Hardness vs. Toughness
Getting the Best of Both Worlds
In the world of 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 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 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 Agri wear parts is finding that magical blend of hardness and toughness that will give a blade long life
and good durability to stresses and impacts during working in the fields. 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.
Perhaps the best way to explain this is by examples. Think of a steel hammer, and a diamond. The diamond is obviously very hard. However it is not tough. The steel hammer, on the other hand, is not particularly hard (you can easily scratch it with many hard substances, including the diamond). However the steel hammer is certainly tough.
Toughness can be increased by adding 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. For us 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 soil condition is there? In addition, what type of failure mode is being experienced now?
In other words, is the blade chipping, wearing out too early, breaking, corroding? With this your detailed information we can recommend other materials that may perform better.
HEAT TREATMENT & TEMPERING
At First, the Basic Shape of the part is Cut or punched from the Steel Sheet or bar.
Next, hardening is a way of making steel harder. The parts are heated up To 850 to 900°C and Cooled Down (quenched) immediately in a Bath of Liquid. This Creates a Chemical Reaction in which steel is transformed from austenite into martensite, and the hardness increases due to this transformation. That Makes the Steel Harder, but also more brittle.
To reduce the brittleness; the material is tempered, usually by heating it to 250–450°C, to achieve greater toughness by decreasing the hardness of the alloy. This results in a hardness of 43–53 HRC and a good balance between sharpness retention, toughness & Exceptional Durability
A higher tempering temperature will yield a somewhat softer material with higher toughness, whereas a lower tempering temperature will produce a harder and somewhat more brittle material, as shown by the figure below.