Papers by Keyword: Austenite

Abstract: The reverse transformation of austenite from pearlite was observed in-situ by using a sophisticated EBSD system. Quantitative information on the nucleation sites and orientation selection of the austenite was obtained. Initially, the nucleation sites were restricted to high-angle grain boundary (HAB) and all the austenite orientations were selected by Kurdjumov-Sachs (K-S) orientation relationship. Latterly, the constraints were relaxed as new nucleation sites were involved. The γ growth was preferentially into the pearlite grains without K-S relation. Nucleation and growth at HAB edge may contribute to γ coarse grains, whereas nucleation at pre-existing γ boundary is important to grain refinement of γ structure.

Abstract: Based on the results of the static crack resistance tests, the contribution of the strain-induced γ→α-transformation into the fructure toughness of steel N18K9M5T with two-phase (α+γ)-structure has been assessed.

Abstract: The high strength silicon steel HY-TUF, applied for manufacturing of the heavy loaded aerospace and engineering parts, was investigated. The effect of the heating temperature in the range 900...1000 °C on the austenite grain size was studied. The steel under consideration had a significant scatter of the austenite grain size. The most intensive growth of the austenite grains was observed in the temperature range 975...1000 °C.

Abstract: With the help of the Thermo-Calc software package, arrays of calculated data were created for carbon concentrations in ferrite and austenite, corresponding to the para-equilibrium of these phases and their para-equilibrium with cementite, as well as for the corresponding temperatures A₁ and A₃. Marked arrays were obtained in wide temperature ranges for ranges of carbon concentrations and the most important substitution alloying elements (Mn; Si; Cr; Ni; Mo), covering the respective ranges for medium carbon and moderately alloyed steels. Analytical formulas were developed on the basis of the reference data arrays for calculating para-equilibrium concentrations of carbon in ferrite and austenite (depending on temperature and chemical composition), as well as temperatures A₁ and A₃ (depending on chemical composition), which allow to reproduce with high accuracy the results obtained using Thermo-Calc.

Abstract: The paper considers theoretical aspects of the kinetics of austenite → ferrite transformation in an Fe–9 %Cr alloy, a common model of diffusionless transformation. In previous studies it was shown that this transformation under isothermal conditions shows a behaviour typical for nucleation site saturation, including the change of the Avrami exponent n (determined as the slow of transformation curve on double logarithmic scale) from 4 to 1. Activation energies determined in two ways: by the ‘nose’ temperature of the normal C-curve and by the slope of the C-curve re-drawn on a reverse temperature scale are unexpectedly similar (272–315 kJ/mole) and not temperature-dependent. But the complete TTT diagrams calculated using these values determined directly from experimental data and the precise formula of Cahn’s solution of grain face nucleated transformation problem do not provide good agreement with experiment in the whole temperature range. This may mean that the theory of site saturation needs some correction.

Abstract: A steel containing Fe-0.2C-2Mn-Si-Al was annealed using intercritical Q&P cycles. Quenching temperature and partitioning time at 440°C was varied. Analysis of microstructure evolution during such Q&P treatments was performed using different characterization tools: FEG-SEM, EBSD, dilatometer and saturation magnetization measurements. Especially, phenomena taking place during partitioning were investigated. The microstructure was analyzed at the end of soaking, before and after partitioning and at the end of the annealing cycle. It was found that bainite transformation happens during partitioning and has a significant effect on the final retained austenite fraction. Quenching temperature has an important effect on both martensitic transformation during cooling and subsequent bainite transformation during partitioning.

Abstract: Advanced High Strength Steel (AHSS) developments have largely focused on automotive applications using metallurgical approaches to develop retained austenite-containing microstructures in a variety of new steels, using the transformation-induced plasticity (TRIP) effect to achieve better combinations of strength and ductility. These efforts have been extended in recent studies to explore the potential to improve wear resistance, using metastable retained austenite to enhance wear resistance for earth-moving and other applications. This paper provides selected highlights of the authors’ efforts to develop wear resistant steels using AHSS processing approaches. Some attractive product/process development opportunities are identified, and it appears that martensite-austenite microstructures produced using “quenching and partitioning” exhibit increased wear resistance.

