Structure of the carbon layer deposited on the steel surface after low pressure carburizing

During low pressure carburizing a carbon layer precipitates on the surface of steel parts. The structure of the carbon layer was tested by means of optical and electron microscopy and Raman spectroscopy as well. It was found out that the carbon layer is composed of fine-crystalline graphite. A sample was carburized (boost step), and was subsequently observed with an optical microscope. The dominant shade of the previously existing austenite grains was gray with few brighter areas (observation with a scanning microscope showed that these areas were darker). Within the gray areas the size of graphite crystallites was 7–20 nm, and within the brighter areas 1–7 nm. The diffusion step led to a change in the grain shade and to a decrease in the size of graphite crystallites. Gray-shaded grains were made of a mixture of gray and dark areas. The areas of homogenous brightness contained graphite made of crystallites of size 1–2 nm, and the areas where the gray shade was not homogenous contained grains of size up to 4 nm.