Increasing Austempering Temperature Research Articles

Influence of Austempering Conditions on Hardness and Microstructure of Bainite in Low-Alloyed Steel

The influence of austempering temperature and time on the microstructure and hardness of a low-alloyed bainitic steel is investigated in the temperature range 275 °C to 375 °C for up to 24 hours. It is shown that the dislocation density and coarseness of the bainitic microstructure are affected by the austempering temperature, while only the dislocation density is significantly affected by the austempering time. The hardness of the steel is estimated based on microstructure–property relations and is in good agreement with the measured hardness. In conclusion, the decrease in dislocation density is the main reason for loss in hardness upon increasing austempering temperature and/or time for the studied temperature range.

Investigation of Mechanical Properties in Ductile Iron with Alloyed and Unalloyed at Time and Temperature on Austempering

Due to the combination of its numerous excellent mechanical qualities, the flexible iron has been used more and more since its invention in 1948. To develop significantly improved characteristics, the unnecessary investigation is being done. The most recent development in the field of flexible iron, or SG iron, is Austempererd malleable iron. At four different temperatures, two different types of spheroidal graphite (SG) cast iron samples with varying copper weight levels were austempered. The temperatures used for austempering were 200°C, 300°C, 350°C, and 400°C. As a component of the austempering time and temperature, the effect of the austempering process (i.e. time and temperature) on the mechanical characteristics of spheroidal graphite iron was investigated. The progress of spheroidal graphite iron's properties was significantly influenced by the pace of cooling and the extinguishing process. The organisation of different stages during isothermal change under varied austempering settings has also been the focus of XRD analysis. By using SEM, graphite morphology has been focused on. For this investigation, samples were obtained from the castings' focal point for XRD analysis. It was discovered that virtually always, it is possible to discriminate between the ferrite (110) and austenite (111) lines. The ferrite (110) line is growing with expanding austempering time and declining with increasing austempering temperature, whereas the highest power of the austenite (111) line is expanding with expanding temperature. Thus, very precise control of the interaction components (austempering duration and temperature) is required for austempering. The results showed that, when compared to other grades (N1) through the various austempering processes used in this evaluation, ADI containing the alloying component copper (grade N2) achieved crucial mechanical qualities.

Evaluation of wear resistance of a novel carbide-free bainitic steel

The effect of isothermal transformation temperatures on wear performances of a newly designed high-silicon carbide-free bainitic steel was studied. Wear tests were performed by a ball-on-disc tribometer using steel spheres as counterpart material. Friction coefficient was provided by the equipment, whereas wear rate was evaluated by the wear volume. Wear tracks on disc surfaces and worn cross-sections were analysed by means FEG-SEM, EDS measurements, XRD, roughness, and hardness tests. Bainitic microstructure offers better wear resistance compared to pearlitic-ferritic microstructure due to higher hardness, strain hardening capability, and strain-induced transformations. Adhesion and tribo-oxidational wear are the main wear mechanisms; the extent of oxidation increases increasing austempering temperature and it is maximum for samples treated at 350 °C and pearlitic-ferritic microstructure.

Effect of Carbon Content on Variant Pairing in Bainitic Low Alloy Steel

Variant pairing in bainite was evaluated in four different commercial low alloy steels with medium to high carbon content. The steels investigated were austempered in the temperature range 275 °C to 450 °C to obtain a bainitic microstructure. It was found that the V1–V6 is the most frequent variant pairing at lower temperature, while it gradually decreases towards intermediate temperatures, and at the highest austempering temperatures the variant pairing with low misorientation boundaries such as V1–V4 and V1–V8 is the most frequent. The preferred variant pairing is the least pronounced after austempering of steels with higher carbon contents at intermediate temperature. Nonetheless, a continuous increase of variant pairing within the same Bain group was observed with increasing austempering temperature for all steels. Furthermore, it was observed that the deviation from the theoretical Kurdjumov–Sachs orientation relationship increases with increasing austempering temperature for all steels.

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%.

