Technical glossary for knife steel
Elements which are intentionally added to the steel to modify the properties. In addition to iron, a stainless steel generally consists of carbon and chromium. It may also contain small quantities of vanadium, molybdenum, nitrogen and other elements.
When iron is heated to above 910°C (1670°F), its microstructure changes to austenite. Austenite is characterized by being non-magnetic, soft and ductile.
After quenching and transformation from austenite to martensite, it is beneficial to retain a small amount of austenite for increased toughness. This is called retained austenite (RA).
Carbides are hard particles that are wear resistant, but at the same time brittle and difficult to grind.
These are formed during the primary production stage and are large, being up to 40 microns in diameter. They are very stable, which means that they do not dissolve into the matrix during heat treatment.
The secondary carbide structure is formed during hot rolling/forging and annealing of the steel. These carbides are small, the average size is about 0.5 microns in diameter. The small carbides contribute to good wear resistance, but without compromising sharpness and regrindability.
The chemical composition is the contents of carbon and other alloying elements added to the iron base of the steel. The composition should be well balanced, not over-alloyed and accurate. The specification tolerances must be tight in order to secure a consistently high quality of the finished knife.
Read more about the chemical composition of knife steel
During austenitizing the carbides are gradually dissolved and carbon and chromium are released and can diffuse in the steel matrix. This enables both high hardness and good corrosion resistance after completed heat treatment.
Deep-freezing to -20°C to -150°C (-4 to -238°F) can be started after hardening, when the material has been quenched to room temperature, in order to increase the hardness. In the hardening recommendations in this guide, Sandvik presents only -20°C and -70°C (-4 to -94°F)as possible deep-freezing temperatures.
The ability to allow deformation without fracture.
Edge performance comprises three elements: Sharpness, edge stability and wear resistance.
This is the ability of the knife edge to withstand edge rolling and edge micro-chipping. Rolled edges and micro-chipped edges are the most common reason for regrinding.
Edge rolling occurs when an edge rolls or folds as a result of being subjected to high forces. Typical behavior for softer steels, since hardness will counteract this behavior.
Edge chipping or micro-chipping
In this process, carbide particles or steel fragments break away from the edge. This usually occurs in brittle steels with large carbides (coarse grades) or extremely high carbide density (powder metallurgical steels).
Hardening is a way of making the steel harder. By first heating the steel to between 1050 and 1090°C (1922 and 1994°F) and then quenching it, the material will become much harder and wear resistant as the microstructure transforms into martensite.
Simultaneous hardening of a large number of products, usually in a vacuum furnace.
Read more about batch hardening
Hardening of individual products in a fairly small furnace or a belt furnace.
Read more about piece hardening
Time-temperature progression for the hardening process.
If the steel is heated to an insufficiently high temperature or for too short a time, an insufficient amount of carbides will be dissolved. This will result in low hardness and inadequate corrosion resistance.
If the hardening temperature is too high or if the heating time is too long, almost all carbides will be dissolved. This will result in low hardness and brittleness of the material.
A steel becomes martensitic when its austenitic structure is rapidly quenched.
Martensitic stainless knife steels become stainless only after heat treatment. Sandvik makes only martensitic stainless knife steels.
Equal to one thousandth of a millimeter.
The microstructure of steels is what distinguishes Sandvik fine-grained steels with a maximum carbide size of 2 microns (average of 0.5 microns) from other knife steels such as 440, D2, etc. that have large primary carbides with a diameter of up to 40 microns.
Read more about knife steel microstructure
Corrosion of stainless steels often takes place in the form of a process known as pitting corrosion. Corrosion starts in places where the protective chromium oxide on the material surface is weak and then penetrates into the material. The attack accelerates when the pit grows.
Non-metallic inclusions will always be a weak point in the steel. They are the starting point for corrosion and the crack initiation point that reduces toughness. Sandvik chromium steels have been used for decades in the health care industry around the world, because of their high purity in terms of non-metallic inclusions.
Read more about knife steel purity
Quenching is the rapid cooling from hardening (austenitizing) temperature to room temperature. When a sufficient quantity of carbides has been dissolved during heating, the material must be cooled quickly to room temperature. The purpose of quenching is to retain the carbon and chromium in solution in the matrix, to ensure maximum hardness and corrosion resistance.
Rockwell C hardness (HRC)
Method used for measuring the hardness of steel. The method consists of impressing a diamond tip into the steel with a force of 150 kg (330 lbs). The depth of the impression is then measured. Sandvik knife steels have a hardness range of 54-63 HRC, depending on grade and heat treatment.
Hardened steels are tempered at 175-350°C (347-662°F) for about 2 hours in order to relieve the brittleness caused by hardening. Higher tempering temperatures yield a somewhat tougher material, whereas a lower tempering temperature produces a harder but somewhat more brittle material.
Tempering temperatures above 350°C (662°F) should be avoided, since this would increase the risk of the material becoming more brittle and its corrosion resistance being impaired.
Resistance of the steel to cracking.
If a steel is quenched too slowly, carbides will have time to precipitate at the grain boundaries, which will lead to reduced corrosion resistance along the grain boundaries. The effect of tempering above 460°C (860°F) is similar. The phenomenon is known as sensitizing.
The steel that bonds the carbides together is called the steel matrix. The chemical composition of the steel matrix is what determines the hardness and corrosion resistance of the steel.
A measure of how long the edge retains its sharpness.