Heat treatment as a method of restoring of the material 15Cr12WNiMoV after operational processes

The article presents the results of studies of the post-operational state of the turbine rotor blade material of the power plant (operating time is about 30000 hours). High-temperature fatigue processes occurring in the heat-resistant steel 15Cr12WNiМoV have been studied. These processes manifested themselves in changing microstructure and mechanical characteristics. Various heat treatment modes were tested to regenerate the structure and restore the mechanical properties of the alloy in order to develop the scientific foundations of the resource-saving technology of turbine blades.


Introduction
One of the main units of gas turbine plants is a gas turbine, and the most critical parts with the highest quality operational requirements are turbine rotor blades. They operate under conditions of simultaneous exposure of centrifugal and vibrational loads and erosivity of high-temperature gas flow. The temperature margin ensuring the structural stability of the alloy is 50-100 °C. Often parts operate at the upper-temperature limits, and in emergencies, there may be critical overtemperature, which affects the operation capability of the material. Therefore, the study of processes of high-temperature fatigue and their reversibility is important both from a scientific and practical points of view. It is necessary for the development of modes of reductive heat treatment as part of resource-saving technologies [1,2].

Methods of research
Samples for studying were cut from the most thermally loaded zone (trailing edge) of the first part of the blades made of the martensitic-ferrite grade of the heat-resistant steel 15Cr12WNiМoV. These blades were used at temperatures up to 780 °С for approximately 30000 hours.
Metal micrographic tests were performed on the micrographic specimens on the «Axio Observer» optical microscope and the «VEGA TESCAN II» electron microscope. Microhardness testings were carried out on a microhardness tester «ПМТ-3» at aт indentation load of 2 N with the State Standard 9450-76 (measurement error was not exceeding 2%). From the experimental microhardness values, the plasticity coefficient of the material was determined by the formula [3]: δ= 1-14,3⋅(1-ν-2ν 2 )⋅(Н/Е), where H is the average value of microhardness, ν is the Poisson coefficient, E is the elastic modulus (E = 216 GPа for the steel 15Cr12WNiМoV [4]). Mechanical pulling tests were carried out on flat samples on a tension testing machine «У10Т».
Fine-needled (sorbitic) martensite of tempering is preserved in the structure (figure 2c) after additional tempering at 700 °С (holding for 5 hours) and furnace cooling (mode 5). The microhardness of the material is 2.0 GPa. a, х500 b, х5000 c, х500 c, х500 d, х10000 After the heat treatment in mode 3, fine-needled martensite or secondary sorbite with the saved martensite orientation in it (needle length: 2.0-4.0 µm) is formed in the structure, which corresponds to assessment 3-2 (figure 4a). The microhardness of the material is 3.2 GPa The application of an additional tempering in mode 7 leads to the growth of needles of martensite (to 8 µm) ( figure 4). The Microhardness of the material is 2.85 GPa.
Heat treatment in mode 4 leads to the formation of medium-needled martensite (assessment 6-7, [4]) ( figure 5a, b). The microhardness of the material is 3.0 GPa. Tempering at 700 °С for 5 hours (furnace cooling) in mode 8 leads to the formation of tempered martensite ( figure 5c) and, as a result, to a slight decrease in the microhardness of the material to 2.7 GPa.
According to the results of metallographical tests, the optimal heat treatment mode (ageing at 740 °С for 5 hours, air cooling and tempering at 700 °С for 5 hours, furnace cooling), which contributes to the formation of a stable microstructure of the mediumneedled martensite, was chosen. Microhardness of the material after ageing at 740 °С for 5 hours with air cooling is 2.4 GPa and doesn't change after the repeated tempering. a b Fig. 4. The microstructure of the steel after the heat treatment: in mode 3 (a) and in mode 7 (b), x500 a, х500 b, х10000 c, х500 Using the experimental values of the microhardness, theoretical estimates of the plasticity coefficient (δ), were made (Fig. 6). This coefficient characterizes the ability of the material to perceive elastic and plastic deformations. To ensure a sufficient level of plasticity of the long-lived metal material, the plasticity coefficient must be at least δ = 0,8. In the hardness range Н min =2.0 GPа; Н max =3.2 GPа plasticity coefficient is within acceptable range: from δ = 0.89 to δ = 0.82. The average hardness value will meet this plasticity criteria. Table 1 shows the results of tests of mechanical properties of steel samples 15Cr12WNiМoV after the operation and reductive heat treatment. Changing the microstructure during the long-term operation leads to an increase in resistance to rupture by 1.33 times (from 740 МРа to 940-1030 МРа) and a decrease in flow limit by 2.1 times (from 590 МРа to 280 МРа). The difference between the values of these limits diminishes severely. The material is in a hardened condition with low ductility (percentage of elongation below the normalized value). The probability of the brittle fracture increases.