極厚高張力鋼のHAZ横割れについて

抄録

On the multipass welds in heavy plates of high tensile strength steels, various types of cracks occur, namely, root cracks, toe cracks and underbead cracks at the heat affected zone and transverse cracks in the weld metals. Extensive studies have been devoted to the factors causing these cracks and the methods of preventing them.
Recently, a new type of cracking has been detected. These cracks occur transverse to the welds in segregated areas of the beat affected zone. So it is considered that the factors and counterplans have to be newly developed.
In this paper, testing methods which simulate actual welding conditions were used. And a U-groove restraint test specimen, which was partially dipped into water, was used as a standard test condition. From this severe condition, the required postheating time and temperature were investigated. Various types of steels were tested with these conditions.
The main results are as follows;
1. The transverse crack at the heat affected zone initiates at segregated regions in the heat affected hardened layer and propagates in the hardened layer of one pass of the multipass weld by hydrogen and residual stress.
2. The occurrence of the cracks is related to the carbon equivalents of the plates, including segregation. The crack occurred in plates over 0.45-0.50% in carbon equivalent, when preheating at about 100°C.
3. Where the plate is 100 mm in thickness and 0.57-0.58% in carbon equivalent, and the heat input is 45 KJ/cm by submerged arc weld, the preheating of 125-150°C and the postheating of 250°C×4h is required to prevent the crack.