Change from a slowly propagating crack such as a fatigue crack or any other types of crack, which was initiated from a stress concentrated region or a weld defect, into a brittle crack at a critical crack length is very significant from a viewpoint of safety of welded steel structures. One of the authors succeeded previously by using the deep notch test specimens newly developed in evaluation of brittle fracture initiation characteristics for a high strength steel under only static tension, without superposition of any factors such as welding residual stress, impact stress, concentrated stress, impact stress, concentrated stress due to structural discontinuity, metallurgical change in notch ductility, etc. The stress-temperature relationships at fracture initiation for various crack lengths can be obtained by using 2 or 3 deep notch test specimens and applying the Griffith-Orowan energy condition. In this paper, the brittle fracture initiation characteristics for various kinds of steel, such as mild steel, WES-HW 36, 40, 50, 63, 70, 80, 90 or 60, 70, 80, 100 kg/mm2 high strength steels, 2.5, 3.5, 9% Ni steels and 33, 37, 58 kg/mm2 yield strength Al-killed quenched and tempered steels as the low temperature steel, which were lately manufactured by leading steel makers in Japan, were investigated.
In order to investigate the distributions of impact value in the heat affected zcne of welded parts of various steel plates, many tensile impact specimens of which diameter is very small were taken out of there. Using these specimens, tensile impact values were measured at every one mm from fusion line to the unaffected base metal. The used steels are mild steel, high tension steels, steels for low temperature use and steels for machine parts. These steel plates were butt welded by CO2-O2 arc welding process. The results obtained are as follows. 1. The decrease of tensile impact value in the heat affected zone corresponds to the hardening of that zone which was cooled rapidly after welding. The more the hardening of the heat affected zone, the more the decrease of tensile impact value becomes remarkable On a 80 kg/mm2 high tension steel and four kinds of steel for machine parts, the decrease of the impact value in the heat affected zone is remarkable. In comparison with the case of these hardenable steels, the decreases of impact value are not considerable on mild steel SS 41, 55 kg/mm2 and 60 kg/mm2 high tension steels, Mn-Si fine grained and 2.5%Ni steels. A 100 kg/mm2 high tension steel is softened by suffering welding arc heat, and impact values in the heat affected zone showed higher value than those of unaffected base metal. 2. The higher the carbon content of steel plates, the more the decrease of impact value and increase of hardness in the heat affected zone become considerable.
In the previous work, it was clarified that the decrease of tensile impact value of the heat affected zone of welded part corresponds to the hardening of that zone by rapid cooling after the welding. So, in this study, it was tried to increase the decreased impact values in the heat affected zone by post heating of the welded plates. In this preliminary study welded plates were heated at 20°C lower than A1 transformation temperature or at 20°C higher than A3 transformation temperature of each steels. As the results of the experiment the following facts were obtained. 1. On mild steel, 55 kg/mm2 and 60 kg/mm2 high tension steel, Mn-Si fine grained steel and 2.5% Ni steel, the distributions of Vickers hardness and tensile impact value of the heat affected zone were little changed by the post heating. 2. On 80 kg/mm2 high tension steel, the maximum hardness of the heat afiected zone was decreased nearly to the hardness of unaffected base metal by post heating, but impact values of that zone was increased. The hardness of base metal was also decreased, and its impact value was increased by post heating. 3. On the three kinds of steels for machine parts, ed. 0.45% C plain carbon steel, chromiummolybdenum and chromium steels, the heat affected zones were softened by post heating. Weld metal and base metal were also softened, and the hardness distribution in welded part became uniform. By means of post heating, impact value of the heat affected zone was also increased.
In the other report, tensile, bending, fatigue, and impact strength are tested on S20C and S45C welded by friction welding, and the result is that, under the properly selected welding conditions, they were almost the same as those of base metal except impact strength. Only Charpy impact strength largely descreased. This report is published to make clear the cause of its decrease. In the case of carbon steel as welded, the causes of its impact strength decrease are commonly considered as the grain growth of structure or the increase of hardness. In the normalized one, however, they have not been finally concluded though they can be supposed to be the faults in welds or to be the fiber structure. Seeing that under the properly selected conditions in the other mechanical tests the strength of welded steel is nearly the same as that of base metal, it is supposed that impact strength decrease was not due to the faults at welds, but has relations with the fiber structure of steel. Some tests are tried on this point. The result is that, when the test pieces cut off at right angles to the rolling direction of steel plate are welded, and also when those of cast steel are welded, they have the same impact strength as that of base metal after the heat treatment. From this result one can see that decrease of Charpy impact strength in friction welding is due not to the faults in welding, but to the fact that the fiber structure is bended in welding. It being due to the fiber structure bended by the axial pressure on the course of welding, it is impossible to remarkably prevent its decrease by improving welding conditions, and one must be contented with getting only the same strength or so as is gotten in lateral flowing direction of steel. As for tensile impact strength, it is considered that its change in friction welding due to flowing direction of steel. But as it is so little, the decrease of impact strength in this case is not so remarkable nor worthy to be discussed.