Mechanical Properties in the Physically Simulated Heat-Affected Zones of 500 MPa Offshore Steel for Arctic Conditions

Offshore steels for the arctic conditions have an increasing demand due to the opening of new oil fields in the Arctic Ocean. However, the requirements for these steels are extremely demanding, as they need to maintain the desired properties in harsh arctic conditions. Additionally, these requirements need to be achieved also in heat-affected zones caused by the welding. In this study the heat-affected zones were created using the physical simulation, so that the zones would be wide enough for reliable mechanical testing. Continuous cast 500 MPa offshore steel was hot rolled in the laboratory hot rolling mill to find out the mechanical properties of the base metal. The physically simulated heat-affected zones were studied using Gleeble 3800. Two different cooling times from 800 degrees C to 500 degrees C (t(8/5)) were used in order to simulate two different welding methods with different heat inputs. Microstructure of both base materials and simulated heat-affected zones were studied using scanning electron microscope and laser scanning confocal microscope. Charpy V-notch impact toughness and hardness profiles were determined of both base material and simulated heat-affected zones. The base metal microstructure was ferritic with some lath-like bainitic features. Minor changes were noted in the microstructure of physically simulated inter-critical heat-affected zone (ICHAZ), while in physically simulated coarse grained heat-affected zone (CGHAZ) the prior austenite grains had coarsened and the transformation microstructure consisted of lath-like features of bainite and in case of a shorter t(8/5) of 6 s, even martensite. It was found out that the critical location regarding the impact toughness in arctic temperatures was found out to be CGHAZ, while the impact toughness of ICHAZ did not differ remarkably from that of the base material. The CGHAZ impact toughness was weaker with t(8/5) = 30 s than with t(8/5) = 6 s indicating that lower heat input welding methods are more beneficial for this material.