TERTIARY CREEP IN A NICKEL FREE AUSTENITIC STAINLESS STEEL

Abstract

Creep behaviour of Nickel free Nitrogen containing Austenitic Stainless Steel is studied. A short primary creep region, a medium duration secondary creep region and a tong tertiary creep region as observed. The creep data is analysed in the light of tertiary creep mechanisms proposed for nicket based superalloys. The effect of aging on creep behaviuor of the steet is investigated. It is observed that tensite and creep properties are adversely affected by increasing the time of aging at 1000°C from 10 hours to 200 hours. Creep weakening effect due to coarsening of precipitates in the structure is dominating over the creep strengthening effect of oxide layer at 1000°C aging of creep samples for different time duration. Fracture occurs by nucleation, growth and coatescence of grain boundary cavities. All types of creep damage mechanisms are playing their role in early onset of tertiary creep regime in following sequence (i) microstructurat degradation (precipitate coarsening) (ii) depletion ofsotute atoms from solid sotution (iii) loss of external section (iv) loss of internal section (cavitation) and (v) environmental attack or oxidation.