海洋结构用超级双相不锈钢焊接接头组织调控及耐腐蚀性强化机理研究

Super duplex stainless steel (SDSS) has excellent corrosion resistance and has been used widely in oceanic environment. However, the existing research shows that in the welded joint of the SDSS, the proportion between austenite and ferrite is difficult to control and the harmful phases, such as intermetallic compounds and nitride, are apt to precipitate. So the corrosion resistance of the welded joint is greatly lower than that of the base metal, which results in the great safety hidden danger and material waste of marine structure. To overcome this technical bottleneck, in this project the microstructure, phase and element distribution in the welded joint of the SDSS will be researched. Based on the acquired welding thermal cycle curves and the extracted key parameters of the weld thermal cycle, then the differential thermal analysis and thermal simulation experiment will be carried out, by which the precipitation behavior of phases is revealed. Then the formation model of welded joint microstructure affected by the gradient superposition of thermal cycle will be built. Then with designing the weld thermal cycle, the influence of the chemical composition and welding parameters on the microstructure of welded joint will be studied, then ensuring the key factors and their action mechanism. The relationship model between the microstructure of the welded joint and its corrosion resistance will be established. The evaluation equation of corrosion resistance based on the proportion and morphology of austenite and ferrite and the element distribution will be presented. The quantitative evaluation criteria for corrosion resistance of the welded joints of SDSS in marine environment can be determined. The parameters for precisely controlling the microstructure of the welded and the reference value for strengthening corrosion resistance will be given, and the match of corrosion resistance between the welded joint and the SDSS can be achieved.

超级双相不锈钢由于其良好的耐腐蚀性能被广泛应用于海洋环境，但其焊接接头中奥氏体和铁素体两相比例容易失调、有害相的析出倾向大，腐蚀性能显著低于母材，导致海洋结构存在极大的安全隐患和材料浪费。本项目针对这一难题，研究超级双相不锈钢焊接接头的组织、物相组成及合金元素分布规律，采集热循环曲线并提取关键热循环参数，进行差热分析和焊接热模拟，揭示研究各相的凝固析出行为，建立梯度叠加热循环作用下的接头组织形成模型；设计焊接热循环曲线，研究化学成分和焊接工艺对焊接接头组织的影响规律，确定焊接组织形成过程中的关键因素及其作用机制；建立焊接接头组织与耐腐蚀性能的关系模型，提出基于接头中奥氏体和铁素体两相比例、形貌以及合金元素分布的耐腐蚀性能评价方程，确定海洋环境下超级双相不锈钢焊接接头的耐腐蚀性能量化评价准则，给出接头组织精确调控参数及耐腐蚀性强化参考值，实现双相不锈钢焊接接头与母材耐腐蚀性能的匹配。

超级双相不锈钢具有优良的力学性能和耐腐蚀性能，其工程应用不断增加，但由于超级双相不锈钢焊接接头中奥氏体和铁素体两相比例易失衡、有害相的析出倾向大，使其耐腐蚀性显著低于母材，导致焊接接头成为超级双相不锈钢结构的薄弱环节。本项目研究了药芯焊丝中合金元素和矿物质粉对焊接工艺性、接头力学性能和耐腐蚀性能的影响规律，构建了基于TiO2-ZrO2-CaF2-NaF中性渣系的超级双相不锈钢药芯焊丝配方体系，开发了超级双相不锈钢焊接用药芯焊丝材料，其熔敷金属的点蚀电位与双相不锈钢母材相当，满足焊接材料与母材金属耐腐蚀性能的匹配要求。研究了焊接热输入、焊接电流密度等参数对超级双相不锈钢焊接接头的组织特征和耐腐蚀性能的影响规律，并采用电弧多层多道焊和激光沉积制造了超级双相不锈钢焊接接头熔敷金属，研究其微观组织演变、成分变化、力学性能及其耐腐蚀行为，分析了热源类型对双相不锈钢焊接接头组织、力学性能和耐蚀性的影响规律，揭示超级双相不锈钢焊接接头组织形成机理。系统研究了电弧多层多道焊超级双相不锈钢焊接接头熔敷金属的成形质量、微观组织、力学性能和腐蚀行为，并对接头中各部分组织及性能的差异进行了分析和讨论，提出影响超级双相不锈钢焊接接头组织特征的关键因素及其作用机制。针对奥氏体含量较低、组织恶化的双相不锈钢及焊接接头，开展了激光表面处理研究，采用光斑尺寸可调的半导体激光器对组织恶化的典型双相不锈钢进行激光表面处理，并结合数值模拟预测温度演变，研究了激光光斑尺寸对温度演变和组织转变的影响，开发了一种通用的恢复焊接接头耐蚀性的激光表面处理方法，实现接头组织双相比例的精确调控及其海洋环境下耐腐蚀性的高效强化。本项目的研究成果经过工程化试验和验证，上升至标准条文，纳入到中国工程建设协会标准《不锈钢结构焊接技术规程》中，将显著推动超级双相不锈钢在结构工程中的应用。