Effect of various alloying elements on the microstructure and repeated deformation behavior of x60 high strength low alloy steel

Repeated deformation behaviors of 10 different X60 based steels were investigated in this study. At room temperature, repeated deformation tests were conducted with f = 0.2 Hz, sinusoidal wave form and Δε_ta a (total strain amplitude) = 0.9%, which are conditions similar to those for coiled tubing application. Different alloys whose microstructures were varied with addition of alloying elements showed different fatigue life. In particular, X60 steel modified by addition of titanium had superior fatigue properties than the base X60 steel. To clarify the relationship between repeated deformation life and microstructure, morphologies of the fractured surfaces and crack propagation path were examined. Most of the cracks propagated smoothly along the ferrite-ferrite interface or through the ferrite, while the crack path was largely changed by pearlite phases, which were relatively harder phases than ferrite. Based on those results, we related the microstructure parameters to repeated deformation behavior.