Abstract
We have examined the influence of mechanical surface finishing on the development of residual stresses, and on the subsequent formation of stress corrosion cracks, in 316Ti austenitic stainless steel after exposure to boiling magnesium chloride. The surface residual stresses of as-received plate, prior to machining, were found to be biaxial and compressive. However, abrasive grinding produced significant compressive stresses in the machining direction but much lower perpendicular stresses. On the other hand, milling produced high biaxial tensile stresses (approaching the ultimate tensile strength, UTS, of the material), which were found to be relatively insensitive to cut depth but to vary as a function of feed rate. On the milled surfaces a distinctive pattern of stress corrosion cracking was evident with longer primary cracks nucleating along the milling direction and secondary, shorter, cracks nucleating perpendicularly. As the surface tensile stress was lower perpendicular to the milling direction, we postulate that the nucleation of primary cracks parallel to machining must be driven by the surface profile after machining (and associated micro-stresses) as much as by the macroscopic residual stresses.
Highlights
Assisted cracking is one of the most harmful localised damage processes and encompasses a wide range of mechanisms that includes, for example: hydrogen induced cracking, hydrogen embrittlement, corrosion fatigue and stress corrosion cracking
For austenitic stainless steels, which are the first-choice workhorse alloys for industrial applications requiring corrosion resistance, a key susceptibility is to stress corrosion cracking in environments containing chloride ions where an applied or residual tensile stress is present
Chloride-induced stress corrosion cracking of ferrous alloys necessarily commences from a component surface because access to the external environment is required and generally occurs on austenitic microstructures, since ferritic phases are relatively immune from such damage
Summary
Assisted cracking is one of the most harmful localised damage processes and encompasses a wide range of mechanisms that includes, for example: hydrogen induced cracking, hydrogen embrittlement, corrosion fatigue and stress corrosion cracking. Cracking generally starts at local defects, which may be microstructural features within the body of the material or, more typically, commence from surface features that are initially present as a consequence of materials processing (e.g. local microstructure, surface roughness) or arise from an in-service damage process such as wear, erosion, or corrosion (e.g. pitting). For austenitic stainless steels, which are the first-choice workhorse alloys for industrial applications requiring corrosion resistance, a key susceptibility is to stress corrosion cracking in environments containing chloride ions where an applied (i.e. service) or residual tensile stress is present. Chloride-induced stress corrosion cracking of ferrous alloys necessarily commences from a component surface because access to the external environment is required and generally occurs on austenitic microstructures, since ferritic phases are relatively immune from such damage. The nature of the material surface and near sub-surface (i.e. microstructure, near-surface residual stress and surface geometry) is critical to the initiation and propagation of stress corrosion cracks
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