湿式水中アーク溶接に関する研究(第2報)

高張力鋼溶接部の衝撃値および疲労強度について

抄録

The knowledge of notch toughness of welded joint-parts is very important for safety of welding structure. The welded parts have metallurgical discontinuity such as weld metal, bond and heat-affected zone, etc.; the Charpy impact test is often used for the valuation of notch toughness for the parts.
It is well known that the fatigue strength of welded parts reduces to less than that of base metal. There are many factors reducing their fatigue strength: for example, geometrical effect of reinforcement, residual stresses and microstructural change in the heat-affected zone.
In this paper, welding experiments are done by using HT60 steel plates and D5003 covered electrodes, and the specimens taken from welded joints in air and in underwater are examined by the impact and the fatigue tests. The results are the following:
(1) In the impact test, the fracture transition temperature does not affect the oxygen and nitrogen absorption, but it does influence the hardness of the notch zone; the lower the hardness goes down, the lower the transition temperature does.
(2) In air and underwater welding, the fatigue fractures occur in the weld metal; the fatigue limit of the joint parts is about 15 kgf/mm² because of hydrogen embrittlement and blowholes in underwater welding, but 23 kgf/mm² in air welding.
(3) The micrography of the fatigue fracture surface shows mainly quasi-cleavage fracutre pattern of hydrogen embrittlement in underwater welding, and dimple fracture pattern in air welding.
(4) In underwater welding, the structure in the neighborhood of the fatigue fracture consists of the ferrite and the pearlite, and the needle-like ones of the ferrite are of the widmanstatten-type. On the other hand, in air welding, it consists of the bainite where the columnar structure is made finer by the effect of the heat treatment due to the multi-pass welding.