OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 boiler has increased from 450 °C to 568 °C, and the pressure has increased approximately six times to 25 MPa, which increases the boiler efficiency. The equipment used to manufacture the boiler consists of a manifold and pipes of various designs. Components such as pressure vessels require low water and steam temperatures. Pressure vessels are mainly exposed to steam and water from the boiler, as well as flue gases from economizers, furnace walls, preheaters and superheaters. The components are mainly made from small diameter steel tubes. Increasing the thermal efficiency of power plants by increasing the operating temperature and pressure of steam entering the turbine has led to the development of a new category of heat-resistant steels. The most commonly used materials in power plants operating at high temperatures and high pressures are ferritic/martensitic steel with improved creep resistance, nickel-based superalloys and austenitic stainless steel [1–5]. Potential candidate materials for ultra-supercritical power plants are Ni-based alloys such as Inconel 617, Inconel 625 and Inconel 740 [3, 4]. These Ni-based alloys have excellent corrosion resistance, good oxidation resistance and high creep strength at 650 °C. However, since Ni, Cr and Mo are the key alloying elements in these alloys, these Ni-based alloys are expensive [5–9]. In addition, these alloys are technically difficult to manufacture. In the mid-1960s, 12% CrMoV steels were developed for thin- and thick-walled power plant components. The operating temperature of such components was 565 °C. The creep strength of 12 % CrMoV steels was achieved by solid solution strengthening and precipitation strengthening. Modern boilers use chromiummolybdenum steels 5Cr-1Mo, 9Cr-1Mo, modified 9Cr-1Mo steels with Nb, V, W or 12Cr, which have better thermal and mechanical properties compared to 300 series austenitic stainless steels. Domestic analogues are steel 0.15C-5Cr-Mo and its modifications 0.15C-5Cr-Mo-V and 12C-8Cr-W-V [10, 11]. Chromium (Cr), tungsten (W) and molybdenum (Mo) are the main alloying elements present in steel and provide better creep resistance at elevated temperature and pressure. The strength of chromiummolybdenum steels is due to its high dislocation density. Materials soften as dislocation density decreases, for example, when dislocations move, meet, and annihilate each other. Steels with a Cr content of 2–13 % maintain dislocation density at high temperatures and, therefore, strength, since the microstructure slows down the movement of dislocations. It is difficult for dislocations to cross grain boundaries, and carbides and precipitates along grain boundaries are relatively immobile and cause dislocation pinning, as shown in [2–5]. Creep is a thermally activated process. It is defined as the slow unsteady deformation of a material under the influence of a constant load. The high operating temperature and pressure requirements of such a modern power plant are leading to the development of creep strength enhanced ferritic (CSEF) and martensitic steels. For nuclear and thermal power plants, creep strength enhanced ferritic (CSEF) steels are considered to be a better material than austenitic stainless steel due to its low coefficient of thermal expansion, good thermal conductivity and high creep strength. Creep occurs due to prolonged exposure of the material to a constant applied stress below yield strength of the material. It is necessary to know the mechanical properties of steel, including reduction in Young’s modulus, yield strength, and reduction in tensile strength at various stress levels and elevated temperatures. To reveal mechanical properties at elevated temperatures, stress-strain relationships should be established. Currently, both steady state and transient state tests are used to measure the mechanical properties at high temperatures. In this case, temperature-dependent physical mechanisms, such as volume diffusion, glide and climb dislocations, are a response to creep phenomena in a crystalline material. Fusion welding (manual arc welding, gas-shielded welding, submerged welding) is a commonly used welding process for steel 0.15C-5Cr-Mo and its modifications 0.15C-5Cr-Mo-V and 0.15C-5Cr-V-W, which includes intense heat input and its dissipation into the base metal due to thermal conductivity [6–9]. Preheating when welding heat-resistant steels prone to hardening helps ensure the quality of the weld and reduces the likelihood of cracking. Before welding steel pipes or plates up to 20 mm thick or more in workshops or on-site, preheating to 300-450°C is commonly used [11–18]. The welding process is usually followed by induction heat treatment to replace the coarse microstructure associated with high heat input during the joining operation with finer pre-austenite grains and fine ferrite phases.
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