G. Junak

Influence of Pearlite Morphology on Creeping Characteristic Curves of R260 Steel

The elements of railway turnouts made from rail sections of R260 high carbon steel must have a high resistance to abrasive and contact fatigue wear, as well as a good resistance to cracking under service loads. These mechanical properties largely determine the suitability of steel for use as railway track components. In this study, two groups of specimens were subjected to tests of mechanical properties and metallurgical analyses. The first group included material obtained from the hot-rolled block section, the rolling end temperature being Tkw = 950°C, while the second group was material after the rolling process with the subsequent 20-minute isothermal annealing at a temperature of 480°C. The microstructure of tested materials (Rm, Rp0.2, A5, Z, HB) was characterized, and their basic mechanical properties and fracture toughness in the KIc plane strain condition were determined. Also the effect of temperature, ranging from-80°C to 100°C, on the KCU impact toughness of R260 steel was established. Based on the SEM observation using the Hitachi S-3400N scanning microscope, it was found that pearlitic structure with a varied distance between cementite plates, equal to 0.29 μm and 0.09 μm, respectively, appears in the hot-rolled R260 steel and in the steel subjected to additional isothermal annealing treatment. The impact of pearlite morphology on the cracking characteristics and basic mechanical properties of materials was analyzed. It was found that at room temperature, the higher fracture toughness (KIc = 66.4 MPa·m1/2) is shown by the steel after isothermal annealing at 480°C, in which less distance between the plates of cementite has been observed in the perlite. The fracture toughness of R260 steel after hot rolling at 950°C was KIc = 48.3 MPa·m1/2. As in the case of fracture toughness, the impact strength of R260 steel after isothermal annealing was significantly greater than the impact strength of steel only after hot rolling. The determined cracking characteristics of R260 steel make it possible to determine the effect of heat treatment on the formation of microstructure and material properties, which determine the service life of rail sections.

Low cycle fatigue life and creep life of material in tube bends made of 15NiCuMoNb5-6-4 steel

The article addresses results of tests of performance characteristics of a tube bend made of the 15NiCuMoNb5-6-4 steel, the latter being for manufacture of feed water pipelines used in power engineering systems. The tube bend was developed by bending with local induction heating, and the optimum tube bending parameters were established based on numerical simulations of the bending process. The mechanical properties of the tube bend were attained through quenching and tempering. Tests of mechanical properties were conducted using samples collected at different zones of the tube bend (the straight one as well as those subject to tension and compression), and compared with properties of the as-delivered tube material. The established basic mechanical properties of the tube bend, required under the applicable standards, were supplemented with results of low-cycle fatigue and creep tests, the purpose of which was to acquire information on the tube bend material service life under conditions of steady and unste...

Fatigue Behaviour of Carbide Precipitation Hardened Austenitic Steels

The paper presents the results of investigations of the microstructure and fatigue behaviour of two newly invented Cr-Ni and Cr-Ni-Mn austenitic steels of 13/13 and 12/8/8 type strengthened through carbide particle precipitation. The specimens of the investigated steels were subjected to tests after heat treatment, i.e. solution heat treatment (1200°C/0.5 h/water) and aged at a temperature of 700°C for 12 h, with cooling in air. The heat treated specimens were then subjected to low-cycle fatigue tests (LCF), carried out at room temperature and at an increased temperature of 600°C. Diagrams of fatigue characteristics of the investigated steels at room temperature as well as at elevated temperature have been worked up. It has been found that during low-cycle fatigue tests, at both temperatures, the investigated austenitic steels indicated a fatigue softening effect. The results of LCF at room temperature showed that the fatigue durability (Nt) of both austenitic steels is located in the range 0.8÷1.3×103 cycles. The results of low-cycle fatigue tests at an increased temperature 600°C indicated that the fatigue durability of the investigated steel was lower, and is located in the range Nt = 0.5÷0.6×103 cycles. It has been pointed out that the investigated austenitic steels are characterized by a stability of structure in conditions of cyclic fatigue.

