Showing papers on "Austenite published in 1989"

Microstructural development during solidification of stainless steel alloys

Abstract: The microstructures that develop during the solidification of stainless steel alloys are related to the solidification conditions and the specific alloy composition. The solidification conditions are determined by the processing method,i.e., casting, welding, or rapid solidification, and by parametric variations within each of these techniques. One variable that has been used to characterize the effects of different processing conditions is the cooling rate. This factor and the chemical composition of the alloy both influence (1) the primary mode of solidification, (2) solute redistribution and second-phase formation during solidification, and (3) the nucleation and growth behavior of the ferrite-to-austenite phase transformation during cooling. Consequently, the residual ferrite content and the microstructural morphology depend on the cooling rate and are governed by the solidification process. This paper investigates the influence of cooling rate on the microstructure of stainless steel alloys and describes the conditions that lead to the many microstructural morphologies that develop during solidification. Experiments were performed on a series of seven high-purity Fe-Ni-Cr alloys that spanned the line of twofold saturation along the 59 wt pct Fe isopleth of the ternary alloy system. High-speed electron-beam surface-glazing was used to melt and resolidify these alloys at scan speeds up to 5 m/s. The resulting cooling rates were shown to vary from 7°C/s to 7.5×106°C/s, and the resolidified melts were analyzed by optical metallographic methods. Five primary modes of solidification and 12 microstructural morphologies were characterized in the resolidified alloys, and these features appear to be a complete “set” of the possible microstructures for 300-series stainless steel alloys. The results of this study were used to create electron-beam scan speedvs composition diagrams, which can be used to predict the primary mode of solidification and the microstructural morphology for different processing conditions. Furthermore, changes in the primary solidification mode were observed in alloys that lie on the chromium-rich side of the line of twofold saturation when they are cooled at high rates. These changes were explained by the presence of metastable austenite, which grows epitaxially and can dominate the solidification microstructure throughout the resolidified zone at high cooling rates.

INCONEL 718: A solidification diagram

Abstract: As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to generate a constitution diagram for INCONEL 718* in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made using the Scheil solidification model, and the fraction of each phase in the eutectic is calculatedvia the lever rule. These are compared with experimentally determined values and found to be in good agreement.

Effects of Deformation Induced Phase Transformation and Twinning on the Mechanical Properties of Austenitic Fe–Mn–Al Alloys

Abstract: Structure and mechanical properties of austenitic Fe–(20 and 30)Mn–(0 to 7) Al alloys in the temperature range between 77 and 295 K have been investigated in relation to the occurrences of phase transformation and deformation twinning. Additions of aluminum to the 20 wt% Mn alloys significantly decreased the γ→e transformation temperature. The yield stress of these alloys increased with increasing aluminum content, whereas the strain hardening of them decreased. This tendency is prominent at low temperatures. In the 30 wt% Mn alloys the yield stress and strain hardening were almost identical regardless of aluminum contents. Additions of aluminum strongly suppress the γ→e transformation and give birth to the occurrence of deformation twinning. Calculated stacking fault energy based on a regular solution approach shows that the austenitic Fe–Mn–Al alloys which have the stacking fault energy approximately larger than 20 erg/cm2 favor the deformation twinning leading to the increase in low temperature ductility.

Martensite formation, strain rate sensitivity, and deformation behavior of type 304 stainless steel sheet

Abstract: The strain and strain rate dependence of the deformation behavior of Type 304 stainless steel sheet was evaluated by constant temperature tensile testing in the temperature range of −80 °C to 160 °C. The strain rate sensitivity, strain hardening rate, and ductility reflected the compctition of two strengthening mechanisms: strain-induced transformation of austenite to martensite and dislocation substructure formation. At low temperatures, the strain rate sensitivity and strain hardening rate correlated with the strain-induced transformation rate. A maximum in total ductility occurred between 0 °C and 25 °C, and the contributions of strain rate sensitivity and strain hardening to independent maxima with temperature of the uniform and post-uniform strains are discussed.

