Stress corrosion cracking of A-286 stainless steel

Alloy A-286 is used as hot-headed screw-type threaded fasteners in sizes from 1/2 inch to 1 1/4 inch diameter. Except for one application at 61,000 psi, the fasteners are stressed to a nominal value of 33,000 psi and exposed to water ranging from 288 degree to 343 degree C. No stress corrosion cracking (SCC) failures have been reported for alloy A-286 in the low stress condition after 13 years of service. The high stress application warranted concern since this stress is in the range where field failures of hot-headed fasteners have occurred. Three different specimen types (uniaxial tensile, precracked compact tension (CT), and modified threaded pins) fabricated from SA-638, Grade 660, Type 1, 2 1/4-inch bar stock were used. Testing was performed in static water environment at 288 degree C (550 degree F) and 343 degree C (650 degree F) for up to 9,000 hours. The water was deaerated with 400 ppm boron, 1 ppm lithium, controlled amounts of N2H2 and O2, and a pH of 7 to 8. Four interim inspections were conducted, along with the final inspection, after 9,000 hours of exposure. Notched (Kt=1.7 and 4.3) and unnotched uniaxial tensile test specimens were loaded to stresses from 118 to 180 ksi. Precracked CT specimens (Kt ranging from 15 to 55 ksi inch) were bolt loaded. Modified threaded pins were stressed to 61 ksi. Except for one notched specimen, no indications of cracking were observed for uniaxial tensile specimens tested. The notched specimen (Kt=1.7) which failed was loaded to a stress of 124 ksi and exhibited intergranular failure over 60 percent of the area