Influences of depositing materials, processing parameters and heating conditions on material characteristics of laser-cladded hypereutectoid rails

Abstract

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.

Introduction

Damages, i.e. wear and rolling contact fatigue (RCF), etc., often develop and accumulate at the contact surfaces during the service lives of rail-wheel components, which are commonly the main reasons for preventative maintenance in modern railway infrastructure. The resulting network downtime and expenditure are, therefore, significant. As an encouraging answer, surface treatment of damaged railway components, particularly the laser cladding technique, has been demonstrated its effectiveness.

Laser cladding is a weld build-up process in which a depositing material is adhered to a base material via utilizing a laser. New or damaged engineering components can be metallurgically bonded with superior depositing materials to gain or regain the desired surface properties. Depending on the nature of the applications, different depositing materials can be selected. Recent studies have demonstrated significant improvements in the performance of the laser treated components. Zhenda et al. (1996) conducted an experimental investigation on laser cladding of WC particle-reinforced Ni-based alloy onto AISI 1020 steel. The tribological results indicated the superior wear resistance of the cladded components due to the presence of the claddings. Sexton et al. (2002) performed laser cladding onto aerospace components made of five separate materials and compared to tungsten inert gas (TIG) welding specimens. They indicated that laser cladding offered obvious advantages for the repair of aerospace components and more superior microstructure, hardness, cracking, porosity and dilution levels compared to the TIG specimens. Abioye et al. (2015) laser-cladded austenitic stainless steel AISI 304 components with Inconel 625 wire and reported that an improvement in corrosion performance of the components made of stainless steel was observed due to the protection of the Inconel 625 claddings. These have validated the beneficial impacts on the performance of the treated components by applying laser cladding technology.

Laser cladding is also being tailored to enhance surface properties and corresponding tribological performance of rails and track components while conserving the properties of the parent rail substrate. Many research groups have applied laser surface engineering techniques, i.e. laser glazing and laser cladding, to various rail grades. Aldajah et al. (2003) reported that the use of laser glazing produced a treated rail with a fine and harder solidified microstructure, i.e. as much as three times harder than the substrate. Using similar technique, Shariff et al. (2010) performed laser-glazing on T-12 rail steels, the standard rail grade in India, and reported a marginal reduction in friction coefficient caused by the laser glazed regions. However, the prevention of undesirable plastic deformation, as known as a batter, might be infeasible due to the presence of such high hardness. Niederhauser and Karlsson (2005) investigated the laser cladding treatment of the B 82 steel plates extracted from the Swedish railway wheel with Co-Cr alloys in terms of the cracking and fatigue behaviour. For the cladded steel plates, a consistent and favourable fatigue behaviour was observed. With increasing strain amplitude, an increase in the number of cracks was detected in the plate substrate along with well-plasticized surfaces and shear bands began to connect over grain boundaries were reported on the cladding. Similarly, Ringsberg et al. (2005) studied the RCF behaviour of a Co-Cr alloy layer cladded on the pearlitic UIC 900A (R260) rail steel and reported that excellent agreement between experimental and numerical results of residual stresses was noticed. The behaviour of fatigue failure was strongly influenced by the process of laser cladding compared to the untreated rail. Under the InfraStar project, Franklin et al. (2005) worked on laboratory tests and Hiensch et al. (2005) conducted actual field tests involving RCF and tribological performance of the UIC 900 A rails after being laser cladded. The results from twin-disc laboratory testing showed that the life cycles of the laser cladded specimens were five times greater than that of the base material. Also, the field test results showed no RCF damage. However, the base material in the unclad condition showed clear RCF damage. The research papers have highlighted the importance and significance of utilizing laser cladding in the future rail maintenance strategies. Therefore, understanding and predicting the mechanical performance and metallurgical characteristics of rail steels after cladding are crucial.

However, previous studies have been limited to hypoeutectoid or eutectoid rail grades, and information on the application of laser cladding to premium hypereutectoid rail grades, such as those commonly used in heavy-haul rail systems, is limited in the open literature. These rail grades are renowned for their superior load bearing capacity, better service durability, smaller crack propagation rate and longer maintenance intervals compared to conventional rail grades.

Previous works on laser cladding of hypereutectoid rails by Lai et al., 2017a, Lai et al., 2017b investigated the effects of cladding direction, heat treatment and carbon dilution on the material properties of a hypereutectoid rail grade using a 410L stainless steel cladding material. The present work focuses on the influence of different cladding materials, processing parameters and heating conditions on microstructural and mechanical characteristics of hypereutectoid rails after cladding. 410L stainless steel, 420SS stainless steel, Stellite 6 and Stellite 21 were selected for the cladding materials. A comparative study of the cladding materials by considering vital aspects, i.e. microstructures, surface defects, hardness, and material strength, was undertaken to determine the preferable cladding material, processing parameters and heating conditions for rail-wheel contact conditions.

To establish functionally graded rails, in this study laser cladding of the cladding materials was applied to 600-mm long sections of premium rail samples utilizing a coaxial fibre laser nozzle. Interpretation of microstructures of both the cladding layers and the HAZ in the rail substrate was achieved based on images of optical microscopy and Scanning Electron Microscopy (SEM). To study and compare the mechanical properties of different depositing materials, shear punch and tensile testing were performed. Indications of wear resistance of cladding layer were obtained via Vickers indentation, thereby, the correlation between the microstructural characteristics and the prediction of wear performance was also established.