Effect of bainite transformation and retained austenite on mechanical properties of austempered spheroidal graphite cast steel

Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Kinetics of Reverse Transformation from Pearlite to Austenite in an Fe-0.6 Mass pct C Alloy and the Effects of Alloying Elements

Substitutional alloying effects on reversion kinetics from pearlite structure at 1073 K (800 °C) in an Fe-0.6 mass pct C binary alloy and Fe-0.6C-1 or 2 mass pct M (M = Mn, Si, Cr) ternary alloys were studied. Reverse transformation in the Fe-0.6C binary alloy at 1073 K (800 °C) was finished after holding for approximately 5.5 seconds. The reversion kinetics was accelerated slightly by the addition of Mn but retarded by the addition of Si or Cr. The difference of acceleration effects by the addition of the 1 and 2 mass pct Mn is small, whereas the retardation effect becomes more significant by increasing the amount of addition of Si or Cr. It is clarified from the thermodynamic viewpoint of carbon diffusion that austenite can grow without partitioning of Mn or Si in the Mn- or Si-added alloys. On the one hand, austenite growth is controlled by the carbon diffusion, whereas the addition of them affects carbon activity gradient, resulting in changes in reversion kinetics. On the other hand, thermodynamic calculation implies that the long-range diffusion of Cr is necessary for austenite growth in the Cr-added alloys. It is proposed that austenite growth from pearlite in the Cr-added alloys is controlled by the diffusion of Cr along austenite/pearlite interface.     

Microstructural development and austempering kinetics of ductile iron during thermomechanical processing

A new method of refining the microstructure of austempered ductile iron (ADI) by thermome chanical processing is investigated. Refinement of microstructure is effected by grain refinement of parent austenite by hot deformation in the austenitizing temperature range, before the austempering treatment. The effects of austenite deformation on the kinetics of austempering reaction and the microstructure development were studied using metallography and X-ray diffraction (XRD), at different austempering temperatures and deformations. The process window for optimum microstructure was determined in terms of the parameters involved. Deformation of 40 to 60 pct could be imparted in the temperature range 900 °C to 1025 °C, resulting in a reduction in the prior austenite grain size by 35 to 50 pct and ferrite size in ausferrite by 70 to 75 pct. The effects of austenitization temperature on the austempered microstructure were also studied.     

The Mechanism of High Strength-Ductility Steel Produced by a Novel Quenching-Partitioning-Tempering Process and the Mechanical Stability of Retained Austenite at Elevated Temperatures

The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T) process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at 473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp ε-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained austenite.     

Strength of initially virgin martensites at - 196 °C after aging and tempering

The compressive strength at -196°C of martensites in Fe-0.26 pct C-24 pct Ni, Fe-0.4 pct C-21 pct Ni, and Fe-0.4 pct C-18 pct Ni-3 pct Mo alloys, all with subzero M temperatures, has been determined in the virgin condition and after one hour at temperatures from -80 to +400 °C. The effects of ausforming (20 pct reduction in area of the austenite by swaging at room temperature prior to the martensitic transformation) were also investigated. For the unausformed martensites, aging at temperatures up to 0 °C results in relatively small increases in strength. Above 0 °C, the age hardening increment increases rapidly, reaching a maximum at 100 °C. Above 100 °C, the strength decreases continuously with increasing tempering temperature except for the molybdenum-containing alloy, which exhibits secondary hardening on tempering at 400 °C. For the ausformed martensites, the response to aging at subzero temperatures is greater than for unausformed material. Strength again passes through a maximum on aging at 100 °C. However, on tempering just above 100 °C, the ausformed materials show a slower rate of softening than the unausformed martensites. The strengthening produced by the ausforming treatment is largest for the Fe-0.4 pct C-18 pct Ni-3 pct Mo alloy, but there is no evidence of carbide precipitation in the deformed austenite to a°Count for this effect of molybdenum.     

