Showing papers on "Austempering published in 1995"
TL;DR: In this article, the effect of changes in the bainite transformation conditions were investigated using two grades of Si-Mn TRIP steels, including one a containing Nb as a microalloy addition.
Abstract: Controlled rolling processes are designed to produce a desired microstructure via the control of hot rolling without subsequent heat treatment. Critical to the success of controlled rolling are the stages that occur after hot deformation, when the steel is cooled to room temperature. This can be divided into two parts: the run out table, where the material is allowed to cool relatively rapidly to a pre-determined temperature, and "coiling" at which point the rolled material is coiled, thus showing down the cooling rate considerably. Controlled rolling schedules generally finish by coiling the steel at temperatures below the bainite transformation start temperature (Bs). Any changes in coiling conditions (temperature and time) in this region can result in variations in bainite characteristics (morphology, size, carbide precipitation, etc.). This in turn, may affect the state of the retained austenite and, consequently, the mechanical properties of Si-Mn TRIP steels, which have bainite as the dominant microconstituent. The effect of changes in the bainite transformation conditions were investigated using two grades of Si-Mn TRIP steels, including one a containing Nb as a microalloy addition. The results reveal that the retained austenite volume fraction was strongly influenced by both bainite formation temperature and hold time, The highest values of total elongation (46 and 33%) and formability index (61180 and 40260 MPa·%) were observed for an intermediate hold time (5 min) and temperature (400°C), respectively. These findings are explained by considering the effect of the bainite transformation on the state of the retained austenite.
70 citations
TL;DR: In this article, microstructural observations and measurements of the retained austenite content, hardness, and unreacted Austenite C content are reported during austempering at 400, 375, 320, and 285°C after austenitising at 920°C.
Abstract: Microstructural observations and measurements of the retained austenite content, hardness, austenite C content, and unreacted austenite content are reported during austempering at 400, 375, 320, and 285°C after austenitising at 920°C for a ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0·007%P, 0·013%S, 0·25%Mo, 0·25%Cu, and 0·04%Mg. The segregation of solute during solidification to the intercellular areas is shown to result in the stage I reactions in the eutectic cell and intercellular areas being separated in austempering time. Evidence is provided of the occurrence of the stage II reaction before the completion of the stage I reaction. The consequence of this sequence of changes is that the processing window as defined for unalloyed irons and known to correspond to optimum mechanical properties is closed for all austempering temperatures.MST/3053
53 citations
TL;DR: In this article, an alloyed ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0.013%S, 0,25%Mo, 0 ·25%Cu, and 0·04%Mg was used for austenitising at 920°C for 120 min.
Abstract: Measurements of ultimate tensile strength, 0·2% proof stress, elongation, and impact energy are reported for an alloyed ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0·007%P, 0·013%S, 0·25%Mo, 0·25%Cu, and 0·04%Mg,for a range of austempering temperatures and times after austenitising at 920°C for 120 min. It is shown that the mechanical properties satisfy the high strength grades of the standard AST MA897 M:1990, but fail to satisfy the higher ductility grades because of poor ductility. This is attributed to overlapping of the stage I and II reactions and the occurrence of the transformation induced plasticity mechanism during deformation, particularly in irons austempered at higher temperatures.MST/3054
51 citations
TL;DR: In this paper, the fracture toughness of unalloyed ADI increases with increase in volume fraction of ferrite in the matrix and reaches a peak when the ferrite content of the matrix is around 65%.
