Showing papers on "Austempering published in 2015"
25 Feb 2015-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, 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.
Abstract: 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 ( γ HC ). 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.
91 citations
TL;DR: In this article, a "disturbed" bainitic austempering process was employed in a Mn-Si-Cr-C low-alloyed steel to reduce the fraction and size of the blocky martensite/austenite (M/A) islands.
61 citations
25 Mar 2015-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: The microstructures and mechanical properties of a novel bainitic bearing steel subjected to austenitizing at 950°C for 40min followed by low-temperature austempering at 200°c for 2h, 6h, 12h and 72h were studied in this paper.
Abstract: The microstructures and mechanical properties of a novel bainitic bearing steel subjected to austenitizing at 950 °C for 40 min followed by low-temperature austempering at 200 °C for 2 h, 6 h, 12 h and 72 h were studied Nano-bainite microstructure with bainitic ferrite plate thickness about 50±15 nm was obtained by austempering at 200 °C for 6 h, 12 h and 72 h Besides nano-scaled bainitic ferrite plates, there were small amount of retained austenite and undissolved carbides in the microstructure The volume fraction of retained austenite is 72% in the samples austempered at 200 °C for 72 h The ultimate tensile strength of samples austempered at 200 °C for 72 h reaches 2373 MPa, which is superior to the sample austempered at 200 °C for 2 h, whose microstructure is almost full martensite The weight loss values are 00100 g and 00273 g for samples austempered for 72 h and 2 h respectively and the wear resistance of the sample austempered for 72 h is about 17 times higher than that for 2 h Because of the excellent strength and wear resistance of the low-temperature bainite, this low-temperature austempering could be a potential technology for manufacturing bearings
53 citations
TL;DR: In this article, the effect of ausforming on the stability of retained austenite in a C-Mn-Si bainitic steel was investigated through metallography, X-ray diffraction and dilatometry.
Abstract: The effect of ausforming on the stability of retained austenite in a C-Mn-Si bainitic steel was investigated through metallography, X-ray diffraction and dilatometry. The geometrical relationships of the amount of bainite transformation and the volume fractions of retained austenite with deformation strains were studied. The results show that the degree of promotion of small strains on bainite transformation is nonlinear because of the dual effects of accelerated nucleation and retarded growth caused by ausforming. The transformed bainite fraction first increased and then decreased with increased small strains. It indicates that there is a maximum degree of the promotional function corresponding to a certain small strain at low temperature. Although small strains promote bainite transformation, a larger quantity of retained austenite exists at room temperature due to the suppressed martensite transformation during the cooling process after bainite transformation. The carbon content in retained austenite increases with the amount of baintie transformation, which contributes to the stability of austenite. Compared with the stabilizing effect due to carbon enrichment, mechanical stabilization caused by ausforming has a decisive effect on determining the volume fraction of retained austenite after isothermal bainite transformation.
44 citations
TL;DR: In this article, the wear resistance of an ADI micro-alloyed with different amounts of boron was analyzed under dry sliding conditions by the pin-on-ring method of an austempered ductile iron.
42 citations
TL;DR: In this article, a Si-Mo-alloyed high-C-Cr bearing steels were formed in low-temperature austempering after partial austenitizing in the intercritical gamma-plus-carbide region.
37 citations
TL;DR: In this paper, the microstructure and the mechanical properties of a low-carbon Al/Si-alloyed carbide-free bainitic steel austempered at temperatures between 300 and 350°C have been investigated by means of optical microscopy, scanning electron microscopy and transmission electron microscope.
Abstract: The microstructure and the mechanical properties of a low-carbon Al/Si-alloyed carbide-free bainitic steel austempered at temperatures between 300 and 350 °C have been investigated by means of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction analysis, and mechanical property tests. The results show that an excellent combination of tensile strength, ductility, and impact toughness is obtained at the austempering temperature of 320 °C—but neither at the lowest temperature of 300 °C nor at the highest temperature of 350 °C. These results are correlated with the features of the constituent phases of the microstructure, especially the amount and size of retained austenite, which are largely dependent on the austempering temperature. The observations are also due to the inconsistent effects of strain-induced martensitic transformation on strength, ductility, and toughness. The results of the present study suggest that an optimized isothermal temperature may also exist for any other bainitic steel, at which, if austempering treatment is carried out, an improved combination of tensile and impact properties can be obtained.
