A carburization technique using a Cr powder layer has been developed to control the diffusion depth of carbon in W-Ni-Fe heavy alloys. The aged heavy alloy samples were covered with a Cr powder layer of about 1-mm thickness and then packed with carbon black powder. The packed samples were heat-treated at 1150 C for 10 minutes in H-2 and then for 50 minutes in N-2. The carburization treatment resulted in the formation of Cr7C3 and Fe3W3C around the tungsten grains from the sample surface with a thickness of 40 to 50 mum. This carburized layer was much thinner than that formed without a Cr powder layer under the same experimental conditions. With the surface carburization, the surface hardness increased by similar to75 pct, from 508 to 888 VHN, and the impact energy decreased by similar to31 pct, from 123 to 85 J. After the carburization treatment, the main fracture behavior in a dynamic torsional test changed from smearing of the matrix to cleavage of the tungsten grains. A high-speed impact test showed that the surface carburization of penetrators induced the formation of many cracks around the penetrator surface, enhanced the self-sharpening, and improved the penetration performance. It appears that the developed technique provides an easy method of carburization without serious deterioration of the toughness of the material.