The object of this study is HSS API 5DP Gr 105 which has been used as drill pipe and was found leak during drilling activity, causing a delay in the delivery of drilling products. The interaction among the high partial pressure of H2S, a high-hardness material, and the injection of high-density completion fluid (HDCF) remains poorly understood, leading to sulfide stress cracking. Despite offering substantial benefits, the detected trace amounts of hydrogen and sulfur indicate a localized corrosion, which can lead to unprecedented drilling shutdowns and consequently impose greater operational costs. Recently, the API 5DP 3-1/2" drill pipe experienced failure with a significant hardness value of 26 HRC, exceeding the standard specified by NACE MR 0175. The material was in service in rich H2S gas well, where HDCF was injected to maintain hydrostatic pressure and serve as a control fluid. Multiple field and laboratory investigations have been undertaken to identify the root cause of this failure, including visual inspections, macrophotography, chemical composition analysis, completion fluid testing, tensile testing, metallography, and SEM-EDX analysis. The shear-slip and step-like markings on the failed material clearly indicate a brittle nature, correlating with a noticeable tensile strength of 907.80 MPa and an elongation limit of 18.18%. The increase in hardness beyond 22 HRC indicates susceptibility to sulfide stress cracking (SSC) where the hydrogen permeation increases with the increasing H2S partial pressure. These facts align with water chemistry analysis results to show S2– and HS– levels exceeding one ppm. Additionally, metallography reveals intergranular cracking in the tempered martensite, likely initiated at a local stress concentrator before propagating and confirmed by scanning electron microscope (SEM) images Copyright © 2026, Authors.