Abstract
Relations between heat treatments and creep rupture lives of 21/4 Cr-1Mo steel for boiler tube used at elevated temperature and under higher pressure were discussed for the purpose of determining the best heat treatment for getting both proper toughness at room temperature and good creep rupture strength at eleveated temperature.Creep rupture strength at 550°C of each specimens was studied in connection with heat treatment, hardness and microstructure. Wihin the range of Vickers hardness 140-190, the specimens having higher hardness have better creep rupture strength than the specimens having lower hardness. From the stand point of microstructure the specimen having ferrite+bainitic structure shows good creep rupture strength.In practical heat treatment process, we must select proper tempering temperature under consideration of the influence of heating for stress relief after welding on the structural change of mother material and also insuring proper toughness for various working abilities at room temperature at the sacrifice of creep rupture strength. We suppose from the base on the above mentioned testing results that conventional after heating temperature ranging from 720°C to 760°C for stress relief is too high for maintaining initial good creep rupture strength of mother material.
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