多量のM<SUB>23</SUB>C<SUB>6</SUB>型炭化物を含むオーステナイト耐熱鋼の高温クリープ中の内部応力

Abstract

Creep test has been performed at temperatures of 873 K and 973 K on an austenitic 21-4N steel with a large amount of M23C6 carbides as well as on a SUS 304 steel without any precipitates. Discussion was then made on the relation between internal stress in the steady-state creep obtained by the strain dip test with application of the extrapolation method and precipitate particles or the dislocation structure.It was found that internal stress increased with an increase of applied stress in both steels, internal stress reached a constant value of 280 MPa at 873 K in the 21-4N steel. This value is close to the Orowan stress at 873 K (σo\fallingdotseq2Gb⁄L0=312 MPa, L0: free spacing between particles). By means of transmission electron microscopy to thin films, unlike in the SUS 304 steel, the cell structure was not formed during the steady-state creep in the 21-4N steel, but the dislocation density around precipitates increased with an increase of applied stress. It was considered from calculations in this study that internal stress in both steels is a long-range force acting on mobile dislocations when they surmount particles by the Orowan mechanism or pass through regions of the high-dislocation density around particles or cell boundaries.The activation energy of creep was 291±8 kJ/mol for the 21-4N steel and 297±19 kJ/mol for the SUS 304 steel, independent of applied stress and temperature. These values were almost equal to that of self-diffusion in gamma iron. An activation area which was of the same order as lb (l: dislocation spacing in a region of the high-dislocation density around particles or at cell boundaries) decreased with an increase of applied stress in both the steels.