大厚度高强钢焊接应力变形分层超声冲击主动调控机制研究

Welding residual stresses and distortions are the important factors that affect the fatigue performance, the in-service safety and the cost of high-strength steel and large-thickness welded structures. However, it is very hard to reduce the internal residual stresses and distortions in large-thickness welded structures. In this project, the layered ultrasonic impact treatment (LUIT) technology is proposed to actively control the internal welding residual stresses and distortions of the high-strength steel and large-thickness welded joints by means of the effects of stress relaxation and surface hardening in the ultrasonic impact treatment (UIT) technology. Firstly, the internal stress distribution is to be measured based on the contour method, and the influencing depth and range that the UIT on stresses will be investigated, then the acting mechanism that the UIT treatment on welding residual stresses will be studied with the investigation on the variation of microstructure and mechanical properties. Secondly, by the aid of the eigenstrain (the source of welding stresses and distortions), the comprehensive regulation of UIT on welding residual stresses and distortions will be investigated, and the relationship among UIT parameters, eigenstrain field and welding stress and distortion fields can be clarified. Finally, based on the previous investigations, the feature of eigenstrain field, stress and strain field after LUIT will be investigated, the effects of subsequence passes on the existed stress field, microstructure and mechanical properties of the treated layers will be discussed, and then the optimized strategies of the LUIT are to be proposed. The implementation of this project can provide the theoretical and technical basis for regulating the residual stresses and distortions in large-thickness and high-strength steel welded structures.

焊接残余应力和变形是影响大厚度高强钢焊接结构疲劳性能、服役安全性及生产成本的重要因素，但大厚度高强钢结构内部焊接残余应力调控及变形矫正非常困难。本项目利用超声冲击处理技术的内部应力松弛效应和表面强化效应，提出分层超声冲击法主动调控大厚度高强钢结构内部焊接残余应力及变形。主要研究内容包括：（1）基于轮廓法测试内部残余应力分布，结合组织和力学性能分析，研究超声冲击处理的影响深度和广度，揭示超声冲击处理对内部焊接残余应力的松弛作用机制；（2）以反映焊接残余应力变形本质的本征应变为桥梁，揭示超声冲击处理对焊接应力变形的综合调控效应，阐明超声冲击工艺、本征应变场和应力变形场的相互关系；（3）基于以上成果，研究分层超声冲击后的本征应变场和应力变形场特征，探讨后续焊道对冲击层应力、组织和力学性能的影响规律，提出优化的分层超声冲击策略。本项目的实施可为大厚度高强钢焊接应力变形主动调控提供理论和技术基础。

焊接接头力学性能恶化和焊接残余应力是引起焊接结构失效的重要原因。超声冲击处理（UIT）通过超声频振动驱动冲击针高速撞击工件，使表面产生塑性变形和压应力，具有强化接头和调控应力双重效果。现有超声冲击处理使用模式仅用于表面处理，超声冲击处理对内部焊接应力和接头力学性能的相关效应研究不够，限制了该方法的应用。. 本项目结合本征应变技术、多次切割轮廓法内部应力测试技术和逐层剥削X射线衍射法表层应力测试技术，研究了表层及内部超声冲击处理对表层到内部焊接应力及焊接变形的作用效果。结果表明，超声冲击处理造成表层2~4mm左右深度的压应力，峰值压应力幅值超过材料屈服强度；与焊后热处理的全局性应力消除效果相比，超声冲击处理对焊接应力的影响为局部性的，对处理区域外以及大于2~4mm深度的原始应力无显著影响；拘束、坡口形式、厚度等对超声冲击处理效果影响不大；1mm深度内初始应力对超声冲击应力无影响；表面/逐层超声冲击处理对内部应力和焊接变形的影响有限。研制了冲剪法设备用于表征局部材料力学性能，研究发现逐层超声冲击处理能促进多道焊接接头的焊缝组织细化、硬度均匀化和接头各区域强度均匀化。结合硬度和组织分析，发现逐层超声冲击处理有利于焊缝组织细化、提高焊缝整体强度以及硬度均匀化分布。其原因在于超声冲击处理的氧化效应和元素转移效应在焊缝中形成了有益针状铁素体的形核质点，从而细化焊缝晶粒并生成针状铁素体，使得焊缝强韧化。基于有限元法模拟了超声冲击处理的动态过程，研究表明振幅和冲击频率决定了冲击处理造成的压应力深度和幅值；较大冲击针和致密覆盖率有利于形成均匀压应力分布。. 上述研究成果实现了从表层到内部应力全貌测试和局部材料力学性能评价，明确了超声冲击处理对焊接应力的交互作用效果及其影响因素，发现了逐层超声冲击处理对焊缝组织和力学性能的积极效果，为焊接残余应力调控、焊接接头强韧化提供了理论保障和技术支持。