31 citations
11 Nov 2015-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effect of boron addition to an Austempered Ductile Iron, in amounts from zero to 120ppm, was analyzed, and it was found that borons have a strong effect on the equivalent carbon content, resulting in an increase on the precipitated graphite volume and a decrease in the dissolved carbon content in the matrix.
Abstract: The present work analyzes the effect of boron addition to an Austempered Ductile Iron, in amounts from zero to 120 ppm. It has been found that boron has a strong effect on the equivalent carbon content, resulting in an increase on the precipitated graphite volume and a decrease in the dissolved carbon content in the matrix. This in turn, increases the ferrite volume fraction in the as-cast conditions from 0.24 in the base alloy to 0.78 for the iron with 120 ppm of boron. Furthermore, a decrease in the nodularity from 100% in the base alloy to 83% with 120 ppm of boron has been observed. During austempering, the transformation to ausferrite was faster and lower volumes of martensite and unstable austenite were detected when boron increased; this promoted lower hardness values, 239 HV for the base iron and 189 HV for the 120 ppm boron alloy. The increase in hardness and strength, typical for the start of bainite formation, were not observed in the boron added irons, but just in the base alloy. Under this basis, it is assumed that at least the addition of 60 ppm of boron extended the optimal processing window. The higher values of strength and ductility were obtained for the alloy with 60 ppm of boron; these results are discussed in terms of the graphitizing effect of boron in these irons and the reduced amount of carbon dissolved in austenite.
28 citations
TL;DR: 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.
27 citations
TL;DR: In this paper, the effect of cutting parameters, namely the cutting speed and the depth of cut, on the surface roughness was investigated in terms of the surface quality of ADI.
Abstract: Austempered Ductile Iron (ADI) is a relatively novel material to the industrial market and it is often reported that ADI is a difficult material for machining. This work mainly focuses on machinability studies of ADI. The Ductile Iron (DI) was austempered at 250 °C for different durations and the process window for austempering was established by studying the microstructure. The microstructural characterization of the material was done using optical microscopy, SEM and XRD. The samples austempered as per the process window were then subjected to turning using a TiAlN-coated tungsten carbide insert to study the effect of cutting parameters, namely the cutting speed & the depth of cut. The effect was investigated in terms of the surface roughness obtained. It has been observed that increasing cutting speed results in decreasing of surface roughness and improvement in surface quality. The surface quality deteriorates with decreasing the cutting speed. Based on the observations it is concluded that when ADI is subjected to turning with TiAlN-coated tungsten carbide insert, the combination of a depth of cut of 2 mm and feed rate of 0.1 mm/rev results in the best surface quality when the cutting speeds in the range 150–200 m/min are employed. The observed machinability behaviour was investigated in light of the microstructure of the material obtained under the given austempering conditions and a structure–property co-relation was established between the two. High-speed cutting with deeper depth of cut not only reduce machining expenses by increasing production volume and rate, but also results in good surface quality of machined parts.
24 citations
Journal Article•
TL;DR: In this paper, the acceleration effect of warm rolling process on bainite transformation at 230oC for a high-carbon, silicon-rich steel with nanostructured bainites was studied.
TL;DR: In this paper, the authors evaluated two types of austempered ductile iron: one with a higher strength and the other with higher ductility, and showed that the thinner material is optimal due to its lower weight and higher mass effectiveness.
TL;DR: In this article, the influence of magnesium on the phase transformation and mechanical properties has been investigated by means of dilatometric and microstructural analysis in low carbon microalloyed steel.
