Effect of post-processing annealing treatments on microstructure development and hydrogen embrittlement in API 5L X70 pipeline steel

Abstract

The mechanical properties of steels are strongly influenced by microstructure and texture developed during thermomechanical processing. Here, different annealing treatments have been performed on pipeline steel at inter-critical temperatures. The aim of this study is to relate hydrogen embrittlement behavior to the microstructural characteristics obtained through one-step, and two-step annealing treatments. Similar grain orientation distributions were observed after the initial hot rolling and the two-step annealing procedures. However, in the later process a dual-phase (i.e. ferrite-martensite) structure was formed; unlike in the starting material where only segregated patches of martensite were observed within a ferritic microstructure. The one-step treated steel was comprised of relatively large and primarily ferrite grains. As a result, the preliminary processing revealed the highest strength, with reduced ductility. The one-step treatment lowered strength and ductility. Although the two-step treated steel showed the lowest strength, it was the most ductile with improved resistance to hydrogen embrittlement. The tempering operation introduced in the two-step process created minimal strain and tempered martensite inside the steel. This reduced the tendency for hydrogen damage. A direct relationship was established between high tensile strength and increased embrittlement risks. Lowering the strength and hardness through double heat treatment cycles delayed the onset of cracking after pre-hydrogen charging.