Dissimilar metal weld failures between low alloy Cr-Mo ferritic steels and austenitic stainless steels made with Ni-base filler metals are typically observed along the fusion line. Such DMW failures often exhibit the onset of damage well before their expected service life. Failure is typically associated with a carbon-depleted region in the ferritic steel and formation of creep voids along a row of so-called Type I carbides. More recently, the formation of Type I carbides adjacent to a carbide-free ferrite band has also been observed in DMWs where the ferritic steel was a 9 pct Cr creep strength-enhanced ferritic steel. However, it has not been completely clarified whether these microstructural features formed during welding, post-weld heat treatment or service. In this study, single pass bead-on-plate welds were prepared on a Grade 91 steel substrate using commonly specified Ni-base weld metals; ENiCrCoMo-1, ENiCrMo-3, ENiCrFe-3, ENiCrFe-2 and EPRI P87 (ENiFeCr-4) and were characterized in the as-welded, post-weld heat treatment (PWHT) and aged conditions. The evolution of the ferrite band was found to form by a diffusion-controlled process and was observed after PWHT and aging associated with regions exhibiting steep concentration gradients in the partially mixed zone (PMZ). In this study, steep concentration gradients were observed along the toe of the weld whereas relatively shallow gradients were observed at the weld bottom-center, resulting in a discontinuous ferrite band along the fusion line during PWHT and aging. The variation in chemical gradients in the PMZ corresponded to different chemical potential gradients, which lead to differences in carbon diffusion during high-temperature PWHT and aging. The results of this study provide insight into potential solutions for minimizing the risk of failure in future applications.

通常沿熔合线观察到低合金Cr-Mo铁素体钢与用Ni基填充金属制成的奥氏体不锈钢之间的金属焊接失败。此类DMW故障通常在其预期使用寿命之前就已经显现出损坏的开始。失效通常与铁素体钢中的碳贫化区域以及沿一排所谓的I型碳化物的蠕变空隙的形成有关。最近，在铁素体钢为9 pct Cr蠕变强度增强的铁素体钢的DMW中也观察到了与无碳化物铁素体带相邻的I型碳化物的形成。但是，还没有完全弄清这些微结构特征是在焊接，焊后热处理或维修过程中形成的。在这个研究中，使用通用的镍基焊接金属在91级钢基底上制备单道镀层焊道焊缝；ENiCrCoMo-1，ENiCrMo-3，ENiCrFe-3，ENiCrFe-2和EPRI P87（ENiFeCr-4）的特性在焊接后，焊后热处理（PWHT）和时效条件下进行了表征。发现铁氧体带的形成是通过扩散控制的过程形成的，并且在与部分混合区（PMZ）中表现出陡峭浓度梯度的区域相关的PWHT和时效之后可以观察到。在这项研究中，沿着焊缝的趾部观察到陡峭的浓度梯度，而在焊缝底部中心观察到相对较浅的梯度，在PWHT和时效过程中沿着熔合线形成了不连续的铁素体带。PMZ中化学梯度的变化对应于不同的化学势梯度，导致高温PWHT和时效过程中碳扩散的差异。这项研究的结果提供了对潜在解决方案的见识，以最大程度地减少将来应用中的故障风险。