Lower Austempering Temperature Research Articles

Nodule count effect on microstructure and mechanical properties of hypo-eutectic ADI alloyed with nickel

Samples of ductile iron alloyed with 0.88 % Ni with a nodule count of 606, 523, and 290 nod/mm2 were obtained from sand cast plates of different thickness in the range from 8.46 to 25.4 mm. The effect of the nodule count was evaluated during the austempering process held at 285?C and austempering times of 15, 30, 45, 60, 70, and 90 min. The volume fraction of high carbon austenite increased when the nodule count increased, however, the carbon content of the high carbon austenite kept almost constant. The process window was narrow, requiring a lower austempering time when the nodule count increased. The combination of a higher nodule count and low austempering temperature allowed obtaining a fine ausferritic microstructure which led to higher Brinell hardness and tensile strength. The process window was determined by XRD measurements and it was in good agreement with the microstructural and hardness evolution as the austempering time increased.

Mechanical properties and wear resistance of ultrafine bainitic steel under low austempering temperature

The mechanical properties and wear resistance of the ultrafine bainitic steel austempered at various temperatures were investigated. Scanning electron microscopy (SEM) and X-ray diffraction were used to analyze the microstructure. The worn surfaces were observed via laser scanning confocal microscopy and SEM. Results indicated that, under low austempering temperatures, the mechanical properties differed, and the wear resistance remained basically unchanged. The tensile strength of the samples was above 1800 MPa, but only one sample austempered at 230°C had an elongation of more than 10%. The weight loss of samples was approximately linear with the cycles of wear and nonlinear with the loads. The samples showed little difference in wear resistance at different isothermal temperatures, whereas the thickness of their deformed layers varied greatly. The results are related to the initial hardness of the sample and the stability of the retained austenite. Meanwhile, the experimental results showed that the effect of austempering temperature on the wear resistance of ultrafine bainitic steel can be neglected under low applied loads and low austempering temperature.

Bainitic Transformation and Mechanical Properties of Low-Carbon High-Strength Bainitic Steels with Mo Addition

Two low-carbon high-strength bainitic steels with/without molybdenum (Mo) addition were designed to investigate the effect of Mo on bainitic transformation, microstructure and properties. The results show that during austempering, Mo addition increases the amount of isothermal bainite transformation and slightly accelerates the bainite transformation kinetics. It is mainly attributed to the fact that there is the ferrite transformation in Mo-free steel but not in Mo-added steel even cooled at a very high cooling rate before austempering. In Mo-free steel, the formation of ferrite leads to the higher carbon content in untransformed austenite with higher stability and thus retards the isothermal bainite transformation kinetics. The ferrite transformation controlled by carbon diffusion is strongly inhibited in the Mo-added steel because Mo addition decreases obviously the diffusion coefficient of carbon and grain boundaries energies. In addition, the promoted effect of Mo addition on isothermal bainite transformation is stronger at lower austempering temperature. Mo addition leads to the more and finer bainite plates and increases the strength of low-carbon bainitic steel. Moreover, the volume fraction of retained austenite first decreases and then increases in Mo-free steel with the increase in austempering temperature, whereas it presents the inverse trend in Mo-added steel. Similarly, during continuous cooling process, Mo addition effectively retards the formation of high-temperature products and thus improves the tensile properties of low-carbon steels.

Wear Behavior of Austempered and Quenched and Tempered Gray Cast Irons under Similar Hardness

In this research, an austempering heat treatment was applied on gray cast iron using various austempering temperatures ranging from 232 °C to 371 °C and holding times ranging from 1 min to 120 min. The microstructure and hardness were examined using optical microscopy and a Rockwell hardness tester. Rotational ball-on-disk sliding wear tests were carried out to investigate the wear behavior of austempered gray cast iron samples and to compare with conventional quenched and tempered gray cast iron samples under equivalent hardness. For the austempered samples, it was found that acicular ferrite and carbon saturated austenite were formed in the matrix. The ferritic platelets became coarse when increasing the austempering temperature or extending the holding time. Hardness decreased due to a decreasing amount of martensite in the matrix. In wear tests, austempered gray cast iron samples showed slightly higher wear resistance than quenched and tempered samples under similar hardness while using the austempering temperatures of 232 °C, 260 °C, 288 °C, and 316 °C and distinctly better wear resistance while using the austempering temperatures of 343 °C and 371 °C. After analyzing the worn surface, abrasive wear and fatigue wear with the presence of pits, spalls, voids, long cracks, and wear debris were the main mechanisms for austempered gray cast iron with a low austempering temperature. However, only small pits and short cracks were observed on the wear track of austempered gray cast iron with high austempering temperature. Furthermore, the graphite flakes were exposed and ground by the counterpart surface during wear tests. Then, the graphite particles would form a tribo-layer to protect the contact surface.

