Testing And Certification

Testing and Certification

Hardness is a characteristic of a material, not a fundamental physical property. It is defined as the resistance to indentation, and it is determined by measuring the permanent depth of the indentation.

More simply put, when using a fixed force (load) and a given indenter, the smaller the indentation, the harder the material. Indentation hardness values are obtained by measuring the depth or the area of the indentation using one of over 12 different test methods.

Hardness testing is used to measure two general characterizations

1. Material Characteristics

  • Test to check material
  • Test hardenability
  • Test to confirm process
  • Can be used to predict Tensile Strength

2. Functionality

  • Test to confirm ability to function as designed.
  • Wear Resistance
  • Toughness
  • Resistance to impact

Hardness Testing Considerations

The following sample characteristics should be consided prior to selecting the hardness testing method to use:

  • Material
  • Sample Size
  • Thickness
  • Scale
  • Shape of sample, round, cylindrical, flat, irregular

Material

The type of material and expected hardness will determine test method. Materials such as hardened bearing steels have small grain size and can be measured using the Rockwell scale due to the use of diamond indenters and high PSI loading. Materials such as cast irons and powder metals will need a much larger indenter such as used with Brinell scales. Very small parts or small sections may need to be measured on a microhardness tester using the Vickers or Knoop Scale.

When selecting a hardness scale, a general guide is to select the scale that specifies the largest load and the largest indenter possible without exceeding defined operation conditions and accounting for conditions that may influence the test result.  

Sample Size

The smaller the part, the lighter the load required to produce the required indentation. On small parts, it is particularly important to be sure to meet minimum thickness requirements and properly space indentations away from inside and outside edges. Larger parts need to be fixtured properly to ensure secure placement during the test process without the chance for movement or slippage. Parts that either overhang the anvil or are not easily supported on the anvil should be clamped into place or properly supported. 

Cylindrical Samples

A correction to a test result is needed when testing on cylinder shapes with small diameters due to a difference between axial and radial material flow. Roundness correction factors are added to your testing result based on the diameter of convex cylinder surfaces. Additionally, it is important to maintain a minimum spacing equal to 2~1/2 times the indentation’s diameter from an edge or another indentation.

Sample Thickness

Your sample should have a minimal thickness that is at least 10x (ten times) the indentation depth that is expected to be attained. There are minimum, allowable thickness recommendations for regular and superficial Rockwell methods

Scales

Sometimes it is necessary to test in one scale and report in another scale. Conversions have been established that have some validity, but it is important to note that unless an actual correlation has been completed by testing in different scales, established conversions may or may not provide reliable information.

MTE offers a wide range of hardness testing to ensure conformity to customer specifications.

The following hardness testing methods are available at MTE…

  • Rockwell (HRA, HRB, HRC)
  • Micro & Macro Vickers (HV0.1 – HV45)
  • Brinell (250 – 3000kg)

We also have a portable hardness tester capable of testing using Vickers, Rockwell and Leeb Hardness scales.  This enables us to test larger and more complex parts that cannot fit on our conventional testers.