Strength of Initially Virgin Martensites at — 196 °C after Aging and Tempering

The compressive strength at —196°C of martensites in Fe-0.26 pct C-24 pct Ni, Fe-0.4 pct C-21 pct Ni, and Fe-0.4 pct C-18 pct Ni-3 pct Mo alloys, all with subzero M_s temperatures, has been determined in the virgin condition and after one hour at temperatures from —80 to +400 °C. The effects of ausforming (20 pct reduction in area of the austenite by swaging at room temperature prior to the martensitic transformation) were also investigated. For the unausformed martensites, aging at temperatures up to 0 °C results in relatively small increases in strength. Above 0 °C, the age hardening increment increases rapidly, reaching a maximum at 100 °C. Above 100 °C, the strength decreases continuously with increasing tempering temperature except for the molybdenum-containing alloy, which exhibits secondary hardening on tempering at 400 °C. For the ausformed martensites, the response to aging at subzero temperatures is greater than for unausformed material. Strength again passes through a maximum on aging at 100 °C. However, on tempering just above 100 °C, the ausformed materials show a slower rate of softening than the unausformed martensites. The strengthening produced by the ausforming treatment is largest for the Fe-0.4 pct C-18 pct Ni-3 pct Mo alloy, but there is no evidence of carbide precipitation in the deformed austenite to account for this effect of molybdenum. This paper is based on a presentation made at the “Peter G. Winchell Symposium on Tempering of Steel” held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.     

Dependence of fracture toughness of austempered ductile iron on austempering temperature

Ductile cast iron samples were austenitized at 927 °C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at 260 °C, 288 °C, 316 °C, 343 °C, 371 °C, and 399 °C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pct retained austenite containing more than 1.8 wt pct carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of K _IC ² against σ _y (X _γ, C _γ)^1/2 resulted in a straight line, as predicted by the model.     

Effect of Austempering Conditions on the Microstructure and Tensile Properties of Low Alloyed Sintered Steel

Because of the combination of strength and toughness, much interest has been focused on austempered sintered low-alloyed steels. Implementation of this treatment for powder metal components has been limited by interaction between the open porosity of the material and the cooling fluid. This work discusses the influence of different austempering environments and parameters on the microstructure and, as a consequence, on the final properties. The sintered steels selected are based on two different prealloyed powders, Fe-1.5Cr-0.2Mo and Fe-1.5Mo, with the addition of 0.6 wt pct graphite. Green samples with medium density (7.3 g/cm³) were sintered at 1393 and 1523 K (1120 and 1250 °C) to ensure a decrease in open porosity. The austempering treatment process requires austenitizing at 1133 K (860 °C) for 2 hours followed by quenching in different baths at 563 K (290 °C). The final strength and ductility are dependent upon the relative amounts of ferrite, pearlite, and bainite phases present in the austempered steel. Discussion of the experimental results compares the as-sintered and the austempered observations as well as the relationship between mechanical properties and the phases present in the final microstructures.     

Influence of the amount and morphology of retained austenite on the mechanical properties of an austempered ductile iron

High Si contents in nodular cast irons lead to a significant volume fraction of retained austenite in the material after the austempering treatment. In the present work, the influence of the amount and morphology of this phase on the mechanical properties (proof stress, ultimate tensile strength (UTS), elongation, and toughness) has been analyzed for different austempering conditions. After 300 °C isothermal treatments at intermediate times, the austenite is plastically stable at room temperature and contributes, together with the bainitic ferrite, to the proof stress and the toughness of the material. For austenite volume fractions higher than 25 pct, the proof stress is controlled by this phase and the toughness depends mainly on the stability of γ. In these conditions (370 °C and 410 °C treatments), the present material exhibits a transformation-induced plasticity (TRIP) effect, which leads to an improvement in ductility. It is shown that the strain level necessary to initiate the martensitic transformation induced by deformation depends on the carbon content of the austenite. The martensite formed under TRIP conditions can be of two different types: “autotempered” plate martensite, which forms at room temperature from an austenite with a quasi-coherent epsilon carbide precipitation, and lath martensite nucleated at twin boundaries and twin intersections.     

