A study of stress relief cracking in [formula omitted] Cr 1 Mo steel—II. The effects of multi-component segregation

Abstract

The examination of stress-relief-cracking in the coarse-grained heat-affected-zone microstructure of 2 1 4 Cr 1 Mo steel has been extended from the study of controlled-purity and (540 ppm) phosphorus-doped alloys [reported in Part I. Acta metall. 28, 869 (1980)] to (500 ppm) tin-doped and commercial-purity alloys. Auger-electron-spectroscopy, electron microscopy, energy-dispersive X-ray analysis and potential-drop crack measurement techniques have been employed to monitor the development of grain-boundary segregation, precipitation microstructure, and crack propagation in each alloy during post-weld heat-treatment. In the controlled-purity alloy an ‘intergranular micro void coalescence’ stress-relief-cracking mode of fracture was observed at temperatures > 880 K. In contrast, an additional ‘low-ductility intergranular-fracture’ mode was observed at lower temperatures in the phosphorusdoped, the tin-doped and commercial alloys. The cracking temperature range was dependent upon material composition. The controlled-purity alloy exhibited a relatively low propensity to stress-reliefcracking, while the susceptibility of the commercial alloy was significantly higher than that for both of the impurity-doped alloys. Fractographic, chemical and microstructural analyses enabled the major processes contributing to each mechanism to be identified, and it was possible to rationalise the overall stress-relief-cracking behaviour in terms of the effects of residual impurities on (i) the relative extent of mechanisms operating in each alloy, and (ii) the rate-determining processes contributing to each mechanism. On this basis, multi-component segregation was found to be severely deleterious, promoting crack-propagation by low-ductility intergranular-fracture at relatively low stress intensities ( e.g. 15 MNm −3 2 for the commercial alloy).