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The kinds and grades of hardness in Mechanical engineering

Hardness means the ability of a kind of material that determines if it can be used in a particular application. There are a couple of  hardness testing methods to test the material’s ability. This paper will introduces briefly the hardness problem in mechanical engineering, introduces the hardness of various kinds of materials and the widely used hardness scale.


What't the meaning of hardness in Mechanical Engineering?

It means the capability of a material to prevent partial plastic deformation led by mechanical indentation or wear. Plastic deformation,named permanent deformation too, reveals that the material does not go back to its original appearance. The ductility, elastic stiffness, plasticity, strain, strength, toughness, viscoelasticity and viscosity decides the hardness ability. The local resistance of a solid to an external object is an index to compare the hardness and softness of various materials. Virous materials commonly have different hardness. The ordinary hard materials are ceramics, diamonds and hard metals such as titanium, while wood and ordinary plastics are softer. Cemented Carbide is one of the most commonly used cemented carbide materials in CNC milling machines.


The types of hardness

Hardness of materials can be evaluated by lots of methods. There are three main hardness measurements: scratch hardness, indentation hardness and springback hardness.

  • 1. Indentation hardness: measures the ability of a sample to withstand surface indentation, as well as the ability of the sample to resist deformation under constant load on a sharp object.If  the indentation hardness is higher, the ability to avoid any deformation greaterly resulting from the application of compression.  In engineering and metallurgical fields, indentation hardness is commonly used. The early indentation hardness test way is to adopt a hard indenter with defined geometry and dimensions to press into the material under specific pressures and to record deformation parameters, such as indentation depth. The most ordinary indentation hardness is that of Brinell, Rockvale, and Vix.

  • 2. Scratch hardness: tests the resistance of a sample to permanent plastic deformation because of the friction with a sharp object. The material which is less affected by scratch has higher scratch hardness. The most commonly used scratch hardness test is the mineralogical mohs hardness tester. The idea is that an object made of a harder material will scratch an object made of a softer material. The idea is to confirm the hardest material a given material can scratch, or the softest material a given material can scratch. When testing coating hardness, scratch hardness means the force required to cut the coating.

  • 3. Rebound hardness: called dynamic hardness as well, measuring the “bounce” of the height of the Diamond Hammer from a fixed height down to the material. A higher rebound hardness of the material will lead to a higher rebound when the hammer drops. This hardness is lined with the elasticity of the material under test. The measuring device for rebound hardness is defined a hardness tester.


Kinds of hardness units 

The hardness units include Rockwell C & B (metal) , Brinell (ball, metal) , Vickers (diamond) , Knoop (diamond) , Elana Meyer (rarely used) , Shore A&D (rubber and softer plastics) and MOHS (minerals) . The international unit of hardness is n/mm2. Pascal units are also used to calculate hardness. Vaious kinds of hardness have different measurement scales. Different methods are used to measure scratch hardness, indentation hardness and springback hardness, including brinell hardness, rockwell hardness and knoop hardness, because these units are borned from these methods, so it’s not a direct comparison. However, there are equivalence of different scales between the hardness conversion tables. The similarity is that, on all these scales, the harder material has a higher value.

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