一种用于异种金属焊接的功能梯度材料的设计和制造方法研究

Dissimilar metal welds are widely used in nuclear and fossil-fired power plants. Premature failure located at the interface between ferritic creep resistant steels and nickel-based weld metal plays a key role in power plants service safety. Great chemical composition gradient between the two materials mentioned above is responsible for premature failure. Functionally graded material (FGM) provides an ideal solution for dissimilar metal welding due to its position-dependent chemical composition. In the present research work, a dual wire GTAW based method is proposed to build FGM used for the fabrication of dissimilar metal welds between 2.25Cr bainitic creep resistant steels and Alloy 800H, which are commonly seen in power plants. The key step for building FGM is the design of the position-dependent composition. In the present research work, two principles are proposed to ensure the successful fabrication of FGM, including functional integrity and structural integrity. For functional integrity, lowering the chemical potential gradient of carbon and thermal stress induced by the mismatch of coefficient of thermal expansion are taken into account. For structural integrity, the considered factors including avoiding the occurrence of solidification crack and brittle phase e.g. intermetallic compounds. Based on the work mentioned above, microstructure and mechanical properties characterization work is planned to conduct, to evaluate the performance of FGM. Some scientific significance should be attached to the present research work due to the guidelines it proposed, and it might also make some contribution to the life-extending of dissimilar metal welds.

异种金属焊接接头在核电、火电机组中有着广泛的应用，产生于铁素体耐热钢与镍基焊缝降金属界面处的早期断裂失效是影响其服役安全性的主要因素，根本原因在于铁素体耐热钢与镍基焊缝金属之间存在较大的成分梯度。使用功能梯度材料连接异种金属是一种理想的解决方案，本项目提出了一种基于双丝GTAW的功能梯度材料的制备方法，用于连接发电机组中常见的2.25Cr贝氏体耐热钢和800H合金。成分设计是该功能梯度材料制备的关键，本项目提出了基于功能和结构完整性的成分设计方法，其中功能完整性包括降低碳的化学势梯度和由热膨胀系数差异导致的热应力，结构完整性包括避免凝固裂纹和脆性相的产生。在此基础之上，拟开展对使用功能梯度材料制造的异种金属焊接接头进行微观组织和力学性能评价。本工作对于完善用于异种金属连接的功能梯度材料的制备具有一定的科学价值，同时对于提高异种金属焊接接头的服役寿命具有重要的现实意义。

使用镍基焊材连接马氏体耐热钢与镍基合金形成的异种金属焊接接头在高超超临界火电机组中有着广泛的应用。由于镍基焊缝与马氏体耐热钢的化学成分差异较大，在镍基焊缝与马氏体耐热钢热的界面附近存在较大的成分梯度，导致界面两侧的微观组织和力学性能差异较大，界面往往成为该类接头蠕变早期失效的位置。本研究以COST E马氏体耐热钢和617B镍基合金作为母材，采用双丝电弧增材制造技术制造了一种新的梯度过渡接头(graded transition joint, GTJ)。对该GTJ进行了化学成分、微观组织和硬度分析，结果表明：相比常规DMWs，GTJ的化学成分梯度被大幅降低；从马氏体耐热钢侧到镍基合金侧，过渡区域的基体组织类型从淬火马氏体转变为马氏体+奥氏体两相共存，最后全部转变为γ镍基组织，硬度分布表现为在淬火马氏体区最高（最高约500 HV），进入两相共存区后急剧下降（低于200 HV），在γ镍基组织区域又缓慢增加（不高于250 HV）。使用动力学计算方法考查了两相共存区在凝固过程中的元素分配行为，发现Ni、Cr和Mo元素倾向于在枝晶界处偏析，降低枝晶界处的马氏体转变起始温度Ms，使得枝晶界处在室温时依旧为奥氏体组织，而枝晶芯部转变为马氏体组织。高温持久试验表明，该GTJ在马氏体耐热钢焊接热影响区和FGM中发生失效，其中位于热影响区中的失效受Ⅳ型开裂、氧化和塑性变形三种因素的共同控制，位于FGM中的失效归因于蠕变过程中局部奥氏体化引起的三向拉应力状态。