Resistance to plastic deformation is referred to as hardness
of the material. A specific type of indenter is used to indent the material and the
determined resistance to indentation is the measure of hardness of that
material. Hardness value is associated with the indentation size which may be
actual surface area or the depth of indentation. Hardness value of a material
depends on other metallurgical properties viz; tensile strength, elasticity,
plasticity, toughness, stiffness etc. Modern Hardness testers are shown below.
How is Hardness determined?
Hardness can be measured in three ways. There are three
methods for the calculation of hardness.
- Indentation – By determining the resistance to plastic deformation.
- Scratch - Resistance of material to fracture/deformation when subjected to scratching or abrasion.
- Rebound – Determination of length to which diamond tip hammer bounce, if it is allowed to drop on a material from a specific height.
Why Hardness test in Welding?
In welding, Indentation method is used to measure the
hardness value. Weldment with hardness above the specific limit may susceptible
to serious cracking. According to quality control plan, weld specimens are
tested so that there is no region in weld that has hardness above the specific
limit. The specific areas where the hardness is measured are;
- Base Metal
- Weld Metal
- Heat Affected Zone
Weld Harndess Examination is classified into two categories;
Macroscopic and Microscopic. Former is used during fabrication where the
specimens of bigger components are prepared. Later is used to investigate the
hardness of smaller sections or the micro components of metal with the help of
microscope and many other modern techniques.
Macroscopic Indentation method includes
1. Brinell Hardness Test
A steel ball of specific diameter is
used to indent the material with specific range of loading force for a
predetermined time (normally thirty seconds). Range of steel ball diameter used
is 2.5-10mm and that of loading force 1-3000kgf. A particular diameter/load
ratio is used for particular type of material in order to get accurate results
for example 30:1 ratio is selected for steel alloys. In this method hardness
value is calculated by dividing the load force by surface area of impression.
For
harder materials tungsten carbide ball is also used in place of steel ball and hardness
up to 600 BHN can be measured. One disadvantage of Brinell Hardness Test is,
too large the indentation due to large ball diameter. This test can’t be
applied for hardness analysis of various HAZ regions where harndess may vary
from region to region. For this, the test is frequently applied for hardness
evaluation of base metals.
2. Vickers Hardness Test
With the same principle to Brinell
Hardness Test, Vickers uses a square based diamond pyramid indenter to make
impression on the material. The 136 degree diamond indenter withstand high load
without any deformation. Also the indentation depth does not have any influence
on hardness, hence load/diameter ratio is not so important in Vickers Hardness
test.
Unlike Brinell, it gives smaller indentations and may be
applied for hardness survey on smaller sections of heat affected zone, weld
runs. The test surface must be flat with the indenter upright to obtain
accurate results and Vickers Hardness Test satisfies this requirement.
3. Rockwell Hardness Test
It is performed by loading the
particular shape and size of indenter on the material to make an impression. Indenter
may be a steel ball or 120 degrees diamond cone and the applying load can be
60kgf, 100kgf or 150kgf.
Procedure
In the start of the test, lower load is applied to make an
impression and after that the required load is applied on the material to make an
indentation. Different scales ranging from A, B, C… K are specified for the
hardness analysis of different materials. For example Scale B is characterized
by employing steel ball indenter and used to measure the hardness of softer
materials while Sale C is characterized by diamond cone and applied on harder materials.
Moreover
Rockwell Hardness Test can also be used to test the hardness of thin materials
like coatings, sheets/strips and ceramics.
Microscopic Examination includes
4. Knoop Test
Like Vickers Hardness, it also uses a pyramid shaped
indenter to make an impression on material but with minor load (ranging from
few gram force to 1 kilogram force). Impression is too small that need some
microscopy aid for visual analysis.
5. Ultrasonic Test
This test is not associated with indentation principle hence
not listed in destructive testing. Here ‘Frequency Resonance Principle’ is used
in this kind of test where a vibrating metallic rod is brought in contact with
the test material. Due to which a change in frequency resonance occurs which is
actually a measure of Hardness.
Moreover potable equipment are also available to test the
hardness of bigger weld components or the weld structure in service. One common
test is Leeb Rebound Hardness Test.
6. Leeb Rebound Hardness Test
Type of a portable hardness where a hammer is rebounded
after striking the material and the length of rebound is measured as the
hardness value. The equipment must be calibrated correctly and personal must be
well trained, in order to get accurate results.
Hardness Value Interpretation
As it is described earlier, different scales are assigned to
different material hardness. Following is demonstration of hardness values with
alphabetic letters.
Examples
- 32 HRC in which hardness is 32 units by Rockwell Hardness with diamond cone using the scale C.
- 438 HV30 in which the hardness value is 438 by Vickers Hardness Test using 30Kgf load.
- 340 HBW in which the hardness value is 340 by Brinell Hardness Test using Tungsten Carbide Ball.
- 55 HB10/500 in which the hardness value is 55 by Brinell Hardness Test using 10mm steel ball by applying 500kg load.