Effect of phosphorus on the formation of retained austenite and mechanical properties in Si-containing low-carbon steel sheet

Abstract: The effect of phosphorus and silicon on the formation of retained austenite has been investigated in a low-carbon steel cold rolled, intercritically annealed, and isothermally held in a temperature range of bainitic transformation followed by air cooling. The steel sheet containing phosphorus after final heat-treatment consisted of ferrite, retained austenite, and bainite or martensite. Phosphorus, especially in the presence of silicon, in steel was useful to assist the formation of retained austenite. Mechanical properties, such as tensile strength, uniform elongation, and the combination of tensile strength/ductility, were improved when phosphorus was increased up to 0.07 pct in 0.5 pct Si steel. This could be attributed to the strain-induced transformation of retained austenite during tensile deformation. Furthermore, two types of retained austenite were observed in P-containing steel. One is larger than about 1 μm in size and usually exists adjacent to bainite; the other one is of submicron size and usually exists in a ferrite matrix. High phosphorus content promotes the formation of stable (small size) austenites which are considered to be stabilized mainly by their small size effect and have a different formation mechanism from the coarser retained austenite in the lower P steels. The retained austenites of submicron size showed mechanical stability even after 10 pct deformation, suggesting that these small austenites have little effect on ductility. The 0.07 pct P-0.5 pct Si-1.5 pct Mn-0.12 pct C steel showed a high strength of 730 MPa and a total elongation of 36 pct.

Nitrogen in austenitic stainless steels

Abstract: Nitrogen alloyed in austenitic stainless steels improves austenite stability, mechanical properties and corrosion resistance. Steels supersaturated with nitrogen (“super-nitrogen steels”) have been investigated, which rival the latest ferritic steels in strength but have potentially greater toughness.

Solidification of high-speed tool steels

Abstract: Gradient solidification and differential thermal analysis (DTA) experiments were used to study the process of solidification and the solidification microstructure of 11 alloys comprising the composition range of customary commercial high-speed steels (with the exception of cobalt-alloyed grades). Also included are a number of experimental high-speed steels alloyed with niobium. The results include the effects of alloy composition and cooling rate on the width of the solidification interval and on the sequence of the solidification reactions; the types of eutectics formed (austenite with M6C, M2C, or MC) and their volume fractions; the chemical compositions of the ledeburitic and primary carbides; and the relation between the chemistry of the carbides and that of the melt. Special attention is given to the formation and composition of heterogeneously nucleated primary MC particles and to the chemistry and stability of eutectic M2C, which is important as a precursor to MC and M6C in the microstructure of finished (hot-worked and heat-treated) material.

Heat transfer during Nd: Yag pulsed laser welding and its effect on solidification structure of austenitic stainless steels

Abstract: Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions. The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur. There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to provide detailed information regarding the heat transfer conditions experienced during welding.

Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam

Abstract: Several Fe-Cr-Ni austenitic stainless steels (Cr wt.%: 13–25, Ni wt.%: 15) were oxidized in steam for 1000 hr at 500–900°C. The oxide scales were examined and categorized with respect to the chromium concentration and the grain size of the base metal. Experiments showed three conditions for the critical bulk Cr concentration and the oxidation temperature at which the oxidation behavior changed drastically. Metallographic examination showed that two of these three conditions resulted from the internal-external transition of Cr2O3 either on the metal surface or along the grain boundaries of the base metal. Attempts were made to interpret these conditions from the available oxidation theories. Atkinson's treatment was employed with some modification to incorporate the grain-boundary diffusion of Cr in the base metal. The calculation basically explained the internal-external transition for the oxidation of these steels.

