Effect of austenitization on austempering of copper alloyed ductile iron

A ductile iron containing 0.6% copper as the main alloying element was austempered at a fixed austempering temperature of 330 °C for a fixed austempering time of 60 min after austenitization at 850 °C for different austenitization periods of 60, 90, and 120 min. The austempering process was repeated after changing austenitization temperature to 900 °C. The effect of austenitization temperature and time was studied on the carbon content and its distribution in the austenite after austenitization. The effect of austenitization parameters was also studied on austempered microstructure, structural parameters like volume fraction of austenite, X _γ , carbon content C _γ , and X _γ C _γ , and bainitic ferrite needle size, d _α after austempering. The average carbon content of austenite increases linearly with austenitization time and reaches a saturation level. Higher austenitization temperature results in higher carbon content of austenite. As regards the austempered structure, the lowering austenitization temperature causes significant refinement and more uniform distribution of austempered structure, and a decrease in the volume fraction of retained austenite.     

Tensile properties of copper alloyed austempered ductile iron: Effect of austempering parameters

A ductile iron containing 0.6% copper as the main alloying element was austenitized at 850 °C for 120 min and was subsequently austempered for 60 min at austempering temperatures of 270, 330, and 380 °C. The samples were also austempered at 330 °C for austempering times of 30–150 min. The structural parameters for the austempered alloy austenite (X _γ ), average carbon content (C _γ ), the product X _γ C _γ , and the size of the bainitic ferrite needle (d _α ) were determined using x-ray diffraction. The effect of austempering temperature and time has been studied with respect to tensile properties such as 0.2% proof stress, ultimate tensile strength (UTS), percentage of elongation, and quality index. These properties have been correlated with the structural parameters of the austempered ductile iron microstructure. Fracture studies have been carried out on the tensile fracture surfaces of the austempered ductile iron (ADI).     

Austempering and austempered ductile iron microstructure in copper alloyed ductile iron

The variation in the austempered microstructure, the volume fraction of retained austenite, X_λ, the average carbon content of retained austenite, C_λ, their product X_λC_λ and the size of bainitic ferrite needles with austempering temperature for 0.6% Cu alloyed ductile iron have been investigated for three austempering temperatures of 270, 330, and 380 °C for 60 min at each temperature after austenitization at 850 °C for 120 min. The austempering temperature not only affects the morphology of bainitic ferrite but also that of retained austenite. There is an increase in the amount of retained austenite, its carbon content, and size of bainitic ferrite needles with the rise in austempering temperature. The influence of austempering time on the structure has been studied on the samples austempered at 330 °C. The increase in the austempering time increases the amount of retained austenite and its carbon content, which ultimately reaches a plateau.     

CARBIDE FORMATION IN AUSTEMPERED DUCTILE IRON ALLOYED WITH NICKEL AND COPPER

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Effect of austempering time on mechanical properties of a low manganese austempered ductile iron

An investigation was carried out to examine the influence of austempering time on the resultant microstructure and the room-temperature mechanical properties of an unalloyed and low manganese ductile cast iron with initially ferritic as-cast structure. The effect of austempering time on the plane strain fracture toughness of this material was also studied. Compact tension and round cylindrical tensile specimens were prepared from unalloyed ductile cast iron with low manganese content and with a ferritic as-cast (solidified) structure. These specimens were then austempered in the upper (371 °C) and lower (260 °C) bainitic temperature ranges for different time periods, ranging from 30 min. to 4 h. Microstructural features such as type of bainite and the volume fraction of ferrite and austenite and its carbon content were evaluated by X-ray diffraction to examine the influence of microstructure on the mechanical properties and fracture toughness of this material. The results of the present investigation indicate that for this low manganese austempered ductile iron (ADI), upper ausferritic microstructures exhibit higher fracture toughness than lower ausferritic microstructures. Yield and tensile strength of the material was found to increase with an increase in austempering time in a lower bainitic temperature range, whereas in the upper bainitic temperature range, time has no significant effect on the mechanical properties. A retained austenite content between 30 to 35% was found to provide optimum fracture toughness. Fracture toughness was found to increase with the parameter (XγCγ/d)^1/2, where Xγ is the volume fraction of austenite, Cγ is the carbon content of the austenite, and d is the mean free path of dislocation motion in ferrite.     

