Stress-Free rails offer stress-free maintenance for track operators

Resulting from almost 30 years of heat-treated rail manufacture and research, the future of rail has been defined with Stress-Free heat-treated rails from Tata Steel. Rail produced using the company’s unique heat treatment process means track operators can enjoy up to five times longer life than is achievable using standard, non-heat-treated rail. What’s more, the opening of a new Tata Steel plant able to produce Stress-Free heat-treated rail in lengths of up to 108m, means greater productivity and reliability for customers.

The Stress-Free heat-treated rails are being exhibited from Sept 23rd to Sept 27th at the Innotrans fair, in Berlin Standard rail is produced by furnace heating and rolling at red hot temperatures. Several stages of rolling are required before the rails are finally cooled and straightened to provide the accuracy required for modern track. The result of the cooling and straightening processes is rail containing high residual stresses.

The area of the rail exhibiting the greatest tension is the base of the foot. Any stress raisers in the material, such as damage or corrosion, are exacerbated by the dynamic loading from passing trains and the track tension needed to prevent buckling. The consequence is that cracks can form and propagate, eventually leading to rail failure. Due to the high residual stresses in conventional rail, it only takes a relatively small flaw to grow a crack.
In recent years there has been a lot of research into rail failure mechanisms and inspection techniques, but predominantly these have focussed on the ‘head’ part of the rail. Foot fatigue cracks are unable to be detected using current methodologies, and as residual stress cracks typically emanate from the foot of the rail, they are often allowed to propagate unnoticed. This situation is what led Tata Steel to address the problem by developing rails with lower foot stresses.

The patented Tata Steel heat treatment process sees the rail move through a set of induction furnaces where an alternating electromagnetic field heats the steel very quickly to approximately 950°C. Using compressed air, the rail is then cooled rapidly, which results in high hardness with a very fine-grained microstructure that delivers an exceptionally high elongation value. A unique attribute of this process is that it produces beneficial residual stress distribution with very low tensile or indeed compressive stresses in the rail foot. This protects against the risk of rail failure from foot defects compared with other in-line heat treatment processes, thus reducing the rate of rail degradation.

Tata Steel Stress-Free rails contain foot residual stress of less than 50 MPa – indeed, they are often compressive, which are in fact beneficial to performance as they help close (or squeeze together) any potential cracks, preventing them from growing further.

This class-leading process innovation at Tata Steel has been rolled out to a whole family of heat-treated rails to suit various specifications in use around the world’s railway network. Products such as SF350 (Stress-Free, 350 HB hardness), for example, are seeing increasing uptake in Europe where their enhanced wear resistance offer up to five times the life of non-heat-treated rail.

Due to its ability to accommodate higher axle loads, Stress-Free heat-treated rail is seen as a natural fit for heavy-haul freight routes. However, it is also finding extensive applications in high density traffic passenger routes, and in other high wear conditions such as tight curves. Here, track operators benefit from reduced risk of corrosion or damage-induced fatigue failure, combined with lower maintenance costs due to the rail’s increased wear resistance.

There is little doubt that this exciting rail product family is shaping the future of rail track engineering. It comes as a result of years of extensive research at Tata Steel, matched by an unerring commitment to investment in the latest technologies. In fact, some €12 million has recently been invested in a new plant for Stress-Free heat-treated rail at Hayange in France, near the border with Luxembourg. The plant is served with steel from Tata Steel’s Scunthorpe, UK plant, and the result of heat treatment is steel that is more wear-resistant and unyielding to rolling contact fatigue than standard steel grades, including the company’s MHH steel grade which provides the highest levels of wear resistance available.

The investment in the Hayange facility has tripled the maximum rail length to 108 m, and more than doubled capacity. This is important as more customers are demanding longer length heat-treated rail to secure a step change in reducing life cycle costs.

The market response has been universally positive. In recent months, Tata Steel has supplied Stress-Free rail for mainline projects across the globe, including Europe, Africa, Asia and the Americas.