Hydrogen permeation behaviour in austenitic stainless steels

Abstract: The permeability, diffusivity and solubility of hydrogen in six types of austenitic stainless steel—316L, 316LN, 21-6–9, 21-9-9, 304 and 321—have been measured by a gaseous permeation technique in the temperature range 200–430°C. The effect of the cold-working and heat treatment conditions and the alloy composition of the materials on hydrogen permeation has been investigated. The results indicate that the permeability and diffusivity of hydrogen in various alloys obey Arrhenius relationships over the experimental temperature range and the hydrogen permeation behaviour is not significantly influenced by cold-working and heat treatment conditions of the materials but is slightly influenced by alloy composition. The difference between the hydrogen permeation behaviour of pure iron, general alloying steels and austenitic stainless steels has been discussed, and a comparison between the present work and data in the literature has been made.

Nucleation kinetics of Ti carbonitride in microalloyed austenite

Abstract: Ti(CN) precipitation data determined by stress relaxation are analyzed using the classical theory of diffusion controlled nucleation. For this purpose, the interface energy γ and strain energy ΔGɛ accompanying nucleus formation are estimated using a new approach, and the driving force for Ti(CN) nucleation is calculated with the aid of a thermodynamic model. The analysis indicates that the critical nucleus is richer in N than the bulk precipitate at equilibrium at a given holding temperature. The results also show that trace amounts of nitrogen dissolved in austenite can significantly increase the chemical driving force for Ti(CN) nucleation and thereby accelerate the rate of precipitation. On the basis of this analysis, a kinetic model is developed for predicting start times (PS) for the strain-induced precipitation of Ti(CN) in austenite. Such predictions are in reasonably good agreement with measuredPS times.

Austenite recrystallization and carbonitride precipitation in niobium microalloyed steels

Abstract: The response of austenite to thermomechanical treatment is investigated in two series of niobium microalloyed steels. Optical and electron metallographic techniques were used to follow the austenite recrystallization and carbonitride precipitation reactions in these steels. The first series of steels contained a constant level of 0.05Nb, with carbon levels varying from 0.008 to 0.25 pct. It was found that a lower carbon concentration results in faster austenite recrystallization, due to a smaller carbonitride supersaturation, which leads to a reduced precipitate nucleation rate. The second series of steels was designed with a constant carbonitride supersaturation, by simultaneously varying the Nb and C concentrations while maintaining a constant solubility product. In these steels, the recrystallization kinetics increase as the volume fraction of Nb(C, N) is reduced and/or as the precipitate coarsening rate is increased. The volume fraction of carbonitrides increases as the Nb: (C+12/14 N) ratio approaches the stoichiometric ratio of approximately 8:1. The precipitate coarsening rate was shown to increase with increasing amounts of niobium remaining in solution in the austenite (i. e., “excess” Nb after precipitation). As expected, recrystallization proceeds more slowly at lower temperatures and after a reduced amount of deformation. An experiment to determine whether Nb atoms dissolved in the austenite could exert a significant solute-drag effect on the recrystallization reaction indicated that 0.20Nb in solution could reduce the rate of recrystallization compared to a Nb-free C-Mn steel. However, this solute effect was smaller than the retarding effect which 0.01Nb can have when it is precipitated in the form of carbonitrides on the austenite substructure after rolling.

Phase transformations of a nitrogen-implanted austenitic stainless steel (X10 CrNiTi 18-9)☆

Abstract: The available results of research work performed on nitrogen-implanted austenitic CrNi steels are contradictory. The following characteristics have been observed: formation of martensite in the austenite as well as complete retransformation of martensite into austenite; solution of nitrogen up to high concentrations as well as precipitation of nitrides at low concentrations; an increase as well as a reduction of the wear resistance. The transformation of austenitic CrNi steels under nitrogen bombardment is apparently not in accordance with a fixed pattern but can go in different directions. Therefore a systematic phase analysis in conjunction with hardness and corrosion tests is indispensable for process development. On the other hand, this system is of particular interest as a model for studying phase reactions in steels with nitrogen implantation because of the variety of transformations. In this paper we first give an outline of the problems encountered in the treatment of austenitic steels with nitrogen ion beams and introduce our programme for the investigation. We then discuss conditions for γ ⇌ α transformations in either direction. In this context we differentiate between pure irradiation impact and alloying effects caused by the constitutional changes. Further investigations cover the formation of nitrides in pure austenite and the development of surfaces with different pretreatment. The analytical methods used are conversion electron Mossbauer spectroscopy, X-ray diffraction, and depth profiling by nuclear reaction analysis.

