This study presents a striking advance in investigating the influence of heat treatment on the microstructure and properties of high strength low-alloyed steels obtained using various technologies. In contrast to the normalization treatment, the application of the thermo-mechanical control process (TMCP) offers higher strength characteristics but less stable properties during consequent high-temperature heat treatment. It has been established that the mechanical properties of both the steels are stable up to 650 °C. With an increase in the treatment temperature, the mechanical properties of the TMCP steel (grade S460M) are strongly degraded, while the normalized steel (grade S355J2) remains stable up to 950 °C. This is attributed to intensive grain growth at a temperature higher than Ac3 for TMCP steel and to the microstructural stability of the normalized steel. It is shown that the structural stability during high-temperature heat treatment is controlled by a number of factors such as heating temperature and holding time, grain growth, accumulated strain, the presence of deformation texture, steel deoxidation, and dissolution/uncontrolled growth of precipitations.
The objective of the work is investigation of the parameters of welding thermal cycle (WTC) in arc surfacing, including heating and cooling stages, on formation of metal structure and properties in overheating zone of the HAZ of high-strength wheel steel of grade 2, containing 0.58 % C. Gleeble 3800 complex was used to conduct heat treatment of model samples at heating rate of 25 to 210 °C/s up to the temperature of 1250 °C with subsequent cooling by WTC (w6/5 = 2.5-64 °C/s), perform metallographic investigations of the structure and plot diagrams of overcooled austenite transformation. At testing by the Implant method influence of cooling rate and structural state of metal in overheating zone of HAZ on critical stress values at delayed cracking of wheel steel was assessed. It is established that incompleteness of the processes of metal austenite homogenizing in arc surfacing, because of its fast heating and limited time of soaking at the temperature above Ac3, has an essential influence on subsequent γ-α transformation in the HAZ metal. This leads to lowering of the critical cooling rate to 20 °C/s, at which not more than 50 % of martensite forms in the structure (w50M). It is shown that the high delayed fracture resistance of HAZ metal on the level of σcr ≥ 0.45σ0.2 can be ensured, provided w6/5 ≤ w50M. Investigation results can be used at specifying the technology of building-up by surfacing of items from high-strength steels. Cracking in welded joints of high-strength steels depends on the state of the structure in the HAZ overheating zone, degree of metal quenching during the welding thermal cycle (WTC) and its plastic properties. This determines the quantity and density of dislocations, depth of running of diffusion and relaxation processes in the quenched metal that essentially influences the processes of crack initiation and propagation at loading and, eventually, the performance of welded joints and metal structures as a whole [1][2][3][4][5].Problems at building-up by surfacing of items from high-strength wheel steels are similar to those arising in welding of alloyed steels. The main of them is prevention of cracking in the HAZ metal. Unlike high-strength alloyed steels, wheel steels do not contain any additional alloying elements, such as chromium, nickel, molybdenum, stabilizing the metal structure under the impact of thermodeformational cycle of welding. The main alloying element in wheel steel is carbon, the content of which is more than 0.50 %. Railway wheels made from such steel have ferrite-pearlite structure. Strength level of wheel metal exceeds 900 MPa at its comparatively low ductility and toughness [6,7].As is known, carbon forms an interstitial solid solution with iron, considerably strengthening the ferrite, and to a much greater extent than alloying elements forming the substitutional solid solution. Carbon solubility in iron is different, depending on the crystalline form, in which iron is present. So, carbon solubility in α-iron (BCC lattice) is equal to less than 0.02 %, and in γ-iro...
Abstract:The railway transport is one of the main types of communications in the world. Analysis of service conditions of the most critical elements of the railway transport rolling stock (rails, wheels and tyres) shows that one of the main factors determining their reliability and service life is the structural-phase state of steel, which is formed in the process of its producing. Complication of service conditions leads to the more severe requirements, specified by the consumers, where the high level of strength and hardness should be combined with high values of ductility and toughness. Realization of these requirements is possible only at the integrated approach to the improvement of technology of their production on the basis of profound knowledge in kinetics of austenite decay processes of structure formation and its contribution to the mechanical properties. With the formation of mainly bainite-martensite structures in the HAZ (heat-affected zone) metal of welded joints of these steels and saturation of this region with diffusive hydrogen their susceptibility to the cold cracking is increased. In this connection this work presents the results of investigations of effect of the WTC (welding thermal cycles) on the nature of structural transformations, hardness, static strength and resistance to the formation of cold cracks in HAZ metal of high-strength carbon rail steel M76. For this purpose, the structure and kinetics of transformation of an overcooled austenite was studied using the advanced methods of physical materials science by the modeling of phase transformations in the Gleeble 3800 unit. It was found that the cause of reduction in mechanical properties of welded joints of steel M76 is the formation of regions with a completely martensite structure in the amount of up to 5%. Results of investigations will be applied for the optimizing the technology and conditions of FBW of the advanced rail steels. 10 Ref., 4 Tables, 11 Figures.
Different characteristics of methods of additive manufacturing of metal products were considered. The prospects of technologies using metal wire as a consumable material were noted. The current state of research works in the field of additive manufacturing of layer-by-layer electric arc volumetric surfacing was shown. 20 Ref., 3 Tables, 10 Figures. K e y w o r d s : additive manufacturing, prototyping, shape formation and structure control, laser surfacing using consumable wire, freeform fabrication using electron beam, electric arc volumetric layer-by-layer surfacing
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