Control of surface carburization and improvement of dynamic fracture behavior in tungsten heavy alloys

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 H₂ and then for 50 minutes in N₂. 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 μm. 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 ∼75 pct, from 508 to 888 VHN, and the impact energy decreased by ∼31 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.