Research On The Phase Transition And Precipitated Carbides Of ZG1Cr10MoWVNbN Heat-resistant Steel

This paper measured the DSC curves of heat-resistant steel ZGlCr10MoWVNbN in temperature increasing and decreasing periods by DSC experiments to analyze the phase transformation kinetics of the materials; the kinds of precipitated phases and the precipitation sequence were also calculated by Thermo-Calc software; Simultaneously, the precipitated carbides before and after aging were extracted by means of anode extraction. Then the type, morphology, size, distribution and the influence of carbides on the performance of steel were discussed through the X-ray diffraction and transmission electron microscope.The results indicate that:The phase transformation of heat-resistant steel ZGlCr10MoWVNbN in the heating stage are divided into Martensiteâ†'Austenite and Martensite + Austeniteâ†'Austenite two stages, and the phase transformation activation energy is 2785 KJ/mol and 875 KJ/mol respectively in this two position; Materials in the cooling stage transform from austenite to marstenite and the phase transformation activation energy is-55KJ/mol. The phase transformation activation energy is negetive value, indicating that the phase transition drive force is infinite and martensitic transformation is spontaneous. Martensitic transformation is reversible for the reaction order n both during the process of temperature rising and dropping are 1, moreover, the reaction mechanism of either martensite transforming to austenite or austenite transforming to martensite is the same, which includes nucleation and growth.The main equilibrium precipitated phases of heat-treatment steel ZGlCr10MoWVNbN between 400℃and 1400℃are M23C6, MX and Laves by simulated calculation of thermodynamics software Thermo-Calc. The equilibrium transformation from austenite to ferrite is from 900℃to 780℃). The initial transformation temperature of MX, M23C6 and Laves are 1200℃,900℃and 700℃respectively.It is showed that the main precipitated phases of heat-resistant steel are M23C6, MX, and Laves by select electron diffraction and XRD analysis. The M23C6 carbides usually precipitate at the boundary or within lath, which morphology is mainly irregular spherical shape or short strip. However, the MX carbides dispersedly distribute in crystals in the shape of small spherical particles or flaky particles. The brittleness of materials increases and microstructure obviously undergo degradation after aging at 600℃for 17,000 hours. The steel performance deteriorates after aging mainly because of the coarsening and nodulizing of carbides on grain boundaries or within crystals.The microstructure of heat-treatment steel ZGlCr10MoWVNbN is the lath martensite and the strengthening mechanisms of the steel include martensite strengthening, solid-solution strengthening and carbides precipitation strengthening. And the strength at high temperature of heat-treatment steel ZGlCr10MoWVNbN can be maintained due to the precipitated MX carbides, which is fine and dispersive in the long-term high-temperature process of aging.