Austempered Specimens Research Articles

Cold work induced stability of retained austenite at elevated temperature in a medium carbon high silicon steel

In the present work, a potential solution has been suggested for arresting the depletion of the retained austenite during prolonged holding at elevated temperature. A medium carbon high silicon steel specimen was austenitized and then air cooled for 20s followed by austempering at 350 °C for 10 min. 20% retained austenite was measured from X-Ray diffraction studies in the specimen and 23.4 ± 1.7% uniform elongation was achieved. On holding the similar austempered specimen at 350 °C for 120 min, all the RA austenite depleted from the microstructure and uniform elongation values dropped to 5.7 ± 0.6%. In further thermomechanical treatments, austempered specimens were cold worked by 4, 7 and 10% at room temperature. The cold worked specimens were further held at elevated temperature (350 °C) for prolonged duration (120 min). Retained austenite content was observed to increase from 14 to 18% with the amount of cold work and tensile elongation value of up to 14% was observed. The decomposition of the retained austenite from the microstructure of cold worked specimens was arrested due to the increase in the dislocation density during cold work applied before holding at the elevated temperature. First principle calculations indicate migration of large number of carbon atoms towards the dislocations at elevated temperature thereby inhibiting the carbide precipitation during decomposition of retained austenite. Application of cold work to the austempered specimens could arrest the depletion of retained austenite at elevated temperature.

Evolution of sub-surface microstructure under linear reciprocating wear of nanostructured bainitic steel

Nanostructured bainitic steels have high strength as well as a combination of significant ductility, fracture toughness and sub-critical fatigue crack growth life. Hence, they can be potentially useful in structures suffering rolling and/or sliding fatigue e.g. rails, mining components, bearings and shafts. However, such applications require high wear resistance which is not solely dependent on the initial strength since repetitive sliding often leads to sub-surface microstructural evolution that alters the tribological response with time. In the current study, three nanostructured bainitic blocks with lath thickness of 45, 54 and 77 nm have been made from steel of composition: Fe-0.86C-1.45Si-1.8Mn-0.47Al-1.88Co-0.23Mo (wt%) by austempering at 250, 300 and 350 °C respectively. We have investigated the friction and microstructural evolution under reciprocating sliding motion of a 10 mm spherical tungsten carbide indenter at 10 kN load and correlated the findings with the initial morphology of bainite. All specimens exhibited hardening after multiple cycles of deformation. However, the steel austempered at the lowest temperature showed the highest sub-surface hardness and lowest friction coefficient and was the most resistant to wear when tested under sliding fatigue for up to 500 cycles. Repeated sliding resulted in formation of two kinds of layers: white etching layer (WEL) and severely deformed layer (SDL). WEL exhibited very fine microstructure whereas the SDL showed alignment of retained austenite and bainitic laths along the direction of highest shear stress. There was a predominant presence of WEL in the specimens austempered at the lowest temperature and SDL in the specimens transformed at the highest temperature. Atomic probe tomography (APT) was used to investigate the distribution of carbon in the deformed layers just below the wear track after 500 sliding cycles for 250 and 350 °C austempered condition. No significant change in carbon has been detected in any of the austempered specimens after wear.

Tribological behavior of heat treated AISI 6150 steel

This research presents an experimental study on the tribological behavior and plastic deformation layer response of quenched-tempered AISI 6150 steel and austempered AISI 6150 steel under sliding with a ball-on-disk type apparatus. Worn surfaces were analyzed by micro-hardness, 3D profilometer, microscopy and scanning electron microscope (SEM). The results showed that the disk surfaces were hardened after tribo-tests due to fresh-martensite formation caused by frictional heating/self-quenching during the sliding tests. The wear loss of the disk specimens was due to smearing plastic deformation resulting in material pile-up along the wear track and debris inside the wear track. The quenched-tempered disk specimens generated more debris than the austempered specimens. The greater amount of wear debris in the quench-tempered samples led to more severe contact stresses between the ball and disk samples which accelerated the smearing of material. As the tempering temperature increases, more cracks were observed for all quenched-tempered disk specimens. More voids and cracks was developed in the near surface region of tempered martensite leading to accelerated material loss through smearing and delamination. Also, more plate-like wear particles were observed on the disks of tempered martensite due to greater surface and subsurface crack fomation and propagation.

