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.
Flame hardening technology uses a heat treatment methodology that’s reminiscent of old blacksmithing procedures. Instead of electrically powered induction coils, an oxy-fuel gas flame takes care of the workpiece hardening work. Capable of applying a focused heat source, this is a process that’s used to target reduced geometrical outlines. It’s also a heat treatment technique that’s available in two different forms.
The Spin Flame Hardening Technique
A stationary oxyfuel flame burns its focused heat source while the workpiece to be heat treated spins in place. The spin rate is adjustable. For example, the process can vary in radial velocity to accommodate the circumference of a gear. As the spur wheel spins at a predetermined velocity, the teeth segments absorb thermal energy and harden. Because of the speed and mass limitations imposed by the technology, this heat treatment method is best suited as a moderate-to-large gear hardening solution.
Applying the Tooth-To-Tooth Process
This time around, the shape of the flame head alters. A single cone-shaped flame sinks deep into each tooth and root. Hardening depths go deeper and are applied more uniformly when this newly configured setup is used. Alternatively, two flames are lined up, one on either side of the tooth ramp. With this dual-flame configuration applied, each tooth is treated faster and that hardened case is laid down more uniformly. Of most note here, this progressive scanning technique incorporates a separate quench mechanism. Scanning with the flame, the head then switches to its quench function.
Alternative Flame Hardened Applications
So, why are there at least three flame treatment techniques in use? Well, as mentioned previously, the spin flame method works better on moderately sized gears, and it can sometimes be used on larger parts, too. Small vehicle parts and medium-sized crane gears receive attention from as many as 8 stationary but perfectly focused oxyfuel heat sources. As far as tooth-by-tooth heat treatment goes, the scanning flame and quenching assembly work together to process very large gear teeth. Large crane assemblies, locomotive gears, massive truck parts, all of these massive power transmission systems have much to gain when their teeth and root sections are individually flame-scanned and quenched.
There’s a flip side to the heat treatment coin, as it were. On one side, the oxy-acetylene heat sources harden specific parts areas. That’s a perfectly adequate feature for a gear hardening equipment rig. However, the temperatures and case hardening depth must be weighed against a number of opposing forces. Worst of those, the parts distortion effect cannot be allowed to impact teeth geometry. Highly reputable heat treatment facilities find the point of equilibrium, which exists between a high-tolerance, case hardened end-line-product and one that’s on the verge of distorting.