Showing papers on "Austenite published in 1999"

Martensite in steel: strength and structure

Abstract: This paper reviews the strengthening mechanisms associated with the various components of martensitic microstructures in steels and other ferrous alloys. The first section examines the experiments and strengthening theories associated with Fe–Ni and Fe–Ni–C alloys, in which the martensite, because of subzero Ms temperatures, can be evaluated with carbon atoms trapped in octahedral interstitial sites. The evaluation of strengthening in these alloys has been limited to interpreting yield strength of unaged, untempered martensite in terms of interstitial solid solution strengthening. The second section reviews strengthening of martensitic Fe–C alloys and low-alloy carbon steels with above-room-temperature Ms temperatures. In these alloys, it is impossible to prevent C diffusion during quenching, and strengthening of martensite becomes dependent on static and dynamic strain aging due to carbon atom interaction with dislocation substructure. In all alloys the dominant strengthening component of martensitic microstructures is the matrix of martensitic crystals, either in lath or plate morphology, but secondary effects due to other microstructural components such as carbides and retained austenite are also discussed.

Characteristic features of shape memory effect and related transformation behavior in Fe-based alloys

Abstract: Shape memory mechanisms, characteristic of Fe-based alloys, are discussed based on the microscopic observations with various scales ranging from micron meter to sub-nanometer. Most of the Fe-based shape memory alloys that may be technologically applicable in the future are associated with the fcc/bct and fcc/hcp transformations. The modes of the stress-induced martensitic transformation and its reverse transformation in these transformation systems, which are directly related to the shape memory effect, are examined in detail and it is concluded that the following conditions are the most important ones to achieve a good shape memory effect. In the case of the fcc/bct transformation, the tetragonality of the bct martensite must be large so that the twin boundary energy in martensite can be lowered, which results in that very fine transformation twins are completely extended through a martensite plate. A low twin-boundary energy in martensite is an important factor to reduce the density of dislocations residing at the austenite–martensite interface and make easy the reverse movement of the interface. In the extreme case of no dislocations at the interface, a thermoelastic transformation will be realized. A higher yield stress for slip in bct martensite is another factor to bring about the reversibly mobile interface. These are deduced from the detailed analysis of dislocations generated behind the reverse-moving interface on heating. In the case of the fcc/hcp transformation, the formation of extremely thin hcp martensite plates with uniform distribution in a deformed sample is most required to achieve a good shape memory effect. For such thin martensite plates to be produced, high densities of stacking faults must preexist on the primary slip system in austenite when an external stress is applied for shape change. A new mechanism for the formation of very thin martensite plates by an applied stress is presented. A low stress for inducing martensite transformation compared with the yield stress for slip in austenite, which results from the so-called ‘training’ treatment, may be a necessary but not sufficient condition for obtaining a good shape memory effect. Future directions of the research on Fe-based shape memory alloys are suggested.

The influence of the substitution of Si by Al on the properties of cold rolled C-Mn-Si TRIP steels.

Abstract: The effect of the substitution of silicon by aluminium on the mechanical properties and the microstructure of cold rolled C-Mn-Si TRIP steels was investigated for different continuous annealing cycles. The mechanical properties were evaluated using tensile testing. It was seen that the Al alloyed steel had very good mechanical properties with an improved formability compared to the conventional C-Mn-Si TRIP steel. The strain hardening behaviour was studied in detail. All the investigated specimens showed a very high n value but their strain dependence was different. For the conventional C-Mn-Si TRIP steel the maximum n value was reached at low strain, while the Al substituted TRIP steel showed a gradual increase of the n value. The latter effect resulted in a larger uniform elongation for the C-Mn-Al-Si TRIP steel. Investigation of the microstructure using color etching and scanning electron microscopy revealed that the Al alloyed steel composition contained a larger amount of bainite with a finer structure than the C-Mn-Si TRIP steel. Furthermore, XRD measurements showed that the Al substitution resulted in a larger volume fraction of retained austenite.

Plasma nitriding of stainless steels at low temperatures

Abstract: To avoid the drop in corrosion resistance of stainless steels in conventional nitriding (precipitation of CrN), low-temperature techniques like ion implantation, plasma immersion ion implantation (PIII, PI3) and low-temperature plasma nitriding were developed. In this investigation, four stainless-steel grades (ferritic: X6Cr17, austenitic–ferritic: X2CrNiMoN22.5.3, austenitic: X8CrNiTi18.10 and X5CrNi18.10) were plasma-nitrided between 250 and 500°C. Nitrogen-enriched layers with a high nitrogen content were produced, leading to a significant increase in surface hardness. X-ray diffraction indicated that CrN did not precipitate if treatment temperatures did not exceed 400°C. ‘Expanded austenite’ formed in the austenitic and duplex steels and E-nitride (Fe2N1−x) in the ferritic steel. The optically visible structure of the nitrided cases is comparable with that of the PIII layers, with higher charging densities being possible in the plasma nitriding. Also, in comparison to conventional ion implantation, large charges and parts with complicated shapes can be treated.

