선박 ․ 해양구조물용 강의 용접금속 특성에 관한 연구

Abstract

The effect of main alloy element on the mechanical properties, charpy impact value and fracture toughness was investigated in the weld metal of YS 460 MPa steels for ship and marine structure. In addition, the characteristics of weld metal, when the weldments were heat treated post weld according to class society rules or AWS code, were investigated for YS 460MPa steel and FH Gr. steel ( LPG cargo tank) applying the flux cored wire developed through those effects. The results obtained from these study are as follows: On lower C content, strength and impact value were increased by reducing Si content, because of higher AF volume fraction. On higher C content, those properties were affected a little by Si content. These phenomena were result from that C affected phase transformation more than Si. CTOD value was indicated higher when Si content was lower and C was higher. This phenomenon was interpreted to come from lower volume fraction of not only PF and FS but also MA phase with a role of crack initiation site by higher C and lower Si contents. The strength of weld metal for YS 460MPa steel was decreased and elongation was increased by PWHT for 140min. at 600℃. These phenomena are resulted from the reduction of acicula ferrite volume fraction by grain growth. Also, Charpy impact and CTOD value were decreased by PWHT. These phenomena are resulted from grain growth as well. Because the grain boundary grown by PWHT can play a role as crack initiation site and make the crack propagate more easily. Although weld metal properties were decreased by PWHT, tensile and impact properties could meet the class societies requirements for welds of YS 460 MPa steel, but decrease of fracture toughness needs to be consider seriously. The effect of PWHT and its conditions ( such as peak temperature and holding time) on the tensile properties and impact toughness of the weld metal for FH Gr. steel(for LPG cargo tank) in relation to microstructure was investigated. On the basis of these studies it was found that PWHT conditions within all class societies affect charpy absorbed energy of weld metal little or no, all PWHT weld metals kept similar level of charpy absorbed energy as as-welded metal down-to -60℃ and finally indicated lower energy value than that of as-welded metal at -75℃. It is because the precipitation of 2nd phase was controlled from welding consumable and the grain size was grown by PWHT.