Characterization of Mechanical Properties of Welds of Q355D Steel by DSI Technique

Temperature variation during the welding will cause different solid-state phase transformations in the welding area and result in a heat-affected zone (HAZ) between the weld metal (WM) and the base metal (BM). Depth-sensing indentation (DSI) is a doable technique to characterize mechanical properties of materials in the HAZ, which is usually not big enough for the fabrication of coupons for traditional tension tests. The basic idea of DSI technique is to push a hard tip into the surface of the material and then reveal its mechanical properties based on the response of the material. In this study, mechanical properties of butt welds of Q355D steel were investigated with DSI tests and reverse analytical approach. Different welding zones, including BM, subcritical HAZ, fine-grain HAZ, coarse-grain HAZ, top pass of WM and middle pass of WM, were distinguished by microstructures observed by scanning electron microscope (SEM). DSI tests were conducted on these welding zones. Mechanical properties were characterized based on load-penetration depth ($P\text{-}h$) curves obtained by DSI tests. These mechanical properties were adopted in finite-element (FE) simulation of DSI tests. $P\text{-}h$ curves obtained by FE simulation agree well with results of DSI tests for all welding zones, especially for the loading curves. The agreement validates the applicability of DSI technique in characterizing mechanical properties of materials. Tension coupon tests were conducted on the BM and the middle pass of WM to verify the accuracy of the DSI technique. Variation of mechanical properties across the welding area were discussed in accordance with microstructures of materials in different welding zones.