Transformation Of Supercooled Austenite Research Articles

Supercooled austenite transformations in a considerable steel for a special engineering products

The advantages, disadvantages and some features of a dilatometric method for studying the transformation of supercooled austenite in steels under continuous cooling are briefly discussed. The continuous cooling transformation diagram of the steel under consideration containing 0.50 wt. % C, 0.25 Si, 0.65 Mn, 0.75 Cr, 1.30 Ni and 0.15 Mo, respectively was plotted based on experimental data obtained upon dilatometer measurements carried out with dif-ferent cooling rates (0.1, 0.3, 1, 5, 30 K/s) on the Linseis L78 RITA dilatometer. According to the obtained dilatometer curves, the degree of transformation was calculated in accordance with the standard method, its disadvantages and possible ways to eliminate them are noted. Additionally for the proper design of the continuous cooling transformation diagram of supercooled austenite metallographic examination and hardness tests were carried out after various heat treatment modes. A qualitative description of the formed microstructures was performed in order to confirm the obtained dilatometer data. The “tangent method” was also employed to determine the critical temperatures upon heating at the rate of 0.1 K/s and the temperature of the of the martensite start temperature upon cooling from the austenitization temperature of 880 °C, based on the literature data the application of the implemented method is justified. Some recommendations are presented as possible methods for selecting the dimensions of parts for which the steel under consideration was planned to be used. Using the example of a billet with diameter of 50 mm, from which samples were machined for research, the possibility of practical application of one of the methods for determining the type of structures formed along the cross section of steel products under continuous cooling from the austenitic region is shown.

Supercooled austenite transformations in a considerable steel for a special engineering products

The advantages, disadvantages and some features of a dilatometric method for studying the transformation of supercooled austenite in steels under continuous cooling are briefly discussed. The continuous cooling transformation diagram of the steel under consideration containing 0.50 wt. % C, 0.25 Si, 0.65 Mn, 0.75 Cr, 1.30 Ni and 0.15 Mo, respectively was plotted based on experimental data obtained upon dilatometer measurements carried out with dif-ferent cooling rates (0.1, 0.3, 1, 5, 30 K/s) on the Linseis L78 RITA dilatometer. According to the obtained dilatometer curves, the degree of transformation was calculated in accordance with the standard method, its disadvantages and possible ways to eliminate them are noted. Additionally for the proper design of the continuous cooling transformation diagram of supercooled austenite metallographic examination and hardness tests were carried out after various heat treatment modes. A qualitative description of the formed microstructures was performed in order to confirm the obtained dilatometer data. The “tangent method” was also employed to determine the critical temperatures upon heating at the rate of 0.1 K/s and the temperature of the of the martensite start temperature upon cooling from the austenitization temperature of 880 °C, based on the literature data the application of the implemented method is justified. Some recommendations are presented as possible methods for selecting the dimensions of parts for which the steel under consideration was planned to be used. Using the example of a billet with diameter of 50 mm, from which samples were machined for research, the possibility of practical application of one of the methods for determining the type of structures formed along the cross section of steel products under continuous cooling from the austenitic region is shown.

Effects of Heat Treatment on Microstructures and Mechanical Properties of a Low-Alloy Cylinder Liner.

Cylinder liners, considered a crucial component of internal combustion (IC) engines, often require excellent mechanical properties to ensure optimal engine performance under elevated temperatures, pressures, and varying loads. In this work, a new low-alloy cylinder liner, incorporating a low content of molybdenum, copper, and chromium into gray cast iron, was fabricated using a centrifugal casting process. Subsequently, the heat treatment processes were designed to achieve bainite microstructures in the cylinder liner through rapid air cooling, isothermal transformation, and tempering. The effects of different air-cooling rates and tempering temperatures on the microstructure evolution and mechanical properties of cylinder liner were investigated. The results revealed that during the supercooled austenite transformation process, rapid air cooling at a rate of 14.5-23.3 °C/s can effectively bypass the formation of pearlitic structures and directly induce the formation of bainite structures. Once the temperature exceeded 480-520 °C, hardness and tensile strength increased with the temperature increase owing to the enhancement of the lower bainite content, the reduction of residual austenite, and the precipitation of the fine hard carbides in the matrix. With temperatures above 520-550 °C, the carbide and lower bainite organization coarsened, thereby reducing the hardness and tensile strength of the material. Therefore, the optimal heat treatment parameters were rapid cooling at 14.5-23.3 °C/s rate to obtain bainite, and tempering of 480-520 °C for finer and more uniform bainite. In addition, the results of the characterization of the mechanical properties of the cylinder liner after heat treatment showed that the hardness, tensile strength, and wear resistance were improved with the refinement of the bainite.

