Effects of Electric Current Pulse Treatment on Phosphide Precipitation Behavior in 304 Stainless Steel

The precipitation behavior of phosphides in melting and solidification of 304 stainless steel under treatment using an electric current pulse (ECP) is studied. The composition and crystal structure of phosphides are confirmed and analyzed via scanning electron microscope and transmission electron microscope (TEM). The results show that the phosphides are composed of Cr, Mn, S, and P in the sample that is quenched at 1773 K without ECP treatment. From the high-resolution TEM images and selected area electron diffraction, the crystal lattice structure of phosphides is the same as that of Cr3P. Mn and S replace some of the Cr and P in the Cr3P lattice because of the replacement solution. Phosphides are hardly detected in the samples that are treated using ECP. ECP treatment inhibits the formation of Cr3P. Phosphide is mainly presented in the form of CrP4 in the samples that are quenched at 1583 K, which contain a large number of nanoaustenite grains. Meanwhile, ECP treatment inhibits the growth of phosphides because of differences in the conductivity between molten steel and phosphides during solidification. The size of phosphides decreases with an increase in current density.