The weld thermal cycle results in significant changes in microstructure and, consequently, mechanical properties of the weld heat affected zone (HAZ). In this paper, hardness variations across the HAZ and for different welding heat inputs (0.5–2.5 kJ/mm), obtained in a Grade 100 microalloyed steell, are explained based on the microstructural observations. Micro- and nano-hardness examination provided hardness profiles across the HAZ and nano-distribution of hardness in each HAZ sub-region, respectively. Optical microscopy and transmission electron microscopy (TEM) were used for evaluation of grain size, phase structure and precipitate type, shape and distribution. Both carbon replicas and thin foils (prepared by focused ion beam technique) were used for TEM. The fine-grained HAZ for all heat inputs was primarily composed of polygonal ferrite, with some regions of twinned martensite in the higher heat input (1.5 and 2.5 kJ/mm) samples. Twinned martensite regions were also identified in the coarse-grained HAZ of the 0.5 kJ/mm sample. Grain size changes were the major cause for the variation of hardness in the fine-grained HAZ; however, large packets of bainitic ferrite and martensite in the coarse-grained HAZ, with small ferrite/martensite laths, were responsible for the relative hardening in the coarse-grained HAZ.

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