Vacuum Treatments: Carburizing, Carbonitriding, Hardening, Tempering, Brazing, Annealing

Vacuum Hardening

Vacuum Hardening is an improvement over Conventional Hardening in that the components’ surfaces are protected from possible negative effects of exposure to a gaseous atmosphere. Vacuum treated material is quenched in gas or liquid, depending on the specification requirements.

The expression “vacuum hardening” is used here to differentiate the process from conventional hardening. It should also be noted that “hardening” is usually referred to as “quenching”.

Vacuum Tempering

Vacuum Tempering is almost always required after hardening (both in an atmosphere furnace as well as in vacuum), to reduce the hardness (and brittleness) of the treated material to a desirable level.

Generally it is not necessary to use vacuum tempering after vacuum hardening, i.e., conventional tempering is most of the time used. Vacuum tempering may be used on high value products, when totally clean surface appearance is required.

The expression “vacuum tempering” is used here to differentiate the process from “conventional tempering”.

Vacuum Annealing

The process is virtually identical to conventional annealing, except, as is usually the case, this premium quality Vacuum Annealing method protects the components’ surfaces from chemical reactions with gases present in the atmospheres present in the conventional process.

Vacuum Brazing

Vacuum Brazing uses a vacuum furnace but it is not, strictly speaking, a heat treating process. Brazing, in general, is a process of joining two components with a metallic bond by briefly liquefying the latter while the assembly is under vacuum conditions.

Vacuum Carburizing

Vacuum carburizing is a state-of-the-art thermal process where carburizing is effected under very low pressures. First the parts are heated in vacuum to above the transformation temperature of the alloy. Then they are exposed to carbon-carrying gas, or gas mixtures, under partial pressure. Nitrex has developed a revolutionary process called “Pulse- Pressure”, a method quickly becoming the industry standard.

Relative to conventional carburizing, the main advantages of the method are:

repeatable results to within ±0.001″ (±25µm)
significantly reduced size changes and distortion
improved fatigue strength
better control of the surface layer chemistry
the process is environmentally friendly

The basic aspects or carburizing in general are described in the Conventional Carburizing section.

Vacuum Carbonitriding

It is a thermal process of simultaneously diffusing both carbon and nitrogen into ferrous alloys under partial pressure. This leads to an extremely hard and wear resistant surface. Vacuum carbonitriding is a significant improvement over conventional gas carbonitriding. The process contains all of the inherent benefits of vacuum carburizing, but also has the additional benefit of precise computer control of surface ammonia content. Furthermore, this process does not require any additional refractory burn-outs so not only is the end product of higher quality, but it is often less expensive than with competing conventional gas processes.