Residual stresses in welded joints of work hardening steels with high manganese content

对于车身结构中高强度结构部件的开发，在延展性和强度之间具有最佳组合的钢材的应用是特别有趣的。高锰含量钢的最终力学性能是通过变形诱发马氏体形成（TRIP效应）或孪晶边界形成（TWIP效应）来实现的。在变形状态下，这些钢结合了高强度和高塑性变形储量。对于焊接结构的设计来说，重要的是，即使在疲劳载荷条件下，焊接接头也必须具有很高的强度，例如由这些材料制成的汽车部件。在这方面，焊接残余应力对强度和疲劳强度行为的影响程度以及第二类和第三类微残余应力对强度和疲劳强度行为的影响程度尚不清楚。此外，微观残余应力发展的各种原因与由此产生的宏观残余应力的稳定性之间是否存在关系尚不清楚。通过回答这些问题，该研究项目将对更好地理解强加工硬化奥氏体钢的加工特性和最终强度做出重大贡献，并为部件设计中定量考虑不同残余应力份额提供改进的基础。该项目旨在了解和描述成形过程中产生的宏观残余应力与焊接后状态之间的相互关系。这将通过系统的实验和焊接工艺链变形的有限元模拟来实现。为此，计划对高锰含量钢的焊接试样，即X40MnCrVAl19 2.5 （TWIP）和X10Mn7 （TRIP 700）进行广泛系统的x射线衍射分析，以分析局部相含量、相特异性织构演变以及各尺度下相特异性残余应力的发展和稳定性。了解工艺参数与由此产生的微观和宏观残余应力之间的关系，将为通过优化热输入和冷却过程以及焊接过程本身来优化由这些合金制成的部件的准静态和循环强度提供基础。在有限元模拟中，将通过对成形和焊接过程中塑性变形的局部计算，并与实验结果相结合，提供与微残余应力的相关性。

For the development of high strength structural components in body construction the application of steels with an optimal combination between ductility and strength is of particular interest. Steels with high manganese contents are highly interesting as the final mechanical properties are achieved through deformation induced martensite formation (TRIP effect) or through formation of twin boundaries (TWIP effect). In the deformed state these steels combine high strength with high plastic deformation reserves. Important for the design of weld constructions from instance in the field of automotive components made from these materials is that the welded joints must exhibit very high strength even for fatigue load conditions. In this regard it remains unclear to what extend welding induced residual stresses will affect the strength and fatigue strength behavior and to what extend the micro residual stresses of 2nd and 3rd kind contribute. Furthermore, it is unknown if there is a relationship between the various causes for the development of micro residual stresses and the resulting stability of macro residual stresses. By answering these questions the research project will make a significant contribution to gain a better understanding of the processing characteristics and the resulting strength of strongly work hardening austenitic steels and provides an improved basis for the quantitative consideration of different residual stress shares for components design. The project aims at the understanding and description of the interrelations between the macro residual stresses induced by the forming process and the state after welding. This will be realized by systematic experiments and through finite element simulations of the process chain deformation-welding.In this regard, extensive systematic X-ray diffraction analyses for welded samples made of steel with high manganese content, i.e. X40MnCrVAl19 2.5 (TWIP) and X10Mn7 (TRIP 700), for analyzing local phase contents, phase-specific texture evolutions and the development and stability of phase-specific residual stresses at all scales are planned. The comprehension of the relationship between process parameters and resulting micro and macro residual stresses will provide the basis for optimizing quasi-static and cyclic strength of components made from these alloys through optimizing the heat input and the cooling course and hence of the welding process itself. For the FE simulations the correlation with micro residual stresses will be provided through local calculation of plastic deformations during forming and welding and coupling with experimental results.