Flame hardening operations extend the operating life of alloy-rich components by adding rigid strength to the microcrystalline backbone of the heat-treated part. The superheated torch produces predictable results, but calculable hardness quotients are obviously contingent on the material build of the product. The differences between low and high carbon steel in flame hardening represent an ideal case study here, so let’s observe the variances that occur when heat treating these two carbon-mutable forms of steel.

An Introduction to Low and High Carbon Steel

Steel is an alloy that is rich in iron. After this primary metallic component, carbon is the next influential element in the mix, plus there are a number of other additives that contribute to the robust characteristics of this structurally adept alloy. Added as the principal alloying constituent, carbon content affects the quality of the metal and its behavior during heat treatment work. A low carbon steel component, for example, adds approximately 0.3 percent of carbon to the mix. In contrast, high carbon steel parts have a higher carbon content rating, a range that varies from 0.3 percent to 1.7 percent. Now, with a figure attached to these two types of carbon steel, we can begin to understand how they behave when flame hardening occurs.
The Flame Hardening of Low Carbon Steel

Soft but ductile, the metal is easy to form, but it requires significant quantities of flame-distributed energy to heat treat the surface and harden the material. The carbon absorbs thermal energy, but, due to the lower content rating, greater temperatures are needed if the required case hardening depth is to be realized. Advantageously, this low carbon candidate is less prone to cracking and distortion than its carbon-rich relative, which is a desirable feature, considering the localized nature of an oxy-acetylene flame hardening torch.

High Carbon Steel Flame Hardening

Logic informs us that a higher carbon content equals a greater hardness rating, but this isn’t always a desirable property, at least not when the feature turns brittle and increases the likelihood of a parts-weakening surface crack. Strong by design, toughness must be maximized while avoiding brittleness. Fortunately, as long as the carbon content sits around the 0.4 percent mark, the heat treatment work will likely proceed efficiently and yield a high-carbon martensitic coating.

Different quantities of thermal energy and carbon-rich atmospheres are used to compensate for carbon content, so the same results can be achieved in both forms of steel. The case hardening process also offsets carbon content by adjusting flame pitch and intensity, and by varying the medium used in the quench stage.