Abstract: The influence of microstructure on the tensile properties and the plane strain fracture toughness (KIc) of unalloyed austempered ductile cast iron (ADI) was examined in room temperature ambient atmosphere. The crack growth mechanism during fracture toughness tests was determined through detailed fractographic studies. Compact tension and round cylindrical specimens were prepared from unalloyed ADI and were given four different austempering heat treatments to produce four different microstructures. Tensile properties and fracture toughness of these four differently heat-treated materials as well as cast materials were determined as per relevant ASTM standards. The results of the present investigation demonstrate that the fracture toughness of unalloyed ADI increases with increase in volume fraction of ferrite in the matrix and reaches a peak when the ferrite content of the matrix is around 65%. Both yield and ultimate tensile strength of the unalloyed ADI was found to increase with increase in volume fraction of ferrite in the matrix. The ductility of ADI, on the other hand, was found to increase with increase in volume fraction of austenite in the matrix. The crack growth mechanism was found to be predominantly by the microvoid coalescence. The crack path appears to connect the graphite nodules along the way.
40 citations
TL;DR: In this paper, X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austenitising kinetics of an alloyed ductile iron containing 0·67%Mn, 0·25%Mo, and 0· 25%Cu.
Abstract: X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austenitising kinetics of an alloyed ductile iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, during austempering at 285 and 375°C after austenitising at 870, 900, and 920°C. The austenitising kinetics show that 120 min is sufficient time to produce afully austenitic matrix. The stage I reaction during austempering occurs in two distinct steps: first in the eutectic cell and then in the intercellular areas. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have only a small effect on the stage II kinetics. Decreasing the austenitising temperature results in a more uniform austempered microstructure and reduces the amount of martensite in the structure. These changes shift the heat treatment processing window for high Mn irons to shorter timesfor austempering at 285°C and come close to, but do not open the processing window at 375°C.MST/3117
34 citations
TL;DR: In this paper, an attempt has been made to evaluate the machinability of Austrian ductile iron alloyed with nickel by calculating the Machinability index based on material removal rate and unit power consumed at various cutting speeds and feeds.
Abstract: Austempered ductile iron (ADI) has found enormous applications in recent years due to its high strength and hardness, coupled with substantial ductility and toughness. The high strength and hardness of ADI have caused many researchers and engineers to doubt the machinability of this material. Many investigations have adopted tool life, tool wear rate, cutting forces, and surface finish produced on a job as general criteria for evaluating the machinability of ADI. In the present investigation, an attempt has been made to evaluate the machinability of ADI alloyed with nickel by calculating the machinability index based on material removal rate and unit power consumed at various cutting speeds and feeds. The results thus obtained are presented in this paper.
34 citations
TL;DR: A series of austempered ductile iron samples were prepared at austempering temperatures of 370, 340, 300, and 270°C, and their sliding dry wear behaviour was studied under applied loads of 110,300, 500, and 800 N as mentioned in this paper.
Abstract: A series of austempered ductile iron samples were prepared at austempering temperatures of 370, 340, 300, and 270°C. Their sliding dry wear behaviour was studied under applied loads of 110, 300, 500, and 800 N. The results obtained showed that the wear resistance is independent of austempering temperature with low applied loads, but shows a strong dependence on austempering temperature at high applied loads (500 and 800 N), reaching a maximum value at a temperature of 340°C. Scanning electron microscopy observations show no evidence of transformation induced plasticity. Wear is explained as being due to subsurface fatigue, with cracks readily nucleated at plastically deformed graphite nodules, the wear controlling mechanism being the crack growth stage when wear shows a dependence on applied load and austempering temperature.MST/2062
34 citations
TL;DR: In this paper, the effects of Cu, Mo, Si contents on the volume fraction of retained austenite of austempered ductile iron (ADI) are analyzed exactly by X-ray diffraction, and the fracture modes of test samples with different volume fractions of retained Austenite are investigated by SEM.
Abstract: In this paper, the effects of Cu, Mo, Si contents on the volume fraction of retained austenite of austempered ductile iron (ADI) are analyzed exactly by X-ray diffraction, and the fracture modes of test samples with different volume fraction of retained austenite are investigated by SEM. It is shown that the retained austenite content increases with the content of copper, decreases with the content of molybdenum, and reaches the maximum with a certain content of silicon. When the retained austenite content decreases, the fracture modes of test samples change from ductile fracture to cleavage fracture.
