Hardness Testing Basics
Hardness is a feature of a given object, and not a basic physical property. Hardness is defined as the resistance to indentation, which can be detected by measuring the depth of the indentation. In a nutshell, the lesser the indentation, the harder the object. Indentation hardness value can be calculated by determining the depth of the indentation using one of the below listed test methods.
Reason for hardness testing:
Hardness testing is performed for two reasons:
- Material features
- Test hardness
- Check material
- To confirm process
- Predict tensile strength
- Wear resistance
- Resistance to impact
- Confirm the capability to function as designed
Considerations for Hardness testing
The following features are considered before choosing the hardness testing method:
- Sample Size
- Sample Thickness
- Gage R & R studies
- The shape of the sample – flat, cylindrical, round, irregular
If the area is smaller, the load will be lighter to create the required indentation. In small areas, it is specifically mandated to meet the thickness requirements and space the indentations away from both inside and outside edges properly. Larger areas must be fixtured ideally to ensure a safe placement during the test procedure without any chance of slip or movement or slippage. The parts that either not supported on the anvil or overhang the anvil should be clamped or supported appropriately.
At times it is mandatory to evaluate in one scale and report in another scale. There are several conversions with proper validity. However, unless a definite correlation has been done by testing in varied scales, the documented conversions may or may not deliver reliable information. It is highly recommended to refer to ASTM scale conversion charts for low and high hardness range of non-austenitic metals. Refer to ASTM standard E140 as well for detailed scale conversion information.
The type of a given material as well as the expected hardness will decide the test method. The materials like hardened bearing steels have small grain sizes which can be measured by the Rockwell scale due to the high PSI loading and the use of diamond indenters. Materials like cast irons as well as powder metals require a larger indenter, which can be measured with Brinell scales. Very small parts or sections should be measured on a microhardness tester using the Vickers Scale.
While choosing a hardness scale, select the scale that denotes the largest load as well as the largest indenter without going beyond the defined operational and accounting conditions that may influence the test result.
The sample should be thick by at least 10x (ten times) the indentation depth that is expected to be achieved. There are permissible thickness recommendations for regular as well as superficial Rockwell methods.
Gage R&R Studies
Gage Repeatability and Reproducibility Studies were established to determine the ability of operators and their instruments to evaluate the tolerance limits of a given sample. In hardness testing, there are essential variables that prevent the use of standard Gage R&R procedures and formulas with original test pieces. Variation in the sample material and the inability to reevaluate the same depth are two important factors that affect GR&R outcomes. To reduce these effects, it is recommended to study highly consistent test blocks to lessen these built-in variations.
Sample Shape – Cylindrical
While testing on cylindrical shapes with fewer diameters, a correction to its outcome is required due to the alteration between the axial and radial material flow. Based on the diameter of convex cylinder surfaces, roundness correction factors are added to the testing result as well. Additionally, it is mandatory to sustain a minimum spacing of 2~1/2 times the indentation’s diameter from one edge or another indentation.
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