Austempering Heat Research Articles

Kinetics of Martensite/Austenite Decomposition during Tempering of Ultrafine Nano-Bainitic Steels.

In this study, the decomposition of a martensite/austenite (M/A) microconstituent in bainitic steels was analyzed using differential scanning calorimetry (DSC) data in conjunction with Kissinger's and Johnson-Mehl-Avrami-Kolmogorov (JMAK)'s formulas. In bainitic steel subjected to austempering heat treatment, the presence of an M/A microstructure adversely affects the mechanical properties. According to the kinetic equations derived, it is observed that after tempering the sample at 600 °C for 4000 s, the generation of each phase reaches its maximum. The SEM images taken before and after tempering reveal extensive decomposition of the M/A constituent in the microstructure. The proportion of the M/A microstructure decreased significantly from about 10% before tempering to less than 1% after. Additionally, the content of residual austenite also reduced nearly to zero. These observations are consistent with the predictions of the kinetic equations.

Genetic algorithm (GA)–backpropagation (BP) network approach for hardness prediction of austempered ductile iron (ADI)

Hardness serves as a crucial indicator for assessing the success of quenching treatment in the steel and iron industry, impacting the processability and wear properties of materials. In the present study, a dataset comprising 125 hardness values of the QT500-7 sample subjected to various austempering heat treatment parameters was utilised to train a neural network model for predicting the hardness of austempered ductile iron (ADI). The established model based on a genetic algorithm and error backpropagation algorithm demonstrates high accuracy in predicting the hardness of ADI if given heat treatment parameters. The mean square error of the model was about 1.019, indicating the reliability and precision of the model in predicting the hardness of ADI based on the specified heat treatment parameters.

A comparative study of flake graphite iron and compacted graphite iron in austempered condition for directional control valves

Demand for lighter materials with the right qualities is unavoidable in the present era of engineering. Compacted graphite iron is a type of cast iron gaining momentum nowadays in foundries due to its distinct characteristics that fall between gray cast iron/flake graphite iron and spheroidal graphite iron. It becomes the right candidate to replace flake graphite iron in many applications. In this research, flake graphite iron (FC −30) and compacted graphite iron-300 grades were studied to enhance its properties through austempering heat treatment process. Initially, the as-cast (non-austempered) flake graphite iron and compacted graphite iron were subjected to microstructural study and mechanical testing. Subsequently, the flake graphite iron and compacted graphite iron were subjected to austempering heat treatment. The various austempering temperatures selected are 270, 320, and 370 °C for different soaking time periods of 30, 60, 90, and 120 min. As cast flake graphite iron specimen (FC-30) exhibited lesser tensile strength (293 N/mm2) than as cast compacted graphite iron (528 N/mm2) and austempered compacted graphite iron (652 N/mm2). With respect to hardness measurement, austempered compacted graphite iron specimens yielded higher hardness of up to 415 BHN. However, the compacted graphite iron specimen austempered at 370 °C for 90 min measured an optimum hardness value of 373 BHN and tensile strength of 630 N/mm2 with less retained austenite (0.3%). This hardness value is considered to be more suitable in machining operation for making hydraulic directional control valves exposed to constant abrasion.

Laser Powder Bed Fusion Fabrication of a Novel Carbide-Free Bainitic Steel: The Possibilities and a Comparative Study with the Conventional Alloy

A newly developed medium-carbon carbide-free bainitic steel was fabricated for the first time utilizing the laser powder bed fusion (L-PBF) technique. Process parameters were optimized, and a high density of 99.8% was achieved. The impact of austempering heat treatment on the bainite morphology and transformation kinetics was investigated by high-resolution microstructural analysis (SEM, TEM, and EDS) and dilatometric analysis, and results were compared with conventionally produced counterparts. Faster kinetics and finer microstructures in the L-PBF specimens were found as a consequence of the as-built microstructure, characterized by fine grains and high dislocation density. However, a bimodal distribution of bainitic ferrite plate thickness (average value 60 nm and 200 nm, respectively) was found at prior melt pool boundaries resulting from carbon depletion at such sites.

