In-situ neutron diffraction was employed in the present investigation to study the effects of holding time on the relaxation of residual strains during Post-Weld Heat Treatment (PWHT) for multi-pass High-Strength Low-Alloy (HSLA) steel welds. Different holding time intervals (0.5, 1 and 3 h) were utilized to evaluate and monitor the residual strain relaxation during PWHT. It was found that the holding time has little effects on the strain relaxations as strain relaxations occurs predominantly during the reheating stage of the heat treatment (~80% of strain relaxation). A similar trend is observed for all three holding time arrangements with the high strain relaxation during the reheating stage followed by linear strain relief may confirm the hypothesis that creep (primary and secondary) is responsible for strain relaxation during PWHT. It further confirms the selected soaking temperature of 600 °C has a significant effect on the stress relaxation.

This finding is in line with the microstructural characterization studies indicating the formation of sub-grains through polygonization in the Heat Affected Zone (HAZ) and Weld Metal (WM) of the heat treated joints. Ex-situ neutron diffraction which was conducted after PWHT confirmed the in-situ neutron diffraction results and showed slightly higher level of residual stresses for the heat-treated specimen with 1/2 hour holding time (~32% of yield strength of the WM). Furthermore, the tensile, hardness and Charpy impact test results support the notion that a shorter holding time is more beneficial, not only for the cost issues but also for the microstructural and mechanical properties of the welded joints. The findings of this study can be used to optimize the current PWHT codes and standards. It can also be used for the validation studies of the finite element modeling of this process.

本研究采用原位中子衍射来研究保持时间对多道高强度低合金 (HSLA) 钢焊缝焊后热处理 (PWHT) 过程中残余应变松弛的影响。不同的保持时间间隔（0.5、1 和 3 小时）用于评估和监测 PWHT 期间的残余应变松弛。发现保持时间对应变弛豫几乎没有影响，因为应变弛豫主要发生在热处理的再加热阶段（~80% 的应变弛豫）。对于所有三种保持时间安排都观察到类似的趋势，在再加热阶段具有高应变松弛，然后是线性应变释放，这可以证实蠕变（主要和次要）是 PWHT 期间应变松弛的原因。

这一发现与微观结构特征研究一致，表明在热处理接头的热影响区 (HAZ) 和焊缝金属 (WM) 中通过多边形化形成亚晶粒。PWHT 后进行的非原位中子衍射证实了原位中子衍射结果，并显示热处理试样的残余应力水平略高，保持时间为 1/2 小时（WM 屈服强度的 32%） . 此外，拉伸、硬度和夏比冲击测试结果支持较短的保持时间更有益的观点，不仅对成本问题，而且对焊接接头的微观结构和机械性能也是如此。本研究的结果可用于优化当前的 PWHT 规范和标准。