Laser metal deposition characterization study of metal additive manufacturing repair of rail steel specimens

ABSTRACT Research on rail steel repair with Stellite 6 powder using a laser metal deposition process was investigated and reported in this study. Rail failures due to excessive wear and other rail head defects can increase the risk of operation and reliability problems in service life. This study examines the deposited material compatibility of a cobalt-based alloy, Stellite 6 with the head-hardened R350HT grade rail steel substrate. Materials characterization work includes metallurgical, wear, and shear properties testing and analysis. Microstructural analysis and hardness measurements of the cross-section from raster scan pattern deposition strategy of Stellite 6 on rail steel substrate surface showed minimal dilution and high hardness of the deposited layer with no presence of defects. Experimental testing of rail wear was conducted using a laboratory ball-on-disc tribometer setup and the wear coefficients for Stellite 6 and rail steel were derived from the wear volume analysis using Archard’s wear model. The material interface bond strength was evaluated from shear testing and analysis to measure the shear interface strength of the metallurgical bond between deposited material on a rail steel substrate.


Introduction
Advances in metal additive manufacturing processes have been made in powder blown Laser Metal Deposition (LMD) methods for metal additive manufacturing repair applications.Robot-assisted LMD process provides a practical solution for metal deposition repair and remanufacturing of worn or damaged steel parts for high value industrial components like turbine blades, oil and gas drill shafts, and automotive engine parts (Sexton et al. 2002;Shepeleva et al. 2000;Siddiqui and Dubey 2021;Torims 2013;Zhu et al. 2021).The deposition of clad material alloy in the form of powder offers a diverse material selection range, relatively low dilution rates, and precise deposition that is suitable for most industrial applications.Laser is used as the energy source to direct heat input across the metal substrate surface to be repaired.The heat input then creates a melt pool layer on the metal substrate surface while powder material from a coaxial powder nozzle head is deposited concurrently onto the melt pool and fused with the substrate forming a single-track layer.Subsequent deposition of overlapping layers allows a build-up of metal repaired surfaces, layer-by-layer with the laser metal deposition process (Toyserkani, Khajepour, and Corbin 2004).
Research on LMD for rail head surface defect repair applications has shown significant promise to improve rail steel performance and service life.Pearlitic steels are commonly used for the manufacture of rails due to their favourable metallurgical properties, wear resistance, and work hardenability (Tyfour, Beynon, and Kapoor 1995).Among the various pearlitic rail steel grades that were designed in accordance to EN 13674 standards, the railway track systems make use of the head-hardened, premium R350HT grade rail steel in curves to withstand severe degradation mechanisms (EN 2011).
The railway industry has been constantly evolving with continued advancements in technology contributing to accelerated growth of the railway transport system (Kerr 2008;Lewis and Olofsson 2009).The higher train speeds and larger axle loads along with increased traffic densities result in larger stresses and strains to be induced on the rails (Cannon et al. 2003;Girsch et al. 2009;Girsch, Jörg, and Schoech 2010).With these aggressive operating conditions, there is a further likelihood for rail steel defects and fractures to occur, leading to the catastrophic failure of train derailment (TMC 226 2012).Rails at curves and crossings are highly susceptible to wear and failure as shown in Figure 1.
Current practice in rail steel maintenance employs a total rail steel replacement maintenance process as a preventive maintenance approach in a rail network when the rail steel wear or damage condition reaches a rail repair or replacement threshold.To complement the total rail replacement maintenance process, rail repair of localized rail head surface defects will allow for service life extension with enhanced wear resistance and interface bond properties.The motivation to study laser metal deposition methods for metal additive repair of rail steel specimens was reviewed in this research study.Clare, et al (2013Clare, et al ( , 2012) ) demonstrated the potential of laser cladding to enhance rail steel service life.Lewis, et al (2015Lewis, et al ( , 2016Lewis, et al ( , 2017) ) evaluated the capability for rail repair and its wear reliability by conducting twin disc tests to study the wear and rolling contact fatigue (RCF) performance of laser cladded rail steel.Abrahams, et al (Lai et al. 2017(Lai et al. , 2018;;Roy et al. 2018Roy et al. , 2019) ) have shown promising results from studies demonstrating laser cladding feasibility on hypereutectoid rail steels and investigating the effects of heat treatment on mechanical properties, wear and RCF performance of the cladded hypereutectoid rail steels.
From the literature review, the powder material alloy chosen for study is the cobalt-chromium alloy known as Stellite 6, which exhibits suitable characteristics for rail steel substrate applications.The scope of this research work will focus on the Stellite 6 deposited material on rail steel specimens.Characterization research on Stellite 6 deposited material microstructure, hardness, wear, and shear interface bond strength study on rail steel specimens.An experimental study on LMD cladding of Stellite 6 powder material on R350HT pearlitic grade rail steel is documented in this report.The clad material properties are assessed in comparison to the rail steel material properties.