Optimizing Hot-Work Tool Steel Microstructure for Enhanced Toughness

Hot-work tool steels play a crucial role in applications exposed to extreme thermal, mechanical, and chemical stresses and require exceptional properties such as high strength, hardness, wear resistance, and toughness. The latter is crucial to prevent an unexpected tool failure due to the formation and propagation of fatigue cracks in demanding environments. In addition, high thermal conductivity is crucial to prevent overheating of the tool and the resulting degradation of the material. This study focuses on a new generation hot-work tool steel with increased Mo and W contents, which has excellent thermal conductivity but limited toughness, as it contains stable Mo-W carbides that remain stable up to 1100 °C. To improve toughness, an alternative heat-treatment method involving austempering at different temperatures was applied. The investigation begins with the characterisation of the chemical composition of the steel, followed by the determination of the martensite-start (MS) and martensite-finish (Mf) temperatures. Based on the results, the researchers established a set of samples for austempering heat treatment. They investigated the effects of different isothermal holding temperatures on the microstructure of the steel and its subsequent mechanical properties. The results show that reduced bainite formation, achieved by austempering at certain temperatures, led to significantly improved impact toughness and moderate hardness. This study also showed a correlation between the isothermal holding temperature and the extent of martensitic transformation, which affected the microstructure and mechanical properties of the steel.

PENGARUH TEMPERATUR AUSTEMPERING TERHADAP SIFAT MEKANIK DAN STRUKTUR MIKRO BESI COR KELABU DENGAN NIKEL 10% UNTUK KOMPONEN BOILER PLTU

Effect of austempering temperature on microstructure and mechanical properties gray cast iron with 10%wt nickel for boiler combustion application has been studied. Cast iron with the JIS FCD 45 standard with a minimum tensile strength of 414 MPa and a hardness of 187 HB is commonly used for this application. To improve its mechanical properties such as strength and hardness, austempering heat treatment usually applied. Austempering was carried out at 250°C, 300°C, 350°C, 400°C, and 450°C for 1 hour, with the same austenization temperature of 850°C for 1 hour. The highest increase in hardness was found at the austempering temperature of 250°C. The hardness was 321.80 HB or 19.30% of the initial hardness of 259.70 HB. In terms of strength, the highest increase occurred at the austempering temperature of 250°C which was 257 MPa or 31.52% of the initial strength of 176 MPa.

Tempered Martensite Embrittlement of the Metallic Fastener After Austempering Heat Treatment and Coating Process Due to the Existence of Inclusion

Tempered Martensite Embrittlement of the Metallic Fastener After Austempering Heat Treatment and Coating Process Due to the Existence of Inclusion

Influence of alloying elements and its effect on austempering of compacted graphite iron – A review

Compacted Graphite Iron (CGI) is a type of cast iron with intermediate properties between grey and ductile cast iron. The vermicular graphite structure in the CGI provides unique properties to manufacture automotive and other engineering components. However, because of its narrow range of metallurgical control, a high volume of CGI production is not attempted in many foundries. The addition of alloying elements and austempering heat treatment process has a significant effect in altering the properties and microstructure of CGI. Hence, the objective of this paper is to review the influence of various alloying elements and austempering processes in manufacturing CGI components. Eventually, the scope for the research on the development of austempered CGI is identified.