Effects of Al addition on bainite transformation and properties of high-strength carbide-free bainitic steels

The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous cooling process (CCP) and isothermal transformation process (ITP). The results indicate that for the CCP treatment, Al addition significantly promoted the ferrite and bainite transformation; however, it did not significantly increase the product of tensile strength and total elongation (PSE). For the ITP treatment, Al addition significantly promoted the kinetics of bainite transformation, and thus, more bainite was formed with Al addition; however, it was found that Al addition resulted in a decrease in tensile strength and an increase in elongation of the tested bainitic steels. Moreover, the effects of Al addition on comprehensive property were profoundly dependent on austempering temperatures. When the austempering temperature was higher (430 °C), PSE significantly increased with Al addition, whereas it decreased at the lower austempering temperature (400 °C). Therefore, it can be concluded that the effects of Al on properties of bainitic steels were more significant at higher austempering temperatures.

Transformation Behavior and Properties of Carbide‐Free Bainite Steels with Different Si Contents

The bainite transformation behavior and properties of low carbon carbide‐free bainitic steels containing different silicon (Si) contents are investigated by two different types of heat treatment processes: isothermal transformation process (ITP) and continuous cooling process (CCP). The results indicate that for ITP and CCP, the transformation kinetics of bainite is retarded and the final bainite amount decreases with increasing Si content. However, both the strength and total elongation improve with the increase of Si content in the range of 1.0–2.0 wt%, resulting in an apparent increment in comprehensive property of bainitic steels due to the more film‐like RA and less carbides. It can be attributed to the increase of shear strength and stability of undercooled austenite and the formation of Cottrell atmosphere, as well as the solid solution strengthening of Si because of higher Si content. In addition, for the same samples, better mechanical properties can be achieved by a lower austempering temperature. Moreover, the increase of Si content resulted in an increase in the temperatures of Ac1 and Ac3.

Inverted multi-step bainitic austempering process routes: Enhanced strength and ductility

An inverted multi-step bainitic austempering process is proposed, where a low austempering temperature step is followed by a high temperature counterpart: after austenitization, the specimens were cooled to the first austempering step at 260 °C for 600 s, then were raised to the second step at 360 °C and held for 1800 s. As comparison, the conventional multi-step bainitic austempering processes were carried out: the first step is at 360 °C for 1800 s, while the second step is at 260 °C for 7200 s. Through inverted multi-step bainitic austempering process, the efficacy of reducing blocky austenite is also achieved and is comparable to that obtained by conventional multi-step bainitic austempering process. But the total time used for inverted multi-step bainitic austempering treatment is reduced markedly due to accelerating bainitic ferrite formation. Furthermore, the novel heat treatment can enhance the strength of a Mn-Si-Cr-C low-alloyed steel without sacrificing ductility, which is attributed to refining austenite and bainitic ferrite blocks.

Effect of Ni Addition on Bainite Transformation and Properties in a 2000 MPa Grade Ultrahigh Strength Bainitic Steel

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.

An investigation on the stability of austenite in Austempered Ductile Cast Iron (ADI)

Austempered Ductile Cast Iron (ADI) has emerged as an important commercial engineering material in recent years because of excellent properties such as high strength with good ductility as well as good fatigue strength and fracture toughness together with excellent wear resistance. These properties are as a result of a microstructure composed primarily of acicular ferrite (α) and high carbon austenite (γHC). The role of the austenite in ADI is a topic that has been discussed extensively in the literature. Several investigators in the past have reported that the austenite in ADI is both thermally and mechanically stable; while others have reported it is neither thermally nor mechanically stable. Moreover, it is not clear whether the austenite formed at lower austempering (or bainitic) temperatures is more mechanically and thermally stable than that produced in the upper austempering (or bainitic) ranges.In this investigation, a comprehensive study was carried out to examine both the thermal and mechanical stability of austenite in the upper and lower austempering (or bainitic) temperature ranges. Compact tension and cylindrical tensile specimens were prepared and austempered at five different temperatures ranging from lower to upper bainitic temperatures. A select number of samples were then cryogenically treated to examine the thermal stability of the high carbon austenite. In order to examine the mechanical stability of austenite the microstructure and mechanical properties of these materials were evaluated as a function of austempering temperature, cryogenic treatment, and mechanical testing. The results of this study show that the austenite formed at lower austempering temperatures (260 and 288°C) was neither thermally nor mechanically stable; while the austenite formed in the upper bainitic temperature range (371–399°C) was partially stable under both thermal and mechanical conditions. Both stress- and strain-induced austenite-to-martensite transformations were observed in the ADI samples during mechanical tests. The test results also showed that cryogenic treatment can improve the mechanical properties without compromising the fracture resistance of the ADI. This is hypothesized to be due to a threshold level for the amount of austenite transformed to martensite; below this level, the amount transformed is not sufficient to produce an observable change in the fracture toughness.

