Carbon: Getting Heated

Carbon’s role in the treatment and properties of steel is best explained through a steel’s fabrication and subsequent heat treatment. A plain carbon steel, up to 0.8% carbon, that has been forged and taken directly from the machine, will show a structure of ferrite (iron) and iron carbide, or a mixture of ferrite and pearlite. Pearlite, which resembles mother of pearl when viewed under a microscope, is a mix of iron carbide and some of the ferrite.  In fact, it is composed of alternating layers of ferrite and iron carbide. The rest of the ferrite has no carbon in it. 

carbon steel under a microscope

When it comes to heat treating steel, it is heated to a temperature of around 850ºC, 1560 ºF, and held there to get everything into equilibrium. This is called soaking, and what happens here, at this temperature, is that the ferrite and pearlite transform into austenite. This is a high-temperature form of iron with a different crystal structure from that of ferrite. All the carbon dissolves in the austenite at this high temperature, and the carbon is said to be in a solid solution. Austenite is named after Sir W.C. Roberts-Austen, an English metallurgist.

1025 carbon steel cylinder

The next stage following soaking is cooling. If the forged shape is complex, there will likely be several different structures within it. As such, it will be advantageous to perform a normalizing treatment, which involves air cooling from 850 C, prior to the next heat treatment. Normalizing removes the stresses from hot working, and in some cases – particularly in lower carbon steels – improves machinability. The structure from normalizing reverts to ferrite plus pearlite. 

If the steel is cooled very slowly in the furnace, it’s being annealed or softened, and the pearlite constituents have the chance to come to a state of near equilibrium, when they will be softer. It should be noted that the slower steel is cooled from a high temperature the softer it will be. This means, of course, that the faster it is cooled the harder it will be.

A plain carbon steel, one with no significant alloying additions, must be quenched in water to harden it. And the reason steel hardens when quenched in water is all to do with martensite. This phase is named after Adolf Martens, a German metallurgist, and is the phase formed when austenite is cooled quickly. It’s super stressed, sort of how we’d feel if, like the Finnish, we went straight from a sauna to a snowbank. The carbon atoms don’t have the time to wander to the places in the ferrite matrix (background) they have when the steel is cooled relatively slowly. They have no choice but to take up different locations, and in so doing they create a very different form of iron/carbon, namely martensite, which is very hard and brittle. Hence the need to temper steels, a reheating process following quenching. A choice of tempering temperature will be determined by mechanical property requirements, plus past experience. There is another phase, midway between martensite and pearlite, called bainite. This was named after Edgar C. Bain, an American metallurgist.  Bainite is not used for industrial carbon steel parts in open die forging or seamless rolled rings.  However, carbon steels in the phases of austenitic and martensitic are widely used for rough machined parts that are then finish machined to drawing dimensions for components within heavy industry.  All Metals & Forge Group can manufacture industrial forged parts using an industry standard material specification to the specific chemistry and mechanical properties needed for machine assembly.