Laser metal deposition setup and materials
A 4-kW fiber coupled diode laser system with a 5 mm diameter laser spot size integrated to a four-axis CNC system is used for precise control of the laser beam's motion relative to the substrate during the deposition process.The other associated components include a laser head, a powder feeding system with a coaxial feeder nozzle, and a chiller system for cooling during the process.A section of the head hardened R350HT rail steel was used as the base material substrate for the experimental study.Stellite 6 powder material was selected due to its excellent tribological properties and proven capability in R260 grade rail steel studies based on literature (Kapoor 2012;Klarstrom and Wu 2004).The chemical composition of Stellite 6 alloy, R350HT grade rail along with the standard R260 grade for comparison are presented in Table 1.The powder material was gas atomized and had a spherical morphology with particle size ranging from 53 to 150 μm.

Laser metal deposition process
The rail section was first pre-machined as a 1 mm depth of material was removed to ensure there was no presence of impurities on the rail head surface.The surface was pre-heated to ensure a crack-free, high-quality deposition is obtained.The LMD process was performed by directly depositing the Stellite 6 material onto the rail steel substrate while the laser beam is scanned across the surface.A schematic of the coaxial powder metal deposition process on a rail head surface is shown in Figure 2. The heat input from the laser source creates a thin melt pool on the rail surface and the Stellite 6 powder discharged via the coaxial feeder nozzle is melted and bonded with the melt pool.Two layers of 1 mm thick multitrack layers with 50% overlap were deposited in a raster scan pattern deposition strategy.The processing parameters for deposition are presented in Table 2.
The final rail head surface post LMD process is shown below in Figure 3 with the shiny layer deposited representing the Stellite 6 hard facing.

Metallurgical procedure
Small samples were sectioned from the LMD surface in a direction perpendicular to the deposited track using wire electrical discharge machining (EDM).Thereafter, the specimens were prepared following a standard metallographic procedure by hot mounting in a resin, ground using SiC abrasive papers (P400, P800, P1200), and polished with diamond paste to a 1 μm surface finish.2% nital solution was then used to etch the specimens.
For materials characterization study, a Light Optical Microscope (LOM) -Carl Zeiss International, Axioskop 2 MAT, a Laser Scanning Microscope (LSM) -OLYMPUS LEXT OLS4100 and a Scanning Electron Microscope (SEM) -JEOL JSM-5600LV were used to examine the cross-sections for characterization of the Stellite 6 layer on rail steel microstructure and heat affected zone (HAZ) region within the rail steel substrate.
Microhardness measurements were performed using a Struers, DuraScan Vickers hardness tester, and the hardness distribution was recorded along the section depth with measurements taken at equidistant positions of 115 μm, extending into the parent rail material.The applied load was 0.3 N over 15 s of indent.The hardness data was then converted from Vickers to Brinell using ASTM standards A370 for compliance with EN 13674 European rail standards (Standard 2000).

Wear test methodology
A laboratory scale ball-on-disc tribometer setup was used to conduct wear performance analysis of plain rail and Stellite 6 deposited rail steel sections.The test procedure and sample dimensions were selected based on ASTM standards G99 (Standard 2017).The test was conducted at room temperature, under dry conditions.The extraction of disc samples from actual rail head sections is shown in Figure 4 schematic illustration.
The extraction procedure involves the removal of an 8 mm thick top layer from the rail head surface to ensure uniformity in the deposited layer thickness.Then 1-mm thick Stellite 6 material is deposited on a rail flat plane.Finally, small disc samples of 30 mm diameter and 10 mm thickness were cut out from the flat plate.
All the disc specimens were polished to a mirror finish in accordance to the standard requirement of at least 0.8 µm surface roughness (Ra).The ball material selected is Aluminium Oxide (Al2O3) of 6 mm diameter and extremely high hardness of 1568 HB since negligible or minimum wear of the ball is desired.The ball and disc samples were cleaned using acetone, followed by a rinse with ethanol, and dried under warm air.A uniform load of 5 N and sliding velocity of 0.025 m/sec was applied, as the total sliding distance covered was 100 m.The applied normal load of 5 N is equivalent to yielding a maximum contact pressure ≈ 1.3 GPa (derived from Hertzian Contact Theory), which is relatively comparable to the wheel rail contact pressure subject to the loading requirements of the railway system in Singapore.
The ball-on-disc procedure involves testing under pure sliding conditions and is valuable in the study of   the deposited Stellite 6 material's wear behaviour relative to the rail steel substrate.Wear assessment and characterisation prior to carrying out actual field trials on track is necessary.The test procedure was then replicated for the R260 grade rail steel with and without Stellite 6, and the results were analysed.

