Behind key industrial products such as steel wire ropes, springs, and tire cords lies a crucial heat treatment technology—sorbitizing quenching (also known as patenting). Through sophisticated microstructural control, this process endows steel with a unique combination of strength and ductility, enabling it to withstand harsh processing and service conditions.
The core purpose of sorbitizing quenching is to obtain sorbite microstructure—a type of extremely fine lamellar pearlite (with a lamellar spacing typically <150nm). This microstructure exerts a decisive influence on subsequent processing and final performance:
Breaking through processing bottlenecks: High-carbon steel wires are prone to fracture due to work hardening during drawing. The fine and uniform sorbite lamellae facilitate slip deformation, allowing large deformation drawing of over 80% without intermediate annealing.
Achieving strength-toughness balance: Sorbite microstructure possesses both high strength (up to 2000MPa or more) and good ductility (elongation >10%), far exceeding ordinary pearlite (strength approximately 600-800MPa).
Optimizing product service life: The fine lamellar structure inhibits crack propagation, significantly improving the fatigue life of steel wires (e.g., elevator cables need to withstand millions of cycles of dynamic loads).
Sorbitizing quenching is an isothermal quenching process, with the core lying in precisely controlling the decomposition of austenite in the pearlite transformation zone:
Austenitization: Steel wires are heated above Ac₃ (usually 850-950℃) to obtain uniform austenite.
Rapid cooling: Cool at a rate of >100℃/s to the sorbite transformation temperature range (approximately 500-600℃) to avoid ferrite or bainite precipitation.
Isothermal transformation: Hold at a constant temperature to allow complete decomposition of austenite into sorbite (typical holding time: 10-60 seconds).
Final cooling: Air-cool to room temperature.
Process key points: The cooling rate must be fast enough to bypass the "C-curve nose," but temperature fluctuation must be strictly controlled within ±10℃ during the isothermal stage; otherwise, lamellar coarsening will lead to performance degradation.
Technical breakthroughs:
Concentration-fog flow effect: In spray quenching, 0.05% carboxymethylcellulose (CMC) solution can increase the cooling rate in the high-temperature zone (>600℃) by 40%, while dynamically regulating phase transformation through droplet evaporation (see Figure 7).
Lamellar structure optimization: SEM shows that the lamellar spacing of lead bath sorbite is approximately 80nm, and that of CMC-quenched sorbite is about 100nm, but fragmented cementite instead improves drawing performance (Figure 6).
High-carbon steel: C content 0.60-0.85% (e.g., SWRH82B, 70 steel), accounting for over 90% of sorbitized steel.
Medium-carbon alloy steel: Spring steels such as 55SiCr and 50CrVA, whose relaxation resistance is improved through sorbitizing.
Stainless steel: Limited to martensitic/precipitation hardening types (e.g., 17-7PH); austenitic steels (e.g., 321) are not applicable due to no phase transformation.
Industry data: The global annual output of sorbitized steel wire exceeds 20 million tons, of which radial tire steel cords account for 35% and high-end spring steel wires account for 20%.
Accelerated environmental protection substitution: The EU has fully banned lead baths, and China is promoting CMC/salt bath (nitrate) substitution technologies, reducing toxicity by 99%.
Intelligent control: The AI parameter optimization system based on orthogonal experiments (see Table 2) enables spray quenching temperature difference control accuracy of ±5℃.
Microstructural ultra-refinement: Rare earth microalloying + ultra-fast cooling technology compresses the sorbite lamellar spacing to below 50nm (strength exceeding 3000MPa).
Sorbitizing quenching is the core process for high-performance steel wires. From lead baths to environmentally friendly polymer quenching, and further to intelligent spray control, technological iteration has always centered on one goal: constructing the perfect balance between strength and toughness among microscale lamellae. With the integration of new materials and digital technologies, sorbitizing treatment will continue to push the manufacturing boundaries from giant bridge cables to micro-medical device springs.
Technical warning: Enterprises adopting traditional lead bath processes need to be alert—47 countries around the world have imposed heavy metal emission taxes. Environmentally friendly processes are not only a technological upgrade but also a survival necessity.
