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Manufacturing and maintenance procedures in the railway industry regularly implement welding and metal deposition operations to produce joints, coatings and repair structures. During these processes, residual stresses arise through the generation of heat affected zones and plastic deformation. This makes accurate measurements of the internal stresses a critical aspect of manufacturing, monitoring, repair and model validation in the develop new metallic coating and joining technologies. Selection of an appropriate residual stress measurement method has many important factors including component size, resolution and the magnitude and location of internal stresses, often resulting in a combination of techniques required to obtain complete assessment of the stress state. This paper offers a review of residual stress measurement techniques for railway components including rail joints and coatings through comparison of destructive and non-destructive approaches, their measurement capabilities, benefits and limitations. A comprehensive discussion of different applications is provided with a summary of facilities available to both research and industry.

The development of a laser cladding repair strategy is critical for the continued growth of heavy-haul railway networks. Premium hypereutectoid rails have undergone laser cladding using a new martensitic stainless-steel alloy, 415SS, developed for high carbon rails after standard cladding metals were found to be incompatible. Non-destructive neutron diffraction techniques were used to measure the residual stress in different layers generated across a dissimilar metal joint during laser cladding. The internal stress distribution across the cladding, heat-affected zone (HAZ), and substrate was measured in the untempered rail, after 350 °C and 540 °C heat treatment procedures and two surface grinding operations. The martensitic 415SS depositions produce compressive stress in the cladding, regardless of tempering procedures, which may inhibit fatigue crack propagation whilst grinding operations locally relive surface stress. Balancing tensile stresses were recorded below the fusion boundary in the HAZ due to thermal gradients altering the microstructure. The combination of 540 °C tempering and 0.5 mm surface layer removal produced a desirable combination of compression in the cladding deposition with significantly reduced tensile stresses in the HAZ. A comparison with the current literature shows that this alloy achieves a unique combination of desirable hardness, low tensile stress, and compression in the cladding layer. Data obtained during strain scanning has been used to determine the location of microstructural changes at the fusion boundary and HAZ through correlation of the stress, strain, full width at half maximum (FWHM), and intensity profiles. Therefore, neutron diffraction can be used for both the accurate measurement of internal residual stress and to obtain microstructural information of a metallurgical join non-destructively.

The properties and microstructure of friction stir processed AA 7075-T651 (Al-Zn-Mg-Cu) Aluminium alloy were studied using various tool designs. Trials were conducted on AA 5005-H34 with the aim of determining the most suitable FSP tool design out of the considered three different pin designs. Fully recrystallised fine microstructure and a defect free processed zone was achieved through the use of some of the FSP pin designs. For FSP 5005-H34 aluminium alloy substantial grain refinement was detected. The initial 192 µm pancake-like microstructure in the AA 5005-H34 base material was refined to 10–20 µm in the processed regions. Similarly, the grain size of around 50 µm in the base material was decreased to 4–10 µm in the processed regions of the FSP of 7075-T651 aluminium alloy. The traverse speed has a greater influence on the microhardness and mechanical properties, compared to the tool rotational speed. Microhardness and mechanical properties were improved as the traverse speed increased, due to the reduction in the precipitate-free zones at the grain boundaries.

Tram or light rail systems are heavily relied upon for passenger transit; however, low-carbon steel grades commonly used in special trackwork, such as in switches, are prone to wear, rolling contact fatigue (RCF), and deformation under cyclic wheel–rail contact. To address this, laser cladding can be used to apply a metal coating to protect the underlying substrate and rebuild the worn rail profiles. Laser cladding may also be applied to remove cracking by rebuilding the rail head. The tribological characteristics of light rail components after laser cladding with Stellite 6 and a newly developed martensitic stainless steel were investigated, using roller-on-disc wear testing. Analysis of the microstructure, mechanical properties, and wear performance was undertaken to develop a comprehensive understanding of the influence of the laser cladding type on the wear and surface fatigue performance. Both cladding alloys significantly improved the tribological performance. These findings were compared to those for a laser cladded hypereutectoid rail type (reported in our previous study). It was found that laser cladding with a suitable alloy was an effective technique for improving the tribological properties, increasing the wear resistance, and increasing the retardation of cracking on both substrates. These findings suggest laser cladding may be used to repair light rail components, and this technique can be optimized to suit different rail grades. This makes laser cladding a flexible and versatile maintenance strategy, in both coating and repair applications, to prolong the operational lifetime of critical components for the railway industry.

