Rockwell Hardness is the Megapixels of Knife Steel Specs

Thanks to Michael Drinkwine for becoming a Knife Steel Nerds Patreon supporter!

Hardness and Megapixels

In the early-to-mid 2000’s with digital cameras and somewhat more recently with smartphone cameras we had the battle of megapixels. The number of megapixels is simply the number of pixels that are captured by a digital camera. When we had 0.3 megapixel cameras the pictures were quite blurry and jumping up to 2 or 3 megapixels made a big difference. However, when comparing 5 to 7 megapixels the quality of the image was much more likely to be controlled by the quality of the lens and sensor than simply the number of megapixels. Despite that, megapixels became an easy marketing point because it is a simple number to present to the public. We haven’t seen rockwell hardness climbing for no reason other than marketing, but it is one of the few simple numbers that are used to advertise for a knife. Therefore it is often misunderstood by knife buyers, and yes, even some knife makers. In this article I cover some simple reasons why hardness is not as important as other factors for predicting most steel properties. And then we get into the nitty gritty with why hardness is not always the same as strength and how heat treatment can affect strength independent of hardness.

Intro to Rockwell Hardness

Rockwell hardness is a simple test for checking the relative strength of materials. It works by indenting steel with a fixed load and measuring the distance that the indenter travels into the steel. It is commonly used by knifemakers, heat treaters, and knife companies. Often the hardness value or a range of hardness is given along with a knife, ie 58-60 Rc or 59 Rc. Hardness correlates well with strength, which tells us how resistant the material is to permanently deforming. With thin edges on knives, resistance to deformation is important to avoid rolled edges. Higher hardness also correlates with higher wear resistance and lower toughness. However, there are times when this hardness value can be misleading. I have summarized a few of those cases below:

Toughness

Higher hardness/strength reduces the toughness of steel. Toughness is the resistance to fracture or chipping which you can read more about in this article. However, there are more factors that control steel toughness than just hardness. For one major example, the amount and size of carbides present in steel greatly controls the toughness of steel, as carbides are very hard brittle particles that promote fracture [1]. Therefore, hardness cannot be used as a proxy for toughness. A 62 Rc steel of one type may be tougher than another at 58 Rc.

Even if we focus on only one steel type, different heat treatments can lead to different levels of toughness at the same hardness. If a steel is austenitized at too-high a temperature, toughness can be greatly reduced. You can read more in this article on austenitizing. In our toughness testing of CruForgeV, we even found a rather sharp drop in toughness even when using an austenitizing temperature recommended by the datasheet, from 14 ft-lbs with 1500°F austenitize to less than 2 ft-lbs with 1550°F:

Wear Resistance

Higher hardness increases wear resistance [2][3][4][5]. However, just like toughness the carbides cannot be ignored. The high hardness of carbides leads to improved wear resistance even when the steel may be at a lower hardness. Wear resistance correlates with slicing edge retention. Therefore, hardness should not be used as a proxy for wear resistance. L6 is a low alloy steel with relatively soft iron carbides (cementite). A2 and D2 contain harder chromium carbides but D2 with its 15% carbide is more wear resistant than A2 which has about 1/3 as much carbide. 10V and 9V have a similar amount of carbide as D2 but have the much harder vanadium carbide for better wear resistance. Cementite is about 1000 on the Vickers hardness scale, chromium carbide 1500 Hv and vanadium carbide 2800 Hv [6]. Note that in the plot below lower is better.

Tensile and Compression Testing

Hardness correlates well with strength. A more complete strength test is called the tensile test where a bar of steel is pulled until it breaks. A compression test is similar but usually uses a cylinder and the steel is compressed until it breaks. The two stress-strain “curves” end up looking similar except for brittle materials which fail prematurely in a tensile test. 

There are three things I want to point out in the tensile test: elastic deformation, yield point, and ultimate stress/strength. Elastic deformation is the period where flexing or pulling a piece of steel and letting go allows it to return to its original position. I wrote all about this behavior in Why Doesn’t Heat Treating Affect Steel Flex? At the yield point the steel permanently, or plastically deforms, so that when you let go the steel doesn’t return to its original shape but stays deformed. The stress (load divided by cross section) required to reach the yield point is the yield stress. The ultimate stress is the stress required to break the material. If we think about this in terms of a knife edge being pressed into a rod, we see the same three regions:

1. Pressing into the rod and letting go leads to the edge returning to its original shape (elastic deformation)
2. Pressing into the rod and letting go leaves a roll in the edge (exceeded the yield stress and permanently deformed the edge)
3. Pressing into the rod until the edge chips (exceeded the ultimate strength)

This is all important because we need to understand what the hardness test is showing us. Hardness of steel usually correlates very well with both yield stress and ultimate stress. However, there are certain cases where yield stress and ultimate stress do not correlate with each other. A steel may have a low yield stress so it is relatively easy to deform but a high ultimate stress. In that case which is the hardness test measuring?

