Effects of Additive Elements on Properties of Fe-Cr-B Gas Atomized Powder with High Hardness

In order to obtain a high hardness shot peening media, the properties of Fe-8%Cr-6.5%B media (Base media) with several additive elements were investigated. Fe-8%Cr-6.5%B-X%M media (X%M=2%Zr, 2%Nb and 2~16%W; Denoted hereafter as X%M media) were produced by gas atomization. Vickers hardness, density and ductility of these media were examined. The properties of Base, 2%Zr, 2%Nb and 2%W media were examined. Vickers hardness of 2%W media was high, but those of 2%Zr and 2%Nb media were low compared to that of Base media. Then, the properties of Base and 2∼16%W media were examined. Vickers hardness of 4%W showed the first peak, then decreased until 8%W. Furthermore, in the range of W addition amount beyond 8% Vickers hardness increased again. 4%W media with the combination of high hardness of 1300 HV, high density of 7.6 Mg/m3 and high ductility exhibited the best performance as shot peening media in this study. From the results of X-ray diffraction, transmission electron microscope (TEM) observations and the evaluations of Vickers hardness after heat treatment at 973 K, it was inferred that 0∼4%W media were strengthened by 250∼300 HV due to supersaturated solid solution of B element with αFe phase. In addition, all media with W addition were strengthened due to solid solution of W element with αFe phase. According to the simulation from the theoretical formula proposed by Ogawa and Asano, it was estimated that the maximum compressive residual stress at carburized steel surface shot peened by 4%W media was approximately 20 MPa higher than that by Base media.