Durability of Tube Bends Made of the 14MoV6-3 Steel under Low-Cycle Fatigue Conditions and Creep at a Temperature of 500°C

The paper contains the results of theoretical and experimental research on tube bending process used in manufacturing of 14MoV6-3 steel tubes. The innovative tube bending process using local induction heating and the results of finite-element numerical analysis of tube bends using Symufact Forming 11.0 software were presented. Numerical analysis covered the changes in pipe bend geometry (ovalization of cross section, wall thickness) and the results were compared with those obtained in industrial conditions. Basic mechanical properties of bended tube (diameter 323.9 mm, wall thickness 40 mm) in the supply condition and after heat treatment were determined using tensile, hardness, impact, low-cycle fatigue and creep tests. It has been shown that 14MoV6-3 steel tube bends manufactured using proposed technology meet the requirements of the PN-EN 10216-2 standard.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of Sintered Stainless Tool Steel

In the paper, the new highly alloyed Cr-V-Mo stainless tool steel produced via the powder metallurgy method was examined. The Elmax steel is a new-generation sintered tool steel characterised by high resistance to wear and corrosion and very good dimensional stability. The purpose of the paper was to analyse the influence of the rate of cooling from the temperature of austenitization and of an additional sub-zero treatment on the microstructure, phase composition and the curve of mechanical properties of steel during tempering. It has been found that a sub-zero treatment decreased the retained austenite fraction in the steel structure after hardening in oil or compressed air. The use of a sub-zero treatment on specimens after hardening had no significant effect on steel hardness after tempering, but it negatively affected the course of bending strength and impact strength of steel. Based on the obtained results it has been found that optimum mechanical properties for the Elmax steel were obtained after hardening from a temperature of 1080°C in compressed air (without a sub-zero treatment) and tempering at a temperature of 180°C.

Research on the Dependence between the Fatigue Life of X20CrMoV12.1 and P91 Steels under the Conditions of the Interactions of Thermo-Mechanical and Isothermal Low-Cycle Fatigue

The paper presents the correlation between fatigue life determined under the conditions of low-cycle fatigue (LCF) and thermo-mechanical fatigue (TMF). Fatigue life values computed using own parameter P and results obtained based on the author’s own research and literature from the publications have been shown. The tests LCF and TMF have been performed for steels used for devices operated in the power engineering industry under the conditions of variable mechanical and thermal interactions X20CrMoV12.1 and P91.

Model of the Deformation Process under Thermo-Mechanical Fatigue Conditions

The paper focuses on the development of a mathematical representation of deformation characteristics under the conditions of an elevated changeable temperature and mechanical loads. The method proposed in the paper is based on the use of characteristics determined in low-cycle fatigue tests at constant temperatures. Hysteresis loops reflecting the behaviour of a material under the conditions of low-cycle loads at an elevated temperature were primarily used. The effect of relaxation on the course of the hysteresis loop was taken into account. The steady state of the material is referred to in the proposed representation. A calculation algorithm was developed to enable the determination of the stress value for subsequent increases of strain over time. The results obtained were compared with experimentally determined characteristics.

Low-cycle fatigue life of steel P92 under gradual loading

The aim of the study was to determine low-cycle fatigue life characteristics of P92 steel used in the power unit components that work under the highest effort. Steel service life was determined in the tests for total fixed strain Δet ranges from 0.6 to 1.2 % and with application of two-stage sequence loading. Low-cycle fatigue tests were conducted at room temperature and at 550 °C on MTS-810 testing machine. The tests under sequence loading were carried out for two strain ranges: Δet = 0.6 and 1.2 %. Sinusoidal load cycles of a coefficient R = −1 were used. Low-cycle fatigue life characteristics were developed with consideration of loading sequence impact. It was found out the fatigue life of an element under sequence loading is strictly correlated with its history of strain, which determines the degree of material microstructure damage. Higher degree of steel damage was found in two-stage tests, in which the larger range of strain Δet = 1.2 % was used as the first one. Fractography analyses of scrap revealed the patterns of fatigue lines and bands typical for this process as well as numerous secondary cracks occurred to the boundaries of former austenite grains. Analytic method of predicting low-cycle fatigue life of an element under two-stage sequence loading is presented. For this purpose, an energy criterion for the material life developed for tests carried out in conditions of low-cycle fatigue was used.