Hardness testing

As a manufacturer of precision metal tubing, testing the hardness of alloys is important to our operation. We measure the hardness of our high quality tubes as part of our quality control procedures. These tests are quick to perform, relatively cheap and often non-destructive to the components being evaluated.

Hardness is the ability of any material to resist surface indentation or scratching. It is not a fundamental property of a material, so its value varies according to the test method used. Consequently, we employ several different testing methods depending on the alloys used and the kind of tubing being manufactured.

The basic principal is that hardness is measured from an indentation produced in the material by applying a constant load on a specific indentor in contact with the component surface for a specified time. When testing thin wall or small diameter tubes, commonly used methods such as the Rockwell test are not ideal as the relatively high load (90kg) can distort surfaces or even punch holes through the tube walls. The ball impression can also fall away due to the curvature of the surface. In this context Vickers hardness testing is seen as more accurate. However when using Vickers tests a form of equivalence with the Rockwell scale is often desired. This correlation is not linear and comparison scales vary subjectively and between different metals.

Fine Tubes has performed detailed research with the UK National Physics Laboratory to find a meaningful comparison between hardness scales using different tests. Below we’ve compiled an outline that explains the most common hardness measurement methods and how they compare.

Hardness Conversion Chart

Comparison of Hardness Scales approx. and Tensile Stress Equivalents approx. (maximum value) in imperial and metric units.

Click here to view the chart

Brinell test

An hardened steel or tungsten carbide ball is pressed into the surface for a standard time (10-15 secs) under a standard load. After removing the load, the circular indentation is then measured in two mutually perpendicular directions taking the average of the two readings. The Brinell Hardness Number (HB) is then calculated from

HB = Applied Load /Surface area of impression (mm²)

With a soft material and a large load it would be possible to push the indentor in so far that it could only produce an impression d = D. In order to obtain accurate Brinell values the relationship d = 0.25D to 0.50D must be maintained. Hence the ball diameter and load applied is specified for the material under test. For steels an example would be:

F/D= 30

The Brinell test has the following limitations:

  1. The impression is large (typically 2-4 mm in diameter) and this may act as a stress raiser in a component. It may also be unacceptable on grounds of appearance on car body panel, for instance, while being acceptable on a car cylinder block.
  2. The large depth of the impression precludes its use on plated or surface hardened components as the impression would also measure the underlying structure.
  3. Very hard materials will deform the indentor, hence the Brinell test is limited to materials of up to 450HB for a steel ball, and 600HB for a tungsten carbide ball.

Vickers test

The Vickers indentor is a square-based pyramid with an included angle of 136°, made from diamond. This is designed to overcome the problems inherent in the Brinell test which uses a spherical indentor. The Vickers hardness (HV) is again a function of the applied load on the indentor and size of the resulting impression in the material being tested.

The advantage of 0 = 136° is that HP ≈ HV, up to about 300. The Vickers Test has the following advantages over the Brinell Test:

  1. It is suitable for hard materials as well as soft materials.
  2. There is no need to use the F/D² ratio for the material to be tested because all impressions are geometrically similar. The only criterion for load selection now is that the impression should be large enough to be read accurately. The Vickers hardness range is proportional, so a material of HV 400 is twice as hard as a material having an HV of 200.

The limitations of the Vickers test are:

  1. The impression is small and difficult to see with the naked eye and so the surface of the component must be polished flat with silicon carbide paper and the component surface must be secured perpendicular to the indentor during the test.
  2. It takes a relatively long time to perform a Vickers hardness test.

Rockwell test

The principle of the Rockwell test differs from that of the others in that the depth of the impression is related to the hardness rather than the diameter of the impression. This greatly speeds up measurement because the Rockwell machine is designed to record the depth of penetration of the indentor. There are many Rockwell scales, but the most commonly used are the:

B-scale (1/16 inch diameter steel ball indentor; 100 kg load), used to measure the hardness (HRB) of non-ferrous metals.

C-scale (a 120° diameter cone indentor called a BRALE; 150 kg load), used to measure the hardness (HRC) of steels.

The advantages of the Rockwell test are:

  1. It can be performed quickly and can be integrated with production lines, providing quality control on a line of 
components.

  2. The impression size produced is between those of the Vickers and Brinell tests and some surface irregularity can be accommodated.

It is not as accurate as the Vickers test, which is usually preferred by technologists in research and development work.

A comparison of the most commonly used hardness tests is shown below

Test Indentor Load Typical Applications
Brinell (HB) 1-10mm diameter steel or tungsten carbide ball Up to 3000 kg for depending upon F/D2 ratio of material Forged, rolled, cast components in ferrous and non-ferrous alloys
Vickers (HV) Square based diamond pyramid 1-120 kg All metal alloys and ceramics needs surface preparation
Rockwell
B-Scale (HRB)
1/16 inch diameter steel ball 10 kg minor Load
100 kg major Load
Low-strength steels and non-ferrous up to HV of 240
Rockwell
C-Scale (HRC)
Diamond cone of Brale 10 kg minor Load
150 kg major Load
All metals with a machined surface finish or equivalent.
High strength steels from HV 240-1 000.

Disclaimer: The information provided is for your guidance only and is not intended for warranty of individual application - express or implied.

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