Transgranular Stress Corrosion Cracking in γ′ -forming Austenitic Steels

Abstract

Tests at 154°C in MgCl₂ solutions at a potential of −0·26 V (SCE), with an applied stress sufficient to give 1% initial strain (i.e., a fairly stringent test condition), have been used to examine the effect of γ′ precipitation on transgranular stress corrosion cracking (SCC).

Precipitation reduced the matrix nickel content and increased slip planarity. This made the alloys with 25% Ni susceptible to stress corrosion and the susceptibility increased with an increase in γ′ size from very small (~2·5 nm) to medium sized γ′ precipitates (~ 9 nm). In this size range the particles were cut by dislocations and deformation was therefore associated with planar slip. A further increase in size to prevent particle cutting and to reduce slip planarity reduced the sensitivity to SCC. Slip planarity was associated with an increase in sensitivity to SCC if the residual matrix Ni content after forming γ′ was 21% or less; however, at a residual Ni content of 31%, slip planarity did not induce SCC. Mo additions of 1·5 or 2·5 wt-% increased SCC. A 10 wt-% Co addition decreased SCC but no further decrease occurred with 20% Co.