Research programs were conducted to select the free-machining additive that would greatly increase the machinability of AISI 8620- and 4140-type steels without adversely affecting their hot workability, chemical homogeneity, and mechanical and metallurgical properties. The additives investigated were sulfur, selenium, tellurium, lead, and bismuth.The principal findings were as follows: 1. Selenium and tellurium were more effective in increasing machinability than were the other additives. 2. Tellurium, even in small amounts, greatly increased scrap loss and conditioning loss. 3. The potent effect of selenium on machinability was obtained with pearlite, bainite, and tempered martensite microstructures. 4. The hardenability, impact properties, fatigue properties, and case-hardening characteristics of steels containing sulfur, selenium, tellurium, and lead were about the same. On the basis of these studies, it is concluded that selenium is the optimum additive, and that the machinability of 8620-and 4140-type steels containing nominally 0.10% selenium is superior to that of commercially available 8620 and 4140 free-machining steels.