HEAT TREATMENT THROUGH HARDENING

Hardening just the surface layer of steels is called case hardening. A very hard case, or "skin" resists wear and is supported by a core of lower hardness, depending on the type of steel, which is more tough and ductile, resisting breakage.

Carburizing and carbonitriding are the two most common types of case hardening processes and are designed for only certain types of steels. When steel at proper temperature is surrounded by certain elements, such as carbon and nitrogen, it will absorb those elements into its surface. The added elements form a "case" which can be very hard.

For a quality job, it is vital for the heat treater to closely control furnace atmosphere, temperature, time, convection system plus the orientation of the parts in the furnace. Selective hardening may be accomplished by masking with high temperature tape, copper paint or copper plating.
CarburizingThis is a form of case hardening in which the furnace atmosphere is adjusted to deposit carbon into work when it is held at critical temperature. This layer of increased carbon can then achieve very high hardness when quenched. Since the sub-surface area has a lower carbon content, it does not harden as much or at all during this process. This leaves a more tough, ductile core than a through-hardening alloy or tool steel with the same surface hardness potential Case hardening is usually specified with a Rockwell C hardness range of three points, such as Rc 58-60, plus a "case depth" within the range of .010" to .080" or more. Interestingly, the rate of carburization will increase in depth either with temperature or over time. Since too high a temperature causes undesirable grain growth and a long period at lower temperature raises costs, a quality heat treater will start with a high temperature and then lower it to refine the grain before quenching.

Since carburized parts are designed for maximum surface hardness, they are sometimes NOT tempered after the hardening quench as is typical. This is especially the case with shallow case depths. In the tough core offsets the brittleness inherent in untempered steel.
Although many types of steels respond to carburizing, classic alloys for this purpose are 8620 and 9310. These "low alloy" steels can not only exceed Rc 60 when carburized, but produce a tough core of good hardness (Rc 30-38). These alloys are the choice of gear makers. Low carbon steels such as 1018 or 1117, while not offering as much toughness, can also be carburized for a low cost solution.