Resistance of 18Cr-18Ni-2Si Stainless Steel to Stress Corrosion Crack Propagation in Boiling Magnesium Chloride

The propagation resistance of a steel cannot be determined by conventional testing procedures if the steel is resistant to crack initiation. Since conventional techniques were used to develop 18-18-2 stainless steel for resistance to stress corrosion cracking, its resistance to crack propagation was determined by means of pre-initiated cracks. Two laboratory heats of steel were prepared, one with the typical 18-18-2 stainless steel composition and the other with essentially the same composition but with high phosphorus and molybdenum contents, to make it susceptible to cracking. The steels were roll-bonded together and sheet specimens were prepared in the annealed, cold worked, and sensitized conditions. U-bend specimens were prepared, half of them with the susceptible steel on the tension side and the other half with the resistant steel on the tension side. The specimens were exposed in boiling magnesium chloride (150 C, 302 F). When cracks appeared, the specimens were retained in the solution for a minimum of 3 to 4 additional weeks to allow the cracks to propagate. No cracks initiated in 18-18-2 stainless steel under any of the conditions tested during five months of exposure. Cracks that initiated in the susceptible steel propagated to the interface and stopped. No cracks were observed that propagated into 18-18-2 stainless steel, even for specimens that cracked in 70 hours and were not removed for five months. These test results indicate that 18-18-2 stainless steel is resistant to crack propagation.