Quenching and tempering methods for knife steel

Cutting is an operation that is carried out quite often during daily activities, such as the preparation of meals, gardening or other works, and industrial processes. The tools to be used for cutting should have a high resistance to mechanical loads, including abrasion resistance, but also must have good workability to restore the required geometry of the blade or good parameters of contact with water and moisture. To ensure proper operating parameters when selecting a knife steel, alloys with properties that can be improved using quenching and tempering or chemical quenching and tempering are often used. This allows the modification of the characteristics of the surface layer, and frequently also the entire internal structure. We can look more closely at the processing carried out under these conditions.

Heat treatment of steel

Heat treatment processes are performed to obtain a change in the material’s internal structure due to its heating to a specified temperature, followed by cooling. The required operations are performed within strictly defined temperature ranges and in a precisely set time. Heat treatment includes annealing, quenching and tempering, as well as solutioning. During quenching and tempering, the structure of steel changes into a martensitic or bainitic one. At high temperatures, in hypo-eutectoid steels, that is, with a carbon content up to 0.8%, austenite is formed, and in hyper-eutectoid steels, that is, with a carbon content of 0.8–1.35%, austenite and cementite are formed. The next processing stage is cooling, during which the austenite transforms into bainite or martensite, depending on the temperature decrease rate. If the process is to be performed faster, the steel is cooled with water, which ensures a decrease of 600°C per second; if the process is to be performed slower, then oil is used to ensure a decrease of 200°C per second. Quenching and cooling are followed by tempering, which is used to ultimately cure the material. It is indispensable for the internal structure to transform from a metastable to a stable state. If required, it is possible to temper at different temperatures. If higher plasticity and slightly lower hardness are needed, we can also carry out high-temperature tempering at 500–600°C. If high elasticity and elevated plasticity are required, then medium-temperature tempering is performed at 250–500°C. If high hardness is required, low-temperature tempering is performed at 100–250°C.

Heat and chemical treatment of steel

To change the properties of steel, we can also enrich the external layer of the material and provide it with elements that improve the material’s properties within a specific range. These processes are jointly called heat and chemical treatment, and involve an increase in temperature to a level at which the atoms of individual substances can penetrate steel. This is possible due to their post-cooling diffusion and incorporation into an alloy’s crystal lattice, and the relevant effects require a suitable concentration. For this purpose, we can use such processes as nitriding, carburising, and cyaniding, as well as chrome hardening and aluminising. Alloy surface parameters change due to the type of element used. For carburising, it is an increase in hardness; for chrome hardening, it is an increase in thermal resistance; and for nitriding, it is an increase in resistance to corrosion-related phenomena. The heat and chemical treatment technologies, and the forms for the supply of individual elements, are different. They include a liquid solution, such as a salt of a given metal, a gaseous phase, or a solid in the form of a powder. To improve the process, it can be performed under decreased pressure or at ionisation.