焼割れ的低温割れ感受性に及ぼす冷却速度の影響および割れ防止条件の提案

中,高炭素低合金鋼の溶接熱影響部における低温割れの研究(第4報)

抄録

The previous paper revealed that quenching crack type cold cracking in HAZ of medium/high carbon low alloy steels can be suppressed by the reduction of hardness and phosphorus, which are the main influencing factors on the cracking. Therefore, in this paper, the changes in hardness, intergranular fracture surface and fracture stress under the various cooling conditions have been studied in detail by the simulated cold cracking test. From the results of this investigation, a criterion has been proposed to prevent this type of cold cracking.
The fracture stress has an excellent correlation with the area fraction of intergranular fracture surface. The fracture stress giving little intergranular fracture surfece is more than about 950 MPa. The hardness has also a good correlation with the area fraction of intergranular fracture surface. The hardness giving little intergranular fracture surface is less than about Hv600. The area fraction of intergranular fracture surface within Hv600 to 700 is largely influenced by cooling rate, i.e. that is fairly low at lower cooling rate, although the hardness is nearly the same compared with the higher cooling rate. Therefore, there is a good correlation between hardness and fracture stress; however, within Hv600 to 700, the fracture stress is fairly high at lower cooling rate because of the effect of cooling rate due to the relation of the hardness and the area fraction of the intergranular fracture surface. By adoption of the condition of the hardness less than about Hv600 or fracture stress more than about 950 MPa, it was thought that quenching crack type cold cracking is avoidable.