Evolution of microstructure and mechanical properties during tempering of M50 steel with Bainite/Martensite duplex structure

The evolution of microstructure and mechanical properties during tempering of M50 steel with bainite/martensite (B/M) duplex structure have been investigated. The microstructural observation shows that the redistribution of carbon atoms and precipitation of transition carbides in the B/M duplex specimens are slightly decreased during the initial stage of tempering, as compared with the fully martensitic specimens. After tempering of 550 °C, not only finer ferrite packets are obtained, but also the precipitation of nano-scale carbides is enhanced for the B/M duplex specimens. The mechanical characterization indicates that the hardness is decreased while the impact toughness is significantly improved when tempering temperature increases to 300 °C. When the tempering temperature increases to 550 °C, due to the decomposition of retained austenite (RA) and precipitation of alloy carbides, the hardness exhibits an obvious improvement while the toughness first decreases and then increases. The impact toughness of the B/M duplex specimens is higher than that of the fully martensitic specimens during the whole tempering process, although they have the same tendency varying with tempering temperature. Furthermore, the final wear resistance is significantly improved in the austempered specimens due to the enhanced precipitation during tempering.

Investigation on Tensile Properties of Austempered SAE52100 Steel

The tensile test is an important test to determine several mechanical properties in which a sample is subjected to a constant strain rate until failure. Properties that are directly measured in a tensile test are ultimate tensile strength, breaking strength, maximum elongation, and reduction in area. The present research compared the tensile properties between austempered SAE52100 steel and quench-tempered SAE52100 steel under equivalent hardness. The experimental results showed that the ultimate tensile strength and yield strength of quench-tempered specimens was higher than austempered specimens. However, the total elongation and area reduction of quench-tempered specimens was lower than austempered specimens. Transgranular quasi-cleavage fracture was found on the austempered SAE52100 steel specimens. While using the austempering temperature of 288 °C, slight plastic deformation with existence of cleavage facets and ductile risers around the edges was obtained. However, while using the austempering temperature of 427 °C, the presence of tear marks on the cleavage facets surrounded by dimples indicated significant plastic deformation or ductile rupture. The fracture analysis could be correlated with the elongation obtained in tensile tests.

Tribological behavior of shot peened/austempered AISI 5160 steel

In order to enhance component performance, material sustainability, and service life, this study presents the tribological behavior of different austempered AISI 5160 steel specimens with shot peening. All the specimens were austempered for 2 h, then followed by shot peening to study the effect of shot peening and bainite morphologies on the wear and friction behavior. The microstructures, hardness, and residual stress of specimens were studied by optical microscopy, scanning electron microscopy, Vickers micro-hardness, 3D-profiler, and X-ray diffraction. Shot peening produced higher hardness on soft austempered specimens, rather than on hard austempered specimens. Compared with non-shot peened austempered disk specimens, the wear reduction of disk specimens with shot peening is up to 73%.

Slip-Rolling Behavior of Ductile and Austempered Ductile Iron Containing Niobium or Chromium

Abstract The use of high niobium alloyed cast iron alloys is a relatively new approach in which the niobium addition intends to improve the properties of the material by the precipitation of hard niobium carbides during solidification. Steels can be replaced by ductile cast iron in some rolling applications, such as gears and cams, in order to reduce material costs. The aim of this work is to evaluate ductile iron alloyed with 1 weight percent (wt.%) niobium for the as cast specimens and with 1.8 wt.% and 2.4 wt.% niobium for the austempered specimens under lubricated slip-rolling tests using mixed/boundary conditions in an Amsler-type machine. Austempered ductile iron (ADI) alloyed with 1 wt.% chromium, or Carbidic ADI, was tested for comparison. For the as cast conditions, the niobium addition resulted in an increase of wear resistance owing to the low contact pressure of these tests. However, for the austempered specimens, the best performance was found for unalloyed ADI. The main factor acting in the initiation and propagation of cracks in ductile iron is the presence of the graphite nodules. The coarse carbides also contributed to the initiation of cracks and spalling of the material.

Microstructure and mechanical properties of austempered AISI 9255 high-silicon steel

ABSTRACTThe present investigation is focused on evaluating the microstructure and mechanical properties of American Iron and Steel Institute 9255 high-silicon steel austempered at different temperatures and durations. Material characterisation was done using a scanning electron microscope and an X-ray diffractometer. Results show the bainite microstructure over a temperature range of 280–400°C. Bainite structure gains coarseness at higher temperatures at 360 and 400°C. A significant improvement in the tensile properties was observed for all austempered specimens; with a maximum tensile strength of 1852 MPa and elongation up to 35%. An excellent strain hardening response was observed from the samples austempered at temperatures of 360 and 400°C. Tensile properties were found to be superior at 15 min of austempering duration for all austempering temperatures.

