Materials Engineering Lab

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01.HardnessTest.ppt
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ME 251
MATERIALS ENGINEERING LAB
Fall 2018

Hardness Test

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What is Hardness?

  • Hardness is the resistance of a material to localized plastic deformation, usually by indentation. It may also refer to:

stiffness or temper

resistance to scratching, abrasion or cutting

  • It is the ability of a material to resist permanent deformation under load
  • The greater the hardness of a material, the greater its resistance to deformation
  • Hardness is designated as a number and has no units

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Applications of Hardness: Prediction of Wear Properties

  • Simple and inexpensive

  • In general, the higher a material’s hardness the better its wear properties

  • Hard materials can be inserted in mechanisms at points of stress to improve the life of the parts
  • For example:
  • Rock crushers have hard plates that make contact with the rock to reduce wear

  • Brick extruding machines use white iron, a hard cast iron, to auger or push clay through dies

Applications of Hardness

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Application of Hardness: Non-Destructive Indicator of Tensile Strength

  • Non-destructive indicator of Ultimate Tensile Strength

  • Charts can relate hardness numbers to tensile strength

Types of Hardness Test

  • Static Indentation Test
  • Scratch Test
  • Damping Test
  • Cutting Test
  • Abrasion Test
  • Erosion Test
  • Rebound Test
  • Plowing Test

Static Indentation Test

  • In these tests, a perpendicular indenter is forced against a material. The dimensions of the deformation zone are used to obtain hardness values.

  • Tests that use this method include: Vickers (HV), Rockwell (HR), Brinell (HB) and Knoop (KHN)

Classification of Static Indentation Tests

  • Hardness measurements can be classified as macro or micro-hardness according to the forces applied and displacements obtained

Macrohardness Tests:

  • Quick, simple methods of obtaining mechanical property data for bulk material from a small sample
  • Macro-hardness tests require an application of loads above 1kgf
  • Common macro-hardness tests include Rockwell and Brinell Hardness tests
  • Not effective for determining coating and surface hardness properties

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Rockwell Hardness Test

  • In Rockwell test, it does not require visual measurements

  • The Rockwell hardness tester determines hardness by measuring resistance to penetration. In this case, the impression depth is measured rather than the indentation diameter

  • With the Rockwell tester, the hardness is indicated directly on the scale attached to the machine.
  • The indenter may either be a steel ball (can be of different diameter) or a spherical diamond-tipped cone with a 120° angle and 0.2 mm tip radius, called a Brale.
  • The type of indenter and the test load determine the hardness scale (A, B, C, etc)

Rockwell Hardness Test

  • With this system, an initial minor load is applied which is followed by a major load
  • On the basis of minor and major load, there are two types of test:

Rockwell

Superficial Rockwell

  • For Rockwell, the minor load is 10 kg, whereas major loads are 60, 100, and 150 kg
  • For superficial Rockwell, 3 kg is the minor load; 15, 30, and 45 kg are the possible major load values
  • Superficial tests are frequently performed on thin specimens

Rockwell Hardness Test

Rockwell Indenter and Load

  • When specifying Rockwell hardness, both hardness number and scale symbol must be indicated

  • The hardness is designated by the symbol HR followed by the appropriate scale identification

  • For example, 80 HRB represents a Rockwell hardness of 80 on the B scale, and 60 HR30W indicates a superficial hardness of 60 on the 30W scale

Type of Material Indenter Load Scale

1) Soft materials such

as copper alloys, soft 1/16" diameter 100 kg B scale

steel, and aluminum alloys steel ball

2) Harder materials,

hard cast iron and 120 degrees 150 kg C scale

many steel alloys diamond cone

For each scale, hardness may range up to 130; however, as hardness values rise above 130 or drop below 20 on any scale, they become inaccurate; and because the scales have some overlap, in such a situation it is best to utilize the next harder or softer scale.

Rockwell Indenter and Load

Precautions for Rockwell Test

  • The indenter and anvil should be clean and well seated
  • The surface to be tested should be clean, dry, smooth and free from oxides (i.e. rust)
  • The surface should be flat and perpendicular to the indenter
  • Tests on cylindrical surfaces will give low readings, with error depending on the curvature, load, indenter and the hardness of the material. Corrections are given in ASTM E140-78
  • The thickness of the specimen should be such that a mark or budge is not produced on the opposite side of the specimen. It is recommended that the thickness be at least 10 times the depth of the indentation. Tests should be made on a material of single thickness
  • The spacing between indentations should be three to five times the diameter of indentation

References

  • Callister Jr., W.D. and Rethwisch, D.G. “Materials Science and Engineering: An Introduction,” 9th Edition, John Wiley & Sons, 2013.
  • Smith, W.F. and Hashemi, J. “Foundations of Materials Science and Engineering,” McGraw-Hill, 2004.

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