The Significant Impact on Microstructure–Properties Relationship of 20Mn2Si2Cr Bainitic Steels Produced by Multistep Austempering Process

To elucidate the relationship between different isothermal processes and toughness of 20Mn2Si2Cr bainite high‐strength steel, conventional austempering (AT), multistep austempering (MAT), and inverse multistep austempering (IMAT) heat treatment processes at different temperatures are conducted. The changes of properties with different processes are analyzed according to microstructure characteristics. The results indicate that the fraction of retained austenite is changed slightly for all processes, while the carbon content in retained austenite is from 1.03 wt% (AT‐1) to 1.24 wt% (IMAT‐2). Toughness is improved significantly by adjusting process parameters, but strength is not greatly affected at the same time. The toughness of AT‐1 specimens is only 98 J cm−2, and it increases to 123 J cm−2 (AT‐2 specimens) by reducing isothermal temperature. Compared with AT‐1 specimens, toughness of MAT‐2 and IMAT‐2 specimens is increased by about ≈40% (138 J cm−2) and ≈65% (162 J cm−2), respectively. It is also noteworthy that the tensile strength varies in a small range from 998 to 1080 MPa. The improvement in properties is severely related to modification of microstructure.

Kinetics of Carbon Enrichment in Austenite during Partitioning Stage Studied via In-Situ Synchrotron XRD

The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation.

Performance Evaluation of Austempered Ductile Iron Camshaft Low Alloyed with Vanadium on an Electric Spin Rig Test

Arbomex S.A. de C. V. is one of the largest worldwide manufacturers of ductile cast iron camshafts, produced by means of the phenolic urethane no-bake sand mold casting method and cold box by stack molding technology. As a result of the development of high-strength ADIs, low alloyed with vanadium, for camshaft manufacturing, previous results were published on the as-cast process and the austempering heat treatments applied to the camshafts. In the present work, camshafts of ADIs, low alloyed with 0.2 and 0.3 wt.% V, were produced at austempering temperatures of 265 and 305 °C. The performance of the new camshafts was evaluated by wear testing to ensure the function and durability of the camshafts by means of the block-on-ring wear test and a valve train system to evaluate the volume loss of material removed and the geometrical changes of the camshaft, respectively. The ADIs heat treated to 265 °C showed a microstructure constituted of fine ausferrite that aided in obtaining the highest wear resistance in the block-on ring wear test. No wear or pitting evidence was detected on the camshaft lobes and roller surfaces after the OEM test protocol during the electric spin ring test at low and high conditions for the ADI alloyed with 0.2 wt.% V heat treated at 265 °C.

Kinetics of Bainite Transformation in Multiphase High Carbon Low-Silicon Steel with and without Pre-Existing Martensite

In the present study, the isothermal decomposition of austenite to bainite in 1.0 wt% carbon, 0.21% silicon steel during the partitioning step of a quenching and partitioning (Q&P) heat treatment has been investigated in a dilatometer in the temperature range of 200 to 350 °C and compared to conventional austempering heat treatment. The bainite transformation was shortened by about 75% in the presence of pre-existing martensite (QP). The kinetics of bainite transformation is described by the well-known Avrami equation. The calculated parameter ‘n’ in the Avrami equation shows that bainite forms in the absence of pre-existing martensite (TT) at a constant nucleate rate, while in the presence of pre-existing martensite, nucleation is interface controlled. The overall bainite transformation activation energy, calculated by the Avrami equation, ranges from 64 to 110 kJ/mol. The outcomes of this investigation provide guidelines for the development of multiphase microstructures, including pre-existing martensite and bainite in high-carbon low-silicon steel, within an industrially acceptable time scale and mechanical performance.

Statistical Data-Driven Model for Hardness Prediction in Austempered Ductile Irons

This research evaluates the effect of temperature and time austempering on microstructural characteristics and hardness of ductile iron, validating the results by means of a statistical method for hardness prediction. Ductile iron was subjected to austenitization at 950 °C for 120 min and then to austempering heat treatment in a salt bath at temperatures of 290, 320, 350 and 380 °C for 30, 60, 90 and 120 min. By increasing austempering temperature, a higher content of carbon-rich austenite was obtained, and the morphology of the thin acicular ferrite needles produced at 290 °C turned completely feathery at 350 and 380 °C. A thickening of acicular ferrite needles was also observed as austempering time increased. An inversely proportional behavior of hardness values was thus obtained, which was validated through data analysis, statistical tools and a regression model taking temperature and time austempering as input variables and hardness as the output variable, which achieved a correlation among variables of about 97%. The proposal of a mathematical model for the prediction of hardness in austempered ductile iron represents a numerical approximation which validates the experimental results at 95.20%.