Abstract: The influence of roughing strain on the extent of austenite recrystallisation in plain carbon steel and Nb-Ti-V microalloyed steel was investigated. Reheating and roughing simulations were conducted on a deformation dilatometer using industrial heating, soaking times, strain and strain rates. The roughing schedules comprised of varying the pass strain magnitude within a typical roughing temperature range. The double stroke method was used to determine the austenite softening fraction. The austenite grain size, prior to and after rolling, was measured on quenched specimens. Grain refinement was achieved in both steels after all rolling schedules. An applied pass strain of 0.15 was sufficient to completely soften the austenite after the first pass and produced the finest recrystallised grain size. This was attributed to sufficient nucleation sites and driving force for recrystallisation compared to lower strains. Partial recrystallisation occurred after the first pass due to the relatively coarse initial grain size. The steel chemistry played an insignificant role in controlling the recrystallisation kinetics at high roughing temperatures. The through-thickness strain distribution calculated from FEM simulations showed that, for a given applied strain, a similar magnitude of deformation is achieved at the centre of a hot-rolled plate.

Abstract: Heusler NiMnGa alloys are often categorized as ferromagnetic shape memory alloys or magnetocaloric materials, which are important for both practical applications and fundamental research. The NiMnGa alloys undergo a series of diffusion and diffusionless transformation from high temperature to low temperature. Among these transformation, martensitic transformation from austenitic phase to martensitic phase is critical in determining the properties of the alloys. Although martensitic transformation is considered diffusionless, diffusion also has important applications in the research of NiMnGa alloysDiffusion couples along with equilibrium alloys have been used to determine the ternary phase diagrams in NiMnGa alloys. Phase diagrams are important in selecting NiMnGa alloys, in particular two-phase NiMnGa alloys for practical applications. Furthermore, the diffusion couples effectively assist in the determination of compositions that exhibit martensitic transformation temperature near room temperature. Diffusion coefficients have been assessed for NiMnGa alloys. Tracer diffusivity of Ni, Mn and Ga was reported in a wide temperature range and followed Arrhenius behavior. Two different activation energies were obtained, corresponding to B2 and L2₁ crystal structure, respectively. Interdiffusion coefficients for NiMnGa alloys with B2 crystal structure are measured, which showed that Ni diffuses the fastest, followed by Mn then Ga. The diffusion coefficients provide useful information for fabricating NiMnGa alloys through diffusional process.A combinatorial approach involving diffusion couples and advance characterization has been developed to investigate the mechanical properties, microstructure and crystallography of NiMnGa alloys rapidly and systematically over a large compositional range. The composition-dependent modulus and hardness for NiMnGa alloys was extracted from the diffusion couples with the help of nanoindentation. Martensitic phases with non-modulated and various modulated crystal structures, and austenitic phase were identified in the interdiffusion zones by transmission electron microscopy. The results demonstrate the capability of using diffusion couples to speed up the discovery of new NiMnGa alloys or other similar alloys showing martensitic transformation.

Abstract: Steel 06Cr18Mn19Mo2N (P900N + Mo) was chosen to study the phase composition and structural transformations occurring in high-nitrogen nickel-free austenitic steels as a result of heat treatments to which they are exposed during production or operation. The methods of light and electron microscopy, X-ray diffraction and dilatometric analysis were used in the work. The heat treatment scheme included hot plastic deformation, quenching and aging over a wide temperature range. It is shown that after the hot plastic deformation and quenching from 1050-1150 °С, and also after quenching with subsequent aging at 300 and 500 °С, the structure consists of austenite and isostructural matrix of nanoscale nitrides CrN. Thermal aging of steel at 700-750 °C causes the formation of Mo₂N nitrides along the grain boundaries, and at 800 °C the decomposition of austenite is accompanied by a discontinuous reaction γγ_depleted + σ with the formation of the χ-phase at prolonged exposures.