"Carburizing produces a harder, stronger layer on the outside of a softer, weaker (and tougher) steel part. The part is heated to a temperature that will cause it to transform to austenite, which has a higher carbon solubility than ferrite. Then, a carbon-containing gas mixture is introduced into the furnace. The carbon diffuses into the steel surface to some depth and results in a concentration profile from the surface to the core that depends on the details of the carbon potential of the gas, temperature, and time. When the part is quenched from high temperature to below a critical temperature that depends on the base composition of the steel and the carbon concentration gradient, martensite will form. Martensite has a less dense crystal structure than austenite or ferrite, so wherever the martensite forms, there is a volume expansion. The volume of the metal atoms expands in the surface layer, starting at the boundary layer between the core and the case and moving outward toward the surface. As the surface layer cools relative to the core, the greater volume attempts to contract but cannot. When the core cools, it attempts to contract and creates compressive stresses in the outer layer, which really doesn't quite fit anymore, on a micro-level. Ideally, you end up with the highest compressive stresses at the surface of the part. If case hardening is not done correctly, you can end up with tensile residual stresses at the surface, which is highly undesirable."