Microstructure and cavitation behaviour of alloyed austempered ductile irons

In this paper, the study of cavitation behaviour of austempered ductile iron (ADI) alloyed with copper, as well as copper and nickel with a fully ausferritic microstructure, is presented. The ADI materials used were austenitized at 900 °C and austempered at 350 °C having an ausferrite microstructure with 16 and 19% of austenite, respectively. The experimental investigations were conducted using the ultrasonically induced cavitation test method. The results show that the cavitation damage was initiated at graphite nodules, as well as in the interface between a graphite nodule and an ausferrite matrix. The cavitation rate revealed that the ADI material alloyed with Cu + Ni austempered at 350 °C/3 h has a higher cavitation resistance in water than ADI alloyed with Cu. An increased cavitation resistance of the ADI material alloyed with Cu and Ni is due to the matrix hardening by stress assisted phase transformation of austenite into the martensite (SATRAM) phenomenon.     

Effect of C and Si on mechanical properties of austempered ductile iron

It is well known that ductile cast iron can be strengthened and toughened by austempering. The tensile strength and the fatigue strength of austempered ductile iron (ADI) are equal to those of forged steel. Previous studies have been aimed at establishing a suitable process to obtain both strength and toughness in ADI.^1,2 These studies focused on the effect of alloying such as Mo, Ni, Mn, Cu, etc. and the austempering conditions such as temperature and holding time. In this study, a new type of ADI with higher toughness and higher elongation was developed as compared with conventional ADI. A new type of ADI with a low carbon content was achieved by reducing the initial carbon content, long annealing and ordinary austempering. The suitable silicon content was found to be 2.5% and effective alloying was 0.25% Mo and 0.7% Cu to obtain maximum impact energy and elongation.     

Effect of austenitizing conditions on the impact properties of an alloyed austempered ductile iron of initially ferritic matrix structure

The effect of austenitizing conditions on the microstructure and impact properties of an austempered ductile iron (ADI) containing 1.6% Cu and 1.6% Ni as the main alloying elements was investigated. Impact tests were carried out on samples of initially ferritic matrix structure and which had been first austenitized at 850,900, 950, and 1000°C for 15 to 360 min and austempered at 360°C for 180 min. Results showed that the austenitizing temperature, T_γ, and time, t_γ, have a significant effect on the impact properties of the alloy. This has been attributed to the influence of these variables on the carbon kinetics. The impact energy is generally high after short t_γ, and it falls with further soaking. In samples austenitized at 850 and 900°C, these trends correspond to the gradual disappearance of the pro-eutectoid ferrite and the attainment of fully developed ausferritic structures. In initially ferritic structures, the carbon diffusion distances involved during austenitization are large compared to those in pearlitic structures. This explains the relatively long soaking periods required to attain fully ausferritic structures, which in spite of the lower impact energy values, have a better combination of mechanical properties. Microstructures of samples austenitized at 950 and 1000°C contain no pro-eutectoid ferrite. The impact properties of the former structures are independent of t_γ, while those solution treated at 1000°C are generally low and show wide variation over the range of soaking time investigated. For fully ausferritic structures, impact properties fall with an increase in T_γ. This is particularly evident at 1000°C. As the T_γ increases, the amount of carbon dissolved in the original austenite increases. This slows down the rate of austenite transformation and results in coarser structures with lower mechanical properties. Optimum impact properties are obtained following austenitizing between 900 and 950°C for 120 to 180 min.     

Effect of Cu content on microstructures and mechanical properties of ADI treated by twostep austempering process

The effect of Cu content on the microstructures and mechanical properties(yield strength,ultimate tensile strength,impact energy,fracture toughness) of austempering ductile iron(ADI) treated by two-step austempering process were investigated. High Cu content in nodular cast irons leads to a significant volume fraction of retained austenite in the iron after austempering treatment,but the carbon content of austenite decreases with the increasing of Cu content. Moreover,austenitic stability reaches its maximum when the Cu content is 1.4% and then drops rapidly with further increase of Cu. The ultimate tensile strength and yield strength of the ADI firstly increases and then decreases with increasing the Cu content. The elongation keeps constant at 6.5% as the Cu content increases from 0.2% to 1.4%,and then increases rapidly to 10.0% with further increase Cu content to 2.0%. Impact toughness is enhanced with Cu increasing at first,and reaches a maximum 122.9 J at 1.4% Cu,then decreases with the further increase of Cu. The fracture toughness of ADI shows a constant increase with the increase of Cu content. The influencing mechanism of Cu on austempered ductile iron(ADI) can be classified into two aspects. On the one hand,Cu dissolves into the matrix and functions as solid solution strengthening. On the other hand,Cu reduces solubility of C in austenite and contributes more stable retained austenite.     

