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Corus Rail has developed a novel technique for the cost-effective repair of discrete defects on the running surface of rail.

The key strength of this technique lies in the replacement of those aspects of the conventional Manual Metal Arc (MMA) process that often result in variability in the quality of the repair with automatic and more controlled operations.

The developed semi-automatic process employs open arc welding with flux-cored arc wire and relies on a low-preheat temperature to proactively control the metallurgical transformations within the Heat Affected Zone (HAZ).

The process has been thoroughly tested and a dedicated unit is currently being manufactured to undertake in-track demonstration in several European networks including France and UK.

The following factors contribute to the cost effectiveness and technical robustness of the new developed process: the move away from the conventional preheating temperature of 350 to 80C has the advantage of faster repair, reduced depth of heat affected zone and more robust microstructure; the use of a standardised removal of the defect area by controlled milling has the advantage of reproducibility and removes the subjective judgement of the operator; the use of a semi-automatic programmed open arc-welding process with flux-cored arc wire ensures control of heat input and predictable operational times; the quality of the weld restored running surface from the developed process is ensured as the repair is extremely resistant to fatigue and has similar wear resistance to that of the standard Grade R260 rail with uniform hardness and microstructures across the weld-restored area.

Corus’ patented repair technique includes four steps.

The defect is first removed by using a portable three-axis rail-milling machine that clamps onto the sides of the rail and ensures a consistent excavation of the identified defect.

This is itself is said to be a significant improvement on the use of manual grinding or flame scarfing – both of which do not give a consistent cavity shape or surface finish to facilitate automatic programmed welding.

Secondly, the adjacent area and the cavity are preheated with a conventional burner.

For Grade 260 rails, the prescribed temperature is between 60 and 80C.

The choice of this temperature is for the control of the microstructure in the HAZ and the programmed square weave pattern of deposition of the subsequent/adjacent beads ensures that the microstructure in the HAZ is fine pearlite and free of any embrittling martensite.

This temperature is suitable for the vast majority of high-carbon rail steels in use today but it may need to be modified for steels that have different transformation characteristics, such as low-carbon carbide-free bainitic steels.

The third stage uses a semi-automatic weld-repair machine, with an open arc-welding process, a Network Rail-approved TN3-0 welding consumable and prescribed welding parameters.

The positioning of the top layer is crucial to prevent the creation of a new Heat Affected Zone (HAZ).

Most of the top weld layer is partially removed by profile grinding.

The fourth and last step consists of restoring and blending the transverse and longitudinal rail profile by grinding, using conventional rail grinders.

A comparative evaluation of the existing MMA technique and the new process was achieved by recording the thermal history of both processes using embedded thermocouples.

Several key conclusions demonstrates the metallurgical robustness of the process.

Despite the use of 80C preheat, the temperature in the HAZ after each deposited weld bead remains above 200C, preventing any transformation to the martensitic microstructure.

The cooling rates in the developed process are almost identical to those in the conventional MMA process for all deposition passes except the first.

The faster rate of 5.2C/s after the first weld bead is also half the critical rate for transformation to martensite.

A crack-free weld deposit interface is apparent with a fully pearlitic microstructure, free from martensite and bainite.

The hardness profile shows that the wear resistance of the bainitic weld deposit will be comparable to that of grade R260 parent rail and ensure a good longitudinal profile.

The weld deposit was subjected to a bending fatigue test with an applied stress range equivalent to three times that expected in service.

Five million cycles were successfully completed without any failure.

The same deposit successfully endured a further 4.3 million cycles at an applied stress range equivalent to eight times that expected in service.

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