低相变温度焊接材料熔敷金属的马氏体相变温度控制及其强韧化机理

Marine engineering equipment is the prerequisite and basis for the development of marine resources. Weld joints are prone to fatigue damage or failure due to the harsh marine environment which includes wind wave and tide flow et al. How to improve the fatigue performance of welded joints and extend their service life has become a bottleneck restricting the development of marine engineering equipment manufacturing industry. The low transformation temperature welding consumables can be used to change the distribution of weld residual stress and improve the fatigue strength of the welding structure. Therefore, it should be a wide application prospect in the field of marine engineering equipment manufacturing. However, the poor low-temperature impact toughness hindered its popularization and application in this field. Based on previous research, this project intends to improve the strengthen-toughening result of deposited metal from the following aspects: the design of deposited metal components, the phase transformation mechanism and the discipline of microstructure formation under the constraints, and control of the precipitation phase during welding process and so forth. The influence of chemical composition, constraints and cooling rate on the critical point and phase transformation expansion of deposited metal would be studied, in order to adjustment and control the residual stress state of welded joints. The influence mechanism of the constraints on the martensitic transformation, the retained austenite stabilization and the carbonitride precipitation process would be clarified, and the technology of improving the low-temperature impact toughness of the deposited metal shall be developed. It is proposed that the guiding production suggestion of flux cored wire should be formulated aiming to satisfy the strength toughness matching and fatigue performance in the actual engineering.

海洋工程装备是海洋资源开发的前提和基础。但是风、浪、流等恶劣的海洋环境导致海洋工程装备极易在焊接部位发生疲劳破坏。如何改善焊接接头的疲劳性能，延长其使用寿命已成为制约海洋工程装备制造业发展的技术瓶颈。低相变焊接材料可以用来改善焊接残余应力的分布状态，提高焊接结构的疲劳强度，在海洋工程装备制造领域有着广阔的应用前景，但低温韧性不足阻碍了其在该领域的推广和应用。本项目以课题组的前期研究为基础，从熔敷金属成分设计、拘束条件下的马氏体相变机理及组织形成规律、焊接过程中的析出相控制等方面来提高熔敷金属的强韧化效果；明确化学成分、拘束条件及冷却速度对熔敷金属相变临界点和相变膨胀量的影响规律，实现对焊接接头残余应力状态的调控；澄清拘束条件对马氏体相变、残余奥氏体稳定化以及碳氮化物析出过程的影响机理，发展提高熔敷金属低温韧性的技术手段；提出满足实际工程强韧性匹配和疲劳性能要求的药芯焊丝指导性生产方案。

风、浪、涌、流等恶劣的海洋环境会导致海洋工程装备在焊接部位发生疲劳破坏，如何改善焊接接头的疲劳性能，延长其使用寿命已成为制约海洋工程装备制造业发展的技术瓶颈问题。研究表明，低相变（LTT）焊接材料可以用来改善焊接残余应力的分布状态，提高焊接结构的疲劳强度，在海洋工程装备制造领域有着广阔的应用前景，但低温韧性不足阻碍了其在该领域的推广和应用。本项目通过药芯焊丝生产线试制金属粉型药芯LTT焊丝，降低了LTT熔敷金属中C、H、O等小原子元素的含量，并针对LTT熔敷金属成分调整与优化、拘束条件下LLT熔敷金属的相变机理及其奥氏体-马氏体相变动力学行为和LTT熔敷金属的强韧化机理进行了理论和试验研究。利用残余奥氏体的变形协调作用和TRIP效应、以及残余奥氏体使裂纹转向或止裂和大角度的马氏体/残余奥氏体相界阻碍裂纹扩展等韧化作用，改善了LTT熔敷金属的韧性在降低焊接残余应力，提高接头疲劳强度的基础上，获得了强韧型匹配良好的高强钢焊接接头。在此基础上，构建了拘束条件下LTT熔敷金属的Ms和Mf预测关系模型；通过对拘束条件下马氏体相变、奥氏体稳定化以及碳氮化物析出过程的分析，发展了提高LTT熔敷金属低温冲击韧性的新途径；最终，在Fe-Cr-Ni-Mo合金系的基础上，确定了LTT熔敷金属的化学成分范围，提出满足实际工程强韧性和疲劳性能要求的药芯焊丝生产方案。本项目通过探讨影响LTT熔敷金属低温韧性的内在机制，在保证LTT焊接材料改善应力分布状态，提高高强钢焊接接头疲劳强度的前提下，研究了改善LTT焊接材料的低温韧性的有效方法和技术手段，这不仅有助于推动我国焊接材料强韧化理论的发展，而且对于提升我国高端自主焊接材料的品牌，打破国外技术垄断也具有重大的意义。