Simple model for austenite grain growth in microalloyed steels

Abstract

Previous models for grain growth are usually based on Beck's formula, which are inadequate for quantitative prediction of austenite grain growth during reheating of as cast microstructures in microalloyed steels. The applications of these empirical grain growth models are limited to some particular categories of steels, such as Nb, Nb–Ti and Ti–V microalloyed steels, etc. In this study, a metallurgically based model has been developed to predict the austenite grain growth kinetics in microalloyed steels. This model accounts for the pinning force of second phase particles on grain boundary migration, in which the mean particle size with time and temperature is calculated on the basis of the Lifshitz–Slyozov–Wagner (LSW) particle coarsening theory. The volume fraction of precipitates is obtained according to the thermodynamic model. The reliability of the model is validated by the agreement between theoretical predictions and experimental measurements in the literature.     

Interfacial reactions in K2Ti6O13(w)/Al2O3/Al composite

Abstract

A pure Al matrix composite, reinforced by potassium titanate whiskers coated with sol–gel alumina, was fabricated by squeeze casting. Good interface bonding was achieved in the coated composite. Interfacial reactions in the composite were found to be less severe than those in an uncoated composite, owing to the barrier effect of sol–gel -Al₂O₃ coating. After the composite was thermally exposed at 530°C for 30 h, the alumina coating still restrained the decomposition of the whisker structure, and thus prevented a decrease in the strength of the composite. However, after a thermal exposure at 700°C for 10 h, the coating was no longer held on whisker surfaces. Mechanical testing showed that tensile fracturing of the coated composite tended to occur at the coating/matrix interface where the interfacial bonding is weaker than at the coating/whisker interface.     

Microstructural aspects of phosphorus grain boundary segregation in low alloy steels

The influence of microstructure on phosphorus grain boundary segregation in 2.6Cr–1Mo–0.3V low alloy steel was investigated. Measurement of the phosphorus grain boundary concentration and characterization of the steel microstructure (substructure) were performed by means of Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). It was found that the phosphorus grain boundary segregation during the aging of the steel at 773 K is faster in as-tempered microstructure than in as-quenched material. The lower dislocation density and the indirectly evidenced lower carbon content in ferrite matrix were considered to accelerate the kinetics of phosphorus segregation.     

Kinetics of grain boundary segregation during recrystallisation annealing

Abstract

In batch annealing and continuous annealing processes, both recrystallisation and grain boundary segregation can occur. In this paper, a simple model is derived which explores the interaction of the boundary migration and segregation processes and considers the application to phosphorus segregation during the annealing of interstitial free steels. The model considers both segregation to a migrating boundary and the segregation which occurs during continuous cooling after the holding period during the anneal cycle.     

Evolution of precipitates,in particular cruciform and cuboid particles,during simulated direct charging of thin slab cast vanadium microalloyed steels

Abstract

A study has been undertaken of four vanadium based steels which have been processed by a simulated direct charging route with processing parameters typical of thin slab casting, where the cast product has a thickness of 50 to 80 mm (in this study 50 mm) and is fed directly to a furnace to equalise the microstructure prior to rolling. In the direct charging process, cooling rates are faster, equalisation times shorter, and the amount of deformation introduced during rolling less than in conventional practice. Samples in this study were quenched after casting, after equalisation, after the fourth rolling pass, and after coiling, to follow the evolution of microstructure. The mechanical and toughness properties and the microstructural features might be expected to differ from equivalent steels which have undergone conventional processing. The four low carbon steels (~0.06 wt-%) which were studied contained 0.1 wt-%V (V – N), 0.1 wt-%V and 0.010 wt-%Ti (V – Ti), 0.1 wt-%V and 0.03 wt-%Nb (V – Nb), and 0.1 wt-%V, 0.03 wt-%Nb and 0.007 wt-%Ti (V – Nb – Ti). steels V – N and V – Ti contained around 0.02 wt-% N, while the other two contained about 0.01 wt-%N. The as cast steels were heated at three equalising temperatures of 1050°C, 1100°C, or 1200°C and held for 30 – 60 min before rolling. Optical microscopy and analytical electron microscopy, including parallel electron energy loss spectroscopy (PEELS), were used to characterise the precipitates. In the as cast condition, dendrites and plates were found. Cuboid particles were seen at this stage in steel V – Ti, but they appeared only in the other steels after equalisation. In addition, in the final product of all the steels, fine particles were seen, but it was only in the two titanium steels that cruciform precipitates were present. PEELS analysis showed that the dendrites, plates, cuboids, cruciforms, and fine precipitates were essentially nitrides. The two Ti steels had better toughness than the other steels but inferior lower yield stress values. This was thought to be, in part, due to the formation of cruciform precipitates in austenite, thereby removing nitrogen and the microalloying elements, which would have been expected to precipitate in ferrite as dispersion hardening particles.     

A new model for radiation-induced grain boundary segregation with grain boundary movement in concentrated alloy system

We have developed a new model for radiation-induced grain boundary migration (RIGM) and radiation-induced segregation (RIS) for austenitic iron-chromium-nickel alloy system. It was assumed that the RIS was induced by diffusional and annihilation processes of excess point defects at the grain boundary, and the RIGM occurred due to rearrangement process of atoms on one of the interfacial planes by annihilation of point defects. The calculated results indicated that the region of RIS was enlarged by the RIGM and asymmetrical concentration profiles were observed around the migrated grain boundary. The present model could explain the RIS behavior with or without grain boundary migration as comparing with our previous experimental results.     

