Hydrogen Assisted Cracking of Ferritic Linepipe steel and Corrosion Behavior of Austenitic TWIP steel

Abstract

Hydrogen embrittlement of steels has been considered as one of the major issues for their industrial applications in the environment containing hydrogen because it drastically decreases the mechanical property of the steel. The hydrogen damage develops by a series of processes of hydrogen adsorption at the steel surface, hydrogen diffusion and trapping in the defects of the steel matrix. Although there have been numerous researches to increase the resistance to the hydrogen assisted cracking problems, it is still needed to investigate and understand the characteristics of hydrogen behavior itself.This thesis is dealing with two different research subjects. One is investigation of the effect of pipe forming on the hydrogen assisted cracking of the American Petroleum Institute (API) X70 graded linepipe steel in the aqueous sour environment containing H2S with respect to the steel thickness and circumferential position of the steel pipe. The hydrogen permeation data show well the effects of the strain level and of the inclusion distribution. The HIC susceptibility increases as the ratio of the thickness (t) to pipe diameter (D) is increased since the level of strain is increased. The thickness of steel pipe affects not only the t/D ratio, but also the size and distribution of the inclusion in the steel. Between the 12 mm and 18 mm thick steel pipes manufactured by the electric resistance welding, the HIC susceptibility is the highest at 180o position of the thicker steel pipe.The other topic is to inspect the corrosion behavior of high manganese fully austenitic TWinning Induced Plasticity (TWIP) steel. The high manganese steel has been attracted by many industries and academic areas because of its excellent strength-elongation balance. However, contrary to many researches dealing with its superior mechanical properties, it is still needed to improve its corrosion resistance since addition of Mn decreases the corrosion resistance detrimentally in aqueous environments. In this study, the corrosion resistance of TWIP steel is investigated with respect to Al content by analyzing the corrosion product formed on the steel surface during cyclic corrosion test (CCT). It is verified the addition of Al improves the corrosion resistance of TWIP steel when exposed to CCT environment. Comparing to the product formed on the non-Al contained TWIP steel, the corrosion product formed on the surface of Al contained TWIP steel has a dense structure which acts a barrier preventing direct contact between the steel surface and environment.