Study of Transformation Kinetics for Austempered Gray Cast Iron

Austempered gray iron (AGI) with its unique ausferrite microstructure is an alternative material to traditional quenched and tempered gray iron with martensite microstructure due to its excellent mechanical properties. This research studies the process of ausferrite formation and kinetic characterization for gray cast iron. Specimens were austenitized at 832°C (1530°F) for 20 min and then held at different isothermal temperatures from 232°C (450°F) to 427°C (800°F) and isothermal times from 20 s to 120 min. The experimental results were analyzed using hardness tests and optical microscopy. Needle-like ausferrite structure was found while using austempering temperatures ranging from 232°C (450°F) to 371°C (700°F). The ferrite plates became coarse with increasing austempering temperature. Feather-like ausferrite was obtained at the austempering temperatures of 399°C (750°F) and 427°C (800°F). Transformation kinetics was utilized to analyze the ausferrite formation, and the activation energy of gray cast iron was found to be 39905.27 (J/mol) with a frequency factor of 6.06457 (1/s).

Tribological behavior of austempered ductile iron (ADI) obtained at different austempering temperatures

The tribological behavior of austempered ductile iron (ADI) obtained at different austempering temperatures is investigated. The microstructure of ADI is consisted of spheroidal graphite, retained austenite and acicular ferrite. The volume fraction of retained austenite and the thickness of acicular ferrite increase with increasing austempering temperature, which results in strength reduction but plasticity increasement. With the increase of austempering temperature, the friction coefficient changes little under low load (5 N) but decreases under high loads (10 N and 15 N), whilst the wear rate increases under low load (5 N) but changes little under high loads (10 N and 15 N). This wear behavior is attributed to the combined effects of hardness and friction coefficient. Additionally, adhesive and abrasive wear are main failure mechanisms for ADI under dry sliding wear condition.

Corrosion Characteristics of Copper-Added Austempered Gray Cast Iron (AGCI).

The aim of this investigation was to assess the corrosion behavior of gray cast iron (GCI) alloyed with copper. Alloyed GCI specimens were austempered isothermally at varying temperatures. After austenitizing at 927 °C, the samples were austempered at different temperatures ranging from 260 to 385 °C with an interval of 25 °C for 60 min. As a result, these samples developed an ausferrite matrix with different percentages of austenite. The resulting microstructures were evaluated and characterized by optical microscope (OM), scanning electron microscope (SEM), and X-ray diffraction (XRD). The corrosion characteristics were determined using potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) of these samples. These tests were carried out in a medium of 0.5 M H2SO4 and 3.5% NaCl solution. It was observed from the potentiodynamic polarization results that with increasing austempering temperature, the corrosion rate decreased. All results of the EIS were in accordance with a constant phase element (CPE) model. It was found that with an increase in austempering temperature, the polarization resistance (Rp) increased. The austenite content was also found to influence the corrosion behavior of the austempered gray cast iron (AGCI).

Microstructure and mechanical properties of copper alloyed austempered grey cast iron

ABSTRACTAustempered grey cast iron (AGI) has emerged as a major engineering material in recent years because of its attractive mechanical properties. The main aim of this investigation is to assess the mechanical properties of copper alloyed AGI. Alloyed grey cast iron specimens are subjected to austempering heat treatment at six different temperatures for four different time periods. The resulting microstructures have been evaluated and characterised by means of light microscope and scanning electron microscope and X-Ray diffraction analysis. The microstructural features of AGI such as austenite content and its carbon content have been also found to influence the hardness, tensile properties and elongation. Both duration of the austempering time and the austempering temperature affect the mechanical properties of AGI. The hardness, tensile strength and ductility initially increase, and thereafter it decreases on longer periods of austempering. On the other hand hardness, tensile strength decreases as increasing austempering temperature, while ductility increases. The best combination of hardness 380BHN and strength 332 MPa; observed at 927°C of austenitising and 260°C of austempering temperature for 60 min.

