Carbonitriding involves the diffusion of both carbon and nitrogen into the base steel. The carbon provides the base metal with a high carbon surface, and the nitrogen provides the case with an added boost of hardenability to insure full case hardness. The addition of nitrogen makes the carbonitriding process especially suited to plain, low carbon steel that would not otherwise respond to standard carburizing.

The type of steel used for a product to be carbonitrided depends primarily on the fabrication method.

• Stampings - The plain carbon steel grades, AISI 1010 through 1020, are most often used for stamping applications. These grades have a high degree of ductility to enhance the fabrication of the product, and are readily carbonitrided. Grades with lower carbon levels, through AISI 1008, though readily formed, can create control problems in heat treatment because of their lower hardenability; if required for maximum formability, it is suggested that carbon levels be held as high as possible, 0.07% minimum.
  
  
• Machined Products - The same plain carbon steels used for stampings are often used for machined products, and are readily carbonitrided. An additional option is the use of resulfurized, free machining steels, such as AISI 1117; the higher sulfur content significantly enhances machining, and the steels respond well to the carbonitriding process.
  
  
• Specialty Materials - Carbonitriding can be used to impart a hard wear resistant case on a wide variety of materials for special applications. These include powdered metal parts, cast irons, as well as other steel grades used to provide a particular balance of core and surface properties.

Carbonitrided cases are typically limited to relatively shallow depths, generally less than 0.030", most often less than 0.015". The primary purpose of carbonitriding is to impart a hard, wear resistant case on the part. Nitrogen diffused into the part with the carbon provides superior hardenability to develop the high hardness case. Carbonitrided cases are also slightly more resistant to softening at slightly elevated temperatures than carburized cases.

The shallower case and typically lower core hardness resulting from carbonitriding are not ideal for high bearing loads; carburizing is better suited for these applications.

Carbonitrided cases are typically specified in terms of either "total" of "effective" case, depending on the case depth required. Shallow cases, to depths of about 0.010", are best evaluated as total case, or the depth of the etched case on a mounted microspecimen. Deeper cases can be evaluated to an effective depth, or the depth to a specified hardness, generally RC 50. Surface hardness after carbonitriding is specified as "file hard" (resistant to a file run across the carbonitrided surface) for shallow cases, or a specified hardness using a superficial Rockwell hardness scale when heavier carbonitrided cases are specified.

The key variables in carbonitriding are time, temperature, and the process environment. Temperatures are typically maintained at 1475 F to 1650 F to insure control of the diffusion process. The higher process temperatures would be used for deeper case specifications. Lower temperatures may be used regardless of case depth requirements to reduce distortion. Time at the processing temperature is controlled to insure the proper development of the specified case.

The process environment is provided by the introduction of methane (for carbon) and ammonia (for nitrogen) to the furnace, in the presence of a controlled carrier gas. By maintaining the proper ratios of the working gases, the processed parts are provided with a thin skin of carbon and nitrogen rich steel.

After carbonitriding, the work is quenched to achieve the full case hardness. Careful consideration of the quench is required because parts are typically used without subsequent grinding operations. Quenching is most often done in oil with oil temperatures selected to achieve the optimum properties with acceptable levels of dimensional change. Hot oil quenching can be used to minimize distortion of intricate parts. Final cleaning with light blast provides a part ready for service.

Milwaukee Gear's heat treating affiliate, Treat All Metals, specializes in quality heat treating, and our carbonitriding capabilities are no exception. Our process equipment is certified to ensure accurate control of the carbonitriding variables, utilizing the best control equipment available. We have capabilities to harden in a variety of quenches to balance the best combination of properties and dimensional control. We can incorporate your gaging in our operations to help insure ready-to-use parts as you receive them.

The Quality Control Metallurgical Lab is well equipped to test your product to your most exacting requirements. When necessary, all process steps and inspections can be certified or substantiated with documentation.

Our technical expertise can also assist you in selecting and successfully applying the best surface hardening process for your application.