Feeling Free Despite LBZ

Abstract

Abstract It is sometimes argued that the occurrence of local brittle zones (LBZs) is a new phenomenon relevant only to modern low-carbon steels, while others claim that such LBZs are also present in the heat-affected zones of the C-Mn steels of the late sixties/seventies. In order to elucidate on this matter, both an “old” normalized (N) steel (1970s) and two modern low-carbon types of steel, normalized (N) and controlled rolled-accelerated cooled (CONRAC) have been investigated and compared with each other. In addition, consideration has been given to the correlation between the level of heat input during welding and the occurrence of local brittle zones. This has led in practice to a restriction of the heat input to 3.5 kJ/mm maximum for the modern low-carbon normalized steel investigated, which was used in recent projects. Furthermore, the engineering significance of LBZs is considered in the light of design aspects. It is concluded that there is little reason for concern with respect to a detrimental effect of LBZs in tubular connections of offshore structures, which is supported by a satisfactory performance of the latter to date.