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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:
- Pressing into the rod and letting go leads to the edge returning to its original shape (elastic deformation)
- Pressing into the rod and letting go leaves a roll in the edge (exceeded the yield stress and permanently deformed the edge)
- 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 read more