Study on Bainitic Microstructure under Low Austempering Temperature and its Wear Resistance of Low-Alloy High-Carbon Steel

A new low-alloy high-carbon steel was designed to obtain the carbide-free bainite under low-temperature austempering. The microstructure, wear resistance under dry sliding friction and the wear mechanism were comparatively studied between samples austempered and low-temperature tempered. Results show that the carbide-free bainitic microstructure composed of bainitic ferrite laths with mean thickness of 120 nm and retained austenite films can be achieved by austempering at 220 °C for 120 h in salt bath after austenitizing at 1000 °C. The wear resistance under dry sliding friction of austempered sample is 19% more than that of low-temperature tempered sample. The wear mechanism of the low-temperature austempered sample is determined as the adhesive wear.

A Comparison of Tensile Strength and Impact Energy of Austempered versus Step Quenched 4340 Ultra High Strength Steel

This study was conducted to determine if austempered 4340 steel had different mechanical properties compared to step quenched 4340 steel. Tensile strength and impact energy was determined at room temperature under identical test conditions. The specimens were cut from a bar with 25 mm diameter and austenitized at 800°C for 60 min and followed by quenching at 430°C for the high austempering temperature to achieve the upper bainite morphology and at 360°C for the lower austempering temperature to achieve the lower bainite morphology. In the case of step quenched, the specimens were first austempered at 430°C and then austempered at 360°C to achieve the mixed structure of upper bainite and lower bainite morphology. The another set of specimens for step quenching, after austenitization were quenched to below Ms (martensite start temperature), followed by heating at 400°C to achieve the mixed structure of tempered martensite and lower bainite and 500°C to achieve the mixed structure of tempered martensite and upper bainite. It is also shown that the best combination of strength and ductility can be achieved by the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation.

Characterization of Inoculated Low Carbon Equivalent Iron at Lower Austempering Temperature

The response of inoculated low carbon equivalent iron to austempering heat treatment has remained as an unchartered area of investigation. The carbon equivalent of inoculated low carbon equivalent iron is much lesser than that of ductile iron, while modulus of elasticity is higher than it. This paper examines and compares the austempering transformations and microstructural changes during austempering of ductile iron and inoculated low carbon equivalent iron with respect to austempering parameters. The optical metallography and X-ray diffraction techniques are used to study the changes in the austempered structure. The alterations in austempered microstructure and structural parameters like, austenite volume fraction (X) and its carbon content (C) at an austempering temperature of 250 °C are studied in the present investigation. It is observed that, austempering heat treatment produces fine ausferritic microstructure, similar to that present in austempered ductile iron at this temperature. However, some significant differences between the austempered microstructure of two alloys are noted.

A New Modified Austempering to Increase Strength and Ductility Simultaneously for UHS Steels

In this paper, a modified up-quenching heat treatment method to the ASSAB 705M steel (ultra high strength steel) is proposed. A low alloy steel (0.33%C), was used to study the effect of isothermal austempering, successive austempering and modified up-quenching austempering heat treatment on the mechanical properties. The specimens, were cut from a bar with 25mm diameter and after achieving the best temperature and time of austenitizing, austenitized at for 60 min and followed by quenching at for the high austempering temperature to achieve the upper bainite morphology and at for the lower austempering temperature to achieve the lower bainite morphology. In the case of successive austempering, the specimens were first austempered at for different periods (500 sec and 60 sec) and then austempered at for 1000 sec to achieve the mixed structure of upper bainite and lower bainite morphology. The specimens selected for up-quenching, after austenitization were quenched to below ( ) for 120 sec. followed by heating at to achieve the mixed structure of tempered martensite and lower bainite and to achieve the mixed structure of tempered martensite and upper bainite for 1000 sec. All of the processes were performed in the salt bath furnaces. Experimental results are presented and the advantages of the modified method are discussed. As well, it is shown that the best combination of strength and ductility can be achieved by the proposed heat treatment method. This modified method, can offer techniques that simultaneously improve not only strength 12 %( compare with results of strength after other heat treatment methods), but also ductility 38 %( compare with results of ductility after other heat treatment methods). While, conventional heat treatment of ultra high strength steels (UHSS) cannot always meet the strict engineering requirements for improved strength and ductility simultaneously. It has been shown that the mixed structure of tempered martensite and lower bainite that has been suggested in this investigation offers a good combination of strength and ductility. The technical reason for this superiority returns back to the fact that it has increased dislocation density. As a result of the increment of the dislocation density, in the morphology, the inter lath carbide (e.g. cementite) decreases, and the intra lath carbide increases. This modified austempering is applicable to all the ultra high strength steels, has noticeable economic advantages because it is simple. Use of this modified austempering for heavy parts, leads to the lightness of heavy parts and combination of the thermomechanical methods with this modified austempering can yield even much more improvements.

