Another observation of the microcracking is that it is generally not seen in carburized steel that is less than the eutectoid line on the iron-carbon equilibrium diagram (steels containing less than 0.77% carbon). Microcracks may occur in the formed case because the carbon content of carburized steel is usually around 0.85% up to 1.1%.
The other influencing factor on this is the alloy content – chromium, molybdenum, vanadium, tungsten and other carbide-forming elements contained in the steel chemistry. This means that if the alloy elements increase, such as a nickel-chrome case-hardened steel, it could well decrease the eutectoid carbon level to below 0.6% carbon, even though the surface carbon is identified at 0.85%.
The next significant step that can contribute to the microcracking is the cooling rate and the positioning of the Ms and Mf temperatures, which are also influenced by the carbon content of the steel. Further, the final microstructure of the quenched carburized steel will contain both martensite and austenite. The amount of residual (retained) austenite that is left will be determined by the available carbon content and the location of the Ms temperature line. The presence and amount of residual austenite will determine the final case transverse hardness traverse from the surface through to the core.
So, there is potential for a double whammy. This is microcracking within the formed case and retained austenite, which will produce a lower-than-expected hardness value. How do we control this? It is only controlled by very careful monitoring of the enrichment gas (assuming gas carburizing) within the furnace atmosphere. In addition, careful selection of the case austenitizing temperature must be taken into account in order to reduce the risk of the microcracking and the retained austenite. Unfortunately, the microcracking is not simply the result of intergranular oxidation (IGO) at the carburized surface, it can also be seen at right angles to the surface of the carburized case.
As has been previously stated, the IGO is a product of gas carburizing and occurs as a result of the formed gas chemistry within the process chamber.
The internal microcracking that is likely to develop in the plate martensite is a result of the compromising atmosphere gaseous diffusion into the surface of the steel and is strongly influenced by the steel analysis, the atmosphere and potential. Further, accurate selection of the case austenitizing temperature is critical along with the quench-medium temperature in relation to the Ms temperature.
Carburizing requires very careful control of both atmospheres and temperature selections for quenching.