The development process and testing methods involved the use of 42mm diameter 65Si2MnWA steel as the test material. The material was heated using a KGPS-500/25 intermediate frequency heater, which has a power capacity of 500kVA and operates at a frequency of 2500Hz. A WFHX-63 digital thermometer was employed to monitor the temperature, with a heating rate of 6°C/s and a target heating temperature ranging between 700°C and 723°C. After heating, the material was rolled on a 200 medium-temperature rolling mill at a rolling speed of 10m/min, followed by air cooling. The final rolling specification resulted in a trapezoidal cross-section of 29mm x 37mm.
In terms of test results, the microstructure of Group A showed flaky pearlite, while Group B exhibited a combination of point-like and flaky pearlite. The microstructure of Group B was finer and more uniform compared to Group A. Both groups had similar graphite carbon content (0.5%), with no significant difference observed. However, the decarburization depth of Group A ranged from 0.07mm to 0.10mm, while that of Group B was between 0.06mm and 0.09mm. Although there was some variation, the overall decarburization levels were comparable. In terms of hardness, Group A had a range of 239–275 HB, while Group B ranged from 239–285 HB, showing close similarity.
The material used was 42mm 65Si2MnWA hot-rolled round bar, which was surface-ground before undergoing five cycles of continuous medium-frequency induction heating to reach temperatures between 700°C and 723°C. It was then rolled into a trapezoidal shape with core dimensions of 22mm–28mm–32mm. Under a magnification of 500x, the metallographic structure revealed fine grains with a small amount of pearlite. The structure showed an extremely uniform distribution of flaky pearlite and fine-grained pearlite, with minimal presence of point-like structures. This indicated excellent mechanical properties and favorable heat treatment characteristics.
The analysis of the layer profile in medium-temperature rolled spring steel highlighted the impact of heating time on graphitization transformation. Spring steel achieves higher yield strength mainly through carbon solid solution in austenite. If graphite forms, it behaves similarly to inclusions, reducing plasticity and fatigue strength. High-silicon steels have higher carbon free energy, increasing cementite instability and promoting graphite formation. According to references, prolonged exposure at 750–800°C or 700–720°C can lead to graphite precipitation. Therefore, avoiding long stays in high-temperature ranges is essential. Medium-frequency rapid heating at 6°C/s and natural cooling after rolling prevent sufficient heat for graphitization.
During trial production, Group A was rapidly heated to 700–723°C and cooled without graphite precipitation. The effect of heating rate on the microstructure showed that faster heating promotes more uniform grain growth. Based on warm rolling theory, heating above the Acl temperature allows recrystallization during rolling, refining coarse grains. These principles confirm the effectiveness of the medium-frequency rapid heating and medium-temperature rolling process.
Surface decarburization significantly reduces mechanical properties and increases the risk of premature fatigue failure. Carbon and silicon content strongly influence decarburization, with higher temperatures and longer times intensifying the effect. Medium-frequency induction heating and medium-temperature rolling help maintain good plasticity, reduce cracking risks during cold deformation, and lower the rolling temperature below Acl, avoiding high-temperature zones. This also shortens the time spent in the recrystallization range, minimizing oxidation.
Comparing the performance of hot-rolled materials, the surface of hot-rolled trapezoidal 65Si2MnWA steel from a domestic mill showed decarburization depths up to 0.45mm, with full decarburization layers reaching 0.09mm. The microstructure included fine, dense pearlite with point-like and small-particle pearlite. However, the service life, machining performance, and heat treatment of these materials were not as favorable as those produced by medium-frequency induction and medium-temperature rolling.
In conclusion, the trapezoidal spring steel processed using medium-frequency induction heating to 700–723°C and medium-temperature rolling offers advantages over traditional hot and cold working methods. The fast heating speed and relatively low temperature effectively address issues like surface decarburization and graphite precipitation. Medium-temperature rolling enhances material plasticity, prevents defects such as cracking, optimizes the microstructure, and improves strength, toughness, and ductility.
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