Material innovation combats rail fatigue

Materials innovation BEEAs winner Howard Smith, a research metallurgist at Tata Steel Europe, outlines the motivation and technology developed for combating rolling contact fatigue on the railway.

The Hatfield rail disaster in 2000 marked a watershed in the safety of not just British, but also global rail networks. It served to highlight the key degradation mechanism leading to premature curtailment of rail life, namely rolling contact fatigue (RCF).

The evermore stringent demands imposed on our railways such as increased axle loads, vehicle speeds and traffic densities have only served to exacerbate the problem. In recent years, conventional pearlitic rail steels have undergone incremental increases in hardness by alloying, heat treatment or a combination of the two, in an effort to address RCF. However, it is now widely accepted that such steels are reaching their limit of exploitation.

Tata Steel research and development in Rotherham has busied itself in recent years to address just this problem. The result has been the HPrail, a new rail steel with a microstructure that has been metallurgically engineered to specifically address the RCF phenomenon, whilst maintaining optimum performance in terms of wear and resistance to plastic deformation.

Since its initial conception less than three years ago, the new HPrail product has already been widely introduced across the UK mainline rail network and initial signs are extremely encouraging. For minimal capital investment and marginal increased alloying costs, the potential financial returns are considerable. It is anticipated that European markets, principally those in France and Germany, will also provide lucrative outlets for HPrail. There is also scope for supplying the new rail grade into India reflecting the parent company's origins of Mumbai, India.

The historically conservative railway industry has always been extremely cautious in adopting new technologies, particularly in the post-Hatfield era. For any rail manufacturer, guaranteeing that a new rail grade will not introduce additional risk into the network is paramount. It was therefore a necessity that the new rail steel undergoes thorough evaluation and product testing before acceptance by network operators for track trials.

Many of the materials that have been designed and developed commercially in recent decades have relied on enhancements in generic properties, for example, hardness, strength and toughness. While such parameters are regarded as influencing the performance of a rail during service, they are not essential to prolonging rail life. Of the plethora of existing steel grades available none were deemed to satisfy fully the specific rail property requirements.

In recent years, Tata Steel, along with its main competitors, has designed alternative steel compositions including heat treated and bainitic types. Although offering attractive property combinations, such steels have always been found wanting in some respect, be it difficulties in their processing, some property deficiency or the ease of welding. Not least, they have proved undesirably, or in some cases prohibitively, expensive.

Nevertheless, these previous experiences provided direction for more recent developments from which the novel concept of HPrail was born. The success of the HPrail development programme was the result of a number of factors. These included a comprehensive appreciation of the market requirements, a combined level of technical expertise that was 'second-to-none' and clever utilisation of key metallurgical concepts and novel processing solutions to develop a rail steel providing a step change in service performance. The success story is all the more impressive in view of the minimal increase in steel costs and that no major capital investment has been necessary.

Meeting the arduous and rail-specific property requirements represented an immense challenge for Tata. The repeated customer demand was for a rail steel that has a significantly reduced susceptibility to RCF. However, this was not to be at the expense of other property attributes such as resistance to rail wear. Ultimately, the key demand was for a rail having demonstrably lower whole life rail costs relative to existing rail steel grades.

In parallel, any new rail steel had to meet the stringent demands stipulated in the current rail specifications. The optimum design of HPrail steel chemistry also necessitated a comprehensive understanding of all stages of rail manufacture. This included steelmaking capabilities and the ability to continuously cast the steel into the feedstock required for rolling to rail. Thereafter the ease of hot rolling the steel and subsequent downstream cold processing of the rolled rails was essential to ensuring a useable product. Finally it was critical to be able to join HPrails by the variety of procedures currently used including flash butt and Alumino-Thermic welding as well as subsequent weld restoration while in service.

Rail maintenance programmes are a huge drain on a rail network's financial resources and considerable input is devoted to optimising rail grinding and renewal strategies. Means of reducing these costs without compromising passenger safety are the subject of continual review. Tata Steel Europe's remit was to develop a rail grade that would address these issues but with minimal capital investment.

The result has been a rail steel that attracts considerably higher profit margins than the minimal increased manufacturing costs relative to those of conventional as-rolled rails. To date, HPrail has been installed in a total of 14 trial sites across the UK mainline rail network. These initial sites carry essentially mixed traffic - passenger and freight - and have been specifically targeted on the basis of their suffering from excessive RCF cracking.

The performance at all the trial sites in terms of a significantly reduced incidence of RCF and increased wear resistance over conventional rail grades has exceeded expectations, so much so that Network Rail is looking to increase its deployment of HPrail at a rate considerably faster than originally intended. Furthermore, as of July 2012, Tata Steel Europe has gained full approval for the provision of HPrail into the UK mainline network as well as trials taking place within two tramway systems.

Howard Smith, Tata Steel Europe

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