Low-temperature toughness properties of 500 MPa offshore steels and their simulated coarse-grained heat-affected zones

Abstract

Offshore steels for cold climate conditions require not only relatively high strength to improve the cost-efficiency of the structures, but also excellent toughness at low temperatures to guarantee the safety of the structures in harsh environments. The most challenging locations to fulfil both requirements are in the welded joints of these steels because the weld thermal cycles cause irreversible changes to the steel microstructure. Titanium and niobium are often used in these steels to form nanoscale precipitates and reduce the austenite grain growth during the thermal cycle. However, there is a risk that in addition to the nanoscale precipitates formed from the solid state, microscale coarse inclusions are also formed from the melt. These inclusions are hard and brittle and thus likely to deteriorate mechanical properties such as fracture toughness and impact toughness. In this study, two experimental 500 MPa offshore steels with different carbon contents and with differing inclusion structures were studied to find out whether microscale titanium-based nitrides significantly deteriorate the impact and fracture toughness of the coarse-grained heat-affected zone (CGHAZ) of these steels at low temperatures. A Gleeble 3800 was used to simulate the CGHAZ with two different cooling rates, the fracture toughness was determined by three-point bend testing, and the impact toughness was determined by Charpy V-notch testing. The study was limited to single-pass welding scenarios. Inclusions were characterized using FESEM-EDS. It was found that both impact and fracture toughness of the steel with coarse titanium-based nitrides were lower and the scatter higher than in the steel with a higher quantity of calcium-based inclusions and without the coarse nitrides. Fractographic examination showed that the failure in the samples with the lowest toughness was initiated by the coarse titanium-niobium nitrides. Thus, to avoid CGHAZ brittleness under cold conditions, it is necessary to mitigate the formation of coarse nitrides by careful control of steelmaking process and continuous casting to avoid the segregation of titanium, niobium and nitrogen.