On the relation between stress-relief annealing, ordering and stress-corrosion cracking susceptibility in three copper alloys

Abstract

The effects of post-forming heat treatments on the scc susceptibility in ammoniacal environments of three new commercial copper alloys (CDA Alloys 619, 638 and 688) have been investigated. The stress-corrosion susceptibility was evaluated semi-quantitatively using pre-formed U-bend specimens exposed to environments of moist ammonia and Mattsson's solution. Short time (30 min) heat treatments at various temps. (within the range 250–600°F) were carried out on the preformed U-bend specimens. During subsequent stress-corrosion testing the specimens were stressed elastically, with little further plastic deformation being introduced. The heat treatments used resulted in differing degrees of residual stress relief in the pre-formed U-bend specimens and also in the formation of differing amounts of order in the alloys; these latter effects were evaluated using differential thermal analysis techniques. The stress-corrosion data, expressed as per cent springback with time, are interpreted in terms of the amount of residual stress relief and the degree of ordering. An optimum stress-relief anneal temp. for maximum stress-corrosion resistance of formed components is recommended for each alloy. Results of a metallographic examination of failed specimens are also reported.