Can Low Carbon Steels Be Heat Treated?

26 October 2018

A newly arrived batch of mild steel indicates this is a low carbon steel. The carbon quotient is noted, then the load is transported onwards. Putting this scenario on pause for a minute or two, we ask the following question: can low carbon steels be heat treated? Still frozen in time, the engineers ponder a workable solution. Problematically, there’s just not enough carbon in the alloy to facilitate a conventional alloy hardening process.

Sourcing an Affordable Alloy 

So why would a client ever select low carbon steel in the first place? For the simple answer, think about the alloy from a financial perspective. It’s a remarkably affordable metal, and it’s not exactly hard to find. From here, though, problems rear their ugly heads. The alloy is let down by its low tensile strength. An unremarkable 0.05 to 0.25% carbon content label came attached to the stack of workpieces, and that value seems like it’s ingrained. Granted, the alloy is workable and weldable, but it’s also relatively weak, possibly even corrosion-prone. Is this really the best a heat treatment facility can do with this load of mild steel? No, there is a solution, but it’ll require some innovative processing.

Can Low Carbon Steel Be Heat Treated? 

At first, the answer is a dubious yes. The workpieces take on a small degree of case hardened strength, but the inner core is still soft, so those hardened surfaces are only skin deep. Cold worked hardening uses mechanical stresses to further toughen the material, but such stresses can be unpredictable. To turn that lowercase “yes” into a big uppercase affirmative, the low carbon steel workpieces require further processing. Now, instead of transporting the parts through an atmospherically neutral furnace, the engineers recruit the services of a carburization station. This heat treatment technique adds carbon to the furnace atmosphere, which then diffuses into the low carbon alloy. The result is a corresponding rise in mechanical strength.

But low carbon steels aren’t porous. There’s not a single alloy that can lay claim to that property, not unless it’s sintered. Therefore, even by diffusing atmospheric carbon during a heat treatment cycle, the workpieces can only expect additional case-hardened depth. A 0.1 – 1.0mm depth-hardened casing is achievable economically by taking this approach. Expect low-carbon steel to be cut and formed in a machine shop, for these processes are commonly applied on mild steels. Beyond machinability, however, limited case hardening is possible in a conventional furnace. Equipped with a carburization station, that depth can reach down as far as 0.1 – 1.0 millimeters economically, but the core remains untouched and thus not structurally sound.  Greater case depths can be achieved but extremely long furnace soak times are required, which negates the cheap cost of the mild steel material.

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