The railroad metal, commonly known as train track steel, is special steel in metallurgical products mainly used for railroad tracks. The rail bears the weight and dynamic load of the train. Its surface wears, and the head is impacted. The rail is subject to large bending stress, too. The complicated press and long-term service bring out damages to the rails.
The main damages of railroad metal
- The side wear of the upper strand and the crush of the lower strand.
- The wave wears caused by insufficient yield strength.
- Brittle fracture, spalling, falling blocks, rail head-splitting, and weld cracks caused by low toughness and plasticity.
Therefore, the basic requirements for rail steel include: wear resistance, crush resistance, brittle fracture resistance, fatigue resistance, and good weldability.
Typical material of train track steel
According to the steel type, rail can be divided into three types:
Carbon steel
carbon steel is a steel rail smelted and rolled with natural raw iron ore. It mainly uses carbon and manganese elements in the ore to increase the strength of the rail. Ordinary carbon train track steel is composed of 0.40%-0.80% carbon and manganese less than 1.30%-1.4%.
Alloy steel
The alloy steel is a steel rail that is smelted and rolled after adding appropriate amounts of alloy elements such as vanadium, titanium, chromium, and tin to the original iron ore. The strength and toughness of this type of rail are higher than that of carbon rail.
Heat-treated steel
The heat-treated steel is a steel rail that is formed by heating and controlling the cooling of the hot-rolled carbon rail or alloy rail. The pearlite structure of the heat-treated rail is more refined than that of the hot-rolled rail, resulting in higher strength and toughness. The hardened rail after heat treatment has a layer of hardening correction at the head of the rail, which greatly improves its mechanical properties so that the service life of the rail can be extended.
The chemical composition of train track steel
| No. | Element | Function |
| 1 | C | Improve the strength, hardness, and wear resistance of the rail. The carbon content of domestic rails is 0.65% to 0.82%. When the carbon content is relatively high, the steel becomes brittle, and its plasticity index will be significantly reduced. At the same time, it will increase the chance of white spots in the steel. |
| 2 | Si | It is easy to combine with oxidation, and can play the role of removing bubbles in the metal. The steel contains an appropriate amount of silicon, which can improve the hardness and wear resistance of the steel. The content of domestic rail steel is generally 0159-0.9%, but too much content will make the steel hard and brittle, and it is easy to produce pores in the weld. |
| 3 | Mn | It is a beneficial element that can improve the strength and wear resistance of steel and increase the toughness of steel. It can remove harmful iron oxide and sulfide inclusions in the steel. The manganese content is generally controlled between 0.6% and 1.54%. Steel with a manganese content of more than 1.2% is called medium manganese steel, and its wear resistance is very high. |
| 4 | Cu | It is a beneficial element. Steel contains a small number of copper compounds, which can improve the fatigue resistance and corrosion resistance of steel. The copper content of domestic steel rails is generally between 0.10% and 0.40%. If the rolling process of the copper-containing rail is not good, fish-like cracks will occur on the surface of the rail |
| 5 | P | It is a harmful element. The biggest hazard of phosphide is to reduce the plasticity and toughness of the steel. Especially at low temperatures, the cold brittleness of the steel increases, which easily leads to broken rails, and its content is controlled at no more than 0.04% |
| 6 | S | The sulfur is a harmful element. It is often left in the steel in the form of granules. When the rail is rolled, it is rolled together with the steel into sheets, which causes delamination or longitudinal cracks in the rail. The amount of sulfur is controlled at no more than 0.05% |
The mechanical property of railway track metal
- Strength
The ability of the rail to resist deformation and damage under load. It is often expressed in terms of strength limit, yield limit, and other indicators. The strength limit (tensile strength) means the metal material resists the tensile load and the maximum stress that can not be destroyed. The yield limit (yield strength) refers to the stress when the metal material can still produce significant plastic deformation without increasing the load. The unit is Mpa.
- Plasticity
The metal material underwent significant deformation without damage under the load and can maintain the deformed shape after the load is canceled. It is often expressed in terms of elongation and shrinkage. Elongation is the percentage of the ratio of the calibrated length to the original calibrated length after the sample is broken. Section shrinkage is the percentage of the ratio of the reduction in the fracture area of the sample to the original enthalpy area.
- Hardness
The ability of a metal material to resist the pressing of another harder object (material) into its surface. According to different measurement methods, it can be divided into Brinell hardness (HB) and Rockwell hardness (HRC).
The practice has proved that there is a certain correspondence between hardness and strength, which can be approximated according to the Brinell hardness value.
Calculate the tensile strength of the material. Such as low carbon steel 6b≈0.36HB, high carbon steel 6b≈0.34HB.
- Resilience
The ability of metal materials to resist impact loads without damage. The toughness of metal materials can be measured by the impact test and expressed by impact toughness value αk in kJ/m2.
- Fatigue strength
Under the action of alternating load, the material fractures. The ability of metal materials to resist fatigue is measured by fatigue strength. Fatigue strength is the maximum stress of a metal material under repeated alternating loads without failure.
The hardness of hot-rolled railway track metal
| Material | Tensile Strenght / Mpa | Hardness / HB |
| U75V | ≥980 | 280~320 |
| U78CrV | ≥1080 | 310~360 |
| U76CrRE | ≥1080 | 310~360 |
| U77MnCr | ≥980 | 290~330 |
The hardness of heat-treatment railway track metal
| Material | Tensile Strenght / Mpa | Hardness / HB |
| U75V | ≥1180 | 320~380 |
| U78CrV | ≥1280 | 370~420 |