Mechanical and Tribological Characteristics of Copper Alloyed Austempered Gray Cast Iron (AGI)

An investigation into the mechanical and tribological behaviour of copper alloyed austempered gray cast iron (AGI) has been carried out at different austempering temperatures. Tensile and wear tests specimens are prepared from cu-alloys as-cast gray iron, and subjected to austempering heat treatment at six different austempering temperature for 1hr respectively. The resulting microstructures are evaluated through optical microscopy, scanning electron microscope (SEM) and X-ray diffraction analysis. In study, the hardness, tensile properties and wear with coefficient of friction (COF) behaviour of all these austempered specimens are determined and examine the relationship of microstructural changes with austempering temperature. However, wear test is carried out using a block-on-roller multi-tribotester with sliding speed of 1.86 m/sec. After wear test, examination of worn surfaces under scanning electron microscope (SEM) shows that delamination and adhesion are the dominant wear mechanisms in the AGI samples. Whereas wear with coefficient of friction (COF) behaviour also found to be dependent on the hardness and tensile properties.

Fatigue Crack Growth Behavior of Austempered AISI 4140 Steel with Dissolved Hydrogen

The focus of this investigation was to examine the influence of dissolved hydrogen on the fatigue crack growth behavior of an austempered low-alloy AISI 4140 steel. The investigation also examined the influence of dissolved hydrogen on the fatigue threshold in this material. The material was tested in two conditions, as-received (cold rolled and annealed) and austempered (austenitized at 882 °C for 1 h and austempered at 332 °C for 1 h). The microstructure of the annealed specimens consisted of a mix of ferrite and fine pearlite; the microstructure of the austempered specimens was lower bainite. Tensile and Compact Tension specimens were prepared. To examine the influence of dissolved hydrogen, two subsets of the CT specimens were charged with hydrogen for three different time periods between 150 and 250 h. All of the CT samples were then subjected to fatigue crack growth tests in the threshold and linear regions at room temperature. The test results indicate that austempering resulted in significant improvement in the yield and tensile strength as well as the fracture toughness of the material. The test results also show that, in the absence of dissolved hydrogen, the crack growth rate in the threshold and linear regions was lower in austempered samples compared to the as-received (annealed) samples. The fatigue threshold was also slightly greater in the austempered samples. In presence of dissolved hydrogen, the crack growth rate was dependent upon the ∆K value. In the low ∆K region (<30 MPa√m), the presence of dissolved hydrogen caused the crack growth rate to be higher in the austempered samples as compared to annealed samples. Above this value, the crack growth rate was increasingly greater in the annealed specimens when compared to the austempered specimens in presence of dissolved hydrogen. It is concluded that austempering of 4140 steel appears to provide a processing route by which the strength, hardness, and fracture toughness of the material can be increased with little or no degradation in the ductility and fatigue crack growth behavior.

Microstructures and mechanical properties of surface and center of carburizing 23Cr2Ni2Si1Mo steel subjected to low-temperature austempering

A multiphase microstructure composed of nano-scaled bainite, martensite, undissolved spherical carbides and retained austenite was obtained on the top surface of carburizing 23Cr2Ni2Si1Mo steel after low-temperature austempered at 200°C, with a dual-phase microstructure of fine martensite lath and carbon-enriched retained austenite in the center of steel. As compared with the conventional oil quenching and tempering (OQ) state, the low-temperature austempering specimens showed a higher surface residual compressive stress and a thicker residual compressive stress layer, and the wear resistance of the top surface of specimens austempered for 4h, 8h and 24h was increased 40%, 58% and 12%, respectively. Moreover, the impact toughness of the center of these austempered specimens is also improved 20.8–33.3% as compared with that of the OQ specimen. Then, it can be deduced that applying the low-temperature austempering treatment for manufacturing carburizing-steel part is beneficial to improving its service performance for the better stress condition, higher wear resistance and higher interior toughness.

