Papers by Keyword: Thermo-Mechanical Treatment

Abstract: The effect of thermo-mechanical treatment on the microstructural evolution of low carbon micro-alloyed high strength steel was studied by combining prestrain with tempering (PST) in this paper. It was found that the prestrain causes the dislocation to plug up around the grain boundary and carbide, resulting in carbide boundary fragmentation. Moreover, it breaks the thermo-dynamic equilibrium between the matrix and carbide, induces the dissolution of carbon in the high energy state, and then changes the distribution of carbon in the matrix. In the subsequent tempering process, the precipitation regularity of carbide was changed, which promoted the precipitation carbide at low temperature. The influence of carbide precipitation on dislocation can be divided into two stages: the first stage was precipitation induced creep, which promoted stress relaxation; the second stage was precipitation pinning dislocation, which improved material strength and inhibited stress relaxation.

Abstract: EUROFER 97 is considered as standard steel for the nuclear applications in the case of high radiation density for first wall of a fast breeder reactors. Based on such consideration the microstructural behaviour of EUROFER 97 after thermo-mechanical processing is fundamental, since such materials properties are interesting also for innovative solar plants. In this paper the effect of thermo-mechanical behavior on the mechanical properties of EUROFER 97 has been analyzed by hot rolling followed by heat treatment on laboratory scale.A strong effect of reheating conditions before rolling on the material strength, due to an increase of hardenability following the austenite grain growth is found. A limited effect of the hot reduction and of the following tempering behavior is found in the considered deformation-range investigated. A loss of impact toughness is detected together with the hardness improvement.Mechanical properties are depending on the tempering temperature and an improvement of tensile yield stress (YS) and ultimate stress (UTS) was determined in tensile test carried on at T=550°C and T=650°C, e.g.: YS increase from about 400 MPa for standard EUROFER 97 [1] to about 550 MPa in samples treated by a tempering temperature of 720°C instead of the standard 760°C for EUROFER 97. Same trend has been found for UTS results.

Abstract: Ti-Ni shape memory alloys were subjected to a rotary forging (RF) from the diameter of 20 or 12 mm to 5 mm at relatively low temperatures of 450 and 350 °C. The structure was studied using X-ray diffraction analysis and transmission electron microscopy. Mechanical properties were determined by uniaxial tensile and Vickers hardness tests, and the maximum completely recoverable strain by a thermomechanical method. The obtained results showed that RF at low temperatures allows manufacturing 3 m-long Ti-Ni rods. A mixed submicrocrystalline and nanosubgrained structure with an average size of structural elements of 118 ± 7 nm after forging from 20 mm, and 126 ± 7 nm after forging from 12 mm at 350 °С provides the highest mechanical (σ_в = 1113 MPa after forging from 20 mm, and 1065 MPa after forging from 12 mm) and functional (maximum completely recoverable strain of 8.0 %, after post-deformation annealing at 400 °С, 1 h) properties as compared to the initial state before RF (700 MPa, and 4.4 %, respectively).

Abstract: Constant increase of energy consumption in modern industry requires construction of heavily loaded pipelines with high throughput capacity. Therefore, high-strength steels should be used for the cost reasons. Additionally, the pipelines are also often used in the areas with cold climate and high seismicity. Therefore, strength and plasticity reduction is unacceptable. Bainitic steels with retained austenite (RA) or martensite/austenite (M/A) constituents meet these requirements. The purpose of this investigation is to determine thermo-mechanical treatment parameters with further accelerated cooling and additional isothermal holding for M/A-phase and mechanical properties formation. Experimental modeling of the production process was carried out using Gleeble HDS-V40 thermo-mechanical simulator. All investigations were realized with two high-strength micro-alloyed steels with different molybdenum and carbon content. Results showed that decrease of temperature and duration of isothermal holding as well as addition of molybdenum promote bainitic microstructure nucleation and reduce grain size and M/A-constituents. All these factors lead to a slight improvement in mechanical properties.

Abstract: The effects of thermo-mechanical training on damping capacity of two types of stainless steels, Fe-18Cr-8Ni (SUS 304) and Fe-25Cr-20Ni (SUS 310S) stainless steels, are studied. The thermo-mechanical training with bending deformation is adopted, since vibration manner in internal friction measurement is bending mode. An anisotropic damping capacity as well as hardness of samples is studied. It is found that deformation induced ε-martensite is observed for trained SUS 304 sample, while deformation twins are formed in the trained SUS 310S sample. It is also found that internal friction of SUS 304 sample is larger than that of SUS 310S sample. Increase in number of training results in an increase in the internal friction and hardness. In addition, anisotropic damping capacity is observed in the samples subjected the thermo-mechanical training. To be concluded, the thermo-mechanical training is useful for enhancement of both damping capacity and strength of SUS 304 and SUS 310S stainless steels.

