Showing papers on "Austenite published in 1987"

Fine phase mixtures as minimizers of energy

Abstract: Solid-solid phase transformations often lead to certain characteristic microstructural features involving fine mixtures of the phases. In martensitic transformations one such feature is a plane interface which separates one homogeneous phase, austenite, from a very fine mixture of twins of the other phase, martensite. In quartz crystals held in a temperature gradient near the α-β transformation temperature, the α-phase breaks up into triangular domains called Dauphine twins which become finer and finer in the direction of increasing temperature. In this paper we explore a theoretical approach to these fine phase mixtures based on the minimization of free energy.

Enhancement of Elongation by Retained Austenite in Intercritical Annealed 0.4C-1.5Si-O.8Mn Steel

Abstract: An excellent combination of elongation over 30% and high strength about 100kgf/mm2 is achieved in processing of a 0.4C-1.5Si-0.8Mn steel by intercritical annealing, rapid cooling into bainite transformation temperature to soak several minutes. This combination is caused by transformation induced plasticity of retained austenite. Sufficient amount of stable austenite is a requisite for the good ductility. For the rapid cooling after annealing, the soaking temperature for the best combination of strength and ductility is immediately above Ac1. On the other hand, a delay before rapid cooling provides good properties if the soaking temperature is near Ac3 and the subsequent cooling is performed at a lower rate before pearlite transformation; in this case the critical cooling rate is reduced. These phenomena are discussed in terms of the growth of ferrite and the diffusion of alloying elements inclusive of Mn during slow cooling.

Effect of composition and process variables on Nb(C, N) precipitation in niobium microalloyed austenite

Abstract: Nucleation theory and the solubility product of niobium, carbon, and nitrogen in austenite have been used to derive equations for the start of Nb (C, N) precipitation as a function of temperature and composition. The predicted curves have been compared with the experimental observations of several authors to determine the effects of thermomechanical processing variables on the density of preferred nucleation sites and to incorporate these in the equations. Good agreement between the predicted and observed forms of precipitation curve is obtained with consistent constants in the equations when account is taken of the influence of different methods of detecting the onset of precipitation. Combining the calculated precipitation start curves with the dependence of recrystallization kinetics on composition and thermomechanical process variables when all niobium is in solution leads to prediction of the lower temperature limit for complete recrystallization and of the upper temperature limit for effecti...

Coupled temperature, stress, phase transformation calculation

Abstract: The quenching of steels involves thermal, mechanical, and structural phenomena and their couplings. In this paper, a coupled thermal, phase transformation, internal stresses calculation model is presented. Especially, the stress-phase transformation interactions (transformation plasticity and kinetics modifications through internal stresses) are taken into account in this model, not only for martensitic transformation but also for diffusion dependent transformation. Using a specific case, the cooling of a cylinder made of eutectoid carbon steel, an analysis of how the stress phase transformation interactions affect the internal stresses, and plastic strain evolutions during cooling are performed. The calculated results show that internal stresses have an important effect on the kinetics of pearlitic transformation. These changes in transformation kinetics modify the levels of the internal stresses themselves and the residual stresses.

The effect of aluminum on the work hardening and wear resistance of hadfield manganese steel

Abstract: A study has been made of the work-hardening and wear resistance of aluminum-modified Hadfield manganese steels ranging in composition from 1.00 to 1.75 Pct carbon and from 0.0 to 4.0 Pct aluminum. Aluminum additions reduced carbon activity and diffusivity in austenites of Hadfield’s composition, increasing the metastable solubility of carbon in Hadfield steel. Aluminum additions inhibited mechanical twinning and, by inference, increased the stacking fault energy of austenite. Increasing carbon in solution in austenite expanded the temperature range over which dynamic strain aging and rapid work hardening occurred. Simultaneous aluminum additions and increased carbon content increased the work-hardening rate and high-stress abrasion resistance of Hadfield steel, but there was an optimum aluminum content beyond which both declined. Maximum work-hardening rate was exhibited by an alloy containing nominally 1.75 Pct C, 13.5 Pct Mn, and 1.3 Pct Al. Improved high-stress abrasion resistance was also found in an alloy containing nominally 1.00 Pct C, 13.5 Pct Mn, and 4.0 Pct Al.