Composition and temperature dependence of tensile properties of austenitic FeMnAlC alloys

Abstract: The effects of aluminium addition (0–4 wt%) to Fe-26MnAl alloys and carbon addition (01 – 03 wt%) to Fe-30Mn-1AlC alloys on tensile properties, especially tensile elongation, were investigated from room temperature to 4 K The transformation of austenite to deformation twins during the tests was very beneficial in enhancing tensile elongation at cryogenic temperatures The amount of deformation twins formed during plastic deformation was not the major factor for maximum elongation, but the optimum work hardening rate by the gradual formation of deformation twins played an important role The maximum elongation peak shifted to lower temperatures with increased aluminium (5 aluminium wt%), but moved to higher temperatures with increased carbon content (03 wt%) The new flow equation σ = Kϵ N exp ( Mϵ ) was applied to calculate uniform elongations The calculated values were in reasonable agreement with the measured values

Application of continuous cooling transformation diagrams for welding of steels

Abstract: In the search for better and more consistent properties in steel welds, the study of microstructure versus mechanical properties has indicated a clear need for a more systematic and detailed presentation of the γ→α phase transformation reaction. The usual method of presenting such data is the continuous cooling transformation (CCT) diagram which relates the composition, cooling rate, and austenite grain size of the material to its γ→α. transformation temperature and resultant microstructure. Many hundreds of such diagrams have been produced throughout the world to describe the transformation behaviour of most grades of commercial steel when subjected to various heat treatment procedures. The majority of these diagrams are designed for typical industrial heat treatment processes, which usually involve re-austenitisation in the low temperature region of the austenite phase field (850–900°C), followed by continuous cooling by quenching, air cooling, or furnace annealing. Unfortunately, such diagrams ...

Kinetics of precipitation in 17–4 PH stainless steel

Abstract: The sequence of precipitation and its kinetics in 17–4 PH (precipitation hardening) stainless steel were studied by observing the electrical resistivity and microstructure of the alloy during isothermal aging at various temperatures in the range 320–600°C. By the absence of an incubation period for the onset of precipitation it is shown that there is no free energy barrier to nucleation. The electrical resistivity of the specimen decreased on prolonged aging approaching a steady value asymptotically with time. The alloys aged above 550°C were found to have higher final resistivity values than those aged at lower temperatures. By transmission electron microscopy, local reversion of the martensite to austenite, attributed to enhanced diffusion and concentration of copper atoms at the lath boundaries, was revealed in the specimens aged at temperatures above 550°C. The kinetics of precipitation in the system obeyed the Johnson–Mehl equation. The activation energy Q of the precipitation process was est...

Matrix microstructure effect on the abrasion wear resistance of high-chromium white cast iron

Abstract: The two-body abrasion resistance of high-chromium white cast iron was investigated as a function of cast iron microstructure. Different microstructures were obtained by means of heat treatment. The chromium and carbon content were chosen in order to have different matrix microstructures (austenitic, martensitic and ferritic) with the same amount of eutectic carbide (M7C3). The results show that the cast iron with an austenitic matrix has the best wear resistance. The good wear resistance of this material is due to strong work hardening of the austenitic matrix resulting in a hardness which exceeds that of other structures. The effect of abrasive paper deterioration on abrasion has also been investigated.

Method of treating a sample of an alloy

Abstract: A method of treating a sample of an alloy which is capable of transforming between martensitic and austenitic phases, to render the alloy pseudoelastic, the method comprising: (a) annealing the alloy at a temperature which is greater than the stress relaxation temperature (T SR ) of the alloy and less than the temperature at which the alloy is fully recrystallized (T x ); and (b) deforming the sample at a temperature which is greater than about the maximum temperature at which the alloy can be made to transform from its austenitic phase to its martensitic phase by the application of stress (M d ), and less than the stress relaxation temperature.

