AISI D6 is a high-carbon high-chromium cold-work tool steel. It has excellent resistance to wear and abrasion, high compressive strength, and good edge holding quality and dimensional stability. These properties make it an ideal choice for use as a tool for blanking, punching, cropping, shearing, trimming and clipping. One of its most promising and important applications is in the form of conical mandrel for expansion of steel tubulars used in oil and gas exploration. Solid Expandable Tubular (SET) technology is being successfully employed for various downhole applications such as slimming of oil and gas wells, and isolation of water-producing zones. The mandrel, made from a special material (generally D6 steel), is hydraulically pushed (or mechanically pulled) through a steel tubular. This cold work expands the tubular beyond its elastic region, causing permanent deformation. To withstand the high level of repeated thermo-mechanical stresses, cone material must have certain specific properties. The work reported in this paper covers heat treatment and mechanical testing of tensile and impact specimens of D6 tool steel. The heat treatment process consists of annealing, hardening/austenitizing, air or oil quenching, and single or double tempering. Mechanical testing includes hardness, tensile properties (elastic modulus, yield strength, ultimate strength, and ductility), and impact strength. Analysis of the test results leads to the identification of an optimal heat treatment sequence for D6 steel that would yield the best combination of desired properties for the expansion mandrel. Optimum material properties would ensure dimensional accuracy of the expanded tubulars and a longer mandrel life.