en Research Paper Research Paper <span style="font-size:11pt"><span style="line-height:107%"><span style="font-family:Calibri,sans-serif"><span new="" roman="" style="font-family:" times="">The investigation of SUP9 steel under the hot-rolling conditions for applications to leaf spring suspension focused on its tensile and fatigue crack growth (FCG) properties. In order to investigate the tensile properties, tensile specimens were fabricated in the longitudinal-transverse (LT) direction. Furthermore, in order to evaluate fatigue crack growth (FCG) behaviour, compact tensile (CT) specimens with different crack plane orientations in both the LT and transverse-longitudinal (TL) directions were employed. Microstructural and fractographic analyses were conducted using optical microscope (OM) and scanning electron microscopy (SEM). The hot-rolling process reduced the interlamellar spacings of Fe₃C, enhancing the tensile properties through strain hardening. A high yield-to-ultimate strength ratio (~0.623) indicates excellent plastic deformation capability and resistance to fatigue crack growth, making SUP9 steel suitable for the leaf spring suspension system. Furthermore, the exponential crack growth rate constant, m, was found to be 3.066 in the TL direction and 3.265 in the LT direction, indicating that cracks propagate more rapidly in the LT orientation. Additionally, non-metallic inclusions, such as spherical oxides and MnS precipitates in LT specimens, were observed to facilitate faster crack growth in the transverse direction.</span></span></span></span><br> &nbsp; SUP9 steel,Hot-rolling,Lamellar Fe3C,Manganese sulphide,Crack growth rate 62 73 http://ijmse.iust.ac.ir/browse.php?a_code=A-10-5472-1&slc_lang=en&sid=1 Mohammad Badaruddin mbruddin@eng.unila.ac.id 1800319475328460019588 1800319475328460019588 No Department of Mechanical Engineering, Faculty of Engineering, Universitas Lampung Ahmad Kurniawan Purga ak.purga92@gmail.com 1800319475328460019589 1800319475328460019589 No Study Program of Magister-Department of Mechanical Engineering, Faculty of Engineering, Universitas Lampung Dwi Asmi dwiasmi82@yahoo.com 1800319475328460019590 1800319475328460019590 Yes Department of Physics, Faculty of Mathematics and Faculty of Mathematics and Natural Science, Universitas Lampung Sugiyanto Sugiyanto sugiyanto.1957@eng.unila.ac.id 1800319475328460019591 1800319475328460019591 No Department of Mechanical Engineering, Faculty of Engineering, Universitas Lampung Slamet Sumardi slametsumardi99@gmail.com 1800319475328460019592 1800319475328460019592 No Research Unit for Mineral Technology, National Research and Innovation Agency (BRIN), Tanjung Bintang, Lampung Selatan-Indonesia Andreas Luki Indratmoko luk@ptg.ac.id 1800319475328460019593 1800319475328460019593 No Polytechnic of Tunas Garuda, Department of Manufactural Engineering, Lampung, Indonesia