Introduction: The surface cladding industry continues to expand and find great success in extending the lifetime of components in heavy wear environments. In applications involving the repair and manufacture of existing parts, hard facing via plasma and directed energy welding allows many high wear components to have dramatically improved lifetimes. Yet these hard-facing techniques have limitations in applications where wear resistance cannot be achieved through increased hardness alone. At the same time, the weld filler-metal industry is seeing great innovation with the introduction of Low Transition Temperature (LTT) Alloys. These materials undergo their martensitic transitions at much lower temperatures. Creating the potential for dramatic reductions in the tensional residual stress experienced by welded components when used. Improving performance and potentially reducing hot cracking.
The confluence of these two technologies creates an opportunity for a new approach to surface cladding and the extension of the lifetime of components in unique high-wear applications. Rail transport industry wheels face intense, high-wear conditions; acting as the primary load-bearing surface for freight transportation. Traditional hard-facing techniques see limited success for this industry as if the wheel becomes harder than the rail, the rail will deteriorate disproportionately. This introduces a higher maintenance cost than wheel replacement. The use of LTT alloys as a surfacing material could provide a solution by improving wear performance through the introduction of compressive surface residual stresses without increasing hardness.
Experimental procedures: A set of alloys of varying compositions were developed to be a representative cross-section of materials of potential use for application onto rail wheel rims. A low alloy material of very similar composition to the base wheel material, a high-alloy material designed to maximize the potential for LTT without exceeding hardness constraints, and an intermediate composition. These alloys are deposited via Plasma Transferred Arc Welding (PTAW) onto plates in order to assess their feasibility as materials to act as a rim surface for locomotives.
Welds were deposited onto low carbon 1/2in steel plates in 4 parallel passes, and also onto a chopped section of rail wheel steel in the same manner. After application, plates were sectioned and prepared for optical micrographs. Micrographs were used to assess the microstructure and quality of the fusion layer. Materials were assessed for hardness and their impact on base material hardness using the Vickers hardness test. Plates are to be tested for residual stresses using the hole drill test according to ASME standards for thick plates in non-uniform stress.
Results and Discussion: Early research results suggested that these LTT alloys could improve the lifetime of repaired parts even beyond the lifetime of as manufactured parts. This is attributed to what literature has determined to be a potential inversion of residual stresses from tensional stress to compressive inside the weld material. This compressive stress is derived from the volumetric expansion associated with the transformation to martensite by the filler material. When this expansion occurs at low temperatures the remaining thermal contraction is too small to absorb the expansion resulting in a final stress state of compression. The application of such materials via PTA has shown promising early results with limited or no hot cracking in plate welds in any of the tested alloys, and microstructures suggesting effective martensitic transitions as well as a successful fusion to the dissimilar base material. Further work is to be explored in the application of such alloys directly to wheel rims, and the functional gradation of the material to ease dilution issues.
Conclusion: As an assessment of the use of PTA for the application of materials to rail wheels for repair and fabrication. A series of alloys were applied to the surface of the locomotive wheel with varying compositions; in an attempt to create a highly wear-resistant overlay with good fusion to the base material. The selected alloys were designed to exhibit some LTT properties to explore the viability of these materials for rail-industry applications. Preliminary results suggest PTA provides a promising option for future applications as a cladding process with these materials. with further development to refine the process and materials required to reach an industrially viable result. There is substantial opportunity for future research to explore optimizing both alloy composition and application processes. Tests applied directly to wheels and assessed for stability and performance will provide the important next step towards industrial adoption.