A thermal shock fatigue study of type 304 and 316 stainless steels

Thermal fatigue crack initiation and propagation promoted by thermal quenches have been studied in AISI 316 and 304 stainless steels and correlated with isothermal strain cycling fatigue. Axially unconstrained specimens of rectangular section were held at bulk temperatures of 2500C to 5000C and symmetrically water-quenched on the narrow faces to give equivalent surface strain ranges from 2.8 x 10-3 to 5.4 x 10-3. Crack initiation in smooth samples showed an apparent threshold at a surface strain range of 2.8 x 10-3 equivalent to a thermal amplitude of 1500C with no cracking being produced in 500,000 cycles. The crack growth in prenotched samples was evaluated by direct observation and by subsequent fractography and showed two modes of growth. The crack growth was strain controlled during the early stages of propagation where the crack tip was within the surface zone under conditions of fully plastic cyclic yield. At greater depths the propagation rates in the remaining elastically cycled material were found to correlate with calculated stress intensity values. In the chosen symmetrical quenched axially unconstrained configuration the crack growth rates decreased towards the centre of the specimen, indicating a crack arrest condition as expected from analysis. (author)