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.     

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.     

Role of the Austenite-Ferrite Transformation Start Temperature on the High-Temperature Ductility of Electrical Steels

The austenite to ferrite transformation start temperature was measured in two low-C, Si-Al grain non-oriented electrical steels using in situ x-ray diffraction and the direct comparison method with the (111)^γ and (110)^α diffracted intensities. It was shown that increasing Al content from 0.2 to 0.6 wt.% in a 0.06 wt.% C, 0.6 wt.% Si and 0.5 wt.% Mn steel increases the Ae₃ temperature from 928 to 955 °C. The results are supported by microstructural observations of isothermally transformed samples and used to discuss the loss of ductility during high temperature deformation (e.g. hot rolling) of this type of materials.     

Formation of an <Emphasis Type="Italic">L</Emphasis>1<Subscript>0</Subscript> superstructure in austenite upon the α → γ transformation in the invar alloy Fe-32% Ni

Structure of a metastable austenitic invar alloy Fe-32% Ni preliminarily quenched for martensite and subjected to α → γ transformation using slow heating to various temperatures (430–500°C) with the formation of variously oriented nanocrystalline lamellar austenite, which was subjected to an additional annealing at 280°C (below the calculated temperature of ordering of the γ phase), has been studied electron-microscopically. An electron diffraction analysis revealed the presence of an L1₀ superstructure in the disperse nickel-enriched nanocrystalline γ phase both after annealing at 280°C and in the unannealed alloy immediately after α → γ transformation upon slow heating to 430°C.     

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.     

Phase stability among the α(A1), β(A2), and γ(D83) phases in the Cu-Al-X system

The phase equilibria among the α(A1), β(A2), and γ(D8₃) phases in the Cu-Al-X systems (X: Ti, V, Mn, Fe, Co, Ni, Zn, Sn, and Sb) at 700 and 800 °C were investigated, and the effects of these alloying elements on the phase boundaries, homogeneous solid solution ranges, and tie-lines among the α, β, and γ phases were accurately determined by the diffusion couple method. The phase stabilities among α, β, and γ phases are discussed in terms of the partition coefficient. In addition, a modified Guillet’s method, which is used to estimate the effect of alloying elements on microstructure, is proposed by taking into account the change of equivalent coefficient with the phase fraction, and applied to predict the effect of alloying elements on the microstructure of the Cu-Al base alloy.     

Phase stability among the α(A1), β(A2), and γ(D83) phases in the Cu-Al-X system

The phase equilibria among the α(A1), β(A2), and γ(D8₃) phases in the Cu-Al-X systems (X: Ti, V, Mn, Fe, Co, Ni, Zn, Sn, and Sb) at 700 and 800 °C were investigated, and the effects of these alloying elements on the phase boundaries, homogeneous solid solution ranges, and tie-lines among the α, β, and γ phases were accurately determined by the diffusion couple method. The phase stabilities among α, β, and γ phases are discussed in terms of the partition coefficient. In addition, a modified Guillet’s method, which is used to estimate the effect of alloying elements on microstructure, is proposed by taking into account the change of equivalent coefficient with the phase fraction, and applied to predict the effect of alloying elements on the microstructure of the Cu-Al base alloy.     

Mathematical model for austenitization kinetics of ductile iron

A mathematical model was developed in the current study to understand the progress of austenitization process in ductile irons. The austenitization time required to produce homogeneous austenite in a two-phase region of austenite and graphite has been estimated in terms of (a) time required for transformation of matrix to austenite and (b) time required for dissolution of graphite in austenite to attain uniform carbon content, which remains in equilibrium with graphite. The time required been related to the structural parameters of cast ductile iron-like radius of graphite nodule, radius of austenite cell, volume fraction of graphite, volume fraction of ferrite in cast matrix, and diffusion constant. The model was used to determine the minimum austenitization time required to achieve homogeneous austenite in three commercial ductile irons when austenitized at a temperature of 900 °C. The results were compared with those obtained. The uniformity of the carbon content in austenite of ductile iron was verified indirectly by measuring microhardness.     

