Heat-treated steel rails are re-engineered. By Rosie Hardy
Steel rails have been in standard use worldwide since they were first laid at Derby Station in 1857. Here in the UK, steel rails with a flat-bottomed shape make up over 16,000km of track across the rail network.
British Steel has played a key part in supplying steel rails for over 150 years. It continues to be a major supplier to the world market and recently provided 57km of steel rails for the Crossrail project in London.
Conventionally, steel rails are manufactured rolling steel into lengths and then heat treating them so that they have the required hardness, strength and other characteristics. The rail lengths are then welded together to form a continuous rail. However, it is also known that the heat from the subsequent welding operation can change the microstructure of the steel rail and cause a lower hardness and loss of strength at the weld region. This lack of structural uniformity leads to localised wear at the welds and a ‘cupping’ in the surface.
To avoid these problems, British Steel has developed a new heat-treating system and method that is able to produce a welded rail with homogenous properties along its entire length. Given the benefits, British Steel has protected this innovative method in the UK and across Europe with European Patent 3186402.
The patent document describes the innovative system and method, which involves heat treating the rail after the rail lengths have been welded together and formed as a continuous rail. This contrasts with the orthodox method of heat treating the rail lengths prior to welding.
After welding two or more rail lengths together to produce a continuous rail, a special heat-treating unit is used to heat treat the entire rail. Every cross-section of the rail is heated above the Ac3 temperature to achieve a fully austenitic microstructure. The temperature of the continuous rail is then held above the Ac3 temperature for a prescribed time ta. After this, a cooling unit is used to reduce the temperature of the rail to a prescribed stop temperature Tstop. This helps to achieve the desired final microstructure and properties along the entire length of the heat-treated continuous welded rail.
The process of heat treating every cross-section of the rail after welding ensures it has a homogeneous microstructure throughout. In other words, the rail has the same hardness and strength properties along its entire length, and the properties of the weld and body of the rail are virtually identical. This enhances the wear-resistance and lowers the residual stress of the rail.
The patented system and method brings further advantages too. For example, it allows for shorter lengths of rail to be used to form the continuous rail, which in turn are easier to transport. Also, by heat treating after the lengths of rail have been welded together to ‘make’ the properties in the continuous rail, the cooling process is more straightforward.
The technology developed by British Steel effectively re-engineers a tried-and-tested heat-treatment process; making continuous welded steel rails that are even more durable. The European Patent gives British Steel exclusive rights to this desirable technology so that it can take full commercial advantage across Europe and restrict competitors.
Rosie Hardy is a patent attorney at European intellectual property firm, Withers & Rogers LLP.