Effect of Bending Process on Microstructure, Mechanical Properties and Crack Formation of 5% Ni Steel

Abstract

The 5% Ni steel is often used to make steel storage tanks to store liquefied natural gas (LNG). Herein, the microstructure and mechanical properties of 5% Ni steel samples during bending were studied through combining scanning electron microscopy, energy dispersive spectroscopy, optical microscopy, X-ray diffraction, and electron backscattered diffractometer methods with tensile tests. The outer and inner arcs underwent tensile and compressive stress, respectively, resulting in a severely deformed microstructure with a high density of dislocation, improving both the tensile and yield strengths. The ductility of the 5% Ni steel samples decreased significantly after bending due to the work hardening and dislocation accumulation. During bending, the shear bands occurred at the surface or subsurface, which were caused by strain localization. Amounts of “harder” grains with high TF and more orange and red KAM areas with high local strain at the outer and inner arcs produced a greater stress concentration than that of the mid-thickness, which can induce crack initiation and propagation due to the large deformation during bending.

Study on the growth mechanism of the internal oxide layer in 9% Ni cryogenic steel

“…As an inevitable phenomenon of high-temperature oxidation for steels during the heating process, oxide scales formed on the surface of steel result in material waste and surface defects. The thermal growth and deformation of the oxide scales during heat treatments in a slab reheating furnace and rolling processes could significantly reduce the properties of products (Utsunomiya et al, 2009;Niu et al, 2022). In addition, due to Ni addition, a complex high-temperature oxide scale on the surface of steel during hot working is formed, which includes an external oxide layer (EOL) and an additional internal oxide layer (IOL).…”