Abstract: The influence of magnesium on the phase transformation and mechanical properties has been investigated by means of dilatometric and microstructural analysis in low carbon microalloyed steel. The thermal simulation experiments were performed on a Gleeble-3800 system equipped with a high-speed deformation dilatometer. The results reveal that, by adding with Mg, the pearlite transformation is delayed, and the pearlite has a scattered distribution even at low cooling rates from 0.1 to 0.5 K/s. It also shows that Mg tends to promote the bainite transformation as a result of the formation of acicular ferrite and granular bainite. Acicular ferrite is attributed to the change of dominant oxide inclusions from inert Al2O3, to xMgO · Al2O3, which serves as an effective nucleant for acicular ferrite. Moreover, bainitic structure obtained by the addition of Mg into the molten steel exhibits the remarkable improvement of toughness and tensile properties.
TL;DR: In this article, the very high cycle fatigue tests were conducted on a kind of high-strength austempered ductile iron (ADI) at 90 Hz and 20 kHz.
TL;DR: In this paper, the analysis of wear behavior of bainitic steels made by austempering from a microalloyed steel MAS2, meant for making railway wheel, and comparison with that of a conventional railway wheel steel, wheel-R19.
Abstract: The present work concentrates on the analysis of wear behavior of bainitic steels made by austempering from a microalloyed steel MAS2, meant for making railway wheel, and comparison with that of a conventional railway wheel steel, wheel-R19. Austempering of the MAS2 steel samples has been performed at different times and temperatures to obtain different morphologies of bainite. Linearly reciprocating dry sliding wear tests of these samples have been carried out at laboratory scale using five different loads. The wear behavior of the bainitic steels has been compared with that of the ferritic-pearlitic steel, wheel-R19. Mechanical properties of the bainitic MAS2 steels are found to be more than that of the wheel-R19 steel. Considerable enhancement in wear resistance of the bainitic steels is attributed to high hardness and strength of the steels. The wear mechanism has been critically analyzed by examining wear track morphology. The wear data gathered have been graphically presented in the form of wear mechanism map to understand the material behavior under different sliding conditions and subsequent morphological variations.
TL;DR: In this article, the microstructure and hardness of a high-strength high-carbon steel containing 0.80% silicon and 0.84% aluminum have been evaluated under different austempering conditions.
TL;DR: In this article, the transformation kinetics, microstructure evolution, hardness and compression properties of four thermo-mechanically processed ductile irons (DIs) having from 0 to 1.7% aluminum.
TL;DR: In this article, the applicability of new materials for parts of the mining machinery is discussed, and the results indicated that each of the tested materials was senstive to the surface hardening effect, which resulted in high wear resistance.
Abstract: The research described in this article is a fragment in the series of published works trying to determine the applicability of new materials for parts of the mining machinery. Tests were performed on two groups of austempered ductile iron – one of which contained 1.5% Ni and 0.5% Mo, while the other contained 1.9% Ni and 0.9% Cu. Each group has been heat treated according to the three different heat treatment variants and then the material was subjected to detailed testing of mechanical properties and abrasion wear resistance, measuring also hardness and magnetic properties, and conducting microstructural examinations. The results indicated that each of the tested materials was senstive to the surface hardening effect, which resulted in high wear resistance. It has been found that high temperature of austempering, i.e. 370◦C, favours high wear resistance of ductile iron containing nickel and molybdenum. Low temperature of austempering, i.e. 270◦C, develops high wear resistance in ductile iron containing nickel and copper. Both these materials offer completely different mechanical properties and as such can be used for different and specific applications.
TL;DR: In this article, it has been shown that, at a carbon content of about 0.10-0.15% and upon additional alloying with silicon and aluminum after such a heat treatment, in steels, the structure of carbide-free bainite is formed and a substantial enhancement in the impact toughness is observed compared to bainites containing carbide precipitates.
