Two-step Austempering Process Research Articles

Effects of two-step austempering processes on the wear resistance and fatigue lifetime of high-carbon nanostructured bainitic steel

The previous research on two-step austempering processes have been mainly focused on the bainitic transformation kinetics, with few details the relationships between the parameters of two-step austempering process and the wear properties and fatigue lifetime. In this paper, the effects of two-step austempering process on wear resistance and fatigue life of high carbon nano-bainite steel were investigated. The results show that the first-step bainite with high volume fraction prolongs the total bainite transformation time. The two-step austempering samples contain finer bainitic ferrite plate and higher volume fraction of retained austenite, as well as resent excellent hardnesses, tensile properties, wear resistance and fatigue lifetime. In particular, the wear resistance of the 50%-290 °C sample is better than that of the 210 °C sample, this is caused by the transformation of the high fraction of retained austenite in the two-step austempering samples into hard martensite and the microstructure refinement zone that forms on the surface of the sample during the wear process. The 50%-290 °C sample have better fatigue properties at low total strain amplitudes (0.7% and 0.8%). When the total strain amplitude is 0.7%, the fatigue life of the 50%-290 °C sample is 57.5% higher than that of the 210 °C sample, which is because of the higher yield strength and finer microstructure of the 50%-290 °C sample. These results support the later study of controlling two-step austempering process for high-carbon nanostructured steels.

Effect of Two-Step Austempering Process on the Microstructure and Mechanical Properties of Low-Carbon Equivalent Austempered Ductile Iron

<div>Low-carbon equivalent austempered ductile iron (LCE-ADI) exhibits high modulus of elasticity than conventional austempered ductile iron (ADI) due to less graphite content. Austempering parameters of temperature and time significantly influence the mechanical properties of LCE-ADI. In the present work, response of the material to two-step austempering in the range of 350–450°C was studied, and a comparison was made to single-step austempering. Reduction in ferrite cell size, increase in % carbon in carbon-stabilized austenite (CSA) and increase in volume fraction of CSA led to increase in tensile strength (10%) and hardness (20%), in addition to improved toughness (10%).</div>

Influence of two-step austempering at different temperatures on mechanical and microstructural properties of AISI 9254 high silicon steel

ABSTRACT This study was undertaken to understand the effects of two-step austempering treatment on an AISI 9254 high silicon steel towards tailoring the properties as desired while simultaneously employing the benefits of high and low austempering temperatures. The samples were initially austenitized at 850°C for 20 min, followed by austempering in a salt bath at the temperatures of 250–270–290°C for 20 min during the first stage. Subsequently, a second stage austempering was carried out by raising the temperature of the salt bath to 300°C at an average heating rate of 0.5°C/min, and the samples were kept in the salt bath for achieving a total austempering time of 120 min including the heating time. A conventional single-stage austempering was also conducted for comparison purposes, in which the austenitization temperature, the austempering temperatures and total time (stage I and stage II, i.e. 120 min) were kept the same for the benchmark samples. In the characterization studies, tensile test, hardness test, XRD analysis, optical microscope and field emission-scanning electron microscope (FE-SEM) equipped with EBSD detector were utilized. The findings of this study indicated that lowering the austempering temperature resulted in refining the structure with a decrease in the amount of austenite. According to the carbon content analysis through XRD patterns, the two-step austempering processes appeared to have considerably increased the carbon content of the austenite irrespective of austempering temperature. The best ultimate tensile strength (U.T.S) of 2194 MPa was achieved in the conventionally austempered sample at the lowest temperature of 250°C, while the best yield strength (Y.S.) of 1753 MPa was reached in the stepped austempered sample initially at 250°C followed by 300°C. In general, two-step austempering process led to a higher yield strength while affecting the ultimate tensile strength and total elongation depending on the austempering temperature.

