Studies On The Microstructures And Properties Of12Cr-W-Mo-Co Martensitic Heat Resistant Steels

With the development of society and economy, the persistent eruptible growths of global energy consumption and environmental pollution have put forward an urgent requirement for high efficient utilization of the fossil energy. Ultra-supercritical (USC) technology with larger capacity and higher steam parameters is one of important ways to address the issue of energy conservation and environment protection, which inevitably requests the development of the heat-resistant steels (HRSs) with high performance in advance. The martensitic steels in9-12%Cr type represent one of the important branches of HRSs. The martensitic HRSs show low cost, excellent thermal conductivity, low expansion coefficients, comprehensive mechanical properties, excellent creep and fatigue resistance, and oxidation resistance. However, the traditional martensitic HRSs will not satisfy the requirement of USC power units with larger capacity and higher steam parameters. In this work, novel12Cr-W-Mo-Co martensitic HRSs have been developed on basis of typical9-12%Cr HRSs. The microstructure, heat treatment technology, room and high temperature mechanical properties and oxidation resistance were systematically investigated. The strengthening and oxidation mechanisms for these12Cr-W-Mo-Co steels were discussed in detail.The new12Cr-W-Mo-Co steels were designed by the adjustment of ferrite-forming elements of Cr, W and Mo and the addition of austenite-forming element of Co. The main elements of these steels are0.1%C,12%Cr,0-5%Co,2.5%W,0.5%Mo,0.2%V,0.05%Nb,0.8%Cu,0.5%Si、0.5%Mn、0.05%N,0.005%B and Fe bal. The HRS bars were prepared by induction melting and forging method. The thermal-equilibrium state microstructures and phase transformation temperatures were studied by Thermo-Calc thermodynamic modeling, thermal expansion and differential scanning calorimeter (DSC). It is shown that the addition of Co suppresses formation of delta ferrite, increases the re-melting temperature of secondary phases, so that improves the microstructure stability. The microstructure of new steel without Co contains a large amount of delta ferrite. With the addition of3%Co, the faction of delta ferrite has been lower than0.2. The delta ferrite is completely suppressed and the microstructure is composed of full martensitic matrix when the Co element was increased to5%Co. On the basis of microstructural characterization, the heat treatment temperatures were determined as:normalizing temperature is from1050℃to1100℃; tempering temperature is from750℃to800℃, and both of the soaking processes were followed by air cooling.Through the combination of heat treatment and composition design, excellent tensile properties and creep resistance for12Cr-W-Mo-Co HRSs were obtained. It is indicated that the strengths of present steels were obvious increased with the increase of Co element. At room temperature, the tensile strength (Rm), yield strength (Rpo.2) and the elongation (A) are772MPa,593MPa and21.84%for12Cr-3Co steel, and887MPa,652MPa and21.07%for12Cr-5Co steel, respectively. At650℃, the Rm and Rp0.2of12Cr-5Co steel reach395MPa and382MPa, respectively. What is more, the Rm and Rpo.2of12Cr-5Co steel are even higher than those of typical martensite steel T/P122by27.4%and22.1%at675℃, respectively. If one evaluates the performance of12Cr-W-Mo-Co steels by the high temperature strengths, the service temperature of the present steels can be increased by50-75℃. In addition,12Cr-3Co steel has excellent short-term creep deformation resistance by comparing the steady-state creep rate with some typical9-12%martensitic HRSs.The strengthening mechanism and microstructure stability were investigated by the analysis of the microstructure of12Cr-3Co steel after heat treatment and creep deformation at650℃. It is shown that the enhancement of mechanical properties result from the martensitic lath, the precipitation of nanoscale M23C6and MX compounds, and the composition strengthening effect of the precipitation and dislocation networks. During the aging treatment at650℃, the martensitic laths of12Cr-3Co steel held relatively stable. The quantity and size of precipitates was increased little by little with aging time, and the coarsening rate of precipitates slowed down after a period of time. The coarsening of secondary phases, especially Laves phase, is closely related with delta ferrite. The excellent short-term creep resistance properties of12Cr-3Co steel is closely related to the increase of precipitates and dislocation density, and the creep failure is due to the fast coarsening of Laves phase.The oxidation behavior and the phase constituent of the scale of12Cr-W-Mo-Co HRSs were systematically investigated at650℃in air and air with10%vapor. And the oxidation mechanism of present steels was also discussed. The atomic structure of the matrix/scale interface was studied by using the Focused Ion Beam (FIB) and high resolution Transmission Electron Microscopy (HRTEM). In air, the oxides of upper scale are mainly inerratic geometrical shape Cr2O3with a little amount of Cu and Mn, and sheet-shaped Cr2O3. The scale is mainly consisted of Cr2O3and a little amount of Fe2O3/Fe3O4(named as (Fe, M)O/(Fe, Cr)2O3) has a spinel structure to prevent the steel matrix from continuous oxidation. The interface between the oxide scale and matrix has coherent or semi-coherent structure. The scale is found to contain nanocrystalline grains, high-and low-angle boundary. In air/vapor, the oxidation rate was accelerated obviously due to the existence of H2O. The oxides near the surface of scale are mainly irregular shape Fe2O3/Fe3O4. The scale has layered structure:the external layer is Fe2O3/Fe3O4, the internal layer is FeO/(Fe, Cr)2O3with spinel structure and enriched with Cr2O3, and the transition layer with Fe-rich, Fe-poor, Cr-rich and Cr-poor regions. It was indicated that the internal layer with spinel structure is the main protective layer for the steel matrix.The oxidation mechanism of12Cr-W-Mo-Co steels was discussed in detail. In air, O was found to directly react with Fe to form the scale of Fe2O3/Fe3O4, where the Cr2O3would form and grow. The scale of (Fe, M)O/(Fe, Cr)2O3with spinel structure and mainly enriched with Cr2O3was formed by the diffusion of O, Fe, Co, Mn and Cu. In air/vapor, the external layer of Fe2O3/Fe3O4and internal layer of FeO/(Fe, Cr)2O3were found to forme by oxidation reaction of H2O and O with steel matrix. The H+ions, originate from the decomposition of H2O, spread into the internal scale and result in the decomposition of FeO/(Fe, Cr)2O3, and production of Fe3+/Fe2+and H2O. The Fe2+/Fe3+ions diffuse onto the surface of the external layer and react with O to form Fe2O3/Fe3O4, resulting in the growth of external scale. H2O formed from the decomposition reaction and O in the environment enter into the steel matrix and form FeO/(Fe, Cr)2O3, so the internal scale grows. Due to the deletion of Fe and Cr during the oxidation process, the concentration gradient of alloy elements near the steel matrix forms through the diffusion and hinder effect of scale, and eventually resulting into the formation of transition layer.