31 citations
TL;DR: In this paper, the effect of alloy elements on the microstructure and property of ADI was investigated, and a PC-controlled vacuum heat treating system was used for the heat treatments.
Abstract: Ductile cast iron has already demonstrated excellent mechanical properties. If given proper austempering, it can exhibit even more outstanding characteristics. The process of austempering for ductile cast iron is similar to steel, and requires an adequate completely, and then rapidly quenching the austenitizing temperature allowing the matrix of ductile iron to be austenitized completely, and then rapidly quenching the austenitized ductile iron down to 300 C--400 C. Caution is required to prevent austenite from transforming into proeutectoid ferrite or pearlite. Finally, the ductile iron must be kept in an isothermal condition for a proper length of time. Many kinds of experimental techniques such as quantitative metallography, magnetic change, dilatometry, X-ray diffraction, electrical resistivity change etc., may be used to measure the phase transformation during the austempering of ductile irons. However, the method of measuring the change of electrical resistivity, not only provides continuous and complete data, but also the time to start and to finish for both stages of the reaction can be significantly determined. In this paper, the effect of alloy elements on the microstructure and property of ADI was investigated. First, the specimens containing Mn, Cu, Ni and Mo were made separately, then a PC-controlled vacuum heat treatingmore » system was used for the heat treatments.« less
30 citations
TL;DR: In this paper, a high Mn ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0.2% proof strength, impact energy, and elongation are defined.
Abstract: The mechanical properties obtainable in a high Mn ductile iron containing 3·52%C, 2·64%Si, 0·67%Mn, 0·007%P, 0·013%S, 0·25%Mo, 0·25%Cu, and 0·04%Mg are defined. Measurements of ultimate tensile strength, 0·2% proof strength, impact energy, and elongation are presented as afunction of austempering time in the range 1–4320 minfor austempering temperatures of 285 and 375°C and austenitising temperatures of 870, 900, and 920°C. The austempering processing window is shown to be openfor the three austenitising temperatures at an austempering temperature of 285°C, and the mechanical properties satisfy the austempered ductile iron (AD I) standard AST MA897 M:1990. Decreasing the austenitising temperature is shown to increase the closure temperature of the processing window, but for the lowest austenitising temperature the window is still closed for an austempering temperature of 375°C. The improvement in ductility obtained by using an austenitising temperature of 870°C is insufficient to satisfy the AD I ...
27 citations
TL;DR: In this paper, the fatigue limits of various spheroidal graphite cast irons were determined under completely reversed plane bending condition, and the relationships between the fatigue limit and microstructural factors such as graphite nodule diameter and matrix hardness were clarified.
Abstract: The fatigue limits of various spheroidal graphite cast irons were determined under completely reversed plane bending condition. The relationships between the fatigue limit and microstructural factors such as graphite nodule diameter and matrix hardness were clarified, and the prediction equations of the fatigue limit of the irons were determined. It is observed that the fatigue limit strongly depends on maximum nodule diameter presumed by using extreme statistics, because fatigue cracks initiate from larger graphite nodules just below the specimen surface prior to others. The fatigue limit can be significantly improved by the hardening matrix only in the specimens with a fine nodule size. Austempered cast iron has a higher fatigue limit than other irons although having the same matrix hardness. As a consequence of the quantitative analysis, the prediction equations of the fatigue limits of spheroidal graphite cast irons are given.
TL;DR: In this article, a stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron, which is illustrated for an iron containing 0.67%Mn, 0.25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, and a second step austempering temperature of 285°C.
Abstract: A stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron The method is illustratedfor an iron containing 0·67%Mn, 0·25%Mo, and 0·25%Cu, using an austenitising temperature of 920°C, afirst step austempering temperature of 400°C for 120 min, and a second step austempering temperature of 285°C The change in the microstructure and phase characteristics with time during the second austempering step are described Related changes in the mechanical properties compared with a single austempering treatment at 400°C are an increase in the ultimate tensile strength from 770 to 970 MN m−2, an increase in elongation from 2·5 to 7·5%, and an increase in the unnotched Charpy impact energy from 40 to 150 JMST/3119
Journal Article•
TL;DR: Still only in its adolescent years, austempered ductile iron presents a horizon of opportunities for foundries as mentioned in this paper, but it is difficult to find suitable foundries for it.
