Why Railhead Hardening Is Used
On curves, gradients and heavy-haul lines, the rail head wears through wheel-rail contact and suffers rolling contact fatigue (RCF) such as spalling, squats and wavy wear. Head hardening raises the surface hardness of the rail so that wear and RCF are controlled, keeping the rail in service longer. Chinese practice is typified by the U75VH head-hardened grade covered by TB/T 2344, and European practice by the R350HT and R400HT grades in EN 13674-1.
Common Railhead Quenching Processes
| Process | Heating and Cooling | Hardened Depth | Typical Use |
|---|---|---|---|
| Induction heating quenching | High-frequency induction coil heats the head to about 850 to 900 degrees Celsius, then water spray cools it | 8 to 12 mm, uniform | Main-line and heavy-haul rails |
| Flame quenching | Acetylene-oxygen flame heating | 5 to 8 mm, shallower | Lines with small traffic volume |
| Contact (electrode) heating quenching | Electrodes heat the head directly | Controllable but equipment is complex | Rarely used in modern practice |
Hardness and Hardened-Layer Requirements
Head-hardened rails are normally specified by surface hardness: U75VH rails achieve about HB 341 to 400, and premium grades target HB 380 to 420, comparable with the R400HT grade. The hardness must not drop suddenly below the surface: the hardness gradient in the hardened layer should be gentle, with a hardness fall of no more than about 20 HB per millimetre from surface to core, to avoid stress concentration. The transition zone between quenched and unquenched rail must be at least 50 mm long so that wheel-rail forces change smoothly. Hardened-layer depth on heavy-haul rails is required to be at least 10 mm.
Effect on Wear Resistance and Service Life
Induction head hardening typically improves rail wear resistance by two to three times on plain line: head-hardened rails serve 10 to 15 years under normal traffic where unhardened rails last 5 to 8 years. The hardened layer also resists wheel-rail contact fatigue, reducing head spalling and wavy wear. Flame-quenched rails, with their shallower layer, give an improvement of about 1.5 times and suit medium-traffic lines where full induction hardening is not economical.
Quality Testing of the Hardened Layer
Hardness: measure surface hardness with a Brinell or Rockwell tester at defined positions, typically three points per metre of rail, and take the average.
Structure: examine the hardened layer metallographically; the requirement is a fine pearlitic structure (at least 90 percent fine pearlite) - martensite is not acceptable because it is too brittle for rolling contact.
Depth: verify hardened-layer depth by ultrasonic testing or section examination, with deviation controlled within plus/minus 1 mm of the design value.
Selecting the Process by Traffic Volume
Heavy-haul lines with annual traffic above 50 million gross tonnes: induction hardening, hardened depth at least 10 mm.
Trunk railways with 20 to 50 million gross tonnes per year: induction hardening (8 to 10 mm) or high-quality flame quenching.
Branch lines below 20 million gross tonnes: flame quenching (5 to 8 mm) balances cost and performance.
FAQ
Q: Why is the hardened layer required to be at least 10 mm on heavy-haul lines?
A: Because wheel-rail contact stresses penetrate several millimetres; a deeper layer keeps the supporting material hard under the contact zone as the surface wears.
Q: Is martensite acceptable in a hardened rail head?
A: No - hardened rail heads should have a fine pearlitic structure; martensite is brittle and forms under different cooling practice, leading to spalling under traffic.
Q: How does hardness relate to wear resistance?
A: Within pearlitic rail steels, wear rate falls as hardness rises, which is why heavy-haul railways specify HB 340 or above on curves.
Q: Can head-hardened rails be welded?
A: Yes, with welding procedures qualified for the grade; the head hardness must be restored or accepted per the weld specification, and weld heat-affected zones are inspected accordingly.
Q: How do I confirm delivery quality of head-hardened rails?
A: Check the mill certificate for hardness results, hardened-layer depth records and ultrasonic test reports, and verify them against the governing standard.