中，高炭素低合金鋼のＲＲＣ試験における焼割れの発生挙動 Ｉ 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

Abstract

In order to investigate the weld cold cracking of medium, high carbon low alloy steels which is considered to be caused by quench cracking, the fundamental behaviors of this type of cracking were studied by means of the RRC test. Namely, HAZ cracking in JIS SNCM439 (0.4C-1.8Ni-0.8Cr-0.3Mo steel) and JIS SK5 (0.8C plain carbon steel) was studied under the conditions of restraint intensity of about 9.8 to 29.4 kN/mm·mm without preheating utilizing GTA welding with austenitic filler wire to eliminate the effect of diffusible hydrogen. Main conclusions obtained are as follows:(1) General behavior of restraint stress after welding of these steels nearly agrees with that of weldable high strength steels in which martensitic transformation reduces restraint stress.(2) The last stage which gives the maximum or saturated restraint stress after welding is not seen even in the low restraint intensity of about 10 kN/mm.mm, because the cracking develops largely before the last stage.(3) Crack initiation estimated with AE technique occurs earlier with increasing the restraint intensity. The highest temperature of crack initiation, however, was about 35- K and 393 K in SNCM439 and SK5, respectively, meaning fairly lower than their Ms temperatures.(4) The stress at the crack initiation was about 400 MPa and 270 MPa in SNCM439 and SK5, respectively irrespective of the restraint intensity.(5) Crack initiation part has an intimate correlation with liquated grain-boundary in HAZ near fusion boundary, and gives very brittle intergranular fracture surface.