Hardness Testing Basics

Hardness Testing Basics

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 value is obtained by measuring the depth or the area of the indentation using one of over 12 different test methods.


Hardness testing is used for 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 consider prior to selecting the hardness testing method to use:

Material
Sample Size
Thickness
Scale
Shape of sample, round, cylindrical, flat, irregular
Gage R & R

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. 

Roundness Correction

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. 


Thickness

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. Refer to ASTM scale conversion charts for non-austenitic metals in the high hardness range and low hardness range. Also refer to ASTM standard E140 for more scale conversion information. 


Gage R&R 
Gage Repeatability and Reproducibility Studies were developed to calculate the ability of operators and their instruments to test accordingly within the tolerances of a given test piece. In hardness testing, there are inherent variables that preclude using standard Gage R&R procedures and formulas with actual test pieces. Material variation and the inability to retest the same area on depth measuring testers are two significant factors that affect GR&R results. In order to minimize these effects, it is best to do the study on highly consistent test blocks in order to minimize these built-in variations.

Newage Testing Instruments hardness testers
operate are ideally suited for these studies. Unfortunately, since these studies can only be effectively done on test blocks, their value does not necessarily translate into actual testing operations. There are a host of factors that can be introduced when testing under real conditions. Newage testers excel at testing in real-world conditions by reducing the effects of vibration, operator influence, part deflection due to dirt, scale, a specimen flexing under load.