Effect of rare earth La on super-cooled austenite transformation of low carbon copper-containing steel and its mechanism

The effect of rare earth element La on the continuous cooling transformation kinetics of undercooled austenite in low carbon copper containing HSLA steel was studied by Formastor-F automatic phase change meter, SEM and TEM. Based on first-principles calculations of Vienna ab-initio simulation package which is under the framework of density functional theory, the effect of La on the transition from fcc-Fe to bcc-Fe was analyzed at atomic scale. The results show that La refined the microstructure, promoted the transformation of granular bainite to lath bainite, and La had a double effect on the phase transformation of low-carbon copper-containing HSLA steel. In the early stage of martensitic transformation, after adding La, the initial martensite transformation temperature Ms decreased from 443 °C to 438 °C, and the CCT curves moved to the lower right, which improved the stability of supercooled austenite. Meanwhile, first-principles calculations results show that the addition of La reduces the driving force of fcc-Fe to bcc-Fe and increases the energy barrier for Fe to undergo martensitic transition through Bain mechanism. The above results show that La delayed the martensitic transition. In the later stage of martensitic transformation, the addition of La decreased the critical transformation speed and improved the hardenability of steel, indicating that La promoted the martensitic transformation.

In Situ Observation of the Grain Growth Behavior and Martensitic Transformation of Supercooled Austenite in NM500 Wear-Resistant Steel at Different Quenching Temperatures.

In situ observations of the austenite grain growth and martensite transformations in developed NM500 wear-resistant steel were conducted via confocal laser scanning high-temperature microscopy. The results indicated that the size of the austenite grains increased with the quenching temperature (37.41 μm at 860 °C → 119.46 μm at 1160 °C) and austenite grains coarsened at ~3 min at a higher quenching temperature of 1160 °C. Furthermore, a large amount of finely dispersed (Fe, Cr, Mn)3C particles redissolved and broke apart at 1160 °C, resulting in many large and visible carbonitrides. The transformation kinetics of martensite were accelerated at a higher quenching temperature (13 s at 860 °C → 2.25 s at 1160 °C). In addition, selective prenucleation dominated, which divided untransformed austenite into several regions and resulted in larger-sized fresh martensite. Martensite can not only nucleate at the parent austenite grain boundaries, but also nucleate in the preformed lath martensite and twins. Moreover, the martensitic laths presented as parallel laths (0~2°) based on the preformed laths or were distributed in triangles, parallelograms, or hexagons with angles of 60° or 120°.

Special Features of Transformations of Supercooled Austenite in Modern Structural Steels

Dilatometric, metallographic and durometric studies of transformations of supercooled austenite in modern commercial and experimental alloy steels under continuous cooling from the austenitization temperature at constant rates of 0.1 – 30 K/sec are performed. Thermokinetic diagrams of transformation of supercooled austenite are plotted and analyzed. The effect of alloying on the temperature and time intervals of formation of structural components in the steels is determined. Statistical analysis of the data obtained is performed to find the dependences of the critical temperatures Ac1 and Ac3 and of the critical quenching rate on the chemical composition of the steel. The behavior of the hardness of the formed structure as a function of the cooling rate is determined. Equations relating the coefficients of the function of logarithmic hardness and the chemical composition of the steels are suggested.

Explosive martensitic transformation of supercooled austenite in CuZr-based thin-film shape memory alloys

CuZr-based alloys are being considered as potential shape memory alloys for use in high-temperature applications. We have conducted a study on the effects of several alloying elements on the shape memory properties of these alloys using polycrystalline thin-film samples. Here we report on the explosive formation of martensite in supercooled CuZr, CuZrNi and CuZrCo samples. This explosive transformation behavior is characterized by the following observations: 1) The high-temperature austenitic phase can be supercooled below the martensite finish temperature Mf. At a critical temperature below Mf, austenite transforms to martensite across the entire sample in less than a microsecond. 2) The critical temperature has a narrow distribution and decreases slightly with higher cooling rate. 3) Observation of supercooling and explosive transformation behavior depends on the temperature history above the austenite finish temperature Af. If a sample is quenched immediately after heating above Af, martensite forms gradually on cooling below Ms; if a sample is allowed to dwell a few seconds above Af, the martensite forms explosively. We suggest that the gradual transformation proceeds by martensite growth on defects that accumulate during successive transformation cycles. If the sample is allowed to dwell at a temperature above Af, however, these defects are annihilated and the transformation is nucleation-limited. Nucleation of martensite then requires significant supercooling. The defect annihilation process is highly sensitive to temperature and has an apparent activation energy of 326 kJ/mol, which is too large for a simple diffusion-limited process. Transmission electron microscopy of CuZrCo samples suggests that the defects may be related to the presence of residual martensite.

