Bainitic Rail Steels

Project Background
Track Trials
Laboratory Wear Tests

Project Background

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:

The Bainite Reaction in Steels: a description of what bainite is and how it forms
Innovation in Rail Steels: an article from Science in Parliament
On The Right Track: an article from Material Eyes
British Steel Alloy Makes Tough Tracks: an article from EPSRC

Track Trials

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.

Laboratory Wear Tests

In laboratory rolling-sliding wear testing, two cylinders are rolled against each other. They rotate at slightly different speeds so there is some slipping between them. This simulates what actually happens in rail-tyre contact, although in the laboratory we accelerate the tests by using a much higher amount of slip than you would get on a rail. This is very much quicker and cheaper than doing track trials; unfortunately the laboratory test results do not always match up with those seen on track. We are trying to find out what causes the differences by comparing the surface appearance and the wear debris in the two types of test. We have done some tests with differing amounts of slip to see how that affects the results; unfortunately it turns out that if you lower the amount of slip to typical levels for rails, you get very little wear occurring at all in the laboratory.

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.

Catherine Pitt
22/1/99