溶着金属の切欠靱性におよぼすTiおよびZrの影響(第1報)

抄録

Recently, with the progress of a large scale construction, various welding methods for heavy steels have been developed and utilized. Most of them, however, use a large heat input in one pass welding, so that the cooling rate of deposit metal is very slow and the deterioration in notch toughness based on growth of the crystal grain hoses a problem.
Although notch toughness of deposit metal is improved by full annealing after welding, addition of some alloy elements and application of supersonic vibration for the refinement of crystal grains under as-welded condition have been tried so far. But it is considered that the latter is limited in the range of application and grain refinement effects, while further examinations are required for the former.
Seeing that macro-and micro-structures of deposit metal closely resemble that of cast steel, the effects of Ti and Zr on the micro-structure and impact value of cast steel were investigated to make some indirect considerations about the effects of those elements on the notch toughness of welding deposit metal.
Both of Ti and Zr have acted as nucleus in the solidification of molten metal and have refined austenite grain. But, under addition of Ti, the hardness of ferrite grain remarkably increased and the impact value not improve. On the other hand, as the result of addition of Zr which was a considerable refining elemement as austenite grain little increased the hardness of ferrite grain and the impact value markedly rose up to 0.04% Zr.
As the average size of the cleavage fracture facets observed in impact fracture face corresponds to that of austenite grains, it is concluded that the improvements of impact value through addition of Zr is attributed to the refinement of austenite grains without increasing the hardness of micro-structure.