Shear test methodology
The interfacial bond strength between Stellite 6 material and the base R350HT rail steel was evaluated by conducting shear tests that conform with the American Standards ASTM A-264 (Standard 2012).The dimensions of the shear blocks and shear test samples were selected with reference to ASTM A-264.For the shear interface bond strength analysis, the test condition was such that the shear direction conformed with the laser metal deposition process direction, parallel to the interface line.A schematic illustration of the shear test setup is presented in Figure 5.
The shear test specimens were subjected to compression loading by applying load 'F' on top of the test specimens which were then sheared along the bonding interface upon completion of the testing process.The shear test experiment was conducted using a universal testing machine at a room temperature of 23°C under displacement-controlled test conditions with a loading rate selected as 1 mm/min.The fixture for the test was designed and fabricated from hardened steel, based on the ASTM A-264 standards.
Since metallic materials in general are proven to exhibit shear strength of approximately 0.577 of their yield strength based on the von Mises Yield Criterion, the test results will then be used for comparison with the shear strength of parent metal to evaluate bond quality (An, Vegter, and Heijne 2009;Rao, Reddy, and Nagarjuna 2011).The fracture mode type for this proposed test method is classified as 'mode II: In-plane shear crack opening' and a typical illustration of this failure mode is shown in Figure 6 (Benham, Crawford, and Armstrong 1996).
A small section of R350HT grade rail was cut into 5 separate segments for the LMD process.Four specimens were deposited with Stellite 6 layer of 1 mm thickness while an additional specimen was deposited with a 2 mm thick layer for comparative study.The dimensions of the shear test specimens are clearly specified in Figure 7.

Microstructural analysis
The interface between the Stellite 6 layer and rail steel substrate obtained from the LSM image shown in Figure 8 displays clear metallurgical bonding with minimal dilution.The Stellite 6 layers were deposited and bonded onto the rail head surface without any presence of defects.No cracking or delamination was observed which can be attributed to the initial preheating of the rail substrate to relieve thermal stresses.
Optical micrograph of the deposited layers and fusion zone between Stellite 6 and R350HT rail substrate is presented below in Figure 9. Based on the lateral displacement measured between the successive tracks deposited and 5 mm bead width, the overlap ratio is approximately 0.5.The cross section can be ideally characterized with the presence of four main features which are the deposited Stellite 6, fusion (transition) zone, heat affected zone (HAZ), and the base material substrate.
Table 3 below shows the size of HAZ and fusion zone which were measured and compared with the Stellite 6 deposited layer thickness in order to evaluate the amount of dilution with the rail steel substrate.
Microstructural evolution across the rail steel substrate, interface, and the deposited Stellite 6 layer are studied and characterized accordingly in this section and the initial observations from LSM images are presented in Figure 10.The influence on hardness and wear resistant characteristics of Stellite 6 can be deduced from the microstructure observed at the clad region.The microstructure of Stellite 6 is separated into the clad zone, transition zone, HAZ, and the base metal zone.
The Stellite 6 deposited layer consists of a dendrite phase rich in cobalt phase solid solution (f.c.c.structure) and an inter-dendritic mixture formed from the eutectic Co and Cr with carbides (h.c.p structure).The intermetallics (Co, Cr, and W elements) react with carbon to form the necessary carbides that contribute to the hardness, abrasion, corrosion, and wear-resistant properties.Dendritic morphology is exhibited at the interface of the cladding layer towards the top surface.The dendrites which are initially formed as cellular and columnar are transformed into the dendritic structure during the solidification process.
During the cladding process, the typical cooling rates are in the range of 10^2°C/s to 10^3°C/s (Hofmeister et al. 1999).The martensitic phase is primarily observed at the transition zone due to the rapid cooling and solidification.The HAZ comprises of diluted elements of the R350HT rail steel substrate and Stellite 6.Heat transfer changes that take place during melt pool formation and pre-heat treatment prior to cladding resulted in a finer pearlite structure at the HAZ compared to the base R350HT.
While there are obvious pores present within the clad structure, further research to optimize the laser metal deposition process parameters and with prudent process control and monitoring has the potential to mitigate these process induced porosity defects.
The SEM image at the interface between the Stellite 6 cladding and the fusion zone is presented and analysed  Rapid cooling in the clad layer leads to the predominant phase being the fine martensitic phase within the columnar dendritic morphology of the clad during fusion.The dendrite morphology formed is dependent on the thermal transient while growth direction is the direction of solidification.Since cooling rates are expected to be highest near the surface, this results in   The composition weightage across the clad rail cross section with the aid of a bar chart, it is evident that Stellite 6 cladding comprised primarily of elements Co, Cr, and W. The dilution of Stellite 6 by iron (Fe) is deduced to lower the hardness of cladding layer which causes reduced corrosion and wear resistance during application in corrosive or wear prone environments.Therefore, the deposition of an inter-layer is often proposed to minimise dilution and mitigate hardness reduction.A mixture of Co, Cr, and Fe elements are observed at the region close to the interface where the % composition of Fe is larger as compared to Cr and Co.The effects of these phase transformations and observed microstructure from the clad to substrate region are then investigated with the microhardness indentation test results obtained.