Property enhancement of cast iron used for nuclear casks

Ductile iron (DI) is a preferred material for use in various structural, automotive, and engineering fields because of its excellent combination of strength, toughness, and ductility. In the current investigation, we elucidate the relationship between the morphological and mechanical properties of DI intended for use in safety applications in the nuclear industry. DI specimens with various alloying elements were subjected to annealing and austempering heat treatment processes. A faster cooling rate appeared to increase the nodule count in austempered specimens, compensating for their nodularity value and subsequently decreasing their ductility and impact strength. The ductility and impact energy values of annealed specimens increased with increasing ferrite area fraction and nodularity, whereas an increase in the amounts of Ni and Cr resulted in an increase of hardness via solid solution strengthening. Austempered specimens were observed to be stronger than annealed specimens and failed in a somewhat brittle manner characterized by a river pattern, whereas the ductile failure mode was characterized by the presence of dimples.

COMPARATIVE STUDIES ON MECHANICAL PROPERTIES OF AISI 4340 HIGH-STRENGTH ALLOY STEEL UNDER TIME- QUENCHED AND AUSTEMPERED CONDITIONS

In the present work, the mechanical properties and microstructures of AISI 4340 high strength alloy steel under different conditions are investigated. Generally time quenching is substituted for surface hardening wherein martensite structure is formed in the case of the softer products towards core but austempering gives always uniform grained bainitic structure without any residual stresses and retained austenite. Austempering is given to hot working dies and springs to obtain maximum elastic limit. To assure uniform finer grains in the specimen normalizing treatment is given prior to other treatments. Samples were prepared and all of them were normalized at 9000C for 2 hrs and then air cooled to room temperature. After normalizing specimens are separated into 2 groups, timed quenching is performed on one set and austempering heat treatment is performed on another set of specimens. During timed quenching, the specimens are heated to 9000C. After reaching 900 0C, the specimens are soaked at that temperature for two hours. The specimens are then taken out of the furnace and quenched in engine oil for less than 4 seconds followed by air cooling. Hardness, microstructure and impact tests were performed on the timed quenched specimens to analyze the hardness value, grain size distribution and the energy absorbed respectively. Another set of the normalized specimens were taken and austempering was carried out on them. During austempering the specimens were heated at 9000C for 2 hrs in one furnace followed by transferring the specimens to the second furnace immediately within 10 seconds, which was maintained at 3200C and the specimens were soaked in the second furnace for 2 hrs and 45 min and then furnace cooling was done Hardness, microstructure and impact tests were again performed and obtained values were compared with timed quenched specimens. Difference in the mechanical properties of AISI 4340 steel at timed quenched and austempered conditions was observed. It is found that grain size of the austempered specimen is higher than the time quenched specimen. It is also observed that the austempered specimen has maximum strength and very good impact energy as compared to that of the timed quenched specimen. As a result mechanical properties like tensile strength, impact strength are improved in the austempered specimens than the timed quenched specimens.

Investigation of microstructural and mechanical properties of austempered steel bar-reinforced ductile cast iron composite

In this study, the effect of reinforcing nodular cast iron with steel bar on impact toughness was investigated. The composite material was produced by the sand mould casting technique. Afterwards, austempering heat treatment was applied to the specimens at two different temperatures of 350°C and 400°C. The samples were evaluated by optical and scanning electron microscopies; then, hardness and Charpy impact toughness tests were conducted at ambient temperature on the cast iron specimens with or without reinforcement. The results revealed that impact toughness of the nodular cast iron increased by reinforcing with the steel bar. Furthermore, austempering heat treatment greatly influenced the impact toughness of reinforced specimens; however, the impact toughness of the austempered composite at 350°C was more than that of the austempered specimens at 400°C.

Investigating the machinability of austempered ductile irons with dual matrix structures

Abstract In this study, the machinability of austempered ductile iron with dual phase structure was investigated. Specimens were intercritically austenitized (partially austenitized) in the two phase region (α + γ) at various temperatures (810, 820 and 830 °C) for 90 min, then quenched in a salt bath held at austempering temperatures of 315 °C or 375 °C for 120 min and then air cooled to room temperature to obtain various ausferrite volume fractions and morphologies. Machinability tests were carried out at various cutting speeds (150, 180 and 210 m min−1), at a constant feed rate (0.30 mm rev−1) and depth of cut at 2 mm after austempering heat treatment. The experimental results showed that austempered specimens from the (α + γ) temperature range exhibited better machinability than as-cast samples. On the other hand, the cutting force was decreased with increasing proeutectoid ferrite fractions. The results also showed that continuity of ausferritic structure along intercellular boundaries plays an important role in determining machinability of austempered ductile iron with different ausferrite volume fractions. Another important result is that higher proeutectoid volume fraction has negative effects on surface roughness.

