Hardness Testing Basics

Learn about considerations, sample sizes, sample thickness, scales and hardness testing methods in this article about the 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 Considerations
The following sample characteristics should be considered prior to selecting the hardness testing method to use: 

• Sample Size 
• Cylindrical Samples 
• Sample Thickness 
• Scales 
• Gage R&R 

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 testing results 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
A sample should have a minimal thickness that is at least ten times the indentation depth is expected to be attained. There are minimum 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. 

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. 


Hardness Testing Methods
Rockwell Test Method, as defined in ASTM E-18, is the most commonly used hardness test method. The Rockwell test is generally easier to perform, and more accurate than other types of hardness testing methods. The Rockwell test method is used on all metals, except in condition where the test metal structure or surface conditions would introduce too much variations; where the indentations would be too large for the application; or where the sample size or sample shape prohibits its use. 

The Rockwell method measures the permanent depth of indentation produced by a force/load on an indenter. First, a preliminary test force (commonly referred to as preload or minor load) is applied to a sample using a diamond indenter. This load represents the zero or reference position that breaks through the surface to reduce the effects of surface finish.

After the preload, an additional load, call the major load, is applied to reach the total required test load. This force is held for a predetermined amount of time (dwell time) to allow for elastic recovery. This major load is then released and the final position is measured against the position derived from the preload, the indentation depth variance between the preload value and major load value. This distance is converted to a hardness number.

Brinell Test Method is defined in ASTM E10. Most commonly it is used to test materials that have a structure that is too coarse or that have a surface that is too rough to be tested using another test method, e.g., castings and forgings. Brinell testing often use a very high test load (3000 kgf) and a 10mm wide indenter so that the resulting indentation averages out most surface and sub-surface inconsistencies. 

The Brinell method applies a predetermined test load (F) to a carbide ball of fixed diameter (D) which is held for a predetermined time period and then removed. The resulting impression is measured across at least two diameters – usually at right angles to each other and these result averaged (d). A chart is then used to convert the averaged diameter measurement to a Brinell hardness number. Test forces range from 500 to 3000 kgf. 

Typically, an indentation is made with a Brinell hardness testing machine and then measured for indentation diameter in a second step with a specially designed Brinell microscope or optical system. The resulting measurement is converted to a Brinell value using the Brinell formula or a conversion chart based on the formula.

Ball indenters diameters can range from 10mm to 1mm. Generally, the lower loads and ball diameters are used for convenience in “combination” testers, like Rockwell units, that have a small load capacity. 

Vickers Test Method, also referred to as a microhardness test method, is mostly used for small parts, thin sections, or case depth work. The Vickers method is based on an optical measurement system. The microhardness test procedure, ASTM E-384, specifies a range of light loads using a diamond indenter to make an indentation which is measured and converted to a hardness value. 

It is very useful for testing on a wide type of materials as long as test samples are carefully prepared. A square base pyramid shaped diamond is used for testing in the Vickers scale. Typically loads are very light, ranging from a few grams to one or several kilograms, although "Macro" Vickers loads can range up to 30 kg or more. The Microhardness methods are used to test on metals, ceramics, composites - almost any type of material.

Since the test indentation is very small in a Vickers test, it is useful for a variety of applications: testing very thin materials like foils or measuring the surface of a part, small parts or small areas, measuring individual microstructures, or measuring the depth of case hardening by sectioning a part and making a series of indentations to describe a profile of the change in hardness. 

Knoop Test Method, also referred to as a microhardness test method, is mostly used for small parts, thin sections, or case depth work. The Vickers method is based on an optical measurement system. The microhardness test procedure, ASTM E-384, specifies a range of light loads using a diamond indenter to make an indentation which is measured and converted to a hardness value. 

It is very useful for testing on a wide type of materials as long as test samples are carefully prepared. A pyramid shaped diamond is used for testing in the Knoop scale. This indenter differs from the pyramid indenter used on a Vickers test. The Knoop indenter is more elongated or rectangular in shape. 

The Knoop method is commonly used when indentations are closely spaced or very near the edge of the sample. The width of the Knoop indentation can provide more resolution for measurement and the indentation is also less deep. Consequently, it can be used on very thin materials. 

In a Knoop test, a predetermined test force is applied with a pyramid-shaped diamond indenter for a specified dwell time period. The indenter used on a Knoop test is pyramid-shaped but more elongated than the indenter used on a Vickers test. After this dwell period, the force is removed. Unlike the Vickers test where the indentation length on the vertical and horizontal axes are measured and averaged, the Knoop method only uses the long axis. This measurement is then converted to a Knoop hardness number using a chart. 

Since the test indentation is very small in a Knoop test, it is useful for a variety of applications such as testing very thin materials like foils or measuring the surface of a part, small parts or small areas, measuring individual microstructures, or measuring the depth of case hardening by sectioning a part and making a series of indentations to describe a profile of the change in hardness.  

Case depth is the thickness of the hardened layer on a specimen. Case hardening improves both the wear resistance and the fatigue strength of parts under dynamic and/or thermal stresses. Hardened steel parts are typically used in rotating applications where high wear resistance and strength are required. The characteristics of case hardening are primarily determined by surface hardness, the effective hardness depth and the depth profile of the residual stress. Gears and engine parts are examples where hardening is used. 

Case depth testing often involves performing a series of hardness impressions from the edge of the specimen towards the center. The hardness progression is plotted on a graph and the distance from the surface to the hardness limit (HL) is calculated.

Learn more about hardness testing and see our full range of hardness testers on www.hardnesstesters.com.