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This paper investigated the metallurgical phenomena of overlapping zone between two laser spots and related effects of pulse frequency (3–15 Hz) on properties of welded joints by pulsed laser welding for DP 590 steel sheets. The welded surfaces were not continuous at 3 and 5 Hz and shallow penetrations, but welded joints with full penetration were obtained at 8–15 Hz, caused by increasing average power. The microstructure evolution of fusion zone (FZ) with pulse frequency increasing was: lath martensite → lath martensite and bainite → bainite and grain boundary ferrite. When pulse frequency was 3–8 Hz, overlapping zone was divided into four areas (FZ of former spot, super-critically HAZ (SC-HAZ ), inter-critically HAZ (IC-HAZ ), sub-critically HAZ (S-CHAZ )) wherein the microstructure changed with the increase of heat input. Hardness distribution of welded surface (3–8 Hz) was in zig-zag pattern and hardness dropped in S-CHAZ and IC-HAZ owing to the different microstructures. An increasing pulse frequency led to a hardness decrease in the cross-sectional of welded joints. For the full penetration of welded joints, tensile test revealed that strength of FZ was superior compared with base metal, and Erichsen value of welded joints could reach to 80% of the base metal when pulse frequency increased to 10–15 Hz. Keywords: Dual phase steel, Laser welding, Pulse frequency, Overlapping zone, Microstructure, Properties

The differences in microstructure and mechanical properties of laser beam welded (LBW) and gas metal arc welded (GMAW) joints of 800 MPa grade Nb-Ti-Mo microalloyed C-Mn steel of 5 mm thickness were studied. The study suggested that the microstructure in welded seam (WS) of GMAW was acicular ferrite and fine grained ferrite, whereas lath martensite (LM) was obtained in WS of LBW, where inclusions were finer and did not act as nucleation sites for acicular ferrite. The microstructure of coarse-grained HAZ (CGHAZ) obtained using the two welding methods was LM and granular bainite (GB), respectively. The original austenite grain size in CGHAZ of LBW was 1/3 of GMAW. The microstructure of fine-grained HAZ and mixed-grained HAZ using the two welding methods was ferrite and M-A constituents, while that of LBW was significantly fine. The hardness of LBW welded joints was higher than the base metal (BM), which was the initiation site for tensile fracture. The tensile fracture location of GMAW welded joints was in WS. The impact toughness of LBW welded joints was excellent and the impact absorption energy was similar to BM.

12Cr–Ni stainless steels containing different carbon contents from 0.004 wt.% to 0.034 wt.% were hot-rolled and air-cooled. Their corresponding microstructures were observed with optical microscope and transmission electron microscope, and the Vickers hardness, tensile and impact tests were also carried out. It was found that the martensitic morphology was significantly influenced by carbon content. The as-received ultra low carbon martensite in the steel containing 0.004 wt.% C is characterized by dislocation cells substructure. The formation of dislocation cells is attributed to high martensite finishing point (above 400 °C) and low interstitial atoms content. On the other hand, the martensite in the steel containing 0.034 wt.% C consists mainly of typical martensite laths because of low martensite finishing point and high interstitial atoms content which hinder dislocation motion. Furthermore, carbon content has an evident effect on the mechanical properties of 12Cr–Ni steels. The hardness and strength of the as-received steels increase with an increase in carbon content, but their elongation and impact toughness decrease with the carbon content. The steel containing 0.004 wt.% C has excellent impact toughness due to the ultra low carbon content in the martensite composed of dislocation cells.