高强度海洋工程钢湿法水下焊接冷裂纹控制机制研究

As it offers significant cost savings and simplicity of the process, underwater wet welding is becoming increasingly important in the repair and maintenance of hydrotechnical constructions. However, owing to the rapid cooling rates and high content of hydrogen present in the weld pool, microstructures with high susceptibility to cold cracks are to be produced in weldment during underwater wet welding. Due to expectations that the need for underwater structural repair and maintenance on offshore production platforms will grow, the demand for producing quality wet joint, including development of wet welding consumables, is being continued to increase. In this study, the metallurgy in underwater wet welding of high strength offshore structural steel EH40 is investigated, and methods for controlling the cold cracking in the underwater wet welds is also studied. In consideration of the fact that carbides of titanium and vanadium can offer strong irreversible hydrogen trapping site and tailor microtructure of weldment, titanium and vanadium powders are added in the electrodes coating for declining cold cracking tendency of the EH40 steel in underwater wet welding. The hydrogen content, cold crack sensitivity, microstructure, fine structure and mechanical properties of the joint are investigated to optimize the titanium and vanadium elements additive amount and reveal the effect of titanium and vanadium elements addition on the diffusible hydrogen content and microstructures of the wet joint. Moreover, the thermal cycles in underwater wet welding of EH40 steel are recorded by thermocouples, as well as simulated using Abaqus software. By revealing the influence of the thermal cycles on microstructures of the weld metal and the HAZ zone, numerical simulation of the weld thermal cycles is utilised in optimising heat input in underwater wet welding.

本项目拟以自制铁素体焊条为研究对象，系统研究EH40钢湿法水下焊接接头微观组织演化规律；并从焊条药皮合金化设计和焊接热输入量设计两方面对高强度海洋工程钢湿法水下焊接冷裂纹控制手段进行了研究，解决焊接冷裂纹这一制约湿法水下焊接技术应用的工程难题。基于Ti、V合金碳化物的强氢陷阱和组织调控作用，本项目拟通过在焊条药皮中添加Ti、V合金粉体改善EH40钢湿法水下焊接冷裂纹敏感性。通过研究湿法水下焊接接头扩散氢含量、冷裂纹敏感性、微观组织与精细结构和力学性能，优化焊条药皮Ti、V合金含量，探讨Ti、V合金元素对EH40钢湿法水下焊接接头扩散氢含量和微观组织的调控机制，丰富湿法水下焊接材料设计理论。通过热电偶温度测量结合Abaqus软件数学建模研究EH40钢湿法水下焊接热过程，分析湿法水下焊接过程中焊缝与热影响区微观组织随温度场演化关系，为湿法水下焊接热输入量优化设计提供理论指导。

冷裂纹是危害高强度海洋工程钢湿法水下焊接安全性的重要因素。本项目以自制铁素体焊条和奥氏体不锈钢焊条为研究对象，研究了焊条药皮类型、药皮合金成分对EH40钢水下湿法焊接接头的组织及性能影响规律，成功消除了高强度海洋工程钢水下焊接冷裂纹。研究结果表明，碱性焊条具有更低的扩散氢含量，然而钛钙型酸性焊条具有更优异的焊接工艺性，更适宜用作水下焊条。水下湿法焊接接头的抗拉强度和冲击韧性随合金元素含量的变化规律不尽相同。对于铁素体焊条，添加适量的Ni元素能抑制焊缝中粗大棒状先共析铁素体的形成，细化多边形铁素体尺寸，Ni粉的最佳添加量为4wt%；添加适量的Ti和V元素能细化焊缝铁素体尺寸并促进针状铁素体生成，从而改善焊缝韧性，Ti粉和FeV50合金粉体的最佳添加量分别为1wt%和0.5 wt%。对于奥氏体不锈钢焊条，添加Ni能提高焊缝韧性，而添加Cr和Mo将促进δ铁素体和σ相的生成，提高焊缝的强度同时大幅降低焊缝韧性。水下焊接工艺的优化研究结果表明：水下焊接熔敷金属扩散氢含量随焊接热输入的增大而增加，160A-180A是水下湿法焊接较为适宜的焊接电流参数，过大的焊接热输入将引发粗晶热影响区马氏体组织过度粗大，造成在熔合线附近生成氢致裂纹。采用混合热源模型，利用abaqus软件对水下湿法焊接温度场进行了模拟计算，计算结果与试验测试结果相符。项目共发表各类学术论文9篇，申报发明专利5项、实用新型专利2项。项目投入经费20万元，支出19.56万元，各项支出基本符合预算。剩余经费0.44万元，计划用于本项目研究后续支出。