The rôle of vanadium in microalloyed steels

Abstract: The overall objective of the present paper is to review the role of V in microalloyed steels with a particular address as to how it affects microstructural evolution and mechanical properties. Its role in thermomechanical controlled processing (TMCP) is emphasised. The review is largely based on work carried out at the Swedish Institute for Metals Research (SIMR) during the last 25 years, but also includes reference to other relevant, published work. A specific aim is to demonstrate the present scientific knowledge of the subject. Therefore the understanding and interpretation of essential phenomena related to microstructure formation and properties are thoroughly examined, ranging from the influence of microalloying on prevention of austenite grain growth and recrystallisation, to precipitation in ferrite and its effect on strength. Within the well-known thermodynamic database, Thermocalc, a special microalloy database has been developed at SIMR, allowing reliable predictions of phase equilibria and thermodynamic functions for phase transformations in microalloyed steels. A comprehensive account is given of the role of V in the most important processing steps that the microalloyed steels are subjected to, viz. TMCP, continuous casting and welding. Compared to the other microalloying elements, Nb and Ti, V exhibits essential differences. In particular, the solubility of its carbonitrides is much larger and the solubility of its nitride is about 2 orders of magnitude smaller than its carbide, contrary to Nb but similar to Ti. For optimal alloy design and thermomechanical processing, proper allowance must be made for these differences. To reach the maximum ferrite grain refinement, ∼4 μm, repeated recrystallisation in a series of rolling reductions is used in TMCP of V-microalloyed steels, so-called Recrystallisation-Controlled-Rolling (RCR), as opposed to traditional controlled rolling of Nb-steels where heavy rolling at low temperatures in the non-recrystallisation regime is the means of attaining grain refinement. RCR presents some important advantages, in particular a more economical hot rolling practice by allowing low reheating and high finishing temperatures. As compared to Nb, V has certain further advantages as a microalloying element due to its greater solubility in austenite. The tendency for hot cracking of cast slabs is much less pronounced and dissolution of coarse V(C,N) compounds is more easily achieved prior to hot rolling than for the corresponding NbC. It is demonstrated that the relatively large solubility of V(C,N) and the much lower solubility of VN than VC makes V an eminent choice for strong and easily controllable precipitation strengthening. A corollary of the difference in VN and VC solubilities is that N becomes an essential microalloying element in V-steels, because it largely determines the density of V(C,N) precipitation and thereby the degree of precipitation strengthening. Moreover, since pure ferrite dissolves more N than C, the total N-content of the steel is normally dissolved in the ferrite before V(C,N) precipitation, whereas only a fraction of the C-content, given by the austenite/ferrite or ferrite/ cementite equilibrium, is dissolved in ferrite. Hence, by precise additions of N, this circumstance facilitates the control of V(C,N) precipitation strengthening.

X-ray diffraction characterisation of low temperature plasma nitrided austenitic stainless steels

Abstract: The nitrided layers produced by low temperature (400–500 °C) plasma nitriding on austenitic stainless steels, AISI 316, 304 and 321, have been characterised by X-ray diffraction, in conjunction with metallographic and chemical composition profile analysis. The thin, hard and corrosion resistant layers exhibited similar X-ray diffraction patterns, but the positions of the major diffraction peaks varied with nitriding temperature and nitrogen concentration profile. The low temperature nitrided layers are predominantly composed of a phase with a face centred cubic (fcc) structure, which is named “S” phase. However, the positions of the diffraction peaks from the “S” phase deviated in a systematic way from those for an ideal fcc lattice. Detailed analysis of the deviation suggested that very high compressive residual stresses and stacking faults were formed in the layers, resulting in a highly distorted and disordered fcc structure. The lattice parameter of the distorted and disordered “S” phase was found to increase with increasing nitrogen concentration.

