So I was at a club meeting recently where we were talking about bowie knives (watch the full video on Youtube) and I asked a few people why they preferred particular steels. Most commonly referred to as carbon/simple steels taking a better edge than stainless steel. When browsing through internet forums and blog posts it’s evident that the general consensus is that carbon steel blades take and hold a better edge. That they are for all intents and purposes, “sharper” than their stainless siblings.
While I myself have found that carbon steels (1084, O1) definitely take an edge more easily than stainless steels, I wondered if that dogma held true today, and if you were prepared to spend the time and effort (with the required skills) when sharpening, could stainless/high alloy steels match or outperform their simple brethren?
What is the difference between stainless and carbon steels?
Steel is an alloy of iron, made primarily of iron and carbon. Other elements can be added to the alloy mixture to impart different characteristics. Stainless steel is a steel that has at least 11-12% chromium added to the composition of the steel alloy.
As “carbon” steels can be both plain/simple as well as high alloy, We’ll try to keep the examples as black and white as possible. ie simple carbon and simple stainless steel so that we are only comparing the difference that chromium brings to the table. Yes, it’s not perfect but it is what is.
Carbides
For the purposes of illustration, carbide-rich steel can be compared to concrete, with the carbides being the aggregate (stones), and the iron/carbon matrix being the cement (cementite).
The actual hardness of individual carbide particles depends on their chemical composition. But, all carbides are not created equal. Some are harder than others [1]. Chromium carbides are about 65/72 HRC, molybdenum and tungsten carbides are about 75-86 HRC, and vanadium carbides are 80/85 HRC.
Smaller carbides are considered better for knife blades as they allow a finer edge and also increase the overall toughness due to there being fewer sites for crack/chip initiation (also referred to as tear/pull-out). This has led to an increase in the popularity of low carbide stainless steels like AEB-L/13C26, 12C27, and 14C28N which have the same advantages as carbon steels when it comes to a fine carbide distribution, ease in finishing, and higher toughness.
Low alloy steels typically used by forging bladesmiths can have good toughness, great ease in sharpening, but the wear resistance and edge retention are relatively low due to the lack of hard carbides.
Wear resistance
The ability of a knife to hold an edge during use is an important subject for both professional and amateur knife makers. The loss of edge radius during use is basically a wear problem that is often ill defined because it is evaluated by qualitative tests that involve cutting speeds, applied forces, and test material that are not consistently held constant. [2]
Steels with high volumes of carbide particles, usually exhibit improved wear resistance (as seen above). These hard little nubs protect the softer steel (cementite/iron carbide) and will allow the blade to hold a working edge for a longer period of time.
Low alloy steels in general (1095, 52100, O1, W2, etc.) have relatively poor edge retention as their wear resistance is affected only by the overall hardness of the steel rather than in together with carbide content.
Sharpening / Edge geometry
When it comes to sharpness or how keen an edge feels, has a lot to do with the geometry of the blade itself. A thinly ground bevel with less “meat” behind the edge (I edge below) may wear down to a larger apex but it will still perform better in cutting tests than a wider wedge / higher angle edge (II edge below). [8]
First is d, the edge thickness/width means less energy is required to initiate a cut. Next is ß, indicating the angle of the edge or primary bevel. α is the secondary bevel. The smaller d, ß, and α are, the better cutting ability and overall edge retention even as the knife dulls with slicing. With a more obtuse edge angle, there is more force required to complete a cut. [3]
The edge utilized the common double bevel geometry. [2]
Generally, very thin edges (8°-10°) dull incredibly quickly because of mechanical damage (edge rolling or microchipping) unless the steel can support such an extreme edge. Steel hardness is related to its compressive yield strength which tells us the stress at which steel begins to deform plastically. This permanent edge deformation causes sharpness deterioration as the plastic deformation grows. Wear-resistant steels do withstand rolling by about 30% better, there is no correlation between the wear resistance and resilience to initial rolling [10].
The secret to sharpening stainless steel more easily has always been in choosing the right tools to sharpen with. A good low grit diamond stone (CBN modern synthetic stones) is recommended for setting your initial angle, especially in the high alloy (vanadium steels). Some tests show that grit doesn’t influence cutting performance as much as you would think, so the grit progression you choose is up to you. Anything over #4000 is “overkill” for most usage cases. A deburring step is always recommended, with or without compound. In the case of carbide-rich steels, stropping without compound may improve the cutting performance as you won’t wear away the supporting steel around the carbides, lessening the amount of pull out. [4]
If you want a wild edge that cuts like crazy the inclusive angle should be about 12°-17° degrees. As the hardness increases 60-63+ HRC you may look at 10° angles. Higher hardness is required to resist edge rolling and deformation but also reduces toughness which increases the possibility of edge chipping. Here a more modern powder metallurgy steel (PM) should have the “edge” as it were, as you generally see an improvement in toughness for any given level of hardness when compared to traditional ingot steel of the same grade.
One of my favourite steels is Bohler’s K110 (D2) which is a semi-stainless, high carbon alloy tool steel. It is a downright PITA to sharpen, but it can be done with time and effort.
Conclusion
Ultimately the world has moved forward and stainless steel can indeed take and hold an edge better than that of its carbon/simple steel counterparts. This is largely due to continued development in stainless and other high alloy steels to meet the demands of the ever-changing world.
- We have found that stainless steel can hold an edge longer than its carbon counterparts due to the presence of carbides in the matrix.
- The high hardness allows both to maintain an initial edge (resist rolling and deformation) as well as resistance to forming a larger burr when sharpening.
- The carbide structure also plays a significant role, as smaller carbides (PM and low alloy steels) lead to higher toughness for better resistance to chipping and also lower possibility of carbide pullout in sharpening.
- Stainless and high alloy steel are inherantly harder to sharpen due to the presence of hard carbides, and therefore take a higher degree of skill, equipment (CBN/diamond abrasives), and or time to achieve a high degree of sharpness.
Some users will want to prioritise sharpness and don’t mind regularly sharpening/honing their knives. In this case, a relatively low allow steel would be appropriate for their needs.
Other users will want to maximise edge retention above all else and hopefully have the skills and equipment to get the best of the high-alloy steels they will no doubt target.
A key takeaway when talking about which steel is better than another steel is the application. We have a large and growing number of steel grades to chose from, each with its own chemical composition that gives them unique properties. It’s up to us as knifemakers, to match these properties to the knives we make in order to maximize their performance.
References:
- Carbide Types in Knife Steels
- Wear tests of steel knife blades
- What is Edge Stability?
- Carbides in Maxamet
- Knife Steel Microstructure
- Testing the Edge Retention of 48 Knife Steels
- Cryogenic Processing of Steel Part 3 – Wear Resistance and Edge Retention
- Maximizing Edge Retention – What CATRA Reveals about the Optimum Edge
- Effect of Steel Hardness on Edge Retention
- Edge Rolling in High Vanadium Knives Sharpened with Aluminium Oxide versus CBN/Diamond
- Edge Stability in Butcher’s and Kitchen Knives as a Function of Edge Angle and Initial Sharpness
