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Corrosion-resistant steels are characterised by increased resistance to acids and corrosion. They are basically divided into the categories of austenitic, ferritic, and martensitic steel.
Martensitic corrosion-resistant steels offer high protection against corrosion in the hardened state, excellent wear resistance, and outstanding cutting edge retention.
Austenitic corrosion-resistant steels offer very good corrosion resistance and very good hot and cold form-ability. They are generally cryogenic steels.
Ferritic corrosion-resistant steels are generally low in nickel and exhibit high resistance to chloride-induced stress corrosion cracking.
Soft Annealing: 800 – 850°C, Furnace cooling <550°C, then air cooling
Hardening: 1000 -1040°C, Oil quenching in a MEDIUM oil, we recommend PQ20 (DO NOT use vegetable oils, it is not designed for this process, and you will not get the constant results you need)
Tempering: 180 – 200°C, (preferred tempering range), for 55 – 58 HRC.
For the tempering diagram see: https://www.lohmann-stahl.de/fileadmin/redaktion/downloads/1.4528.pdf
Among the modern non-powder steels, the martensitic, cobalt-alloyed N690 steel from the Austrian-Swedish company Böhler-Uddeholm AG is currently the most popular. And in the past few years, it has almost superseded its ancestor - classic American 440C steel, which is very similar in composition, but contains more molybdenum and cobalt. According to the manufacturer, the steel is intended for production of cutting tools, including knife blades, cutting surgical instruments, rotary knives for the meat-processing industry, corrosion-resistant roller bearings, pistons for refrigeration machines, etc. The main qualities of N690 are: good strength and ductility, high wear resistance, good machinability, dimensional stability, polishability and high corrosion resistance.
The main advantage of this steel in the production of knives is undoubtedly its balanced composition. This alloy is well resistant to corrosion, can be heat-treated to 60 HRC and retains high impact toughness. However, like 440C, N690 is inferior in sharpness retention to its popular competitors, such as D2 or PGK. The closest to it in composition, apart from 440C, are Japanese VG-10, AUS-10, as well as Russian 95X18. However, more advanced manufacturing techniques and higher quality give Bohler's products a certain edge. In particular, the use of cryogenic treatment with liquid nitrogen is recommended for hardening. This results in the removal of residual stresses and reduces the steel's tendency to chip, which is often found on its analogues steels when working hard with rough materials.
This steel is often used on folding knives as well as on various fixed-blade camping and hunting knives. It should be noted, however, that for camping knives it is more suitable, as the steel does not have a high retention of sharpness when used on complex and quite abrasive materials, such as bear skins or large hoofed animals. In camping conditions, on the other hand, the steel performs very well. In a moderate climate this steel does not rust, withstands considerable stresses on the edge of the blade, and can be used for chopping. This steel is also suitable for folding knives, primarily because of its high corrosion resistance and good ductility.
Steel composition:
The carbon (C) content is about 1.08%, it increases the strength of the steel and gives the metal the necessary hardness. Carbon is the most important element of the alloy and the amount of carbon in N690 is the golden mean for a non-powder alloyed metal.
The amount of chromium (Cr) is about 17.30%. Chromium affects the anti-corrosion properties of steel and to some extent increases its wear resistance. The amount of chromium in N690 is very high which guarantees excellent corrosion resistance.
The steel contains a sufficiently high percentage of molybdenum (Mo), about 1.10% - which prevents the fragility and brittleness of the blade and gives it the necessary rigidity.
One of the most important elements in modern steels is vanadium (V), the amount of which in N690 is not high - 0.10%. It is responsible for strength, impact toughness and wear resistance. Vanadium also enhances the properties of chromium and makes the knife steel inert to aggressive chemical environments.
The alloy also contains 1.50% of cobalt (Co). It enhances the mechanical properties of the steel.
It is easy to sharpen N690 steel with TSPROF sharpeners. The maximum hardness of the steel is 60 HRC. This makes it possible to work on such blades practically with any abrasives: aluminium oxide (Boride T2 and PC series), silicon carbide (Boride CS- HD, TSPROF CS), as well as diamond bars of the Venev Diamond Plant. The N690 can be sharpened quickly and is easy to deburr if the quality of the heat treatment is maintained.
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