Papers by Keyword: Bainite

Abstract: During the hot rolling of bainitic steels, time and temperature must be controlled within narrow limits to avoid undesirable ferritic or martensitic phase fractions. In order to design a reliable process window for the production of bainitic steels, the effects of the different process parameters on the phase transformation and the final properties of a microalloyed and a non-microalloyed steel were investigated. Thermomechanical tests with the possibility of producing secondary samples were conducted to analyze the influence on the mechanical properties strength and toughness. Transmission electron microscopy (TEM) and electron probe micro analysis (EPMA) were used to investigate the origin of the differing properties. In particular, it has been found that thermomechanical rolling of the microalloyed steel leads to an improvement in strength. This is partly due to the transformation kinetics and partly to strain-induced precipitations. Further, the hardening behavior is affected by the secondary phase within the bainitic matrix configured through the cooling strategy. Coarse Martensite/Austenite (MA) structures reduce toughness, whereas finely dispersed MA islands increase the hardening potential. Furthermore, the results from the material experiments were used to develop a rate model in combination with a nucleation model to predict the kinetics of the phase transformation and the shape of the bainitic microstructure.

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: Recently, due to the requirements of lightweight and safety, the grade of 980MPa high-strength steel has the demand of high hole expansibility and high yield strength. Due to the large difference of hardness between the soft ferrite and hard martensite, the traditional DP980Y dual phase steel has poor hole expansibility. In order to improve the hole expansibility of DP980Y dual phase steel, the best way is to modify the microstructure into a single-phase to eliminate the large difference of hardness. In this paper, the steel of nearly full bainite microstructure with small amount of ferrite and M/A constituents was studied. Compared to the DP980Y dual phase steel, it was found that this modified steel with a single-phase microstructure has the same grade of 980MPa of tensile strength, but can achieve the demand of higher yield strength and hole-expansion ratio. This study shows reducing the amount of ferrite can increase the homogeneity of matrix with the single phase to improve the hole expansibility. In addition, the use of lower bainite transformation temperature and lower carbon content has the higher hole-expansion ratio due to the less amount of M/A constituents.

Abstract: The aim of this study was to determine the effect of non-isothermal tempering on microstructure evolution in large-size slabs. Using high-resolution dilatometry, three different cooling rates (from 0.08 to 3°C/s) representative of different regions from the surface to the core of the slab were experimentally simulated, and then tempering was carried out for each starting microstructure. A combination of light and electron microscopy and X-ray diffraction analyses were employed to accurately analyze different phenomena occurring during the tempering process, specially, the identification of different microstructures (bainite, martensite and retained austenite), and the determination of the percentage of retained austenite for each experimental condition were considered. Experimental results revealed that the microstructure after the cooling rate of 0.08°C/s consisted of bainite and some retained austenite. For the cooling rate of 0.3°C/s, martensite plus bainite was detected, and when the cooling rate was increased to 3°C/s, a martensitic microstructure was obtained. Analysis of dilatometry curves indicated that tempering behavior varied significantly with the starting microstructure. Martensite tempering was accompanied with a length decrease due to the decomposition of medium-carbon martensite to low-carbon martensite plus carbides. Tempering of bainite and retained austenite resulted in a remarkable length increase.

Abstract: Commercial production of high strength steel plates by the quenching and tempering (Q&T) route requires control of alloy design and heat treatment parameters to achieve the desired strength and toughness through thickness. Plates with different thicknesses (up to approximately 100 mm) are produced for applications in the energy and power or lifting and excavation sectors. For thick plate the difference in cooling rate through thickness affects the as-quenched microstructure with martensite, auto-tempered martensite and lower and/or upper bainite being present. The different as-quenched microstructures can show a different response to tempering which affects the final strength and toughness.In this study the starting microstructure of a low alloy 0.17 wt% C Q&T steel has been varied using isothermal heat treatment at 430 °C to create mixed martensite and lower bainite microstructures (nominally 25:75; 50:50 and 75:25 percentages). The effects of tempering at 600 °C for times between 0.5 and 16 hours on the carbide precipitates and hardness of the mixed microstructures have been investigated and compared to the tempering response of single phase (martensite and lower bainite) microstructures. It has been found that the hardness decrease due to tempering is larger in the martensitic structure than the bainitic structure due to more rapid carbide coarsening. The as-quenched hardness of the mixed microstructures can be predicted by a rule of mixtures using the single phase properties. The tempering response of the mixed microstructures is discussed.

