Abstract
The carbon equivalent of heavy thick plate of high strength steel is usually high in order to obtain sufficient strength, and the effect of the segregation is notable due to a small reduction ratio. Moreover, there are two disadvantages in heavy thick plate; firstly, high three dimensional residual stresses develop during welding; secondly, hydrogen accumulates due to the difficulty of its emission. So the precautions against various kinds of cracking during welding are required. The same factors exist when practising gas cutting for edge preparation before welding. Therefore, it is probable that the edge crack due to gas cutting occurs in heavy thick plate of 50 kgf/mm2 class high strength steel.
In this paper, the features and the factors affecting edge cracks during edge preparation by gas cutting were studied in the case of heavy thick plate of 50 kgf/mm2 class high strength steel.
The main results are as follows;
1. Hydrogen gets into the heat affected zone from environment during gas cutting. Reducing the cutting speed or increasing the hydrogen atomic ratio of the cutting gas enables more hydrogen to enter the heat affected zone and this increases the risk of cracking.
2. The tempered martensite embrittleness due to heat cycle by bevel cutting assists hydrogen induced edge cracking.
3. In case of the steel of 0.45% carbon equivalent and 145 mm thickness, the crack occurred at the segregated portions in plate caused by both hydrogen and tempered martensite embrittleness.
