A transmission electron microscope study of 1 Pct Mn ductile iron with different austempering treatments

A transmission electron microscope (TEM) equipped with an energy dispersive spectroscopy (EDS) system was used to study the bainitic reaction in a conventional and a successive austempering process for 1 wt pct Mn ductile iron. In the case of conventional austempering, the specimens were full austenitized at 900 °C and then austempered at 375 °C (high austempering temperature) and 315 °C (low austempering temperature) for different periods. In the case of the successive austempering process, following austempering at 375 °C for different periods, specimens were austempered at 315 °C, and subsequently quenched in ice water. The TEM-EDS study showed that carbide precipitation in the ferritic and retained austenitic component of bainite is a function of the local concentrations of the alloying elements, austempering time, and temperature. After a short time at high austempering temperature, carbide-free bainite forms near graphite nodules. Longer austempering time or lower austempering temperature encourages carbide precipitation in the bainitic ferrite. A long austempering time at high temperature leads to decomposition of retained austenite to ferrite and carbide. A rough inspection shows that the precipitated carbides in the ferritic component of specimens austempered at low temperature lie at an angle of about 40 to 50 deg to the sheaf axis.     

Fracture Characteristics of Austempered Spheroidal Graphite Aluminum Cast Irons

The fracture characteristics of austempered spheroidal graphite aluminum cast iron had been investigated. The chemical content of the alloy was C 3.2, Al 2.2, Ni 0.8 and Mg 0.05 (in mass percent, %). Impact test samples were produced from keel blocks cast in CO2 molding process. The oversized impact samples were austenitized at 850 and 950 ℃ for 2 h followed by austempering at 300 and 400 ℃ for 30, 60, 120 and 180 min. The austempered samples were machined and tested at room temperature. The impact strength values for those samples austempered at 400 ℃ varied between 90 and 110 J. Lower bainitic structures showed impact strength values of 22 to 50 J. The fractures of the samples were examined using SEM. The results showed that the upper bainitic fracture revealed a honey Comb-like topography, which confirmed the ductile fracture behavior. The lower bainitic fractures of those samples austempered for short times revealed brittle fracture.     

Characterization of Inoculated Low Carbon Equivalent Iron at Lower Austempering Temperature

The response of inoculated low carbon equivalent iron to austempering heat treatment has remained as an unchartered area of investigation. The carbon equivalent of inoculated low carbon equivalent iron is much lesser than that of ductile iron, while modulus of elasticity is higher than it. This paper examines and compares the austempering transformations and microstructural changes during austempering of ductile iron and inoculated low carbon equivalent iron with respect to austempering parameters. The optical metallography and X-ray diffraction techniques are used to study the changes in the austempered structure. The alterations in austempered microstructure and structural parameters like, austenite volume fraction (X) and its carbon content (C) at an austempering temperature of 250?°C are studied in the present investigation. It is observed that, austempering heat treatment produces fine ausferritic microstructure, similar to that present in austempered ductile iron at this temperature. However, some significant differences between the austempered microstructure of two alloys are noted.     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Fracture of steels containing pearlite

The relative effects of pearlite and spherodite on ductile, cleavage, and fatigue failure are summarized. Neither the cleavage strength nor the fatigue endurance limit appear to depend directly on cementite contentper se. Spherodized steels cleave less readily than ferrite/pearlite steels. Ductile fracture resistance is lowered considerably by both types of cementite, pearlite being more deleterious. Ferrite/pearlite steels appear to exhibit slower fatigue crack growth rates at low stress intensity levels than high strength steels. At high stress intensity levels the behavior is reversed. Slip-incuded cracking of carbide lamellae appears easier than that of spherodized carbides. In ductile fracture situations the crack spreads progressively through a pearlite colony via preferential cracking of carbides and rupture of the intervening ferrite accompanied by large local shear strains. Fatigue fracture proceeds with formation of frequent branches, preferentially along the pearlite colony interface. This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

Creep crack growth of HK40 steel: Microstructural effects

Creep crack growth tests at 871 °C have been performed on compact tension specimens of HK40 steel having different microstructures. The skeleton-shaped carbides on the grain boundaries have a higher resistance to crack growth than the blocky-shaped carbides. The secondary carbide size and distribution explicitly affect crack growth behavior. There exists a critical size of the secondary carbides. With an increase in the secondary carbide size, the resistance to crack growth increases up to the critical size and decreases beyond the critical size.     

