The Hardenability of the Clamping Plate Material and the Uniformity of Clamping Force for Large-Section Rails
Why does clamping large-section rails impose higher hardenability requirements on pressure plates?
Large-section rails (e.g., >=75kg/m) have a rail head width exceeding 80mm, requiring a corresponding increase in the pressure plate's load-bearing cross-section. Insufficient hardenability results in only the surface forming high-strength martensite during quenching, while the core remains soft ferrite-pearlite. During bolt tightening, uneven elastic deformation occurs between the surface and core, concentrating clamping force at the plate edges and leaving insufficient force at the rail head center. This uneven force fails to constrain large-section rail displacement, easily causing track diseases.
(ratio of maximum to minimum clamping force over the effective length) is used. For plates with adequate hardenability, the coefficient <=1.2 (uniform distribution). For insufficient hardenability, it can exceed 1.5. For example, a 75kg/m rail plate with poor hardenability may have an edge clamping force of 12kN but only 7kN at the center-far below design requirements and ineffective for clamping.
; carbon is fundamental, and alloys are critical. Common materials are 45# steel or 40Cr steel-40Cr has far superior hardenability due to chromium addition. Alloys like chromium, manganese, molybdenum, and vanadium reduce the critical cooling rate, enabling martensite formation in the core during quenching. For large-section rails, medium-carbon alloy steels (e.g., 42CrMo) with higher alloy content are used for enhanced hardenability.
Martempering involves heating the plate to the austenitizing temperature, cooling it in molten salt or hot oil slightly above the Ms point to equalize core and surface temperatures, then air-cooling to room temperature. This process , avoiding structural differences from rapid surface cooling and slow core cooling. Martempering produces a uniform tempered sorbite structure throughout the plate, ensuring consistent elastic modulus and uniform deformation under load-guaranteeing even clamping force for large-section rails.
How to preliminarily judge plate hardenability qualification via on-site hardness testing?
cross-sectional hardness test is used: sample plates are cut along the load-bearing cross-section, polished, and tested for hardness at the surface, 1/2 thickness, and core using a Rockwell durometer. For qualified plates, hardness values at the three positions are nearly identical (deviation ≤HRC 3). A core hardness ≥HRC 5 lower than the surface indicates insufficient hardenability and structural differences. Additionally, a clamping force test with a distribution coefficient >1.2 indirectly confirms poor hardenability-such plates are strictly prohibited for large-section rail installation.