Si-Mn spring steel (JIS SUP 9) is primarily employed in the construction of coil springs, particularly those used in soil cultivators deployed in harsh environments. This is due to its specific properties, which allow it to resist permanent deformation, and also provide rapid and effective vibration relief. Si-Mn spring steel (JIS SUP 9) for use in such projects is normally subjected to oil quenching and heat treatment, in order to strengthen its resistance to both impact and fatigue. Unfortunately, oil quenching is associated with environmental issues, most notably pollution and the contamination of wastewater with oil. Therefore, the objective of this study was to describe an alternative aqua quenching for use with this heat treatment. However, aqua quenching has traditionally conferred a lower tensile laod than does oil quenching. This problem of lesser tensile load was solved by properly controlling the density and temperature of the aqua quenching liquid, as well as careful mixing. As a result, it has proven possible to achieve results with aqua similar to those conferred by oil quenching, with regard to chemical components, tensile load, and surface hardness. The tensile load results achieved by oil quenching were 13,145 Pa and 17,177 Pa, and the tensile loads achieved with aqua quenching were 15,392 Pa, 16,804 Pa, 17,923 Pa, and 18,668 Pa. Therefore, the results of our experiments demonstrated the appropriacy and efficacy of this aqua quenching technique as a component of heat treatment for Si-Mn spring steel (JIS SUP 9).