Stress relaxation cracking (SRC) is considered one of the major failure mechanisms for 347H stainless steel welds at elevated service temperatures or during post weld heat treatment (PWHT), especially within the heat-affected zone (HAZ). This work focuses on the characterization of SRC susceptibility within 347H physically simulated arc welded HAZ at elevated temperatures. A four-step SRC thermomechanical test in combination with finite element modeling (FEM) of the welding and testing processes is developed to establish a susceptibility map for HAZ. The test first runs a thermal cycle with three different peak temperatures (1335, 1275, and 1150 °C) to duplicate representative HAZ subzone microstructures, followed by time-to-failure examination under a variety of pre-stress (260–600 MPa) and pre-strain conditions (0.03–0.19) as a function of reheat temperatures between 750 and 1050 °C. With the aid of FEM, SRC susceptibility maps are generated to identify the threshold stress, plastic strain, and creep strain as a function of test temperature. It was found out that HAZ subzone with a lower peak temperature (1150 °C) appears to be slightly less susceptible to SRC than the other two subzones that experienced higher peak temperatures. Generally, time-to-fracture reduces with increasing initially applied stress and strain for all test temperatures. The pre-stress thresholds decrease from about 500 to 330 MPa as the testing temperature increases from 800 to 1050 °C, while the corresponding initial plastic strain thresholds reduces from 0.15 to 0.06. The SRC susceptibility was also evaluated through the Larson–Miller Parameter (LMP) analysis as a function of plastic strain, initial stress and starting stress upon reaching the testing temperature, respectively. The 1050 °C test with a high pre-applied strain (0.1) exhibits an extremely short time to failure (t = 3 s) that lies outside the general trend in LMP analysis. Additionally, it was identified that a plastic strain above 0.07 is identified to significantly reduce the bulk creep strain tolerance to fracture and therefore increases SRC susceptibility. Hardness measurement and fractography analysis indicated that the strain aging of niobium carbonitrides and other potential phases in conjunction with intergranular precipitates contributes to an increase in microhardness and increased intergranular cracking susceptibility.

中文翻译：

---

347H不锈钢电弧焊缝应力松弛裂纹：热影响区敏感性评价

应力松弛裂纹 (SRC) 被认为是 347H 不锈钢焊缝在高温下或焊后热处理 (PWHT) 期间的主要失效机制之一，特别是在热影响区 (HAZ) 内。这项工作的重点是表征高温下 347H 物理模拟电弧焊接 HAZ 内的 SRC 敏感性。四步 SRC 热机械测试与焊接和测试过程的有限元建模 (FEM) 相结合，旨在建立 HAZ 的磁化率图。该测试首先运行具有三个不同峰值温度（1335、1275 和 1150 °C）的热循环，以复制代表性 HAZ 分区微观结构，然后在各种预应力 (260–600 MPa) 下进行失效时间检查和预应变条件 (0.03–0.19) 作为 750 至 1050 °C 之间再热温度的函数。借助 FEM，生成 SRC 磁化率图，以识别阈值应力、塑性应变和蠕变应变作为测试温度的函数。结果发现，峰值温度较低 (1150 °C) 的 HAZ 子区域似乎比其他两个峰值温度较高的子区域更不易受 SRC 影响。一般来说，对于所有测试温度，断裂时间随着初始施加的应力和应变的增加而缩短。随着测试温度从800℃升高到1050℃，预应力阈值从约500MPa降低到330MPa，而相应的初始塑性应变阈值从0.15降低到0.06。还通过拉森-米勒参数 (LMP) 分析评估了 SRC 敏感性，分别作为塑性应变、初始应力和达到测试温度时的起始应力的函数。具有高预施加应变 (0.1) 的 1050 °C 测试表现出极短的失效时间 (t = 3 s)，这超出了 LMP 分析的总体趋势。此外，还发现塑性应变高于 0.07 会显着降低整体蠕变应变断裂耐受性，从而增加 SRC 敏感性。硬度测量和断口分析表明，碳氮化铌和其他潜在相的应变时效与晶间析出物一起导致显微硬度的增加和晶间裂纹敏感性的增加。