Austempering Heat Research Articles

Prediction of Microstructure in ADI Castings

Abstract Tests were carried out on samples of low-alloy ductile iron with additions of Ni, Cu and Mo, subjected to austempering heat treatment. The samples were austenitized at 850, 900 and 950 °C, and then austempered at T = 210, 240, 270, 300 and 330 °C. The ausferritizing treatment was carried out in a salt bath for the time τ = 2 - 8 hours. Additionally, tests and studies covered samples subjected to the ausferritizing treatment at 270 °C with the time of holding castings in a bath from 2 to 24 hours. Evaluation covered the results of the ADI microstructure examinations and hardness measurements. The ADI matrix morphology was identified counting the average number of ausferrite plates and measuring their width and spacing. The regression equations HB = f (τ, T) and τ = f (HB, T) were derived to establish the, so-called, “process window”, allowing obtaining a priori the required microstructure of ADI and, consequently, the required mechanical properties, mainly hardness, shaping the functional properties of castings, abrasion wear resistance – in particular.

Advanced Properties of Ausferritic Ductile Iron Obtained in As-Cast Conditions

In previous studies, the process parameters to obtain ausferritic ductile iron in as-cast conditions by means of engineered cooling were defined, that is, without an austempering heat treatment. This material was fundamentally characterized, and its mechanical properties were determined. It was demonstrated that obtaining fully ausferritic microstructures by means of engineered cooling was feasible and that the properties met the requirements of the conventionally produced austempered ductile iron. Additionally, an experimental model was developed to define the optimal processing parameters of castings presenting different thermal moduli, in terms of chemical composition, temperatures and time parameters. The aim of the present work is to go into detail about the physical properties of the ausferritic as-cast materials. The chemical composition of the samples was defined by means of the experimental model. The isothermal transformation temperature was changed from 300 to 400 °C, while the other process parameters (shakeout temperature and isothermal transformation time) remained constant. Due to the excellent strength/toughness ratio of these materials, they are prone to being used on different applications such as automotive suspension components, rail components in low temperature environments and pumps and engines exposed to corrosive marine conditions among others. With the aim of responding to this demand, an advanced characterization of the material’s low temperature, corrosion or dynamic properties was performed on this work. These results were compared to the conventionally heat-treated austempered ductile iron as well as other nodular iron ferritic–pearlitic grades found in the literature.

Low-Temperature Toughness of the Austempered Offshore Steel

This study investigates low-temperature toughness of the offshore steel with two different austempering heat treatments. Toughness of upper bainite is significantly lower than that of lower bainite. Low impact toughness of the upper bainite is caused by the presence of martensite–austenite (MA) and increasing the amount of low-angle lath boundaries in upper bainite package. EBSD equipped in the SEM demonstrates an effective approach to analyze the misorientation angle of lath boundaries in upper bainite packet.

Effect of nodule count and austempering heat treatment on segregation behavior of alloying elements in ductile cast iron

The equilibrium partition ratio, k, has been measured for Mn, Mo, Si, Ni and Cu in a ductile iron with composition (wt.%): 3.45C, 0.25Mn, 0.25Mo, 2.45Si, 0.5Ni and 0.5Cu with different nodule counts obtained from different section sizes of 13, 25, 75 mm in the as cast, austenitized (at 870 °C for times 1, 4 and 6 hours) and austempered (at 375 °C for times 1 to 1,440 min) samples. Results show that Mn and Mo segregate positively at cell boundaries, but Si, Ni and Cu concentrate in an inverse manner in the vicinity of graphite nodules and there is a depletion of these elements at cell boundaries. Segregation curves for Ni and Cu are more smooth than for Si. Carbide formation has been observed at cell boundaries. Based on the results, the partition ratios for all elements decrease with increasing the nodule count. More carbide with coarser morphology has been observed in the microstructure with a lower nodule count. Austenitization for a longer time can decrease partition ratio, but cannot eliminate it entirely. Increasing the austenitization temperature has the same efect. Austenitizing parameters have no significant effect on carbides volume fraction. The kinetics of austempering is faster in higher nodule counts and subsequently better mechanical properties including higher ductility, strength and toughness have been observed for all austempering conditions studied.