Experimental study of the thermal stability of austempered ductile irons

A nonisothermal annealing was applied to austempered Ni-Cu-Mo alloyed and unalloyed ductile irons to determine the thermal stability of the ausferritic structure. Differential thermal analysis (DTA) results were used to build the corresponding stability diagrams. The transformation starting temperature of the high carbon austenite was found to be strongly dependent on the austempering temperature, the heating rate, and the chemical composition of the iron. The Ni-Cu-Mo alloying elements and high austempering temperature increased the stability. The transformation of the austenite to ferrite and cementite is achieved via the precipitation of transition carbides identified as silico-carbides of triclinic structure.     

Corrosion behavior of nickel alloyed and austempered ductile irons in 3.5% sodium chloride

In this study, the nickel alloying and austempering effects on corrosion behavior of ductile irons were investigated. The microstructure of austempered ductile iron (ADI) was analyzed by XRD, and the polarization corrosion tests were conducted using 3.5 wt.% NaCl solution. The results showed Ni-alloyed as-cast has less nodule counts than the unalloyed one; therefore, the former is more corrosion resistant than the latter. For the ADI, the nickel addition increases the retained austenite content, resulting in having better corrosion inhibition than the unalloyed ADI. Comparatively, the order of corrosion resistance in 3.5 wt.% NaCl solution is as follows: 4%Ni-ADI > ADI > 4%Ni-DI > DI.     

The effect of segregation on the austemper transformation and toughness of ductile irons

The effect of segregation of alloying elements on the phase transformation of ductile iron during austempering was investigated. Four heats, each containing 0.4%Mn, 1% Cu, 1.5% Ni, or 0.4% Mo (wt%) separately, were melted; then three different sizes of casting bars (3,15, and 75 mm diameter) were poured from each heat. The distribution and the degree of segregation of certain elements were quantitatively analyzed using an electron microprobe. A personal computer (PC)-controlled heat treating system was used to measure electrical resistivity, and the information on resistivity variations was used to analyze the effect of segregation on phase transformations during austempering. Also, Charpy impact and Rockwell hardness tests were performed to determine the effect of segregation on properties. Results of the electron microprobe analysis showed that the degree of segregation of alloy elements increases with an increase in diameter of the casting bars (i.e., an increase of solidification time of castings). The degree of segregation of alloy elements, represented by segregation ratio (SR) (the maximum concentration of element in cell divided by the minimum concentration of element in cell), varied linearly with the casting modulus (M) (volume of casting divided by surface area of casting). Regarding the segregating tendency among alloy elements, positive segregating elements Mn and Mo showed more segregation than the negative segregating elements Si, Cu, and Ni. In addition, segregation of Mo was more significant than Mn, and that for Cu was greater than Ni and Si. Between the time of finishing the first stage and beginning the second stage of bainite reaction in ductile irons, there is a significant “processing window,” At;, for austempering to obtain optimum mechanical properties. From the electrical resistivity data, it was observed that the austempering temperature plays a major role in the processing window. There was a narrow window at 400 ‡C but a larger one at 350 ‡C. Additionally, the microsegregation of alloying elements led to variation of the time of phase transformation for various regions in the grain cells of ductile iron which caused the processing window to decrease. The span of the processing window decreased with an increase in degree of segregation. There was no significant difference in the hardness of the alloys in various diameter specimens. However, the impact toughness was significantly affected by the segregation. The impact values in 15 mm specimens with less degree of segregation were greater than those in 75 mm specimens with significant segregation. The Ni, Cu, and Mn alloys that were austempered to complete the first stage of bainite formation had approximately the same impact values for all diameter samples. The Mo alloy upon austempering produced no bainite, but it had much untransformed retained austenite in the intercellular regions and, therefore, had lower impact values.     

