Effects of Adding Carbon to Iron during Heat Treatment

15 May 2019

Carbon isn’t a metal. Actually, it’s a sooty black substance, and it’s not that strong. However, appearances can be deceiving, as every engineer knows only too well. Crushed by incredible pressures, carbon crystals become diamonds, which are the strongest naturally occurring substance known to man. In light of this most helpful material feature, by diffusing carbon into iron, heat treatment engineers know they can create some incredibly strong alloys.

Sourcing Carbon Steel Billets

In its rawest form, post-beneficiation, iron ore possesses a ferrous-rich structure. Ferrite has a cubic crystal structure. Soft and malleable, this post-extracted type of iron isn’t very hard. It’s only during the forging phase that the ferrite gains a percentage of carbon strength, which arrives as a coke-rich forging additive. Strangely enough, the carbon content can exceed usable levels here, so an oxygen blasting system is used to remove the excess.

Heat Treatment: Adding Carbon to Iron

This is where carbon diffusion technology takes charge of the iron hardening process. Supplied as charcoal or as a gaseous carbon monoxide atmosphere, the carburizing work ramps up carbon content. Now the ferrite microcrystalline structure shares alloy real estate with cementite. This hard iron carbide compound forms a laminated (pearlite) grain within the heat-treated workpiece. Just as a check, the total amount of carbon inside this laminated sub-structure barely tops the 0.50% mark. It’s only at the 0.87% level, after more of the ferrite has turned into pearlite, that the carbon steel becomes harder. Too bad, as the cementite/pearlite crystals begin to dominate, that laminated structure starts to exhibit a not-so-desirable tendency towards becoming brittle.

Explaining Transformative Heat Pauses

Energy is a non-destructive resource. Somehow, though, as a carbon-enriched workpiece gets hotter inside a heat treatment furnace, it stops radiating thermal energy. The energy is going somewhere else. Instead of cooking the workpiece, the heat is triggering a series of complex changes within its crystal structure. Bonds are changing, carbon is diffusing, and physical properties are gaining momentum. Fundamentally altered, the brittle pearlite and cementite laminates dissolve. The carbon atoms link to and strengthen the steel crystals until the carbon iron attains gamma phase hardness.

A whole range of austenitic carbon steels takes shape as the carbon dissolves. Again, small percentages of coke are used to produce low and medium carbon steels. The high content variants rarely use more than 1.7% carbon because that 0.87% notch point constitutes a critical baseline level. Tough, hard and malleable at this critical temperature plateau, further improvements are possible as long as the carbon content doesn’t exceed the 1.5 to 1.7% threshold. It’s here, with the pearlite laminate dissolved, that the workpiece should be tempered and quenched so that it stays as ductile as it is hard.

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