Properties investigation of austempered ductile iron

This work concerns microstructural and mechanical properties of an austempered ductile cast iron (ADI). The ductile iron material was produced by the sand mould casting technique. Afterwards, austempering heat treatment was applied to the specimens at two different temperatures of 250°C and 350°C. Austempered Ductile Irons (ADIs) were produced successfully by different two-stage heat treatments, to obtain favorable microstructure and hardness. The microstructure and hardness obtained by such variable heat treatments were compared. The austempering temperature and time were found to be decisive parameters in obtaining a desired ADI microstructure.

Microstructures and Mechanical Properties of Austempering SUS440 Steel Thin Plates

SUS440 is a high-carbon stainless steel, and its martensite matrix has high heat resistance, high corrosion resistance, and high pressure resistance. It has been widely used in mechanical parts and critical materials. However, the SUS440 martempered matrix has reliability problems in thin plate applications and thus research uses different austempering heat treatments (tempering temperature: 200 °C–400 °C) to obtain a matrix containing bainite, retained austenite, martensite, and the M7C3 phase to investigate the relationships between the resulting microstructure and tensile mechanical properties. Experimental data showed that the austempering conditions of the specimen affected the volume fraction of phases and distribution of carbides. After austenitizing heat treatment (1080 °C for 30 min), the austempering of the SUS440 thin plates was carried out at a salt-bath temperature 300 °C for 120 min and water quenching was then used to obtain the bainite matrix with fine carbides, with the resulting material having a higher tensile fracture strength and average hardness (HRA 76) makes it suitable for use as a high-strength thin plate for industrial applications.

Wear behaviour of bainitic rail and wheel steels

The paper presents a short review on the past research and recent trends in the development of rail and wheel steels using bainitic morphology from the angle of mechanical and wear performances. The enhancement in mechanical properties as well wear resistance of the bainitic rail and wheel steels is very promising. In addition, the mechanical behaviour of a bainitic rail steel and a wheel steel along with their linearly reciprocating dry sliding wear characteristics in comparison to the existing pearlitic rail and ferritic–pearlitic wheel steels used in Indian Railways is discussed. The steels were subjected to austempering heat treatment at different temperatures and durations in the bainitic region as determined from the TTT diagrams of the respective steels. Bainitic steels show better mechanical properties and wear resistance due to the typical morphology of bainitic microstructural.

Effect of 10% ausforming on impact toughness of nano bainite austempered at 300 °C

The volume fraction and morphology of the microstructural constituents are extremely important when discussing the impact toughness of nanostructured bainitic steels. On the other hand, microstructural features can be influenced by plastic deformation of austenite prior to isothermal bainitic transformation. This paper aims to investigate the effect of 10% ausforming process before austempering heat treatment on impact toughness of nanostructured low temperature bainitic steels austempered at 300°C. Results indicate that the volume fraction and morphology of high carbon retained austenite within the microstructure are important factors affecting the impact toughness energy. However results showed that the role of bainitic sheaves cannot be ignored due to their valuable effect of arresting/deflecting the cracks. It has been found that bainitic sheaves grow at more limited crystallographic variants in ausformed samples which can influence the value of the impact energy absorbed during straining the samples.

Effect of prior austempering heat treatment on the microstructure, mechanical properties and high-stress abrasive wear behaviour of a 0·33% C dual-phase steel

