Microstructure and Charpy impact properties of 12-14Cr oxide dispersion-strengthened ferritic steels

This paper describes the microstructure and Charpy impact properties of 12-14 Cr ODS ferritic steels fabricated by mechanical alloying of pure Fe, Cr, W, Ti and Y2O3 powders in a Retsch ball mill in argon atmosphere, followed by hot isostatic pressing at 1100 degrees C under 200 MPa for 4 h and heat treatment at 850 degrees C for 1 h. Weak Charpy impact properties were obtained in the case of both types of as-hipped materials. In the case of 14Cr materials, the weak Charpy properties appeared related to a bimodal grain size distribution and a heterogeneous dislocation density between the coarse and fine grains. No changes in microstructure were evidenced after heat treatment at 850 degrees C. Significant improvement in the transition temperature and upper shelf energy of 12Cr materials was obtained by heat treatment at 850 degrees C for I h, which was attributed to the formation of smaller grains, homogenous in size and containing fewer dislocations, with respect to the as-hipped microstructure. This modified microstructure results in a good compromise between strength and Charpy impact properties. (C) 2007 Published by Elsevier B.V.