Hardness is one of metallic properties of castings and is defined as the resistance of the permanent mechanical deformation. Hardness testing is a quality test used in foundries to measure the properties of cast metals and their suitability for different applications. Its popularity is due to the non-destructive nature of the test, and its relationship with other mechanical properties. Hardness tests are performed after casting, cleaning and heat treatment of casting products. In the same heat treatment process, the hardness of the casting depends on the amount of carbon in the material. Foundries infer the tensile stress of a material based on the hardness test result. The higher the carbon contents, the higher the hardness of the material is. High hardness affects its cutting performance and makes machining processing difficult. However, if the hardness is too low, the strength of the casting parts and the wear resistance are reduced.
Benefits of hardness tests:
Ability to verify the heat treatment of a part in heat-treating operations and case depth analysis
Ability to determine if a material has the necessary properties for its intended use
Non-destructive test alternative for effectively qualifying and releasing material or components for target applications
Low cost and quick performance
There are different methods to test the hardness of the material including Rockwell hardness test, Brinell hardness test, Vickers hardness test, and Knoop hardness test. The choice of testing methods depends on the size, structure and hardness of products.
Three Methods of Hardness Testing
Brinell hardness test
The origins of the Brinell hardness test stretch back to 1900. In the early years of the test, the results were strongly influenced by the perspective of the operator. Different operators would come up with different results leading to high variability in measurement. However, with the introduction of electronic measuring equipment, the level of consistency has significantly improved.
Brinell hardness test is applicable to cast iron, non-ferro alloy, all kinds of annealed and tempered steels. It is not suitable for samples or work pieces that are too hard, too small, too thin or whose surface is not allowed to have large indentation. Brinell hardness ranges from 8 to 650HBW.
Rockwell hardness test
When the sample is too small or the Brinell hardness (HB) is greater than 450, we use the Rockwell hardness measurement.
Rockwell hardness is an index of hardness value determined by the indentation plastic deformation depth, with 0.002 mm as a hardness unit. The Rockwell test has two stages. The equipment applies a preliminary test force to the sample using a diamond or ball indenter. The purpose of this stage is to break through the surface of the metal and reduce the effect of surface finish on the final result. The operator measures a baseline indentation depth at this point. After holding the preload for a set time, a major load is then applied. Again, the force is held for a pre-set time, before reducing it to the preload force again. Once the time has elapsed, the operator takes a depth measurement of the indentation. The Rockwell hardness number is based on the difference between the baseline and final depth measurements. In the Rockwell hardness test, using different pressure heads and test forces will produce different combinations, corresponding to a ruler with different Rockwell hardness.
Vickers hardness test
Vickers hardness test is performed by pressing a square conical diamond compressor with the angle of 136 ° opposite on the surface of pressure-material. After maintaining the specified time, measuring the diagonal length of the indentation, calculating the surface area of indentation, then dividing the load value by the surface area of the material indentation is the Vickers hardness value (HV).
How To Controls The Hardness Testing Quality
We have two kinds of hardness testers, Brinell and Rockwell hardness testing equipment.
We test hardness of products for each batch after heat treatment both in house and outsourced. The performance of heat treatment is qualified and based on the hardness testing results and mechanical testing results.
Before testing casting parts’ hardness, inspectors calibrates the hardness equipment according to rules. Both Brinell hardness testers and Rockwell hardness testers are calibrated yearly by the third party, which are the same as all the in house inspection equipment, on the basis of IATF16949 certificate’s requirements.