To overcome the previous limitations imposed by standard laser cladding alloys, a novel iron-based laser cladding alloy, SS415, is developed and investigated. The wear and rolling contact fatigue resistance properties of the novel SS415 laser cladded on a premium railway hypereutectoid rail are experimentally investigated using a custom-made roller-on-disc test machine. The obtained results are compared with our previously published results of non-clad rail and the standard iron-based cladding alloys, namely, 410L and SS420, specimens under the same conditions. The accumulated wear rate and the pileup area were quantified with the aid of an optical profilometer. The obtained results were correlated to in-depth wear surface characterization using optical microscopic images. The experimental results showed that the novel SS415 exhibits favorable hardness variation, more ductile behavior, promising work hardenability, and a 95% reduction in the wear rate, in comparison to the non-cladded samples. Moreover, the SS415 cladding resulted in less surface damage and the lowest degree of third-body abrasive wear modes, after the completion of the rolling contact cycles. Based on this investigation, the novel SS415 is a promising iron-based laser cladding alloy that offers the best wear and fatigue performance compared to both 410L and SS420 cladded samples. Additionally, this novel alloy offers significantly improved mechanical and tribological properties compared to the two standard alloys.

The effects of different cladding materials, processing parameters and heating regimes on the underlying microstructural features and mechanical properties of laser-cladded premium rails were investigated by using a hypereutectoid rail grade as a substrate, which is extensively used in heavy-haul rail systems. Cladding materials of 410L, 420SS, Stellite 6 and Stellite 21 with single and double depositions were considered for the comparative study of different cladding materials and processing parameters. To ensure the constant thickness of the claddings for comparison purposes, transverse speed and powder feed rate were modified concurrently in the ranges of 1000–1200 mm/min and 3–4 RPM, respectively. Two heating conditions, i.e. preheating only (HTA) and a combination (HTB) of preheating and post weld heat treatment (PWHT) were applied after the preferable parameters for each cladding material were obtained. The most suitable cladding material for rail-wheel contact was established by assessing all crucial aspects, i.e. surface defects, hardness, microstructural and mechanical properties. Process parameters for each considered cladding material were determined to achieve no surface defects. For cladding layers, application of HTA was not able to significantly modify the microstructures of the deposits, whereas HTB was observed to cause severe cracks in Co-base alloys, i.e. Stellite 6 and Stellite 21. In the heat affected zones (HAZs), irrespective of the cladding materials, the formation of untempered martensite was not avoided by the application of preheating at 350 °C. Consequentially, cracking in the HAZ was observed. An uncracked and desirable microstructure in the HAZs was established using HTB, regardless of the depositing materials. The addition of a second layer did not change the thickness of the HAZs but refined the HAZ’s microstructures. Shear punch testing (SPT) and Vickers hardness testing were utilized to characterize mechanical properties for the considered cladding materials and good correlations with the obtained microstructural morphologies were shown.