Low temperature plasma carburising of austenitic stainless steels for improved wear and corrosion resistance

Abstract: A novel plasma surface engineering process has been developed for engineering the surfaces of austenitic stainless steels to achieve combined improvements in wear and corrosion resistance. The process employs carbon as the major alloying species in the glow discharge of a plasma, facilitating the diffusion of carbon into the surfaces of various austenitic stainless steels and forming a precipitate free layer supersaturated with carbon at relatively low temperatures. This low temperature plasma surface alloying process produces a surface engineered layer several tens of micrometres thick, which has a high hardness together with excellent wear and corrosion resistance. The present paper describes various aspects of the novel process, concerning processing, structures, properties, and applications to engineering components.

Bainite transformation of low carbon Mn–Si TRIP-assisted multiphase steels: influence of silicon content on cementite precipitation and austenite retention

Abstract: Studies dealing with TRIP-assisted multiphase steels have emphasized the crucial role of the bainite transformation of silicon-rich intercritical austenite in the achievement of a good combination of strength and ductility. The present work deals with the bainite transformation in two steels differing in their silicon content. It is shown that both carbon enrichment of residual austenite and cementite precipitation influences the kinetics of the bainite transformation. A minimum silicon content is found to be necessary in order to prevent cementite precipitation from austenite during the formation of bainitic ferrite in such a way as to allow stabilisation of austenite by carbon enrichment. (C) 1999 Elsevier Science S.A. All rights reserved.

Martensite and deformation twinning in austenitic steels

Abstract: The structural changes that occur in highly-alloyed austenitic steels during plastic deformation have been studied as a function of temperature and plastic strain. Stress-assisted and strain-induced martensites can occur, but also, deformation twinning can be identified by transmission electron microscopy. In the high-Mn austenite of Hadfield steel deformation is controlled by slip and twinning over a wide temperature range. In high-Ni and more complex alloyed austenites (such as in TRIP steels) deformation involving martensite formation, both stress-assisted and strain-induced, but near room temperature and above the influence of mechanical twinning, is also apparent.

Structural characteristics of low temperature plasma carburised austenitic stainless steel

Abstract: A low temperature plasma carburising process has recently been developed to engineer the surfaces of austenitic stainless steels to achieve combined improvements in wear and corrosion resistance. The present paper discusses the structural characteristics of the carburised layers produced on AISI type 316 steel at temperatures between 400 and 600°C. It was found that at low temperatures (<520°C), the carburised layers produced were precipitation free and comprised a single phase, which had a face centred cubic structure and was identified as expanded austenite owing to the supersaturation of carbon in austenite. The carburised layer was in a deformed and distorted state. High densities of twins, stacking faults, and dislocations were found in the expanded austenite. The degree of lattice expansion was estimated and was found to vary with processing temperature and depth in the layer. Precipitation of carbides (mainly Cr7 C3 ) occurred when the carburising temperature was relatively high (for exampl...

Structure and composition of expanded austenite produced by nitrogen plasma immersion ion implantation of stainless steels X6CrNiTi1810 and X2CrNiMoN2253

Abstract: The structure and phase composition of the modified surface of two different stainless steels (austenitic steel X6CrNiTi1810 and austenitic–ferritic steel X2CrNiMoN2253) after 3 h plasma immersion ion implantation of nitrogen at 400°C were compared using XRD, SEM, TEM and Mossbauer spectroscopy. Both steels show a surface layer with very similar composition. The major phase found is expanded austenite. Very fine precipitates in the near surface region are most likely decomposition products (ferrite/martensite and CrN) of the metastable expanded austenite. The stability of the expanded austenite in the austenitic–ferritic steel is lower and the amount of decomposition products is higher under the same treatment conditions.

Composition, microstructure, hardness, and wear properties of high-speed steel rolls

Abstract: The effects of alloying elements on the microstructural factors, hardness, and wear properties of four high-speed steel (HSS) rolls fabricated by centrifugal casting were investigated. A hot-rolling simulation test was carried out using a high-temperature wear tester capable of controlling speed, load, and temperature. The test results revealed that the HSS roll containing a larger amount of vanadium showed the best wear resistance because it contained a number of hard MC-type carbides. However, it showed a very rough roll surface because of cracking along cell boundaries, the preferential removal of the matrix, and the sticking of the rolled material onto the roll surface during the wear process, thereby leading to an increase in the friction coefficient and rolling force. In order to improve wear resistance with consideration to surface roughness, it is suggested that a reduction in the vanadium content, an increase in solid-solution hardening by adding alloying elements, an increase in secondary hardening by precipitation of fine carbides in the matrix, and formation of refined prior austenite grains by preaustenitization treatment be employed to strengthen the matrix, which can hold hard carbides in it.