Carbonitride precipitation in niobium/vanadium microalloyed steels

Abstract: A detailed study of carbonitride precipitation in niobium/vanadium microalloyed steels is presented. A thermodynamic model is developed to predict the austenite/carbonitride equilibrium in the Fe−Nb-V-C-N system, using published solubility data and the Hillert/Staffansson model for stoichiometric phases. The model can be used to estimate equilibrium austenite and carbonitride compositions, and the amounts of each phase, as a function of steel composition and temperature. The model also provides a method to estimate the carbonitride solution temperatures for different steel compositions. Actual carbonitride precipitation behavior in austenite is then examined in two experimental 0.03Nb steels containing 0.05V and 0.20V, respectively. Samples were solution treated, rolled at 954°C (20 pct or 50 pct), held isothermally for times up to 10,000 seconds at 843°C, 954°C, or 1066°C, and brine quenched. The process of carbonitride precipitation in deformed austenite is followed by analytical electron microscopy (AEM) of carbon extraction replicas. Precipitates are observed at prior-austenite grain boundaries, and also within the grains (presumably at substructure introduced by the rolling deformation). Analysis of the grain-boundary and matrix precipitate compositions by AEM indicates that the grain-boundary precipitates are consistently richer in vanadium than the matrix precipitates, although compositional trends with holding time and temperature are similar for the two types of precipitates. The compositions of both the grain-boundary and matrix precipitates are not significantly influenced by the rolling reduction or the holding time at temperature. As predicted by the thermodynamic model, the precipitates become more vanadium-rich as the vanadium level in the steel is increased and as the temperature is reduced. The agreement between the measured and predicted precipitate compositions is quite good for the grain-boundary precipitates, although the matrix precipitates are consistently more niobium-rich than predicted by the model.

Effect of rapid solidification on stainless steel weld metal microstructures and its implications on the Schaeffler diagram

Abstract: An investigation was carried out to determine the effect of rapid solidification on the weld metal microstructure of austenitic stainless steels and its implication on the ferrite constitution diagram. A wide variety of stainless steels were laser beam welded at different welding speeds and laser power levels. The results indicate that both weld pool cooling rate and the postsolidification solid-state cooling rates have a profound effect on the microstructures. For the steels investigated, the microstructures ranged from duplex austenite ([gamma]) + ferritic ([sigma]) to fully austenitic or fully ferritic. These microstructures were found to be sensitive to both cooling rates and composition. The observed results are rationalized based on rapid solidification theory. Observations of this investigation indicate that solidification rates and postsolidification cooling rates have a profound effect on the observed microstructures, thus making it impossible to predict the microstructures of rapidly cooled weld metal from the conventional constitution diagrams. The influence of the observations made in this investigation on the Schaeffler diagram is demonstrated, and possible corrections to the constitution diagram incorporating the cooling rate effects are proposed. 23 refs., 17 figs., 3 tabs.

Fracture toughness of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract: The fracture toughness of two experimental silicon-containing steels in the bainitic condition has been measured and related to the microstructural state of the steels. The optimum bainitic microstructure for high strength and high toughness combinations consists of bainitic ferrite and thin interwoven laths of retained austenite instead of cementite, this condition being achieved through the silicon addition to the steels. The thin films of retained austenite are thermally and mechanically stable and act to reduce the effective fracture grain size and also possibly help to blunt propagating microcracks; blockier volumes of retained austenite are unstable and hence not beneficial to toughness. The two experimental steels achieved strength and toughness values equal to, or better than, some commercial steels in the martensitic condition.MST/528

Effect of cooling rate on grain size of ferrite in a carbon steel

Abstract: Ferrite grain refinement by accelerated cooling has been studied in a carbon steel. The size of ferrite grains dα formed by continuous cooling transformation from polygonal austenite has been measured as a function of cooling rate and austenite grain size dγ. In the cooling rate range studied (q= 0·05–5 K s−1), dα was found to be proportional to q−0·26dγ 0·46. The mechanism of grain refinement by accelerated cooling is discussed, and it is shown that this occurs in the transformation where the ratio of nucleation to growth rate increases with a decrease in temperature. The austenite grain size dependence of ferrite grain size is shown to become progressively large as the nucleation mode changes from homogeneous to grain surface to edge to corner. A theoretical estimation of ferrite grain size formed by continuous cooling transformation was attempted on the basis of nucleation and growth rates. In the alloy studied, ferrite grain size was theoretically estimated to be proportional to q−0·17dγ 0·33...