Effects of first-step controlling on ultra-fine bainitic steel produced by two-step austempering process

Two-step austempering process has been demonstrated to be an effective method to accelerate the bainitic transformation at low temperature, but the rule of first-step controlling is still unclear. This paper comprehensively studies the effects of the first-step transformation on bainitic transformation, microstructural evolution and the related mechanical properties of ultra-fine bainitic steel obtained by two-step processes. Results show that the two-step austempering process notably accelerates the bainitic transformation, with the maximum reduction in total transformation time of 93.1% when the first-step bainitic fraction is controlled to be 22%. The thickness and length of bainitic ferrite plates and the size of blocky retained austenite of the two-step austempering samples are smaller than those of the conventional first-step austempering sample. Although the hardness and strength of the two-step austempering samples decrease slightly as compared with traditional one-step sample, they also meet the requirements of the product, such as bearing, especially the sample with 22% and 50% first-step fractions, which achieve higher toughness and yield strength, respectively. This study provides a theoretical support on process design of ultra-fine bainitic steel, based on short-period heat treatment process, optimized microstructure and mechanical properties.

Insights into effect of first-step austempering temperature on the microstructure and properties of austempered ductile iron

Austempered ductile iron (ADI) is a revolutionary material with high strength and hardness combined with excellent ductility and toughness. The discovery of a two-step austempering process has resulted in superior combination of all the mechanical properties. Therefore, ADI has many different applications in varying fields. One notable application of ADI is in transmission components for new energy vehicles. As the automotive continues to develop, ADI transmission components with higher damping properties are required, because the damping properties of ADI affect the vibration and noise of the transmission system. However, the mechanism behind the damping of ADI, as well as a full characterisation of the damping properties, has not been thoroughly investigated in the literature. Therefore, it is necessary to reveal the influencing factors and mechanism of the damping properties of ADI. In this paper, the effect of first-step austempering temperature on the microstructure, mechanical properties, and damping properties of ADI was investigated. The results revealed that the tensile strength, yield strength, and elongation of ADI all initially increased, until reaching a global maximum value (1320 MPa, 1230 MPa, and 13.2%, respectively) when the first-step austempering temperature was 280 °C, followed by a decrease in value. Interestingly, the impact energy of ADI initially decreased before reaching a global minimum and then increasing to a maximum value of 93.5 J, when the first-step austempering temperature reached 320 °C. With increasing strain amplitude, the internal friction values (Q−1) of ADI treated with different first-step austempering temperatures all increased gradually, and the Q−1 values of ADI with first-step austempering temperatures of 240 and 260 °C were higher than those of other samples. Moreover, the damping properties of ADI varied with frequency with obvious S-K peaks and Ge peaks.

Microstructure and mechanical behaviors of grinding balls produced by dual matrix structure two-step austempering process

The austempered ductile iron (ADI) grinding balls were austenitized at 900 °C for 2 h and then incomplete quenched in a sodium silicate solution for 100 s, after that austempered in an isothermal furnace at 230 °C for 1 h. Different from the two-step austempering process, this process can prepare a two-phase structure through the incomplete quenching stage and austempering stage, so we called it a dual-matrix structure (DMS) two-step austempering. This work aims to investigate the effect of different concentrations of sodium silicate (30 wt.%, 40 wt.%, 50 wt.% and 60 wt.%) as the incomplete quenching medium on the microstructure and properties of ADI. The microstructure, mechanical properties and wear resistance of ADI were analyzed using SEM with EDS, hardness testing (rockwell) apparatus and impact-toughness tester, as well as friction wear testing and impact wear testing. The result shows that the martensite and lower ausferrite phase in the DMS grinding balls gradually transform to lower ausferrite and upper ausferrite with increasing concentration of quenching liquid. As the sodium silicate concentration increased from 30 wt.% to 60 wt.%, the lower ausferrite volume fraction has initially increased and then decreased. DMS two-step austempering with a 40 wt.% sodium silicate produced the grinding balls with lower ausferrite and a small amount of tempered martensite that has the best overall mechanical properties and wear properties, which were suitable for application of large grinding machines. Besides, this work developed a new method to replace the salt bath austempering process, which was environmentally friendly.

