What is Hardness Measurement?
Hardness measurement refers to a range of methods for measuring a surface’s resistance to a body that is pressed or struck into it.
The measured value will depend on the test object’s:
- Tensile strength and yield point
- Modulus of elasticity
- Dimension
- Surface finish
- Material homogeneity
Furthermore, the geometry of the indentation body, the force (load) on the indentation body, the speed during indentation or impact, and the load time will play a role. Therefore, the hardness measurement value must be accompanied by a unit indication that unambiguously determines these parameters.
Applications for Hardness Measurement
Hardness measurement is used for control of items where hardness/strength is a significant property, e.g., gears, bearing tracks on shafts, toughened shafts, welds, etc.
Hardness measurement is also a simple control in the delivery situation, for sorting or for control of heat-treated items.
Hardness measurement is often used to control the material condition after an unintended (over-)heating, e.g., in operational accidents and fires. In addition to measuring equipment for metals, hardness equipment exists for plastic and rubber, textiles, paint films, etc.
There are two types of devices in hardness measuring:
Stationary Hardness Testers
The stationary devices, which can only measure on items of limited size or cut-out samples;
As a general rule, stationary devices are the most accurate but are partly limited by the size of the item they can handle to measure. Most often, it is cut-outs of cross-sectional samples as part of procedure and production tests that are measured in stationary systems.
Portable Hardness Testers
The portable devices can measure directly on the surface of larger items and are therefore not limited by the size or geometry of the item.
There are a number of standardized conversions between different hardness units when measuring unalloyed and low-alloy steel. Examples of this include: DIN 50150, ISO 4964, and ASTM E 140-84.
As mentioned, the Vickers measuring apparatus provides a very accurate measurement. However, these devices are often so large that it is not practically possible to measure at material transitions or other irregular geometries.
Therefore, measuring the hardness in, for example, weld seams up to flanges, bends, and dimension jumps can pose problems.
Method Principles for Hardness Measurement
Brinell
Ball body ø 2.5 - 5.0 and 10.0mm, steel or hard metal. Loads from about 1kg to 3,000 kg. Speed less than about 0.5 mm/sec
Brinell is mostly used for larger test items and leaves a larger impression in the item which is measured optically.
- Used for larger items
- Good for porous and inhomogeneous surfaces
- Suitable for testing of forged and cast items
The measurement result is found by measuring the resulting ball impression in the item.
The unit is denoted HB or possibly HBS or HBW depending on ball diameter (mm)/load (kg) and load time (sec).
The measurements are standardized in ISO 6506
Brinell with Impact Load
The ball diameter is most often 5mm and the load occurs by a blow.
In one type of equipment, the load is controlled by a calibrated fracture pin that breaks at a fixed load as seen in the figure below:
Brinell Brinell with impact load, principle for type with calibrated fracture pin. A = Indentation body, B = impact/fracture pin
Rockwell C
Diamond cone tip with a cone angle of 120° and a rounded tip. The tip (indenter) makes an impression in the material with a calibrated pressure force and the depth of the hole is measured to determine the hardness.
- Most often used for larger test items
- Often used for quick tests of metallic materials
- Developed for production control
- Quick results
The load occurs in two steps: Preload 10 kp (98N), total load 150 kp (1.470N).
The measurement result is found as 100 minus the difference in indentation depth under preload before and after the total load, measured in a unit of 0.002 mm. - The unit is denoted HRC.
The measurement is standardized in ISO 6508.
Vickers
Four-sided diamond pyramid with a top angle of 136°.
Vickers is a hardness test that can be used for all solid materials, including metal.
- Can be used to test all solid materials (including metal)
- Suitable for many types of tests and applications
- Can measure individual specific welds
The measurement result is found by measuring the average diagonal length of the square impression.
The hardness is denoted HV (Hardness Vickers) and the load is indicated in kp.
The measurements are standardized in ISO 6507/1 and 6507/2.
Rockwell B og N
Rockwell is also found in Rockwell B where the indentation body is a ball with a diameter of 1/16" and a total load of 100 kp (980.7N) and Rockwell N (Superficial) with a smaller preload and lower total load often 3kp (28.4 N) preload and 15, 30 or 45 kp (147, 294, 441 N) total load.
Shore
The Shore method is based on dynamic measurement of the energy absorbed in the test object when a body with a ball tip hits it.
UCI Hardness Measurement
Measurement is done by pressing a vibrating rod with a diamond pyramid tip down into the surface.
The hardness value is a function of the contact area between the diamond and the test object.
Equotip
Equotip measures by an induction principle the exit speed and return speed at a given height above the surface of the test object. The impact element is fired from a spring.
Hardness Measurement Method Development
Hardness measurement started with the development of the Brinell and Rockwell principles, among other things for the control of cannon and rifle barrels. Since then, the other methods have been developed to cover measurement needs within other strength levels and other materials. Some of the methods, especially the newer UCI and Equotip, have been developed to meet the need for both quick measurements and smaller and thus more mobile devices for use on site.
Hardness measurement is widely used today to control the strength properties of items either according to a direct hardness specification for the given item or from the expected hardness of a material of the type in question. The control is carried out both as production, input, and damage control.
For ideal hardened and annealed steel items, there is a direct correlation between the hardness after Meyer (HM) and tensile strength. The difference between HM and Brinell HB is very small as mentioned and therefore the correlation is used directly on Brinell measurement results. The Brinell measurement result divided by 0.3 gives a value very close to the steel’s tensile strength.
The correlation can be used with greater uncertainty on normalized steel. However, it cannot be used on cold-formed/deformation-hardened items. Metals without or with very limited linear elastic work curves will not show this correlation. This applies, for example, to austenitic steel, aluminum, and copper.
Certain damage mechanisms have proven to be directly dependent on the material’s strength. Most relevant in these years are cold cracks-hydrogen cracks, for example in connection with welding; and stress corrosion as a result of hydrogen sulfide in acidic oil and gas environments. Investigation results have formed the basis for specifying maximum permissible hardnesses. The hardness requirements are most often 325 or 350 HV at risk of hydrogen cracks and 22 HRC or converted 248-250 HV at hydrogen sulfide-induced stress corrosion.