X12CrMoWVNbN10-1-1 heat-resistant steel is widely used in the manufacture of large castings and forgings such as ultra-supercritical steam turbine rotors, main steam valve bodies, and gas turbine disks. It is a typical (9% to 12%) Cr (mass fraction) heat-resistant steel, with low thermal expansion coefficient, high thermal conductivity, good high-temperature creep performance, and corrosion resistance. It has high endurance strength at about 600 ℃, and is widely used in the manufacture of high-temperature service components of ultra-supercritical generators, For example, large castings and forgings such as high-pressure rotors and steam valve bodies in steam turbine units [1]. However, in practical applications, it has been found that the elongation of X12CrMoWVNbN10-1-1 steel at 400 ℃ is lower than room temperature, and its plasticity is poor. This is obviously not conducive to the widespread application of X12CrMoWVNbN10-1-1 steel in practical production within a wider temperature range, laying a safety hazard for production
At present, many domestic and foreign researchers have focused on the heat treatment process of X12CrMoWVNbN10-1-1 steel Yang Gang et al. [2] studied the effect of quenching and tempering cooling rates on the mechanical properties of the steel at room temperature. Chilukuru [3] studied the effect of precipitation and coarsening of carbonitrides on creep strength during long-term high-temperature creep at 650 ℃. G Kutz et al. [4] studied the effect of heating processes on precipitation of strengthening phases in the steel. Tao et al. [5] The effect of high temperature tempering above 570 ℃ on the precipitation behavior of precipitated phases in this steel has been studied. However, there has been little research on the mechanical properties of X12CrMoWVNbN10-1-1 steel within the temperature range of about 400 ℃. In this paper, mechanical tests have been conducted on X12CrMoWVNbN10-1-1 steel at 300~600 ℃, and the microstructure of tensile specimens at various temperatures has been observed and analyzed to explore the effect of temperature on the mechanical properties and microstructure of X12CrMoWVNbN10-1-1 steel.
1.Experimental Materials and Methods
The X12CrMoWVNbN10-1-1 steel used in the experiment was taken from the steam valve body of the ultra supercritical steam turbine, and its chemical composition is shown in Table 1. The valve body was melted in an alkaline electric furnace, refined in a ladle, and refined in a vacuum induction furnace, and then cast into a casting at about 1560 ℃. After heat treatment, it was produced. The heat treatment process is 1 050 ℃ annealing furnace cooling+1 100 ℃ normalizing air cooling+740 ℃ tempering.
Cut from experimental material φ 5 mm × 25 mm tensile specimens were subjected to tensile testing on a SANS universal testing machine under high temperature conditions of 300, 350, 400, 450, 500, 600 ℃. High temperature transient tensile testing was conducted in accordance with the standards specified in GB/T4338-2006 High Temperature Tensile Testing of Metallic Materials, with the values of 2 × Conduct tensile testing at a strain rate of 10-4s-1. During high-temperature transient tensile testing, first heat the tensile sample to the test temperature at 10 ℃/min, and hold it at this temperature for 1 hour before conducting uniaxial tensile testing. Then, observe the fracture morphology of the tensile sample, and take a sample near the fracture for microscopic observation and analysis
The sample was successively polished with 400 # to 2000 # abrasive paper, and polished. After polishing, it was etched with a mixture of 5 g of FeCl3, 25 mL of HCl, and 25 mL of ethanol. The metallographic structure was observed under an OLYMPUS DSX500 metallographic microscope. The scanning structure and tensile fracture were observed using a Zeiss Ultra Plus field emission scanning electron microscope. A 0.5 mm thin slice was cut along the cross section about 5 mm from the fracture and ground to 50 mm μ M thick, punched out φ A 3 mm circular plate was thinned using a double jet electrolytic polishing method to prepare a TEM sample. The electrolyte was a mixed solution (volume fraction) of 95% CH3COOH and 5% HClO4, and the electrolysis temperature was below - 30 ℃. TEM observations were performed on a FEI Tecnai G20 transmission electron microscope.
2. Results and Discussion
The high-temperature tensile test results of X12CrMoWVNbN10-1-1 steel can be seen that within the test temperature range, when the temperature is below 400 ℃, the strength of the material decreases slowly, and even when the tensile strength is at 350 ℃, there is a slight increase. As the temperature increases, the rate of strength reduction gradually increases. Unlike the change in strength values, within the range of 300 to 600 ℃, the elongation of the material decreases first and then increases rapidly, with the elongation at 400 ℃ being 14.2%, Minimum reached.
The morphology of the tensile fracture surface of X12CrMoWVNbN10-1-1 steel at some temperatures. Within the test temperature range, the fracture mode of the material is ductile fracture, with a large number of dimples distributed on the fracture surface. The fracture surface of the samples at 300 ℃ and 400 ℃ has small and dense dimples, but some large dimples appear in the samples at 300 ℃, indicating good toughness. After the temperature rises to 500 ℃, the dimple size increases significantly, It indicates that the toughness gradually increases, and there is a good corresponding relationship between the tensile fracture and the change in material plasticity.
