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)(7)C-3 and (Fe4.3Cr2.5Mo0.1)C-3. The initial secondary carbide precipitated is (Fe,Cr)(23)C-6 after heat-treating at 853 K for 10 h. There are MoC, Fe2MoC and epsilon-carbide precipitating, and (Fe,Cr)23C6 transforms to M3C after 16 h at 853 K. The e-carbide and (Fe,Cr)(23)C-6 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)(23)C-6, followed by epsilon-carbide, MoC and Fe2MoC. In addition, there are two in situ transformations from (Fe,Cr)(23)C-6 and epsilon-carbide to M3C carbides. (C) 2004 Elsevier Inc. All rights reserved.