The steel of a knife blade is heated during forging, which gives it its hardness. Quenching in an oil bath produces the hardened blade steel, but it is not yet usable as it is. The high brittleness and internal stresses must be eliminated by dosed and uniform reheating during tempering.
Dosed and controlled cooling
Hard materials that are brought to a formable processing state by heating are not ready for use after cooling. In the case of glass and metals, stresses and chemical-physical reactions occur during the cooling process, leading to brittleness and susceptibility to fracture. Controlled reheating during the cooling phase prevents or neutralizes these processes.
In the case of blown glassware, gradual cooling in a curing oven is required. Steel blades of knives are also cooled in a controlled manner in a furnace. This tempering should begin immediately after quenching. Complete heating is a prerequisite for tempering in order to “calm” the material molecules. During this process, the hardness of the blade decreases somewhat, which should already be taken into account when choosing the steel or alloy.
This is how tempering works
The hot knife blade, freshly quenched in an oil bath, is conveyed into the tempering furnace without delay. In principle, any furnace shape in which a specific and controllable temperature can be generated is suitable for use as a furnace. If this temperature control is possible, a forge fire can also be used for tempering.
The blade of the knife is placed in the already preheated furnace or fire. Before this, the so-called scale layer formed during the forging and quenching process must be coarsely removed with 120 to 200 grit emery paper. The knife blank is placed on a base with as few support points as possible. The furnace must supply top and bottom heat to the same extent.
Checking the process
On average, it takes one to two hours to temper a knife blade. 200 degrees Celsius can be selected as the straightening temperature. Observation of the color development of the steel serves to control and, if necessary, readjust the temperature. Straw yellow to golden yellow coloration is ideal. If the coloration darkens, the temperature must be lowered. Common knife steels range in tempering temperature from 160 to 230 degrees Celsius.