SM520B 강의 예열온도 변화에 따른 용접부 균열에 관한 연구

A Study on Welding Crack of SM520B Structural Steel Caused by Adjustment of Preheat Temperature

It is easy to initiate the crack at weld zone because thick plate is relatively faster on cooling rate and is higher on confining stresses than thin plate, thus steel materials are becoming thick plate with structures have become bigger along with industrial development. Cold crack is divided into the transverse crack occurred inside weldments and the longitudinal crack induced at heat affected zone, which is comes from diffusible hydrogen, remained stress, susceptible microstructure. It is well known that the susceptible microstructure such as martensite, lower-bainite is overly sensitive to hydrogen assisted cracking in microstructure aspects. In this study, SM 520B steels which are used at construction were butted welded to make comparative study physical properties at welds with variation of preheating temperature. As a result, the preheating effect was a much safer than when no preheating in many ways. In case of no preheating, microstructure is more likely to form bainite which lowers toughness in base metal and weld zone due to fast cooling rate, thus hardness value is much higher as 324Hv compare to 219 Hv when preheating. Moreover, the transverse crack which judged typical cold crack not only generated on surface of weld-bead about 14 to 20mm in length but formed hardening structure at weld.