Microstructure evolution of ultra-fine grain low-carbon steel tubular undergoing radial expansion process

Tubular expansion is a cold metal forming process where diameteral change is achieved by propagating a conical mandrel through the tubular either by mechanical pull or hydraulic push. Cold metal forming alters post-expansion mechanical and microstructural properties of tubular material, which may lead to premature failure during operation. In order to prevent tubular from failure, its post-expansion material and mechanical properties must be investigated thoroughly. Initial grains morphology, distribution of phases, and subsequent variation in material and mechanical properties due to expansion process of low-carbon LSX-80 steel tubular are investigated in the current study. The observed microstructure is typical of high strength steels with a mixture of carbon-poor and carbon-rich regions. A noticeable volume fraction of martensite phase was also observed. Presence of smaller grains in the material is a clear indication of the application of grain refinement mechanism to improve strength and toughness. Microhardness and Charpy impact tests were done on unexpanded and expanded sections of tubular in order to determine their mechanical properties. In addition, fractographic analysis was accomplished and obtained results showed that the morphology of the fractured surface was nearly alike at the macroscopic scale throughout the range of expansion ratios considered in this study. However, at the fine microscopic scale, the fractured surface was mostly ductile at low expansion ratio, while it was mainly brittle at large expansion ratio. Hence, an expansion ratio in the vicinity of 15% is highly recommended for the current tubular material in order to have adequate safe margin for down-hole application. An alternative material has to be selected and/or developed in order to realize the goal of achieving higher expansion ratio (≥30%) while preserving the tubular structural integrity after expansion.