Microstructure Evolution and Wear Resistance of Cu-Bearing Carbidic Austempered Ductile Iron after Austempering

The influence of the austempering process on microstructure evolution, mechanical properties, wear resistance, and corrosion resistance of Cu-bearing carbidic austempered ductile iron (CADI) was investigated. The results show that with an increase in the austempering temperature, the amount of acicular ferrite decreases, the volume fraction and carbon content of retained austenite increase gradually, the hardness and tensile strength decrease, and the impact toughness and corrosion resistance increase. Wear tests show that as the austempering temperature increases, the wear resistance of Cu-bearing CADI is improved and decreases when the temperature exceeds 300 °C. Comparing to the conventional single-step austempering process, two-step austempering process (namely added pre-quenching at 260 °C) significantly improves the wear resistance of Cu-bearing CADI by more than 16% without compromising toughness.

Effect of two-step austempering process on carbidic ductile iron

The two-step austempering process when applied to ductile iron, results in significant improvement in mechanical properties due to the strengthening of the ausferitic matrix. The present work was undertaken to investigate the effect of the two-step austempering process on carbidic ductile iron. Carbidic ductile iron is a new member to the ductile iron family, it is produced by alloying the ductile iron composition with chromium. In the present work, the carbidic ductile iron thus produced, was austempered by both conventional and two-step austempering route. For both processes, all specimens were austenitized at 975°C and soaked for 180 min. In the conventional process, the specimanwas quenched in salt bath held at to 350°C, 400°C, and 450°C and soaked at respective temperatures for 180 min. Whereas, in the two-step austempering process, specimenwere first quenched in salt bath heldat 250°C. After stabilization at 250°C, the salt bath temperature was raised to 350°C, 400°C, and 450°C. After the respective temperature was attained, the specimens were soaked for 180 min. The austempered samples obtained by both processes were analyzed with the help of SEM, XRD, and TEM. Two-step process resulted in finer ausferitic matrix than the conventional process. The XRD analysis revealed an increase in carbon content of austenite and volume fraction of Carbon Stabilized Austenite (CSA) in sample austempered by a two-step process. The TEM analysis indicated that the lath width of austenite and ferrite was small in samples austempered by two-step process than the conventional process, due to a high degree of super-cooling.

Microstructure and mechanical properties of two-step Cu-alloyed ADI treated by different second step austempering temperatures and times

Austempering ductile iron (ADI) is an attractive material due to its excellent comprehensive mechanical properties. However, the deficit in elongation and toughness always threatens its security application. Two-step austempering process is an effective way to improve elongation and toughness simultaneously. In the present work, the influence of the amount, morphology and distribution of ferrite and austenite on mechanical properties of ADI under different second-step austempering parameters has been analyzed. Results show that the amount of austenite and its carbon content decrease with increasing of second-step temperature. Carbide begins to precipitate as second-step austempering temperature reaches 380 °C. These factors together influence the mechanical properties of two-step Cu-alloyed ADI. Impact energy and fracture toughness are strongly affected by second-step austempering temperature, and are dramatically decreased with increase of second-step austempering temperature. Elongation remains constant when the second-step temperature is below 360 °C, and then it is rapidly decreased with further increase of second-step temperature. Strength is slightly influenced by second-step temperature. Ferrite morphology is not influenced by second-step austempering duration, while blocky retained austenite size is slightly decreased with the increasing of second-step austempering time. The amount of retained austenite is decreased while the carbon content of retained austenite is increased with the extending of second-step austempering time. The substructure of austenite is transformed from dislocation to twin when second-step austempering time exceeds 60 min. Strength and elongation are improved slightly with extending of second-step time. Impact energy and fracture toughness initially decrease with the extending of second-step time, and then remain constant when the time is longer than 60 min. This is a result of austenite content decreasing and carbon content of austenite increasing. The second-step austempering time mainly influences austenite content and its carbon content, which is a result of carbon diffusion behavior variation.

