Higher Austempering Temperatures Research Articles

Influence of Austenite Stability on the Machinability of Different Grades of Austempered Ductile Iron (ADI) During Dry Turning Machining

This work evaluated the machinability characteristics of three different austempered ductile irons using three machinability indicators: cutting force, tool wear, and surface roughness. Additionally, this paper addresses the interplay among the production conditions of ADIs, microstructural development, the TRIP effect arising from transformations of the retained austenite into martensite, and the machinability behavior of ADIs. Various characterization studies including hardness measurement, X-ray diffraction analysis, light and electron microscope were used. Machinability results indicated that the increase in cutting force is always associated with a corresponding increase in the martensite content when the cutting depth was increased from 0.5 to 2.0 mm. Such correlation is more evident with ADI grade austempered at high austempering temperature, 375 °C (ADI-375). It was noticed that although ADI-375 has only 0.46% martensite in the structure before machining, this ratio increases with the highest depth of cut of 2.0 mm to reach about ~ 23% martensite with a corresponding increase in the cutting force. Ferrite content and morphology have a great impact on the overall surface finish of each grade as it leads to deteriorating the surface characteristics of the investigated alloys. This could explain the highest roughness values across all processing experiments obtained in the ADI grade austenitized in the intercritical region (IADI), which mainly contains the highest ferrite content. Tool wear and chip characteristics were also studied in this work.

Effects of aluminium addition and austempering temperatures on Al-alloyed ductile iron microstructure and mechanical properties

The research investigates the effect of varying amounts of aluminium (1.05, 1.575, 2.29, 3.02 and 3.74 wt.%) addition and heat treatment (austempering at 300, 350, 400 oC) on the microstructure and mechanical properties of ductile cast iron alloys. The graphitizing effects of the Al alloy and varied austempering temperatures on hardness, tensile strength, and impact toughness of the ductile cast iron (DCI) were evaluated. The results of the influences of Al addition and heat treatment on the properties of the DCI determined were presented as graphical data while the microstructures were done by using a scanning electron microscope (SEM). The microstructures revealed that the addition of Al into the matrix brings about the precipitation of ferrite around the graphite nodules. The combined effects of the increase Al content and austempering temperatures produced greater hardness values on the Al-alloyed DCI samples than the as-cast sample. The hardness value for the entire sample ranged between 27.25 to 57.03 BHN. Tensile strength increased with an increase in Al content and lower austempering temperatures, whereas, the impact toughness increased with an increase in Al content and higher austempering temperature.

The Role of Microstructure on Damage Tolerance in Nano-Bainitic Steels

Three nanostructured bainitic steels have been produced by austempering at 250, 300 and 350°C respectively after a similar austenitization treatment at 950°C. It has been found that a decrease in austempering temperature causes increase in content of bainitic ferrite (BF) as well as refinement of BF and retained austenite (RA) resulting in strengthening of the steel. However, results of sub-critical fatigue crack growth rate tests on these steels revealed that the steel transformed at higher austempering temperature which showed coarser morphology of BF and RA and also higher content of RA are more damage tolerant. It appeared that the thickness of BF plates and its volume fraction in the microstructure plays a major role in enhancing the strength whereas improved damage tolerance has been attributed to higher content of ductile phase RA. Scanning electron microscopy, nano-indentation and surface profilometry techniques revealed that the larger plastic zone size, higher amount of strain energy consumed during austenite to martensite transformation (TRIP effect) and tortuous crack path are the main factors for slower fatigue crack propagation rates in steel with the coarser morphology and higher retained austenite content.

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.

Machinability study of manganese alloyed austempered ductile iron

Austempered ductile iron (ADI) is a heat-treated ductile cast iron with ausferrite microstructure. ADI has high strength and hardness with good ductility and toughness. However, because of the high strength and hardness of ADI, there is a concern regarding its machinability. In the present study, machinability of ADI is assessed using tool life and the surface roughness. Machining tests are conducted using Taguchi experimental design. The effects of machining parameters, heat treatment parameters, on manganese alloyed ADI in the dry turning operation are studied and it was found that austempering temperature and cutting speed have the major contributions on tool life, whereas feed rate has the major contribution on surface roughness of the material within the range of values under study. Metallographic study has revealed coarser structure at higher austempering temperature of 420� which resulted in better tool life. 0.64 wt% of manganese in the ADI has resulted in lower surface roughness value.

Toughness dependence of nano-bainite on phase fraction and morphology

Yield strength of nano-bainitic steels is enhanced while retaining significant ductility with a refinement in the bainitic lath thickness and increasing its volume fraction. This study makes an effort to improve toughness of nano-bainite by investigating the effect of content, size and morphology of constituent phases when austempered at 250 °C, 300 °C and 350 °C. Improved impact and fracture toughness at higher austempering temperature has been observed primarily due to higher content of ductile phase austenite in spite of its coarser morphology. Micrographs of the fracture surface show quasi-cleavage fracture for all the conditions.

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.