Effect of aging and cold rolling on the microstructure and mechanical properties of a new Fe-Mn-Al-Cr-C duplex alloy

The microstructural changes that occur during aging and cold rolling of a new Fe-Mn-Al-Cr-C duplex alloy have been investigated. Two treatments were developed to produce either a good combination of tensile strength and ductility (σ _u =800 MPa, σ _y =525 MPa, and A=46%) or a high strength (σ _u =1340 MPa, σ _y =1200 MPa, and A=15%) with a ductile type of fracture after aging at 320 °C. Aging between 550 °C and 700 °C led to a significant decrease in strength and ductility due to the precipitation of the brittle βMn phase. However, aging above 750 °C showed a considerable increase in strength and ductility due to the precipitation of very fine grains of ferrite within the austenite phase.     

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.     

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).     

Deformation behavior of a low carbon steel in α+γ range under electric current heating

The effect of electric current (EC) heating on deformation and phase transformation behavior is studied for plain low carbon steel. It is shown that EC heating does not sensibly influence the deformation resistance of austenite. During deformation at the upper temperatures of the α+γ range the EC-effect is positive (flow stress decreases upon application of EC) while at the lower temperatures of the α+γ range the EC-effect is negative. This is accompanied by variations in the work hardening rate of the same sign. The EC-effect in the α+γ range is more pronounced at lower strain rates when the contribution of the EC heating to the overall temperature of the specimen is higher. EC heating combined with deformation leads to an increase in the A_r3 temperature. The magnitude and the sign of EC-effects on deformation resistance and transformation behavior in the α+γ range are related to the differences in electric resistivity and deformation resistance between the emerging and the parent phases. A negative difference in electric resistivity and deformation resistance accelerates the phase transformation and leads to a positive EC-effect, and vice versa.     

Development of low thermal expansion superalloy

Alloy 903 and Alloy 909 are well-known Fe-Co-Ni-Al-Ti-Nb alloys with controlled low thermal expan-sion, but they have some properties that can be improved. To improve stress-accelerated grain boundary oxidation embrittlement of Alloy 903 and instability of theγ phase of alloy 909, two new alloys with good stress-rupture ductility, high creep-rupture strength, high tensile strength at high temperature, and good controlled thermal expansion were developed. These property improvements were accomplished by the combination of optimizing the Fe-Co-Ni ratio of the matrix and stabilizing theγ phase with the addition of aluminum.     

The influence of nickel and copper on the austempering of ductile iron

In the present investigation, the effect of alloying elements on the austempering process, austempered microstructure, and structural parameters of two austempered ductile irons (ADI) containing 0.6% Cu and 0.6% Cu/1.0% Ni as the main alloying elements was investigated. The optical metallography and x-ray diffraction were used to study the changes in the austempered structure. The effect of alloying additions on the austempering kinetics was studied using the Avrami equation. Significantly more upper bainite was observed in the austempered Cu-Ni alloyed ADI than in Cu alloyed ADI. The volume fraction of retained austenite (X _γ), the carbon level in the retained austenite (C _γ), and the product X _γ C _γ in an austempered structure of Cu-alloyed ADI are higher than in Cu-Ni-alloyed ADI. The austempering Kinetics is slowed down by the addition of Ni.     

Phase transformations of 24Cr-14Ni-0.7Si stainless steel under different aging conditions

This study discusses the development of a phase transformation in 24Cr-14Ni-0.7Si stainless steel after aging under various aging temperatures, times, and N₂/Air ratios. The observation of OM indicated that the initial state of δ-ferrite in the test material appeared as complete dendrite structures at short aging times and then exhibited lacy and dispersed structures when the aging time increased. This led to a gradual austenitization transformation as the nitrogen/air ratio increased, accelerating the δ/σ phase transformation and retarding the δ/γ phase transformation at the same time. The δ/σ phase transformation was dominant when the aging temperature was 800 °C. A line scanning analysis of the EPMA showed that the X-ray spectrum of Cr at the δ/γ interphase boundary was raised. In addition, Si showed lower X-ray spectrum energy after the δ/γ phase transformation. Clearly, Si had a stabilizing effect on the δ-ferrite and σ-phase. Furthermore, it had the fastest precipitation ratio for the δ/σ phase transformation at 800°C among all aging temperatures.     

