How Is Steel Hardened? Cooling It Fast Enough to Trap the Carbon

Steel is iron with a small proportion of carbon, and iron has two crystal arrangements that are stable at different temperatures. At room temperature it takes a body-centred cubic form called ferrite, which dissolves very little carbon. Heated above roughly 723 degrees it transforms into a face-centred cubic form called austenite, whose more open structure dissolves far more carbon, so the carbon that was locked in compounds spreads through the iron. What happens next depends entirely on the cooling rate. Cooled slowly, the iron reverts to ferrite and the carbon has time to move out and form layered structures, giving soft ductile steel. Cooled very rapidly by quenching in water or oil, the iron transforms before the carbon can move anywhere, trapping it in a distorted crystal structure called martensite, which is extremely hard and extremely brittle because its own internal strain leaves no room for the planes of atoms to slide over each other.

The main processes are all combinations of heating to a particular temperature and cooling at a particular rate:

• •Annealing, heating and then cooling very slowly, usually in the furnace, which produces the softest and most workable condition and relieves internal stress
• •Normalising, heating and cooling in still air, which gives a uniform fine-grained structure and moderate strength, used to reset a part after forging or welding
• •Hardening, heating and quenching rapidly to form martensite, which is hard and so brittle that a fully hardened part will frequently crack or shatter in use
• •Tempering, reheating the hardened part to a lower temperature and holding it, which allows a controlled amount of carbon to precipitate out, trading some hardness for a large gain in toughness, and is essentially never omitted
• •Case hardening, adding carbon or nitrogen to the surface at high temperature and then hardening, producing a hard wearing skin over a tough core, which is what gears and camshafts need
• •Austempering and martempering, quenching into a bath held at an intermediate temperature to produce different structures with better toughness for a given hardness

Several factors interact and getting any of them wrong produces a part that fails. Carbon content sets the maximum achievable hardness, and steel below about a third of a percent carbon cannot be hardened usefully by quenching at all, which is why mild steel structural sections are not heat treated. Alloying elements including chromium, molybdenum and nickel increase hardenability, meaning how deeply the hardening penetrates, which matters because a thick part quenched in water hardens at the surface while the interior cools too slowly. Quench severity must be matched to the steel and the section, since quenching in water when oil was required causes cracking from the thermal shock and the volume change as martensite forms, which is around four percent and generates enormous internal stress. Tempering temperature directly sets the final properties, and before thermometers smiths judged it by the oxide colours that appear on clean steel as it warms, running from pale straw for a cutting edge through blue for a spring, a technique still taught.

Almost all of this was known empirically long before any of it was understood. Smiths in India, the Middle East and Japan developed sophisticated treatments over centuries, including the Japanese practice of coating a blade in clay of differing thickness before quenching so that the edge cools quickly and hardens while the spine cools slowly and stays tough, producing both a hard cutting edge and a shock-resistant body in one piece, along with the visible hardening line and the curvature that develops as the edge expands during transformation. Wootz steel from South Asia, traded as Damascus steel, had properties that were not reproduced in Europe for centuries and whose characteristic patterns have been linked to carbide banding and, in some analyses, to trace elements in particular ore deposits. The scientific explanation arrived only with metallurgical microscopy in the late nineteenth century, which revealed the structures the smiths had been manipulating, and the practical instructions they had passed down turned out to be substantially correct.

Heating steel above about 723 degrees converts iron to a structure that dissolves carbon, and the cooling rate decides what forms next: slow cooling gives soft steel while rapid quenching traps the carbon as martensite, which is very hard and brittle. Tempering afterwards recovers toughness at the cost of some hardness and is essentially never skipped. Carbon content limits achievable hardness, alloying controls how deeply it penetrates, and smiths judged tempering by the oxide colours on clean metal.