High Carbon Steel Hardening Process

10 November 2017

Carbon is the element upon which all of life is founded. Also, in heat treatment technology, carbon is the primary alloying element in the steel hardening process. Not surprisingly, there are variations on this theme. For example, high carbon steel is a dense alloy, but it’s also a brittle metal. Let’s follow this hardening process and apply some heat treatment wizardry so we don’t end up creating an inelastic product.

A “By the Numbers” Viewpoint

First of all, we lay out the figures. Hardened steel contains between 0.61 percent and 1.6 percent of carbon. That key element makes the alloy hard and unworkable. The metal typically refuses to be bent or welded when it’s this hard. With that beyond hard mechanical characteristic in mind, the heat treated part tends to enter the furnace in its final form. Drawn into this fiery environment, the metal glows orange hot inside the 820°C furnace. Alternatively, a section of the workpiece is flame hardened selectively by this austenitising heat. In this latter case, we could be talking about gear teeth or the working edge of a chisel.

High Carbon Heat Treatment

Produced in an open-hearth crucible, approximately one-percent of the hardened steel contains carbon. Now we’re back at the heat treatment furnace. The high carbon steel part has been exposed to a massive thermal envelope. Pushed to 820°C, the hot workpiece is fully transformed into an austenitic state. Next, the internal crystal structure is quenched in a brine solution. The goal here is to uniformly cool the hard part without adding internal stresses or changing the dimensions of the part. A balance is struck. The high-percentage carbon blends with the manganese and the changed crystalline structure of the iron to produce a hardened steel.

Illustrating Some Fringe Benefits

This finished component is dense and hard, durable and dimensionally consistent. In addition to those key mechanical properties, the part features superior shape memory. So, on top of providing tool edges and hard-wear surfaces, this rigid steel is commonly used in the manufacturing sector as a spring strengthening tool. Picture it, the strong alloy supporting a massive vehicle, a railway locomotive, or some other gargantuan construct. Every time the spring is stressed and stretched, the heat treated metal pulls the coiled alloy segment back into its original form.

Case hardened steels are heat treated in furnaces while wholly hardened high carbon steels are born in an open-hearth crucible. Cast and poured into shape, these parts are entirely rigid. In fact, they’re so rigid that a fracture will take place if the metal is incautiously bent, which is why the heat treatment cycle is so important here as a hardness-to-ductility balancer.

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