When machining hardened steel (greater than or equal to 45 HRC) with polycrystalline cubic boron nitride (PCBN) tooling, the cutting speeds used produce high temperatures in tire primor-v shear zone, which are sufficient to plasticize the workpiece. The paper initially reviews the effect of workpiece hardness and calling speed on chip formation. work-piece sur face integrity and cutting,forces. Equations are detailed,for determining the primary shear zone temperature, the proportion of heat conducted into the workpiece and the shear, flow stress. Following on from this, experimental work is presented involving the orthogonal machining of AISI H13 hot work the steel with PCBN tooling. Tests were carried out over a range of cutting speeds with work-pieces of different hardness, in order to provide cutting force, shear angle, chip morphology arid primary shear :one thickness data. The shear flow stress decreased with increasing cutting speed and/or workpiece hardness. Willi the AISI H13 heat treated to 49 +/- 1 HRC, the stress magnitude changed more significantly with cutting speed and the proportion of heat conducted as was from the workpiece approached 99 percent at 200 m/min. Shear localized chips were produced with white unetched layers due to intense heat generation followed by rapid cooling.