Investigations on the influence of internal nitridation on creep crack growth in alloy 800 H

For two batches of Alloy 800 H with Al contents of 0.02 and 0.34 pct, creep crack growth was investigated at 1000 °C in air and in an Ar-5 pct H₂ atmosphere. TheK concept, the net-section stress concept, and theC ^* concept of creep fracture mechanics were applied when plotting the experimental results. TheC ^* concept proved to give the best correlation between load parameter and crack growth rate. A good agreement was observed between the experimental results and the model calculations for crack extension by constrained diffusive cavity growth. Strong internal nitridation, which occurred in the air tests and which had been shown to increase the creep strength in creep rupture tests, did not show any significant influence on the creep crack growth rate in comparison with tests in an Ar-5 pct H₂ atmosphere, in which no internal nitridation was observed. Also, the differences in the Al contents of the two batches did not play a role. This behavior is explained by the fact that neither the nitride particles nor the particle matrix interface is a particularly weak site in the material. It also becomes obvious that theC ^* concept can be rather insensitive to precipitation strengthening effects, if these only affect the parameterB in Norton's creep law. M. WELKER, formerly with DECHEMA