"Developments in the Control of Weld Haz Properties in Modern Microalloyed Steels "

by "Frank Barbaro, Lenka Kuzmikova, Madeleine Du Toit "

Publisher - "University of Wollongong, Australia "

Category - Engineering & IT

"A microalloy addition of titanium has been the traditional approach to minimise austenite grain coarsening in the weld HAZ through the grain boundary pinning action of TiN precipitation. The high thermal stability of TiN precipitates was the basis for control of grain growth compared to other microalloy additions. However, steelmaking procedures required to produce the optimum precipitate size distribution can be difficult to achieve consistently. Increased additions of niobium in modern high strength pipeline steels have now demonstrated increased control of HAZ microstructures with improved fracture toughness. The present paper details the demonstrated benefits of increased Nb content in steelmaking, pipe making and field welding applications. These improvements are related to improved grain size control and microstructural development as a response of the steel to reheating and subsequent cooling. Welding is the most important fabrication technique and imposes the greatest challenge in resisting the deterioration of the mechanical properties, mainly toughness. Hence the current paper mainly addresses the microstructure property relationships, which occurs in the weld zone of high strength steel grades with different microalloy designs. Real welds and thermo-mechanical simulation techniques were utilized to evaluate the critical coarse grained HAZ (CGHAZ). Simulations were calibrated using real weld thermal cycles, to quantify the influence of alloy design and specifically the role of Nb on weld zone properties. The results reveal that the fracture toughness of the simulated CGHAZ in steel with increased additions of niobium is superior to that of traditional microalloyed steel grades. Toughness was related to the subtle difference in the bainitic HAZ microstructure and most importantly a difference in austenite grain size. "

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