Effect of the cross-rolling and austempering heat treatment on the microstructure, texture, and mechanical properties of a low alloyed steel used in rotary cutting tools

Effect of the cross-rolling and austempering heat treatment on the microstructure, texture, and mechanical properties of a low alloyed steel used in rotary cutting tools

Mechanical Behaviors of Microalloyed TRIP-Assisted Annealed Martensitic Steels under Hydrogen Charging.

Transformation Induced Plasticity (TRIP)-assisted annealed martensitic (TAM) steel sheets with various microalloying additions such as niobium, vanadium, or titanium were prepared on laboratory scale and subjected to a double-quenching and austempering heat treatment cycle. Slow strain rate tensile (SSRT) was tested on the investigated TAM steels with and without hydrogen charging to reveal their tensile behaviors and hydrogen induced embrittlement effects. Microstructure observations by scanning electron microscope (SEM) are composed of a principal annealed martensitic matrix and 11.0–13.0% volume fraction of retained austenite, depending on the type of microalloying addition in the different steels. SSRT results show that these TRIP-assisted annealed martensitic steels under air media conditions combine high tensile strength (>1000 MPa) and good ductility (~25%), while under hydrogen charging condition, both tensile strength and ductility decrease where tensile strength ranges between 680 and 760 MPa, down from 1000–1100 MPa, and ductility loss ratio is between 78.8% and 91.1%, along with a total elongation of less than 5%. Hydrogen charged into steel matrix leads to the appearance of cleavage fractures, implying the occurrence of hydrogen induced embrittlement effect in TAM steels. Thermal hydrogen desorption results show that there are double-peak hydrogen desorption temperature ranges for these microalloyed steels, where the first peak corresponds to a high-density dislocation trapping effect, and the second peak corresponds to a hydrogen trapping effect exerted by microalloying precipitates. Thermal desorption analysis (TDS) in combination with SSRT results demonstrate that microalloying precipitates act as irreversible traps to fix hydrogen and, thus, retard diffusive hydrogen motion towards defects, such as grain boundaries and dislocations in microstructure matrix, and eventually reduce the hydrogen induced embrittlement tendency.

Development and properties of austempered low alloyed white cast iron

Abstract This study examined the response of low-alloy white cast iron to austempering heat treatment. In addition, it investigated the microstructure and mechanical properties of austempered low-alloy white cast iron. The low-alloy white cast iron specimens were austenitized at 900 °C, followed by quick quenching into a salt bath at 375 °C, and held there for 15 to 120 minutes for austempering heat treatment. Microstructural features were studied by optical, scanning electron microscopes, and XRD analysis. The mechanical properties were determined by hardness and unnotched Charpy impact toughness tests. As a function of those austempering times, a microstructural map was constructed to show how the transformation products develop, quantitatively. The experimental results showed that the austempering heat treatment produced a microstructure consisting of eutectic carbides + ausferritic structure in low-alloy white cast iron. It can be concluded that the low-alloy white cast iron can be austempered, similar to ductile cast irons. Improved hardness and impact toughness values have been obtained in austempered low-alloy white cast iron.

Morphology and wear of high chromium and austempered ductile iron balls as grinding media in ball mills

High chromium balls are recognized as better grinding media in terms of wear rates than forged steel balls, which are conventional grinding media in the milling process of iron ore. In this research work, the wear rate of high chromium balls and austempered ductile iron (ADI) balls as crushing media in a ball mill are compared. ADI are prepared by austenitizing the spheroidal graphite (SG) iron balls at 920 °C for one hour, and step austempering heat treatments were given, which includes the first step austenitizing at 300 °C for 15 min, followed by a second step austenitizing at 400 °C for 60 min. The wear rates were estimated when both balls were used separately by maintaining the same machining conditions and when the balls are mixed. The grinding wear conduct of both materials is evaluated for wear loss in wet grinding conditions. The experimental results reveal that the performance of ADI balls is better than high chromium balls when tested separately and mixed. Results also indicate that the wear rates/revolutions will decrease when the operating period increases.

