Hardness Is a Gradient, Not a Single Number
A rail is described in purchasing documents by one hardness figure, but the metal that actually carries the wheel load is a layered structure. On a 60 kg/m or 75 kg/m flat-bottom rail the running surface, the upper gauge-corner fillet and the two lower fillets behave differently, and hardness falls as the measurement position moves from the surface toward the neutral axis. For a heat-treated rail the difference between the surface and a position 25 mm below it can exceed 80 HBW; for an as-rolled rail the spread is smaller but still measurable.
Wear is a near-surface process. Contact pressure is concentrated in the first few millimetres of the head, so the properties of the outer layer govern profile loss, while the core governs resistance to bending fatigue. A rail with a shallow, sharply stepped hardened layer can spall; a rail with a smooth, sufficiently deep gradient wears slowly and keeps its profile longer.
How the Gradient Is Produced
Three production routes create the hardness profile, and they give different gradients:
| Route | Typical head hardness | Gradient character |
|---|---|---|
| As-rolled pearlitic rail (U71Mn type) | 260-300 HBW | Shallow, chemically driven |
| Offline head hardening (reheat and spray quench) | 340-390 HBW at surface | Deep, 20-30 mm, controlled |
| Low-alloy / micro-alloyed rail (U75V type) | 280-340 HBW | Uniform through the head |
In offline head hardening the head is reheated by induction or by the residual heat of rolling, then quenched with compressed air and water mist so that the cooling rate at the surface exceeds the critical rate for pearlite but stays below the rate that would form martensite. The result is fine pearlite with a small interlamellar spacing, which is the metallurgical reason why hardness and wear resistance rise together. Ferrite content has the opposite effect: soft free ferrite at prior-austenite boundaries lowers the local hardness and becomes the initiation site for rolling contact fatigue.
Measuring the Distribution
Hardness distribution is verified by Brinell testing on a cross-section of the head, and the test positions are fixed by the product standard rather than chosen by the supplier. The Chinese rail standard GB/T 2585 controls the chemical composition, the tensile properties and the dimensional tolerances of 38 kg/m to 75 kg/m rails, while EN 13674-1 classifies flat-bottom rails of 46 kg/m and above into grades such as R260 (260-300 HBW) and R350HT (350-390 HBW). In both systems the acceptance value is tied to a stated position below the running surface, so a certificate that quotes hardness without stating the measurement depth cannot be compared with another certificate.
Practical checks a buyer should request with the mill certificate: the testing depth, the number of indents, whether the upper fillets were included, and whether the tensile test was taken from the head or the web. Tensile strength and hardness must move together in a pearlitic rail; where the certificate shows R260 class hardness with a strength figure below 880 MPa, or R350HT class hardness with a figure below 1175 MPa, the data set is inconsistent.
From Hardness to Wear Resistance
Wear resistance in pearlitic rail steel scales with the hardness of the layer in contact, but the relationship is only useful inside a hardness window. Below roughly 260 HBW, wear rate rises steeply on curves with high cant deficiency. Above roughly 400 HBW the near-surface layer becomes sensitive to crack initiation, and in wet or contaminated conditions the gain in wear resistance can be offset by a shorter fatigue life.
Three practical consequences follow. First, curve severity should drive grade selection: a head-hardened rail on a high-degree curve outlasts an as-rolled rail of the same weight by a wide margin, and the cost per gross tonne is usually lower even at a higher purchase price. Second, the depth of the hardened layer matters as much as the surface value, because grinding removes material at every pass and the rail must still meet the minimum hardness after several grinding cycles. Third, the hardness of the head must be matched to the hardness of the wheel: an excessive hardness difference concentrates wear on the softer partner, and mismatched couples generate shelling earlier than a balanced pair.
Specification Guidance
When hardness distribution is written into a purchase order, state the grade, the measurement depth, the minimum surface value, the minimum value at the deepest specified position and the acceptance method. Add the required camber and straightness, the drilling pattern for the fishplate or fastening system, and the ultrasonic inspection requirement for the whole length. These clauses remove the freedom that allows two suppliers to deliver rails that both pass a nominal hardness figure but behave differently in track.
Frequently Asked Questions
Q: Why does hardness fall from the rail surface to the web?
A: Because only the head is quenched or alloyed for wear resistance. The web and base are left softer and tougher so that they can absorb bending stress without brittle fracture.
Q: What hardness range should be specified for R350HT rail?
A: EN 13674-1 defines the R350HT grade by a nominal head hardness in the 350-390 HBW band, measured at the standard position on the head. Always quote the measurement depth with the value.
Q: Does a hard rail always last longer?
A: No. Above roughly 400 HBW the fatigue penalty can exceed the wear benefit, especially on wet or contaminated track. Grade selection should follow curve radius, axle load and traffic density.
Q: How deep should the hardened layer be?
A: Deep enough that the rail still meets the minimum hardness after the grinding cycles planned over its life. For offline head hardening a depth of 20-30 mm is common practice.
Q: Can hardness distribution be judged from a mill certificate alone?
A: Only if the certificate states the measurement depth, the number of indents and the test standard. Without those details the figures are not comparable between mills.
Q: Which rail standard covers 75 kg/m heavy rail?
A: GB/T 2585 covers hot-rolled rails for railway use, including the 75 kg/m section, and fixes composition, tensile properties and dimensional tolerances.