Control of weld HAZ properties in modern high strength pipeline steels
Critical performance of modern high strength linepipe is related to the ability of the steel to maintain mechanical properties in the weld heat affected zone (HAZ). The region most susceptible to mechanical property degradation is the coarse grained HAZ, however in multipass welds, the intercritically reheated CGHAZ (ICCGHAZ) also presents challenges to maintain toughness. Currently Ti is employed to minimise austenite grain coarsening through the grain boundary pinning action of TiN precipitates. This is effective because of the high thermal stability of TiN but control of the precipitate size distribution is very much dependent on alloy design and processing conditions to ensure final weld HAZ properties, particularly toughness. This can be difficult to maintain and alternative methods are required to further improve performance of the weldments. It is now evident that increased additions of Nb in modern high temperature processed (HTP) steels have demonstrated increased control of HAZ microstructures with improved fracture toughness [1, 2]. The present paper details the microstructure - property relationship of two pipe steel grades with different alloy designs. Evaluation of the critical CGHAZ was achieved by simulation techniques, calibrated using real weld thermal cycles, to determine the influence of alloy design and specifically level of Nb on weld zone properties. The results reveal that the fracture toughness of the simulated CGHAZ in the HTP steel is superior to that of a conventional microalloyed pipeline steel grade. Toughness was related to the distribution of martensite-austenite (M-A) constituent and the effective grain size which appeared to correspond to prior austenite grain size as evidenced by examination of cleavage facet size (CFS) on fractured Charpy specimens.