Shot peening to prevent the corrosion cracking of austenitic stainless steels

Abstract

Rockwell International and Metal Improvement Company have conducted a joint test program to develop shot peening as a technique for preventing corrosion cracking in austenitic stainless steels. Initial laboratory-scale tests demonstrated the feasibility of preventing stress corrosion cracking by shot peening to impose compressive stresses on the surface of the work pieces. Conventional U-bend test specimens, when peened, survived 1000 h tests in the boiling 42 pct magnesium chloride stress corrosion test. Unpeened reference specimens commonly fractured within one or two hours in this test. Component tests were conducted to demonstrate the practicality of the peening process for sizes and shapes that typify components in a reactor piping system. Pipe sections and cold worked, hexagonal tubes were tested. In all components, unpeened sections developed stress corrosion cracks within a few hours in the magnesium chloride test; in contrast, the shot peened surfaces survived hundreds of hours. It was discovered at Rockwell International that intergranular corrosion can be prevented in austenitic stainless steels by severe shot peening prior to exposure to sensitizing temperatures. For this purpose, the surfaces must be severely cold worked by the shot peening to break up surface grains and grain boundaries. Two nondestructive testing techniques show promise as methods for measuring the stresses or cold work imparted on the surface of the work piece by peening. In one method, eddy currents are used to measure differences in electrical properties induced by cold working of metals. Another is a magnetic technique which measures the changes in magnetic properties due to the transformation of austenite to ferrite by cold working.