The present study investigates the effect of prior austempering heat treatment on the microstructure, mechanical property and high-stress abrasive wear response of 0·33% carbon dual-phase (DP) steel. DP steels were produced by intercritical annealing of the as-received steel (DP-AR) and of the steel subjected to prior austempering heat treatment (DP-AT). Prior austempering heat treatment results in the refinement of martensite phase in DP-AT steel, which is responsible for generation of less strain and microcracks in the vicinity of martensite phases. Therefore, this steel exhibits improvement in ductility with a marginal loss in strength. On the other hand, DP-AR steel shows coarse martensite phase which promotes strain accumulation and thereby generation of microcracks. These microcracks are found to be responsible for poor ductility in this steel. Furthermore, abrasion tests were carried out at varying sliding distances at 7 N applied load. DP treatments result in improved overall wear response. However, prior austempering heat treatment results in improvement in wear resistance in DP-AT steel at all sliding distances except at initial period. This is attributed to finer morphology of martensite and the absence of microcracks along with higher strain hardening in DP-AT steel. However, the improved wear resistance at initial period of DP-AR steel is attributed to considerably higher strain hardening index at low strain as compared to DP-AT steel.

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.

Numerical Simulation of Austempering Heat Treatment of a Ductile Cast Iron

This paper presents a coupled thermo-mechanical-metallurgical formulation to predict the dimensional changes and microstructure of a ductile cast iron part as a consequence of an austempering heat process. To take into account the different complex phenomena which are present in the process, the stress-strain law and plastic evolution equations are defined within the context of the associate rate-independent thermo-plasticity theory. The metallurgical model considers the reverse eutectoid, ausferritic, and martensitic transformations using macro- and micro-models. The resulting model is solved using the finite element method. The performance of this model is evaluated by comparison with experimental results of a dilatometric test. The results indicate that both the experimental evolution of deformation and temperature are well represented by the numerical model.

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

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

Nanoausferritic matrix of ductile iron

Austempered ductile iron is known for its excellent mechanical properties resulting from special phase composition and austempering heat treatment. Typical microstructure consists of ferrite plates of micrometre size submerged in untransformed austenite matrix. It has been recently shown that by use of appropriate chemical composition of cast iron and well targeted heat treatment parameters, it is possible to reduce ferrite plates width to submicron or even nanometric size. This creates the potential to achieve even higher mechanical properties of austempered ductile iron. The paper describes the influence of applied heat treatment parameters on microstructure of selected austempered ductile iron grades. Conditions necessary to reduce size of phases to a nanometric scale by heat treatment in austempered ductile iron are discussed.

Simple expressions to estimate the Manson–Coffin curves of ductile cast irons

Due to the possibility of improving and adjusting their mechanical properties using austempering heat treatments, ductile cast irons are very attractive materials for structural applications. However, for this class of materials, very few fatigue data are available for designers in the International Standards. Therefore, at least in the preliminary design phase, simple expressions to estimate the fatigue properties of these materials, taking advantage of their static properties, can be useful for designers. In the recent literature, a simple method was proposed to estimate the strain-life curve of steels based only on the Brinell hardness and the elastic modulus of the material. In this paper, the possibility of extending this approach to ductile cast irons is discussed based on a set of more than 130 fatigue data obtained by the author on ferritic, pearlitic, isothermed and austempered ductile irons.

Thermal Stability of Nanocrystalline Structure In X37CrMoV5-l Steel

AbstractThe aim of the study was to investigate the thermal stability of the nanostructure produced in X37CrMoV5-1 tool steel by austempering heat treatment consisted of austenitization and isothermal quenching at the range of the bainitic transformation. The nanostructure was composed of bainitic ferrite plates of nanometric thickness separated by thin layers of retained austenite. It was revealed, that the annealing at the temperature higher than temperature of austempering led to formation of cementite precipitations. At the initial stage of annealing cementite precipitations occurred in the interfaces between ferritic bainite and austenite. With increasing temperature of annealing, the volume fraction and size of cementite precipitations also increased. Simultaneously fine spherical Fe7C3carbides appeared. At the highest annealing temperature the large, spherical Fe7C3carbides as well as cementite precipitates inside the ferrite grains were observed. Moreover the volume fraction of bainitic ferrite and of freshly formed martensite increased in steel as a result of retained austenite transformation during cooling down to room temperature.

