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.
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.
- 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.