**Development Conditions and Methods**
The test material used was 42mm diameter 65Si2MnWA steel. An intermediate frequency heater, model KGPS-500/25, with a power of 500kVA and a frequency of 2500Hz was employed for heating. A digital thermometer, model WFHX-63, was used to monitor the temperature, with a heating rate of 6°C/s and a target temperature range of 700–723°C. The material was then processed on a medium-temperature rolling mill with a rolling speed of 10m/min. After rolling, the material was air-cooled. The final rolling dimensions were 29mm by 37mm.
**Test Results and Microstructure Analysis**
The microstructural analysis showed that Group A had a flaky pearlitic structure, while Group B exhibited a more refined and uniform mixture of punctate and flaky pearlite. Both groups had similar graphite carbon content (0.5%), with no significant difference between them. The decarburization depth in Group A ranged from 0.07 to 0.10mm, whereas in Group B it was between 0.06 and 0.09mm. There was no substantial change in decarburization levels between the two groups. In terms of hardness, Group A had a range of HB 239–275, while Group B ranged from HB 239–285, showing close similarity.
**Metallographic Observations**
After surface grinding, the 42mm 65Si2MnWA hot-rolled round bar was subjected to five cycles of medium-frequency induction heating at 700–723°C, followed by five-stage rolling to produce a trapezoidal core with dimensions of 22mm–28mm–32mm. At 500x magnification, the metallographic structure revealed fine grains and a small amount of pearlite. The structure was characterized by flaky pearlite (0.08–0.5272 mm) and fine-grained pearlite, which was extremely uniform. This indicated excellent mechanical properties and heat treatment performance. The layer profile was consistent across samples.
**Analysis of Medium-Temperature Rolling and Graphitization**
In spring steel, a higher yield strength is achieved through carbon dissolved in austenite. However, if graphite precipitates, it behaves similarly to inclusions, reducing plasticity and fatigue resistance. High-silicon steels have a higher free energy of carbon, making cementite less stable and promoting graphitization. According to reference data, prolonged exposure at temperatures between 750–800°C or 700–723°C increases the risk of graphite precipitation. Therefore, long-term exposure in high-temperature ranges should be avoided. The medium-frequency induction heating process used here involved a rapid heating rate of 6°C/s, followed by natural cooling after rolling, preventing sufficient thermal energy for graphitization.
**Effect of Heating Rate on Structure**
During the quenching heat treatment, the original microstructure of spring steel must be spheroidized to achieve optimal mechanical properties. From the perspective of metal dynamics, a faster heating rate leads to more uniform grain refinement. Additionally, according to warm rolling theory, heating the material above the Acl temperature during medium-temperature rolling allows for recrystallization and grain refinement during deformation. These principles underpin the success of the medium-frequency rapid heating and medium-temperature rolling process.
**Impact of Decarburization and Silicon Content**
Surface decarburization significantly reduces mechanical properties and can lead to premature fatigue failure. Carbon and silicon contents strongly influence decarburization, with higher temperatures and longer exposure times increasing the tendency for decarburization. The medium-frequency induction heating and medium-temperature rolling method reduced the rolling temperature below the Acl point, minimizing time spent in high-temperature zones and reducing oxidation on the surface.
**Comparison with Hot-Rolled Materials**
When comparing the surface and core microstructures of hot-rolled trapezoidal 65Si2MnWA steel produced by a domestic mill, the differences were clear: (1) the hot-rolled material showed a decarburization depth of 0.45mm, with a full decarburization layer of 0.09mm and semi-decarburization; (2) it contained short, dense, and fine-grained pearlite, with some point-like and small-particle pearlite; (3) the service life, machinability, and heat treatment performance of hot-rolled materials were inferior to those of the medium-frequency induction heated and medium-temperature rolled material.
**Conclusion**
The trapezoidal spring steel produced using medium-frequency induction heating to 700–723°C and medium-temperature rolling offers distinct advantages over traditional hot and cold working methods. The fast heating speed and relatively low temperature effectively prevent decarburization and graphite precipitation on the surface. Medium-temperature rolling enhances the material's plasticity, avoids cracking during deformation, refines the microstructure, and improves overall strength, toughness, and ductility. This innovative process provides a superior alternative for producing high-quality spring steel.
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