Characterization of borides formed on impurity-controlled chromium-based low alloy steels

Abstract

This study reports on characterization of borides formed on impurity-controlled steels essentially containing 0.3% C, 0.20 P, 0.5% Cr; 0.3% C, 0.02% P, and 1.0% Cr. The formation of borides on the impurity-controlled chromium-based low alloy steel substrates was performed using thermochemical boronizing technique at 940 degrees C for 5-7 h. The boronizing molten salt bath, in which samples were immersed, consisted of borax, boric acid, and ferro-silicon. The presence of non-oxide ceramic borides (FeB, Fe2B, CrB, Cr2B) was confirmed by the classical metallographic technique, scanning electronmicroscopy and X-ray diffraction analysis. A wavelength dispersion X-ray dot map showed that boron diffused into the substrate, i.e, the diffusion layer is thicker than that of the coating layer. The hardness of non-oxide ceramic borides formed on the surface of steel. substrate were measured using a Vickers microhardness tester with a load of 2 N. The hardness of non-oxide ceramic borides was over 1500 HV. Depending on boronizing time, the thickness of borides ranged, from 25 to 200 mu m and the fracture toughness of non-oxide ceramic borides ranged between 3.42 and 4.40 MPa m(1/2). It was observed that chromium has a negative role on the fracture toughness and boride layer thickness. Metallographic analysis revealed that the borides formed on the surface of steel have a columnar structure. (C) 1999 Elsevier Science S.A. All rights reserved.