An Atom Probe Study of Kappa Carbide Precipitation and the Effect of Silicon Addition

The influence of silicon on κ-carbide precipitation in lightweight austenitic Fe-30Mn-9Al-(0.59-1.56)Si-0.9C-0.5Mo cast steels was investigated utilizing transmission electron microscopy, 3D atom-probe tomography, X-ray diffraction, ab initio calculations, and thermodynamic modeling. Increasing the amount of silicon from 0.59 to 1.56 pct Si accelerated formation of the κ-carbide precipitates but did not increase the volume fraction. Silicon was shown to increase the activity of carbon in austenite and stabilize the κ-carbide at higher temperatures. Increasing the silicon from 0.59 to 1.56 pct increased the partitioning coefficient of carbon from 2.1 to 2.9 for steels aged 60 hours at 803 K (530 °C). The increase in strength during aging of Fe-Mn-Al-C steels was found to be a direct function of the increase in the concentration amplitude of carbon during spinodal decomposition. The predicted increase in the yield strength, as determined using a spinodal hardening mechanism, was calculated to be 120 MPa/wt pct Si for specimens aged at 803 K (530 °C) for 60 hours and this is in agreement with experimental results. Silicon was shown to partition to the austenite during aging and to slightly reduce the austenite lattice parameter. First-principles calculations show that the Si-C interaction is repulsive and this is the reason for enhanced carbon activity in austenite. The lattice parameter and thermodynamic stability of κ-carbide depend on the carbon stoichiometry and on which sublattice the silicon substitutes. Silicon was shown to favor vacancy ordering in κ-carbide due to a strong attractive Si-vacancy interaction. It was predicted that Si occupies the Fe sites in nonstoichiometric κ-carbide and the formation of Si-vacancy complexes increases the stability as well as the lattice parameter of κ-carbide. A comparison of how Si affects the enthalpy of formation for austenite and κ-carbide shows that the most energetically favorable position for silicon is in austenite, in agreement with the experimentally measured partitioning ratios.     

Constitutive Modeling of the Mechanical Properties of V-added Medium Manganese TRIP Steel

In this study, medium Mn transformation-induced plasticity steel with the composition Fe-0.08 pct C-6.15 pct Mn-1.5 pct Si-2.0 pct Al-0.08 pct V was investigated. After intercritical annealing at 1013 K (740 °C), the steel contained coarse-grained ferrite and two ultrafine-grained (UFG) phases: ferrite and retained austenite. The material did not deform by localized Lüders band propagation: it did not suffer from this major problem as most UFG steels do. Localization of plastic flow was shown to be suppressed because of a combination of factors, including a bimodal grain size distribution, a multiphase microstructure, the presence of nanosized vanadium carbide precipitates, and the occurrence of the deformation-induced martensitic transformation of retained austenite. A constitutive model incorporating these effects was developed. The model was used to identify the factors which can lead to a further improvement of the mechanical properties of the UFG medium Mn TRIP steels.     

Alloying Element Partition and Growth Kinetics of Proeutectoid Ferrite in Hot-Deformed Fe-0.1C-3Mn-1.5Si Austenite

Alloying element partition and growth kinetics of proeutectoid ferrite in deformed austenite were studied in an Fe-0.1C-3Mn-1.5Si alloy. Very small ferrite particles, less than several microns in size, were formed within the austenite matrix, presumably at twin boundaries as well as at austenite grain boundaries. Scanning transmission electron microscopy–energy-dispersive X-ray (STEM-EDX) analysis revealed that Mn was depleted and Si was enriched in the particles formed at temperatures higher than 943 K (670 °C). These were compared with the calculation of local equilibrium in quaternary alloys, in which the difference in diffusivity between two substitutional alloying elements was assumed to be small compared to the difference from the carbon diffusivity in austenite. Although the growth kinetics were considerably faster than calculated under volume diffusion control, a fine dispersion of ferrite particles was readily obtained in the partition regime due to sluggish growth engendered by diffusion of Mn and Si.     

In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s⁻¹) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form.

