INTRODUCTION & BACKGROUND
The thermal aging and consequent embrittlement of materials are ongoing issues in cast and duplex stainless steel. 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 either 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 several 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 BWR operating conditions. The measurement 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 hours at 430 and 400 °C. Stress Corrosion Crack (SCC) growth rates have been measured for material in the as-welded, 1000 hour/400°C and 5000 hour/400°C aged weld metal @ 288°C in high purity water containing 300ppb of Oxygen. Fracture toughness (JIC) have been measured in the 5000 hour/400°C aged condition and estimated in the other conditions. Crack growth rates for material in the as welded and aged for 5000 hours @ 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 at toughness values significantly below (<50%) the room temperature fracture toughness for material that contains an SCC "precrack". In-situ fracture toughness with a fatigue precrack, is still significantly below the air values. This behavior, termed "environmental fracture" requires further investigation.
Stainless steels are often joined by welding. The welding process requires that a specified amount of d ferrite be present in the weld to prevent hot cracking. While the elimination of hot cracking is essential for a sound weld, the d ferrite can be unstable with respect to a number of microstructural changes including precipitation of Cr-rich a?, Ni and Si-rich G phase and ¿2 phase.1 At lower temperatures (= 400°C) it has been found that the precipitation of the Cr-rich a? phase is often preceded by a spinodal decomposition in the ferrite phase. Additionally, residual a-ferrite can also be present that can also undergo these transitions.1 Spinodal decomposition is a process of phase separation in which a chromium rich phase separates from the iron rich phase in the ferrite.2 The resulting microstructure will have a sinusoidal composition profile with an average wavelength. Typical values in d-ferrite have been experimentally measured at about 5 nm.3 The above processes are also operative in welds in either cast or wrought stainless steels where d ferrite is present. While embrittlement can occur after several hundred hours of aging at 475°C-primarially due to a? precipitation, the spinodal decomposition reaction is also operative at lower temperatures, where ductility reductions have been observed after several tens of thousands of hours at temperatures from 300-450°C.4,5
