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 Ｖ 斜めＹ型溶接割れ試験法による焼割れ的低温割れ発生の防止条件の検討

Abstract

The effect of preheating on quenching crack type cold cracking in HAZ of JIS SNCM439 and SK5 has been studied by the y-slit cracking test utilizing GTA welding with duplex stainless filler wire without the effect of diffusible hydrogen. Furthermore, the validity of the crack free condition which was proposed in the previous paper by the simulated cold cracking test under the various materials and cooling conditions has been also studied by use of the results of the y-slit cracking test.In SNCM439, it was noted that cracking do not occur at somewhat low preheating temperature, although the hardness and microstructure (martensite with some retained austenite) are nearly the same as that of without preheating. Therefore, it was found that cracking in this steel can be avoidable by adoption of lower preheating even in the range of cooling rate without bainite. In SK5, cracking has a tendency to decrease gradually with decreasing in hardness through the increase in formation of troostite and bainite by increasing preheating temperature.According to the crack free condition of the hardness less than about Hv600 or the fracture stress more than about 950 MPa suggested in the previous paper, it was confirmed that cracking do not occur under the condition of the restraint intensity 15 kN/mm⋅mm in the y-slit cracking test. Consequently by use of the relationship between the hardness and the fracture stress, the critical hardness in HAZ for cracking can be easily estimated for various restraint intensities in actual weldedjoint.