Abstract: Still only in its adolescent years, austempered ductile iron presents a horizon of opportunities for foundries.
TL;DR: In this article, the thermal stability of SG cast iron has been studied at high temperature, and it has been shown that the microstructures obtained by austempering at low (300°C and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures.
Abstract: Spheroidal graphite (SG) cast iron is often plasma nitrided for corrosion resistance, and plasma nitriding has been proposed as a surface engineering treatment to improve wear resistance. However, the microstructure of austempered SG iron comprises constituents that may be unstable at nitriding temperatures. Therefore, the thermal stability of austempered SG cast iron has been studied at high temperature. Differential scanning calorimetry shows that microstructures obtained by austempering at low (300°C) and intermediate (380°C) temperatures, and which contained retained austenite, underwent a large exothermic transition during heating to typical nitriding temperatures. The transition began at approximately 470°C and peaked at 510–520°C, and was due to the decomposition of retained austenite to ferrite and cementite. A microstructure obtained by austempering at a higher temperature (440°C), and which consisted entirely offirst and second stage bainite, was stable up to nitriding temperatures. Afte...
1 Dec 1995
TL;DR: In this article, the authors applied thermal infrared imagery to rotating bending test specimens of austempered ductile iron, an alloy whose fatigue limit is, due to the high scatter dispersion of the data points and the long testing period required, generally difficult to determine by the traditional technique.
Abstract: Previous work by the authors showed that the endurance limit of specimens, or mechanical components, can be predicted using thermal infrared imagery. The new technique enables the determination of the fatigue strength limit in a comparatively short period of time (few thousands cycles), and using very few specimens (theoretically only 1). The present work applies this technique to rotating-bending test specimens of austempered ductile iron, an alloy whose fatigue limit is, due to the high scatter dispersion of the data points and the long testing period required, generally difficult to determine by the traditional technique. This material exhibited higher fatigue strength than the familiar nodular cast iron. This was confirmed by the results derived from the traditional Wohler test and the new technique, and supported by the data gathered from literature.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
TL;DR: In this paper, the transformation of a ductile iron at 410 °C for different times, after austenitization for 30 minutes at 900 °C, is analyzed in detail, and a certain amount of martensite is observed after quenching not only for short transformation times but also for intermediate times.
Abstract: The transformation of a ductile iron at 410 °C for different times, after austenitization for 30 minutes at 900 °C, is analyzed in detail. Upper bainite and a high volume fraction of austenite are formed for intermediate annealing times. A certain amount of martensite is observed after quenching not only for short transformation times but also for intermediate times. The formation of the martensite on cooling after intermediate transformation times is due to the decrease in carbon concentration of the retained austenite because of the homogeneous precipitation of epsilon carbides within. This homogeneous precipitation of epsilon carbide inside austenite is unambiguously observed. The epsilon carbide, pre-precipitated in austenite, which transforms to martensite on cooling, continues growing in the martensite after transformation. For long times of austempering at 410 °C, some complex large carbides or silicocarbides are formed, probably from the epsilon carbide, which result in the total decomposition of austenite.
01 Apr 1995-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, transmission electron microscopy (TEM) energy-dispersive X-ray analysis (EDXA) was used to study some aspects of bainitic reaction in high Mn austempered ductile iron with reference to carbide precipitation in Bainitic ferrite.