Tensile deformation of two experimental high-strength bainitic low-alloy steels containing silicon

Abstract: Two silicon-containing low-alloy steels, Fe–0·2C–2Si–3Mn and Fe–0·4C–2Si–4Ni (nominal wt-%), isothermally transformed in the bainitic temperature range (~400–250°C) have been deformed in tension. The bainitic microstructures in these steels contain appreciable amounts of retained austenite (instead of interlath cementite), and the behaviour of this phase during tensile deformation, and its apparent influence on the mechanical properties, has been examined. In particular, it is shown that provided the retained austenite exists in an interlath, thin-film morphology it has appreciable mechanical stability. Larger volumes of retained austenite have less mechanical and thermal stability, forming plate martensite structures and also undergoing deformation twinning. The effects of these variations on tensile strength and ductility are discussed.MST/527

Optimum Microalloying of Niobium and Boron in HSLA Steel for Thermomechanical Processing

Abstract: For application to thermomechanical processing (TMP) with acceleated cooling, effect of a combined addition of boron and a grain-refining element such as niobium, titanium or vanadium in very low carbon steels was studied in terms of mechanical properties and microstructures. It was found that, although boron as a single addition has little effect on the properties, the combined addition of niobium and boron improves the strength and toughness balance. A separate addition of boron produces coarse Fe23(CB)6 precipitates at austenite grain boundaries after rolling and boron is consumed to invalidate the effect of γ-α transformation suppression. Niobium addition to boron steel suppresses the precipitation of Fe23(CB)6 and strongly retards the γ-α transformation to form a finegrained bainitic structure. Titanium has the same effect as that of niobium, but vanadium does not. The strengthening and toughening mechanism by the combined addition of niobium and boron was also investigated by examining the effect of alloying elements on the minimum recrystallization temperature of austenite during rolling, and γ-α transformation behavior.

