Analysis of super Cr13 stainless-steel internal fracture growth effects during skew mill piercing process

The seamless tube piercing process of super Cr13 martensitic stainless steel is addressed in this paper by comparing industrial piercing tests, laboratory hot tensile tests for material behavior characterization, and finite element simulations. Particular focus is given to the effect of process parameters on internal fracture growth and geometrical homogeneity of the tube. The geometrical non-homogeneities found on industrial pierced tubes are closely related to the change of flow stress of super Cr13 stainless steel during the piercing process. This way, the non-uniform strain rate along the tube's cross-section during piercing affects the fracture growth and, consequently, the thickness homogeneity of the tube, which varies from tip to tail. Due to obtained results, the mean value of pierced tube perimeter is 660 mm, while the mean value of the tube perimeter in the FEM simulation was predicted to be 2 % more (675 mm). The contact temperature of the plug and piercing tube was predicted at 1100 °C, and the mean value of pierced tube diameter in the simulation (210 mm) was 1.5 % more than the mean value of the actual tube (207 mm). The morphology of δ-ferrite affects the origin of primary crack initiation during the piercing process. Short and thick grains of δ-ferrite created at low strain rates that lengthen with increasing strain rate affect fracture sensitivity at the tip of the plug, favoring the non-homogeneity of the material.