High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides

Abstract

We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered κ-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L′1₂ ordered precipitates (so called κ-carbides) resulting in high strength (yield stress ～1·0 GPa) and ductile (elongation to fracture ～30%) steels. Novel insights into dislocation–particle interactions in a Fe–30·5Mn–8·0Al–1·2C (wt-%) steel strengthened by nanoscale κ-carbides are discussed.     

Microstructure and properties of carbide free bainite railway wheels produced by programmed quenching

Abstract

A kind of carbide free bainite steel was used to produce railway wheels. The alloy design, manufacture process, microstructure and behaviours are summarised. The novel steel is distinctively superior in excellent combination of strength and toughness compared with the traditional pearlite type wheel steels. The details of the microstructure of novel steel have been investigated in nanoscale.     

Microstructural evolution of oxide-dispersion-strengthened Fe–Cr model steels during mechanical milling and subsequent hot pressing

Oxide-dispersion-strengthened (ODS) ferritic steels of Fe–9Cr–0.3Y₂O₃ and Fe–9Cr–0.2Ti–0.3Y₂O₃ (in mass) incorporating nanoscale oxide particles, were produced by mechanical milling (MM) followed by hot pressing (HP). Microstructural evolution of these two types of ODS steels were structurally characterized at each step of the elaboration processes by means of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), and optical microscope. The observations of structure of the mixed powders and the nanoscale oxide particles in both ODS steels after MM indicate that the initial powders, coupled with the original yttria powders, get fractured by severe plastic deformation and ultrafine bcc grains (~20 nm) of the matrix and Y₂O₃ nanocrystals with irregular edges are formed during MM. The addition of titanium (Ti) promotes the refinement of bcc grains, Y₂O₃ nanocrystals and the formation of amorphous phase of Y₂O₃ during MM. TEM observations of these two Oxide-dispersion-strengthened (ODS) steels exhibit a very fine structure of micrometer-scale grains in which large number of nanoscale oxide particles are distributed after HP process. The observation of some unreinforced domains without the nanoscale oxide particles indicates that there still exist inhomogeneous areas, although the size of those oxide particles reaches nanoscale. Threshold stress of the HPped Fe–9Cr–0.2Ti–0.3Y₂O₃ steel with the relatively homogeneous dispersion was carefully evaluated on the basis of higher magnified images of the nanoscale oxide particles. Different values of threshold stress were obtained due to the various dispersions of the nanoscale oxide particles within different areas. That may be the reason why the threshold stress cannot be clearly obtained by the results of creep tests.     

Constitutive modelling of carbon and alloy steels

Abstract

Semi-empirical models for the constitutive behaviour of steels often fail to predict the flow stress with sufficient accuracy. A simple neural network structure 3 : 4 : 1 is able to model flow behaviour better than other models available in the literature. It has been developed for four carbon steels, two microalloyed steels, an austenitic stainless steel and a high speed steel.     

Development of de-alloyed zones during oxidation: effects on microstructure and spallation behaviour

Abstract

The development of de-alloyed zones during oxidation of martensitic and austenitic steels and Ni based superalloys has been reviewed and the influence of de-alloying on local creep strength has been assessed. The de-alloyed zones in martensitic steels have similar, possibly higher, strength than the bulk material, whereas in Ni based superalloys the de-alloyed zone is significantly weaker than the bulk alloy. The effect in austenitic steels varies according to the strengthening phases present in the alloys: the de-alloyed zone is weaker in alloys strengthened by chromium carbides and/or γ′ but has similar strength in alloys strengthened by niobium carbide.     

Effect of vanadium on cast carbide in high speed steels

Abstract

The effect of vanadium (0–4%) on the morphology and amount of eutectic and eutectoid carbides in high speed steels has been investigated using scanning electron microscopy and image analysis. It was found that vanadium promotes the formation of MC carbide and M₂C carbide, but inhibits the formation of M₆C carbide. In the vanadium free steels, the eutectic carbide consists solely of skeletal M₆C. For each steel composition, there is a critical vanadium content at which the skeletal eutectic changes to lamellar eutectic and the critical value decreases as the molybdenum content of steel increases. The effect of vanadium on the total amount of eutectic carbide differs in tungsten alloyed and molybdenum alloyed high speed steels. The δ eutectoid has a rodlike morphology in tungsten high speed steels; δ eutectoid is not present in Mo–W or molybdenum high speed steels. Increasing the vanadium content leads to an increase in the size of eutectic and eutectoid carbides.