Investigation on grinding wear behaviour of austempered ductile iron as media material during comminution of iron ore in ball mills

An attempt has been made to assess the grinding wear behaviour of austempered ductile iron (ADI) as media material in comminution of Kudremukh haematite iron ore in a ball mill. Spheroidal graphite (S.G) iron balls were austenitised at 900°C for one hour and austempered at 280°C and 380°C for different time durations. These materials were characterized by measuring hardness, carrying out X-ray diffraction analysis, studying microstructures using scanning electron microscope (SEM). Grinding wear behaviour of ADI was assessed during wet grinding at different pH of the mineral slurry. The wear resistance of ADI was compared with that of forged En 31 steel balls under similar grinding conditions. It was found that ADI balls austempered at 280°C for 30 minutes which contains lower bainite registered superior wear resistance. It was also noted that the wear resistance of ADI was more at higher pH range of the slurry.     

The effect of the austenitizing heat-treatment variables on the fracture toughness of high-speed steel

The influence of austenitizing treatment and tempering on the fracture behavior of high-speed steel (DIN 1.3333) has been investigated. The fracture behavior has been characterized by determining the K _IC and J _IC values via the performance of modified compact tension (CT) and single edge notched (SEN) tests. The micromechanisms of crack initiation and propagation have been studied by metallographic examination of the fractured specimens. The results indicate that austenitizing conditions of temperature range 1050 °C to 1190 °C and time 0.25 to 6 minutes and tempering at 550 °C to 650 °C up to 150 minutes alter the microstructure and, subsequently, the fracture toughness. It was found that cracking occurs by nucleation at the interface of matrix/vanadium-enriched large carbides, where sulfur is segregated and where linkage of the microcracks bridges ductile ligament of voids at small Mo + W enriched carbides. The improvements of the fracture toughness and hardness by short austenitizing time of 15 to 75 seconds at 1190 °C are attributed to (1) the optimum distribution of a dense network of small carbides, (2) the lack of grain growth as the boundaries are pinned down by these small carbides, and (3) the retained austenite at a level up to 16 vol pct transformed to martensite.     

Examination of Microstructure and Mechanical Properties of Austempered Ductile Iron (ADI) As Per Austempering Temperature and Time

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.     

Tribological Performance of Austempered and Tempered Ductile Iron

Austempered ductile iron with its unique ausferritic structure is produced by an isothermal heat treatment process. Austempered ductile iron is a potential material to substitute for traditional steel castings and forgings in current industry due to its excellent mechanical properties. The tempering process is frequently used to enhance the ductility and toughness of a material and reduce residual stress. In this research, the phase transformation of austempered ductile iron was studied by applying various tempering temperatures with constant holding duration. It was found that the ausferritic structure was decomposed into dispersive cementite particles after receiving a tempering temperature of 538 °C or higher. The specific amount of retained austenite was analyzed by X-ray diffraction. The wear resistance of tempered austempered ductile iron was investigated by using a ball-on-disk sliding test configuration. The results were compared with conventional quenched and tempered ductile iron under equivalent hardness. Both austempered ductile iron and tempered austempered ductile iron samples had better wear resistance than quenched and tempered ductile iron. The results presented in this research can be utilized as a reference in the tempering treatment of austempered ductile iron material for future applications.     

Microstructure and fracture of 52100 steel

The fracture behavior of 52100 steel hardened and tempered to R_C62 has been investigated as a function of austenitizing over the temperature range from 800 to 1100°C. Specimens were homogenized at 1150°C and either furnace cooled or isothermally transformed at 580°C to produce a pearlitic microstructure prior to austenitizing for hardening. Furnace-cooled specimens developed a proeutectoid carbide network that did not dissolve during subsequent austenitizing below A_cm . The residual proeutectoid carbides and the carbide-free martensite-austenite structure between them controlled fracture and produced K_IC of 19 MPa \ m^1/2, the highest determined in this investigation. The specimens isothermally transformed prior to austenitizing below A_cm produced a microstructure of fine spherical carbides dispersed throughout a fine martensitic matrix and did not contain residual proeutectoid carbides. The transgranular fracture of the latter specimens by microvoid coalescence around the closely spaced spherical carbides resulted in the lowest values of fracture toughness, 14 to 16 MPa\ m^1/2, determined in these experiments. Austenitizing above A_cm caused solution of all carbides, a gradual coarsening of the austenitic grain size, a transition to plate martensite, and an increase in retained austenite. Fracture toughness increased slightly with increasing austenitizing temperature above A_cm despite the fact that fracture propagated primarily along the austenitic grain boundaries. The improved fracture toughness, verified by scanning electron microscopy of the fatigue crack-overload fracture interface, is believed to be caused in part by transgranular crack propagation during the first stages of crack extension that are most important in determining K_1C.     

