The Effects of Heat Input on Tensile and Fatigue Properties of Submerged Arc Welded ASTM A709 Grade 50 Steel

The purpose of this study was to investigate the effects of heat input on tensile and fatigue properties of submerged arc welded American Society for Testing and Materials (ASTM) A709 Grade 50 steel, which was supplied as plates with dimensions of 500 mm × 250 mm × 20 mm by Hitachi Canadian Industries (HCI) Ltd., Saskatoon. Identical heat input (2.75 kJ/mm) was used to weld one side of the plates, while various heat inputs (single wire: 3.03 kJ/mm and 3.43 kJ/mm; tandem wires: 4.11 kJ/mm and 4.56 kJ/mm) were used for the second side. The weld properties were evaluated using visual inspection, ultrasonic inspection, hardness measurements, tensile and fatigue testing, weld bead geometry measurement, optical microscopy and scanning electron microscopy. The results obtained showed that heat input affected the microstructure, hardness and weld bead geometry of ASTM A709 Grade 50 steel welds appreciably, but had little effect on the tensile and yield strengths. Increase in heat input slightly reduced the amount of acicular ferrite within the weld metal zone, and resulted in coarser microstructures in the coarse-grained heat-affected zone. The hardness of the coarse-grained heat-affected zone and the weld metal zone increased with increasing heat input. The penetration depth, heat affected zone size, bead width, bead reinforcement, penetration area, and nugget area of the weld increased with increasing heat input for both single wire and tandem wire welding, but the contact angle decreased. Electrode melting efficiency increased with increasing heat input for single wire welding, but the plate melting efficiency only changed within 4% between single wire and tandem wire welding. Percent dilution remained practically unchanged with increasing heat input. In general, the fatigue strength of the welds was slightly higher than that of the parent metal at room temperature. The fatigue strength of ASTM A709 Grade 50 steel and its welds increased slightly at -20°C and -30°C compared to room temperature. At room temperature, the welds showed slightly higher fatigue strength than the parent metal