<![CDATA[Leaf springs are essential components in heavy vehicle suspension systems, serving to absorb shock loads and maintain vehicle stability. Prolonged service life can cause degradation of mechanical properties, which affects both safety and suspension performance. This study aims to analyze and compare the mechanical properties of used SUP9 and SUP9A steel leaf springs through Micro Vickers hardness testing and microstructural observation in accordance with ASTM E384-17. Samples were taken from a Hino Dutro (SUP9) and a Mitsubishi Canter (SUP9A), then prepared and analyzed using ImageJ software for phase quantification. The results showed that the hardness values of SUP9 (246 HV = 234 HB) and SUP9A (214 HV = 204 HB) were below the JIS G 4801 standard range (363–429 HB). The microstructure of SUP9 consisted of 80% pearlite, 15% ferrite, and <5% martensite, while SUP9A contained 70% pearlite, 25% ferrite, and <5% martensite. The higher pearlite content in SUP9 contributed to greater hardness, whereas the higher ferrite content in SUP9A increased ductility but reduced wear resistance. These findings indicate that both materials have undergone softening due to tempering effects from repeated loading, potentially reducing their ability to withstand shock loads and accelerating fatigue failure. Technical recommendations include re-heat treatment or component replacement to restore suspension performance and improve vehicle operational safety]]>
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