Standard Basis for GB Light Rail
Light rail in the Chinese standard system is a hot-rolled tee section lighter than the mainline railway rail sizes, and the product standard that governs the profile and the dimensional tolerances is GB/T 11264, which covers hot-rolled light rails in the small mass range that includes the 8 kg per metre size. Heavier mainline sections are covered by separate rail standards, so the first decision in any procurement is which product standard and which nominal mass apply. The second decision is the material, and for light rail the answer is usually a general-purpose structural carbon steel rather than a rail-specific steel. The two grades that appear on most orders are Q235B to GB/T 700-2006 and, where more strength or better response to welding is needed, Q355B to GB/T 1591-2018. The material specification, not the profile, is what determines whether the rail will survive the way it is loaded.
Why Q235B Is the Standard Material for the 8 kg Size
Q235B is a general-purpose carbon structural steel whose designation carries its own meaning: Q stands for yield strength, 235 is the minimum specified yield strength in megapascals, and the B suffix denotes the quality grade, which fixes the impact requirement and the maximum sulphur and phosphorus levels. A minimum yield strength of 235 MPa is modest by rail standards, but the 8 kg per metre section is used at low axle loads and low speeds, where the controlling factor is not fatigue of a heavy rail head but the ability to provide a continuous running surface that can be bolted or spiked down, cut, drilled and re-laid repeatedly.
| Property (GB/T 700-2006, Q235B) | Value |
|---|---|
| Minimum yield strength, thickness 16 mm and under | 235 MPa |
| Tensile strength | 370-500 MPa |
| Elongation | 26 % minimum |
| Carbon, maximum | 0.20 % |
| Silicon, maximum | 0.35 % |
| Manganese | 1.40 % maximum |
| Phosphorus, maximum | 0.045 % |
| Sulphur, maximum | 0.045 % |
The combination of these values explains the popularity of the grade. Elongation of 26 % minimum means the rail tolerates a degree of bending and local deformation during transport and laying without cracking, and the moderate carbon level keeps welding and cutting within the capability of an ordinary site workshop. The chemistry also keeps the cost low, because the steel can be produced from readily available raw material without special refining, which matters when a project needs several hundred metres of light track rather than a few lengths. Q235B is not, however, a rail steel in the metallurgical sense: it has none of the hardness control that a heavy rail head receives, so it should not be used where wheel loads or traffic frequency are high.
When the Material Choice Moves Up to Q355B
Where the same 8 kg per metre section has to carry a heavier locomotive, sit on a steeper alignment, or survive a longer service period without re-gauging, the material is usually raised to Q355B to GB/T 1591-2018, a low-alloy structural steel with a minimum yield strength of 355 MPa, a tensile range of 470-630 MPa and a minimum elongation of 20 % for the common thickness range, with carbon limited to 0.20 % and manganese to 1.60 %. The higher strength allows a slightly higher axle load for the same profile, and the alloy content also improves the response of the steel to weathering. The trade-off is cost and, in thin sections, a slightly higher sensitivity to welding and forming, so the advantage only pays off where the load actually justifies it rather than as a general upgrade.
Chemistry, Weldability and Field Practice
For both grades the carbon equivalent is the number that governs site fabrication. With carbon limited to 0.20 % and manganese limited in the ranges above, Q235B is welded without preheat in ordinary ambient conditions at the thicknesses used for rails and base plates; Q355B with the same carbon limit also welds readily but benefits from dry electrodes, a controlled heat input and, in cold weather, a modest preheat. The practices that protect light rail on site are the same for either grade: drill rather than burn fishbolt holes, because an oxy-fuel cut leaves a hard, cracked edge that will initiate a failure; avoid welding across the rail web at mid-span; support the rail continuously on sleepers or on a prepared base rather than at intervals; and keep the rail ends square and deburred so that joint bars seat properly. Rails stored outdoors should be stacked clear of standing water and separated so that the underside is not permanently wet.
Checks Before and After Delivery
A light rail order should be checked against three groups of facts. First, the profile: nominal mass, rail height, base width, head width and web thickness measured at a defined point, against the values in the applicable profile standard. Second, the material: a certificate stating the grade, heat number, chemical composition and mechanical properties, with the yield, tensile and elongation values consistent with the standard for that grade. Third, the condition: straightness and camber over the full length, freedom from laps, seams and excessive corrosion, and fishbolt holes that match the joint bar pattern. On site, the gauge, the joint gap, the sleeper spacing and the degree of support decide whether the installation meets its expected life far more than the steel grade does, and these are worth checking with a gauge bar and a level before the track is signed off.
Frequently Asked Questions
Q: What material is used for GB 8 kg light rail?
A: Q235B general-purpose carbon structural steel to GB/T 700-2006 is the standard choice, with Q355B to GB/T 1591-2018 used where higher strength is required.
Q: What does the designation Q235B mean?
A: Q indicates yield strength, 235 is the minimum yield strength in megapascals, and B is the quality grade, which sets the impact requirement and the maximum sulphur and phosphorus content.
Q: What yield and tensile strength does Q235B provide?
A: A minimum yield strength of 235 MPa for thickness up to 16 mm, tensile strength of 370-500 MPa and a minimum elongation of 26 % under GB/T 700-2006.
Q: Which profile standard applies to 8 kg per metre light rail?
A: GB/T 11264 covers hot-rolled light rails in this mass range. The order should quote the standard, the nominal mass and the delivery length, since profile tolerances are defined by that standard.
Q: Can Q235B light rail be welded on site?
A: Yes, at the thicknesses used for rails and base plates Q235B welds without preheat in normal ambient conditions. Holes should be drilled rather than flame-cut, because a burnt edge is hard and crack-prone.
Q: Why would a project choose Q355B instead?
A: Where the axle load, the gradient or the required service life exceeds what the lower-strength grade can carry at the same section. The gain is strength and weathering behaviour; the cost is a higher material price.