Dependence of strength–ductility characteristics on thermal history in lowcarbon, 5 wt-%Mn steels

Abstract: As a result of investigations into the improvement of the strength–ductility characteristics of some C–Mn steels, it has been found that the strength–ductility combination (expressed numerically as tensile strength × uniform elongation) of as hot rolled 0·1C–5Mn steels, with or without 2%Si, is greatly improved if an optimum reheating treatment in the range 650–675°C is applied to the steel. Since there is a direct connection with a large increase in the work hardening rate, it is very likely that this improvement is the result of the transformation induced plasticity of well stabilised austenite which has been formed by the reheating treatment.MST/798

Fracture and fatigue crack propagation properties of hardened 52100 steel

Abstract: Good toughness in hardened 52100 ball bearing steel is important in order to prevent premature fracture during mounting or service of bearing elements. Steel cleanliness, residual copper content, and carbon content effects have been investigated in relation to fracture mechanics properties, and it was observed that only the carbon content has any relevance for the range of compositions investigated. The effect of hardening and tempering temperatures for conventional furnace-hardening techniques on toughness was investigated, theKlcbeing generally much less sensitive to these parameters than blunt notch toughness testing. Cold deformation of the material prior to martensitic hardening significantly increased the blunt notch toughness. Thermal grain refining treatments did not give the same improved blunt notch toughness as observed for prior cold deformation. Short austenitization cycles (ten seconds) for martensitic hardening resulted in microstructures with high retained austenite contents. This microstructure resulted in higher fracture toughness and retardation of the crack growth rates, the mechanism being associated with transformation toughening in the plastic zone. Inductive tempering of martensitic-hardened 52100 was observed to result in similar blunt notch toughnesses as compared to furnace tempered material of the same hardness. A poor correlation between fracture toughness and blunt notch toughness was observed, particularly for the unstable structures,i.e., microstructures with high levels of retained austenite. Fracture toughness does not represent the intrinsic toughness of high carbon martensite with related high contents of retained austenite.

Effects of alloying elements on NaCl-induced hot corrosion of stainless steels

Abstract: Various stainless steels were examined to investigate the effects of alloying elements on NaCl-induced hot corrosion properties at temperatures below its melting point. The corrosion rate increases with increasing testing temperature. Austenitic stainless steels exhibit better hot corrosion resistance than ferritic stainless steels. Aluminium is effective in improving the hot corrosion resistance of ferritic stainless steel. Silicon improves the hot corrosion resistance of austenitic stainless steel, and nickel is also effective. Corrosion is increased when Cr 2 O 3 reacts with NaCl to form Na 2 CrO 4 instead of a protective scale. It is considered that austenitic stainless steel with high silicon content forms SiO 2 which is not liable to react with NaCl.

Small area MXPS- and TEM-measurements on temper-embrittled 12% Cr steel

Abstract: Martensitic stainless steel is widely used as corrosion resistant material exhibiting occasionally temper-embrittlement failures after a thermal treatment in the temperature range of 450 ~ 550 ~ The origin of these failures is predominantly the grain boundary segregation of trace elements which causes a reduction of the cohesive forces and thus an intercrystalline embrittlement of the alloys [1]. This process depends on the composition, the microstructure and the thermal and mechanical treatment of the steel. Specifically, the precipitation or dissolution of carbide phases at the grain boundaries triggers the segregation process of phosphorus which is the most critical element for temper embrittlement of chromium steels [2]. This short communication deals with the reversible temper embrittlement of a X-10-Cr-I 2 martensitic steel with 220 ppm phosphorus. Previous investigation of intercrystalline fracture surfaces by Scanning Auger Electron Spectroscopy have shown that the P concentration at the grain boundaries increases with a growing chromium carbide coverage at the grain boundaries [3]. The objective is to identify the carbide phase that is linked to the segregation process. From TEM investigation it is known that Cr23C6 carbides are located at the prior austenite grain boundaries which so far were considered to be the crack planes. Recent studies revealed smaller Cr7C 3 carbide precipitates at subboundaries between the martensite plates very close to the prior austenite boundaries. These Cr7C3 carbides were surrounded by a stress field which is a typical sink for P atoms. After a deembrittling annealing (15 min 625 ~ the very small Cr7C3 carbides were completely dissolved and the phosphorus enrichment of the fracture surfaces significantly reduced [4]. For a better understanding of these reversible temperembrittlement phenomena it is therefore of particular interest to identify the carbides that are present at the fracture surface of temper-embrittled steel. For an unambiguous identification of the type of carbides at the fracture surface small area monochromated photo electron spectroscopy (MXPS) seems at present to be the most appropriate technique. The pronounced chemical shift of the C Is peak clearly distinguishes between different metal carbides and

