Pack carburizing of steels

This process can be used with less skilled personnel and is used for large parts that may not be processed using conventional carburizing.

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Pack carburizing is a very old method of carburizing steel, but it is still practiced in many tool rooms and machine shops. It is a simple process and can be performed by heat treaters of different skill levels. This method has been used for large gears, shafting, knives, and many different parts. The basic process is shown in Figure 1.

In this process, parts are packed in a container and surrounded by pack carburizing compound. Parts are then heated to 900-950°C (1,650-1,740°F) for two to 36 hours depending on the depth of case required. The parts are then removed from the furnace and allowed to air cool. Parts are removed from the pack carburizing container and heated to the austenitizing temperature under atmosphere. The parts are then quenched and tempered to the desired hardness.

Figure 1: Typical pack carburizing process.

The pack carburizing compound used is usually a mixture of ground wood charcoal and coke, sized to pellets of 3 to 6 mm (0.12 – 0.25”). Hardwood charcoal is more reactive than coke but has poorer mechanical properties. Coke has better hot strength and better thermal conductivity. An energizer is usually added to the pack carburizing compound such as calcium carbonate (CaCO3), sodium carbonate (Na2CO3), or potassium carbonate (K2CO3). A binder is used, typically an oil or molasses, to bind the carbonates to the coke and charcoal.

The carburizing process follows the general sequence of:

These reactions are like those occurring in gas carburizing, with the exception that wood charcoal and coke are supplying the carbon. The activator reduces the amount of CO2 at the surface of the part. In fact, before natural gas was common, wood charcoal generators were used to provide carburizing and neutral atmospheres.

Practical Considerations

Most modern furnaces can provide adequate temperature control for pack carburizing. Box furnaces, car-bottom, and pit furnaces are all used for pack carburizing. Box furnaces are loaded manually or with mechanical assistance. Car-bottom furnaces are used for very large parts, with loading of the parts and pack carburizing containers with a fork truck. Pit furnaces are also used for heavy items, using minimum floor space. Extensive technical training of operators is not required because these furnaces are simple to operate.

The containers are usually supported in at least three locations and blocked above the hearth. Setting the pack carburizing containers directly on the hearth is not a good idea, as the bottom of the hearth is typically colder than the rest of the furnace. Blocking above the hearth allows the container to reach a uniform temperature for uniform carburizing.

Containers used for pack carburizing can be made of carbon steel or heat-resisting alloys. Carbon steel tends to be oxidized rapidly and have a short life, but the low cost and ease of fabrication is often the deciding factor over heat-resistant Fe-Ni-Cr alloys.

Most containers are made from sheet or plate. Adequate rigidness, using ribs or corrugated panels, is necessary to provide long life during repeated cycles of heating and cooling. The design of the containers should be such to minimize sagging during extended period at elevated temperature. Lifting hooks or lift eyes are usually incorporated in the design of the container to facilitate material handling.

Lids for containers are usually sheet metal or plate to facilitate sealing. The lid must be tight enough to minimize burning of the compound but not so tight that it prevents excess gas from escaping. Often, lids are sealed with clay cement.

It is often good practice to condition the containers prior to first use by pack carburizing the interior of the container to prevent the container being carburized during use, instead of the part.

Surface Carbon Content

In carburizing, the available carbon potential is related directly to the ratio of carbon monoxide to carbon dioxide. The use of energizers such as calcium carbonate increases the available carbon monoxide at the surface, and thus more carbon. For park carburizing, the rate of carbon monoxide is fixed and always exceeds the rate required to supply carbon to the surface. In fact, the carbon potential is approximately that of the saturation limit of carbon in austenite. The carbon potential is directly dependent on the carburizing temperature. If a surface carbon of 0.8% carbon is required, then the carburizing temperature is approximately 815°C (1,500°F). As more carbon is needed at the surface, the temperature is increased. This is the simplest way of achieving control of the carbon potential.

While carbon potential is the amount of available carbon at the surface, it does not describe the rate of diffusion of carbon into the steel. This is governed by Fick’s second equation of diffusion (solved for carburizing of steel) [1]:

Where Cs is the surface concentration of carbon, C0 is the initial carbon concentration in steel, D is the diffusion coefficient of C in austenite, Cc is the carbon concentration as a function of time, t, and distance x from the surface at x =0. Higher temperatures promote faster carburizing and increase the case depth. This is shown in Figure 2.

Figure 2: Effect of time of pack carburizing on the case depth and carbon gradient of SAE 3115 steel carburized at 925°C [2].

Pack carburizing is difficult for maintaining good case depth control. Very often, the case depth variation will exceed 0.25 mm (0.010”). This process is not used for light case depths, but is used for case depths greater than 2 mm.

Packing

Packing for pack carburizing is critical for proper process control. In the container, the packing compound is layered on the bottom to a depth of approximately 25-50mm (1-2 inches). The parts are then stacked on top of the layer of pack carburizing compound. Sometimes part supports are added to reduce distortion of the parts. Parts are usually packed 25-50 mm (1-2 inches) apart. Pack carburizing compound is then applied to the part, completely covering the parts and supports. The compound is lightly tamped until level, to make sure the parts are completely covered. Finally, the container is filled with the final layer of compound to a minimum depth of 50 mm (2 inches). The actual amount needed depends on the part size and time of carburizing. Experience will dictate the final thickness needed. Sometimes a thermocouple is added to the container to determine when the part is up to the desired temperature.

Advantages and Disadvantages

Pack carburizing has several advantages. The first is minimal technical training of heat-treating operators is required. There are minimal equipment and operating costs, with those being the cost of the pack carburizing compound, the container, and furnace. Many different types of furnaces can be used, and no generated atmosphere is required since the pack carburizing compound provides the atmosphere.

Pack carburizing is dirtier than atmosphere carburizing. Shallow case depths are not recommended, and the case depth is not consistent. Pack carburizing is not as easily controlled as to carbon potential as normal gas carburizing. Because the parts are inserted into a steel container, it takes a long time to heat the part, the container, and compound. This contributes to very long cycle times.

Conclusions

In this article, we have described the process of pack carburizing and its advantages and disadvantages. This process can be used with less skilled personnel and is used for large parts that may not be processed using conventional carburizing.

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References

  1. J. I. Goldstein and A. E. Moren, “Diffusion Modeling of the Carburization Process,” Met. Trans. A, vol. 9A, no. 11, pp. 1515-1525, 1978.
  2. ASM International, “Pack Carburizing,” in ASM Handbook: Steel Heat Treating Fundamentals and Process, vol. 4A, J. Dossett and G. E. Totten, Eds., Materials Park, OH: ASM International, 2013, pp. 560-564.