High temperature alloys : their exploitable potential

Abstract: List of Contents.- Preface.- Opening Session.- Chairman: M. Van de Voorde, C.E.C., Joint Research Centre, Petten Establishment, Petten, NL..- Welcome and Opening of the Conference:.- Opening Thoughts - the Future of High Temperature Materials.- E.D. Hondros, Director of the Joint Research Centre, Petten Establishment, Petten, NL..- Keynote paper:.- The Capability of Current Materials in Relation to Future gas Turbine Needs.- G. Meetham, Rolls Royce Ltd., Derby, UK..- Session 1: Theoretical and Practical Limits of HT Alloys. Sub-session 1.1.: Chairman: S. Amelinckx, SCK-CEN, Mol, B..- Mechanical Properties of Nickel Base Alloys in Relation with their Microstructure (Abstract).- Modelling Creep and Fracture in Engineering Alloys.- High Temperature Creep Mechanisms in Single Crystals of Some High Performance Nickel-Base Superalloys.- Dislocation Microstructure in PM Astroloy and Ma6000 after HTLCF.- Ferritic Fe-Ni-Al Alloys for High Temperature Applications.- Metallographic Indication of Methane in Steel Cavities.- Rapporteur's synthesis.- Sub-session 1.2.: Chairman: G. Sainfort, CEA, Grenoble, F..- The Corrosion Behaviour of Alloys in High Temperature Gaseous Environments.- The Limits to the Protective Effect of Oxide Scales on High Temperature Materials Which are Subjected to Strain.- Relaxation of the Mechanical Stresses Developed Through Oxide Scales During Oxidation of Metals.- The Effects of Cerium and Cerium Oxide on the High Temperature Oxidation of Iron-Chromium Alloys at Iooo DegreesC in 100 Torr O2.- The Carburisation Behaviour of Steels for Petrochemical Plant.- An Evaluation of Fe-Cr-Al Alloys in Simulated Coal Gasifier Environments from 850-1050 DegreesC.- Environmental Resistance oF Incoloy alloy Ma956.- Identification of Oxide and Sulphide Corrosion Products Using Raman Microscopy.- The Sulphidation Behaviour of Vanadium, Niobium and Molybdenum.- The Influence of Alloy Composition Upon The Corrosion Behaviour of 25Cr-35Ni-Fe Alloys Exposed to Mixed Gaseous Oxidising/Sulphidising/Carburising Atmospheres.- High Temperature Sulphidation of Aluminised Alloys: Protective Properties of Alumina Layers.- The Laser Raman Microprobe - A New Technique for the Identification of Corrosion Scale Constituents.- Rapporteur's Synthesis.- Session 2: Potential for Development in Alloys and Processing. Sub-session 2.1.: Chairman: J. Levy, Ecole des Mines, Paris, F..- Steel and Nickel Base Alloys.- Production and Properties of Large Nimonic Alloy 901 Discs for Land-Based gas Turbines.- Development of 12Cr-1Mo-1W-V-Nb Steel for Elevated Temperature Applications.- Service Experiences of 12Cr-1Mo-W-V-Nb Steel and 9Cr-2Mo Steel in a Power Boiler.- An Austenitic Heat Resisting Steel Tube Developed for Advanced Fossil-fired Steam Plants.- Hot Working of Duplex Stainless Steels and Nickel-Iron Superalloys.- Development of a Duplex Structure 12Cr-1.5Mo-1W Steel for Elevated Temperature Applications.- The Influence of Powder Processing Variables on the Structure and Properties of Hiped Low Carbon Astroloy.- Sensitivity of Mechanical Properties of Nickel Base Alloys to Cooling rates.- Effects of Cobalt on the Microstructures and Mechanical Properties of the Fe-Ni-Co Base Superalloy Refractoloy 26.- Properties and Applications of Hot Formed O.D.S. Alloys.- Influence of Regeneration Treatments on Creep Rupture Lifes of Turbine Blades.- Rapporteur's Synthesis.- Sub-session 2.2.: Chairman: J.F. Conde, Admiralty Marine Technology, Poole, UK..- Coatings - their Potential and Limits (Abstract).- Diffusion Phenomena Between Coating and Base Material in Ni-Base Superalloy.- The High Temperature Oxidation Behaviour of the Single Crystal Ni-Base alloy M002 Mod. with and without Protective Coatings.- Rapporteur's Synthesis.- Session 3: Engineering Considerations. Sub-session 3.1.: Chairman: J. Schijve, Technical University, Delft, NL..- Thermo-Mechanical Loading Constraints (Abstract).- Fatigue and Microstructure.- Low Cycle Fatigue Behaviour of Inconel Alloy 617.- Influence of Heat Treatment on Fatigue of Inconel 718 Alloy at 650 DegreesC.- Rapporteur's Synthesis.- Sub-session 3.2.: Chairman: J.R. Donati, Electricite de France, Moret, F..- Factors Governing Design Rules for HT Components.- Analysis of Exhaustion and Damage on Creep-Exposed Components.- Corrosion and Creep of Nickel-Base Alloys in Steam Reforming GAS.- The Creep Ductility and Fracture of Carburised Alloy 800H at HIGH Temperatures.- Qualification of Stationary Creep of the Carbide Precipitating Alloy 800H.- Rapporteur's Synthesis.- Session 4: Future Outlook. Chairman: H. Nickel, K.F.A., Jiilich, FRG..- The Potential and Problems of Engineering Ceramics (Abstract).- Contribution to the Quantification of the Development Potential of High Temperature Alloys.- Rapporteur's Synthesis.- Final Discussion.- Rapporteur's Synthesis.- List of Participants.