Microhardness analysis
The microhardness measurements in Figure 13 showed an increasing hardness distribution from the interface to the cladding surface.An average hardness of 420 HB was noted within the clad owing to the formation of the carbide hardening phases in the CoCr alloy matrix.This also proves that the alloying elements with primary carbide phases have a significant influence on the microstructure as well as hardness.Maximum hardness of 763 HB was observed at the HAZ, in the transition zone closer to the clad region due to the presence of the fine needle-like martensitic phase.The high hardness of the cladding as compared to the base rail (325 HB) is reflective of the excellent wear resistance that is expected of Stellite 6 material albeit the presence of Fe dilution effect.However, the peak hardness values observed in the HAZ are not desirable and thus the cladding will have to be tested for delamination by a brittle fracture in the laboratory and on-track reliability tests.The hardness distribution is reflective of the microstructural analysis and phase characterisation along the  different regions of the clad-rail cross section from the deposited cladding layer, to the interface and the bulk rail steel substrate.

Wear analysis
Figure 14 shows the wear tracks on the R350HT and R260 disc specimens for both with and without cladding which were analysed using an optical microscope.
The track width was measured and used to evaluate the total wear volume for both cladded and plain rail steel disc specimens in accordance to the wear volume formula from ASTM G99.
where V d represents the disc volume loss in mm 3 , R is the wear track radius (mm), d is the wear track width (mm), and r is the ball radius (mm).The measured wear track width and wear volume are shown in Table 4.
As expected, the wear volume calculated for the plain rail steel disc specimens was predominantly larger in    5.The surface topography in 2D and 3D along with the wear profile of the disc specimens are presented in Figure 15.
A comparison of the measured wear volume and the estimated wear volume based on the ASTM model is presented in Figure 16.While there are some variations between the two sets of data, it is to be noted that the ratio of volume loss is relatively consistent -Plain R350HT wear ≈ 1.50 times of Clad R350HT, Plain R260 wear ≈ 2 times of Clad R260.
With the known applied load of 5 N and sliding distance of 100 m, the wear rates can be determined for the cladded and plain rail disc specimens and are shown in Table 6.
The Archard's model is generally used to describe material loss and the dimensionless wear coefficient k, can be obtained from this model to characterize wear resistance of the specimen types above.The contributions of sliding velocity and contact pressure parameters are isolated to wear rate in the calculations and therefore provide a better understanding of the transitions in wear conditions.The primary difference between the wear rate calculated above and the wear coefficient derived from Archard's wear model is that it takes into account the hardness of the softer contact body (disc specimen in this case) which is an influential factor in wear and tribology studies.The wear coefficients in Figure 17 are synchronous with the wear rates obtained wherein the k value for cladded specimens is approximately half that of the rail steel specimens.It is apparent that the wear rates and hence wear coefficients decrease significantly when the disc specimen has laser cladded layer deposited.It can be inferred that the microstructure and hardness of the cladded layer influences the wear resistance of rail steel.The higher wear coefficient indicates low wear resistance and the Stellite 6 cladded rail steel disc specimens exhibit superior wear resistance in comparison to the plain rail steel specimens.
The wear behaviour and damage mechanisms are analysed to further ascertain and examine the difference in wear coefficients and the wear resistant property of Stellite 6 and the R350HT grade rail steel material.
Observations from the SEM images in Figure 18 of the wear track sections show that both abrasive and adhesion wear are present.For the cladded surface, the wear track is relatively uniform with some wear debris build-up on the circumference of the track.Delamination is identified as the dominant wear mechanism.Due to contact with the hard alumina ball surface, the Stellite 6 layer is observed to delaminate from the disc surface.Delamination occurs as the ball starts to plough into the disc surface over time with the uniform applied load.On the contrary, a severe form of wear is observed on the surface of the plain rail steel surface.
The abrasive grooves are more well-defined on the track and there are thick oxide layers formed at certain regions of the wear track.The wear damage mechanisms observed in Figure 19 can be correlated to the wear performance of the cladding surface and rail steel substrate surface without cladding deposited.The difference in wear coefficients can be associated to the severity of wear conditions between the clad and rail steel which is representative of the wear resistance of Stellite 6 in comparison to the rail steel.