Abstract: Mechanical properties of chromium-nickel-molybdenum steels with the carbon contents of 0.1 to 0.4% after slow continuous cooling in the bainitic range have been determined, and the structure has been investigated. It has been shown that, at a carbon content of about 0.10–0.15% and upon additional alloying with silicon and aluminum after such a heat treatment, in steels, the structure of carbide-free bainite is formed and a substantial enhancement in the impact toughness is observed compared to bainite containing carbide precipitates. The enhancement in the level of toughness is related to the presence in carbide-free bainite of an appreciable amount of retained austenite enriched with carbon.
1 Jan 2015
TL;DR: In this article, the acceleration effect of warm rolling process on bainite transformation at 230 o C for a high-carbon, silicon-rich steel with nanostructured bainites was studied.
Abstract: In this paper, the acceleration effect of warm rolling process on bainite transformation at 230 o C for a high-carbon, silicon-rich steel with nanostructured bainite was studied. The warm rolling temperature was determined by simulation experiments using a Gleeble 3500 thermal and mechanical testing system. It is found that the bainite transformation will take more than 50 hours to complete at the same isothermal temperature without warm rolling, while the incubation time can be shortened from 6 hours to 0.5 hours when warm rolling was conducted within a temperature range 300-600 o C. In addition, the quantitative X-ray analysis in dicated that the volume fraction of retained austenite after warm rolling (total strain 0.20) and isothermal transformation for 20 hou rs was about 33.8% , keeping a same level compared with the austempering bainite transformation process (no strain) which austenite volume fraction was about 32%. Low temperature bainite transformation can be accelerated by warm rolling effectively without compromising strength. And the ultimate tensile strength of warm rolling nanostructured bainite steel is 2127 MPa. © 2014 The Authors. Published by Elsevier Ltd. Selection and Peer-review under responsibility of the Chinese Materials Association in the UK (CMA-UK). This is an open acces s article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/)
TL;DR: In this paper, a comparative study of the corrosion resistance of bearing steel 67SiMnCr6-6-4 after two kinds of nanostructuring treatments was performed.
Abstract: The paper describes a comparative study of the corrosion resistance of bearing steel 67SiMnCr6-6-4 after two kinds of nanostructuring treatments and two kinds of conventional quenching and tempering treatments. The nanostructuring treatment consisted of austempering with an isothermal quenching at 240◦C and 300◦C. The conventional heat treatment consisted on quenching and tempering at 350◦C for 1 h and quenching and tempering at 550◦C for 1 h. Time and temperature of tempering was chosen so that the hardness of both samples (nanostructured as well as quenched and tempered) was similar. The microstructure of steel after each heat treatment was described with the use of transmission electron microscopy (TEM). It was shown, that the austempering conducted at 240◦C produced homogenous nanobainitic structure consisting of carbide-free bainite plates with nanometric thickness separated by the layers of retained austenite. The austempering at 300◦C produced a sub-micrometric carbide-free bainite with retained austenite in form of layers and small blocks. The conventional heat treatments led to a tempered martensite microstructure. The corrosion resistance study was carried out in Na2SO4 acidic and neutral environment using potentiodynamic and electrochemical impedance spectroscopy (EIS) methods. The corrosion resistance of nanostructured steel samples were compared to the steel samples with tempered martensite. The obtained results indicate, that the corrosion resistance of bearing steel with nanobainitic structure is similar to steel with tempered martensite in both acidic and neutral environment. This means that the high density of intercrystalline boundaries in nanobinite does not deteriorate the corrosion properties of the bearing steel.
TL;DR: In this article, the influence of applied heat treatment parameters on the microstructure of selected austempered ductile iron grades is discussed, and conditions necessary to reduce size of phases to a nanometric scale by heat treatment in Austempered Ductile Iron are discussed.
Abstract: 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.
Journal Article•
11 Mar 2015-World Academy of Science, Engineering and Technology, International Journal of Materials and Metallurgical Engineering
TL;DR: In this article, a special heat treatment process named austempering on AISI 4340 steel is carried out to enhance mechanical properties such as hardness, tensile strength and impact strength.
