The compatibility of the yield strength ratio of rail bolts with the long-term preload retention of the fastening system
Why is strict control of the yield strength ratio (YSR) required for track bolts, rather than simply pursuing high tensile strength?
High tensile strength only represents the bolt's fracture limit, while YSR determines its elastic reserve capacity. Track bolts endure long-term alternating vibration loads after tightening; an excessively high YSR (e.g., >0.9) results in an extremely narrow elastic working range, where minor overload or plastic deformation pushes the bolt into the yield stage, causing permanent preload loss. An excessively low YSR (e.g., <0.75) means the bolt's tensile strength is underutilized, leading to material waste and excessive elastic deformation. Thus, controlling YSR is key to balancing strength and elasticity and ensuring long-term preload stability.
What are the optimal YSR ranges for track bolts under different service conditions?
For conventional line bolts, the optimal YSR range is 0.80-0.85, balancing elastic reserve and material utilization for moderate vibration loads. High-speed line bolts require a wider elastic range due to high vibration frequency, with an optimal range of 0.78-0.82 to avoid elastic fatigue from high-frequency vibration. Heavy-haul line bolts endure large static loads, requiring higher yield strength to resist plastic deformation-their optimal range is 0.85-0.88, enhancing overload resistance while ensuring elasticity.
What typical preload failure mode occurs in bolts with an excessively high YSR during service?
The typical failure mode is "yield-type preload loss". Under repeated train impact, alternating stress on the bolt overlaps with initial preload; when the combined stress exceeds the bolt's yield strength, micro-plastic elongation occurs. This irreversible plastic deformation causes continuous preload decay over time-even repeated retightening cannot restore the initial preload. Ultimately, bolts fail due to excessive plastic deformation, leading to fastener loosening-a more covert failure mode than fracture, prone to causing mass accidents.
How does "quenching and tempering" in heat treatment precisely control the YSR of track bolts?
Quenching and tempering is the core process for YSR control. Adjusting quenching temperature and holding time refines the bolt's martensitic structure, increasing tensile strength. Adjusting tempering temperature controls martensite tempering degree, altering yield strength. For example, to reduce YSR, tempering temperature can be appropriately increased to form a more uniform tempered sorbite structure, reducing yield strength with a smaller decrease in tensile strength. In production, orthogonal experiments determine process parameters to ensure each batch of bolts has a YSR within the design range.
How to quickly screen bolts with unqualified YSR using torque-angle curves on-site?
In bolt tightening tests, a qualified YSR bolt exhibits a clear three-stage torque-angle curve: "elastic stage - yield stage - strengthening stage", with a stable elastic stage slope and a distinct yield plateau. Bolts with an excessively high YSR have an extremely short elastic stage, almost directly entering the yield stage with no obvious elastic reserve. Bolts with an excessively low YSR have a smaller elastic stage slope, with the torque value at the end of the elastic stage far below design requirements. On-site, portable torque-angle testers can sample incoming bolts; batches with abnormal curves require full re-inspection and are strictly prohibited from use.