Effect of Thermal Processing Routes on the Microstructure, Mechanical Properties, and Sustainability of an Automotive Spring Steel: A Green Engineering Approach

This study presents a comprehensive comparison of the microstructural evolution, mechanical behavior, and environmental impact of SAE 9254 spring steel processed via conventional quenching & tempering (QT) and induction quenching & tempering (IQT). IQT, characterized by rapid thermal cycles and localized heating, produced a refined microstructure with smaller prior austenite grains (∼9 μm), higher retained austenite (γR ∼8%), and lower dislocation density compared to QT. Despite ∼3% drop in ultimate tensile strength (σUTS), IQT significantly enhanced ductility, exhibiting a 19% higher reduction of area (RoA) and an 8.2% greater elongation in comparison to QT. High-cycle fatigue tests were conducted at 40–80% of the respective yield strength (σYS) under stress ratios of R = 0.2 and 0.4, representative of service conditions in maneuvering and payload vehicles, and revealed fatigue strength improvements of 36% and 64%, respectively, for IQT relative to QT. Fractographic analysis indicated predominantly surface-initiated failures, with fisheye fractures observed in selected cases. QT samples exhibited inclusion-driven crack initiation, whereas IQT failures originated from matrix regions associated with Mn and Si segregation. An industrial-scale sustainability assessment demonstrated that IQT reduces annual energy consumption by 18.25 GWh and CO2 emissions by approximately 15,000 tons compared to QT. For equivalent production capacity, the IQT route requires approximately 67% less energy than the conventional QT process, indicating a substantial improvement in process energy efficiency. These quantified reductions highlight the energy-efficient potential of IQT and align with broader sustainability objectives, including the United Nations Sustainable Development Goals (SDGs) 9, 12, and 13. Collectively, the findings establish IQT as a mechanically robust and environmentally progressive alternative for the manufacturing of ultrahigh-strength spring steels.