Low alloy high tensile steels are of limited use in welded structures under alternating load, because welded joints in those steels have not greater fatigue strength than similar joints in mild steels. It has been believed that a metallurgical factor is responsible for the low fatigue strength of the welded joint. Nevertheless it has been made clear recently that all metallurgical microstructures in the heat-affected zone (HAZ) of welds are not weak to fatigue and not very notch-sensitive. Only in the welded joint of larger heat-input than the proper welded condition for the steel, the softening zone of HAZ may decrease the fatigue strength by a few kg/mm2. In order to examine the effect of the external geometry of welded joints, investigations were carried out using simulated weld reinforcement specimens. In these previous papers, the external geometry of reinforcement was not carefully observed, so that the effect of a very small toe radius upon the fatigue strength has been ignored. A reduction in the fatigue strength as large as 70% remained unclarified. The present research was devoted to studying the effect of the external geometry of the reinforcement on fatigue strength. For this purpose, a careful observation of the external geometry of the toe of the reinforcement was performed. It was found that the acute flank angle θ ranged from about 20 to 60 deg and the radius of curvature R at the toe of the reinforcement ranged from about 10 to 0.02 mm. Fatigue tests under pulsating tension were conducted on simulated butt-welded specimens of weldable high tensile steel. The profile of the weld reinforcement was controlled by varying θ and R. How the geometry of the reinforcement toe influences the fatigue strength of the welded joint has been revealed. The stress concentration factor at the toe of the reinforcement with 60 deg flank angle and 0.02 mm toe radius, affecting the fatigue crack initiation, was deduced from photo-eastic experiments and determined to be 3.5 by the finite element method. The fatigue notch factor of the mild steel was 1.9 and that of 80 kg/mm2 class high tensile steel was 3.2 for the stress concentration factor of 3.5. From these values, the butt-welded joint in these steels can be explained well. The main reason for the very poor fatigue strength of the welded joint is the stress concentration associated with the severe discontinuity of the external geometry at the toe of the weld reinforcement.
Study was made on the effect of alloying elements on notch toughness of duplicated weld heat-affected zone heated up to 1350°C corresponding to the thermal cycles of weld bond produced with larger heat input. Effect of alloying elements is almost similar to that clarified by the study of single-pass submergedarc weld bond. Increased carbon decreases notch toughness of duplicated weld heat-affected zone, while nickel, manganese and chromium improve notch toughness. Increase of aluminium up to 0.16% improves notch toughness of single-cycle duplicated weld heat-affected zone contrary to the tendency in submerged-arc weld bond, which is assumed to be free from the weld metal effect. Faborable effect of aluminium on notch toughness is presumed to depend on combining effect of decreasing free nitrogen and inhibiting austenite grain growth in heat-affected zone. However, the notch toughness of multi-cycle weld heat-affected zone has suitable range of aluminium, 0.07% approximately. Small quantities of vanadium and titanium have little effect on notch toughness of duplicated weld heat-affected zone. Summing up a series of studies on single-pass submerged-arc weld bond and duplicated weld heat-affected zone, 0.08% C-0.4% Si-1.3% Mn-Ni-1.0% Cr-0.4% Mo-0.05% V-0.15% Al type steel quenched and tempered to 80 kg/mm2 tensile strength level is amenable to large heat input welding such as submerged-arc process.
The reason why most of the coated electrodes can give a sound weld metal.even under water but low hydrogen type electrode and semi-automatic CO2 welding are apt to produce blowholes in an underwater weld metal is investigated in this report by oscillograms, chemical analysis of gases in blowhole and the configuration, the microstructure and the hardness of deposited bead made under water as compared with those in air. Results are as follows; 1) Arc of ilmenite, high titania, lime titania, high iron oxide-iron powder and high cellulose type electrodes is stable and their droplet transfer in arc is spray type, while underwater arc of low hydrogen type electrode and semi-automatic CO2 welding has a considerably long arc-disappearing time on oscillogram and it seems difficult to reignite arc. 2) Main component of blowhole gas in an underwater deposit is hydrogen, which is very different from the case of usual welding in air. 3) Microstructure of the heat affected zone of an underwater weld of mild steel is bainitic-martensitic and its hardness is very high. 4) The cause of blowhole formation by low hydrogen electrode and CO2 welding is probably attributed to their insufficient reignition of arc on a cold base metal under water, which makes solidification rate of weld metal so extraordinarily fast that a large amount of hydrogen produced by arc under water can not escape from molten metal.