Carbon steel is iron alloyed with carbon, and its behaviour is governed mainly by the carbon content. The four conventional types, in order of increasing carbon, are low carbon, medium carbon, high carbon and ultra-high carbon steel. Carbon content raises strength and hardness but lowers ductility, toughness and weldability, which is why material selection is always a balance rather than a race to the highest number.
The Four Types of Carbon Steel
| Type | Carbon content | Typical properties | Common uses |
| Low carbon (mild steel) | Below 0.3% | Soft, ductile, easily welded and formed | Structural sections, pipes, car bodies, general fabrication |
| Medium carbon | 0.3-0.6% | Balance of strength and ductility | Gears, axles, shafts, machine parts |
| High carbon | 0.6-1.0% | Hard, wear resistant, less ductile | Springs, wires, cutting tools, rails |
| Ultra-high carbon | 1.0-2.0% | Extremely hard and brittle | Dies, punches, specialist cutting edges |
How Rail Steel Is Classified
Rail steel is produced in three families. Carbon rails are smelted and rolled from iron ore-based heats, relying mainly on carbon and manganese for strength; a typical ordinary rail steel contains roughly 0.40-0.80% carbon with manganese at about 1.0-1.4%. Alloy rails add small amounts of alloying elements such as vanadium, titanium, chromium or niobium to raise strength and toughness beyond the carbon grades. Heat-treated rails are carbon or alloy rails that are heated and then cooled under controlled conditions to refine the pearlite structure; the rail head develops a hardened layer that greatly improves wear resistance and extends service life.
Chinese Rail Grades and Mechanical Properties
Hot-rolled rails (GB/T 2585)
| Grade | Tensile strength (MPa) | Hardness (HB) |
| U75V | at least 980 | 280-320 |
| U78CrV | at least 1080 | 310-360 |
| U76CrRE | at least 1080 | 310-360 |
| U77MnCr | at least 980 | 290-330 |
Heat-treated rails
| Grade | Tensile strength (MPa) | Hardness (HB) |
| U75V (heat treated) | at least 1180 | 320-380 |
| U78CrV (heat treated) | at least 1280 | 370-420 |
The European and UIC families use their own designations: R260 and R350HT under EN 13674-1, and 700, 900A and 1100 grades under UIC 860, where the number is the minimum tensile strength in MPa.
Choosing the Right Rail Steel
Select the grade from the traffic: heavy axle loads and sharp curves demand harder, higher-strength heads, which is where heat-treated rails earn their premium. Straight, lightly loaded lines can run standard carbon grades at lower cost. Weldability also matters: high-carbon and alloy rails need controlled welding procedures, and the rail's chemical composition must be compatible with the welding process used on the project. For fastening components, spring steels such as 60Si2MnA sit in the high-carbon range, and clip hardness after heat treatment is typically 42-50 HRC.
Common Misconceptions
Higher carbon is not automatically better: ultra-high carbon steel is too brittle for structural and dynamic parts. Rails are not made of low carbon steel, which would wear out in months. And the phrase alloy steel does not mean stainless steel: alloying elements are added in small percentages to improve strength and toughness, not for corrosion resistance.
FAQ
Why is rail steel always high carbon?
Because the rail head must resist wear and the rail as a whole must resist bending and fatigue. High carbon gives the hardness and strength required, while the specific carbon and manganese levels are tuned for weldability and toughness.
What is the difference between carbon rail and alloy rail?
Carbon rail relies on carbon and manganese for strength; alloy rail adds small amounts of vanadium, titanium, chromium or niobium to push strength and toughness higher.
What does heat treatment do to a rail?
Controlled heating and cooling refine the pearlite structure and create a hardened layer at the rail head, typically raising tensile strength by 150-200 MPa and markedly improving wear life on curved track.
Is U75V better than U71Mn?
U75V has higher strength and better wear resistance and is a common choice for heavy-haul and high-speed lines. U71Mn remains widely used for conventional lines where its weldability and cost balance well.
Can high carbon steel be welded easily?
No. High carbon steels need controlled preheat, qualified procedures and post-weld inspection, which is why rail welding (flash butt, thermite or aluminothermic) is a specialist operation with approved parameters.
What hardness should spring clips have?
Railway spring clips are typically quenched and tempered to 42-50 HRC. Lower hardness risks plastic deformation and loss of toe load; higher hardness risks brittle fracture under impact.