Analyses of HCP/D019 and D019/L10 phase boundaries in Ti-Al-X (X=V, Mn, Nb, Cr, Mo, Ni, and Co) systems by the cluster variation method

The cluster variation method (CVM) based on the octahedron and tetrahedron approximation was applied to the calculation of γ(TiAl, L1₀)/α ₂(Ti₃Al, D0₁₉) phase equilibrium in the Ti-Al-X (X=V, Mn, Nb, Cr, Mo, Ni, and Co) systems. The antiphase boundary (APB) energy, the long-range order (LRO) parameter, and the substitution behavior of the γ(TiAl, L1₀) were calculated. The results of calculations were in good agreement with the data determined experimentally.     

Dynamic Ferrite Transformation Behaviors in 6Ni-0.1C Steel

Phase transformation from austenite to ferrite is an important process to control the microstructures of steels. To obtain finer ferrite grains for enhancing its mechanical property, various thermomechanical processes followed by static ferrite transformation have been carried out for austenite phase. This article reviews the dynamic transformation (DT), in which ferrite transforms during deformation of austenite, in a 6Ni-0.1C steel recently studied by the authors. Softening of flow stress was caused by DT, and it was interpreted through a true stress–true strain curve analysis. This analysis predicted the formation of ferrite grains even above the Ae₃ temperature (ortho-equilibrium transformation temperature between austenite and ferrite), where austenite is stable thermodynamically, under some deformation conditions, and the occurrence of DT above Ae₃ was experimentally confirmed. Moreover, the change in ferrite grain size in DT was determined by deformation condition, i.e., deformation temperature and strain rate at a certain strain, and ultrafine ferrite grains with a mean grain size of 1 μm were obtained through DT with subsequent dynamic recrystallization of ferrite.     

Effect of the austenite stability on the fracture mode and mechanical properties of aging nonmagnetic alloys after different strengthening treatments

Aging austenitic alloys with a stable (N26Kh5T3; M _s < −196°C) and metastable (N25Kh2T3; M _s = −130°C) austenite have been investigated after employing different methods of heat and thermomechanical treatments, namely, (1) aging at a temperature T _a = 600°C (A); (2) strengthening using phase-transformation-induced hardening (“phase naklep” (PN)) and subsequent aging at T _a = 600°C (PN + A); (3) deformation to 30% (D) at T _d = 600°C after preliminary aging at the same temperature (A + D); and (4) PN + A + D. In this case, the alloy with stable austenite has not been strengthened by phase naklep. Structure, fracture mode, ultimate strength, yield strength, relative reduction of area, and relative elongation have been studied depending on the duration of aging τ_a upon these strengthening treatments. It has been established that the physicomechanical properties of the alloys depend not only on τ_a, but also on the testing temperature. It is shown that all above physicomechanical characteristics of the alloys under consideration are affected substantially by the austenite stability.     

Analyses of HCP/D019 and D019/L10 phase boundaries in Ti-Al-X (X=V, Mn, Nb, Cr, Mo, Ni, and Co) systems by the cluster variation method

The cluster variation method (CVM) based on the octahedron and tetrahedron approximation was applied to the calculation of γ(TiAl, L1₀)/α ₂(Ti₃Al, D0₁₉) phase equilibrium in the Ti-Al-X (X=V, Mn, Nb, Cr, Mo, Ni, and Co) systems. The antiphase boundary (APB) energy, the long-range order (LRO) parameter, and the substitution behavior of the γ(TiAl, L1₀) were calculated. The results of calculations were in good agreement with the data determined experimentally.