Heat treatment process and strength improvement of fishplate
- What are the steps of heat treatment processes commonly used for fishplates?
Annealing is a pretreatment step. Heat the fishplate to 800 - 850℃, keep it warm for 2 - 3 hours, then cool slowly to below 300℃. The purpose is to eliminate forging stress, make the structure uniform, and lay the foundation for subsequent treatment. Q235 fishplates must go through this step. Quenching is a key process. Heat the fishplate to 850 - 900℃ (adjust with material), keep it warm for 1 - 2 hours, then cool quickly with water to form martensite structure on the surface, improve hardness. The hardness of 45# steel fishplates can reach HRC45 - 50 after quenching. Tempering follows quenching: heat to 500 - 600℃, keep warm for 2 - 3 hours, then air cool to reduce internal stress, adjust hardness to HRC35 - 40, balance strength and toughness, avoid brittle fracture. Heavy - haul railway fishplates must strictly control the tempering temperature. Surface quenching is for rail head contact parts: heat to 900 - 950℃ by high - frequency induction, cool quickly, make the 1 - 3mm surface layer hardness reach HRC50 - 55, keep internal toughness, improve wear resistance. High - speed railway fishplates often use this process.
- What impact do heat treatment parameters have on fishplate strength?
Too low quenching temperature (＜850℃) will lead to incomplete austenitization, insufficient fishplate hardness (HRC＜35), 15% - 20% decrease in tensile strength, unable to meet heavy - haul needs. If the quenching temperature of 45# steel is insufficient, the bearing capacity will decrease significantly. Insufficient holding time will lead to inadequate structure transformation, undissolved carbides inside the fishplate, large strength fluctuation. The tensile strength difference of fishplates in the same batch may exceed 10%. Ensure holding time ≥1 hour. Too high tempering temperature (＞600℃) will reduce hardness too much (HRC＜30), decrease strength by 20% - 30%, but improve toughness. Ordinary railway fishplates can appropriately increase the tempering temperature to extend service life. Too slow cooling speed will increase pearlite structure, decrease hardness and strength. The water cooling speed during quenching must be ≥50℃/s to ensure sufficient martensite formation on the surface, otherwise the fishplate is easy to wear.
- How to detect the quality of fishplates after heat treatment?
Hardness test is the basis. Measure 3 points on different parts of the fishplate with a Rockwell hardness tester. The qualified range of 45# steel fishplates is HRC35 - 40. A deviation of more than ±2HRC is unqualified to ensure strength. Tensile test samples to detect tensile strength: Q345 fishplates need ≥510MPa, 45# steel ≥600MPa, elongation after fracture ≥15%. Observe the fracture: ductile fracture (fibrous) is qualified, brittle fracture (crystalline) needs rework. Metallographic analysis checks the internal structure: after quenching and tempering, it should be tempered sorbite, uniform and fine, without network carbides or Widmanstatten structure, otherwise it will affect strength and toughness. 100% of high - speed railway fishplates must undergo this test. Fatigue test: after 2 million cycles of load, the fishplate has no cracks, and the strength retention rate ≥90%. Heavy - haul railway fishplates must pass this test to ensure fatigue resistance.
- What are the differences in heat treatment processes among fishplates of different materials?
The heat treatment of Q235 steel fishplates is simple, only needing annealing + low - temperature tempering, with hardness controlled at HB180 - 220, meeting medium load, low cost but limited strength. Q345 steel needs complete quenching + tempering: quenching temperature 850 - 880℃, tempering 550 - 600℃, hardness HRC30 - 35, tensile strength 510 - 600MPa, suitable for medium load lines, with moderate process complexity. 45# steel needs high - temperature quenching + medium - temperature tempering: quenching temperature 880 - 900℃, tempering 500 - 550℃, hardness HRC35 - 40, 20% - 30% higher wear resistance than Q345, suitable for heavy - haul railways. Low alloy high - strength steel (such as 16Mn) needs to control the quenching cooling speed (≥60℃/s) and add temper brittleness inhibitors, hardness HRC35 - 40, 15% - 20% better fatigue resistance than 45# steel, commonly used in high - speed railways.
- How do heat treatment processes adapt to fishplate needs of different lines?
Ordinary railway fishplates focus on toughness, using "annealing + low - temperature tempering" with hardness HB180 - 220, tensile strength 375 - 500MPa, meeting medium load, low cost and easy processing, Q235 steel is sufficient. Heavy - haul railways need high strength, using "quenching + medium - temperature tempering + surface quenching" with hardness HRC35 - 40, surface layer HRC50 - 55, tensile strength ≥600MPa, 45# steel or low alloy steel, able to bear repeated loads of large axle loads. High - speed railways require high strength + high toughness: quenching temperature 880 - 900℃, tempering 550 - 600℃, hardness HRC35 - 40, fatigue strength ≥300MPa, surface quenching layer 1 - 2mm, ensuring wear resistance and impact resistance, low alloy steel fishplates adapt to this demand. Urban rail transit fishplates need to balance strength and processability, using "overall quenching + high - temperature tempering" with hardness HRC30 - 35, easy to drill, and meet fatigue needs of frequent start - stop. Q345 steel performs well after this treatment.