Investigation into the Microstructure and Mechanical Properties of Thin Wall Austempered Gray Cast Iron (TWAGI)

The main aim of this investigation is to assess the mechanical properties of thin wall (3 mm thickness) copper alloyed gray cast iron. Thin wall alloyed gray cast iron specimens are subjected to austempering heat treatment at six different temperatures for four different time periods. As a result, these samples develop an ausferrite matrix with excellent mechanical properties. The resulting microstructures have been evaluated and characterized by means of optical microscope and scanning electron microscope (SEM) and X-Ray diffraction analysis. The hardness and tensile properties of all these specimens are determined and correlated with the microstructure. The hardness, tensile strength and ductility initially increase, and thereafter it decreases on longer periods of austempering. On the other hand, hardness and tensile strength decreases with increasing austempering temperature, while ductility increases.

Effect of austempering temperature on microstructure and properties of austempered ductile iron

The influence of austempering temperature on microstructure and properties of Austempered Ductile Iron were studied in this paper. The purpose was to improve comprehensive performance of Austempered Ductile Iron for adapting to slurry pump transflux parts. The nodularisation grade of graphite was 2-3 magnitude and the size of graphite was 6-7 magnitude. After austempered at different temperatures for 120 min, the microstructure mainly consisted of acicular ferrite, retained austenite and spheroidal graphite. With increasing austempering temperature, tensile strength improved firstly and then decreased, impact toughness increased, the wear resistance became worse, and the corrosion resistance increased. When the heat treatment process was 900°C × 90 min + 250°C × 120 min, the comprehensive performance was better (tensile strength: 1710 MPa, elongation: 2%, impact toughness: 40 J/cm2, Brinell hardness: 467 HBW, wear weightlessness rate: 0.17 mg/m) than the requirements in ASTM A897-06.

Influence of austempering temperature and salt bath agitation on microstructure and mechanical properties of austempered ductile iron

The influence of austempering temperature and salt bath agitation on microstructure and mechanical properties of austempered ductile iron was studied. The as-cast specimens of ductile iron were subjected to heat treatments consisting of austenitisation at 900◦C for 1 h followed by austempering at temperatures ranging from 230 to 450◦ C in a salt bath with and without agitation. The volume fraction of retained austenite increases with increasing austempering temperature to about 395◦C and decreases at higher austempering temperatures. The austempering at lower temperatures leads to the formation of finer microstructures. The agitation of the salt bath leads to the formation of the higher volume fraction of ausferrite at the expense of the retained austenite compared to that of the specimens prepared without salt bath agitation. Vickers hardness decreases with increasing austempering temperature and increasing volume fraction of the retained austenite. Room temperature impact energy increases with increasing austempering temperature to about 395◦C and then decreases at higher temperatures. The impact energy increases with increasing volume fraction of the retained austenite. The proposed mathematical models are of industrial importance for prediction of Vickers hardness, impact energy and volume fraction of the retained austenite as functions of austempering temperature and agitation or no agitation of the salt bath.

Effect of austempering temperature on microstructure and properties of austempered ductile iron

The influence of austempering temperature on microstructure and properties of Austempered Ductile Iron were studied in this paper. The purpose was to improve comprehensive performance of Austempered Ductile Iron for adapting to slurry pump transflux parts. The nodularisation grade of graphite was 2-3 magnitude and the size of graphite was 6-7 magnitude. After austempered at different temperatures for 120 min, the microstructure mainly consisted of acicular ferrite, retained austenite and spheroidal graphite. With increasing austempering temperature, tensile strength improved firstly and then decreased, impact toughness increased, the wear resistance became worse, and the corrosion resistance increased. When the heat treatment process was 900°C × 90 min + 250°C × 120 min, the comprehensive performance was better (tensile strength: 1710 MPa, elongation: 2%, impact toughness: 40 J/cm2, Brinell hardness: 467 HBW, wear weightlessness rate: 0.17 mg/m) than the requirements in ASTM A897-06.

