Observation and Analysis of Phosphides in Steels Using a Non-aqueous Electrolyte-Potentiostatic Etching Method

Abstract

Phosphides in carbon steel and stainless steel are studied using the non-aqueous electrolytepotentiostatic etching method. The results are summarized as follows.
(1) Niobium phosphide is found in austenitic stainless steel (SUS 347). The phosphide is fine and eutectic and located in fracture surface of the steel strained at high temperature by Gleeble Test Device. According to the X-ray diffraction analysis of the residues extracted from the steel, the phosphide is identified as Nb₈P₅ (orthorhombic: a₀=2.62 nm, b₀=0.947 nm, c=0.346 nm).
(2) Titanium phosphides are found in continuously cast slab of ferritic stainless steel (SUS 430). The phosphides are needle-, plate-like and dendritic. From the X-ray diffraction analysis of the phosphides, Ti₃P(Cr₃PN type, orthorhombic), Ti₄P₃(cubic) and TiP(hexagonal) are found.
(3) Titanium phosphide is found in carbon steel (0.014%P-0.16%Ti). The phosphide is identified as Ti_∼1.7P(orthorhombic) by X-ray diffraction analysis of the extracted residues. Ti/P ratio of the residues is about 1.2. The phosphide precipitates above 873 K and the maximum of the precipitation is found in the temperature range from 973 to 1073 K.