To develop a laser cladding technique for repairing rail surface damages due to rolling contact, wear and rolling contact fatigue characteristics of a set of laser cladded rails were investigated using a roller-on-disc test machine. Three deposition materials 410L, SS420 and Stellite 6, were chosen to clad a premium hypereutectoid steel rail under two different heat treatment processes. In the first heat treatment, only preheating at 350 °C was conducted and in the second heat treatment, preheating at 350 °C, post-heating at 350 °C (1 h) then slow-cooling to room temperature was conducted. Preheating the substrate was insufficient to prevent martensite formation resulting from the rapid cooling rate, whereas post heat treatment was beneficial for refining the lamellar spacing and eliminating martensite formation in the clad layer and heat affected zone. Following the roller-on-disc tests, wear loss was calculated from wear track profiles using a laser optical profilometer. The level of surface degradation, surface cracking and spalling was investigated using an optical microscope. Experimental results revealed that SS420 cladding had the highest wear resistant behaviour but severe surface cracks and spalling were found in the worn area. Stellite 6 cladding showed similar wear resistance as the parent substrate rail and the best fatigue resistance behaviour among the three cladded rail samples. Based on this research, Stellite 6 is the most promising deposition material for repairing rails by laser cladding.

The impacts of preheating conditions and carbon dilution on the microstructural and mechanical properties of laser cladded rails using single and double cladding layers have been investigated for a hypereutectoid steel grades typically used under heavy haul conditions. The microstructures in the HAZ showed that formation of martensite, which has a detrimental effect on behaviour in wheel-rail contact, was successfully inhibited by increasing the length of the preheated region using a preheating temperature of 350 °C. Dilution of carbon from the hypereutectoid substrate was observed and its effect on the microstructures of the 410L ferritic stainless-steel deposits was investigated. The formation of ferrite in the 410L cladding layers was attributed to the very low carbon content, and no carbide formation was observed on boundaries of the ferritic grains. The thickness of dilution band was determined to be approximately equal to the thickness of the first cladding layer. Texture measurement obtained by EBSD showed a random trend owing to the formation of martensite in diluted bands. Strong solidification fibre texture was developed for double deposition, particularly in the second deposit. Mechanical characterization of the 410L deposits undertaken in terms of Vickers microhardness, shear and tensile yield strengths, and ultimate tensile and shear strengths were correlated with the observed microstructural morphologies.

In this study, the effects of cladding direction, preheating and post heat treatment on microstructural and mechanical properties of laser cladded rail repairs are presented. Laser cladding of a premium hypereutectoid rails grade with 410L stainless steel powder were conducted using a fibre laser gun with a powder feeder. Two different cladding directions and different heat treatment regimes were investigated. An excellent microstructural consistency was established across the railhead and its heat affected zone (HAZ) by changing cladding direction and using a heat treatment consisting of pre-heating and post-heating. The microstructure of the cladding layer and HAZ were characterized by optical microscopy and SEM. Phase identification and distribution were investigated by using XRD, EDS, and EBSD. Indications of the mechanical and tribological performance of the cladding layer in wheel-rail contact were obtained via shear punch tests and Vickers indentation, which demonstrated great correlation with the obtained microstructure.

Plasma sprayed ceramics exhibit nonlinear stress-strain behaviour under tension and compression due to porosity and microcracks existing in their microstructure. The modelling done in the previous works, was reported under compression and tensile loading of YSZ. In the present study, it was extended to the effect of microstructural parameters on tensile behaviour of YSZ and validates the experimental results and introduces the variation of stress with microstructural parameters. In this paper, the nonlinear tensile behaviour of YSZ plasma sprayed material is modelled phenomenologically by obtaining a relationship between microcrack density and tensile strength. The governing equations in this model are used from the previous works done on compression behaviour of plasma sprayed materials. In the present work, an empirical relationship between tensile stress and crack density is proposed based on the experimental stress strain curves obtained from the uniaxial tensile tests on freestanding YSZ coating. The variations in maximum strength corresponding to a strain and elastic modulus were also studied for different microcrack densities and porosity. Further, the effect of porosity and crack density on mechanical behaviour of YSZ coatings is extrapolated at different combinations of porosity and crack densities ranging from 0.1–0.5 per unit volume. The result obtained using the proposed numerical model was found to be in good agreement with the obtained experimental results and the non-linear tensile behaviour of YSZ could be modelled phenomenologically.

