RAILWAY infrastructure managers invest millions every year in grinding and milling rails to avoid the potential problems that can emerge as a result of rolling contact fatigue (RCF). An increasingly common phenomenon due to greater traffic levels, RCF is the result of over-stressing of the rail caused by thousands of wheel-rail interface interactions, and was cited as the cause of Britain's devastating Hatfield derailment in 2000.
Dipl-Ing Dr Norbert Frank, Voestalpine's head of technology, says the company recognised an opportunity in the market during the mid-2000s to develop a new steel grade for rails that could eliminate this problem altogether and reduce maintenance costs significantly.
The result is Dobain, which is described by Voestalpine as the world's first maintenance-free rail with respect to RCF management, and is currently around half way through extensive track testing in Germany and Austria with a view towards certification for use in these two countries. Discussions are also progressing with other leading European infrastructure managers about further track testing and certification processes.
Frank says Dobain was developed to tackle the continuing problem of headchecks which are a typical sign of surface RCF and tend to appear in under-maintained medium to wide-curvature track. Headchecks result from fine cracks on the surface of the rail following prolonged use. While different types of pearlitic steel grades and anti-headcheck profiles are available, they all remain susceptible to headchecks at some stage of their life and at present the only way of countering this problem is to grind or mill the rail at regular intervals.
Previous track tests have shown that the harder the rail steel, the slower the crack initiation and propogation rate. However, Frank says that Voestalpine found during development that even steels with a pearlitic microstructure, which forms following premium heat treatment, and a hardness of up to 440BHN, will still develop headchecks although they are proven to extend rail life significantly while offering increased maintenance intervals compared with previous steel grades such as non heat-treated R260.
High strength pearlitic rail is now considered a standard in Europe following the recommendations of the InnoTrack project with rails of this type widely accepted by infrastructure managers during studies conducted between 2006 and 2009. While pearlitic steels are now the benchmark, Voestalpine says that a heat-treated bainitic steel, which was found in early studies to offer increased resistance against crack initiation and propagation due to its refined microstructure, was recognised as holding the greatest potential for producing a maintenance-free rail.
As a result, Frank says that Voestalpine experimented with three different metallurgical steel types over a period of three to four years with the aim of understanding how these materials might react under certain conditions. The objective of the research was to identify a grade that would produce a headcheck-free rail as well as to develop industrial-scale production processes for the new rail type at its Donawitz plant in Austria.
Ultimately the engineers settled on a heat-treated bainitic steel grade with 340-410BHN, with the hardness adjustable depending on the loads to which the rail will be exposed. Frank says while this is similar to the properties of pearlitic steel grades, the added strength is achieved by the dispersed nature and the orientation of the constituents pearlite and cementite in this particular microstructure. Tests have shown that it provides sufficient hardness and strength to neither produce headchecks nor show signs of RCF under the stress of intensive use on mixed-traffic, high-speed or light rail lines over the 30-year life of a rail.
Bainitic is not a unique steel grade for use in rails. Other procedures have been developed that can produce a similar bainitic microstructure. However, they have proved to cost almost twice as much as the Voestalpine method due to the need to add additional alloys which also decreases weldability and conductivity, while increasing thermal expansion creates additional stresses in the rail. They also are not proven to produce a completely headcheck-free rail. By utilising Voestapline's HSH heat-treatment process, the rail is dipped into a liquid medium to accelerate the cooling process. Here a certain time-temperature regime must be maintained to control the material transformations that will achieve the RCF-resistant bainitic microstructure through an effective in-line heat treatment rather than expensive alloying.
A specially-designed rolling load stand that uses a 1:1 wheel-rail setup is aiding the development of Dobain. The system simulates in the laboratory the passage of thousands of sets of wheels over the rail which is much faster than a typical track test which can take up to five years. Frank says that this process has sped up the testing process by up to 10 times due to its capability to simulate different conditions almost at the push of a button and to run continuous tests.
"The self-developed rolling stand took 10 years of development to reach the stage we are at now for controlling the wheel-rail interface," Frank explains. "The system is entirely computerised and automated, and it enables us to select different conditions and different loads that are proven to produce headchecks consistently on conventional rail after only 30,000 cycles. However, we have found with bainitic steel even after 750,000 passes at the same load conditions that it remained completely headcheck-free."
The results of these tests show specifically that Dobain bainitic steel has a useful life at least double that of standard-grade steel. Rail grinding is only required once or twice during the lifetime of the rail to maintain profile, but not to eliminate headchecks. In comparison, R260 grade requires grinding, milling or planing more than 10 times before a complete replacement is carried out which would be necessary twice during the equivalent life of a Dobain rail. Voestalpine's R350HT, which has a pearlitic structure, has a similar life-span to Dobain but again the testing found that the rail will require grinding at least 10 times during this period to eliminate headchecks and avoid RCF.
"We can clearly see from these tests that the life-cycle cost of using Dobain compared with existing rail is going to be a lot smaller," Frank says. "You are still going to get to get 25, 30 or 40 years of life depending on track curvature and loads, but during this period the rail will not develop any headchecks so there will be no cost for RCF grinding. The upfront cost of course will be more, but the total cost across the life of the rail will be much lower which will be of great benefit to the customer."
The success of the rolling stand tests has enabled Voestalpine to secure German Federal Railway Authority (EBA) and the European Railway Agency's (ERA) Technical Specification for Interoperability approval for application of Dobain on high-speed and conventional mainline railways. Track testing is also now underway in cooperation with DB Networks and Austrian Federal Railways (ÖBB) as part of the certification process to meet the standards required by these infrastructure managers for their domestic networks.
Frank says that the tests with DB Networks have been underway for 1.5 years of a five-year programme with the rails located in what he describes as "highly stressed areas" of track which are regularly used by freight trains and ICE high-speed sets. So far the Dobain track, as expected, has not produced any headchecks. It is the same in Austria where the tests began last year.
When the tests are concluded within the next three to four years and certification is secured, Frank says that Voestalpine hopes to offer Dobain to the market as a rail for new and replacement projects. "Our target is to produce 120m-long Dobain bainitic rails that will be available in all profiles," Frank says. And if the results of the tests continue to hold up, it is likely to be an intriguing choice for infrastructure managers that are continually looking for ways to reduce costs and minimise maintenance schedules which can severely impact availability and quality of services.