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This study investigated the effect of hydrostatic test procedure on the micro structure of a steel pipe material and its effect on the mechanical properties of the materials due to the distortion of the microstructure of the steel material. Thus, the study aims to establish whether there is any significant change in these properties of the steel pipeline after the procedure. In The study, two pipeline spools of 76.2 mm (3-inch) and 101 mm (4-inch) were subjected to pre-determined maximum allowable operating pressures and designed hydrostatic pressures at designated data points. Thereafter, the spools were cut and the samples prepared for experimental tests and analysis. Series of test were carried out and the results obtained showed that there is significant change in the grain size (by 22.6% for the 3-inch and 17.5% for 4-inch pipes) and grain arrangement for the tested specimen with pronounced changes observed in the tensile strength, fatigue strength, hardness and microstructure of the hydrostatically tested specimens.