Influence of Yogic Treatment on Quality of Life Outcomes, Glycemic Control, and Risk Factors in Diabetes Mellitus: Randomized Controlled Trial

In this study, the effect of reinforcing nodular cast iron with steel bar on impact toughness was investigated. The composite material was produced by the sand mould casting technique. Afterwards, austempering heat treatment was applied to the specimens at two different temperatures of 350 °C and 400 °C. The samples were evaluated by optical and scanning electron microscopies; then, hardness and Charpy impact toughness tests were conducted at ambient temperature on the cast iron specimens with or without reinforcement. The results revealed that impact toughness of the nodular cast iron increased by reinforcing with the steel bar. Furthermore, austempering heat treatment greatly influenced the impact toughness of reinforced specimens; however, the impact toughness of the austempered composite at 350 °C was more than that of the austempered specimens at 400 °C.

The effect of matrix structure on the fatigue behavior of austempered ductile iron

The primary objective of this study has been to investigate the fatigue behavior of the austempered unalloyed ductile iron. Ductile iron specimens were austenized at 900 °C for 110 min and then one group specimens were austempered at 230, 330, 430 °C for 60 min, and another one group specimens were austempered at 330 °C for 30, 60, 120 min to obtain different microstructures. Rotating bending tests (10 million cycles) were conducted on the ductile iron and the austempered specimens selecting completely reversed cycle of stress ( R = minimum stress/maximum stress = −1). The highest fatigue strength was found in the bainitic structure (ausferritic). The presence of martensite or carbide phases in the structure of austempered ductile iron reduces the fatigue strength.

Dry sliding wear of low alloyed austempered ductile iron

A multiple low alloyed ductile iron with 0.8 wt-%Ni and 0.25 wt-%Mo was austempered in single and two step processes at 300 and 400°C for 120 min. Specimens were used to study the effect of austempering conditions on the wear behaviour of this material. Sliding wear tests were carried out using a pin on disc apparatus, the tes tmaterials rubbing under dry atmospheric conditions against a surface of hardened steel (55 HRC) at speeds of 0.6, 0.7 and 1.0 m s-1 and normal loads of 15.82 and 22.84 N. Test durations were 30, 60, 90 and 120 min. Scanning electron microscopy was used to examine the worn surfaces of test specimens. It was found that two step austempered specimens exhibited wear resistance that was higher than that of specimens austempered at 400°C, and almost as high as that of specimens austempered at 300°C. These two step austempered specimens, moreover, gave the highest impact energy and showed the best combination of mechanical properties. During two step austempering, the first stage reaction (formation of ausferrite) was completed in the intercellular area before the undesired second stage reaction (precipitation of carbides) had started in the eutectic cells. The two step treatment resulted in a duplex structure: upper and lower bainitic ferrite without formation of carbides. This structure was responsible for the improvement of mechanical properties and the good wear resistance. The results show that a well balanced choice of smaller additions of multiple alloying elements can reduce the negative effects of segregation and resulting structural inhomogeneity. MST/5472

Thermal effects due to tempering of austenite and martensite in austempered ductile irons

Typical compositions of austempered ductile cast irons in different metallurgical states were studied with the aim of making a detailed assignment of thermal effects seen using differential scanning calorimetry (DSC) to transformations occurring in the alloy phase constitution and microstructure. In austempered specimens the transformation of retained austenite is revealed by an exothermic reaction at temperatures above 450°C which depend on the content of alloying elements. Martensite transformations are seen in a wide temperature range starting at ∼100°C and giving exothermic and endothermic overlapped DSC signals due to relaxation, carbon diffusion, and tempering. It is concluded that DSC is a powerful tool for detecting tiny traces of martensite in austempered materials.

Mechanical properties of austempered ductile iron: Iio, Y., Yoshino, S. and Ogino, Y. Kubota Tech. Rep. Apr. 1988 (20), 1–6 (in Japanese)

The primary objective of this study has been to investigate the fatigue behavior of the austempered unalloyed ductile iron. Ductile iron specimens were austenized at 900 °C for 110 min and then one group specimens were austempered at 230, 330, 430 °C for 60 min, and another one group specimens were austempered at 330 °C for 30, 60, 120 min to obtain different microstructures. Rotating bending tests (10 million cycles) were conducted on the ductile iron and the austempered specimens selecting completely reversed cycle of stress (R = minimum stress/maximum stress = −1). The highest fatigue strength was found in the bainitic structure (ausferritic). The presence of martensite or carbide phases in the structure of austempered ductile iron reduces the fatigue strength.