In the world of industrial razor blades, hardness is a desired quality. Usually, the higher the hardness, the greater the resistance to friction and abrasion and the longer the life of the blade. How do we define hardness? How does this differ from toughness?
Hardness vs. Toughness: Generally as hardness increases, toughness decreases. Toughness is desirable when blades are heavily impacted, hardness when a blade is exposed to corrosive or abrasive materials.
Hardness is related to the amount of carbon in steel. Often the lower the carbon, the higher the toughness. Also, some steels do not perform at lower hardness as they were designed for use at higher hardness.
Indentation hardness tests are primarily used in engineering and metallurgy fields. The tests work on the basic premise of measuring the critical dimensions of an indentation left by a specifically dimensioned and loaded indenter.
Hardness is a characteristic of a solid material expressing its resistance to permanent deformation. The Rockwell or Vickers hardness scales are most commonly used in the industrial blade industry.
Toughness on the other hand is the maximum amount of energy a material can absorb before fracturing, which is different than the amount of force that can be applied. Toughness tends to be small for brittle materials, because it is elastic and plastic deformations that allow materials to absorb large amounts of energy.
The key with industrial razor blades is finding that magical blend of hardness and toughness that will give a blade long life and good durability to stresses and impacts depending on the application. Tool steels like M2, D2 have these most ideal properties. However, you can create ideal qualities with different alloys of stainless or carbon steel as well.
Hardness of steels is typically achieved by heat treating processes. The carbon in the steel combines with various elements in the steel like Vanadium, Chromium, Molybdenum, Silicon etc. to form carbides and other crystalline structures. Steel by its nature becomes harder (and more brittle…less tough) when it is heat treated and quenched.
With Steels, putting together the right mix of materials is key, but how you treat or temper the material also plays a role in the toughness and hardness properties.
As we explained in American Cutting Edge’s January/February newsletter on coatings, often the combination of a “hard” steel and a wear-resistant coating can offer the user the best of both worlds.
One great example of extreme hardness but little to no toughness is carbide razor blades. While they have high endurance for cutting, they withstand very little impact or side load pressure.
Even with “exotic materials” like this you can achieve some level of toughness by working with different binders like cobalt or nickel. Each will add toughness as their concentration increases in the material make up. But you sacrifice some hardness for this.
To combat its lack of toughness, carbide is often brazed onto softer steels, allowing the steel to absorb the vibrations and impact while the harder carbide can retain its extreme wear ability with out fracturing.
For American Cutting Edge to really help a prospect with material selection, we need to know as much as possible about the application: what is being cut, what type of process is being used (i.e. slit cut, crush cut, shear cut, etc), is the application corrosive in any way?, is there a contamination that may influence the process? In addition, what type of failure mode is being experienced now? In other words, is the blade chipping, getting dull, breaking, corroding? By giving your salesperson as much information as possible, we can recommend other materials that may perform better.
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