Calculation of the Ti(C y N1−y )−Ti4C2S2−MnS-austenite equilibrium in Ti-bearing steels

Abstract: A thermodynamic model is presented for the equilibria among various precipitates (Ti(C y N1−y ), Ti4C2S2, and MnS) and austenite containing six alloying elements (C, Mn, N, S, Si, and Ti). This model is applied to four microalloyed steels with Ti levels of 0.05, 0.11, 0.18, and 0.25 pct. The calculations show that the Ti in these steels cannot be completely dissolved over the austenite temperature range. However, the compositions of the undissolved Ti carbonitrides differ significantly from pure TiN, as 10 to 40 pct of the nitrogen is replaced by carbon. An expression for the Gibbs energy for the formation of Ti4C2S2 in austenite is estimated. The present predictions are compared with those of the Hudd, Jones, and Kale (HJK) model; considerable differences are observed at temperatures below 1250°C.

Effect of magnetic fields on martensitic transformations in ferrous alloys and steels.

Abstract: The recent works carried out by the authors' research group on magnetic field-induced martensitic transformations are reviewed, which are concerned with various kinds of ferrous alloys, such as Fe-Ni poly- and mono-crystals, invar and non-invar Fe-Ni-C polycrystals, discordered and ordered Fe-Pt polycrystals, ausaged Fe-Ni-Co-Ti polycrystals and paramagnetic Fe-Mn-C polycrystals. The works clarified influences of composition, the existence of grain boundaries, crystal orientation, invar characteristic, thermoelastic nature and austenitic magnetism on the magnetic field-induced martensitic trnasformations. In the work on the ausaged Fe-Ni-Co-Ti alloy, the appearance of ''magnetoelastic martensitic transformation'' was newly found. By taking into account the influences of composiiton, grain boundaries, crystal orientation, invar characteristic, thermoelastic nature and austenite magnetism, a new and exact equation was proposed to generally expalin the shift of Ms temperature as a function of critical magnetic field to induce martensitic transformations in those alloys, which consisted of three terms of the Zeeman energy, high field susceptibility energy and forced volume magnetostriction energy. The new and exact equation was experimentally verified to hold in all the alloys studied.

On microstructure-property correlation of thermally aged type 316L stainless steel weld metal

Abstract: This paper deals with the microstructural changes and consequent deterioration in the room temperature tensile properties of type 316L stainless steel weld metal when exposed to elevated temperatures (773 to 973 K) for prolonged periods (up to 5000 hours). The microstructure-property correlation derived in this study is based on a variety of techniques: Magne-Gage, electrochemical extraction, X-ray diffraction, tensile testing, and both optical and electron microscopy. It has been established that the amount and morphology of the sigma phase are the key factors in determining the changes in the strength levels, total elongation, and extent of work hardening. The amount and morphology of sigma, in turn, is seen to depend on the relative kinetics of the various transformations, such as dissolution of delta-ferrite, growth of carbides,etc., shape changes in sigma, and the relative stabilities of the phases at the corresponding temperature of aging. The complicated dependence of the tensile properties on the microstrutural changes has been explained with direct quantitative evidence.