Evaluation of creep rupture strength of high nitrogen ferritic heat-resistant steels using small punch creep testing technique

The development of next generation ultra-heat-resistant ferritic steels for the future advanced thermal power plants which operate in excess of 700 °C are currently in progress in Japan. The small punch (SP) creep testing technique which uses miniature specimens has been applied to evaluate creep rupture strength of newly developed high nitrogen ferritic steels. The SP creep tests have been carried out on different grades of high nitrogen ferritic steels in the temperature range of 600-800 °C, under different loads in the range of 70-400 N, using specimens of dimension ∅8 mm x 0.5 mm. The SP creep rupture results of high nitrogen ferritic steels have been compared with that of conventional steel (Gr.91). The high nitrogen steels exhibited higher creep rupture strength when compared to the Gr.91. The creep rupture results obtained from SP creep tests were correlated to the uniaxial creep rupture results obtained from conventional creep tests using a stress conversion coefficient of 2.05. The paper investigates the applicability of SP creep testing technique for the evaluation of creep rupture strength of high nitrogen ferritic heat-resistant steels.