Laser beam welding of quenched and tempered ASTM A514 Gr.B Steel

Quenched and tempered steels can combine high yield and tensile strengths with good notch toughness, ductility and weldability. These steels are commonly utilized on welded constructions and are particularly convenient whenever the increase of mechanical resistance gives rise to a remarkable reduction of weight. As well known, toughness and soundness of these steels can be affected by welding operations, which induce metallurgical modifications through the heating cycles. In general it is recognized that the slower is the cooling rate during welding operations, the deeper is the alteration and the reduction of toughness; consequently welding process with controlled heat input are preferred. Laser Beam Welding (LBW) process, among others, offers low thermal input and is established as joining technique in several manufacturing sectors; a broad field of utilization is covered, for instance in the automobile and in the shipbuilding industry. The expected advantages of LBW technique are the improved mechanical properties due to reduced fusion zone and heat affected zone (HAZ), less distorsion and residual stresses, easy automation of welding operations. In this paper, a preliminary investigation is done on bead-on-plate welds on steel ASTM A 517 Gr.B, focusing the alteration of microstructural and mechanical properties. Welds were obtained with LBW process and, for comparison, with manual arc process (SMAW). Welds with LBW was carried out without any filler metal, whereas a consumable electrode of proper composition was utilized with SMAW. Experimental work included macrographic examination, optical metallography and microhardness survey. The results show the formation of a mixed bainitic-martensitic structures in the Heat Affected Zone (HAZ) with both welding processes, due the relatively high cooling rate of the re-austenitized region close to the melted zone. LBW technique allows an advantageous reduction of areas of both fusion and heat affected zones, if compared to SMAW process, giving rise to less distortion and residual stresses. The HAZ is about 0.35 mm wide with LBW process, whereas is about 1.5 mm wide with SMAW process. In spite of higher cooling rates typical of LBW process, hardness values are of the same order of magnitude in both cases.