C. L. Davis

A review on non-destructive evaluation of rails: State-of-the-art and future development

Abstract Rails are systematically inspected for internal and surface defects using various non-destructive evaluation (NDE) techniques. During the manufacturing process, rails are inspected using automated optical cameras and eddy current sensing systems for any surface damage, while the presence of internal defects is assessed through ultrasonic inspection. Similarly, ultrasonic transducers and magnetic induction sensors have been extensively used by the rail industry for the inspection of rails in-service. More recently, automated vision techniques and hybrid systems based on the simultaneous use of pulsed eddy current probes and conventional ultrasonic probes have been introduced for the high-speed inspection of rail tracks. Other NDE techniques, such as electromagnetic acoustic transducers, laser ultrasonics, guided waves, and alternating current field measurement probes, are also under development for application in the rail industry. This paper comprehensively reviews NDE methodologies in use around Europe and North America for rail defect detection. This includes a detailed overview of the background theory and the techniques used to incorporate condition data into maintenance procedures. It also presents a review of the current state-of-the-art in NDE of railways coupled with a discussion of future developments and novel inspection methodologies in the field.

Influence of processing history on mesotexture and microstructure-toughness relationship in control-rolled and normalised steels

Abstract This paper studies the effect of processing history on grain boundary misorientation angles in steels and investigates the relationship between microstructure and impact transition behaviour. For normalised steel, fracture is directly related to the metallographic two-dimentional microstructure, however, for thermomechanical controlled rolling steel grain boundary misorientation needs to be taken into account.

Preliminary study of the inhomogeneous precipitate distributions in Nb-microalloyed plate steels

Three as-thermomechanically controlled rolled (TMCR) microalloyed steels containing Nb levels between 0.023 and 0.057 wt% have been characterised in terms of phase balance and ferrite grain size distribution (optical microscopy and image analysis). In addition, transmission electron microscopy (TEM) has been carried out on carbon extraction replicas from sub-surface and mid-thickness positions to determine fine precipitate size distribution and areal number density in ferrite and pearlite. Precipitate distributions were also determined after simulated re-heating schedules. The ferrite grain size distributions have been related to precipitate distributions and rolling schedules, whilst the development of precipitate distributions has been considered based on solidification, reheating and deformation behaviour.

Cleavage initiation in the intercritically reheated coarse-grained heat affected zone: Part II. Failure criteria and statistical effects

In part I of this article, cleavage initiation in the intercritically reheated coarse-grained heat affected zone (IC CG HAZ) of high-strength low-alloy (HSLA) steels was determined to occur between two closely spaced blocky MA particles. Blunt notch, crack tip opening displacement (CTOD), and precracked Charpy testing were used in this investigation to determine the failure criteria required for cleavage initiation to occur by this mechanism in the IC CG HAZ. It was found that the attainment of a critical level of strain was required in addition to a critical level of stress. This does not occur in the case of high strain rate testing, for example, during precracked Charpy testing. A different cleavage initiation mechanism is then found to operate. The precise fracture criteria and microstructural requirements (described in part I of this article) result in competition between potential cleavage initiation mechanisms in the IC CG HAZ.

Measurement of phase transformation in steels using electromagnetic sensors

Abstract An electromagnetic (EM) sensor, capable of detecting the formation of ferromagnetic ferrite from paramagnetic austenite below the Curie temperature, has been developed and assessed. The long term aim of this work is to develop a method for monitoring microstructure online during strip steel processing. In the present paper, the initial results of variation in trans-impedance with microstructure obtained for three different types of steel with varying carbon contents are discussed. It was found that the EM sensor can successfully detect the formation of ferrite below the Curie temperature, but trans-impedance values are affected by the presence of a decarburised ferrite ring that forms around the specimens tested in a furnace. It was also found that the trans-impedance value is monotonically (non-linearly) related to ferrite volume fraction, and depends on the morphology and distribution of the ferromagnetic phase and, hence, is influenced by the prior austenite grain size.