Retained Austenite

When steel is heat treated it is heated up to a high temperature where a phase called austenite is formed, followed by quenching the steel rapidly to form the hard phase called martensite. In certain cases a certain amount of austenite is “retained” after quenching. Learn more about quenching and retained austenite in this article. Retained austenite is much softer than martensite. Higher austenitizing temperature generally means higher hardness because more carbides are dissolved and more carbon is in solution to strengthen the martensite. However, higher austenitizing temperature also means more carbon and alloy in solution which stabilizes more retained austenite. Therefore at a certain point the hardness no longer increases with higher austenitizing temperature but decreases because of excessive retained austenite, such as in this example with Uddeholm Caldie [7]:

If excessive retained austenite is formed and the hardness is somewhat reduced vs the target, the knifemaker may reduce the tempering temperature to maintain the same hardness as when the optimal austenitizing temperature is used. However, the yield stress is changed by the retained austenite content as well [8]:

The heat treatments above used a constant 200°C (392°F) temper for the 19-28% retained austenite conditions but changed the austenitizing temperature with 1020, 1050, and 1075°C. The 4% retained austenite steel used 1020°C along with a high temperature temper, 525°C (977°F) which greatly reduces the retained austenite. Those three heat treatments led to a similar hardness, 60 Rc, but led to a significant difference in yield stress because of retained austenite. Therefore, a knifemaker or company may produce knives with a target hardness assuming that the hardness value correlates with resistance to edge rolling, but that may not be the case.

For one more example, here is a study that compared yield stress and hardness and found that retained austenite in 52100 led to a similar behavior [9]:

Tempering of Steel

Another case where yield stress and ultimate stress may not perfectly correlate is with tempering of steel. With untempered steel there is a low yield stress relative to its ultimate stress and hardness [10]. With low temperature tempering the ultimate stress is reduced but the yield stress is increased. With further tempering the yield stress will eventually reach a point below the original untempered state due to the overall softening of the steel.

This increase in yield stress is seen with tempering temperatures up to about 250°C (482°F). Over this low tempering temperature range, hardness and ultimate stress is reduced but yield stress actually increases, as seen here with 4350 steel [11]:

Tensile or compression testing of tool steels is pretty difficult to find, but here is a plot showing similar behavior with D2 steel [12]:

So just like with retained austenite the knifemaker may be deciding to temper at lower temperatures in an attempt to increase strength but the property they are changing may be different than what they think they are. Reducing the tempering temperature from 400 to 300°F may increase hardness and ultimate stress but reduce yield stress. Therefore the resistance to edge rolling may not be increased despite the increase in hardness.

Strength vs Geometry

Regardless of what strength level a steel is heat treated to, there is still a balance with design. Thinner edge geometries require higher strength to avoid edge rolling. As a knifemaker you must test knives to ensure the steel and heat treatment is sufficient for supporting the design and if not changes must be made. As a knife buyer there has to be some trust that the knifemaker has appropriately designed the knife. As a knife user the sharpening process, such as choosing the edge angle, can be modified to enhance the properties for the use case. If the knife will be used for slicing soft materials only, then a more acute edge can likely be used. When the edge begins chipping or rolling then you know you’ve gone too fine. If a knife will be used more roughly then a more obtuse edge may be appropriate. Regardless of the design the user still has some responsibility for ensuring the strength, toughness, and geometry combination is sufficient for the task at hand. 

Summary

Hardness is a measure of strength, but is not a perfect measure of it. Higher yield strength means better resistance to edge deformation but rockwell hardness does not always accurately measure yield strength. Eliminating retained austenite increases the yield strength of steel even when the hardness is the same. Retained austenite can be reduced by ensuring the austenitizing temperature is not too high, or using cryo or high temperature tempering. Knifemakers should be careful when selecting very low tempering temperatures (<400°F) when trying to increase strength because the higher hardness may not mean a higher yield stress. While higher hardness correlates with lower toughness and better wear resistance, other factors such as carbide volume, size, and type are often more important than hardness. Knifemakers, buyers, and users all have a responsibility for ensuring the strength is sufficient for supporting the knife geometry for its intended use. 

---

[1] https://www.uddeholm.com/files/PB_Uddeholm_vanadis_8_english.pdf

[2] https://cartech.ides.com/datasheet.aspx?i=102&E=125&FMT=PRINT

[3] https://cartech.ides.com/datasheet.aspx?i=102&E=129&FMT=PRINT

[4] https://cartech.ides.com/datasheet.aspx?i=102&E=127&FMT=PRINT

[5] https://cartech.ides.com/datasheet.aspx?i=102&E=111&FMT=PRINT

[6] Theisen, W. “Hartphasen in Hartlegierungen und Hartverbundstoffe.” (1998).

[7] https://www.uddeholm.com/app/uploads/sites/54/2018/05/Tech-Uddeholm-Caldie-EN.pdf

[8] Rehan, Muhammad Arbab, Anna Medvedeva, Berne Högman, Lars‐Erik Svensson, and Leif Karlsson. “Effect of Austenitization and Tempering on the Microstructure and Mechanical Properties of a 5 wt% Cr Cold Work Tool Steel.” steel research international 87, no. 12 (2016): 1609-1618.

[9] Park, W., M. R. Hilton, A. R. Leveille, and P. C. Ward. “Microstructure, fatigue life and load capacity of PM tool steel REX20 for bearing applications.” Tribology & Lubrication Technology 55, no. 6 (1999): 20.

[10] Swarr, Thomas, and George Krauss. “The effect of structure on the deformation of as-quenched and tempered martensite in an Fe-0.2 pct C alloy.” Metallurgical Transactions A 7, no. 1 (1976): 41-48.

[11] Krauss, George. Steels: processing, structure, and performance. Asm International, 2015.

[12] Roberts, G A, and Robert A. Cary. Tool Steels. Beachwood, Ohio: American Society for Metals, 1980.