Control of austenitic transformation in ductile iron aided by calculation of Fe-C-Si-X phase boundaries

To control austenite transformation of ductile iron, thermodynamics procedures were used to calculate the Ae₃, the Gr/γ (A_cm), and the A₁ phase boundaries of high Mn and Ni-Cu-Mn alloyed iron as a function of austenitization temperature. The results of calculation show that segregation of Mn in the intergraphite regions reduces the carbon content of austenite at the Ae₃ phase boundary to the lowest value. If one ignores the effect of substitutional alloying elements on the nucleation of austenite, the austenite should first nucleate in the cell boundaries and then grow to the graphite nodules. In addition, the calculated results show that the A₁ temperature is the lowest in the intercellular region of a high Mn alloy. Therefore, if the austenitization temperature is not sufficiently high, only those parts of the matrix that have the A₁ phase boundary below the austenitization temperature transform to austenite, and dual formation of the α and γ phases will occur. By using the procedure introduced in this study, the volume fraction of each phase can be evaluated by calculating the A₁ phase boundary as a function of intergraphite distance. In the case of Ni-Cu-Mn alloy, Ni stabilizes austenite, which lowers the Ae₃ phase boundary. In this alloy, carbon content of austenite at the Ae₃ phase boundary is lower near the graphite nodule and higher in the intergraphite regions. However, the variation of carbon content of austenite at the Ae₃ phase boundary in the matrix of this alloy is much lower than in the high Mn alloy.     

Identification of Mg<Subscript>2</Subscript>Cu particles in Cu-alloyed austempered ductile iron

In the present work, the Mg₂Cu precipitates in copper-alloyed austempered ductile iron (ADI) were identified by analyzing techniques such as TEM and SEM with EDS. It was revealed that, in castings made of ADI-containing copper, highly dispersed particles of Mg₂Cu are formed, whose size does not exceed <1 μm. The research work was carried out on ductile iron that was austenitized at 900 °C, followed by austempering at 380 °C. The microstructure was investigated using various techniques, including optical microscopy, XRD, SEM, and TEM. In addition to this, the exhibited impact properties of castings with Cu, Ni, and Cu+Ni were also determined. This study casts a new light on the formation of the structure of Cu-alloyed ADI. The highly-dispersive and brittle Mg₂Cu particles that are located in the vicinity of the graphite nodules have a negative effect on the impact properties of ADI. It has also been shown that impact strength decreases from levels of 160-180 J (for copper-free ADI) to 90-120 J (for copper-and copper-nickel-alloyed ADI).     

等温处理工艺对等温淬火球铁显微组织和硬度的影响

上世纪70年代,通过奥氏体等温淬火开发出抗拉强度大于1000MPa、伸长率大于15％的高强度、高韧性等温淬火球铁。利用正交试验法,研究了等温淬火工艺参数对等温淬火球铁显微组织及硬度的影响。结果发现,在设计的试验工艺内全部可以得到以针状铁素体和富碳奥氏体为基体的等温淬火球铁组织;在等温淬火工艺中,等温淬火温度对试样硬度影响最为显著,其次是奥氏体化温度与奥氏体化时间,而等温淬火时间对于试样硬度的影响最小。     

The Strain-Hardening Behavior of Partially Austenitized and the Austempered Ductile Irons with Dual Matrix Structures

In the current study, an unalloyed ductile iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.%, and 0.047 Mg wt.% was intercritically austenitized (partially austenitized) in two-phase regions (α + γ) at different temperatures for 20 min and then was quenched into salt bath held at austempering temperature of 365 °C for various times to obtain different ausferrite plus proeutectoid ferrite volume fractions. Fine and coarse dual matrix structures (DMS) were obtained from two different starting conditions. Some specimens were also conventionally austempered from 900 °C for comparison. The results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high-carbon austenite (retained or stabilized austenite)) has been developed. Both of the specimens with ∼75% ausferrite volume fraction (coarse structure) and the specimen with ∼82% ausferrite volume fraction (fine structure) exhibited the best combination of high strength and ductility compared to the pearlitic grades, but their ductility is slightly lower than the ferritic grades. These materials also satisfy the requirements for the strength of the quenched and tempered grades and their ductility is superior to this grade. The correlation between the strain-hardening rates of the various austempered ductile iron (ADI) with DMS and conventionally heat-treated ADI microstructures as a function of strain was conducted by inspection of the respective tensile curves. For this purpose, the Crussard-Jaoul (C-J) analysis was employed. The test results also indicate that strain-hardening behavior of ADI with dual matrix is influenced by the variations in the volume fractions of the phases, and their morphologies, the degree of ausferrite connectivity and the interaction intensities between the carbon atoms and the dislocations in the matrix. The ADI with DMS generally exhibited low strain-hardening rates compared to the conventionally ADI.     

