The necessity for this article comes from the common misconception of stainless steels as illustrated by comments from Bob Engnath in an article on steels used by knifemakers1 :
... 440A and 440B are similar alloys, often confused with 440C, but not worth a damn for knife making use.
Steels such as 420HC, AUS-6A and AEB-L fall under similar condemnation and this is a viewpoint promoted by many makers. However this judgement is opposed by many including distinguised individuals such as ASM metallurgist Dr. J. Verhoeven. Quoting from his book on metallurgy for knifemakers 2 :
... Uddeholm AEB-L and Sandvik 12C27, along with the similar steels of Table B1, (DD400 and AUS6) provide the best combination of properties desired in a knife blade ...
This arguement is supported by knifemakers such as Roman Landes and Ulrich Gerfin who also support Verhoeven's viewpoint on ideal stainless blade materials. Well respected knifemaker P. J. Tomes also uses 12C27 in his high end stainless steel knives such as illustrated by the picture on the right. Recently manufacturers such as BRK&T have also started offering knives in 12C27 and there is no question of them "not being worth a damn".
What is the root cause for this harsh criticism of those steels? The reason is noted clearly by Enganth in comments on usenet group rec.knives 3 :
When you use a high alloy steel, like stainless, it has to have quite a bit more carbon content than a plain. carbon steel, in order to make it harden to a degree that will make a good knife.
In detail, chromium in steel will bind to carbon forming hard aggregates called carbides, these are the large white regions in the picture in the right 4 and the carbon bound up in these carbides is not free to hardened the steel. However, concerning 440A, when the steel is properly austenized at 1100 C, 0.2% of the chromium will remain in carbides with 0.48% of the carbon in the steel. This 0.48% of carbon will allow a final hardness of 59 HRC when the steel is quenched in oil and given a cold treatment and tempered at 150C. When the amount of chromium is reduced as for example in AEB-L which has the same carbon content as 440A but a lot less chromium then given the same heat treatment there is more carbon in the steel and less in the carbides and thus AEB-L can achive a hardness of 65 HRC and will have superior wear resistance than the low alloy steels such as 52100.
Why do so many individuals condemn those steels in spite of the actual materials facts which state otherwise? The reality is they are speaking the truth from what they have experienced. Steels such as 420HC, AUS-6A and 440A can be fine blanked due as they have a small volume of carbides. This means that they are much cheaper to make knives from as blanking allows the shape of the knife to be formed in one step. Because of this they are used in inexpensive knives where they are given heat treatments which are less than ideal and often they can be as much 4-6 HRC points softer than optimal. This means a 440A blade at 54 HRC is compared to a 440C knife at 59 HRC and of course the edge on the 440A knife rolls and dents easily and is difficult to sharpen well as it forms large burrs under the stones.
However when heat treated optimally this class of stainless steels which includes AEB-L, 12C27, DD400 and AUS-6A have properties as described by Dr. Verhoeven 2 :
Note the last contention specifically, it is simply an issue of edge stability. 4, 5. High carbide steels such as 440C, S30V and ZDP-189 will suffer from carbide as has been shown clearly in the picture in the right which is from Landes research. Landes directly measured how much damage steels took when the edge was microloaded and the high carbide steels took large amounts of damage when the edge angles were small as the carbides were torn out by the edge loads 5. Note clearly the edge angle condition. Steels such as AEB-L, 12C27 and AUS-6A are thus superior choices in knives sharpened at acute angles, 8-12 degrees per side, when the desired performance is taking and holding a very high sharpness. Note as well that while steels such as 12C27 have lower wear resistance than steels like ZDP-189, the wear resistance of 12C27 is actually superior to 1095 when both are of the same hardness 6.
To understand in detail the performance of a stainless steel which has a low alloy content aside from chromium, simply find its spot on the graph on the right2. There are three general rules for the behavior of steels on that graph. As a steel moves in the vertical line which is with increasing carbon content, then it gets harder and less corrosion resistant. As it moves horizontally, with more chromium content, it will get softer but more corrosion resistant. Lastly, the further a steel to the right of the carbon saturation line the high the amount of carbides and the greater the wear resistance but the lower the edge stability. Note steels which are on the same "tie-lines" have the same hardness and corrosion resistance, but with increasing carbide volume as they move away from the carbon saturation line. As steels get significant amount of other alloying elements such as molybdenum and vanadium then a similar analysis can be performed but this is outside of the scope of this article.
As with all steels, when attempting to judge the performance of stainless steels from the alloy content it is critical to note not just simply the amount of carbon in regards to the ability to harden the steels but the other elements. For stainless steels the amount of chromium is most critical. Steels such as AUS-6A and 12C27 can offer very high performance for knives if the desired ability is to offer optimal cutting ability and high sharpness edge holding, similar to alloy steels such as 52100.
1 : Bob Engnath, Steels used by knifemakers, 1997.
2 : Dr. J. Verhoeven, Metallurgy of Steel for Blade Smiths and Others Who Forge Steel,1997
3 : Bob Engnath, Knife Steels - Why is 440A so bad?,1997
4 : Dr. R. Landes, Messerklingen und Stahl, 2. Auflage, Wieland Verlag, Bruckmühl, Germany. Copyright 2006
5 : C. Stamp, Edge Stability, CutleryScience.com, 2007
6 : P. Ericson, Sandvik 12C27-stainless steel for edge tools, Steel Research Centre, Sandvik, Sweden (1981).
|Written: Sept. 2007||Updated: Aug. 2007||Copyright (c) 2007 : Cliff Stamp|