Thermal barrier coatings (TBCs) with yttria stabilized zirconia (YSZ) as topcoat, are used to provide thermal insulation for the hot sections of gas turbine engines and other high temperature components. The aim of the present study is to evaluate the in situ tensile properties of plasma sprayed freestanding ZrO2 8 wt% Y2O3 top coat of ~250 μm thickness. Air plasma sprayed films were prepared, using pre-fabricated templates, in the form of tensile specimens. Digital image correlation with optical imaging was used to obtain full-field deformation, and a strain resolution less than 0.01% was achieved. Further, the probability of failure in these coatings was investigated using Weibull analysis. The tensile stress vs. Strain curves of the coatings exhibited a non-linear behavior, with an elastic modulus of 12 ± 4 GPa, and failure strength of ~16 ± 4 MPa with a Weibull modulus (m) of 5. The bending strength and flexural modulus of these coatings were higher, and found to be ~69 ± 12 MPa with ‘m’ of 10 and 37 ± 8 GPa, respectively. Successive unloading-reloading experiments indicated the absence of non-linearity in tensile behavior with increase in number of cycles demonstrating the inhibition of frictional sliding of microcracks. Furthermore, fractography indicated intersplat shear and decohesion as probable failure mechanisms.

Residual stress is one of the critical parameters affecting the fatigue behaviour of tribological components, which can be introduced by a thermo-mechanical process such as laser cladding. In this study, the residual stress distribution of laser cladded rails was evaluated using a neutron diffraction technique. The substrate rail for the laser cladding was hypereutectoid rail steel used in Australian heavy-haul railway track, and the cladding materials were 410L (a low carbon content stainless steel alloy) and Stellite 6 (a Co-based alloy). The cladding materials were selected based on their high wear, corrosion and fatigue resistance properties. This study measured the residual stress in full-scale laser cladded rails where the residual stresses were measured in the cladding layer, heat affected zone (HAZ) and substrate zone of the railhead. A new sample preparation strategy was developed to quantify the residual stresses in the full-scale rails with high spatial resolution. Higher compressive residual stress was found in the cladding layer, which may have resulted from the martensitic transformation occurred in that region. Tensile stresses occurred in the HAZ to a depth of 4 mm, which might be mainly caused by thermal contraction and volumetric change in the microstructure. The addition of a second cladding layer did not significantly affect the magnitude of the residual stresses, but the peak tensile residual stress shifted to a deeper location from the surface, which is beneficial in resisting wear. Post-cladding heat treatment significantly reduced the undesirable high residual stress from the cladding layer and HAZ.

Refers to: Quan Lai, Ralph Abrahams, Wenyi Yan, Cong Qiu, Peter Mutton, Anna Paradowska, Xiya Fang, Mehdi Soodi, Xinhua Wu Effects of preheating and carbon dilution on material characteristics of laser-cladded hypereutectoid rail steels Materials Science and Engineering: A, Volume 712, 17 January 2018, Pages 548-563

Local material properties of the cladding layer, heat affected zone and substrate of laser cladded hypereutectoid steel rails were evaluated by uniaxial tensile tests on miniature specimens. Three laser cladding materials, i.e. 410 L, SS420 and Stellite 6, and two different heat treatment conditions were considered. To examine any possible anisotropic behaviour, specimens machined along the longitudinal and transverse laser cladding directions were tested. All the cladding layers showed similar or higher yield and ultimate tensile strengths, but lower elongation than the non-clad rail. Application of post-heat treatment significantly improved the elongation of all the cladding layer specimens. Elongation increments of 70%, 192% and 90% were respectively obtained from the 410 L, SS420 and Stellite 6 post-heat treated cladding layers. The post-heat treatment also marginally increased the yield and ultimate tensile strengths in this region. The improvement in tensile properties was associated with a more favourable microstructure, which resulted in a more dimpled morphology as depicted in the SEM images of the fractographic analysis of the tested specimens. While other properties are similar, both cladding layer and HAZ specimens showed significant different elongation values between the longitudinal and transverse directions in some cases, which indicates certain anisotropy in those materials’ ductility.