Abstract: The first results of the complex researches by NChI KFU, KAMAZ and RC Modificator of influence of heat treatment on CGI structure and properties are presented in the article. It is shown that austempering of CGI raises its strength characteristics more than twice and, thanks to good foundry properties, exclusively highs the resistance of deformation, good damping capacity and machinability, allows to expand CGI applications, in particular, instead of the alloyed SGI, including ADI.

Abstract: Structure-texture states in 18Cr-9Ni austenitic stainless steel after long-term operation of the tube at high temperatures and neutron irradiation have been investigated with orientation microscopy (EBSD). In the examined samples, cut out at the external surface, a significant concentration of α-phase with the lattice close to bcc has been detected. Phase transformation shows prominent crystallographic direction, caused by initial orientation of austenite grains and tensile stress effect, normally directed at a tangent to its external surface. High-angle boundary spectrum with the most prominent coincidence site lattice (CSL) boundaries, Σ3, Σ11, Σ25b, Σ33с Σ41с, is typical for α-phase. Thus, it can be claimed that austenite transformation was carried out by shear (bainite, taking into account high temperature) mechanism, according to orientation relationships (OR), intermediate between Kurdjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W). Shear γ-α transformation began in austenite on twin boundaries (CSL Σ3), and was carried out in the range determined by initial orientation of γ-phase crystals and effective stress value. Based on high density of CSL boundaries Σ3 in α-phase it has been suggested that its nuclei are represented not by single crystallites, but crystallite couples in twin misorientation.

Abstract: Electroless nickel (Ni) and composite nickel coatings (Ni+TiN) are investigated in this study. EFFTOM -Nickel Method for electroless nickel plating is applied. Nanosized TiN particles (50nm) are used as a strengthening material. The coatings are plated on ductile cast iron samples. The wear tests of the coatings by the classic model TABER ABRASER “disc to disc” are carried out. The observation of the samples microstructure by means of an optical metallographic microscope GX41 OLIMPUS and the microhardness measurements by Knoop Method are performed. The wear resistance, the thickness and microhardness of the samples before and after thermal processing at 290°C, 6 h are defined.

Abstract: Novel steels with high hardenability were proposed to replace the conventional HY-TUF steels for the large parts manufacturing. The chemical composition of the steels under consideration was, mass. %: C – 0.16...0.18; Cr – 2.35...2.55; Mn – 0.67...1.99; Si – 0.76...1.03; Ni – 1.17...2.31; Mo – 0.34...0.47; S. P < 0.025. The dilatometer experiments revealed that during the continuous cooling of the steels with the constant rates 0.1... 30 °C/s only martensite and bainite transformations occurred. for conventional HY-TUF steel ferrite and pearlite formed after cooling with the rates 0.1...0.3 °C/s.

Abstract: The most widely used equation for analytical description of the transformation kinetics of the metastable solid solutions (the steel austenite in particular) is Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation [1]. However the practical analysis of the experimental isothermal bainite transformation kinetics often gives significant deviation from the conventional theory [2]. This problem can be solved by the derivation of an analytical function which would provide the best fit of the experimental results. Two analytical approaches describing the kinetics of bainite transformation in steels 300M and D6AC are proposed. The first one is based on an approximation of the experimental ln (-ln (1-Р)) vs. ln τ dependence by a second order polynomial function. The second approach is based on the solution of the differential equation y(x) = ay’(x)+b, where x= ln τ, y(x) = ln(-ln(1-P)). A comparison between the proposed approaches and Kolmogorov - Johnson - Mehl – Avrami equation is conducted. The adequacy of the two analytical models is estimated using Fisher ratio test.