TEM study on precipitation and transformation of secondary carbides in 16Cr-lMo-lCu white iron subjected to subcritical treatment

摘要

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)_7C_3 and (Fe_(4.3)Cr_(2.5)Mo_(0.1)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, Fe_2MoC and e-carbide precipitating, and (Fe,Cr)_(23)C_6 transforms to M_3C 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 e-carbide, MoC and Fe_2MoC. In addition, there are two in situ transformations from (Fe,Cr)_(23)C_6 and s-carbide to M_3C carbides.