L51451 Shot Peening and Grit Blasting on the Stress-Corrosion Cracking Behavior of Line-Pipe Steel

Laboratory experiments and field studies conducted in the Stress-Corrosion-Cracking Phase of the NG-18 program have demonstrated that there are a number of factors that influence the initiation and growth rate of stress-corrosion cracks in pipeline steels. Chief among these are: (1) the susceptibility of the steel, (2) the stress level and the strain rate, (3) the chemical environment in contact with the steel surface, (4) the electrochemical potential established on the steel surface, and (5) the temperature of the steel/environment system. These five factors are strongly interrelated in rather complex ways. However, it has been recognized that, for stress-corrosion cracks to initiate or grow, each of the five factors must be present concurrently and at appropriate values.As a result of these requirements, modification of any one of the factors is often sufficient to prevent or appreciably retard stress-corrosion cracking. Result: The objectives of this study were to demonstrate the beneficial effect of shot peening and grit blasting; determine the minimum amount of grit blasting needed to increase the stress-corrosion threshold stress to at least80 percent of the specified minimum yield stress (SMYS); and determine practical parameters to control the degree of grit blasting and to determine the effect of oxidation (rusting) after blasting on potential stability in the stress-corrosion cracking range. Laboratory experiments were carried out to investigate the effect of shot peening and grit blasting on the resistance to stress-corrosion cracking of pipe steels. It was demonstrated that both shot peening and grit blasting could increase the resistance to stress-corrosion cracking. However, since grit blasting is a more commonly used method of treating pipe-steel surface than shot peening, the emphasis in this report was placed on grit blasting.