In the design of a downhole isolation tool for multi-stage fracturing in the oil and gas industry, a setting component, called slip, was used to set the tool in the casing prior to the hydraulic fracturing operation. The material of the slip is made of gray cast iron with surface hardening requirement. This study investigated the performance of slips treated by induction hardening versus flame hardening. The slip treated by induction hardening produced low hardness and insufficient affected layer. On the other hand, flame hardening generated satisfactory results of case hardening layer by 0.762 mm (0.030 in.) thickness with 50 HRC minimum hardness. The Type E graphite in the raw material was transformed to Type A in the flame hardening process, which is favored in the case hardened layer. The effect of different treatment processes on the affected layer and their microstructural response in gray cast iron was discussed in this study. The isolation tool using the slip treated by flame hardening, together with other proven components, showed successful performance of 82.7 MPa (12 ksi) pressure holding at 177 °C (350 °F) for high pressure and high temperature downhole applications.