Claire Davis

Analysis of the Lift-off Effect of Phase Spectra for Eddy Current Sensors

This paper presents an analytical model that describes the inductance change when a double air-cored coil sensor is placed next to a conducting plate. Analysis of the analytical model reveals that the phase signature of such a sensor is virtually liftoff independent. This finding is verified by numerical evaluations. This paper also finds that the phase signature of a ferrite U-cored sensor can be approximated by that of a double air-cored sensor of similar size and, therefore, possesses a similar liftoff-independent property. Measurements made with a sample U-cored sensor next to plates of nonmagnetic and magnetic materials verified the theoretical results.

Cleavage initiation in Ti–V–N and V–N microalloyed ferritic–pearlitic forging steels

The effects of silicon (0.53 and 1.05 wt.%) and titanium (<0.002 and 0.022 wt.%) on microstructure and mechanical properties of vanadium microalloyed medium carbon steels heat treated after rolling to simulate the thermal cycle of hot forging have been determined using room temperature tensile tests, impact tests, optical microscopy and scanning electron microscopy (SEM). Silicon was found to increase strength values whilst titanium had a strong refining action on prior austenite grain size. Room temperature Charpy ‘U’ notch impact energies were all on the lower shelf; ductile–brittle transition temperatures, determined from fracture appearance in Hounsfield impact tests, ranged from 100 to 145°C, scaling with material strength. Initiation in the Charpy tests was by microcracking of coarse (Ti,V)(C,N)-containing single or multi-phase inclusions except in the low strength, titanium-free case when the absence of a completely continuous grain boundary ferrite layer allowed matrix microstructure initiation by interfacing pearlite colonies to occur.

The Development of a Multi-frequency Electromagnetic Instrument for Monitoring the Phase Transformation of Hot Strip Steel

This paper describes the development of an instrument to analyze the phase transformations of hot strip steel using an electromagnetic sensor. The sensor exploits variations in the electrical conductivity and magnetic permeability of the steel to monitor microstructure evolution during processing. The sensor is an inductive device based on an H-shaped ferrite core, which is interrogated with a multifrequency impedance analyzer containing a digital signal processor. Online fast Fourier transform was performed to abstract the multifrequency inductance changes due to the microstructural evolution of the sample. An overview of the instrument and measurements from a range of carbon steel samples are presented. The results verify the ability of the instrument both to monitor the microstructural changes and to reject variations in liftoff distance between the sensor and the hot strip. It is believed that this is the first time this result has been reported with tests on hot steel samples

Predicting the Life of Steel Rails

Abstract A model of plastic strain accumulation, wear, and rolling contact fatigue (RCF) crack initiation in rail steel has been developed. Local to the contact zone, material is subject to severe cyclic stresses taking it beyond yield and leading to incremental accumulation of plastic deformation (ratcheting). This model is based on a ratcheting law derived from twin-disc, rolling-sliding contact experiments and can simulate thousands of ratcheting cycles with corresponding strain hardening. The model is being further refined to account for detailed microstructural changes. Sections of worn and fatigued rail, removed from service, have been metallurgically analysed. To obtain further data on rail-steel deformation and RCF crack initiation, twin-disc tests have been performed using discs cut from across a railhead and wheel rim. Two heat treatments were applied to some rail discs to investigate the effect of pro-eutectoid ferrite phase distributions and volume fractions. Tests were run to failure (defined by an eddy current crack-detection system) and to percentages of fatigue lives. Micro- and nano-hardness tests, and microstructural observations, have been used to suggest a micromechanism of fatigue crack initiation for the model. Application of this model will contribute to reduced maintenance costs and an improved understanding of RCF development.

Fracture behaviour in medium-carbon Ti–V–N and V–N microalloyed ferritic-pearlitic and bainitic forging steels with enhanced machinability

Abstract The microstructure, tensile and impact behaviour of medium-carbon Ti–V–N and V–N microalloyed resulphurised forging steels with and without Cr and Ca additions have been determined before and after forging up to 75% reduction. Titanium additions refined the prior austenite grain size resulting in ductile failure becoming fully transgranular rather than mixed inter- and transgranular. However, the titanium additions also resulted in the presence of coarse (Ti,V)(C,N) particles which acted as cleavage initiation sites reducing toughness by increasing the fracture appearance transition temperature. Ductile fracture mode variation did not affect upper shelf energy, which was determined by inclusion volume fraction (reduced by Ca additions) and, to a lesser degree, by the inclusion size and shape (modified by Ca addition and forging). The addition of Cr increased the hardenability of the steels resulting in the presence of bainite in the otherwise ferrite and pearlite microstructure. A small fraction of bainite (7%) was found to improve the strength levels however a large fraction (83%) was less effective resulting in an increase in the tensile strength but a decrease in the proof strength. The amount of bainite did not affect the fracture properties of the steels with the other microstructural features of inclusion volume fraction and type having a dominant effect.