镍、铜低合金奥贝球铁特性研究

阐明了含Ni、Cu质量分数分别为1.5%和0.8%的奥贝球铁的凝固/相变和性能特点。测定了凝固过程的微分热分析曲线、膨胀仪曲线、绘制了奥氏体连续冷却较变曲线和等温转变动力学曲线，对比分析了该成分铸铁的淬透性。测定了Ni、Cu合金化奥贝球铁的常温力学性能。     

Influence of alloying elements Cu,Ni and Mo on mechanical properties and austemperability of austempered ductile iron

Abstract

Austempered ductile iron (ADI) is the most recent development in the nodular iron family. The austempering treatment produces a unique microstructure, ausferrite, which provides high mechanical strength combined with ductility, toughness, and good fatigue and wear resistance. The effect of alloying elements Cu, Ni and Mo on the mechanical properties and austemperability of ADI is reported. The mechanical strength and toughness decreased with the addition of Mo, but both wear resistance and austemperability increased with Mo content.     

Effect of Retained Austenite on Properties of Austempered Ductile Iron

Austempered ductile iron (ADI) exhibits a favourable combination of strength and toughness, and has been used as a substitute for quench-tempered or carburise-quenched steel. A characteristic feature of bainite transformation of cast iron, as opposed to carbon steel, is that precipitation of carbide is suppressed by the high concentration of silicon. Thus, a favourable structure, consisting of bainitic ferrite and retained austenite without carbide, can be provided by the optimum austempering treatment. Such microstructure and the mechanical properties of the iron are significantly affected by the conditions of the austempering treatment and the chemical composition. In this study, several grades of ductile iron were austempered under various conditions. The relationship between the impact strength, the quantity of retained austenite and the isothermal transformation curve was investigated. The stability of the retained austenite is considered important, because ADI contains a large amount of retained austenite which contributes to the improvement of ductility and toughness and which may transform to martensite when held at low temperature or subjected to stress. In this study, the stability of the retained austenite at low temperatures was examined by holding or stressing to establish the relations between transformation and temperature, stress and strain.

When the austempering time is short, the untransformed austenite partially transforms to martensite during air cooling, due to the lower carbon content, resulting in lower impact strength. As the austempering time increases, the untransformed austenite is stabilised by carbon-enrichment and there is little transformation to martensite, resulting in a large amount of retained austenite and higher impact strength. When the austempering time becomes much longer, the carbon-enriched austenite decomposes, presumably to bainitic ferrite and carbide, decreasing impact strength. In increasing the silicon content, precipitation of carbide in bainite is suppressed and both the maximum impact value and the content of retained austenite increase. The decreasing rates after the maxima through an additional isothermal holding becomes smaller.

By holding at temperatures down to –40°C, the decrease in retained austenite and the increase in hardness are both small. The retained austenite is stable under stress lower than that required to cause plastic deformation. Compressive stress hinders the martensitic transformation, because the transformation is accompanied by volume expansion.     

Enhancement of Fatigue Properties of Ductile Irons by Successive Austempering Heat Treatment

The aim of this study is to evaluate the effects of austempering heat treatment on the microstructure, mechanical properties, and bending fatigue behavior of an alloyed ductile iron with chemical composition of 1.6 wt.% Ni, 0.47 wt.% manganese and 0.6 wt.% copper. Based on the results of tensile and impact tests, as well as metallographic studies, optimum heat-treating cycles were determined and applied on the standard fatigue specimens. The results showed that the fatigue strength of specimens austempered successively was practically comparable to those austempered at high temperatures and considerably greater than those austempered at low temperatures.