Effect of Nitrogen on the High Temperature Creep Behavior of 9Cr-2Co Steel

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Constant load creep tests were conducted at 873, 898 and 923 K under an initial stress of 50, 60 or 70 MPa to examine the effect of nitrogen addition (0.06, 0.11, 0.16 and 0.19 mass%) on the creep behavior of a 9mass%Cr-2mass%Co ferritic steel. Extraction residue analysis showed that 240 to 410 ppm of nitrogen was present as solid solution. The rest of the nitrogen existed as nitride precipitates. The creep rate decreased while creep rupture life increased with an increase in nitrogen content. It is also observed that the higher the nitrogen, the shorter the creep rupture strain. A state equation for creep could be represented as; ε<sub>0</sub>=<I>A</I><sub>0</sub>σ<SUB>0</SUB><SUP>n</SUP>exp(-<i>Q</i><sub>0</sub>/<i>RT</i> ), where ε<sub>0</sub> is the imaginary initial strain rate defined in the text, <i>A</i><sub>0</sub> is a constant, <i>n</i> is the stress exponent and <i>Q</i><sub>0</sub> is the apparent activation energy. The mean values of <i>n</i> and <i>Q</i><sub>0</sub> were 4.5 and 360 kJ/mol, respectively. The Orowan stress for specimens crept at 873 K and 70 MPa was estimated from the measurement of mean interparticle spacing obtained by TEM study. The resultant values are 15, 22, 30 and 38 MPa for specimens containing 0.06, 0.11, 0.16 and 0.19 mass% nitrogen, respectively.

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  • ISIJ international  

    ISIJ international 37(2), 181-187, 1997-02-15 

    The Iron and Steel Institute of Japan

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