stresses can be generated. During tempering the material progresses towards its equilibrium by diffusion of carbon, carbide precipitation, dissolution of retained austenite as well as residual stress relaxations. As a consequence of these mechanisms, the material receives ductility required for many applications and the life time of the heat treated component will be increased. Although modelling and simulation of heat treatment is well-established in many fields of application, tempering is rarely considered despite its particular importance regarding the resulting performance characteristics of hardened steel parts. The objective of this project is to investigate the tempering and the preceding martensitic hardening process as well as their interactions and hence to develop a physically based model describing the occurring mechanisms. The characteristics of interests in modelling are the resulting microstructure, hardness, mechanical properties and residual stresses as well as the distortion of the parts. The different processes are to be investigated at different steels and heat treatments, whereat certain processes predominate in each case. The formation of carbides and mechanical properties are focused by the IWM of the RWTH Aachen based on the through hardening of the high-alloyed steel X40CrMoV5-1. Interactions between the formation of martensite, the stabilization of austenite and tempering processes which already occur during quenching (autotempering) as well as their effects on the subsequent tempering are analysed using the through heat treated bearing steel 100Cr6 at the IWT Bremen. The inhomogeneous microstructure in consequence of short time austenitization and short time tempering, as occurring in inductive surface hardening, are investigated using the tempering steel 42CrMo4 at the IAM-WK of the KIT. Finally, based on these collaborations a modelling is enabled containing all single effects during tempering.