Influence of casting heterogeneities on microstructure and mechanical properties of austempered ductile iron (ADI)

An investigation was carried out to examine the influence of as-cast heterogeneities on the resulting microstructure and mechanical properties of an austempered ductile cast iron (ADI). A commercial hypereutectic ductile iron alloy with 3.59% C, 2.68% Si, 0.24% Mn and 0.46% Cu (in wt. %) was produced in a coreless induction furnace. The specimens were obtained by casting the alloy into Y-block molds. The austempering heat treatments consisted of pre-heating at 500°C, followed by austenitizing step at 900°C and 930°C during 60 minutes. Austempering was carried out in molten metal baths at temperatures of 300°C and 370°C for 30 minutes. Microstructural characterization was carried out by light optical microscopy (LOM) with image analysis, scanning electron microscopy (SEM-FEG) and X-ray diffraction with Rietveld refinement. The mechanical properties were evaluated by tensile and Vickers hardness tests. The results of this investigation indicate that as-cast heterogeneities have a detrimental effect on the transformed microstructure and mechanical properties of ADI. During austempering, non uniform distribution of graphite nodules and segregation of alloying elements at cell boundaries resulted in transformation gradients throughout the metallic matrix. Intercellular regions displayed inhomogeneous microstructures containing eutectic carbides and martensite. Yield strength, tensile strength and hardness decrease for increasing austempering temperature. Fracture surfaces showed a rapid transition from ductile to cleavage mode at intercellular regions, containing solidification carbides.

Some Aspects on the Welding Characteristics and Formation of Microstructures in a Newly Developed Coated Electrode for Austempered Ductile Iron (ADI)

In this study, six different coated electrodes were manufactured to select compatible one for ADI and only two electrodes were further studied (Trial 3 and Trial 4) to check their compatibility. Welding conditions were established to obtain crack free weld deposit on DI plate by varying preheat (200°C, 300°C and 400°C for lhour) as well as PWHT (maintaining same preheat temperature) at constant heat input using two selected electrodes. Only preheating at 300°C was possible to obtain sound weld for Trial 3, whereasboth preheat (300°C) and PWHT (300°C) were required for Trial 4. Although the as-deposited microstructures of FZ and PMZ consisting of ledeburitic carbide and alloyed pearlite in both the electrodes were same, the amount of microstructural constituents varied significantly. More ledeburitic carbide and less alloyed pearlite were observed in Trial 3 compared to Trial 4. However, microstructures of HAZ for both the electrodes showed bainitic and pearlite. Austempering heat treatment comprising of austenitization at 900°C for 2 hr and austempering at two different temperatures (300°C and 350°C) for different holding time was carried out for both the weld deposits. Interestingly, for both the weld deposits austempered microstructures show bainitic ferrite and retained austenite with graphite nodules. Increasing austempering temperature from 300°C and 350°C the lower bainite transformed to upper bainite and vol. % of retained austenite was also increased.

Microstructures and Mechanical Properties of a Wear-Resistant Alloyed Ductile Iron Austempered at Various Temperatures

To further improve the mechanical performance of a new type of alloyed bainitic wear-resistant ductile iron, the effects of the various austempering temperatures have been investigated on microstructure and mechanical behaviors of alloyed ductile iron Fe-3.50C-1.95Si-3.58Ni-0.71Cu-0.92Mo-0.65Cr-0.36Mn (in weight percent). This alloyed ductile iron were firstly austenitized at 1123 K (850 °C) for 1 hour and then austempered in a salt bath at 548 K, 573 K, and 598 K (275 °C, 300 °C, and 325 °C) for 2 hours according to time–temperature–transformation diagram calculated by JMatPro software. The microstructures of austempered wear-resistant ductile irons consist of matrix of dark needle-like ferrite plus bright etching austenite and some amount of martensite and some dispersed graphite nodules. With increasing the austempering temperature, the amount of ferrite decreases in austempered ductile iron, while the amount of austenite and carbon content of austenite increases. There is a gradual decrease in hardness and increase in compressive strength with increasing austempering temperature. The increased austenite content and coarsened austenite and ferrite can lead to a hardness decrease as austempering temperature is increased. The increased compressive strength can be attributed to a decreased amount of martensitic transformation. The alloyed ductile iron behaves rather well wear resistance when the austempering is carried out at 598 K (325 °C) for 2 hours. Under the condition of wear test by dry sand/rubber wheel, the wear mechanisms of austempered ductile irons are both micro-cutting and plastic deformation.