The Effect of Cooling Conditions on Martensite Transformation Temperature and Hardness of 15% Cr Chromium Cast Iron.

The research reported in the paper concerned the conditions of cooling high-chromium cast iron with about 15% Cr content capable to ensure completeness of transformation of supercooled austenite into martensite in order to obtain high hardness value of the material and thus its high resistance to abrasive wear. For testing, castings were prepared with dimensions 120 mm × 100 mm × 15 mm cast in sand molds in which one of cavity surfaces was reproduced with chills. From the castings, specimens for dilatometric tests were taken with dimensions 4 mm × 4 mm × 16 mm and plates with dimensions 50 mm × 50 mm × 15 mm for heat treatment tests. The dilatometric specimens were cut out from areas subject to interaction with the chill. The austenitizing temperature and time were 1000 °C and 30 min, respectively. Dilatograms of specimens quenched in liquid nitrogen were used to determine martensite transformation start and finish temperatures TMs and TMf, whereas from dilatograms of specimens quenched in air and in water, only TMs was red out. To secure completeness of the course of transformation of supercooled austenite into martensite and reveal the transformation finish temperature, it was necessary to continue cooling of specimens in liquid nitrogen. It has been found that TMs depended strongly on the quenching method whereas TMf values were similar for each of the adopted cooling conditions. The examined cooling variants were used to develop a heat treatment process allowing to obtain hardness of 68 HRC.

The Chemical Composition Effect of Medium-Carbon Chromomanganesemolybdenum Steel on the Structure of Drill Pipes

Dilatometric studies on medium-carbon Cr–Mn–Mo 40KhGMA steel with various chemical compositions are performed. Thermokinetic diagrams for the transformation of supercooled austenite are plotted, and the temperature-time ranges for the formation of structural components are determined. The metallographic study of the steel microstructure have been carried out after cooling at different rates. Steel hardness depending on the tempering temperature and mechanical properties in the heat-treated state are studied. By using a numerical simulation, cooling paths under heat treatment have been determined for a drill pipe with a wall thickness of 30 mm.

Transformations of Supercooled Austenite in Promising High-Hardenability Machine Steels

We study special features of the transformation of supercooled austenite under the continuous cooling of Si – Mn steels with reduced contents of nickel as compared with traditionally used machine steels. The temperature ranges of the phase and structural transformations running under the conditions of heating and cooling of steels are determined by the dilatometric method. We plot the thermokinetic diagrams of transformations of supercooled austenite. The microstructural components formed in the investigated steels are analyzed both qualitatively and quantitatively. We also propose the chemical compositions of promising steels characterized by a high stability of supercooled austenite and high hardenability.

A Study of Phase Transformations in High-Chromium Ferritic-Martensitic Steels by Magnetometry

Phase transformations in high-chromium ferritic-martensitic steels are studied using magnetometric analysis of the magnetic field. A designed, certified and experimentally tested facility for measuring the magnetic permeability of small (pea-size) specimens is described. The facility makes it possible to determine the temperatures of magnetic transformations and, in some cases, the thermal effects of phase transformations. Examples of application of the developed method for determining the characteristics mentioned are given and used to plot diagrams of isothermal transformation of supercooled austenite. An analysis of pairs “mechanical properties – parameters of diagrams of isothermal transformation” is performed, and the dependences obtained are analyzed. The formulated features of the parameters of the diagrams of isothermal decomposition may be used for developing high-chromium ferritic-martensitic steels with high refractoriness. The limits of the applicability of the developed methods and of the used facility are presented.

Importance of Thermokinetic Diagrams of Transformation of Supercooled Austenite for Development of Heat Treatment Modes for Critical Steel Parts

The role of plotting of diagrams of isothermal and thermokinetic transformations of supercooled austenite in the development of heat treatment processes of steels is discussed. Specific examples of the necessity of plotting of thermokinetic diagrams of transformations in steels for solving critical production problems are considered.

