Papers by Keyword: Thermomechanical Treatment

Abstract: This work presents results of experimental researches of influence of various high-temperature thermomechanical treatment (HTMT) modes in 35KhGSA steel – material applied in rock-destroying cutters – on hardness and wear rate for the most common abrasive rock in mining operations – sandstone. It has been found out that HTMT leads to increase of steel hardness and reduction of wear rate compared to heat treatment applied at manufacture of cutters. The maximal increase of hardness (by 23%) and reduction of wear rate (by 38%) corresponds to the mode of distortion accumulation of ~1,5 at 900 °С at water quenching and tempering at 230 °C. A conclusion that thermomechanical treatment is an effective way to increase the wear resistance of steels applied in the conditions of abrasion wear has been drawn.

Abstract: Spheroidal Graphite Irons (SGIs) are ductile cast irons with toughness and ductility comparable to those of carbon steel. In particular, high silicon Solution Strengthened Ferritic (SSF) SGIs are developed to provide higher strength with excellent ductility suitable for structural applications. The main characteristics of these materials lie in the graphite particles inclusions whose morphology and count greatly influence the mechanical properties and more specifically the fatigue crack initiation and propagation behaviour of the SGI components. In this work, SGIs specimens have been subjected to various thermomechanical treatments in order to analyse the influence of these treatments on the microstructure of the material. Observations of degenerated forms of graphite particles alongside the spheroidal nodules in the microstructure were then used as a basis for correlation with damage mechanisms at the microscale. In static tensile testing, it was observed that the matrix-nodule interface decohesion and plastic deformation of the ferrite matrix were the dominant damage mechanisms. In separately performed fatigue crack initiation and fatigue crack propagation tests, it was confirmed that the graphite particle shape played a decisive role in crack initiation and propagation. The results of the microstructural characterization have been implemented in a computational model for further study of the influence of the microstructure on the fatigue behaviour of these materials.

Abstract: The major challenge in a heat-resistant steel is to generate thermally stable microstructures that allow increasing the operating temperature, which will improve the thermal efficiency of the power plant without diminishing strength or time to rupture. The strengthening mechanism in tempered martensitic 9Cr steels comes mainly from the combination of solid solution effect and of precipitation hardening by fine MX carbo-nitrides, which enhance the sub-boundary hardening. This work is focused on the effect of ausforming processing on MX nanoprecipitation, on both their distribution and number density, during the subsequent tempering heat treatment. The creep strength at 700 oC was evaluated by small punch creep tests. The creep results after ausforming were compared to those obtained after conventional heat treatment concluding, in general, that ausforming boosts the creep strength of the steel at 700 oC. Therefore, conventional ausforming thermomechanical treatment is a promising processing route to raise the operating temperature of 9Cr heat-resistant steels.

Abstract: Nb is a classical microalloying element in the design of thermomechanical treatments in low carbon steels for flat products applications. However, its use in medium-high carbon grades, as occurs in hot rolling of bars, is less common. This is, in part, because of the diversity of characteristics required to those grades of steels and the less knowledge about the function of Nb in these cases. Consequently, less information is reported concerning thermo-mechanical processing of Nb microalloyed steels in long products applications. In this case, it is necessary to consider the singularities related to these processes, such as the short interpass times and the wide range of chemical compositions usually applied on these products. Short interpass times result in high strain rate values that can lead to metallurgical changes on the mechanisms occurring during the hot rolling must be considered. Moreover, the high Carbon contents applied in long products, usually between 0.20–0.40%, can influence the Nb solubility and precipitation in each stage of the process: prior to hot rolling on the reheating furnace, during the process and after hot rolling, depending on the cooling strategy adopted and on the post-rolling heat treatments that can be applied. This paper analyses different singularities associated with the use of Nb microalloying for long products. Several aspects, such as the partial or complete dissolution of the Nb prior to hot rolling, its role in the control of austenite microstructure and its incidence in the final microstructure and mechanical properties, will be considered.

Abstract: The wire rod has high relevance due to its wide application as a raw material for steel wire and wire processed in cold rolling mills and drawing. The control of process variables, such as cooling rate, coiling temperature, rolling speed is essential for obtaining the microstructures and therefore mechanical properties of the material. The purpose of this work is to study the behavior of the microstructure and mechanical properties with the variation of thermomechanical treatment in the hot rolling of wire rod. The steels used in this study were the microalloyed niobium steel equivalent to ASTM A913 grade 50 and carbon steel equivalent to ASTM A510 grade 1013. Despite the carbon steel has higher equivalent carbon to microalloyed steel, was found higher yield strength (σ_e) in the microalloyed niobium steel. Thus, by applying appropriate thermomechanical treatment in microalloyed steel is possible to improve mechanical properties mainly due to grain refining.