     

Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel

BlastAlloy160 (BA-160) steel, with a nominal composition of Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct), is strengthened by Cu-rich precipitates and M₂C carbides. This alloy was subjected to several weldability tests to assess its susceptibility to certain weld cracking mechanisms. Hot ductility testing revealed a liquation cracking temperature range (LCTR) of 148 K (–125 °C), which suggested moderate susceptibility to heat-affected zone (HAZ) liquation cracking. The enrichment of Ni and Cu was measured along the prior austenite grain boundaries in the simulated partially melted zone (PMZ) and was consistent with similar enrichment at interdendritic boundaries of the simulated fusion zone (FZ). Good wetting and penetration of liquid films along the austenite grain boundaries of the PMZ was also observed. Associated with that finding were thermodynamic calculations indicating a completely austenitic (face-centered cubic) microstructure at elevated temperatures. In testing to determine reheat cracking susceptibility, ductility values of 41 to 78 pct RA were established for the 723 K to 973 K (450 °C to 700 °C) temperature range. The good ductility values precluded susceptibility to reheat cracking according to the test criterion. Dilatometric measurements and thermodynamic calculations revealed the formation of austenite in the reheat cracking temperature range, which was attributed to the high Ni content of the BA-160 alloy.     

Thermal dependence of austempering transformation kinetics of compacted graphite cast iron

The evolution of the relative fraction of high-carbon austenite with austempering time and temperature was analyzed in a compacted graphite (CG) cast iron (average composition, in wt pct: 3.40C, 2.8Si, 0.8Mn, 0.04Cu, 0.01P, and 0.02S) at five different austempering temperatures between 573 and 673 K. Samples were characterized by Mössbauer spectroscopy, hardness measurements, and optical microscopy. During the first stage of transformation, the kinetics parameters were determined using the Johnson-Mehl’s equation, and their dependence with temperature in the range from 573 to 673 K indicates that the transformation is governed by nucleation and growth processes. The balance between growth-rate kinetics and nucleation kinetics causes the kinetics parameter (k) to have a maximum at ≈623 K of 3.9×10^?3(s^?1). The evolution of the C content in the high-carbon austenite was found to be controlled by the volume diffusion of carbon atoms from the ferrite/austenite interface into austenite, with a dependence of t ^0.40±0.05 on the austempering time (t).     

Formation of Delta Ferrite in 9 Wt Pct Cr Steel Investigated by <Emphasis Type="Italic">In-Situ</Emphasis> X-Ray Diffraction Using Synchrotron Radiation

In-situ X-ray diffraction (XRD) measurements using high energy synchrotron radiation were performed to monitor in real time the formation of delta ferrite in a martensitic 9 wt pct chromium steel under simulated weld thermal cycles. Volume fractions of martensite, austenite, and delta ferrite were measured as a function of temperature at a 10 K/s heating rate to 1573 K (1300 °C) and subsequent cooling. At the peak temperature, the delta ferrite concentration rose to 19 pct, of which 17 pct transformed back to austenite on subsequent cooling.     

Simulation of Ferrite Formation in Fe-C Alloys Based on a Three-Dimensional Mixed-Mode Transformation Model

A three-dimensional mixed-mode (MM) transformation model accounting for both soft impingement and hard impingement was developed that calculates the growth kinetics of ferrite grains in an austenite matrix. The simulations are compared to the kinetics of ferrite formation in high-purity Fe-C alloys for which phase-transformation kinetics were measured isothermally by dilatometry at several temperatures in the range of 973 K to 1043 K (700 °C to 770 °C). The interface mobility is obtained from the best fit of the data at 1023 K (750 °C) for which the nucleus density N is estimated from the final microstructure. Subsequently, the experimental ferrite kinetics in Fe-0.36C at the other temperatures are simulated. The values of N extracted from the fits can be described with a nucleation model. The significance of the MM calculations is rationalized by comparing the results for Fe-0.17C with simulations assuming purely diffusion-controlled (DC) and purely interface-controlled (IC) growth. Comparison of simulated fraction curves for Fe-0.57C with the three models demonstrates that the transformation in high-carbon steels is essentially DC.     

