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°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°C, it can also occur at lower temperatures where ductility reductions have been observed after tens of thousands of hours at 300°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°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°C and 400°C. Stress corrosion crack (SCC) growth rates have been measured for as-welded and 5,000-h/400°C aged weld metal at 288°C in high-purity water containing 300 ppb of oxygen. Fracture toughness (JIC) have been measured in the 5,000-h/400°C aged weld metal and estimated in the other conditions. Crack growth rates for material in the as-welded and aged metal for 5,000 h at 400°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 “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.