Abstract: The effect of severe plastic deformation using MaxStrain (MS) device which is a part of the Gleeble thermo-mechanical simulator of rolling and forging processes on the structure and functional properties of Ti–50.0 at.% Ni shape memory alloy has been studied. The use of the MS module allows performing SPD of the material under isothermal conditions with precise control of the deformation parameters. The deformation temperature was lowered from 370 to 330 °C. The accumulated true strain varied from e=4.6 to 9.5. Structure features were studied by the transmission electron microscopy. The maximum completely recoverable strain was determined by a thermomechanical method using a bending mode for strain inducing. A mixed submicrocrystalline and nanosubgrained structure with average grain/subgrain size below 100 nm was formed using SPD at 330 °C. A very high completely reсoverable strain (9.3%) was obtained against a reference treatment (2.5%).

Abstract: A final thermomechanical treatment (FTMT) including peak aging and subsequent dynamic aging was proposed to prepare 7055 Al alloy sheets. The optimization was based on nine well-planned orthogonal experiments. Three main processing conditions in the thermomechanical treatment for obtaining the optimum synthetic properties of 7055 (i.e. preheating temperature, final rolling temperature and deformation degree) were investigated. It was shown that the final rolling temperature is the most important factor among the three parameters, and the optimum properties (yield strength: 651 MPa, ultimate tensile strength: 660 MPa) of 7055 Al alloy sheet can be gained with preheating at 140oC and 40% deformation at 170oC. With dynamic aging, grain boundary precipitates became discontinuous without much coarsening of matrix precipitates, while they were continuously distributed after T6 aging. The present optimal FTMT process can improve the intergranular / exfoliation corrosion resistance without sacrificing the strength compared to T6 tempering. The present FTMT process as a good alternative can produce high-strength Al alloy sheets with high strength and good corrosion resistance efficiently and economically.

Abstract: A novel thermomechanical treatment (TMT) was proposed for increasing the strength of 6156 aluminum alloy while maintaining a high elongation, which is based on artificial aging at low temperature, cold rolling and natural aging. The corresponding tensile properties and microstructure of 6156 aluminium alloy under different treatments were investigated. The yield strength and the ultimate tensile strength are increased by over 50 MPa over those of T3, while a high elongation rate is maintained. The fracture mechanism of T4 and TMT state alloy are typical ductile fracture, while that of T6 and T8 state alloy is a mixture of ductile fracture and shear fracture. For the TMT processed 6156 alloy, though no obvious precipitates can be observed in the matrix, tangled dislocations around primary phase in the matrix, as well as a high density of dislocations piled up at the grain boundary were observed. The mechanism of the novel TMT, by which the mechanical properties of the alloy is greatly improved, is the synergistic effect of composite structures, including dislocation substructures, the complex of Mg-Si clusters/vacancies, as well as GP zones.

Abstract: Casting of steel with different aluminum content was carried out. Thermo-mechanical process was carried out at 1200°C. A cross-sectional area reduction of 95% was done using hot forging hammer. Two regimes of thermomechanical processes were adopted in this study. First regime was intercritical annealing (between AC₁ and AC₃ to obtain 50% austenite and 50% ferrite) followed by rapid quenching in salt bath to the martensite zone for 15minutes to produce ferrite, martensite and carbon saturated austenite and to prevent the whole transformation of austenite to martensite. The second regime was reheating 50oC above AC₃, followed by rapid quenching in salt bath to the martensite zone (for 15 minutes) to get partially partitioned martensite and super saturated retained austenite. The initial tensile strength after hot forging is 1067MPa and 942MPa for alloys 1 and 2 respectively. The tensile strength after intercritical annealing is 1621MPa while after quenching from austenite to martensite zone with austenite partitioning is 2113MPa. Elongation after intercritical annealing and austenite partitioning is null due to bad shapes and distribution of ferrite with sharp corners.

Abstract: The average plastic strain ratio (the R-value) and the anisotropy parameter |ΔR| calculated from the measured texture of AA1050 Al alloy sheet treated by the heavy asymmetric rolling by 84% reduction in thickness and subsequent annealing for 1 h at 500 °C, followed by light rolling by 10% or 20% reduction in thickness and the subsequent annealing for 1 h at 500 °C increased by 1.52 times that of the non-processed specimen and reduced to 1/12 times that of the non-processed specimen, respectively.