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 the Austempering Process on the Microstructure and Mechanical Properties of 27MnCrB5-2 Steel

AbstractThe effect of austempering parameters on the microstructure and mechanical properties of 27MnCrB5-2 steel has been investigated by means of: dilatometric, microstructural and fractographic analyses; tensile and Charpy V-notch (CVN) impact tests at room temperature and a low temperature.Microstructural analyses showed that upper bainite developed at a higher austempering temperature, while a mixed bainitic-martensitic microstructure formed at lower temperatures, with a different amount of bainite and martensite and a different size of bainite sheaf depending on the temperature. Tensile tests highlighted superior yield and tensile strengths (≈30%) for the mixed microstructure, with respect to both fully bainitic and Q&T microstructures, with only a low reduction in elongation to failure (≈10%). Impact tests confirmed that mixed microstructures have higher impact properties, at both room temperature and a low temperature.

Characteristics of the Transformation of Retained Austenite in Tempered Austempered Ductile Iron

Controlling the amount of retained austenite is a concern in austempered ductile iron formation. Retained austenite has a strong influence on austempered ductile iron properties, such as hardness and wear resistance. In this research, the characteristics of the transformation of retained austenite were investigated as a function of the number of tempering cycles. The hardness of the austempered ductile iron samples was measured, and the specific amount of retained austenite was analyzed by x-ray diffraction (XRD). Wear tests were conducted on a ball-on-flat sliding fixture. The tempering process was found to have no effect on the hardness of the austempered ductile iron samples. This may be due to retained austenite being partially converted into brittle quenched martensite during the tempering process. However, tougher tempered martensite was also formed from existing martensite. The two effects seemed to offset each other, and no significant differences occurred in overall hardness. XRD analysis showed that under the same austempering temperature and holding time, the amount of retained austenite decreased with additional tempering cycles. Also, with the same holding time and tempering cycles, less retained austenite was contained in the matrix at higher austempering temperatures. This was due to more high carbon content austenite and needle-like ferrite being present in the austempered ductile iron matrix. In addition, tempered austempered ductile iron exhibited significantly higher wear resistance as compared to traditionally treated ductile iron.

Dry sliding wear of Ni alloyed austempered ductile iron

Measurements of dry sliding wear are presented for ductile irons with composition Fe-3.56C-2.67Si-0.25Mo-0.5Cu and Ni contents of 0.8 and 1.5 in wt.% with applied loads of 50, 100 and 150 N for austempering temperatures of 270, 320, and 370 °C after austenitizing at 870 °C for 120 min. The mechanical property measurements show that the grades of the ASTM 897M: 1990 Standard can be satisfied for the selected austempering conditions. The results show that wear resistance is independent of austempering temperature with an applied load of 50 N, but there is a strong dependence at higher austempering temperatures with applied loads of 100 and 150 N. Observations indicate that wear is due to subsurface fatigue with cracks nucleated at deformed graphite nodules.

Microstructure and mechanical properties of Al-alloyed ductile iron upon casting and austempering

In the current study, Al was added to ductile iron (DI) in the range of 0·68–2·70 wt-% and its influence on the mechanical properties and on the microstructure at different section thicknesses was investigated. It was observed that increasing Al addition up to 2·70 wt-% improves the hardness and strength in the as-cast condition. However, this effect was completely reversed after austempering, where the mechanical properties decreased gradually with increasing Al content. In conclusion, Al reduced the strength of DI after austenitising at 900°C. Higher austempering temperatures may be required to achieve the desired properties depending on the Al content of the as-cast DI.

In Situ Study of the Influence of Nickel on the Phase Transformation Kinetics in Austempered Ductile Iron

Phase fractions and austenite carbon contents in austempered ductile iron samples with three different nickel contents were determined by in situ neutron diffraction. The samples were austenitized at 1178 K (905 °C) for 30 minutes and austempered for 3.5 hours at temperatures between 523 K and 723 K (250 °C and 450 °C) using a mirror furnace. Based on the in situ neutron diffraction studies, plateau times were derived, which determine the end of stage I reaction. The austenite contents increase for higher austempering temperatures when the austempering times are selected properly, considering the accelerated phase transformation at higher temperature. Appropriate austempering times were derived for austempering temperatures between 523 K and 723 K (250 °C and 450 °C). Increased nickel contents lead to higher austenite phase fractions. Moreover the retarding effect of nickel on the phase transformation was quantified. The plateau values of phase fraction and the according austempering times were converted to TTT diagrams. The evolution of the austenite carbon content shows a maximum at 623 K (350 °C) austempering temperature. This can be explained by temperature-dependent carbide precipitation and carbon diffusion into lattice defects. Fine carbides within the ferrite could be found by preliminary APT analysis.

