Case hardening is a metallurgical technique that increases the hardness of the surface of a component to a certain, calculable depth. This provides a hard-working surface but allows the interior of the part to remain ductile and tough. One such technique to case harden a component is carburization.

Carburization is used most often for components that have high wear, such as gear teeth, fasteners, locks, camshafts, railroad wheels and many others. In particular, these components need to be resistant to wear, but also need to undergo repeated, cyclic stresses. Because of these cyclic stresses, the component should not be made entirely from a harder material that might be more likely to fail in brittle fashion or through fatigue.

These tank wheels are ready to be loaded into a furnace and carburized to give them a hard, wear resistant surface while maintaining a ductile core. Source: Picryl.

The process itself also creates a shallow gradient between hard, wear resistant material and soft, ductile material. The shallow gradient means there is no sharp barrier between the two materials, as that can lead to cracking and spalling, especially under impact or thermal cycling.

What is diffusion?

Before one can understand carburization, they must first understand diffusion. Diffusion is the natural process in which gradients are made shallower, or the barriers between physical properties diminished.

Consider what happens if a hot coffee cup is left on the table. There is a sharp thermal gradient between the coffee cup and the rest of the room. Heat transfers from the coffee cup to the ambient air, cooling the coffee but warming the surrounding environment. Eventually, the coffee cup and the ambient air are in equilibrium at some temperature between coffee temperature and room temperature. This cooling is non-linear, as the “steepness” of the temperature gradient is a factor in the equation.

Diffusion occurs due to concentration gradients, not thermal gradients. If a block of lead and a block of gold were attached to each other in close contact, eventually, some of the gold atoms would migrate into the lead block and vice versa. The end result would be a contiguous block of half lead and half gold.

One can imagine the random motion of an atom in this circumstance. As it jiggles around due to Brownian motion, sometimes it jumps to a nearby vacant spot. This vacant spot could be an actual vacancy (missing atom) nearby, or it could be in an interstitial site between atoms, if the migrating atom is small enough to fit there. This motion does not just happen with one atom, or one jump. The longer one waits, and the more energy these atoms have to move around (meaning, the higher the temperature), the more quickly this process occurs.

Ultimately, the diffusive process relies on the time, temperature and several constants involving the materials and how they are interacting. The constants involve the likelihood of motion, based on the sizes and the bond strengths of the different materials, and they are often determined empirically, rather than calculated.

Carburization

Carburization is a specific example of diffusion. In this case, carbon is the atom of interest, and its penetration into a steel alloy. Steel alloys are more ductile and fatigue resistant, but often lack the requisite hardness for high-wear applications.

Cast iron alloys are typically not carburized, as they already have a relatively high carbon content (2% to 4%), and so there is some difficulty in migrating more carbon into the surface. Furthermore, cast irons and their high carbon content are already brittle, and so they lack the toughness and fatigue resistance offered by steel alloys. This can be confusing; cast iron actually has less iron in it than steel.

Carbon is a smaller atom that can sit in the interstitial sites between the iron atoms. This allows for the formation of cementite (Fe₃C) that is much harder than the iron alone. Interstitial diffusion occurs much more rapidly than replacement diffusion, where an atom takes the place in the crystal structure of another atom. Therefore, carbon diffusing into iron happens much more quickly than the lead and gold example, and this is reflected in the diffusion constant.

The receiver to this reproduction Browning Model 1886 rifle was case hardened through carburization. These were desirable in the old west due to the dusty environment and the constant exposure to blowing sand. Furthermore, the coloration made it an attractive and expensive-looking item. Source: Flickr.

Carburization process

In order to carburize components, they must be placed in an environment where they are exposed to carbon. The most common way to perform this is to gas carburize the parts, where they are placed in a carbon monoxide atmosphere, though they can be coated with a carbon-bearing salt or coated with a graphite powder and a few other materials in a solid carburization process.

In all cases, the process occurs much more quickly if the parts are heated. Typically, the exposure to carbon occurs inside a furnace, and the metallurgist controls the time and temperature to which the parts remain in the furnace. It is an optimization problem — longer times mean lower temperatures can be used to achieve the same result, but sometimes, more rapid throughput at higher temperatures is required.

Typically, a carburization depth (distance from surface) at a certain concentration is specified, and the metallurgist alters the process parameters to reach this mark. It can be verified through destructive testing through sectioning and spectrography, or ground and hardness tested at different depths. Eventually, surface hardness can be used to approximate the carburization, but occasional destructive testing is still necessary.

There is an equation that relates these properties:

Where C_s is the initial carbon content of the steel, C_x is the carbon content of the steel at depth x, and C_o is the concentration of the carbon to which the steel is exposed (the atmosphere). The erf is the error function (related to a Gaussian distribution), and y is where all of the important variables factor in:

Fig4

Where x is the depth, D is the diffusion constant (found empirically, but also a function of temperature) and t is the time.

Engineering challenges

Carburization is one technique to alter the surface chemistry of materials to extend their lifetime and their usefulness into new applications. The metallurgist may only have a few variables under their control: time and temperature in the furnace, and maybe part geometry and carbon monoxide concentration through various means. They must carefully weigh out the variables and constantly check their calculations against empirical data to ensure the optimized carburization treatment.