Abstract: A 1 wt.% Mn ductile iron austenitized at 900 °C for 90 min and austempered at 375 °C for different periods was used to study some aspects of bainitic reaction in high Mn austempered ductile iron with reference to carbide precipitation in bainitic ferrite. Transmission electron microscopy (TEM) energy-dispersive X-ray analysis (EDXA) study shows that precipitation of carbide in the ferritic component of bainite is a function of the local concentration of alloying elements. In other words, in the region near graphite where si segregates and there is negative Mn segregation as well as carbon, the bainitic ferrite is carbide free. However, in the intercellular region where Mn segregates and Si is depleted, the ferritic component of bainite occurs together with very fine and almost uniformly distributed carbide. Furthermore, TEM-EDXA results show that the increase in Mn content not only delays stage I (the initial transformation of austenite to ferrite and retained austenite) of the bainitic reaction, but also delays stage II (decomposition of retained austenite to ferrite and carbide).
TL;DR: In this paper, the effect of a modified austemper on the tensile properties of 0·52%C steel has been studied for the purpose of developing the mechanical properties of upper bainitic steel.
Abstract: The effect of a modified austemper on the tensile properties of 0·52%C steel has been studiedfor the purpose of developing the mechanical properties of upper bainitic steel. The modified austempering treatment involved intercritical annealing at 1018 K in the two phase region offerrite (α) and austenite (γ) followed by austempering at 673 K and subsequent water cooling. The results have been compared with those obtained from conventionally austempered steel, and quenched and tempered steel with a similar ultimate tensile stress. The modified austempered steel consisted of a mixed structure of upper bainite and 10 vol.-% ferrite in which ferrite appeared as layers along the rolling direction. The modified austempering treatment wasfound to significantly increase the product of ultimate tensile stress and total elongation, and also the notch tensile stress at 193 K. Conventional austenitising at 1173 K followed by subcritical annealing at 998 K in the two phase region of ex and y, and then austemper...
Patent•
20 Mar 1995
TL;DR: In this paper, a method for producing a brake component involves providing a cast gray iron rotatable brake component where the gray iron has a carbon content between 3.4% and 4.0%.
Abstract: A method for producing a brake component involves providing a cast gray iron rotatable brake component where the gray iron has a carbon content between 3.4% and 4.0%. The brake component is subjected to an austempering heat treatment process. Then it is subjected to a re-tempering process to provide a microstructure which consists of spheroidized pearlite carbon in a matrix of bainitic and austenitic ferrite.
TL;DR: In this article, the tensile characteristics of ADI with upper bainitic structure were studied by varying the austenitization temperature (1173 and 1223 K) and austempering temperature (643 and 693 K) of a spheroidal graphite cast iron which contains 3.6 mass% C and 2.4 mass% Si.
Abstract: The 300 to 693 K tensile characteristics of ADI with upper bainitic structure were studied by varying the austenitization temperature (1173 and 1223 K) and austempering temperature (643 and 693 K) of a spheroidal graphite cast iron which contains 3.6 mass% C and 2.4 mass% Si. The experimental results show that the flow stress decreases with increasing austenitization and austempering temperatures. The tensile characteristics can be divided into two temperature regimes. In the lower temperature regime below about 550 K, the presence of acicular ferrite should be the main factor which leads to the common features of dynamic strain aging including strengthening, elongation drop and flow instability. The flow instability, which occurs in the temperature range of about 373 to 493 K, is more distinct with coarser microstructure. The absence of flow instability for the specimens with the finest microstructure is probably caused by its nature of high dislocation density. In the higher temperature regime above 550 K where the flow stress decreases with increasing tensile temperature, the elongation starts to drop with temperature from about 600 K. The elongation drop, which should be due to the formation of quasi-continuous carbide particles from austenite during tensile deformation, can sometimes give rise to the appearance of quasi-cleavage fracture.
TL;DR: In this article, heat treatments were used to obtain upper and lower bainitic compacted graphite cast irons, and the effects of these heat treatments on the mechanical properties, fracture toughness, and fatigue crack growth rate of the irons were evaluated.
