New knowledge will change rail maintenance
Danish research has solved the riddle of the large quantity of cracks observed in European rails in the past 10-15 years.
By Anne Kirsten Frederiksen
Just over 20 years ago, Banedanmark switched to using rails of a harder type of steel—so-called head-hardened steel—in particularly exposed locations. These rails were used in curves, where the wear is high, and where it was thus desirable to extend the service life of the rails.
But around 2010, Banedanmark—like many other European railway operators—discovered that unusually many defects occurred in the head-hardened rails.
“We found it puzzling, as the hard steel was, in fact, precisely to ensure greater durability. Even though the head-hardened rails only represent six per cent of the total number of rails in Denmark, they still accounted for up to 15-20 per cent of our maintenance costs. We therefore stopped using rails of this type after a few years, and we instead went back to an older rail type, made of steel that is less hard,” says Carsten Jørn Rasmussen, Track and Welding Engineer with technical system responsibility for the rails in Banedanmark.
The problem was the same in other European countries, which all experienced the same defects and thus incurred disproportionately large expenses for maintenance of head-hardened rails.
Suspicion by pure chance
A few years later, Carsten Jørn Rasmussen had a suspicion—by pure chance—of what the cause of the defects in the head-hardened rails could be.
This occurred in connection with an examination of a rail in a completely different context. The rail was cut through and analysed. It turned out that martensite had formed in an area in the rail where this was not to be expected.
Martensite is a microstructure in steel which is very brittle and hard, and which is formed in connection with heating followed by rapid cooling. Cracks in rails typically occur in connection with the formation of martensite.
“Until now, the general understanding has been that martensite is formed when the wheels of the train touch the rail and have contact with it. But there had been no contact between wheel and rail in the examined area of the rail head. However, the area of the rail had been ground, and we therefore had the suspicion that the damage could be connected with the grinding of the rails that is done as part of the maintenance,” says Carsten Jørn Rasmussen.
Grinding of rails is done as a preventive measure to extend the service life of the rail. Through such grinding, a tiny amount—around 1/10 mm—of the rail is ground off, thus removing any small cracks before they becoming bigger. This is done using a grinding train which has round grindstones mounted underneath it.
When the train moves forward, the grindstones grind the rails. Grinding generates intense heat of more than 700 degrees. Immediately afterwards, the steel cools down again very quickly, as the rest of the rail and the surroundings have not been heated.
DTU performed analyses
There were subsequently an increasing number of indications that the grinding might be the cause of the increased number of defects in head-hardened rails. To acquire more knowledge about this, Banedanmark entered into a partnership with DTU on the performance of a number of analyses.
Rails are obviously not so easy to examine. But Banedanmark found some rails at a station that were not in use. Banedanmark ran a grinding train on the rails and could subsequently cut out rail sections and send them for analyses.
Senior Researcher Hilmar Kjartansson Danielsen from DTU Wind Energy conducts research into steel, including bearings in wind turbine gearboxes where steel rolls on other steel. He and Professor Dorte Juul Jensen from DTU Mechanical Engineering are among the DTU researchers who have worked with Banedanmark to determine the cause of the problem.
“We’ve analysed the rails with both 3D scanning and microscopy. Using these methods, we were able to ascertain that there were marks across the rails which were repeated at regular intervals, for example precisely 29 mm. Grindstones are made of hard ceramic materials consisting of many abrasive grains. When an abrasive grain was larger than the others, a mark was formed in the rail, which we could ascertain was repeated at regular intervals,” says Hilmar Kjartansson Danielsen.
Danish knowledge in demand
Based on scannings, the researchers found that brittle martensite was formed at the grinding marks on the surface of the rail, resulting in the initiation of small cracks in the rail surface. The grinding—which had previously be regarded as beneficial and as extending the service life of the rail—thus turned out to be too aggressive and was instead found to contribute to the creation of cracks.
Extensive research has been conducted into this field in many European countries to find a solution to the problem, but the cause of the large number of defects in head-hardened rails was thus discovered by researchers in Denmark.
“The problem with the many defects in rails is found throughout Europe and has major economic and operational consequences. Therefore, there is great interest in our results, among both the players responsible for railway tracks in the other European countries and the enterprises which help maintain the rails, for example through grinding. Less aggressive grindstones for use in maintenance have thus already now been developed,” says Carsten Jørn Rasmussen.
Banedanmark has long since ceased laying new head-hardened rails, and the existing head-hardened rails are no longer being ground, but milled instead. Among other projects, Banedanmark is currently working on developing a different and less aggressive maintenance method in a partnership with one of the rail-grinding enterprises.
source: Technical University of Denmark