Improved quality of weld repair of rail defects at reduced costs from Corus

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 novel 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). Given that an average cost per repair or short replacement rail can run into several thousands of euros and that the occurrence of wheel rail interface defects is likely to increase with the evident increase in levels of traffic on most railways, the importance of the new process is easy to understand.

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 running of carriage wheels on rails creates high and complex stress patterns within the rail/wheel contact patch, leading to surface degradation. The wide range of track design, wheel profiles, and types of traffic can result in a variety of surface defects that reduces the life of the rail. Defects such as squats and wheelburns occur even in the most modern and well maintained railway networks and, as a broad general rule, every network develops one such defect each year, every two kilometres. The replacement of such defects with a short rail section is expensive and not always desirable as it introduces two new discontinuities in the track in the form of two aluminothermic welds (exothermic reaction using aluminium as the reducing agent) that destroy the advantages obtained with long hot-rolled rail (up to 120 metres). The alternative conventional technique for the repair of such defects is the Manual Metal Arc (MMA) welding process. Although the technique is used many industries, it is heavily reliant on the competence of the welder, is time consuming, and is prone to internal defects such as porosity that subsequently grow through fatigue, and if not detected by ultrasonic inspection, result in rail breaks.

The following factors contribute to the cost effectiveness and technical robustness of the new developed process: 1) The move away from the conventional preheating temperature of 350°C to just 80°C has the advantage of faster repair, reduced depth of heat affected zone, and more robust microstructure. 2) The use of a standardized removal of the defect area by controlled milling has the advantage of reproducibility and removes the subjective judgement of the operator. 3) 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´ new 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. It ensures a consistent excavation of the identified defect. This is itself 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 80°C. 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 (UK) 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 just 80°C preheat, the temperature in the HAZ after each deposited weld bead remains above 200°C, preventing any transformation to the martensitic microstructure (as the martensite start temperature is 160°C for grade 260 rails).

-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 5.2 °C/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.

For further information, e-mail: laure.lafare@corusgroup.com or view website: www.corusrail.com  Refer to page 88

Custom torque sensors now available on standard lead times

A UK manufacturer of torque sensors is offering custom designed static/reaction torque transducers on similar lead times to its standard units, with only a small premium on the standard list price.

Applied Measurements Ltd, based in Aldermaston, is able to design and manufacture custom static torque sensors to meet any customer requirement, including different flange diameters, unusual sizes of square drive, special sealed versions and shorter, more compact designs.

According to Peter Lewis, Managing Director at Applied Measurements: "Many manufactured products require fatigue testing to ensure that they can operate for a guaranteed minimum period of time. An accelerated life test, for example, may be carried out on a drive shaft to determine the torque limits. The fatigue testing can sometimes be an afterthought. If this is the case, some kind of customised sensor is then required before the torque sensor can be coupled to the test unit. This is what Applied Measurements specialises in."

Whilst Lewis concedes that there are plenty of companies able and willing to supply static (reaction) torque sensors from their standard range, many do not offer customised designs, let alone on lead times that are similar to standard product.

Applied Measurements manufactures two types of static torque sensor as standard, a square drive mounting type and a flange mounted unit. The DTD-S square drive mounting sensor is designed specifically for measuring direct torque and is ideal for use in the calibration or testing of torque tools (screwdrivers, spanners, etc) in a quality assurance and inspection environments. The sensor is constructed from stainless steel and is protected to IP65. For fast, easy connection, the sensor is supplied with an integral, robust bayonet lock military connector. The DTD-S is available from 10Nm up to 50,000Nm as standard, with custom versions rated to 200,000Nm.

The DTD-F range of flange mounting static torque sensors are also designed to measure direct torque, but are ideally suited to fatigue test applications. Again, the units are finished in stainless steel and are protected to IP65. The sensor is provided with an integral, robust bayonet 'lemo' connector for fast, easy connection.

As Lewis continues: "The flange mounting sensors are normally used in applications where there are two mating faces, one driving or applying the torque and the other resisting the torque. For any torque sensor to work, there has to be a reaction or load to generate the resistance to motion and hence torque."

"While we offer customers a range of standard torque sensors, we also have the ability to design and manufacture custom versions, where the standard unit does not suit the application. It could be the size and shape of the sensor that needs changing, but could also include the need for odd size ranges that fall between the standard increments."

Lewis says this could include making the sensor longer, shorter, broader, or with different flange diameters and thicknesses. Units with different hole spacing or with an odd size of square drive may be requested. As an alternative to the square drive unit, Applied has also custom designed round shaft units with keyways. By creating a tubular design and applying the gauges internally, it is possible to create high integrity sealed units for wet environments. Other designs have included two-axis measurement of combined force and torque, pressure-compensated models or complete submersion units.

Applied Measurements has supplied custom static torque sensors to a variety of customers and industry sectors, including aerospace, marine, oil and gas, robotics, machine building and plastic moulding machines.

For further information on Applied Measurements' range of static torque sensors, email: info@appmeas.co.uk   Refer to page 70