Shear analysis
The load vs displacement plots for the specimens tested are provided in Figure 20.As the load gradually increases to about 2.5 kN, shearing starts to occur, and thereafter as the load rises sharply and peaks at ≈ 20 kN where the specimens fail before the load drops to zero.The minor variations in the load where shear begins and when the peak load shear failure occurs can be associated with the cladding quality.It is established that the shear strength is calculated as 57.7% of yield strength based on the Von Mises Yield criterion.
The average shear strength of the cladded specimens is evaluated to be approximately 581 ± 25 MPa which corresponds to ≈ 76% of the yield strength and 32% higher than the shear strength of R350HT rail steel.The average shear strength is also significantly higher than the minimum of 140 MPa as specified in the ASTM A-264 standards.The shear stress vs displacement plot was derived as shown in Figure 21.
Results obtained from the shear test experiment are presented in the following Table 7 below.The test results indicate that the shear strength evaluated for all the Stellite 6 cladded specimens exceeds that of corresponding properties of R350HT rail steel, thereby verifying the existence of a strong metallurgical bond between the cladding layer and rail substrate material.Preliminary assessment indicates that delamination is unlikely to occur when a cladded rail track is subjected to train load due to the strong interfacial bond.This proves that there is sufficient capability to meet the performance requirements in laser cladding repair & remanufacturing of rail steel.Images of the failure location and the fracture surface morphology after the shear test are presented for analysis.Figures 22-26 show the photographs along with LSM optical micrographs of the fracture surfaces of the sheared test specimens.The shear strength test specimens were examined to fracture at the interface bond line and complete fracture of the cladding from the base rail steel plate was observed for all except one specimen.It has to be noted that this break-up that occurred during the shearing process indicates total detachment of the clad layer from the rail steel substrate leading to interfacial delamination.
The initial shear crack is originated at the bond interface, as the shear crack then primarily tears from the bond interface and then propagates within the rail steel substrate side where the strength is lower than that of the cladding layer side.The dominant cracks then propagated in the direction parallel to the applied loading.
Based on the study of the optical micrographs and analysis of the fracture surface morphologies of the specimens, it is evident that shear initiated along the edge where the specimen was loaded.The crack was developed in the substrate metal but did not reach the bond interface line as examined from the LSM images, particularly in Figure 22 where the smooth crack surface implied a clean break during failure.Figure 23 shows a non-uniform fracture of the surface which is also closer to the interface line in comparison to the failure of the specimen (1).Multiple pores are present near the clad interface in Figure 24 where the fracture is observed to have occurred even closer to the bond interface line.The porosity could have also resulted in a lower shear bond strength relative to the other two specimens (1) and (2).In Figure 25, the tested specimen shows fracture initiating in the cladding rather than at the interface or the rail substrate.The additional test specimen (5) which was deposited with 2 mm thick cladding layer (Figure 26) showed fracture occurring at the interface with a minimal amount of base material still attached at the cladding interface.Tearing marks can be seen that were generated from the point of shear.Interfacial delamination was the primary mode of fracture that occurred due to the pure shear phenomena.
The results and observations from the analysis showed the main fracture site was observed in the parent metal side as the crack initiated and propagated along the bond interface line.The interface exhibited good bonding quality in majority of the test specimens examined.

