Stress Corrosion Cracking Crack Growth Behavior of Type 316L Stainless Steel Weld Metals in Boiling Water Reactor Environments

Abstract

Thermal aging and consequent embrittlement of materials are ongoing issues in cast and duplex stainless steels. Spinodal decomposition is largely responsible for the well-known "475 degrees C" embrittlement that results in drastic reductions in ductility and toughness in cast materials. This process is also operative in welds in cast or wrought stainless steels where delta ferrite is present. While the embrittlement can occur after several hundred hours of aging at 475 degrees C, it can also occur at lower temperatures where ductility reductions have been observed after tens of thousands of hours at 300 degrees C. The effect of thermal aging on mechanical properties, including tensile, toughness, fatigue, and static crack growth, has been investigated at room temperature and in 288 degrees C high-purity water simulating boiling water reactor (BWR) operating conditions. The measurements of tensile, microhardness, and Charpy-impact energy show an increase in strength and a decrease in impact energy after aging for up to 10,000 h at 430 degrees C and 400 degrees C. Stress corrosion crack (SCC) growth rates have been measured for as-welded and 5,000-h/400 degrees C aged weld metal at 288 degrees C in high-purity water containing 300 ppb of oxygen. Fracture toughness (J(IC)) have been measured in the 5,000-h/ 400 degrees C aged weld metal and estimated in the other conditions. Crack growth rates for material in the as-welded and aged metalfor 5,000 h at 400 degrees C have been measured and are generally within the scatter band for wrought material, although the aged material data fall at the high end. Unusual in situ unstable fracture behavior has been experienced for material that contains an SCC &apos;precrack" at toughness values significantly below (<50%) the room temperature fracture toughness. In situ fracture toughness with a fatigue precrack is still significantly below the air values. This behavior, termed "environmental fracture," requires further investigation.