Chromium oxide film
Known, that high corrosion resistance of stainless steels, including those, of which make stainless knives, gives very thin, chromium-rich oxide film. This film is so thin - its thickness is about 2 nanometers or 0,002 μm - that it is transparent. This oxide film protects the underlying steel from further chemical interaction with the environment..
When such a protective film forms on steel, say, that steel passivation has occurred or, that the steel is in a passivated state. The ability of steels to passivate increases with an increase in the chromium content in it.. For, for steel to be considered stainless, it must contain at least 12 % chrome.
Stainless knives – only from martensitic steels
Stainless steels are ferritic, martensitic, austenitnye, as well as stainless steel transitional, intermediate types between them. Of all types of stainless steels for making knives only martensitic ones are used - only they are able to provide the required level of hardness. As their name suggests, martensitic stainless steels are heat treated to a martensitic structure.
Features of heat treatment of martensitic stainless steels
The hardening heat treatment of martensitic stainless steels is basically similar to the heat treatment of ordinary carbon and low carbon steels.. It also includes three stages: austenitization (heating for hardening), quenching and tempering.
However, heat treatment of martensitic stainless steels is performed much more slowly for two reasons.:
- carbides, typical for these steels, dissolve in austenite much more slowly, than conventional cementite carbides due to the presence of chromium in them;
- thermal conductivity of these steels is much lower, than conventional carbon steels.
The first factor requires an increase in the duration of heating for hardening., to ensure the dissolution of carbides in austenite. Due to the second factor, higher temperature gradients appear in the product., which can lead to cracking and warping even when heated. Therefore, it is usually recommended to subject complex products to intermediate low-temperature heating to smooth out temperature differences and only then complete heating for hardening..
Effect of carbon content on the hardness of martensite
As for conventional carbon and low alloy steels, the hardness of untempered martensite, its carbon content:
- at carbon content 0,06-0,14 %: 38-49 HRC;
- at carbon content 0,2-0,4 %: 44-54 HRC;
- at carbon content 0,65-1,2 %: 56-61 HRC.
Carbides in stainless steels
Corrosion resistance of martensitic stainless steels is generally lower, than ferritic and austenitic stainless steels. This is due to the presence of an increased carbon content in them.. The thing is, that high carbon content lowers chromium, which could dissolve in austenite. Besides, high carbon content increases the possibility of carbide formation (Cr,Fe)23C6, which "takes" a significant amount of chromium.
When a small carbide particle is formed (Cr,Fe)23C6, then the matrix around it is ferrite or austenite – depleted of chromium. If this impoverishment is, that the chromium content falls below 12 %, then the steel becomes susceptible to corrosion.
Another chromium carbide also plays a role in stainless steels. – (Cr,Fe)7C3.
Requirements for steels for stainless knives
To provide the knife with optimal properties, the steel must have:
- high level of hardness;
- even distribution of fine carbides for high blade wear resistance and blade sharpening ability;
- chromium content, which provides corrosion resistance.
To do this, two basic requirements must be met:
- the austenitic phase of the steel before quenching should have a carbon content of about 0,6 % or higher to create hardness in the range 63-64 HRC;
- the austenitic phase of the steel before quenching must have a chromium content 12 % and higher to achieve high corrosion resistance.
Popular steels for stainless knives
A large number of steels are used in the manufacture of stainless knives in the world.. Not all of them are optimal in terms of the above requirements..
FROMrarer carbon and chromium content in thirteen popular knife steels:
1) 440A (AISI standard): 0,7 and 17,0 %;
2) 440IN (AISI standard): 0,85 and 17,0 %;
3) 440FROM (AISI standard): 1,1 and 17 %;
4)12C27 (company Sandvik): 0,6 and 13,5 %;
5) AEB–L (company Uddeholm): 0,65 and 12,8 %;
6) DD400 (company Minebea): 0,61 and 12,9 %;
7) 425M (Crucible): 0,54 and 14,2 %;
8) 154CM (Crucible): 1,05 and 14,0 %;
9) ATS55 (Hitachi): 1,0 and 14,0 %;
10) ATS34 (Hitachi): 1,0 and 14,0 %;
11) OUT6: 0,6 and 13,8 %;
12) AUS8: 0,73 and 13,8 %;
13) OFF10: 1,0 and 13,8 %.
Disadvantages of knives made of 440A steels, 440B and 440C
With that said, popular American steels (ANSI) 440A, 440B and 440C are not optimal for making knives.
Average content of carbon and chromium - grade and actual in austenite when heating steel for hardening to 1100 ºС is, respectively:
- steel 440A: 0,70/17 % and 0,48/15,1 %;
- steel 440V: 0,85/17 % and 0,52/14,6 %;
- steel 440C: 1,07/ 17 % and 0,56/13,6 %.
For all three alloys, the chromium content in austenite is reduced compared to the grade content. 17 %, but remains above content 12 %, necessary for good passivation. Carbon content in austenite decreases, albeit insignificantly, below level 0,6 %, necessary to achieve hardness 63-64 HRC. Besides, turns out, the alloy with the highest carbon content - steel 440C - usually contains unfavorable coarse particles of primary carbide, which are formed during the solidification of the original steel ingot.
Optimal steels for stainless knives
Only four steels from the above list are optimal. These include the following steels (highlighted in bold in the general list):
- steel 12C27 (Sandvik);
- steel AEB-L (the firm of Uddeholm);
- DD400 steel (Minebea company);
- steel AUS6.
Among other steels, these steels provide the best combination of properties for the material of the knife blade.:
- Hardened hardness level 63-64 HRC, which provides high resistance to wear.
- The level of chromium in austenite when heating steel for hardening is more than 12 %, which provides good corrosion resistance.
- Presence of evenly distributed fine carbides in steel, which provides high resistance to wear, as well as the absence of large primary chromium carbides, which reduce the blade's ability to sharpen.
Source: John D. Verhoeven, Steel Metallurgy for Non-Metallurgists, 2007