Differences in Hardenability of National Standard Rail Materials and Their Impact
- What are the differences in hardenability between common materials of national standard rails?
U71Mn rails have medium hardenability, with a hardened layer depth of 8-12mm after quenching, suitable for ordinary railway main lines. U75V rails have better hardenability, with a hardened layer depth of 10-15mm and more uniform hardness distribution, suitable for heavy-haul railways. U78CrV rails have the best hardenability, with a hardened layer depth of 12-18mm, which can withstand higher contact stress and are often used in high-speed railways and mountainous multi-curve lines.
- How does hardenability affect the hardness of rail heads?
Rails with good hardenability (such as U78CrV) can have a surface hardness of HB380-420, and the hardness decreases gently from the surface to the core, with the core hardness still ≥HB300. Rails with poor hardenability (such as some U71Mn) can reach HB360-390 on the surface, but the core hardness may be lower than HB280, leading to fatigue cracks in the rail head. Uneven hardenability will cause local hardness deviation of the rail head to exceed HB30, exacerbating local wear.
- What are the differences in hardenability requirements for rails in different line types?
High-speed railway lines require stable hardenability of rails, with a hardened layer depth deviation ≤2mm to ensure uniform distribution of wheel-rail contact stress. Heavy-haul railways, due to large axle loads, require higher rail hardenability, with a hardened layer depth ≥12mm to resist greater rolling stress. Ordinary railway branches have lower hardenability requirements, with a hardened layer depth ≥8mm to meet daily operation needs.
- What early damages can insufficient hardenability cause to rails?
It will reduce the wear resistance of the rail head surface, and excessive rail head wear (depth ≥3mm) may occur after 1-2 years of operation. In curve sections, rails with insufficient hardenability are prone to plastic deformation, forming "fat edges", and the wear speed of the rail head side is 20%-30% faster than normal. After long-term use, network cracks may appear inside the rail head, reducing the overall strength of the rail.
- How to check whether the hardenability of rails is qualified?
Cut rail head samples, measure the hardness distribution along the section after quenching, and the hardened layer depth must meet the corresponding material standards. Use a metallographic microscope to observe the microstructure, ensuring uniform pearlite lamella spacing in the quenched area and no obvious undissolved carbides. For each batch of rails, sample 3% for testing. If the hardenability deviation exceeds 10% of the standard value, the entire batch must be downgraded for use.