The authors present a numerical study of a start-up of a boiler with a thick-walled element subjected to thermomechanical loading. The significance of calculations of real heat transfer coefficients has been demonstrated. Fluid dynamics, mechanical transient thermal and static structural calculations have been conducted in both separate and coupled modes. Strain-stress analyses prove that the effect of the heat transfer coefficient changing in time and place in comparison with a constant one as recommended by standards is the key factor of fatigue calculations.
The experimental material consisted of semi-finished products of high-grade, medium-carbon constructional steel with: manganese, chromium, nickel, molybdenum and boron. The experimental material consisted of steel products obtained in three metallurgical processes: electric and desulfurized (E), electric and desulfurized with argon-refined (EA) and oxygen converter with vacuum degassed of steel (KP). The production process involved two melting technologies: in a 140-ton basic arc furnace with desulphurisation and argon refining variants, and in a 100-ton oxygen converter. Billet samples were collected to analyze: relative volume of impurities, microstructure and fatigue tests. The samples were quenched and austenitized at a temperature of 880o C for 30 minutes. They were then cooled in water and tempered by holding the sections at a temperature of 200, 300, 400, 500 and 600o C for 120 minutes and air-cooled. Fatigue tests were performed with the use of a rotary bending machine at a frequency of 6000 cpm. The results were statistical processed and presented in graphic form. This paper discusses the results of microstructural analyses, the distribution of the relative volume of impurities in different size ranges, the fatigue strength characteristics of different production processes, the average number of sampledamaging cycles and the average values of the fatigue strength coefficient for various heat processing options.
Paper presents the assessment of impact of heat treatment on durability in low-cycle fatigue conditions (under constant load) in castings made using post-production scrap of MAR-247 and IN-713C superalloys. Castings were obtained using modification and filtration methods. Additionally, casting made of MAR-247 were subjected to heat treatment consisting of solution treatment and subsequent aging. During low-cycle fatigue test the cyclic creep process were observed. It was demonstrated that the fine-grained samples have significantly higher durability in test conditions and , at the same time, lower values of plastic deformation to rupture Δϵpl. It has been also proven that durability of fine-grained MAR-247 samples can be further raised by about 60% using aforementioned heat treatment.