The stress relaxation behaviour of 21–4N, a manganese-stabilized austenitic stainless steel, is investigated in terms of the metallurgical state, the application of multiple strain levels during ‘stepped’ stress relaxation testing at 700 °C, the strain level during isostrain stress relaxation tests at 538 °C and 700 °C, and the correspondence with results from constant-load creep tests. The results indicate that for isostrain stress relaxation tests the stress relaxation rate is similar for strains that span both elastic and plastic strain levels. A transition in the stress relaxation behaviour occurs at a stress level approximately equivalent to the tensile stress–strain proportional limit; below this transition the stress–strain rate relationship, or the time predicted for 1 per cent creep strain, obeys a creep power law type of equation. Stress relaxation testing successfully delineates the difference between the creep resistances of two different metallurgical conditions with similar tensile properties using fewer specimens and requiring less time. The time to 1 per cent creep strain determined from the analysis of stress relaxation results is always less than the actual time to 1 per cent creep strain during constant-load creep tests.
