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 7e20 nm, and within the brighter areas 1e7 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
1e2 nm, and the areas where the gray shade was not homogenous contained grains of size up to 4 nm.