Bauschinger effect in Nb and V alloyed line-pipe steels

Abstract The UOE process is used for cold forming of large diameter steel line-pipes. Pipe strength has been found to increase (work hardening) or decrease (Bauschinger effect) after the UOE process compared to the plate depending on the steel grade, plate and pipe processing history. The steel chemistry, through the presence of microalloy precipitates, and prior processing, through the size and distribution of microalloy precipitates and presence of retained work hardening, affects the magnitude of the Bauschinger effect. In this paper the microstructures of two (Nb and Nb–V alloyed) steel plates, in terms of (Nb,V)(C,N) particle distributions and dislocation densities, have been related to the Bauschinger parameters in the as rolled and annealed initial conditions. The Bauschinger stress parameter increases with microalloy particle number density and dislocation density increase and the relative importance of the two effects is discussed.

Visualization and Modelling to Understand Rail Rolling Contact Fatigue Cracks in Three Dimensions

The article presents an extensive survey of experimental data on rolling contact fatigue (RCF) crack shape and propagation characteristics in rails removed from service, where such cracks are angled to the rail axis. The data include re-analysis of previously published experimental data to extract crack shape information and new experimental work on crack shapes at different stages in the early RCF life. Periods from initiation (ratcheted ‘flake cracks’) have been considered through very early growth to the limit of one prior austenite (PA) grain and on to rail-head visual cracks. Techniques included multi-sectioning through single cracks and crack zones, on used rail and test discs, to build up real three-dimensional (3D) data on crack shapes and propagation characteristics. This data have been compared with the UK rail system guidance charts relating to visual crack length and respective vertical depth; all data fell within the indicated guidance zones. The configuration of such angled cracks, typically found in curves, so aligned due to the vector of both lateral and longitudinal traction, rather than just axially, was identified as an important case for modelling. A fracture mechanics-based model has been developed to predict modes I and II stress intensity factors for such cracks covering multiple PA grains. An important geometry effect is revealed by which a contact approaching a crack angled to the rail axis is effectively ‘offset’ from the approach direction considered in 2D models, thereby resulting in lower predicted peak stress intensity factor values, compared with 2D, for the prediction of crack growth rates.

Mechanical Property Development During UOE Forming of Large Diameter Pipeline Steels

Mechanical properties of large diameter welded steel pipes depend on the thermomechanically controlled rolled (TMCR) plate microstructure and UOE pipe-forming cold deformation sequence. Strength from plate to pipe may increase (work-hardening) or decrease (the Bauschinger effect). Bauschinger effect parameters depend on steel composition and plate processing history. The present study is examining two pipeline grades: X60 (Nb-alloyed) and X65 (Nb- and V-alloyed). Mechanical properties are determined by grain refinement, solid solution, precipitation strengthening mechanisms, and work-hardening (work-softening). The reverse deformation yield drop increases with an increase in the precipitate particle volume fraction and pre-strain. Annealing, leading to a decrease in the dislocation density, reduces the yield drop. The Bauschinger parameters are being quantitatively related to the particle type, size, and volume fraction, and the dislocation density.

The Influence of Wheel/Rail Contact Conditions on the Microstructure and Hardness of Railway Wheels

The susceptibility of railway wheels to wear and rolling contact fatigue damage is influenced by the properties of the wheel material. These are influenced by the steel composition, wheel manufacturing process, and thermal and mechanical loading during operation. The in-service properties therefore vary with depth below the surface and with position across the wheel tread. This paper discusses the stress history at the wheel/rail contact (derived from dynamic simulations) and observed variations in hardness and microstructure. It is shown that the hardness of an “in-service” wheel rim varies significantly, with three distinct effects. The underlying hardness trend with depth can be related to microstructural changes during manufacturing (proeutectoid ferrite fraction and pearlite lamellae spacing). The near-surface layer exhibits plastic flow and microstructural shear, especially in regions which experience high tangential forces when curving, with consequentially higher hardness values. Between 1 mm and 7 mm depth, the wheel/rail contacts cause stresses exceeding the material yield stress, leading to work hardening, without a macroscopic change in microstructure. These changes in material properties through the depth of the wheel rim would tend to increase the likelihood of crack initiation on wheels toward the end of their life. This correlates with observations from several train fleets.

Detection of crack growth in rail steel using acoustic emission

Abstract Increased traffic speeds and axle loads on modern railways enhance rail track degradation. To eliminate track failure due to rail defects, a condition monitoring system requires methods for the early detection of defects which grow in service. Acoustic emission (AE) monitoring is the only non-destructive technique which might be applied online to study the defect growth under traffic loading. However, a high level of traffic noise and a limited signal from crack growth, especially at low crack growth rates, significantly complicate the AE signal analysis. In the present work, the AE monitoring of rail steel fatigue was carried out in a ‘noisy’ laboratory environment using different methods of signal analysis. Signal parameters of AE for machine noise, sample deformation and crack growth were identified. The crack growth related AE signature was found to be dependent on fracture mode.