Abstract: In the present paper, heat treatments were used to obtain upper and lower bainitic compacted graphite cast irons. The effects of the percentage of compacted graphite, the matrix structure, and the specimen thickness on the mechanical properties, fracture toughness, and fatigue crack growth rate of the irons were evaluated. Optical microscopy, scanning electron microscopy, and X-ray diffraction were used to correlate the microstructural features with the observed properties. It was found that austempering not only greatly increases the strength of the compacted graphite irons, it also enhances the plane strain fracture toughness values by ~15–60% of that of the as cast (ferritic–pearlitic) material. The crack growth rate was also reduced by more than 30%. Plane stress fracture toughness values were found to increase with decreasing specimen thickness for all matrixes and percentages of compacted graphite.MST/1982
Patent•
10 Feb 1995
TL;DR: In this article, the authors presented a method to produce high proof stress austempered spheroidal graphite cast iron increased in yield ratio without accompanying deterioration in elongation by subjecting the cast iron to austempering treatment, executing rapid cooling, furthermore applying it with specified strains and thereafter executing specified reheating treatment.
Abstract: PURPOSE:To produce high proof stress austempered spheroidal graphite cast iron increased in yield ratio without accompanying deterioration in elongation by subjecting spheroidal graphite cast iron to austempering treatment, executing rapid cooling, furthermore applying it with specified strains and thereafter executing specified reheating treatment. CONSTITUTION:Spheroidal graphite cast iron, e. g. constituted of, by weight, 3.5 to 4.2% C, 2.0 to 3.3% Si, <=0.8%. Mn, 0.020 to 0.060% Mg, <=0.1% P and about <=0.02% S, and the balance Fe with inevitable impurities is heated to an austenitic region, is thereafter rapidly cooled to an austempering treatment temp. and is held. The spheroidal graphite cast iron subjected to the austempering treatment is rapidly cooled by water cooling treatment or subzero treatment and is thereafter applied with strains of 0.2 to 3.0%. After that, this spheroidal graphite cast iron is reheated to 250 to 560 deg.C (523 to 833K) for 5sec to 5min.
TL;DR: In this article, the effect of overstressing in rotary bending fatigue on the fatigue properties of an annealed and austempered ductile iron containing 1.5 Ni-0.3 Mo was investigated.
Abstract: Studies have been conducted on the effect of overstressing in rotary bending fatigue on the fatigue properties of an annealed and austempered ductile iron containing 1.5 Ni-0.3 Mo. For various R ratios S-N curves were determined and the fatigue limit estimated. It was found that the fatigue limit was a function of the level of overstressing and cycle ratio. In the case of austempered samples a beneficial effect of overstressing was observed at a certain level of overstressing. This was related to the work hardening behaviour of the austenite phase. In annealed samples, a reduction in the fatigue limit was observed at all levels of overstressing.
Patent•
27 Jun 1995
TL;DR: In this paper, a cutter is made of spherical graphite cast iron by casting, and its Rockwell hardness (HRC) is higher than or equal to 50, and the product is ground and finished to be a manufactured article.
Abstract: PURPOSE:To reduce production man-days, form easily a cutter of a complex shape, and reduce costs. CONSTITUTION:A cutter is made of spherical graphite cast iron by casting, and its Rockwell hardness (HRC) is higher than or equal to 50. To produce it, a primary product made of the spherical graphite cast iron is formed in a casting process (A). In the graphite application, (1), of a heating process (B), graphite dissolved in starch is applied to the primary product. In heating, (2), the primary product is heated to 800 deg.C-950 deg.C to be transformed to austenite. In primary cooling, (3), this primary product is put in a bath of tin heated to 250 deg.C-300 deg.C, to be subjected to so-called austempering treatment. In secondary cooling, (4), the primary product is quenched in water, so that martensite is formed and the Rockwell hardness (HRC) is increased to 50 or higher. Thereafter, in a grinding process (C), the product is ground and finished to be a manufactured article.
