Hot crack assessment during welding using novel LTT weld filler materials

采用低温焊材是降低高强钢焊接过程中拉伸残余应力的一种创新方法。在之前的一个研究项目中，已经对LTT焊材的可焊性、转化动力学和残余应力形成进行了研究。结果表明，与传统焊材相比，某些LTT合金由于其凝固和偏析行为而表现出更高的热裂纹敏感性。在研究项目的第一个资助期内，考虑到整个材料体积，采用层析检查进行全面的热裂纹评估，以评估所建立的MVT热裂纹试验的结果。研究结果表明，与仅评估表面附近裂纹的传统MVT试验相比，热裂纹网络发展的三维信息提供了一个扩展的评估基础。为了对LTT合金进行综合评价，必须考虑材料体积。使用微CT的层析分析方法已被证明是有效的。然而，出现了一些问题，这些问题鼓励申请人提交为期两年的后续申请，以实现使用层析成像方法系统描述LTT系统中热裂纹形成的目标。在后续研究中，将研究在必要的样品制备过程中已被证明松弛的残余应力在多大程度上影响微CT分析的结果，以及所选程序的可重复性如何。将三维热裂纹网络的局部方向作为焊接参数的函数作为微CT的扩展量进行分析，并用作热裂纹敏感性的评估工具。从材料科学的角度来看，必须澄清两种LTT合金体系CrNi和CrMn的相反热裂纹敏感性是如何随着热输入的增加而引起的。通过在淬火成形膨胀仪上进行凝固试验，将相关影响变量材料和载荷从焊接工艺规范中分离出来。要确定关键参数，最终确保LTT合金的安全使用。

The use of Low Transformation Temperature (LTT) welding consumables is an innovative way of reducing tensile residual stresses during welding of high-strength steels.In a previous research project, investigations on weldability, transformation kinetics and residual stress formation of LTT welding consumables have already been carried out. It turned out that some of the investigated LTT alloys show an increased hot crack sensitivity compared to conventional welding consumables due to their solidification and segregation behavior. In the first funding period of the research project, tomographic examinations were used for a comprehensive hot crack evaluation considering the entire material volume to evaluate the results of the established MVT hot crack test. During the investigations it was successfully shown that in comparison to the conventional MVT test, in which only the cracks near the surface are evaluated, the 3D information for the development of the hot crack networks represent an extended evaluation basis. For a comprehensive evaluation of the LTT alloys the consideration of the material volume is necessary. The approach of tomographic analysis using µCT has proven to be effective. Nevertheless, several questions appeared which have encouraged the applicants to submit a two-year follow-up application to achieve the objective of systematic description of hot crack formation in the LTT system using tomographic methods. Within the continuation, it is to be investigated to what extent the residual stresses that have been proven to relax during the necessary sample preparation affect the result of the µCT analyses and how the reproducibility of the selected procedure is. The local orientation of the 3D hot crack networks as a function of the welding parameters is to be analyzed as an extended quantity of the µCT and used as an evaluation instrument for the hot crack susceptibility. From a materials science point of view, it must be clarified how the opposite hot crack sensitivity of the two LTT alloy systems CrNi and CrMn is caused with increasing heat input. By means of solidification tests in a quenching and forming dilatometer, the relevant influencing variables material and load are isolated from the welding process specifications. Critical parameters are to be identified to ultimately enable the safe utilization of LTT alloys.