It’s tough to case harden low carbon steels. Simply put, more carbon equals more hardness, a feature that even the casual observer can appreciate. One solution to this processing puzzle is to introduce more carbon. In essence, the low carbon surface is infused with a supplementary carbon source. Consequently, as the additional carbon is artificially introduced, the super-heated workpiece is slowly cooled. Let’s examine the intricacies of this process.

Carburization Facilitated Case Hardening 

Just to underscore the issue, it’s hard to case harden a low carbon steel when carbon is the required process instigator. Logically, we need more carbon. We need to add carbon-rich iron shavings, carbon powder, or some other sooty medium to the heat treatment procedure. Cycled several times, the carbon medium binds itself to the surface of the carbon-low alloy until it’s case hardened. Referred to as a cementing or carburizing stage, the operation basically enhances the technique by introducing the case hardening element.

Furnishing the Carbon Medium 

Depending on the furnace configuration, several carbon supplementing techniques are on hand at this stage of the project. A gas carburization process injects carbon monoxide into a heated furnace. Alternatively, there are molten liquid carbons and salted baths, which diffuse the element into the surface of the workpiece. Deposited and cemented, the slow cooling surface assumes its case hardened characteristics. Finally, we’ll use pack carburizing as a work-in-motion example, a method that illustrates the finer points of the carbon supplementation cycle.

Pack Carburization Proceduralism 

Uniformly heated until the steel part glows red-orange, the component is immersed in its carburizing medium. This time, instead of a salted bath or a gaseous environment, it’s a bone charcoal mix that covers the processing part. Still furnace-hot, the workpiece is run through this cycle several times. The charcoal is binding itself to the low steel component, cementing the carbon to the part’s surface. We’re not quite done just yet. The charcoal detritus needs to be rinsed and the part hardened via a water cooling station. Finally, the annealing stage takes place, which means the subtly altered microcrystalline surface should be left to slowly cool. Further tempering work is typically carried out before the case hardening project is considered complete. Typically, the additional stages call for reheating, cooling, and water or oil quenching.

Here’s a process that can’t run on heat alone. A secondary element is required to facilitate low carbon case hardening, a literal second element. Supplied as a supplementary reservoir of sooty carbon, as a gaseous carbon monoxide compound, or simply as wood charcoal, the subsidiary medium increases surface carbon content, then ultimately cements that element to the low carbon alloy until it’s properly case hardened.