The effects of strain amplitude and low cycle fatigue (LCF) behavior on nodular cast iron with two-step austempering (TSA) process

The strain amplitude analysis of low cycle fatigue (LCF) on nodular cast iron processed with TSA aimed to observe the fatigue behavior and fatigue cracking phenomenon until the failure. The LCF test followed the ASTM A606 standard and it was performed at room temperature with strain ratio = −0.1, strain rate 2.5 × 10–3s−1, various strain amplitudes of 0.002, 0.0025, 0.00275 and 0.003 mm mm−1, and TSA holding time of 5, 10, and 15 min. LCF test results were shown in the hysteresis loops curve (s versus e) and peak stress versus number of cycle curve. LCF test showed that the 0.0025 mm mm−1 strain amplitude at 5- and 10-minutes TSA generated the highest cycles compared to 0.002, 0.00275 and 0.003 strain amplitudes. The 0.0025 strain amplitude and 5 min TSA had 10845 cycles and 10 min TSA had 7414 cycles. Therefore, 0.002, 0.00275 and 0.003 strain amplitudes had total cycles which were below the above cycles.

Effect of Cu content on microstructures and mechanical properties of ADI treated by two-step austempering process

The effect of Cu content on the microstructures and mechanical properties (yield strength, ultimate tensile strength, impact energy, fracture toughness) of austempering ductile iron (ADI) treated by two-step austempering process were investigated. High Cu content in nodular cast irons leads to a signifcant volume fraction of retained austenite in the iron after austempering treatment, but the carbon content of austenite decreases with the increasing of Cu content. Moreover, austenitic stability reaches its maximum when the Cu content is 1.4% and then drops rapidly with further increase of Cu. The ultimate tensile strength and yield strength of the ADI frstly increases and then decreases with increasing the Cu content. The elongation keeps constant at 6.5% as the Cu content increases from 0.2% to 1.4%, and then increases rapidly to 10.0% with further increase Cu content to 2.0%. Impact toughness is enhanced with Cu increasing at frst, and reaches a maximum 122.9 J at 1.4% Cu, then decreases with the further increase of Cu. The fracture toughness of ADI shows a constant increase with the increase of Cu content. The infuencing mechanism of Cu on austempered ductile iron (ADI) can be classifed into two aspects. On the one hand, Cu dissolves into the matrix and functions as solid solution strengthening. On the other hand, Cu reduces solubility of C in austenite and contributes more stable retained austenite.

Effect of Two-Step Austempering Process on Transformation Kinetics of Nanostructured Bainitic Steel.

The two-step austempering process has been reported to be an effective method to accelerate the bainitic transformation process by introducing martensite (Q-M-B). However, in this study, it was found that the Q-M-B process reduced the incubation time, but the transformation duration remained nearly unchanged. The notably reduced activation energy barrier for nucleation of bainitic ferrite on the preformed martensite should be responsible for the reduced duration time of the Q-M-B process. A process that both of the two steps were above, Ms (Q-B-B), has been demonstrated to increase transformation rate and improve the amount of bainitic ferrite, which probably results from the additional hysteresis free energy provided by the first quenching process.

TEM Analysis of Austempered Ductile Iron Processed Through Conventional and Two-Step Austempering Process

A transmission electron microscope was used to study the morphology of austenite and ferrite in austempered ductile iron obtained through conventional and two-step austempering process. In this investigation, two compositions of ductile iron with carbon equivalent 4.3% (alloy 1) and 3.1% (alloy 2) were selected. Both alloys were austenitized for 60 min at 900 °C. In conventional treatment, after austenitization, specimens were quenched into salt bath at 400 °C and austempered for 120 min. In two-step process, first specimens were quenched into salt bath at 250 °C and then temperature was raised to 400 °C and specimen was austempered for 120 min. The TEM study showed lamellar arrangement of ferrite and austenite in both the austempering processes. The width of the ferrite and austenite lamina was found to be smaller in specimen austempered by two-step process than by conventional austempering process.

