Effect of creep deformation on the microstructural evolution of 9Cr-1Mo heat-resistant steel

초록

Creep tests of 9Cr-1Mo heat-resistant steel were carried out at 550, 600, and 650 degrees C under various stresses ranging from 70 to 270 MPa. The creep stress exponent was determined from the relationship between the minimum creep rate and applied stress, which decreased with increasing temperature. The creep activation energy was calculated to be 653 kJ/mol under 200 MPa. To investigate the coarsening behavior of precipitates, replica films were extracted from crept specimens and analyzed using transmission electron microscopy (TEM). After tempering, MX and M23C6 precipitates were observed. Laves phase, absent in the as-tempered specimen, was found to precipitate in the specimen tested for 4454 h at 550 degrees C and under all testing conditions at 600 degrees C, whereas no precipitation occurred at 650 degrees C. The size of all precipitates increased with creep rupture time, and those in the gage portion were larger than in the grip portion. In addition, the hardness in the gage portion decreased more significantly than in the grip portion. This is attributed to the faster development of subgrains by the stress-driven recovery process in short-term exposure at high temperatures.

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