Heat treatment with gas quenching has already been an established heat-treatment process for two or three decades in the field of full hardening. At first, it was limited to the hardening of high-alloyed tool steels whose alloy structure enabled them to be hardened satisfactorily with a rather slow gas cooling rate.
The enhanced quenching action achieved with gas pressures above 10 bar has allowed successful extension of gas quenching to the field of low alloyed tool steels, steels for hardening and tempering, antifriction-bearing steels, and case-hardening steels. The capability to carburize and gas-quench in vacuum furnace installations has provided the industry with a new, environmentally friendly case-hardening process.
The Process
Like plasma carburizing, vacuum carburizing can also be performed in a vacuum furnace system. Vacuum carburizing can be succinctly described by the following key points:
• Carburizing gas is propane.
• Pressure ranges up to 20 mbar (absolute).
• Temperature range is usually 900 to 1050°C, but higher temperatures are also possible.
Once the charge has been heated to the carburizing temperature under a neutral atmosphere (vacuum or nitrogen), propane is admitted into the evacuated heating chamber. Propane very rapidly undergoes 100% dissociation into more stable hydrocarbons and hydrogen. Carbon is also released and diffuses through the surface of the steel or component.
Vacuum carburizing is characterized by a high carbon mass flow rate, which carburizes the surface layer to near the carbon saturation limit within a short treatment time. In the subsequent diffusion phase, no more carburizing gas is fed in — rather the existing carbon diffuses farther into the steel in accordance with the diffusion law until the desired carbon profile has been attained.
Process Comparisons
In contrast to protective-gas carburizing, vacuum carburizing can be performed with substantially higher case carbon contents. The case carbon percentage is already over 1.3% after a short period of carburization and then is held at 1.4 to 1.5% at 930°C, which is about 0.2% higher than in protective-gas carburizing with a carbon level just below the sooting limit. The higher case carbon content in vacuum carburizing results in shortened treatment times, even at the same carburizing temperature. Raising the carburizing temperature results in a further considerable time savings.
Vacuum carburizing systems readily permit a carburizing temperature of over 1050°C, although the heat treatment racks made of heat-resistant cast steel (which are currently in use) are no longer usable at such high temperatures. With racks made of CFC (carbon-fiber-reinforced carbon), the limit is shifted to much higher temperatures. CFC material can only be used in an oxygen-free atmosphere such as that prevailing during vacuum carburizing. Of course, the carburizing action at the point of contact with the component must be taken into consideration.
The grain growth of case-hardening steels also does not permit such high temperatures over a long period of time. The vacuum furnace offers a pearlitizing treatment to refine the grain. Despite the time cost for pearlitizing, the result in comparison to the time required in a multipurpose protective-gas chamber furnace is a time savings of about 4 h for the case hardening of a 25% Cr Mo 4 steel to a case depth (550 HV) of 1.7 mm.
Distortion
Vacuum carburizing with gas quenching offers a potential for reduced parts distortion. A large number of experiments, conducted primarily on transmission parts, have shown that the scatter of the dimensional and shape changes after gas quenching is narrower than after oil quenching.
For example, a clutch body (O.D., 84 mm; I.D., 50 mm; height, 15 mm; mass, 0.2 kg each) made of 16% Mn Cr 5 with a case depth (550 HV) of 0.4 to 0.8 mm was tested. The study evaluated so clutch bodies after case hardening in the vacuum furnace (quenching with helium at 20 bar) and, for comparison, 50 others were evaluated after case hardening in the protective-gas furnace (oil quenching). The radial run-out of the clutch bodies was measured in the soft and hard states. The difference is illustrated in Figure V.1.
Advantages
Case hardening in vacuum heat-treatment systems with gas quenching offers the user many advantages in comparison to conventional protective-gas carburizing with oil quenching. Parts are clean and dry after treatment requiring no washers or management or disposal of liquid waste. Leidenfrost phenomenon is avoided and with more uniform quenching, distortion is minimized. Vacuum carburizing also allows for carburizing at up to 1000°C.
As a protective atmosphere, vacuum can prevent case oxidation and eliminate toxic off-gases. The vacuum carburizing process also provides a high carbon mass flow rate with low consumption of carburizing gas. With regard to productivity, the vacuum process can be integrated into a production line without the burdening requirements for fire-protection and fire-extinguishing systems, excessive heat removal to the surroundings, or extensive exhaust gas handling.
In determining the carbon mass flow rate, it becomes clear that in the first few minutes of carburizing in this process (up to 30 min) there is a very high carbon mass flow rate of up to 100 g/m2h. The case-hardening steel can be carburized up to its limit of solubility in the surface layer without any sooting occurring. The system technology also makes it possible to carburize at temperatures above 1000°C. These two factors result in an enormously shortened process duration.
The carburization results are comparable with those of the protective-gas process with regard to case depth, case carbon content, and surface hardness. The advantages for component quality lie in reduced distortion. Investigations of various transmission parts have shown that the scatter of the dimensional and shape changes can be narrowed with gas quenching in comparison to oil quenching. The clean surface of the component and the absence of case oxidation after heat treatment are additional advantages of this technology.
