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The Factors That Will Influence Micro-Cracking in Carburized Steel Cases
One of the interesting facts about micro cracks is that the track will not form in the lath martensite. However they will form in plate martensite. Another observation of the micro cracking is that it is generally not seen in carburized steel that are less than the eutectoid line on the iron carbon equilibrium diagram. That is steels which contain less than 0.77% carbon. It is at levels of carbon content above the 0.77% at one may experience the formation of micro cracks in the formed case simply because the carbon content of carburized steel is usually is around a carbon potential level 0.85% up to say 1.1%. The other influencing factor on this is of course the alloy content all the 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 which can contribute to the micro cracking is of course the cooling rate, and the positioning of the Ms and Mf temperatures which are also influenced by the carbon content of the steel. Further to this the final micro structure of the quenched carburized steel will contain both martensite and austenite. The amount of residual 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. Which is micro cracking within the formed case and retained austenite which will produce a lower than expected hardness value. How do we control this?
We can only control this by very careful monitoring of the enrichment gas (assuming gas carburizing) within the furnace atmosphere. In addition to this, careful selection of the case austenitizing temperature must be taken into account in order to reduce the risk of the micro cracking and the retained austenite. Unfortunately the cracking is not confined to that, which can be seen as a result of into granular oxidation (IGO.) at the carburized surface in the form of also micro cracks, but 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 micro cracking that is likely to develop in the plate martensite, is as a result of the compromising atmosphere gaseous diffusion into the surface of the steel, and is strongly influenced by the steel analysis, and the atmosphere, and potential. Further to this, accurate selection of the case austenitizing temperature is critical along with the quench medium temperature in relation to the Ms temperature.
Carburizing does require very careful control of both atmospheres and temperature selections for quenching.
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