Get the answer of: What Controls Surface Hardness of Steels?
Surface hardness is mainly dependent on the carbon dissolved in austenite at the carburising temperature. When the carbon content at the surface particularly of alloy case hardening steel exceeds 0.70%, the Ms temperature decreases sharply.
This means that the amount of retained austenite after quenching increases which results in decreased surface hardness. Fig. 8.21 illustrates that maximum hardness is obtained at a depth of 1 mm from the surface. The value of maximum hardness is less if Ni content increases in steel, i.e., carbon content corresponding to maximum hardness decreases. In Cr-Mn steel, the maximum hardness occurs at 0.70% carbon, whereas it is 1% C for Cr – Mo steel.
The lower surface hardness is due to retained austenite. Normally, large amount of retained austenite may not be desirable in machine components, tools. In gas carburising, it is possible to control surface carbon content to a level of 0.7-0.8%. For practical reasons, the temperature of the furnace is lowered to about 830°C to 845°C before directly quenching.
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This lowering of temperature leads to:
(i) Formation of carbides
(ii) Amount of carbon in austenite get decreased somewhat
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(iii) Lower temperature reduces the danger of distortion.
Lot of experience is required to decide about the amount of retained austenite, which can give best results for a component. Large amount of retained austenite in gears can prove to be beneficial. Sub-zero treatment particularly for alloy steels reduces the amount of retained austenite. Surface hardness increases as the quenching temperature is lowered, and the sub-zero treatment cannot effect this hardness.