Mechanical Characterization of Austempered Ductile Iron Obtained by Two Step Austempering Process

Austempered ductile iron (ADI) is known to have a good combination of mechanical properties due its unique ausferrite microstructure. The strength of ADI is mainly a function of the austempering temperature and the stability of ausferrite matrix. To increase the stability of the ausferritic matrix, two stage austempering processes was developed. During this investigation, in the Ist step, ductile iron specimens were austenitized at 900 °C for 60 min followed by quenching to 250 °C in salt bath. In the IInd step, after quenching at 250 °C, the salt bath was gradually heated to 350 °C, 400 °C and 450 °C respectively where specimen were soaked for 120 min. The tensile strength and impact strength were evaluated according to ASTM standards. The results were compared with that obtained by conventional austempering process by quenching directly into salt bath at 400 °C for 120 min. Both tensile and impact strength were found to have improved by two step austempering process. During Ist stage of austempering, martensite was observed while during IInd stage of austempering microstructures revealed acicular ferrite and carbon stabilized austenite. The fractographic examination revealed mixed type of fracture mode and intergranular fracture was seen under SEM. It was further observed that the tensile strength decreased whereas the impact strength increased with IInd stage of austempering temperature.     

The Coexistence of Two Different Pearlites,Lamellae of (Ferrite + M<Subscript>3</Subscript>C), and Lamellae of (Ferrite + M<Subscript>23</Subscript>C<Subscript>6</Subscript>) in a Mn-Al Steel

Two different pearlites after two separate eutectoid reactions were observed in an Fe-19.8 Mn-1.64 Al-1.03 C (wt pct) steel. The steel specimens were processed under solution heat treatment at 1373 K (1100 °C) and received isothermal holding at temperatures from 1073 K to 773 K (800 °C to 500 °C). The constituent phase of the steel is single austenite at temperatures between 1373 K and 1073 K (1100 °C and 800 °C). At temperatures below 1048 K (775 °C), M₃C and M₂₃C₆ carbides coprecipitate at the austenitic grain boundaries. Two different pearlites appear in the austenite matrix simultaneously at temperatures below 923 K (650 °C). One is lamellae of ferrite and M₃C carbide, and the other is lamellae of ferrite and M₂₃C₆ carbide. These two pearlites are product phases from two separate eutectoid reactions, i.e., austenite → ferrite + cementite and austenite → ferrite + M₂₃C₆. Therefore, the supersaturated austenite has decomposed into two different pearlites, separately.     

Thermal-Treated Pitches as Binders for TiB2/C Composite Cathodes

The effect of thermal treatment on the microstructure and properties of pitches and thermal-treated, pitch-based TiB₂/C composite cathodes were investigated. Thermal treatments were performed at 473 K, 523 K, 573 K, 623 K, and 673 K (200 °C, 250 °C, 300 °C, 350 °C, and 400 °C), respectively. The results show that the aromaticity of the treated pitches increases with an increasing thermal treatment temperature, and subsequently, the coking value and quinoline-insoluble (QI) content increase from 60.62 wt pct to 79.09 wt. pct and from 8.97 wt pct to 32.54 wt pct when the treatment temperature increases from 473 K to 623 K (200 °C to 350 °C). The volume fraction of coalesced mesophase in semicoke decreases with an increasing thermal treatment temperature, and after 673 K (400 °C) is reached, the coalesced mesophase is almost invisible. The bulk density and compressive strength of modified pitch-based cathodes increase with an increasing thermal treatment temperature from 2.24 g cm⁻³ to 2.39 g cm⁻³ and from 24.21 MPa to 54.85 MPa, whereas open porosity decreases from 34.62 pct to 27.06 pct. Both electrical resistivity and electrolysis expansion ratio first decrease and then increase with an increasing thermal treatment temperature, and the lowest values (45.63 μΩ m and 0.65 pct) are achieved at 573 K (300 °C). Compared with those of the parent pitch-based cathode, the properties of the modified pitch-based cathodes had improved significantly. The mechanisms of the improvements are discussed in the text.