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Additive manufacture of railway crossing for Network Rail

Additive manufacture of railway crossing for Network Rail

Introduction

As part of the Network Rail research and development strategy to optimise and transform the railway, Network Rail called on TWI to assist with a research project on additive manufacturing (AM) of railway crossing. The work, which came under the In2Track project within the European Commission funded Shift2Rail programme (Grant Agreement 730841), was designed to establish which additive manufacturing processes would be most suited to achieve the required enhanced material properties where the train wheels contact with rails at railway crossings

Railway crossings are subject to greater wear than other areas of track, which has led to the use of austenitic manganese steel (AMS) due to the good combined wear and impact properties. AMS crossings are usually supplied in a soft cast state and harden into a wear resistant surface as train wheels pass over them. However, the casting process can lead to defects that can develop into fatigue cracks.

The repair and replacement of worn rails create substantial costs, particularly at crossing points that experience high stresses. In addition, track downtime causes disruption to traffic, so any reduction in this through durability improvements and the subsequent increased length of service are welcome.

Objectives

The project aimed to assess the viability of additive manufacturing processes to improve the performance of railway crossings while also optimising fracture toughness and wear/plastic deformation resistance.

To achieve these objectives, the project sought to identify a preferred deposition / additive manufacturing process and suitable feedstock, optimise the chosen process through a series of trials and metallurgical testing in line with railway standards requirements, and produce a demonstrator part.

In order to meet these objectives, the work was broken down into five distinct tasks:

Processes and requirements definition
Process down selection
Process optimisation
Production of demonstrator part
Final report (To present a project summary, highlight key findings, provide recommendations for future work)

Work Carried Out

Initial Trials

The first task required the processes and requirements of the project to be defined, including a literature survey and a gap analysis for additive manufacturing applications for railway crossings. This survey identified a justification for continuing with the research project with initial deposition trials to take place on S460NL steel grade in 100mm thickness x 50mm width x 300 mm length dimensions.

A gas metal arc welding variant known as cold metal transfer, plasma arc welding, and laser powder deposition processes were all investigated for the initial deposition trials, and filler wire and metal powders were selected in accordance with AWS A5.21: Specification for bare electrodes and rods for surfacing. Once initial trials were completed, one of the three deposition processes would be selected for further trials.

Visual inspection, bend tests, charpy impact testing, Vickers hardness tests as well as macrostructure and microstructure tests were carried out in accordance with relevant standards. The initial trials also examined the suitability of feedstock.

The results found that the metal active gas (MAG) welding process was most able to deposit weld layers of acceptable quality on the carbon steel substrate using AMS flux cored filler wire as a feedstock. The plasma arc welding trials proved inconclusive as the filler wire was deemed unsuitable for this process, while the laser powder deposition trials were incomplete. As a result, MAG process was chosen as the best process to undergo further trials.

Process Optimisation and Production of Demonstrator Parts

Network Rail supplied TWI with a mock-up railway crossing to be used as a deposition trial test block. The weld deposit was built directly onto the crossing ‘nose’ and ‘wing’ profiles with a welding procedure developed using a minimum preheat temperature of 50°C applied using ceramic heating mats and a maximum interpass temperature of less than 300°C. Five 25mm weld layers of approximately 210mm and 125mm lengths were built using multi-layer passes with the robotic MAG system. Mechanical tests were then conducted in accordance with the requirements of EN ISO 15614-1:2017 and BS EN 15689:2009 and fatigue tests were also performed to assess the overall deposit weld quality.

The tests determined that the chosen process could successfully produce an additive manufactured weld deposit using an AMS metal-cored filler wire as the feedstock. The deposit hardness and the fatigue performance exceeded the minimum requirements and the non-destructive examination (NDE) and mechanical tests were deemed acceptable to BS EN 15689 and ISO 5817 standards. However, the impact bend tests did not meet the BS EN 15689 requirements, which indicated that the weld deposit impact toughness falls short of the specification required for the rail/wheel contact area.

With the process optimisation complete, Network Rail provided a crossing mock-up with a plane surface (without the ‘nose’ and ‘wing’ profiles machined on it). Meanwhile TWI continues work to produce a demonstrator part using the optimised welding procedure with a few modifications, including building the ‘wing’ and ‘nose’ profiles directly onto the plane crossing surface and a minimum preheat temperature of 100°C to reduce the peak hardness in the HAZ of the substrate. Offline programming (OLP) software was used to develop a robot program for controlling the weld deposition path.

Conclusion

The project demonstrated that wire and arc additive manufacturing can be used for the fabrication of a railway crossing, although the AMS weld deposit impact toughness needs to be improved for the part to reach acceptable service requirements for the rail / wheel contact area. However, the hardening potential of the weld deposit and fatigue performance at the weld deposit/substrate interface were positive. There are also potential applications for additive manufacturing to be used for repair as well as fabrication.

TWI made recommendations for further work to improve the impact toughness of the weld through using an AMS solid wire consumable with optimised composition or tailor the composition of the present metal-cored wires for AM application. TWI also recommended an investigation into the use of a lower heat input to improve the impact toughness of the AM weld deposit, while Network Rail may consider an investigation into joining the AM fabricated crossing to a pearlitic rail leg to determine the weldability of the crossing and its incorporation into the existing rail track infrastructure.

You can find out more about TWI’s work in the rail industry sector here.

Close-up of the test block with weld layers applied

Dr. Usani Ofem Principal Project Leader – Arc Welding Engineering

Usani Ofem is a Welding Engineer and Principal Project Leader at TWI in the Arc Processes, Fabrication and Welding Engineering Section. He joined TWI in 2014 and has over 15 years’ of experience fusion welding processes and welding engineering activities. Usani has studied Materials Engineering and his MSc and PhD were carried out in Cranfield University, UK at the Welding Engineering and Laser Processing Centre. He is highly experienced in the provision of arc welding solutions for both ferrous and non-ferrous alloys and specialises in duplex stainless steel and underwater hyperbaric welding. As a Principal Project Leader, he is currently providing R&D, and welding engineering consultancy support to TWI member companies worldwide. He is experienced in developing processes and welding procedures for specific applications, including wire and arc additive manufacturing, feasibility studies, process comparison and optimisation. He also works closely with equipment and consumables manufacturers to study the viability, benefits and limitations of recently developed technologies. He is experienced in the application of various national and international welding codes and standards including ASME, DNV, API, AWS, BS/EN/ISO series standards, and ABS Rules.

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