Abstract: The multiscale modelling of the behaviour of metal alloys during processing is often limited by the computing power required to run them. The Agile Multiscale Methodology was conceived to enhance the designing and controlling of complex multiscale models through an automatic run-time adaptation of its constitutive sub-models. This methodology is used to simulate the behaviour of an 6082 aluminium alloy during its thermomechanical treatment. The macroscopic deformation, the work-hardening and the state of precipitation are computed in different modules, allowing the coupling of several software solutions (DEFORM^TM2D and © MatCalc) through an external storage of the relevant data.

Abstract: Abstract. Residual stress gradients across the wall of seamless steel tubes influence decisively the mechanical stability and reliability of automotive and industrial constructions. Irreversible bending moments imposed on the tubes induce gradual and asymmetric elasto-plastic deformation across the tube cross-sections which result in very complex residual stress distributions. The aim of this contribution is to present a novel methodology as well as complementary modeling approach to assess the three-dimensional distribution of triaxial residual stresses in bent steel tubes. The stress characterization was performed using high energy X-ray diffraction at the HEMS beamline of PETRAIII synchrotron source in Hamburg as well as using laboratory Drill-hole method. For the complementary modeling of the stress distribution, a FEM software package DEFORM ^HT was used. The results reveal that the stress gradients across the tube wall are primarily influenced by the martensite profile predetermined by the parameters for thermo-mechanical treatment of the tubes. The tube bending causes the formation of continually varying compressive and tensile stresses across the tube circumference whereas the stress magnitude across the wall thickness scales again with the martensite appearance. Finally the results document the importance of the cooling process control and the influence of the applied bending radius on the resulting stress distributions as well as related mechanical parameters like fracture toughness and fatigue behavior.

Abstract: The microstructure of pre-aged AA7085 rolled plate was studied by means of hardness tests, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electrical conductivity tests. The results show that supper refined and homogeneous precipitates were formed during pre-aging, but the hardness of the alloy was still relatively low. There occurred a large number of dislocations when the warm deformation was introduced, and the size of the intragranular precipitates firstly increases and then decreases and then increases with the dislocation density increasing, which was caused by the role of dislocations on inhibiting the formation of GP zone and promoting the nucleation and transformation of η phase. Meanwhile, the distribution of grain boundary precipitates also changed from continuous chain to coarsening interrupted distribution and the precipitate free zone (PFZ) broadened obviously.

Abstract: Modelling the behaviour of metal alloys during their thermo-mechanical processing relies on the physical and mathematical description of numerous phenomena occurring in several space scales and evolving on different characteristic times. Although it is possible to develop complicated multi-scale models, it is often simpler to simulate each phenomenon separately in a single-scale model and link all the models together in a global structure responsible for their good interaction. Such a structure is relatively difficult to design. Both efficiency and flexibility must be well balanced, keeping in mind the character of scientific computing. In that context, the Agile Multiscale Modelling Methodology (AM3) has been developed in order to support the object-oriented designing of complex numerical models [. In this paper, the application of the AM3 for designing a model of the metal alloy behaviour is presented. The basis and some consequences of the application of the Object-Oriented design of a sub-models structure are investigated. The object-oriented (OO) design of a 3 internal variables model of the dislocations evolution is presented and compared to the procedural one. The main advantages and disadvantages of the OO design of numerical models are pointed out.

Abstract: Taking into consideration increased quantity of accessories used in modern cars, decreasing car’s weight can be achieved solely by optimization of sections of sheets used for bearing and reinforcing elements as well as for body panelling parts of a car. Application of sheets with lower thickness requires using sheets with higher mechanical properties, however keeping adequate formability. The goal of structural elements such as frontal frame side members, bumpers and the others is to take over the energy of an impact. Therefore, steels that are used for these parts should be characterized by high value of UTS and UEl, proving the ability of energy absorption. Among the wide variety of recently developed steels, high-manganese austenitic steels with low stacking faulty energy are particularly promising, especially when mechanical twinning occurs. Beneficial combination of high strength and ductile properties of these steels depends on structural processes taking place during cold plastic deformation, which are a derivative of SFE of austenite, dependent, in turn on the chemical composition of steel and deformation temperature. High-manganese austenitic steels in effect of application of proper heat treatment or thermo-mechanical treatment can be characterized by different structure assuring the advantageous connection of strength and plasticity properties. Proper determinant of these properties can be plastic deformation energy supply determined by integral over surface of cold plastic deformation curve. Obtaining of high strength properties with retaining the high plasticity has significant influence for the development of high-manganese steel groups and their significance for the development of materials engineering.