Science: What Happens When Knife Blades Are Hardened?

Heating steel with a medium to high carbon content to a high temperature and then cooling it rapidly imparts hardness to it. Back yard steel workers are able to perform hardening with some precision by following specific steps. But what happens to the metal when it undergoes hardening?

Hardening: Step by Step

Demonstration of the hardening process is available on the internet; there are several videos. In one, the narrator holds a steel knife blade at a 90 degree angle with a pair of pliers in his gloved hand. Then he takes an oxyacetylene torch, set to a neutral flame, and heats the blade to a red hot state. Heat is applied evenly to the surface, so no melt spots or other structural defects are imparted to the steel.

When the entire blade is glowing cherry red, the narrator immediately plunges the knife into a waiting can of old motor oil for rapid cooling, a method called quenching. The blade is swirled around in the oil, so it cools evenly even as flames shoot up and out. When the flames stop and the blade has visibly cooled, it finishes cooling in the air. After the blade has cooled to the touch, the narrator demonstrates the hardness with a file. The file, although able to grip and file the non-hardened shaft, can do nothing more than slide across the surface of the blade; the blade hardened.

Atomic Changes in the Metal

When steel, a combination of iron and carbon, is heated to the critical point (named the Curie point for the temperature at which a material’s permanent magnetism becomes induced magnetism), the crystals in the iron change from ferrite (magnetic) to austenite (non-magnetic). When the crystals become austenite, they can hold more carbon. The carbon dissolves when it’s heated, filling in empty spots around the iron crystals. At the point of saturation, the steel is quenched.

Slow cooling of steel is called annealing. Steel is annealed to make it softer and more easily worked. Annealing also helps smooth out any machining deformities. A knife blade can have its cutting edge put on after the steel is annealed; then it is heated to cherry red and quickly cooled. During quenching, when the temperature cools to a low enough point, the steel tries to return to its low temperature crystal structure, but carbon atoms have taken up the free space of the crystal lattices, so the metal cannot return to its previous state. This phase is called martensite. It is extremely hard.

Why Tempering is Necessary

There are tradeoffs to the hardening, however. When steel is hardened, atoms that could previously slip and slide across and around each other when force was applied can no longer do so. Instead of bending and giving, the metal now breaks or shatters. With hardness comes brittleness. Tempering the steel takes out some of the hardness and gives back toughness.

As soon as the hardened steel has cooled to room temperature, it is tempered. In tempering, the steel is heated for a longer time with a low temperature, and then it is slowly cooled. A standard home oven is often used, since it can reach the low temperature range needed. Ideally, the steel develops the balance of toughness and hardness as required for the uses of that particular steel.

Steel is a versatile metal with an interesting atomic structure. The back yard steel worker can create his perfect knife blade with only a small amount of knowledge of the atomic changes each processing step is creating. Annealing, hardening and tempering are all stages steel can go through on its way to becoming the perfect metal for the job.

Further information