Grzegorz Korpala

Effect of Hot Rolling and Thermal Treatment on the Microstructure Evolution of Microalloyed Bainitic Steels for Pipeline

Heavy plate for pipelines, which are used in earthquake-prone areas, must have among other a good ductility. The ductility is needed to prevent cracking in the case of local plastic deformation. The bainitic steels with retained austenite or martensite meet these requirements. The aim of this investigation is the determination of the thermo-mechanical treatment parameters with which such microstructures can be generated during the heavy plate production or an additional heat treatment. Experimental modelling of the production process for heavy plate was realized on a Gleeble HDS-V40 thermo-mechanical simulator. Moreover, the microstructure evolution processes during cooling on the round out table were investigated by dilatometry. The investigations on the formation of the microstructure during following accelerated cooling and heat treatment combination were carried out. All investigations were realized with two high-strength micro-alloyed steels, one of which was additionally alloyed with molybdenum. Results revealed that the decreasing of the temperature and duration of the heat treatment as well as addition of molybdenum reduce the size of grains and promotes the nucleation of the bainitic microstructure.

Effect of Cooling and Isothermal Holding on the Amount of Martensite/Austenite (M/A) Constituents, Microstructure and Mechanical Properties of Microalloyed Pipeline Steel

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.

Modelling the influence of carbon content on material behavior during forging

Nowadays the design of single process steps and even of whole process chains is realized by the use of numerical simulation, in particular finite element (FE) based methods. A detailed numerical simulation of hot forging processes requires realistic models, which consider the relevant material-specific parameters to characterize the material behavior, the surface phenomena, the dies as well as models for the machine kinematic.This data exists partial for several materials, but general information on steel groups depending on alloying elements are not available. In order to generate the scientific input data regarding to material modelling, it is necessary to take into account the mathematical functions for deformation behavior as well as recrystallization kinetic, which depends alloying elements, initial microstructure and reheating mode. Besides the material flow characterization, a detailed description of surface changes caused by oxide scale is gaining in importance, as these phenomena affect the material flow and the component quality. Experiments to investigate the influence of only one chemical element on the oxide scale kinetic and the inner structure at high temperatures are still not available. Most data concerning these characteristics is provided for the steel grade C45, so this steel will be used as basis for the tests. In order to identify the effect of the carbon content on the material and oxidation behavior, the steel grades C15 and C60 will be investigated. This paper gives first approaches with regard to the influence of the carbon content on the oxide scale kinetic and the flow stresses combined with the initial microstructure.Nowadays the design of single process steps and even of whole process chains is realized by the use of numerical simulation, in particular finite element (FE) based methods. A detailed numerical simulation of hot forging processes requires realistic models, which consider the relevant material-specific parameters to characterize the material behavior, the surface phenomena, the dies as well as models for the machine kinematic.This data exists partial for several materials, but general information on steel groups depending on alloying elements are not available. In order to generate the scientific input data regarding to material modelling, it is necessary to take into account the mathematical functions for deformation behavior as well as recrystallization kinetic, which depends alloying elements, initial microstructure and reheating mode. Besides the material flow characterization, a detailed description of surface changes caused by oxide scale is gaining in importance, as these phenomena affect the mater...

Influence of Deformation Controlled Strain Rate on Tensile and Compression Behaviour of Magnesium and Steel Wire

This article demonstrates the difference in the flow curves of an AZ31 magnesium alloy and S235JR structural steel wire caused by non-linear strain rates during uniaxial tensile and compression testing at elevated temperatures. Throughout tensile deformation, the traverse velocity of the testing machine has to be adapted according to the current elongation of the specimen, thus accelerating, to ensure a constant strain rate during the admission of the stress-strain curve. The equivalent is necessary during compression testing, where the traverse velocity of the testing machine needs to decelerate ensuring a constant strain rate. Nevertheless, tensile and compression tests are performed with constant traverse velocity, which lead to divergent flow curves in comparison to deformation controlled traverse velocities. The results of the research show the difference in flow behaviour of magnesium and steel wire, when the temperature and strain rate are varied in conjunction with constant and deformation controlled traverse velocities.

Development of a Procedure for Low-Temperature Testing of Wire Materials and Basic Research into the Behaviour of Materials in the Temperature Range of -50 to -80 °C

New technical applications and the ongoing infrastructural and industrial development of regions with extreme climatic conditions place ever greater demands on the properties of the materials used. On the one hand conventional materials can often meet such demands only to a limited extent whilst, on the other, a lack of experience means that sometimes no solid conclusions can be drawn regarding their suitability under extreme conditions. The examination of the influence of extreme environmental conditions on the behaviour of the material and the development of innovative materials with a tailored profile of properties is therefore one of the main tasks of modern material research as well as the material manufacturing and processing industry.

Metal Forming Aspects of Cyclic Severe Plastic Deformation of Ti-Ni Shape Memory Alloys Using MaxStrain Device

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%).

Bainitic Steels, their Characteristics and Applications

Abstract. Excellent combination of mechanical properties makes bainitic steels very attractive for commercial application. The most potential benefit of bainitic steels is found in the lightweight design of car bodies. The chemical composition, productions process and the austenite state control the properties of those steels. To reach the target of the correct component design it is important to focus on transformation kinetics. High carbon steels have been investigated, which contain approximately 0.5% C, 1.5% Si, 1.5% Mn, 0.9% Cr and 1.5% Cu. It’s expected to form a carbide free bainitic microstructure due to the Si addition. The residual austenite in the microstructure of high carbon bainitic steels ensures the forming ability besides the high strength. Maximum tensile strength is 1650 MPa and elongation is around 30%. Those steels can be used to produce large scale components.

The Influence of the Initial State on the Softening and Precipitation Kinetics in Hot Metal Forming

Energy-efficient production in today’s metal industry includes the usage of casting heat to allow hot deformation directly after solidification for saving time and cost of expensive reheating processes. Due to the different initial state this processing route also leads to differences in deformation behavior (strain, flow stress) and softening and precipitation kinetics compared to a reheated material. This is caused by the different microstructures in the material directly after solidification, showing a much more coarsed grain structure and a higher amount of dissolved microalloying elements. Therefore, a material directly deformed after solidification usually shows a retarded softening behavior accompanied by precipitation processes. The different behaviour of a reheated material and a material directly deformed after solidification is shown in this work. The different modes of action for micro-alloying elements in different initial states are compared at the example of steels with different chemical compositions. Differences in deformation behaviour were simulated with semi-empirical models including specific coefficients to consider the processing parameters strain, strain rate, temperature, and chemical composition. The models are capable of describing the retarded softening caused by a superimposed precipitation kinetic leading to a typical plateau. The results of these models are compared with established physical models.