Cross-Sectional Grain Size Homogeneity Effect on Structural Steel Fatigue Performance in Air and Hydrogen Environments

Structural steel mechanical properties of strength and ductility for a given microstructure are predominately driven by the average ferrite grain/packet size and by the through-thickness homogeneity of the ferrite grain/packet size in the final product. Fatigue performance, a ductility property, in air for applications of wind towers, bridges or high-rise buildings along within environments of high-pressure gaseous hydrogen for various pipeline systems is critical to the end-use design. Fracture and fatigue testing of a commercially produced low carbon 20 mm thick API X60 Sour Service steel had been completed which showed good and stable performance when compared to other commercially produced pipeline and structural steel microstructures. This commercially produced API steel was reported as “Alloy D” in prior published work. The microstructure was predominately polygonal ferrite with industrial quality of steel cleanliness, minimum of microstructural banding and a small but relatively homogenous through-thickness grain size required for a successful API X60 Sour Service specification/application. Based on the initial fatigue performance reported for the “Alloy D” through-thickness microstructure a more comprehensive study on the effect of the through-thickness grain size/homogeneity on fatigue was initiated. To isolate and study this effect of the, laboratory developed samples of a low carbon API X60 Sour Service steel with the same alloy design as “Alloy D”, which is characterized by a predominately single-phase polygonal ferrite microstructure with excellent cleanliness and no microstructural banding. Two sets of steels were made with the only difference being variations in average through-thickness and homogeneity of the final ferrite grain size.