FRACTURE CRITERIA FOR HYDROGEN AND TEMPER EMBRITTLEMENT IN 9CRLMO STEEL

A reduction in ductility and toughness is observed on thermal ageing o f 9Cr1Mo steels in the temperature range 400-650-degrees-C. Previous i nvestigations using tensile tests have shown a synergistic effect of h ydrogen on such temper embrittlement. To investigate the mechanisms un derlying this effect of hydrogen a series of fracture tests have been undertaken on a commercially produced 9Cr1Mo steel using blunt-notched (SENB) specimens tested in four-point-bend. The results were analysed to determine the maximum local tensile stress sigma(F) and the local stress and strain levels at the point of crack initiation as a functi on of test temperature, hydrogen content and thermal ageing. Local fra cture stress, rather than sigma(F), was relatively constant with test temperature, and it is proposed that this parameter, together with lo cal fracture strain, controls the failure mechanism. A dual stress/str ain criteria model is proposed, and it is suggested that both hydrogen and temper embrittlement reduce the local fracture stress, indicating an influence on cohesive strength for both mechanisms of embrittlemen t.