Cutting mechanism during machining of hardened steel

Abstract: Chip segmentation and the tool forces involved during cutting of hardened steel are discussed. AISI 4340 steel was machined on an engine lathe to study chip morphology, tool forces, and the surface generated. It was found that chip segmentation occurs when the hardness of the steel exceeds a certain value, and that the tool forces associated with chip segmentation are very high. A transformed layer of untempered martensite and retained austenite was produced when the cutting conditions were severe.MST/469

Microstructure and mechanical properties of medium-carbon ferrite–pearlite steel microalloyed with vanadium

Abstract: Microstructural analysis and mechanical testing have been carried out on medium-carbon steels to which additions of vanadium in the range 0·075–0·6 wt-% were made. The steels were either continuously cooled or isothermally heat treated after austenitization. Vanadium carbide precipitation in the proeutectoid ferrite regions of the microstructure and, more unusually, also in the pearlitic ferrite lamellae, were identified by transmission electron microscopy. Moreover, in both ferrite phases the precipitates are aligned in rows, indicative of interphase precipitation at the austenite/ferrite transformation interface. These observations are discussed in terms of the various mechanisms that have been proposed for the interphase precipitation reaction. In the alloys studied the vanadium additions were found to increase the strength of the steels by up to 100%, but to reduce the ductility and notched impact resistance. The most useful combination of increased strength with reasonable ductility and impac...

MICROSTRUCTURE AND WEAR PROPERTIES OF LASER CLAD Fe-Cr-Mn-C ALLOYS.

Abstract: The laser surface cladding technique was used to formin situ Fe-Cr-Mn-C alloys on AISI 1016 steel substrate. In this process mixed powders containing Cr, Mn, and C with a ratio of 10∶1∶1 were delivered using a screw feed, gravity flow carrier gas aided system into the melt pool generated by a 10 kw CO2 laser. This technique produced ultrafine microstructure in the clad alloy. The microstructure of the laser surface clad region was investigated by optical, scanning, and transmission electron microscopy and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic or retained austenite phase. In preliminary wear studies the laser clad Fe-Cr-Mn-C alloys exhibited far superior wear properties compared to Stellite 6 during block-on-cylinder tests. The improved wear resistance is attributed to the fine distribution of metastable M6C carbides.

Strain induced martensite formation in stainless steel

Abstract: The Conversion Electron and X-ray Mossbauer studies of the surface of Type 316 stainless steel at 400 K, 300 K, and 100 K show that both the substitutional and interstitial elements perturb the cubic symmetry at the iron site. The single peak of austenite is a superposition of at least five quadrupole split doublets whose magnitudes and intensities depend on the type and concentration of the impurity elements. However, when the surface of the stainless steel is plastically deformed, a layer of martensite about 5000 A thick is formed on the austenite base. This layer consists of a mixture of 31 pct martensite with the rest being the original austenite. The magnetic environment of the iron in this martensite is controlled by the concentration of alloying elements, and the distribution of the hyperfine fields is determined by the number of nearest and next nearest neighbor impurity atoms. The magnetic field decreases linearly at first as the number of nearest neighbors increases and then follows a nonlinear trend for a number of nearest neighbors. The temperature dependence of the sublattice magnetization is different for each number of neighbors, and a Curie temperature has been estimated for each site.

Interlaboratory Evaluation of ASTM Practice for X-Ray Determination of Retained Austenite in Steel with Near-Random Crystallographic Orientation (E 975)

Abstract: A standard practice (ASTM E 975) was developed by Task Group E04.0602 for the X-ray diffraction determination of retained austenite in steel. Results of interlaboratory studies of retained austenite samples performed before and after the practice was developed are described. The final interlaboratory study using ASTM E 975 and ASTM E 173 as a guide to the study resulted in an estimate of repeatability within the same laboratory of 3% retained austenite for a 95% confidence limit, and an estimate of reproducibility between laboratories of 4% retained austenite for a 95% confidence limit.

