The Influence of Material Factors, Including Cold Work on the Susceptibility of Stainless Steels to Stress Corrosion Cracking

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Abstract

The University of ManchesterFaculty of Engineering and Physical SciencesABSTRACT OF THESIS submitted by ISMAILA IDOWU AHMEDFor the degree of DOCTOR OF PHILOSOPHYand entitled THE INFLUENCE OF MATERIAL FACTORS, INCLUDING COLD WORK ON THE SUSCEPTIBILITY OF STAINLESS STEELS TO STRESS CORROSION CRACKINGDate of submission 08th/12/2011

Layman's Abstract

THE UNIVERSITY OF MANCHESTERAbstract of thesis submitted by: Ismaila Idowu Ahmed for the degree of Doctor of Philosophy and entitled: The Influence of Material Factors, Including Cold Work on the Susceptibility of Stainless Steels to Stress Corrosion Cracking. The main objective of the thesis was to gain better understanding of key parameters associated with Cold Work (CW) and their possible effects on Stress Corrosion Cracking (SCC) susceptibility of Austenitic Stainless Steels (ASS) cold rolled to different degrees. The microstructural characterisation of the cold rolled ASS was carried out using optical microscopy to determine and correlate the average grain size with the level of CW. The assessment of martensite development during the CW was carried out using the neutron diffraction technique. The effects of CW levels and strain paths on the lattice strain evolution during the in-situ loading and on the mechanical failure of cold worked ASS were studied. The electrochemical behaviour of cold rolled ASS was also studied. Finally, The SCC susceptibility of ASS was investigated using the Slow Strain Rate Testing (SSRT) techniques. The post-mortem analyses of the failed samples were carried using the optical and Scanning Electron Microscopy (SEM). The study showed that the average grain size decreases with CW and reaches minimum at 20%CW. The smallest and the largest grain size occurred consistently on the Longitudinal (L) and Short-transverse (S) plane respectively. Evidence of martensite development was only found during the plastic deformation at cryogenic temperature and none was observed at ambient temperature. The study showed that the strength of material increases with the level of CW. The Bauschinger effect occurred when the strain path is reversed and its magnitude is independent on whether the tensile or compressive prestraining comes first or last but rather dependent on the amount of CW. The correlation between the CW levels and the lattice strain evolution during the in-situ loading showed that, the lattice strain increases with prestrain and reaches saturation in the material prestrained to 20%CW.The result of the mechanical failure test showed that, 20% cold rolled material loaded along the L and Transverse (T) directions showed a gradual failure, whilst the material loaded along the S direction exhibited a rapid failure. The SEM micrographs suggest that materials loaded along the L and T direction failed with the characteristic features of pure ductile failure while the specimen loaded along S direction showed mixed features of the ductile and brittle failure. The electrochemical properties of the cold rolled materials are more affected by sample orientation than the levels of CW. The short-transverse plane was observed to be most noble whilst the longitudinal plane was the least noble. The results of the SSRT in the chloride environment showed that the plastic elongation, the ultimate tensile strength and the time to failure decrease as the applied potential increases. The post-mortem analysis of the failed samples with the SEM showed that, the fracture surface contained region of ductile failure characterised by dimples, and region of SCC with secondary cracks along the loading axis. Whilst the cross sectional analysis, showed evidence of predominant transgranular stress corrosion cracks. The study found that SCC susceptibility of the ASS is directly linked to strain heterogeneity and directional anisotropy caused by cold working.Month and year of submission: December, 2011

Keyword(s)

cold work, stainless steels, prestrain, strain paths, peak width Bauschinger effect, in-situ, deformation, SCC, Lattice Strain, microstrain, twinning, internal strain, martensite, ferrite, austenite, directional anisotropy, yield strength, load cycle, FWHM, 304L, current density, corrosion, cleavage, ductile, microvoid.

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