Conclusions
This research study reports on LMD materials characterization, wear, and shear testing and analysis to characterize the wear durability and shear strength performance of Stellite clad metal repair deposited on rail steel specimens.Experimental characterisation results from the LMD process with Stellite 6 powder for metal additive repair on rail steel components were conducted and the following conclusions can be made from this study.Metallurgical study of the LMD process of Stellite 6 material with R350HT rail steel substrate described observations from the cross-sectional analysis.The clad-substrate interface displayed clear metallurgical bonding with minimal dilution as Stellite 6 clad layers were deposited onto the rail head surface without any presence of defects.No cracking or delamination was observed which can be attributed to the initial preheating of the rail substrate to relieve thermal stresses.
The clad region showed columnar dendritic morphology which is in agreement with the earlier research observations of Stellite 6 microstructure.At the transition (fusion) zone, the martensitic phase was primarily observed due to the rapid solidification and cooling during the laser cladding process which can cause embrittlement.Results from the EDS measurements further verified the presence of Co-rich dendritic and inter-dendritic mixture of carbide phases of Co, Cr, and W, in the deposited cladding layer.
Results from the hardness distribution profile at Clad: 420 HB, HAZ: 763 HB, Rail Base Metal: 325 HB, was representative of the microstructural analysis and phase characterisation along the different regions of the cladrail cross section from the deposited cladding layer, to the interface and the bulk rail steel substrate.However, the peak hardness values observed in the HAZ are not desirable and thus the cladding will have to be tested for delamination by a brittle fracture in the laboratory and track reliability tests with a running train.
Overall, the cladded specimens exhibited wear volume significantly lower than the plain rail steel specimens.Wear for both the cladded R350HT and R260 is comparable with the R260 grade giving slightly higher wear volume results.Wear volume estimated using the ASTM wear equation yielded similar results to actual wear measured using a surface profilometer.Wear coefficients obtained from the Archard's wear model prove the superior wear resistance of Stellite 6 cladding in comparison to the plain head hardened R350HT and standard R260 grade rail steel.Wear coefficients derived from the Archard's wear model Shear testing and analysis of the clad-substrate material interface bond strength were studied for assessment of delamination.Shear load (stress) Vs Displacement plots were derived for which shear failure of the clad material specimens occurred.Results from the study indicated that delamination is unlikely to occur when cladded rail steel is subjected to train load due to the strong interfacial bond.The shear strength test specimens were examined to fracture at the interface bond line and complete fracture of the cladding from the base rail steel plate was observed for all except one specimen.The crack was developed in the substrate metal but did not reach the bond interface line.
Shear initiated along the edge where the specimen was loaded as tearing marks were observed, generating from the point of shear.Interfacial delamination was the primary mode of fracture that occurred due to the pure shear phenomena.
Encouraging results from the materials characterization studies and the wear and shear mechanics testing and analysis demonstrate that LMD clad repair of rail steel components have significant potential for future deployment for rail steel repair operations.

Figure 5 .
Figure 5. Schematic illustration of shear test setup in accordance to ASTM A-264.

Figure 7 .
Figure 7. Shear test specimen dimensions, (right) Test specimen extracted from the Stellite 6 deposited rail steel section.

Figure 10 .
Figure 10.Microstructure at the respective regions of the cross-section (a) Stellite 6 clad, (b) Transition zone where the clad fuses with the base rail, (c) Heat affected zone (HAZ), (d) Base R350HT.

Figure 11 .
Figure 11.Interface between Stellite 6 clad layer and fusion zone, (a) Columnar and dendritic phase of Co matrix (b) Needle-like martensitic phase.

Figure 13 .
Figure 13.Hardness distribution profile across the clad rail cross section.

Figure 14 .
Figure 14.Optical micrographs of the wear track on each specimen.

Figure 16 .
Figure 16.Wear volume loss of the disc specimens.

Figure 17 .
Figure 17.Wear coefficient k, for cladded rail and plain rail.

Figure 18 .
Figure 18.SEM images of cladding and rail steel wear track section.(a) Stellite 6 Cladded Rail Steel; (b) Plain Rail Steel.

Figure 20 .
Figure 20.Load vs Displacement plot of the four shear test specimens.

Figure 19 .
Figure 19.SEM images showing presence of abrasive, adhesive and oxidative wear mechanisms on wear track.
Figure2.Laser metal deposition process schematic on rail head surface.

Table 2 .
Laser metal deposition process parameters.

Table 4 .
Wear track width and wear volume.

Table 5 .
Measured wear depth and wear volume.

Table 6 .
Wear rate for the cladding and plain Rail Steel.

Table 7 .
Shear test results summary.
Figure 21.Shear Stress vs Displacement plot of the four shear test specimens.