Effects of the Destabilisation Heat Treatments on the Precipitation of Secondary Carbides and Their Effect on the Corrosion of 27 wt.% Chromium White Cast Iron

Abstract

A 27 wt.% Cr white cast iron has been subjected to various destabilization heat treatments. The transformation of the matrix phase as well as the precipitatiopn of secondary carbides by destabilization heat treatments, which have been clearly determined in this paper. The results revealed that in the destabilization temperature (about 880°C), the matrix phase is enriched with elements such as C and Cr, due to dissolution of eutectic carbides at high temperature. The secondary carbides were precipitated along grain boundary of C, Cr- rich matrix, they grew up within the matrix phase. When the destabilization temperature increases up to 1000°C the number, volume and size of secondary carbides also increase, respectively (secondary carbide size up to 2,22 µm). At 1050°C/3 h, the size of secondary carbides reduce significantly with a high distribution density in the matrix phase (grain size reduce to below 0,8 µm). At 1100°C and holding time for 3 hours, secondary carbides were dissolved into the matrix, and therefore, reduce the number and grain size of secondary carbides. Effects of secondary carbides on corrosion of alloy were determined by polarization test of alloys in H₂SO₄ 5 vol.% solution at room temperature, by the depth of corrosion layer and by microstructure analyzing of corroded surfaces of alloys. The 1050°C/3 h alloy is the best corrosion resistance between the tested alloys with a large amount of fine secondary carbides and uniformly distributed within the matrix.