In-situ measurement of phase transformation kinetics in austempered ductile iron

Austempered ductile iron (ADI) alloyed with 0.42% Mn and 0.72% Cu was heat treated in a mirror furnace and the phase transitions were studied in-situ by neutron diffraction. The heat treatment consisted of austenitisation at 920°C and isothermal austempering at 400°C, 350°C and 300°C, respectively. Due to the growth of ferrite platelets, the austenite content decreases rapidly at all temperatures within the first 15–20min and reaches a stable plateau after 35min (400°C) to 80min (300°C).The carbon content of the residual austenite, which was monitored and characterised by the change of the lattice parameter, increases up to 1.6wt.% caused by redistribution from the newly formed ferrite. While at higher austempering temperatures this takes place almost parallel to the phase transformation, at 300°C the redistribution of carbon to austenite lags behind considerably.Furthermore the neutron data revealed an austenite peak asymmetry during austempering which is attributed to successive phase transformation. It results temporarily in two fractions of austenite, an initial low-carbon and an enriched high-carbon modification.

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.

Wear Performance of Cu-Alloyed Austempered Ductile Iron

An investigation was carried out to examine the influence of structural and mechanical properties on wear behavior of austempered ductile iron (ADI). Ductile iron (DI) samples were austenitized at 900 °C for 60 min and subsequently austempered for 60 min at three temperatures: 270, 330, and 380 °C. Microstructures of the as-cast DI and ADIs were characterized using optical and scanning microscopy, respectively. The structural parameters, volume fraction of austenite, carbon content of austenite, and ferrite particle size were determined using x-ray diffraction technique. Mechanical properties including Vicker’s hardness, 0.2% proof strength, ultimate tensile strength, ductility, and strain hardening coefficient were determined. Wear tests were carried out under dry sliding conditions using pin-on-disk machine with a linear speed of 2.4 m/s. Normal load and sliding distance were 45 N and 1.7 × 104 m, respectively. ADI developed at higher austempering temperature has large amounts of austenite, which contribute toward improvement in the wear resistance through stress-induced martensitic transformation, and strain hardening of austenite. Wear rate was found to depend on 0.2% proof strength, ductility, austenite content, and its carbon content. Study of worn surfaces and nature of wear debris revealed that the fine ausferrite structure in ADIs undergoes oxidational wear, but the coarse ausferrite structure undergoes adhesion, delamination, and mild abrasion too.

Saving energy by modification of Tomita heat treatment method for AISI 4340 steel

In this paper a modified up-quenching heat treatment method (Tomita method) for AISI 4340 steel (ultra high strength steel) is proposed. A low alloy steel (0.33%C), was used to study the effect of isothermal 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 800oC for 60min and followed by quenching at 430oC for the high austempering temperature to achieve the upper bainite morphology and at 360oC for the lower austempering temperature to achieve the lower bainite morphology. The specimens selected for up-quenching, after austenitization were quenched to below Ms (martensite start temperature) (320oC) for 120sec, followed by heating at 500oC to achieve the mixed structure of tempered martensite and upper bainite for 1000sec. The specimens selected for modified up-quenching, after austenitization were quenched to below Ms for 120sec, followed by heating at 400oC to achieve the mixed structure of tempered martensite and lower bainite for 1000sec. It is also shown that the best combination of strength and ductility can be achieved by the modified heat treatment that has been suggested in this investigation by the decrement of heat energy for the austenitization temperature, up-quenching temperature and time.

Effect of Austempering on Plastic Behavior of Some Austempered Ductile Iron Alloys

A numerical description relating microstructure to elastic and plastic deformation behavior would make it possible to simulate the mechanical behavior of complex cast components with tailored material properties. Limited work and data, however, have been published regarding the connection between microstructure and plastic behavior of austempered ductile irons (ADIs). In the current work, the effects of austempering temperature and austempering time on the strength coefficient and the strain hardening exponent of the Hollomon equation were investigated for two ADI alloys. The results show that the plastic behavior is highly dependent on the combination of austempering temperature and austempering time. It was found that as the austempering temperature increases, both the strength coefficient and the strain hardening exponent initially decrease, but after reaching a minimum at the critical austempering temperature, they show a plateau or an increase. The effect of the austempering time on the plastic behavior depends on the austempering temperature. At low austempering temperatures, the strength coefficient and the strain hardening exponent decrease with increased austempering time, whereas at higher austempering temperatures, they show little time dependence. These relations are explained by the microstructural transformations that take place during the austempering heat treatment.

Influence of the Austempering Temperature on the Microstructure and Crystallography of a Carbide-Free Bainitic Steel

We elaborated two carbide-free bainitic steels with different microstructures through specific heat treatments and alloy design. EBSD analysis was used to point out major differences in these microstructures. In-situ characterizations of the bainitic transformation were performed by high energy synchrotron diffraction to go further into the study of each phase characteristics. The elaborated microstructures exhibited various phase fractions of bainitic ferrite, retained austenite and blocks of martensite and retained austenite. Moreover, the volume fraction of retained austenite increased with higher austempering temperatures. On the other hand, the austempering temperatures showed a strong influence on the kinetics of the bainitic transformation. Isothermal transformation under Ms showed a two stage transformation which led first to the formation of self-tempered martensite and then to bainitic ferrite. Furthermore, the evolution of the austenitic cell parameter showed enrichment in carbon ruled by diffusional mechanisms.