TEM study on precipitation and transformation of secondary carbides in 16Cr–1Mo–1Cu white iron subjected to subcritical treatment

High chromium white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The subcritical heat treatment can destabilize the austenite by precipitating chromium-rich secondary carbides and other special carbides. In the as-cast condition the eutectic carbides are (Fe,Cr)7C3 and (Fe4.3Cr2.5Mo0.1)C3. The initial secondary carbide precipitated is (Fe,Cr)23C6 after heat-treating at 853 K for 10 h. There are MoC, Fe2MoC and ɛ-carbide precipitating, and (Fe,Cr)23C6 transforms to M3C after 16 h at 853 K. The ɛ-carbide and (Fe,Cr)23C6 accomplish transformation to M3C and the matrix changes from martensitic to pearlitic after 22 h at 853 K. Thereby, in the subcritical heat treatment process, the initial secondary carbide precipitated is (Fe,Cr)23C6, followed by ɛ-carbide, MoC and Fe2MoC. In addition, there are two in situ transformations from (Fe,Cr)23C6 and ɛ-carbide to M3C carbides.