Properties of Steels and Weldments for Low Temperature Applications
Authors: Karlsen M. , Kulbotten I.M. , Bakken K.A. , Stiklestad E. 
Published: December 2011

The study involves Charpy V and CTOD testing and microstructure examination of a 50mm thick 0.05C–1.3Ni type S420 HSLA steel and associated weldments. The testing plates were welded using fabrication practice welding with normal welding equipment to simulate realistic working conditions. The plates were welded using two heat input conditions; 3.5 kJ/mm of submerged arc welding (SAW) for test panels A and B, and 1 kJ/mm of flux cored arc welding (FCAW) for panels C and D. Different welding consumables were applied for each test panel. The tests revealed excellent impact toughness properties of the base material with minimum values above 230 J down to −80°C. Moreover, acceptable results were obtained in the fusion line and weld metal for all panels tested at -40°C. At -60°C the Charpy values from the fusion line were all above 52 J and the CTOD values, except one for panel A, were above 0.2 mm. However, a significant scatter in properties was observed in the weld metal. In general, a marked drop in impact toughness was seen in the root compared to the CAP, which may be attributed to the welding consumables applied and the harsh weld configuration. Moreover, the highest CTOD properties were achieved from the two panels that were welded using the lowest heat input. The study has shown that acceptable properties of welded HSLA steels can be achieved at -60°C for certain weld combinations, even though the welding has been carried out under severe conditions. However, it was disclosed that future efforts should focus on improving weld metal toughness to enlarge the working range of steels for low temperature applications.

Introduction

Extraction of oil and gas resources in Arctic areas is characterized by extreme challenges from remote locations, poorly developed infrastructure, limited support systems, a highly sensitive environment and harsh weather conditions. Some of these challenges must be addressed through the application of robust materials, such as steels that are able to withstand low temperatures, large temperature variations and potentially large deformations from frost heave, ice and icebergs.

Today there are commercially available high alloyed structural steels with very good low temperature properties. However, costs may prohibit large scale applications of such steels. Instead, high strength low alloy (HSLA) steels have been developed for wide range of oil and gas installations due to their beneficial mechanical properties, adequate weldability and low costs [1]. The steels receive their mechanical properties by the formation of a fine grained microstructure and favourable transformation products following thermo-mechanical controlled processing (TMCP) [2]. However, the operational properties of steels depend on the base material, the heat affected zone and the weld metal. For example, coarse grained regions and brittle phases may form in the heat affected zone during welding that can cause brittle failure. Therefore, it is important to restrict the formation of such regions and phases to obtain the target microstructure following welding.