Ultrafine Ductile and Austempered Ductile Irons by Solidification in Ultrasonic Field

In this research, ultrasonic melt treatment (UST) was used to produce a new ultrafine grade of spheroidal graphite cast iron (SG iron) and austempered ductile iron (ADI) alloys. Ultrasonic treatment was numerically simulated and evaluated based on acoustic wave streaming. The simulation results revealed that the streaming of the acoustic waves propagated as a stream jet in the molten SG iron along the centerline of the ultrasonic source (sonotrode) with a maximum speed of 0.7 m/s and gradually decreased to zero at the bottom of the mold. The metallographic analysis of the newly developed SG iron alloy showed an extremely ultrafine graphite structure. The graphite nodules’ diameter ranging between 6 and 9 µm with total nodule count ranging between 900 to more than 2000 nodules per mm2, this nodule count has never been mentioned in the literature for castings of the same diameter, i.e., 40 mm. In addition, fully ferritic matrix was observed in all UST SG irons. Further austempering heat treatments were performed to produce different austempered ductile iron (ADI) grades with different ausferrite morphologies. The dilatometry studies for the developed ADI alloys showed that the time required for the completion of the ausferrite formation in UST alloys was four times shorter than that required for statically solidified SG irons. SEM micrographs for the ADI alloys showed an extremely fine and short ausferrite structure together with small austenite blocks in the matrix. A dual-phase intercritically austempered ductile iron (IADI) alloy was also produced by applying partial austenitization heat treatment in the intercritical temperature range, where austenite + ferrite + graphite phases coexist. In dual-phase IADI alloy, it was established that introducing free ferrite in the matrix would provide additional refinement for the ausferrite.

Enhanced strength and plasticity in a novel 55Si2MnMoV spring steel via austempering

A series of austempering heat treatments were carried out on the designed new spring steel to study the influence of various parameters on the microstructure and mechanical properties of 55Si2MnMoV steel. Experimental results indicate that excellent comprehensive mechanical properties can be obtained at 275 °C for 30 min, with ultimate tensile strength (UTS) and product of strength and elongation (PSE) being 2144 MPa and 25.26 GPa%, respectively. Transmission electron microscope (TEM) shows that the multiphase microstructure is composed of lath bainite ferrite (BF), martensite, and retained austenite (RA), and the RA with film-like structure is mainly distributed between bainite laths, while the fine carbides are embedded in the matrix. As the austempering temperature increases, the grain size increases gradually, while the strength decreases slowly. Austempering at 325 °C will produce a long-term transformation induced plasticity (TRIP) effect during tensile strain, making total elongation (TE) reach the maximum. While having ultra-high strength and good plasticity, the average rotating bending fatigue strength of the sample at 275 °C is 952 MPa and the fatigue ratio is 0.444, which is mainly determined by the reasonable partition of the multiphase microstructure at low temperature.

Weldability of austempered rail steel using the flash-butt process

Abstract In this study, bainitic microstructure was formed via heat treatmenton R260 rail steel, which is generally used in railways. Bainitic steel, which is considered more advantageous than current rail steel, waswelded by flash butt welding, which is often used for joining rails andthe mechanical and microstructure of the samples were thenexamined and compared. Bainitic structural steel obtained by austempering heattreatment with normal rail steel was welded by flash butt welding. Flash-butt welding parameters were kept constant during the experiment. The welding capabilities of the joints were compared and the results wereevaluated. It was determined that the bainite structure obtained as a result of austempering heat treatment changes the microstructuralproperties of the samples and affects the mechanical values ​of the joints.