Homogeneous formation of epsilon carbides within the austenite during the isothermal transformation of a ductile iron at 410 °C

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.     

The effect of carbides on fracture toughness of steels of ferritic matrix

The authors evaluated the effect of the volume fraction and the dispersion rate of cementite on fracture toughness of ferrite. The investigations were performed at -196°C on five types of carbon steels containing 0.028–1.22% of C in which cementite was coagulated at 700°C for 1–8 h from the quenched state. It was determined that the fracture toughness of steel increases very strongly up to the content of carbides of about 7% by volume. At the same time, hardness and strength of these steels grow. First of all, this is the result of size reduction of ferrite grains by fine carbides. These carbides, distributed almost exclusively on grain boundaries, can only participate in the transmission of the crack to the neighbouring grain. At larger contents of carbides, their dispersion rate decreases while their number in the grain volume grows. Fine carbides from inside of the grains set the path of easy cracking on the boundaries with the ferritic matrix while the coarse carbides crack in front of the fracture. As a result, the steel fracture toughness decreases. The fracture development by means of carbides is less harmful than on the carbide/matrix boundaries.     

The effect of testing temperature on fracture toughness of austempered ductile iron

The effect of testing temperature (− 150 °C, 25 °C, and + 150 °C) on the fracture toughness of austempered ductile iron (ADI) was studied. Specimens were first austenitized at 900 °C for 1.5 hours and then salt-bath quenched to 360 °C or 300 °C, for 1, 2, or 3 hours of isothermal holding before cooling to room temperature. The resulting matrices of the iron were of upper-ausferrite and lower-ausferrite. It was found that raising the testing temperature to 150 °C from ambient improved the fracture toughness by 18, 30, and 7 pct for the as-cast/lower-ausferrite ADI/upper-ausferrite ADI, respectively. Lowering the testing temperature to −150 °C produced a decrease of −15, −35, and −48 pct. Optical microscopy, X-ray diffraction analysis, and scanning electron microscopy (SEM) fractography were applied to correlate the toughness variation with testing temperatures.     

Mechanical behaviour of an austempered ductile iron

Austempering of a ferrite-pearlitic grade of ductile iron was carried out to assess the potential use of the material for crank shaft application reported. A commercial material was austempered at 340°C to realize the properties. The austempered ductile iron gave good strength although the ductility values were lower. The material developed had complete ausferritric structure free of pearlite. The various phase constitution and phase transformation associated with the treatment and during mechanical deformation was examined. Using XRD analysis the volume fraction of the austenite in the matrix was estimated. The various aspects of processing a commercial cast iron during ausetmpering, the phase transformation, microstructural evolution have been examined along with the property of the material. The mechanical behaviour of the material and the scope for further improvement is discussed.     

Effects of sic content on fatigue crack growth in

An experimental study has been conducted with the purpose of examining the fatigue crack growth characteristics of cast aluminum alloy matrix composites reinforced with different vol- ume fractions of silicon carbide particles. Particular attention has been paid to developing com- posite microstructures with similar matrix aging condition, precipitation, matrix strength, reinforcement particle size distribution, and interfacial characteristics but with different con- trolled amounts of reinforcement particles. Fatigue crack growth experiments have been con- ducted using constant stress amplitude methods with a fixed load ratio as well as constant K_max control involving a varying load ratio. The development of crack closure and the microscopic path of the crack through the composite microstructure are monitored optically and using the electron microscope in an attempt to examine the mechanisms of fatigue fracture. The results indicate that an increase in SiC content results in the suppression of striation formation in the ductile matrix. Although ductile matrix failure involving the formation of striations in the low SiC content composite or of void growth in the high SiC content composite is evident, the results also show that fracture of the reinforcement particles plays a significant role in dictating the rates of fatigue crack growth. Detailed quantitative analyses of the extent of particle fracture as a function of the reinforcement content have been performed to elucidate the mechanistic origins of fatigue resistance. The propensity of particle fracture increases with particle size and with the imposed value of stress intensity factor range. While discontinuously reinforced metal- matrix composites with predominantly matrix cracking are known to exhibit superior fatigue crack growth resistance as compared to the unreinforced matrix alloy, the tendency for particle fracture in the present set of experiments appears to engender fatigue fracture characteristics in the composite which are inferior to those seen in the unreinforced matrix material. Particle fracture also results in noticeable differences in the microscopic fracture path and causes a reduction in crack closure in the composites as compared to that in the matrix alloy. The results of this work are discussed in light of other related studies available in the literature in an attempt to develop a mechanistic perspective on fatigue crack growth resistance in metal-matrix composites.     

Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites: Part I. Tensile and fatigue behavior

Flexural fatigue behavior was investigated on titanium (Ti-15V-3Cr) metal matrix composites reinforced with cross-ply, continuous silicon carbide (SiC) fibers. The titanium composites had an eightply (0, 90, +45, -45 deg) symmetric layup. Fatigue life was found to be sensitive to fiber layup sequence. Increasing the test temperature from 24 °C to 427 °C decreased fatigue life. Interface debonding and matrix and fiber fracture were characteristic of tensile behavior regardless of test temperature. In the tensile fracture process, interface debonding between SiC and the graphite coating and between the graphite coating and the carbon core could occur. A greater amount of coating degradation at 427 °C than at 24 °C reduced the Ti/SiC interface bonding integrity, which resulted in lower tensile properties at 427 °C. During tensile testing, a crack could initiate from the debonded Ti/SiC interface and extend to the debonded interface of the neighboring fiber. The crack tended to propagate through the matrix and the interface. Dimpled fracture was the prime mode of matrix fracture. During fatigue testing, four stages of flexural deflection behavior were observed. The deflection at stage I increased slightly with fatigue cycling, while that at stage II increased significantly with cycling. Interestingly, the deflection at stage III increased negligibly with fatigue cycling. Stage IV was associated with final failure, and the deflection increased abruptly. Interface debonding, matrix cracking, and fiber bridging were identified as the prime modes of fatigue mechanisms. To a lesser extent, fiber fracture was observed during fatigue. However, fiber fracture was believed to occur near the final stage of fatigue failure. In fatigued specimens, facet-type fracture appearance was characteristic of matrix fracture morphology. Theoretical modeling of the fatigue behavior of Ti/SCS-6 composites is presented in Part II of this series of articles. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Environmentally induced crack propagation behavior of 2219 aluminum: microstructural effects

Alloy 2219 has been evaluated under corrosion fatigue conditions. The effect of the micro-structures present in the T851 and T6 conditions on crack propagation rates has been determined. Tests were performed on compact tension specimens in air and in NaCI solutions at 23 and 70°C. The corrosion fatigue behavior of the material under these conditions was evaluated by studying the crack propagation kinetics and also crack fractography.The results of this study show that 2219 aluminum is resistant to corrosion fatigue in aqueous solutions containing up to 10% NaCI. However, the material is more resistant in the T6 than in the T851 condition. In the absence of stress corrosion cracking, the initial stages of the crack growth were characterised by ductile fracture while a mixed fracture mode dominated the final stages of the growth. At low loading frequencies (0. 1 Hz), crack branching and blunting were observed when tests were performed at 70°C.     

Comparison of High-Temperature Properties and Thermal Shock Resistance of Austempered Ductile Irons (ADI) with Those of Pearlitic Ductile Cast Irons

High-temperature strength and thermal shock resistance of austempered ductile iron (ADI) in high temperatures because of instability of ausferrite phase has been less interest. The aim of this study is to investigate the tensile properties of ADI and pearlitic ductile cast iron by using the short-time tensile test in high temperatures. Tensile test was conducted in temperatures of 298 K, 673 K, 873 K, and 1073 K (25 °C, 400 °C, 600 °C, and 800 °C). Thermal shock test also was conducted by using the molten lead bath at 1273 K (1000 °C). In this experiment, samples of pearlitic ductile cast iron and ADI were divided in two groups; that after immersing in the molten lead bath for 25 seconds, one group was cooled in the air and other one was quenched in the water. Results showed that strength and thermal shock resistance of ADI samples are higher than those of the pearlitic ductile cast iron.