Rail Steels

This article provides an overview of the most important aspects of rail steels, including their microstructure, manufacturing process, head-hardening methods, and the classification of different grades.

Challenges

Rail fatigue cracking is one of the main causes of railway track failures, while excessive wear and changes in rail profile are the primary reasons why rails need to be replaced. Rail steels must therefore combine high resistance to abrasive wear and fatigue with high hardness and sufficient ductility, meaning good resistance to cracking.

Pearlitic Microstructure

Rail steels generally have a pearlitic microstructure, which provides high wear resistance while maintaining sufficient ductility and resistance to cracking. This is in contrast to martensite, which offers excellent wear resistance but is relatively brittle.

Producing the desired pearlitic structure requires a precisely controlled chemical composition and carefully controlled cooling rates during heat treatment. How is this achieved?

Manufacturing

Rail steel is typically produced in a basic oxygen furnace, followed by vacuum degassing and continuous casting into large blooms. Vacuum degassing and ladle treatment make it possible to reduce the hydrogen content to below 2.5 ppm and achieve a chemical composition within very narrow tolerances.

The blooms are then reheated and rolled into the final rail profile. The finished rails may be air-cooled or surface hardened (continuous method).

Head hardening – after leaving the final rolling stand at approximately 1000°C, the rail, positioned with its head facing upward, enters the cooling system. After passing beneath temperature sensors, the rail head is sprayed with water. The pearlitic transformation occurs after the rail leaves the cooling system.

A computer continuously controls the cooling rate in different areas of the rail. The aim is to achieve the required refinement of the pearlitic structure in the centre of the rail head without cooling the surface so rapidly that bainite or martensite forms.

The rails are subsequently straightened and inspected for defects using ultrasonic testing, eddy-current testing and laser inspection. During cooling and straightening, the rails move along a series of rollers.

Classification

Classification by tensile strength – rail steels can be divided into:

• normal rail steels, Rm > 700 MPa,
• wear-resistant rail steels, Rm > 880 MPa,
• high-strength rail steels, Rm 1080–1200 MPa.

Tensile strength Rm and chemical composition – for rail steels, the relationship between Rm and chemical composition is as follows:

Rm = 227 + 80% C + 87% Si + 115% Mn + 133% Cr + 891% P + 614% V

Nomenclature, on the other hand, is based primarily on hardness. According to PrEN 13674-1 + A1:2008, rail steel grades are designated by the letter R, followed by the minimum HBW hardness and, where applicable, an additional variant designation, for example R260Mn.

The grades are listed below. One notable feature of rail steels is the very narrow tolerance range for their chemical composition.

Normal Rail Steels R200 and R220

These are the least expensive rail steel grades, but they also offer the lowest wear resistance. Their maximum hydrogen content is 3 ppm, compared with 2.5 ppm for more wear-resistant grades.

R200 rail steel:

• Chemical composition: C=0.5; Si=0.37; Mn=0.95; P<0.035; S<0.022; Al<0.004; N<0.009; O<20ppm; H<3ppm
• Tensile strength Rm > 680 MPa
• Hardness = 200–240 HBW
• Elongation A > 14%

R220 rail steel:

• Chemical composition: C=0.55; Si=0.40; Mn=1.13; P<0.025; S<0.017; Al<0.004; N<0.009; O<20ppm; H<3ppm
• Tensile strength Rm > 770 MPa
• Hardness = 220–260 HBW
• Elongation A > 12%

Wear-Resistant Rail Steels R260 and R260Mn

These grades are used on railway lines subject to high axle loads, heavy traffic or tight curves.

R260 rail steel:

• Chemical composition: C=0.71; Si=0.37; Mn=0.95; P<0.025; S<0.017; Al<0.004; N<0.009; O<20ppm; H<2.5ppm
• Tensile strength Rm > 880 MPa
• Hardness = 260–300 HBW
• Elongation A > 10%

R260Mn rail steel – a high-manganese variant:

• Chemical composition: C=0.65; Si=0.38; Mn=1.5; P<0.025; S<0.017; Al<0.004; N<0.009; O<20ppm; H<2.5ppm
• Tensile strength Rm > 880 MPa
• Hardness = 260–300 HBW
• Elongation A > 10%

High-Strength R320Cr Rail Steel

R320Cr contains approximately 1% chromium, which gives it high hardness and excellent wear resistance. Similar steels were widely used before 1985. However, as bolted track joints have gradually been replaced by continuously welded rail, this grade has become less common because of its relatively poor weldability.

R320Cr rail steel – with a high chromium content:

• Chemical composition: C=0.7; Si=0.8; Mn=1.0; Cr=1.0; V<0.18; P<0.025; S<0.017; Al<0.004; N<0.009; O<20ppm; H<2.5ppm
• Tensile strength Rm > 1080 MPa
• Hardness = 320–360 HBW
• Elongation A > 9%
• Poor weldability

High-Strength Rail Steels R350HT and R350LHT

R350HT rail steel – a heat-treated grade:

• Chemical composition: C=0.76; Si=0.37; Mn=0.95; P<0.02; S<0.017; Al<0.004; N<0.009; O<20ppm; H<2.5ppm
• Tensile strength Rm > 1175 MPa
• Hardness = 350–390 HBW
• Elongation A > 9%

R350LHT rail steel – a low-alloy heat-treated grade:

• Chemical composition: C=0.76; Si=0.37; Mn=0.95; Cr<0.3; P<0.02; S<0.017; Al<0.004; N<0.009; O<20ppm; H<2.5ppm
• Tensile strength Rm > 1175 MPa
• Hardness = 350–390 HBW
• Elongation A > 9%

Rail Steels with Extremely High Wear Resistance: Austenitic 14% Mn Steel

For turnouts, rail crossings and other exceptionally heavily loaded sections of track, where the use of such an expensive material is economically justified, rails made from high-manganese steel are used.

This type of steel undergoes significant work hardening during service and can reach a hardness of more than 400 HBW after relatively little use.

• Approximate chemical composition: C=0.75–0.9; Mn=13.0–14.0; Si=0.2–0.4
• Tensile strength Rm: 820–970 MPa
• Initial hardness: 180–210 HBW
• Hardness after work hardening > 400 HBW
• Elongation A: 40–60%