Specimens of Type 304 stainless steel ruptured at temperatures between 600°and 800°C and with rupture periods up to about ten years have been examined metallographically in order to deduce creep fracture mechanisms and make creep fracture mechanism maps.
The steel has showed four different creep fracture mechanisms: transgranular creep fracture, wedge-type cracking at triple point, intergranular cavitation, and cracking at sigma/austenite interface. The rupture strength in the fracture field of the cracking at sigma/austenite interface shows marked decrease with increasing time to rupture. Stress index of time to rupture and apparent activation energy for rupture in this fracture field are in the ranges of 1.4-2.0 and 160-170 kJ/mol, respectively, suggesting that the rupture life in this field is controlled mainly by means of grain boundary diffusion growth of the cracks. Metallographic observations indicate that relatively high aluminum content (about 0.05 wt%) may be responsible for early formation of the grain boundary cracks due to precipitation of AIN associated with the grain boundary sigma phase.