Effect of Heat Treatments on Austempered Ductile Iron

This work presents the influence of austempering heat treatment carried out in one-step and two-step processes on the microstructures and mechanical properties of ductile cast iron. The samples were extracted from as-cast pieces and heat treated by austempering. For the one-step process the samples were heated at 910°C for 90 min for austenitization and cooled in salt bath at a temperature of 300°C for 30 min. For the two-step process the samples were cooled from 910°C to 245°C, kept at this temperature for 5 min in salt bath, then heated in another salt bath at a temperature of 300°C for 30 min. The samples were analyzed by optical microscopy and mechanical tests. After the one-step austempering, microscopic analysis of the samples showed ausferrite microstructure matrix and graphite in nodules surrounded by fine pearlite. For the two-step austempering, the presence of ausferrite matrix with graphite in nodules and retained austenite was observed. As to mechanical properties, the results showed that, with the two-step process there was gain (4.7%) in the average hardness and loss (3.5%) in the impact resistance. The microhardness of the ausferrite was 6.2% higher in the one-step austempering when compared to the two-step process.

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.

Sliding wear behavior of austempered ductile iron microalloyed with boron

This work analyses the wear resistance under dry sliding conditions by the pin-on-ring method of an austempered ductile iron (ADI) micro-alloyed with different amounts of boron. The main observed effect of boron was to accelerate the transformation of austenite to ausferrite during the austempering heat treatment; this caused lower values of retained austenite in the irons with boron additions. Lower retained austenite amounts, in turn, caused higher wear rates since the amount of austenite able to transform to martensite by plastic deformation is lower. It was observed that wear loss is a function of the iron hardness. For short austempering times higher values of hardness were obtained which produced lower wear; and the main observed wear mechanism was oxidation. On the other hand, for longer austempering times hardness decreased but the ductility was improved (more ausferrite, less martensite). With the increase in load, a higher deformation under the worn surface was observed and wear was slightly increased. It was observed that boron additions increased the carbon diffusion and accelerated the ausferrite transformation promoting a more homogeneous microstructure. Due to this, a higher ductility but lower values of hardness were obtained, which changed the wear behavior. The higher hardness values of the base alloy are related to a low friction coefficient and lower wear. On the other hand, boron added irons were softer than the base alloy, but the higher volume of graphite acting as a lubricant and the finer matrix contributed to give wear resistance, giving a good balance of mechanical properties.

Microstructure, Mechanical Properties, Toughness, Wear Characteristics and Fracture Phenomena of Austenitised and Austempered Low-Alloyed Ductile Iron

Conventional ductile iron (DI) and austempered ductile iron (ADI) alloys were successfully produced. The alloying elements—Ni, Mo, Cr and Mn were added to get as-cast low-alloyed ductile iron (LADI) followed by austempering heat treatment. Hardness was measured for all investigated alloys. A hardness conversion table was deduced for irons investigated. Highest values of calculated quality index (QI) were for ADI alloys implying higher material performance. Impact energy values were attributed to microstructure and tensile properties. Wear characteristics of selected ADI alloys showed comparable values with LADI. Micro-hardness values were used to identify the micro-constituents and the work-hardened layers. Fracture modes were revealed and fracture surface observations were done by SEM. Due to higher toughness and QI, and cheap price, it is suggested that rolling mills may be produced from ADI, specially heat-treated instead of LADI.

Acoustic emission study of the plastic deformation of quenched and partitioned 35CrMnSiA steel

Acoustic emission (AE) monitored tensile tests were performed on 35CrMnSiA steel subjected to different heat treatments. The results showed that quenching and partitioning (Q-P) heat treatments enhanced the combined mechanical properties of high strength and high ductility for commercial 35CrMnSiA steel, as compared with traditional heat treatments such as quenching and tempering (Q-T) and austempering (AT). AE signals with high amplitude and high energy were produced during the tensile deformation of 35CrMnSiA steel with retained austenite (RA) in the microstructure (obtained via Q-P and AT heat treatments) due to an austenite-to-martensite phase transformation. Moreover, additional AE signals would not appear again and the mechanical properties would degenerate to a lower level once RA degenerated by tempering for the Q-P treated steel.