Abstract: In this study a special heat treatment process named austempering on AISI 4340 steel is carried out. Heat treatment on steel is carried out to enhance mechanical properties. In this regard, it is considered essential to undertake a study to evaluate different changes occurred in AISI 4340 steel in terms of hardness, tensile strength and impact strength at different austempering temperatures and cooling times and achieving the best combination of these improved mechanical properties for better and optimum utilization of this grade of steel. By using software Design Expert DOE is formulated with Taguchi orthogonal arrays comprising of L18 (3*3) with 03 factors and 03 responses to be calculated. Results of experiments are analyzed via Taguchi method. Signal to noise ratio of responses are carried out to determine the significant factors among the 03 factors chosen for experimental runs. Overall analysis showed that impact factor along with hardness is improved to great extent by austempering process.
TL;DR: A hardness conversion table was deduced for irons investigated in this article, where the alloying elements were added to get as-cast low-alloyed ductile iron (LADI) followed by austempering heat treatment.
Abstract: 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.
TL;DR: In this paper, the authors analyzed the mechanical properties and microstructural characteristics of as-cast ductile iron austenitized at 900°C for 90min and afterward austempered over a range of temperatures to obtain distinctive microstructures.
Abstract: Austempered ductile iron is a heat treated form of as-cast ductile iron. The heat treatment process-austempering, was developed with the intent of improving the strength and toughness of ferrous alloys. It offers a range of mechanical properties superior to those of other cast iron, and shows excellent economic competitiveness with steels and aluminum alloys. The main aim is to analyze the mechanical properties and microstructural characteristics of as-cast ductile iron austenitized at 900 °C for 90 min and afterward austempered over a range of temperatures to obtain distinctive microstructures. The samples were austempered for durations of 60, 90 and 180 min at each austempering temperature of 340, 360, 380, and 400 °C. The influence of these austempering temperatures and times on the microstructure and tensile properties were investigated at room temperature.
TL;DR: In this article, the authors analyzed the feasibility of ADI feasibility to substitute the conventional ductile iron in automotive components manufacture, based on machinability criteria comparison of thrust force, torque and cutting power of two ADI grades (ISO 800-10 and ISO 1.050-6).
Abstract: The aim of this work is to analyze the austempered ductile iron (ADI) feasibility to substitute the conventional ductile iron in automotive components manufacture. This analysis was based on machinability criteria comparison of thrust force, torque and cutting power of two ADI grades (ISO 800-10 and ISO 1.050-6) with FE 70003 pearlitic ductile iron as-cast. Cutting speed, feed rate and cutting fluid application technique, besides the work material, were varied in the drilling tests. The tool used in the tests was the K20 cemented carbide twist drill coated with multilayers of TiN/TiAlN with diameter of 10 mm. The results showed that the ISO 800-10 ADI grade was the material with highest machinability followed by FE 70003 pearlitic ductile iron as-cast and then by ISO 1.050-6 ADI grade. It was concluded that the machinability of the materials studied is thereafter closely related to their microstructures. Therefore, ADIs obtained by DI partially austenitized have excellent potential to substitute ductile irons based on the machinability criteria evaluated.
TL;DR: In this article, 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 was presented.
Abstract: 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, wit...
Abstract: The aim of this study is to determine the susceptibility to hydrogen embrittlement in X37CrMoV5-1 steel with two different microstructures: a nanocrystalline carbide-free bainite and tempered martensite. The nanobainitic structure was obtained by austempering at the bainitic transformation zone. It was found, that after hydrogen charging, both kinds of microstructure exhibit increased yield strength and strong decrease in ductility. It has been however shown that the resistance to hydrogen embrittlement of X37CrMoV5-1 steel with nanobainitic structure is higher as compared to the tempered martensite. After hydrogen charging the ductility of austempered steel is slightly higher than in case of quenched and tempered (Q&T) steel. This effect was interpreted as a result of phase composition formed after different heat treatments.