The Effect Of Processing Variables On Stress Corrosion Cracking In 9ni 4co Steel Alloy

The Effect of Processing Variables on Stress-corrosion Cracking in 9ni-4co Steel Alloy

Since all current aircraft strctural steels exhibit some degree of stress-corrosion cracking (SCC), the recently developed steels, HP 9-4-30 and HP 9-4-45, were investigated to define their limits of stress-corrosion susceptibility or immunity. Stress-corrosion testing variables included HP 9-4-30 Martensite (220/240 KSI), HP 9-4-45 Martensite (260/280 KSI), and HP 9-4-45 Bainite (260/280 KSI); normal and severe grinding and drilling, flash and fusion welding, stress relief, 400 F service exposure, shot-peening, and cadmium-plating. Effects of microstructures, pre-existing cracks, surface conditions, and residual stresses (X-ray diffraction) were determined. HP 9-4-30 Martensite was immune to SCC in the as-heat-treated, H.T. (heat treated) and ground, H.T. and drilled, H.T. and shot-peened, and H.T. and cadmium-plated conditions, but was susceptible in the H.T. and severely ground, H.T. and severely drilled, H.T. and fusion-welded, and H.T. and pre-cracked conditions. The SCC threshold for tempered martensite was above 205 KSI and between 90 and 120 KSI for untempered martensite. HP 9-4-45 Martensite was very susceptible to SCC for all variables tested, except shot-peening (immune). The cadmium-plated condition was the most susceptible. The SCC threshold for tempered martensite was between 89 and 127 KSI. HP 9-4-45 Bainite was immune to SCC in the as-heat-treated, H.T. and shot-peened, and flash welded and H.T. conditions, but susceptible in the H.T. and cadmium-plated, and H.T. and pre-cracked conditions. Severe grinding and severe drilling conditions contained SCC only in the martensenic surface layer.

Stress-corrosion Cracking in High Strength Steels and in Titanium and Aluminum Alloys

Technical Abstract Bulletin