Effect of carbon and phosphorus addition on sintered density and effect of carbon removal on mechanical properties of high density sintered steel

Abstract

This paper provides data on the effect of carbon and phosphorus levels on the density of liquid phase sintered steel and the impact of subsequent carbon removal on the mechanical properties. After sintering die pressed samples composed of liquid forming additives and coarse water atomised powder at 1250°C or below, followed by postsintering decarburisation, densities of >95% relative density and non-brittle microstructures are achieved. Tensile testing shows the important effect of the microstructure on the mechanical properties. Ductility is improved by the post-sintering decarburisation, corresponding to elongation to fracture of 12% for certain compositions. Apparent diffusion coefficients for carbon were also estimated.     

Stress-induced non-equilibrium grain boundary segregation of phosphorus in a Cr–Mo low alloy steel

Grain boundary segregation of phosphorus under a 40 MPa tensile stress at 520 °C in a 0.025 wt.% P-doped 2.25Cr1Mo steel, which has already been thermally equilibrated, is examined using Auger electron spectroscopy. The segregation of phosphorus during stress-ageing has a non-equilibrium characteristic, i.e. it is non-equilibrium segregation. The segregation level first increases with increasing stress-ageing time until about 0.5 h and then diminishes with further increasing stress-ageing time, leading the boundary concentration of phosphorus to return to its thermal equilibrium value after ageing for about 15 h. Therefore, the critical time for this non-equilibrium grain boundary segregation of phosphorus is about 0.5 h at which the segregation is peaked. At this critical time, the boundary concentration of phosphorus is about 20.5 at.%, which is about 4.5 at.% higher than its thermal equilibrium level. Xu's kinetic model for stress-induced grain boundary segregation [T.D. Xu, Philos. Mag. 83 (2003) 889–899; T.D. Xu, B.-Y. Cheng, Prog. Mater. Sci. 49 (2) (2004) 109–208] is used to analyse the experimental results, demonstrating that the measured data may be well simulated by the model.     

Precipitation in V bearing microalloyed steel containing low concentrations of Ti and Nb

Abstract

The paper describes the precipitation behaviour in a thermomechanically processed V bearing microalloyed steel containing small additions of Ti and Nb (0·007–0·008 wt-%) using analytical transmission electron microscopy. An intriguing aspect is the significant precipitation of titanium and niobium at these low concentrations, contributing to strength. A high density of multimicroalloyed precipitates of (V, Nb, Ti)(C, N) are observed instead of simple TiN, TiC, and NbC precipitates. They are characterised as cuboidal (45–70 nm), spherical (20–45 nm), irregular (20–45 nm), and fine (10–20 nm). Estimation of solubility products of carbides and nitrides of V, Nb, and Ti implies that the precipitation of titanium occurs primarily in austenite. Interphase precipitation of niobium occurs during austenite to ferrite transformation, while complete precipitation of vanadium takes place in the austenite–ferrite region close to completion of transformation. Substoichiometric concentrations of Ti and Nb, the presence of nitrogen, and the mutual extensive solubility of microalloying carbonitrides explains the formation of core shell (triplex/duplex) precipitates with highly stable nitrides ((Ti, Nb, V)N) in the core and carbides ((Ti, Nb, V)C) in the shell. The qualitative stochiometric ratios of triplex and duplex carbonitrides were Ti_0·53Nb_0·35V_0·12 and Ti_0·6V_0·4, Nb_0·51V_0·49 and Ti_0·64Nb_0·36. Extensive precipitation of fine carbides on dislocation substructures, and sub-boundaries occurred. They were generally characterised as vanadium carbide precipitates with ordered cubic L1₂ structure and exhibited a Baker–Nutting orientation relationship with the ferrite matrix. M₄C₃ types of carbides were also observed similar to the steel, having high concentrations of Ti and Nb.     

Review: Interaction of precipitation with recrystallisation and phase transformation in low alloy steels

Abstract

The processes of precipitation, restoration and phase transformation can interact in complex ways during thermomechanical processing of microalloyed steels to profoundly alter their structures and properties. Precipitation in austenite during hot deformation can strongly modify the kinetics of recovery and recrystallisation, subsequently affecting the nucleation and growth of ferrite during cooling. For steels containing strong carbide/nitride formers, interphase precipitation (IP) can occur in ferrite at the austenite/ferrite interface, conferring significant coherency strengthening. Much of what is known about this phenomenon is attributable to the impressive research efforts of Robert Honeycombe and his colleagues at Cambridge.     

Influence of vanadium on grain boundary segregation of phosphorus in iron and iron-carbon alloys

The influence of vanadium on grain boundary segregation of phosphorus has been studied in iron and iron-carbon alloys by means of fracture experiments in a scanning Auger microprobe. The emphasis here is to study the effects of vanadium on the interaction processes operative under circumstances when structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation form simultaneously. It is emphasized that to predict and analyse the behaviour of an alloy, it is important to consider atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the principal determining factor in the scavenging or retardation of migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially reacts with carbon in view of higher interaction for carbon as compared to phosphorus. A consequence of this is the increase in the grain boundary concentration of phosphorus. In such a situation the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides, serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the sitecompetition model. An effort is also made to examine the behaviour of segregating elements in terms of whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.     

Evolution of grain boundary structure in submicrometer-grained Al-Mg alloy

This paper presents high-resolution electron microscopy studies of grain boundary structures in a submicrometer-grained Al-3%Mg solid solution alloy produced by an intense plastic straining technique. The studies include the effect of static annealing on the grain boundary structure. Many grain boundaries are in a high-energy nonequilibrium state in the as-strained sample. The nonequilibrium character is retained on some grain boundaries in samples annealed at temperatures below the onset of significant grain growth. The effect of electron irradiation on the grain boundary structure also is examined.     

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.