STRESS CORROSION CRACKING AND HYDROGEN EMBRITTLEMENT OF FERRITIC STAINLESS STEELS - EFFECTS OF THERMOMECHANICAL TREATMENTS.

摘要

The influence of thermomechanical treatments on the stress corrosion crack initiation and hydrogen assisted crack growth of 26Cr-1Mo alloys in boiling 42% LiCl solutions and 5% H(,2)SO(,4) solutions has been investigated using a uniaxial constant load fixture.; Annealing at temperatures where only recovery processes operate, reduces the susceptibility of prestrained low interstitial 26Cr-1Mo alloy (E-Brite) to SCC in boiling chloride solutions. Both slip step height and density, which are the major parameters controlling the breakdown of protective films in these alloys, are reduced substantially by annealing near 300(DEGREES)C for 1 hour.; Continuous exposure of unfilmed metal to the aggressive environment is provided during SCC due to creep; this causes reduction in overall repassivation rate. Grain coarsening and prestraining operations result in larger slip step height and density due to increase in dislocation density.; Crack propagation potential for as-received (mill-annealed) E-Brite during SCC in boiling chloride solution is the protection potential for the localized corrosion.; Crack growth in SCC of these ferritic stainless steels is by hydrogen embrittlement mechanism. Hydrogen charging under load causes failure for E-Brite only in the grain coarsened and prestrained condition, in contrast to 26-1S (a high interstitial alloy) which fails in all but as-received condition.; Fracture morphological correlations exist between the brittle delayed failure due to hydrogen and SCC of these ferritic alloys. A region of hydrogen embrittlement ahead of a crack front is identified. Crack growth proceeds discontinuously due to hydrogen in these alloys. Interaction of plastic deformation and hydrogen produces crack arrests and stepped structure on the fracture plane.; In HE, crack initiates at grain boundaries for 26-1S in all failing conditions. However, for E-Brite, nucleation of cracks occur in isolated regions across the whole cross-section of the alloy sample, especially when forced to failure after hydrogen charging under load; distributed embrittled regions initiate at several levels and link up by fast fracture in between them.; Strain rate plays a major role in crack propagation by hydrogen. Exhausting creep, prior to hydrogen charging, decreases susceptibility of these ferritic alloys to hydrogen embrittlement.