Many problems occurs during carburising of steels. Some of these are: 1. Low Surface Hardness 2. Low Case Depth 3. Non-Uniform Case Depth 4. Soft Spots 5. Sooting 6. Distortion.
Problem # 1. Low Surface Hardness:
There are two main reasons of low surface hardness of a carburised component- Presence of large amount of retained austenite or decarburisation. There is a steep increase of retained austenite as the carbon content of the surface becomes higher than 0.7%. In an effort to increase the case depth in short time, the high carbon potential of the carburising atmosphere may end up having surface carbon of 1.2 to 1.4%. The presence of alloying elements further increases the amount of retained austenite in hardened steel.
By increasing the hardening temperature, more amount of alloying elements get dissolved in austenite, which lower the Ms temperature, i.e., increase the amount of retained austenite. The best way of reducing the retained austenite is to give cold treatment to just hardened steel. If slowly cooled carburised steel is reheated to austenitising temperature, some reduction of surface carbon occurs due to diffusion of carbon away from surface, which too decreases the retained austenite.
The atmosphere in pack-carburising box may become decarburising as the temperature decreases, which lowers the carbon content of the surface. The quenched hardness can thus become low. Carburise the component for a short time again and use faster cooling to avoid decarburisation.
Problem # 2. Lower Case Depth:
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If low temperatures are used, diffusion is slow, to result in insufficient case depth, or a lean solid carburiser was used or the amount of enriching hydrocarbon added was less.
Problem # 3. Non-Uniform Case Depth:
This in pack carburising could be due to improper packing of components, or uneven heating of the container, or use of very tine carburising powder. If some surface is not easily in contact with the carburising gas or is in a section of furnace where temperature is less, can lead to non-uniform case depth. Many times the presence of oxidised layer or some greasy substance can also lead to non-uniform case depth.
Problem # 4. Soft Spots:
The uncleaned surface of component may have scale on the surface or dirt sticking to it to prevent or decrease adsorption of carbon-all these may result in soft spots. Sometimes excessive grinding-heat may soften the surface to give soft spots. The increased temperature of coolant like water or too small an amount of coolant makes it hot soon or improper agitation may also lead to soft spots.
Problem # 5. Sooting:
Soot formation takes place on surface of components and the parts of the furnace during gas carburising. Thus, sooting interferes with carburising process as well as discolours the surface of the component. The main cause is the addition of large amount of hydrocarbon gas in the furnace. The flow of this gas should be properly controlled.
Problem # 6. Distortion:
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Too rapid rate of heating or cooling, higher case depth, larger gradient in carbon in smaller depth can lead to distortion apart from the facts considered in the section on hardening.
Cracking and exfoliation on carburised parts occur due to super-saturation of carbon in case, which leads to much differential properties in the neighbouring layers of steel. The cementite network helps in initiating the cracking to exfoliate high carbon region from the lower carbon region.