Precipitation of an intermetallic phase with Pt2Mo-Type structure in Alloy 625

Abstract: The microstructure of Alloy 625, which has undergone prolonged (∼70,000 hours) service at temperatures close to but less than 600 °C, has been characterized by transmission electron microscopy. The precipitation of an intermetallic phase Ni2(Cr, Mo) with Pt2Mo-type structure has been observed in addition to that of the γ″ phase. Six variants of Ni2(Cr, Mo) precipitates have been found to occur in the austenite grains. These particles exhibit a snowflake-like morphology and are uniformly distributed in the matrix. They have been found to dissolve when the alloy is subjected to short heat treatments at 700 °C. The occurrence of the Ni2(Cr, Mo) phase has been discussed by taking the alloy chemistry into consideration. Apart from the intermetallic phases, the precipitation of a M6C-type carbide phase within the matrix and the formation of near continuous films, comprising discrete M6C/M23C6 carbide particles, at the austenite grain boundaries have been noticed in the alloy after prolonged service.

Light microscopy of carbon steels

Abstract: Containing over 1,200 representative micrographs and the information and explanatory text that makes them really useful, including composition, condition, etchant, magnification, and more than 100 graphs and tables, this 'how to' book not only gives everyday working examples, but also discusses the relationship between the constitution, metallurgy, and microstructure of various carbon steel products. Contents: Nomenclature of Phases and Constituents; Phase Transformations; Low-Carbon Irons and Steels; Annealing and Normalizing; Spheroidization and Graphitization; Austenitization; Transformation of Austenite; Tempering of Martensite; Welding; Surface Oxidation, Decarburation and Oxidation Scaling; Glossary of Terms; EtchingMethods; ConversionTables; Index.

The Influence of Microstructural Aspects on the Service Behaviour of Advanced Power Plant Steels

Abstract: There is a world-wide substantial demand to increase the application temperature and design stresses of advanced creep resistant ferritic-martensitic 9-12 % Cr steels to increase the efficiency of thermal power plants. They show, compared to austenite grades, favourable physical properties-like good thermal conductivity and low coefficient of thermal expansion coupled with better service behaviour. In the frame work of the European research action COST 501 intensive investigations of these types of steels were performed. These alloys show during service conditions pronounced microstructural changes in the aged as well as in the stressed conditions. Therefore an extrapolation of short term creep tests to long times-i.e. 100 kh-using conventional methods is not successful because of changes in the acting creep mechanism governed by microstructural development. Therefore an intensive investigation and modelling of the microstructure of W- and Mo-alloyed advanced 9-12 % Cr steels, mainly G-X12 CrMoVWNbN 10-1-1, was applied to understand the mechanisms. Hardness tests, optical microscopy, SEM, TEM methods and thermodynamic equilibrium phase diagram calculations were performed. Very promising is the use of the EFTEM (Energy filtering TEM) method to quantify the precipitation sequence of the different types of precipitate: M23C6, MX, Laves Phase and Z-Phase as a function of time, temperature and straining. A new approach ("Graz Model") was proposed for the improvement of the creep resistant alloys in terms of the α/γ-transformation and the Curie temperature respectively, which influence the microstructural stability. Both were found as important influencing factors. Experimentally it was found, that the alloys showing the highest creep resistance i.e. the Japanese grade NF616/P92 and the alloy B2, one of the COST 501 activity, show the highest degree of microstructural stability after creep exposure. Further investigations and modelling is necessary to develop an optimised alloy composition of advanced 9-12% Cr steels.

Effect of retained austenite on the microstructure and mechanical properties of martensitic precipitation hardening stainless steel

Abstract: The effect of retained austenite (γ) on the microstructure and mechanical properties of a martensitic precipitation hardening stainless steel was experimentally investigated, whose chemical composition was Fe-1.8Cu-15.9Cr-7.3Ni-1.2Mo-0.08Nb-low C, N (mass %). The microstructures of all specimens consist of a typical lath martensite with interlath films of the retained γ, which is not reverted with aging. Cu-rich precipitates which may contribute to precipitation hardening can not clearly be observed. The tensile properties and Charpy absorbed energy are linearly approximated to the amount of retained γ as follows: 0.2% Y.S. (MPa) = 1192.3 − 13.6 × γ%, T.S. (MPa) = 1250.1 − 9.3 × γ%, El. (%) = 12.16 + 0.43 × γ%, R.A. (%) = 64.25 + 0.14 × γ%, and A.E. (J) = 72.5 + 0.8 × γ%. The introduction of retained γ is not beneficial to the fatigue limit. An excellent combinations of strength, ductility and toughness obtained in the present work is attributed to the introduction of retained γ and also to the chemical composition of the specimen used.