Anomalies in carbon concentration determinations from nanostructured bainite

There are large discrepancies in the measured concentrations of carbon in the ferrite and austenite within nanostructured bainite and carbide-free bainitic steels in general. The concentrations are usually measured on the basis of lattice parameters determined using X-ray diffraction, lattice imaging in transmission microscopy, or using the atom probe technique, i.e., time-of-flight mass spectroscopy. We examine here a number of difficulties with these methods and assess the role of defects, crystal symmetry and the heterogeneous distribution of carbon in interpreting the experimental data. Issues where experimental and theoretical work is needed are identified.This is the accepted manuscript. The final version is available at http://www.maneyonline.com/doi/10.1179/1743284714Y.0000000655

Solution to the Bagaryatskii and Isaichev ferrite–cementite orientation relationship problem

The Bagaryatskii and Isaichev orientation relationships between cementite and ferrite are closely related but not identical. They cannot easily be distinguished using ordinary electron diffraction methods and precise methods indicate that the Bargaryatski orientation does not exist. The issue is important when considering the mechanism by which cementite forms during the tempering of martensite or the formation of lower bainite, where the iron and substitutional solutes are unable to diffuse during the course of precipitation. It is demonstrated here that just one of the orientation relationships is consistent with the mechanism of precipitation at low temperatures, and is associated with much smaller deformations than the other.Non

Is low phosphorus content in steel a product requirement

An attempt is made, on the basis to published literature, to assess the amount of phosphorus that might be present in the steels without making it susceptible to grain boundary embrittlement. Embrittlement occurs when the general resistance to plastic flow is comparable to the stress required to separate crystals at their boundaries. A criterion is developed that enables a simple assessment to be made of the tendency to embrittle as a function of yield strength and the fraction of grain boundary sites that are covered by phosphorus. The latter is also sensitive to chemical composition, since some elements such as carbon segregate preferentially to phosphorus, giving rise to site competition that can permit a greater tolerance to the impurity. Other solutes affect the embrittling potency by different mechanisms, not all of which are clear.This is the author accepted manuscript. The final version is available from Maney at http://www.maneyonline.com/doi/abs/10.1179/1743281214Y.0000000261

Niobium in Microalloyed Rail Steels

Rail steels rely primarily on possessing adequate wear and rolling contact fatigue resistance. These properties, together with the toughness, can in principle be optimized by implementing thermomechanical processing assisted by controlled niobium additions. The purpose of the current work is to develop a Nb-microalloying strategy in the context of high-carbon pearlitic and cementite-free bainitic steels. The conventional methods do not leave the critical regions of a rail section in a suitably processed state. An attempt has been made for the first time, to create a pancaked austenite grain structure, with an examination of the consequences on the final product. One of the major difficulties is to ensure that niobium does not segregate during manufacturing, since niobium is a strong carbide former and rail steels traditionally contain large carbon concentrations. Niobium solubility in austenite has been assessed critically and thermodynamic calculations for microsegregation have been taken into account. The aim is to ensure that any primary niobium carbide precipitated from solute-enriched liquid during non-equilibrium solidification, can be taken into solution in austenite during reheating, to mitigate potential effects of coarse precipitates on the final mechanical properties. Rail steels containing 0.01-0.02 wt% Nb have been designed and characterised. In as-cast condition, primary niobium carbides as large as ~10 µm can be observed, which dissolve slowly during reheating. An attempt has been made to develop a model to estimate the dissolution kinetics of the carbides. Dissolved niobium in reheated austenite precipitates during hot deformation as fine niobium carbides (<50 nm) which inhibit austenite recrystallisation by pinning the austenite grain boundaries. Nb-microalloying increases the ‘no-recrystallisation temperature’ of deformed austenite during multi-pass compression tests. The topology of grain deformation has been analysed in terms of stereological calculations and dilatometric experiments have shown that transformation kinetics tend to accelerate when the austenite is deformed below the no-recrystallisation temperature, however the effect is relatively small. The microstructure and mechanical properties of the as-rolled Nb-microalloyed steels have been characterised along with their rolling-sliding wear performance and compared with their non-microalloyed counterparts. Increased austenite grain boundary area and increased dislocation activity due to pancaking, hinder bainite growth which leads to an increased retained austenite volume fraction. This in turn, leads to slightly improved ductility, improved toughness and improved wear resistance in Nb-microalloyed bainitic alloys. Microstructural refinement in Nbmicroalloyed pearlitic alloys does not have any significant effect on tensile and toughness properties, but wear resistance improves significantly. A Bayesian neural network model has been developed to estimate the wear of rails. Predicted trends have been found consistent with metallurgical experience and the perceived noise levels are consistent with reasonable repeatability of the wear testing method used. The model can be applied widely to estimate wear because of its capacity to indicate uncertainty, including both the perceived level of noise in the output, and an uncertainty associated with fitting the function in the local region of input space

Atomic mechanism of the bainite transformation

Abstract I have on previous occasions shown how we can be surprised and delighted by new discoveries in steels, which at the same time may be useful. However, my focus in this lecture is purely on some basic science so that a well-founded understanding of mechanisms can lead to ever greater advances. The composite structure that is known colloquially as bainite is arguably the most interesting of all of the essential microstructures that occur in steels, where the manner in which atoms move is seminal to the design of steels. Therefore, I take the liberty to indulge myself and talk only of theory on this occasion.</jats:p

The first bulk nanostructured metal.

Nanotechnology has become an overused adjective, but there has been justified excitement in the context of structural materials. A class of iron alloys has been discovered in which a high density of strong interfaces can be created by heat-treatment alone. The packing of interfaces is so large, and the fact that there is an intrinsic work hardening mechanism in the structure, leads to remarkable properties. The genesis of this structure, its commercialization, the new science associated with the discovery, and its limitations are all explored in this short review