MST/1264     

Critical time and temper embrittlement isotherms

Abstract

The critical time in a non-equilibrium segregation isotherm will induce a critical time in the relative temper embrittlement isotherm, at which when a steel is held, a maximum extent of embrittlement will occur. This suggestion has been confirmed for the non-equilibrium cosegregation to grain boundaries of Ti, Sb, and Ni in low carbon Ni–Cr steels and for phosphorus non-equilibrium segregation to grain boundaries in some steels.     

Nature of large precipitates in titanium-containing HSLA steels

Abstract

Samples of concast Nb–V, Nb–V–Ti, and Nb–Ti steels have been examined using analytical electron microscopy. It has been shown that the addition of titanium to niobium-containing high-strength low-alloyed (HSLA) steels results in undesirable precipitation in the steels, i.e. precipitation of large precipitates with various morphologies. The composition and distribution of the large precipitates has been determined. The thermal stability of these large precipitates has been assessed by carrying out various heat treatments such as annealing at and quenching from temperatures between 1050 and 1250°C, and using heat cycles which simulate the heat affected zone in welds. It has been found that some of these large precipitates are stable up to 1150°C and some up to the melting point of the steels. The significance of these observations is discussed in terms of the addition of titanium to HSLA steels.

MST/641     

Effect of silicon content on microstructure and toughness of simulated heat affected zone in titanium killed steels

Abstract

hree steels having different silicon contents were prepared to study the microstructure and toughness of the thermally simulated heat affected zone (HAZ) in titanium killed steels. For a low silicon addition, the oxygen content in the molten steels decreased remarkably. This in turn caused a change in the inclusion phase from predominantly titanium oxide to titanium nitride (TiN), the change being accompanied by two major microstructural modifications. The austenite grain size became refined and the quantity of intragranularly nucleated acicular ferrite decreased. The microstructural change was found to cause coarsening of Charpy fracture surfaces and deterioration of HAZ toughness of the steels. The minor change of silicon content therefore has a profound influence on the properties of titanium killed steels.

MST/1503     

Modelling precipitation sequences in powerplant steels Part 2 – Application of kinetic theory

Abstract

New kinetic theory capable of dealing with the simultaneous precipitation of several phases has been applied to a variety of creep resistant power plant steels. It has been demonstrated that the model has the ability to predict the vast differences in precipitation kinetics reported in the published literature for power plant steels. New experimental results on precipitation in a 9Cr 1Mo type steel are reported and shown to be consistent with theoretical predictions.     

Diffusion of chromium in ferritic and austenitic 9–20 wt-%chromium steels

Abstract

Radiotracer ⁵¹Cr diffusion experiments were conducted on 9–20 wt-% chromium ferritic and austenitic steels. Volume diffusion coefficients have been determined in the temperature range 881–1281 K, and triple product values of grain boundary diffusion between 795 and 1281 K. Compared with dilute solid solutions, high ratios of grain boundary and volume diffusion activation energies have been obtained. This is discussed in view of the chemical composition of the grain boundaries measured by Auger electron spectroscopy. Furthermore, in the case of ferritic steels the effects of α–γand paramagnetic–ferromagnetic phase transitions are illustrated, while for austenitic steels a classical Arrhenius relationship has been found in the investigated temperature range.     

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     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

Characterization of Al rich oxide layers on austenitic stainless steels by fluorescence spectroscopy

Abstract

An Al rich oxide passivation technique has been developed to improve the corrosion resistance of the stainless steel to ozone added ultra-pure water. Fluorescence spectroscopy was applied to the study on the selective oxidation of aluminum containing austenitic stainless steel in low oxygen atmosphere at 1,353K. It was found that in the thermal oxidation under low oxygen pressure, the minor alloying constitution of Al resulted in the formation of thin oxide layers. The frequency shifts of fluorescence spectra show that the compressive stresses exist in the oxide layers as a result of the difference of the thermal expansion coefficients between substrate steels and α-Al₂O₃, and depends on the thickness of the oxide layers. It is confirmed that pure α-Al₂O₃ protective layers grown on the stainless steels, which remain stable and attached to the stainless steels in ozone added ultra-pure water. These act as a diffusion barrier and protect the stainless steels from the metal dissolution.     

