Comparison of stress relief cracking in A 508 2 and A 533 B pressure vessel steels

The mechanisms of, and susceptibility to, stress relief cracking (SRC) were studied in A 508 class 2 and A 533 B pressure vessel steels by means of fixed displacement, load relaxation tests on notched specimens in pure bending at 823–923 K in vacuum. The specimens had previously been given heat affected zone simulation treatments in the range 1323–1573 K. The mechanisms of SRC were found to be the same as reported separately for A 508 2 steel, which was found to be generally more susceptible to SRC than A 533 B steel because of the faster formation of microcracks and coalescence of microcracks into a macrocrack. This can be rationalized in terms of the higher creep strength in A 508 2 steel owing to the presence of chromium, and the higher concentration of intergranular sulphur or reprecipitated sulphides owing to the lower manganese content of A 508 2 steel, which results in less stable sulphides. These factors are discussed in the framework of a recently proposed model for the brittle mode of SRC.