[Technical Frontier] Research on the microstructure changes in thermal aging of 12Cr steel for turbine rotors World Metal Guide High Cr-type heat-resistant steel is used in boiler materials and turbine rotor materials for thermal power generation equipment. In order to understand

[Technical Frontier] Study on the microstructure changes in thermal aging of 12Cr steel for turbine rotors

World Metal Guide

High Cr-type heat-resistant steel is used in boiler materials and turbine rotor materials for thermal power generation equipment. In order to understand the high temperature intensity, it is important to understand the growth and disappearance mechanism of precipitates at high temperatures. In boiler materials, there are many reports on the life and precipitation behavior of creep . On the other hand, turbine rotor materials are independently developed by each company, so the chemical composition is different. It is difficult to say that sufficient data has been accumulated regarding the precipitation behavior. In this paper, a thermal aging test was carried out on the turbine rotor using 12Cr steel, and the changes in the microstructure accompanied by thermal aging were investigated.

test materials are 12Cr steel for high-pressure and medium-pressure turbine rotors, and the chemical composition is shown in Table 1. The test material was air-cooled at 1050°C×35min, 1 tempering at 550°C×1.5h and 2 tempering at 650°C×20h. The aging temperature and aging time are respectively varied for the material to make thermal aging materials. The aging temperatures are 600℃, 625℃ and 650℃, and the aging time is 1000h, 3000h and 10000h. Observe the microstructure of unaging materials and thermally aged materials to measure Vickers hardness. In microstructure observation, precipitates were identified by elemental analysis using scanning transmission electron microscope .

Figure 1 is the hardness measurement result of each aging material. According to the tendency to soften the thermal aging, the higher the aging temperature, the longer the aging time, and the more significant the hardness decreases. Figure 2 is the STEM observation results of the unaging material and the aging treatment material of 650°C×10000h. The unaging material in Figure 2(a) is the tempered martensite tissue, with high density distributions of tens of nanometers and M23C6 from tens of nanometers to hundreds of nanometers. At different fields of view, tens of nanometers of fine MX were also found. The aging material of 650°C × 10000h in Figure 2 (b) is the replies of the dislocation lower tissue martensite tissue . Like the unaging materials, M2X, M23C6 and MX were also found, and there was no difference in the types of precipitated phases before and after aging. It has been reported that in high Cr-type heat-resistant steel, the Laves phase and the Z phase precipitate through long-term aging at high temperatures, but these phases were not found in this test. One of the reasons is that the chemical composition is different.

▲ "World Metals Guide" Issue 45 B01

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