Processing of Nanostructured Austempered Ductile Cast Iron (ADI) by a Novel Method

Austempered ductile cast iron (ADI) has emerged as a major engineering material in recent years. It has a unique microstructure consisting of bainitic ferrite and high carbon austenite. In this investigation, nanostructured austempered ductile cast iron (ADI) consisting of bainitic ferrite and high carbon austenite was developed by applying a unique process consisting of austenitization and simultaneous high temperature plastic deformation at the same austenitizing temperature followed by austempering. It was theorized by these authors that further refinement of this of ADI in the nanoscale region could possibly further enhance the mechanical properties of ADI and nanostructured ADI can be potentially a useful structured material. The influence of plastic deformation, single step and two-step austempering process on the resultant microstructure and mechanical properties of nanostructured ADI has been examined. Test results indicate that the application of high temperature plastic defor

Sliding wear and corrosion behaviour of alloyed austempered ductile iron subjected to novel two step austempering treatment

Austempered Ductile Iron(ADI) is an exciting alloy of iron which offers the design engineers the best combination high strength-to-weight ratio, low cost design flexibility, good toughness, wear resistance along with fatigue strength. The two step austempering procedure helps in simultaneously improving the tensile strength as-well as the ductility to more than that of the conventional austempering process. Extensive literature survey reveals that it’s mechanical and wear behaviour are dependent on heat treatment and alloy additions.Current work focuses on characterizing the two-step ADI samples (TSADI) developed by novel heat treatment process for resistance to corrosion and wear. The samples of Ductile Iron were austempered by the two-Step Austempering process at temperatures 300°C to 450°C in the steps of 50°C.Temperaturesare gradually increased at the rate of 14°C/Hour. In acidic medium (H2SO4), the austempered samples showed better corrosive resistance compared to conventional ductile iron. It has been observed from the wear studies that TSADI sample at 350°C is showing better wear resistance compared to ductile iron. The results are discussed in terms of fractographs, process variables and microstructural features of TSADI samples.

Two-step austempering of high performance steel with nanoscale microstructure

It is crucial to eliminate the thermally and mechanically less stable austenite blocks as much as possible if enhanced mechanical properties are demanded in high performance nanostructured bainitic steels. Step-austempering would be an efficient procedure in this regard. This article aims to investigate the strength-ductility- impact toughness combinations in nano-bainite after two-step austempering process comparing with those obtained after conventional isothermal bainite transformation. It has been shown that large austenite blocks further decomposed to bainite after step-austempering process which in turn decreased the average volume fraction of austenite, increased its mechanical stability and generally refined the final microstructure. Step-austempering increased the hardness value and enhanced the yield strength and ultimate tensile strength properties. Additionally, the higher mechanical stability of retained austenite in step-austempered samples increased the elongation level and improved the impact toughness at earlier stages of the second step of transformation. The important point is, it has been found that ductility and toughness properties were influenced by transformation time at the second stage of austempering which approved the idea that not only the mechanical stability and morphology of retained austenite but also its volume fraction must be taken into account when applying a multi-step austempering heat treatment in nanobain steels.

Evaluation of the Machining Aspects of Austempered Ductile Iron

Exceptional properties, such as wear resistance, high strength and ductility, predestine austempered ductile iron (ADI) to substitute for forged steel, aluminum or ductile iron in technical components and parts. The advantages of ADI can be ascribed to the austenitic-ferritic matrix produced in a two-step austempering process. Even though ADI is already established for some industrial applications, machinability in the heat treated condition still hinders a more extensive usage. The machining of ADI is still ambitious, and especially the metallurgical aspects, such as the influence of the chemical composition and the cast skin, have not been taken into account in recent machining investigations. In this paper, the influence of the above mentioned metallurgical aspects on machinability are described and adapted machining strategies to counteract these obstacles will be presented.

Influence of Two-Step Austempering Process on the Fracture Toughness of a Low Carbon Low Alloy (LCLA) Steel

In this investigation a new low carbon and low alloy (LCLA) (Third generation AHSS) steel with exceptional combination of yield strength and fracture toughness has been developed. The steel has been processed using a novel two step austempering process. The investigation also examined the influence of a two-step austempering process on the microstructure and mechanical properties of this steel. Test results show that by two-step austempering process significant improvement in yield strength and fracture toughness can be obtained in this steel.

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.

Mechanical Properties of a Medium Carbon Low Alloy Steel Processed by Two Step Austempering Process

A new bainitic steel with a combination of exceptionally high yield strength and fracture toughness has been developed. This steel has been synthesized by austempering a medium carbon low alloy steel by a novel two-step austempering process. The influence of this two-step austempering on the microstructure and the mechanical properties of this new steel have been examined.