The effect of chromium in high carbon bearing steels

Abstract: Chromium is present in the 52100 bearing steel composition in the range 1.30 to 1.60 wt pct. Chromium has a significant influence on the spheroidization of cementite, finer carbides being formed due to chromium additions. The ferrite to austenite transformation temperatures are increased due to chromium. The microchemistry of the cementite in 52100 changes during heat treatment; the chromium content of secondary carbides is generally lower than that in the spheroidal (FeCr)3C produced by soft annealing. The rate of carbide dissolution is controlled by the rate of chromium diffusion from the carbide-matrix interface. Also, the chromium content of the residual, spheroidal (FeCr)3C increases during austenitization. The effect of substitution of chromium in bearing steel compositions is discussed. In view of the beneficial effect of chromium, only substitution by similar strong carbide forming elements should be considered for bearing steels.

Improved lower temperature fracture toughness of ultrahigh strength 4340 steel through modified heat treatment

Abstract: A modified heat treatment has been suggested whereby lower temperature plane-strain fracture toughness (KIC) of 4340 ultrahigh strength steel is dramatically improved in developed strength and Charpy impact energy levels. The modified heat-treated 4340 steel (MHT-4340 steel) consists of a mixed structure of martensite and about 25 vol pct lower bainite which appears in acicular form and partitions prior austenite grains. This is produced through isothermal transformation at 593 K for a short time followed by an oil quench (after austenitizing at 1133 K and subsequent interrupted quenching in a lead bath at 823 K). The mechanical properties obtained at room temperature (293 K) and 193 K have been compared with those achieved using various heat treatments. Significant conclusions are as follows: the MHT-4340 steel compared to the 1133 K directly oil-quenched 4340 steel increased theKIC values by 15 to 20 MPa • m1/2 at increased strength and Charpy impact energy levels regardless of the test temperature examined. At 193 K,KIC values of the MHT-4340 steel were not less than those of the 1473 K directly oil-quenched 4340 steel, in whichKIC values are significantly enhanced at markedly increased strength, ductility, and Charpy impact energy levels. The MHT-4340 steels compared to austempered 4340 steels at 593 K, which have excellent Charpy impact properties, showed superiorKIC values at significant increased strength levels irrespective of test temperatures. The lower temperature improvement inKIC can be attributed to not only the crack-arrest effect by acicular lower bainite but also to the stress-relief effect by the lower bainite just ahead of the current crack.

Process for the production of a strip of a chromium stainless steel of a duplex structure having high strength and elongation as well as reduced plane anisotropy

Abstract: Process for producing steel strip of duplex structure wherein cold rolled strip of chromium stainless steel comprising, in addition to Fe, 10.0% to 20.0% of Cr, to 0.10% C, to 0.12% of N, the (C+N) being 0.01% to 0.20%, to 2.0% of Si, to 4.0% of Mn to 4.0% of Ni and to 4.0% of Cu, the {Ni+(Mn+Cu)/3} being not less than 0.5% but not more than 5.0%, is continuously passed through a heating zone where it is heated to form a two-phase of ferrite and austenite and the heated strip is cooled at a cooling rate sufficient to transform the austenite to martensite. The product has high strength and elongation reduced plane anisotropy and hardness of at least HV 200.

The precipitation sequence of Ni 3 Ti in Co-free maraging steel

Abstract: TEM, microdiffraction, and X-ray microanalysis were used to study the precipitation processes in Co-free maraging steel. Austenite crystals were found to nucleate in the martensite matrix as the first step in the precipitation sequence of Ni3Ti. The austenite reversion is a result of Ni segregation. Ni3Ti nucleates in the austenite. The shape and orientation of Ni3Ti is determined by the austenite precursor. The same sequence occurs for heterogeneous nucleation on dislocations and grain boundaries. At the later stages of growth Mo is incorporated in the Ni3Ti lattice.

Effects of Nitrogen on the Selective Dissolution of a Duplex Stainless Steel

Abstract: Nitrogen (N) has been shown to exert a beneficial influence on localized corrosion resistance of austenitic stainless steels. With duplex stainless steels (SSs), N plays a dual role of controlling the austenite to ferrite ratio and increasing corrosion resistance. In addition, on rapid cooling from high temperatures, N can form deleterious precipitates in the ferrite reducing corrosion resistance. The corrosion resistance and corrosion modes are quite different in N and non-N duplex SSs. These differences are illustrated for a 25%Cr duplex SS.