Underclad cracking in nuclear pressure vessels was of significant concern in the 1970s and 1980s before mitigating adjustments were made both to steel compositions and manufacturing practice. Unfortunately, the cracking mechanisms are still not well understood, and this can undermine confidence when changes to cladding operations are under consideration. In this work, Charpy-sized test coupons of 18MND5 steel were subject to a range of thermal cycles that can be experienced by the substrate immediately adjacent to the interface with the overlay. The Charpy test results are considered in combination with analysis of the samples through field emission gun (FEG) scanning electron microscopy (SEM) and wavelength dispersive X-ray spectroscopy (WDXS). The findings suggest that the heat affected zone subject to the coarse grained plus intercritical thermal cycle, before post weld heat treatment, has a peculiar and potentially brittle microstructure. However, in a steel with no obvious macrosegregation, no clear evidence of intergranular cracking in the early stages of the post weld heat treatment could be found. The second part of the work focuses on the effects of segregation regions in large forgings, more specifically ghost lines, on the tendency for underclad cracking. Ghost line regions that subsequently form a coarse grained heat affected zone (CGHAZ), and are then heated to just under the temperature, seem to be the most prone to intergranular cracking. Cracking susceptibility appears to be associated with elevated concentrations of alloying and impurity elements at grain boundaries.

中文翻译：

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反应堆压力容器下包层热影响区的物理模拟研究晶间裂纹

在对钢成分和制造实践进行缓解调整之前，核压力容器的下包层裂纹在 20 世纪 70 年代和 80 年代备受关注。不幸的是，裂纹机制仍然没有得到很好的理解，这可能会损害在考虑改变熔覆操作时的信心。在这项工作中，夏比尺寸的 18MND5 钢测试试片经受了一系列热循环，紧邻覆盖层界面的基材可以经历这些热循环。夏比测试结果与通过场发射枪 (FEG) 扫描电子显微镜 (SEM) 和波长色散 X 射线光谱 (WDXS) 对样品进行的分析相结合。研究结果表明，在焊后热处理之前，受到粗晶加临界热循环影响的热影响区具有特殊且潜在的脆性微观结构。然而，对于没有明显宏观偏析的钢，在焊后热处理的早期阶段没有发现明显的晶间裂纹证据。该工作的第二部分重点研究大型锻件中偏析区域（更具体地说是鬼线）对下包层裂纹倾向的影响。随后形成粗晶热影响区 (CGHAZ) 的鬼线区域，然后加热到略低于该温度，似乎最容易产生晶间裂纹。裂纹敏感性似乎与晶界处合金元素和杂质元素浓度升高有关。