Abstract The aim of this paper is to evaluate the machinability of hardened steels at different levels of hardness and using a range of cutting tool materials. More specifically, the work was focused on the machinability of hardened AISI 4340 high strength low alloy steel and AISI D2 cold work tool steel. The tests involving the AISI 4340 steel were performed using two hardness values: 42 and 48 HRC; in the former, a coated carbide insert was used as cutting tool, whereas in the latter a polycrystalline cubic boron nitride insert was employed. The machining tests on the AISI D2 steel hardened to 58 HRC were conducted using a mixed alumina-cutting tool. Machining forces, surface roughness, tool life and wear mechanisms were assessed. The results indicated that when turning AISI 4340 steel using low feed rates and depths of cut, the forces were higher when machining the softer steel and that surface roughness of the machined part was improved as cutting speed was elevated and deteriorated with feed rate. Abrasion was the principal wear mechanism acting when turning the 42 HRC steel, whereas diffusion was present when machining the 50 HRC steel. Turning AISI D2 steel (58 HRC) with mixed alumina inserts allowed a surface finish as good as that produced by cylindrical grinding. The flank wear of the mixed alumina tool increased with cutting speed and depth of cut, presenting a considerably higher tool wear rate when using at a cutting speed of 220 m/min and feed rate of 0.15 mm/rev, which resulted in tool failure by spalling.