Creep Fracture Mechanism Maps Based on Creep Rupture Tests up to about 100 000 h for Type 316 Stainless Steel

Abstract

Specimens of Type 316 stainless steel with rupture periods up to about 100 000h have been examined. metallographically in order to clarify creep fracture mechanisms and effects of microstructure on the mechanisms.
Main results obtained are as follows.
(1) Creep fracture mechanism maps constructed experimentally show the fields of dominance of each fracture mechanism : transgranular creep fracture, wedge-type cracking at triple point, intergranular cavitation, and cracking at interface between intermetallic phases and austenite matrix. The maps also indicate that this steel in power plants is used within the field of the cracking due to the intermetallic phases of sigma and chi.
(2) The creep fracture mechanisms are subject to the influence of precipitation behavior at grain boundary: grain boundary M₂₃C₆ promotes the transition from the wedge-type cracking to the intergranular cavitation, and grain boundary sigma or chi phases lead to the grain boundary cracking.
(3) Higher aluminum content than about 0.03% leads to the precipitation of AlN associated with the grain boundary sigma or chi phases, and this AlN is thought to cause early formation of the grain boundary cracks, resulting in the marked reductions in rupture strength and ductility at longer time.