Effect of deformation and Nb segregation on grain size bimodality in HSLA steel

Abstract Bimodal ferrite grain sizes (mixed coarse and fine grain bands) have been observed in Nb microalloyed thermomechanically controlled rolled (TMCR) steel plates and are undesirable as they can reduce toughness. This paper examines the role of rolling deformation on the formation of bimodal grain structures in reheated continuously cast slab material with initial uniform or bimodal austenitic grain structures. The slab material contains solute rich and solute poor regions, due to interdendritic segregation, which have been shown to cause bimodal austenite grain structures during reheating within a certain temperature range. It is known that deformation in the partial recrystallisation region can result in a mixed coarse and fine grained structure. Theoretical calculations (based on the Dutta–Sellars model) and deformation experiments indicated that the segregated microalloying elements (particularly Nb) can promote bimodality during deformation by affecting the local recrystallisation kinetics over a much wider range of temperatures than for a homogeneous material.

Segregation behaviour in Nb microalloyed steels

Abstract Four continuously cast slabs from two sources were characterised by optical microscopy and scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDS) in the as cast state and after normalisation close to the Ae 3 temperature. For all four slabs the composition of the majority of the slab depth (>80%) was the bulk composition and in this region microsegregation, particularly of Nb, was found to agree with predicted partition ratios from equilibrium thermodynamics. For two of the slabs, which gave final solidification as single phase austenite, equilibrium thermodynamics were also able to predict the compositional centreline macrosegregation. In the other two cases, this simple approach was not successful, which has been ascribed to solidification as a more complex phase mixture (austenite and δ-ferrite) or more complex processing during solidification (e.g. soft reduction).

Monitoring microstructure changes in rod online by using induction spectroscopy

Abstract This paper describes the application of a new multifrequency sensor system to an industrial rod mill. The system operates by analysing the response of the rod to an excitation signal which contains a range of frequencies. The sensor system is able to successfully monitor the transformation of the material. Sensor configurations that could be deployed online have been considered and a prototype system has been built and tested in the Stelmor section of the Scunthorpe Rod Mill fitted in the enhanced cooling zone. Initial results are shown, demonstrating that the phase spectra is essentially unaffected by variations in lift-off and the lay pattern of the rod on the run-out table. The system was also able to detect different phase spectra from different grades of steel and also to detect features of the process which are known to affect microstructure.

Impedance spectroscopy for remote analysis of steel microstructures

Abstract The current state of impedance spectroscopy as an online technique in hot strip mills to determine the progress of phase transformations in steels is reviewed. Electromagnetic sensors have the advantage of being non-contact probes and the use of multi-frequency techniques allows the variation of phase composition with depth to be profiled.

Predictability of Charpy impact toughness in thermomechanically control rolled (TMCR) microalloyed steels

Abstract A number of empirical equations, based primarily on average metallographic grain size and carbide thickness, exist to predict the Charpy impact transition temperature in steels. Some of these commonly used equations, successful for normalised steels, have been shown in this paper to be inadequate for predicting the transition temperature for thermomechanically control rolled (TMCR) microalloyed steels. Thermomechanical control rolling can produce clusters of small grains with low angle grain boundaries, i.e., the steel shows mesotexture. The cleavage facet size in TMCR steels has been found to be significantly larger than the optical grain size and it was also observed that individual cleavage facets can be comprised of multiple grains. In contrast, it was observed for a heat treated steel that the facet size matches the optical grain size and that individual facets consist of single grains. It is concluded that in TMCR steels, the average microstructural unit experienced by the crack front is larger than the optical grain size because mesotexture causes groups of closely orientated grains to be treated as single effective grains. This paper shows that the 50% and 27 J impact transition temperatures can be predicted for TMCR steels using the existing equations if mesotexture and grain boundary carbide size are taken into account.