Laboratory Wear Tests
Rail tracks have always been made out of pearlitic steel. It is necessary for rail steels to be cheap and very wear resistant and pearlite fulfills these requirements. Pearlite consists of alternating lamellae of soft iron and very hard iron carbide (also known as cementite). The smaller the spacing between cementite layers, the harder and more wear resistant the steel is. Unfortunately there is a practical limit to how small the spacing can be made and current wear-resistant rail steels are approaching it. Pearlitic rail steels are also very brittle and hard to weld so researchers started to look at bainitic and martensitic steels instead. However the wear resistance was almost always inferior to pearlitic steels at the same hardness level.
A carbide-free bainitic steel designed to be a tough rail steel by my supervisor, Dr. Harry Bhadeshia, surprisingly turned out to have excellent wear resistance. In laboratory tests it was even better than wear resistant pearlitic steels. I am currently studying the wear behaviour in the laboratory and in full-scale track trials in the hope of discovering what makes it so much better than pearlite and other types of bainite.
For more details about bainite and the carbide-free rail steel try these:
British Steel have put some of their experimental bainitic rails into track at their Scunthorpe Works. The rails are in the torpedo route where the axle load is 50 tonnes. The bainitic rails have been laid on a curve because the wear in curves is much greater than in straight track. I am studying the wear behaviour of these rails through wear rates and scanning electron microscope examination of the worn surfaces and wear debris. The trains on this track carry molten iron between the blast furnace and the converter and go past every ten to twenty minutes so I can't cut pieces out of the rails to study. Instead I make cellulose acetate replicas of the surfaces which works well provided you time it properly between trains. I then coat them with gold and examine them in the SEM. The technique has a surprisingly good resolution.
We have also done experiments with rails and tyres of varying hardnesses. Interestingly it seems that increasing the hardness of the tyre has an even more dramatic effect on the wear rate of both the rail and the tyre than improving the rail steel does. Tyre steels are usually much softer than typical wear resistant rail steels. Here is a bar graph of results for pearlitic rails run against tyres of varying hardness if you are curious.