Effect of Low-Carbon Steel 20GFL Heat Treatment Regime on Structure and Mechanical Properties

Results are given for a study of the effect of heat treatment parameters on supercooled austenite transformation kinetics in steel 20GFL. Specimen microstructure is studied in specimens 10 × 10 mm in cross section with the following regimes: soaking at 860 and 940°C, cooling in an air stream 3–8 m/sec through a channel 100 mm in diameter. The change in microstructure features in the course of high-temperature soaking at 940°C for 30 and 60 min after specimen cooling in salt water is determined. It is established that in cast steel there is chemical composition microliquation inhomogeneity with respect to carbon and manganese, which decreases during soaking (60 min) and is accompanied by austenite grain growth. Critical cooling rates are determined with which a structure forms containing upper and lower bainite. A dependence is obtained for impact strength KCV−60 on soaking temperature and time in the γ-region. The effect is demonstrated of alloying with vanadium on the possibility of stimulating the formation of globular lower bainite components.

Experience in Improving Silicon Steel Component Heat Treatment Quality

Inspection of the heat treatment section is performed on the basis of studying results of supercooled austenite transformation in silicon steel and modeling for heat treatment in a Heat Treatment Solution (SYSWELD) program. Deviations are revealed for the hardening production process, after whose avoidance uniform distribution of structural components and hardness is achieved through the cross section of components, and this provides an improvement for finished object set of properties.

High-Temperature Thermomechanical Treatment of Alloyed Structural and Tool Steels

The effect of hot plastic deformation on decomposition of supercooled austenite in some structural and die steels with different contents of carbon and alloying elements is investigated. It is shown that high-temperature plastic deformation accelerates the transformation of supercooled austenite by the diffusion mechanism and decelerates the development of bainitic transformations by raising the hardenability of the steels. The general laws of formation of mechanical properties under high-temperature thermomechanical treatment of structural and die steels after low- and high-temperature tempering are studied.

Heat Treatment Technology for High-Strength Engineering Steel Variable Cross-Section Components

Heat treatment parameters providing a required level of mechanical properties for different component parts are determined on the basis of experimental data about supercooled austenite transformation in engineering chromium-molybdenum steel, and also results of numerical modeling of variable cross-section components of this steel in a QForm 7 program.

Nucleation Laws of Supercooled Austenite Transformation

It is significant theoretically to study the nucleation laws of the phase transformation products of the supercooled austenite. The different iron and steel materials were used to study the nucleation of pearlite, bainite and martensite by QUANTA-400 environmental scanning electron microscope and JEM-2100 transmission electron microscope. The results show that, from the high-temperature zone to the low-temperature zone, the nucleation of the transformation products of the supercooled austenite is a gradually evolutionary process from two-phase nucleation to single-phase nucleation. Pearlite nucleates in the austenitic grain boundary, bainite nucleates preferentially in the grain boundary and sometimes in the grain interior and martensite nucleates preferentially in the interface and generally in the grain interior. The preferential nucleation sites are the interface, and with the decrease of the transformation temperature, they gradually shift to the defects in the grain interiors, which accord with the general rules of the phase transformation nucleation.

Structure formation in rebar strengthened by discontinuous quenching

In the layer adjacent to the surface of rebar hardened by discontinuous quenching, abnormal pearlite colonies are formed in the eutectoidal transformation of supercooled austenite, with decrease in the carbon content. Plate-pearlite colonies are formed after the depleted austenite is enriched by the deposition of excess ferrite.

Methodological aspects of plotting of thermokinetic diagrams of transformation of supercooled austenite in low-alloy steels

Aspects of plotting of thermokinetic diagrams of transformation of supercooled austenite in low-alloy steels are analyzed critically using published data and results of own studies. Special attention is devoted to the methods of processing and visual representation of experimental data. Recommendations on representation of results for developing unified approaches to plotting and interpreting the diagrams obtained are developed.

Modelling of CCT diagrams for engineering and constructional steels

Abstract The paper presents the methodology of modelling using the neural networks and regression method of the relationship between the chemical composition and austenitizing temperature, and the supercooled austenite transformation kinetics during the continuous cooling. The worked out model makes possible to calculate a CCT diagram for the steel with a known chemical composition and analysis of the influence of particular elements on the characteristic points and transformation curves of the supercooled austenite. In order to work out the methods the set of experimental data worked out on the basis of information available in the literature consisting of 300 CCT diagrams made for constructional and engineering steels was used. Model presented in the paper features a useful assistance in selection of the chemical composition of steel for machine elements subjected to heat treatment.