Prediction of surface hardness after ferritic nitrocarburising of steels using artificial neural networks

Abstract

A model for prediction of the surface hardness of structural steels after nitrocarburising has been designed. The model is based on a backpropagation feedforward artificial neural network. The performance of the model was checked, using data from the published literature. Good correspondence between predicted from the artificial neural network (ANN) and experimental data was observed. The effects of the chromium, manganese, and nickel contents in the steels on the hardness were studied. Using the model the surface hardnesses after nitrocarburising for some commonly used steels are also predicted. The ANN model has been designed with the aim of optimising the application of nitrocarburising technology.     

Evolution of textures in hot rolled Nb - Ti and V - Nb microalloyed steels

Abstract

A considerable texture gradient in the through thickness direction was observed during hot rolling of Nb - Ti and V - Nb microalloyed steels. The most intense deformation texture for Nb- Ti steels was {113}〈110〉 at all depths, whereas for V - Nb steels the plane was shifted to {115}〈110〉 ; the angular difference between {113} and {115} is about a degree. The recrystallisation texture of austenite, {100}〈001〉 , transformed into {100} 〈011〉 component in the ferrite and indicated an increase in the intensity with increase in depth for both Nb - Ti and V - Nb steels. However, the intensity of this {100}〈011〉 texture was less for Nb - Ti steels compared to V - Nb steels at all depths. The reduced intensity of {100}〈011〉 texture in Nb -Ti steels is likely to be the reason for the superior formability and improved toughness of Nb - Ti steels as compared with V - Nb steels. The {100}〈011〉 type of texture has an undesirable effect on the edge formability of steels.     

Evaluation and review of simultaneous transformation model in high strength low alloy steels

Abstract

This work briefly describes and evaluates one of the most complete transformation models, which deals with the non-isothermal decomposition of austenite. The model, that does not consider the effect of precipitation on phase transformations, has been experimentally validated in high strength low alloy steels in order to evaluate how it works for microalloyed steels, where precipitation may play an important role. It has been found that the simultaneous transformation model is able to predict with an excellent agreement in microalloyed steels the formation of microstructures consisting of ferrite plus pearlite. However, the bainite formation is not successfully described by the model. The calculations incorrectly predict the formation of martensite instead of bainite in many situations.     

Aluminium nitride precipitation in multiple microalloyed pipeline steels

Abstract

The precipitation of aluminium nitride has been studied in pipeline steels containing aluminium, niobium, nitrogen, titanium, and vanadium. It was found that the titanium to nitrogen ratio had a dominant influence on the formation of aluminium nitride as reflected in the high value of aluminium in these precipitates when the ratio was low. Thermomechanical deformation of the steels appeared to promote aluminium nitride precipitation which occurred preferentially in the coarser particles. The precipitates of multiple microalloyed steels were found to be complex and their occurrence was interpreted in terms of isomorphology and miscibility. The overall steel analyses have been utilised in charting the sequence of precipitation.

MST/793     

Mechanistic approach for prediction of creep deformation,damage and rupture life of different Cr–Mo ferritic steels

Abstract

A mechanistic approach based on finite element analysis of continuum damage as proposed by Kachanov has been used to assess and compare creep deformation, damage and rupture behaviour of 2·25Cr–1Mo, 9Cr–1Mo and modified 9Cr–1Mo ferritic steels. Creep tests were carried out on the steels at 873 K over a stress range of 90–230 MPa. Modified 9Cr–1Mo steel was found to have highest creep deformation and rupture strength whereas 2·25Cr–1Mo steel showed the lowest among the three ferritic steels. Creep damage in the steels has been manifested as the microstructural degradation. 2·25Cr–1Mo steel was more prone to creep damage than 9Cr–steels. Finite element estimation of creep deformation and rupture lives were found to be in good agreement with the experimental results.     

Development of hypoeutectoid graphitic steel for wires

Abstract

Hypoeutectoid graphitic steels with BN as prime nucleating site for graphite nucleation were developed for improvement of drawability of wires. Al₂O₃ also acted as nucleation site for graphite. The steel chemistry has played an important role for BN formation and thus dictated the behaviour of graphite formation and tensile properties after graphitising annealing. The steels are with a low yield ratio of ～0·5 while percentage elongation is in the range of 23–25% and percentage reduction in area is in the range of 45–51%.