Rails and Rail Steel
Rails and Rail Steel
Rail steel is used to make rails for railway lines and for other uses such as tracks for moving equipments like cranes, transfer cars etc. Heavier rails carry heavier and faster trains on the tracks. The rails represent a substantial fraction of the cost of a railway track. Worn, heavy rail from a mainline is often reclaimed and downgraded for re-use on a branch line, siding or yard or rerolled in rerolling mills to produce other steel products. Rail steel is hot rolled steel of a specific cross sectional profile (an asymmetrical I- beam) designed for use as the fundamental component of railway track. The rail profile is the cross sectional shape of the rail perpendicular to the length of the rail (Fig 1).
Fig 1 Typical cross section of a rail
The importance of the rail steel can be known from the fact that even after years of service and high stress, there is no difference between the grain structure of a used rail and a new rail. Age, traffic and weather do not change its basic properties. All stresses are relieved through heating in the used rail prior to being rerolled. This rerolling decreases the grain size of the used rail steel and hence improves its resiliency.
Earlier wooden rails were used on horse drawn wagon ways. By 1760s strap iron rails, which consisted of thin strips of cast iron fixed onto wooden rails came into use. These were superseded by cast iron rails that were flanged (i.e. ‘L’ shaped) and with the wagon wheels flat. In 1789, the edge rails where the wheels were flanged were introduced and, over time it was realized that this combination worked better. The earliest of this in general use were the so-called cast iron ‘fishbelly’ rails from their shape. Rails made from cast iron were brittle and broke easily. They could only be made in short lengths which would soon become uneven. In 1820 as rolling techniques improved, wrought iron rails were introduced and replaced cast iron rails. These rails contributed significantly to the explosive growth of railroads in the period 1825-40. The cross-section of these rails varied widely from one line to another. The parallel cross-section which developed in later years was referred to as Bullhead. The first steel rails used anywhere in the world were laid in Derby station on the Midland Railway in 1857. Steel is a much stronger material, which steadily replaced iron for use on railway rail and allowed much longer lengths of rails to be rolled.
Composition and metallurgy of rail steel
The basic requirement of rail steel is that it should be hard, wear resistant and crack resistant. This is achieved by steel composition and cooling of the hot rails. These properties of steel are achieved through control of carbon (C) and manganese (Mn) contents. Carbon content of rail steel can go up to a maximum of 0.82 % and manganese content up to a maximum of 1.7 %. The normal rails are made of steel containing 0.7% C and 1% Mn, which are called as C-Mn rail steel. From a strength perspective (ultimate tensile strength of 880 MPa or 90 Kg/ Sq mm), the C-Mn rail is popularly known as 90 UTS rail or Grade 880 rail. This grade of rail steel is also known as wear resistant grade.
The microstructure of rail steel consists fully of pearlite which comprises a mixture of relatively soft ferrite and a hard, brittle iron carbide called cementite. Ferrite and cementite takes the form of roughly parallel plates in a lamellar structure. Due to this structure rail steel achieves a good resistance to wear because of the hard carbide and some degree of toughness as a result of the ferrite’s ability to flow in an elastic/plastic manner. The hardness and tensile strength of steel are determined by the proportion of pearlite in its microstructure. They are also determined by the ‘fineness’ of the pearlite structure. The shorter is inter lamellar spacing the higher are the hardness, wear resistance and tensile strength of the rail steel. The microstructure of the rail steel is shown in Fig 2. The ferrite is black while the cementite is grayish blue.
Fig 2 Microstructure of rail steel
The fineness of the pearlite structure (inter lamellar spacing) of rail steel is controlled by the rate at which the hot rail cools. Heat treatment methods are available to control the cooling rates and control the microstructure which in turn controls the hardness and strength of the rail steel. Rails are to be free from internal cracks which are caused by hydrogen trapped in the molten metal as it cools. Hydrogen needs removal either by vacuum degassing of the liquid steel before it is cast or by letting hot rails cool down very slowly. Rail steel should also be resistant to fatigue or surface cracking so that it can have a long fatigue life for this rail steel needs improved cleanliness.
The following refinement of some basic features around the fundamental structures has been carried out
- Very accurate control of the chemistry of the rail steel with carbon and manganese at optimum level.
- Control at low levels of the potential embrittling compounds such as sulphur, phosphorus, nitrogen and hydrogen etc.
- Steel is made cleaner with fewer numbers of stress-concentrating entrapped inclusions from the steel making and casting process.
The rails are subject to heavy contact cyclic loading that accompanies increased size and loading of the wagons, increased size and speed of the trains over the last few decades. These increasing demands require manufacturing and metallurgical approaches that offset wear and other types of failure that limit rail life. An early type of rail failure was associated with entrapped hydrogen that produced shatter crack or flakes in heavy rail sections, but that difficulty has been effectively controlled cooling and by vacuum degassing of liquid steel.
In the last 20 years there has been increase in strength of the rail steel mainly to cater for the heavy axle loads. Rail steel of around 130 to 140 Kg/Sq mm is now being used in large quantities. They are finding increasing use in tight curve/high wear situations. This high strength is being achieved by making the spacing between the pearlite lamellae finer by controlling the growth rate of pearlite.
Alloying elements such as chromium and nickel can be added to some rail steels to improve properties. Alternatively, the rail can be cooled quickly to reduce the time available for diffusion. The rail can also be heat treated. Combination of alloying and heat treatment is possible and a range of grades can be produced.
Rails not only wear but they break also. The inherent toughness of rail steel is poor as a result of the presence of the brittle carbide phase. Fracture can occur from relatively minor stress concentrating features inside the rail or on the surface due to the manufacturing defect or handling damage. The rail breakage has a high replacement cost and can be very disruptive to the railway network.
Pearlitic rails have been developed almost to their limit. Now rail steel with bainitic structure has been developed with suitable alloying additions and with proper heat treatment (generally with an intermediate cooling). This structure also contains ferrite and cementite like pearlite but in this case the ferrite is semi coherent with the high temperature austenite phase from which it was formed. The bainitic structure of rail steel is shown in Fig 3
Fig 3 Bainitic structure of rail steel sample
For similar hardness levels the bainitic grade shows a reduction in rate of the wear of the rail track as well as the reduced wear of the wheel. Further presently the formation of very fine surface cracks by a phenomenon of rolling contact fatigue is of increasing concern. The new bainitic grade rail steel has shown dramatic improvements in comparison to the traditional pearlitic rail in this respect. The whole wheel/rail test done on two types of rail steels has shown crack initiation at two hundred and twenty thousand cycles on pearlitic rail, whereas after one million cycles the bainitic rail steel was crack free when the test was terminated. The bainitic steel has considerable potential in terms of life in track, reduced maintenance, increased safety, with anticipated reduction in rail fractures.