Evolution of Microstructures During Austempering of Ductile Irons Alloyed with Manganese and Copper

The influences of relatively high manganese (0.45 through 1.0 wt pct) and copper (0.56 through 1.13 wt pct) contents on microstructure development and phase transformation in three austempered ductile irons have been studied. The experimental ductile irons alloyed with copper and manganese are found to be practically free from intercellular manganese segregation. This suggests that the positive segregation of manganese is largely neutralized by the negative segregation of copper when these alloying elements are added in appropriate proportions. The drop in unreacted austenite volume (UAV) with increasing austempering temperature and time is quite significant in irons alloyed with copper and manganese. The ausferrite morphology also undergoes a transition from lenticular to feathery appearance of increasing coarseness with the increasing austempering temperature and time. SEM micrographs of the austempered samples from the base alloy containing manganese only, as well as copper plus manganese-alloyed irons, clearly reveal the presence of some martensite along with retained austenite and ferrite. X-ray diffraction analysis also confirms the presence of these phases. SEM examination further reveals the presence of twinned martensite in the copper plus manganese-alloyed samples. The possibility of strain-induced transformation of austenite to martensite during austempering heat treatment is suggested.

Improvement in fracture toughness of austempered ductile iron by two-step austempering process

Ductile cast iron samples were austenitised at 900°C and subjected to two types of austempering called as conventional austempering and two-step austempering. Five different temperatures, 280, 300, 320, 350, 380 and 400°C, with an austempering time of 2 h, were chosen for conventional austempering. For two-step austempering process, the first step temperatures were 280, 300 and 320°C. The samples were austempered at each of these temperatures for different times, i.e. 10, 20, 30, 45 and 60 min, and then upquenched to higher temperature of 400°C for 2 h. Fracture toughness and tensile studies were carried out under all these austempering conditions. During conventional austempering, the fracture toughness initially increased with increasing austempering temperature, reached a peak value of 63 MPa m1/2 and dropped with further increase in temperature. During the two-step austempering, fracture toughness was found to increase with increasing first step time. The curve shifted to higher values of fracture toughness as the first step temperature was decreased and the maximum value of 78 MPa m1/2 was obtained. The results of the fracture toughness study and the fractographic examination were correlated with microstructural features such as bainitic morphology, the volume fraction of retained austenite, and its carbon content. Ferrite lath size and stability of the retained austenite were found to influence the fracture toughness.

The effect of bainite morphology on the mechanical properties of a high bainite dual phase (HBDP) steel

4340 steel bars were austenitized at 850 °C for 1 h followed by heating at 700 °C for 90 min and quenching into a salt bath at the temperature range of 300–450 °C for 1 h to obtain dual structures with 34 vol.% fraction ferrite and various bainite morphologies. SEM studies showed that by increasing the austempering temperature, bainite morphology varies from lower to upper bainite. Tensile, impact and hardness tests revealed that increasing the austempering temperature from 300 to 400 °C leads to a reduction in yield and ultimate tensile strength, hardness, uniform and total elongation and impact energy. But in dual phase steel produced by austempering at 450 °C, yield and tensile strength and hardness increased and severe reduction in total elongation and impact energy obtained. Fractography of tensile specimens showed brittle behavior for this austempering temperature. Fatigue test results showed that fatigue limit decreases with increasing austempering temperature from 300 to 400 °C. Finally, fractography studies showed cleavage fracture at the surface of fatigue specimens austempered at 400 °C, which confirms the tendency to brittle behavior.

In situ x-ray observation of bainitic transformation of austempered silicon alloyed steel

The in situ x-ray diffraction observations of the bainitic transformation were conducted by using the high-temperature x-ray diffraction technique. The volume fraction and carbon content of austenite depend on the transformation temperature. The d{110} value of bainitic ferrite decreases with increasing austempering temperature, which is related to the decrease of carbon concentration in bainitic ferrite. Asymmetry diffraction peaks are obtained for samples at the early stage of transformation at any austempering temperatures. This asymmetry diffraction peak after the formation of bainitic ferrite could be attributed to a heterogeneous distribution of carbon in different regions of austenite and show that two types of austenite with different carbon contents, low-carbon austenite (γLC) and the high-carbon austenite (γHC), exist during the transformation. The microstructure after cooling down to room temperature is presented to show the effectiveness of the x-ray diffraction analysis.