Corrosion resistance of TiN/TiAlN-coated ADI by cathodic arc deposition

Because of its low cost, high tensile strength, fatigue resistance, and wear resistance, austempered ductile iron (ADI) has been utilized in a wide variety of mechanical applications. However, in opposition to the versatility of the above-mentioned mechanical properties, the shortcomings of ADI are its surface hardness (Hv: 396) and corrosion resistance. And, because its relatively low austempering temperature falls in the range of Ms–450 °C (Ms: the martensite start temperature), ADI cannot be case hardened using traditional high temperature heat treatment. This study explored the effects of TiN and TiAlN coatings on ADI, with respect to surface hardness and corrosion resistance. The process employed for coating TiN/TiAlN onto ADI involved variable target current low temperature cathodic arc deposition (CAD). The coating characteristics and corrosion resistance of TiN/TiAlN-coated ADI were analyzed for further understanding of the feasibility of the low temperature CAD coating process. The analysis showed that TiN and TiAlN films could successfully be coated onto ADI through CAD technology without altering the unique microstructure of ADI. Utilizing a variety of target currents in the coating process yielded an acceptable level of adhesion while raising the surface hardness of ADI by a factor of 2–4 (Hv: 396 versus 1356–1910). It was observed that as target current increased, the surface roughness of the specimens also increased. TiN/TiAlN-coated ADI specimens exhibited higher corrosion resistance than uncoated ones when both were immersed in separate solutions of both 3.5 wt% NaCl and 10 vol.% HCl. Conclusively, the application of TiN and TiAlN films to ADI through the coating process of low temperature CAD is a viable surface modification for ADI applications that require increased surface hardness and corrosion resistance.

Effects of successive austempering on the tribological behavior of ductile cast iron

A 0.75 wt.% Mn ductile iron with different nodule counts was austempered by conventional and successive austempering processes at 315 and 375°C for different periods. Specimens with optimum mechanical properties were used to study the effect of austempering process on the wear behavior of austempered ductile iron (ADI). Sliding wear tests were performed in a pin-on-ring wear tester with the test materials rubbing under dry atmospheric condition against a surface hardening tool steel ring at speeds of 0.6 and 1.28 m s −1 and normal loads of 100, 200 and 300 N. To study worn surfaces of test specimens, optical and scanning electron microscopy (SEM) were used. It was found that specimens austempered at 315°C (LAT, lower austempering temperature) show the highest wear resistance while specimens austempered at 375°C (HAT, higher austempering temperature) show the lowest. Specimens austempered by successive austempering process (HLAT, high–low austempering temperature) with hardness about equivalent to HAT specimens, in addition to improvement of mechanical properties, show higher wear resistance than that of HAT specimens. Duplex structure, upper and lower bainite, along with higher carbon content of retained austenite and reduction of UAV (untransformed austenite volume) can be the main factors which improve mechanical properties and wear resistance of HLAT specimens. The results showed that specimens with longer solidification time have lower wear rate than specimens with shorter solidification time, supposedly due to effect of longer solidification time on the reduction of nodule count and heavier segregation of alloying elements. SEM study of worn surfaces suggests that delamination could be the probable wear mechanism in this investigation.

A transmission electron microscope study of 1 Pct Mn ductile iron with different austempering treatments

A transmission electron microscope (TEM) equipped with an energy dispersive spectroscopy (EDS) system was used to study the bainitic reaction in a conventional and a successive austempering process for 1 wt pct Mn ductile iron. In the case of conventional austempering, the specimens were full austenitized at 900 °C and then austempered at 375 °C (high austempering temperature) and 315 °C (low austempering temperature) for different periods. In the case of the successive austempering process, following austempering at 375 °C for different periods, specimens were austempered at 315 °C, and subsequently quenched in ice water. The TEM-EDS study showed that carbide precipitation in the ferritic and retained austenitic component of bainite is a function of the local concentrations of the alloying elements, austempering time, and temperature. After a short time at high austempering temperature, carbide-free bainite forms near graphite nodules. Longer austempering time or lower austempering temperature encourages carbide precipitation in the bainitic ferrite. A long austempering time at high temperature leads to decomposition of retained austenite to ferrite and carbide. A rough inspection shows that the precipitated carbides in the ferritic component of specimens austempered at low temperature lie at an angle of about 40 to 50 deg to the sheaf axis.