TL;DR: In this paper, the influence of casting defects on the fatigue strength was examined through fractography and it was found that the fatigue crack initiated at the micro porosity and the fatigue life was mutually related to a presented parameter Ka=2S √(πa), where S is the stress amplitude and as the dimension of micro defect initiating a fatigue crack.
Abstract: In order to estimate fatigue reliability of austempered ductile iron (ADI) with austempered ferrite-retained austenite dual structures, the influence of casting defects on the fatigue strength was examined through fractography. Fatigue testing as conducted under tension-tension loading, R=0. 05. It was found through fractography that the fatigue crack initiated at the micro porosity. This, the fatigue life, N, was mutually related to a presented parameter Ka=2S √(πa), where S is the stress amplitude and as the dimension of micro defect initiating a fatigue crack. By comparing ADI with spheroidal graphite iron of full pearlite matrix structure in using Ka-N diagrams, it could be understood that austempered Ferrite-retained Austenite dual structures had higher resistance against the meso scopic crack initiating at the micro porosity than the full pearlite matrix structure.
TL;DR: In this paper, a computer-controlled system for measuring electrical resistance has been developed and used to study the isothermal transformation of austenite in a ductile iron (3.31 % C, 3.12 % Si, 0.22 % Mn, and 0.55 % Cu).
Abstract: A computer-controlled system for measuring electrical resistance has been developed and used to study the isothermal transformation of austenite in a ductile iron (3.31 % C, 3.12 % Si, 0.22 % Mn, 0.55 % Cu). The ability of the technique to follow the isothermal decomposition of austenite was established by measurements on an AISI4340 steel. The times at which the austenite decomposed to primary ferrite, pearlite, and bainite were accurately detected. In the ductile iron, the formation of pearlite and of bainite was easily detected, and an isothermal transformation diagram was constructed from the results. The temperature range for the formation of bainite is especially important in producing austempered ductile iron (ADI) and was mapped. An initial stage of decomposition of austenite to ferrite and high-carbon austenite is followed by a time delay; then the high-carbon austenite decomposes to bainite. The formation of ADI requires austempering to a structure of ferrite and high-carbon austenite, then quenching to retain this structure, thus avoiding the formation of bainite. This is achieved by isothermal transformation into the time-delay region. For the ductile iron studied here, this time region was about 2.6 h at 400 °C and increased to 277 h at 300 °C.
1 Jan 1995
TL;DR: Carbide formation in ADI alloyed with Ni and Cu has been studied by use of TEM as discussed by the authors, and the results show that η carbide precipitates in the bainitic ferrite when austempered at 350℃.
Abstract: Carbide formation in austempered ductile iron (ADI) alloyed with Ni and Cu has been studied by use of TEM. The results show that η carbide precipitates in the bainitic ferrite when austempered at 350℃.for 6h, but no carbide was found at the austenite/.ferrite interfaces. When austempered at 300℃, two kinds of. s carbides appear in the bainitic ferrite when austempering time reached I h, and X carbide also precipitates at the austenite /ferrite interfaces when austempering time was extended up to 6h. The bainitic. ferrite and the retained austenite follow the Nishiyama- Wasserman relationship,
TL;DR: Very high cycle fatigue tests were conducted on austempered ductile iron finished by buffing and that finished by electropolishing (called material EA) for comparison of what please clarity as discussed by the authors.
Abstract: Very high cycle fatigue tests were conducted on austempered ductile iron finished by buffing (called material A) and that finished by electropolishing (called material EA) for comparison of what please clarity. An S-N curve of rotating bending fatigue of material A represented a characteristic of a two-stage bending resembling extremely high-strength materials. This phenomenon clarified the occurrence of different fracture mechanisms through the intermediary of nonpropagating cracks. In material EA, the structural sensitivity was lower owing to the removal of the work hardening layer on the material surface. Consequently, its S-N curve of rotating bending fatigue was unexceptional, because mechanical properties of its matrix structure resemble those of its pearlitic structure. Fatigue stress amplitudes of both materials were σw = 382 MPa.