Tech Showcase: Which Hardness Test Works?
On a macroscale, the hardness of a material as a whole is used for comparison with service experience or a general property. One way of determining a material’s hardness on a macroscale is through indentation testing, a measurement of a material’s resistance to permanent deformation. Applicable methods of indentation testing are the Brinell, Rockwell and Vickers hardness tests. Indentation testing is used for applications on a microscale as well, testing constituents or grains of metal to provide the most useful method for data correlation. Hardness tests that must be carried out on a microscale may be thought of as measurements of a particle’s hardness; the Knoop and microcharacter hardness tests can be used for this purpose. In general, there is no simple relationship between the results of the different indentation tests, although conversion tables for hard steels do exist.
Another type of testing at a macroscale is rebound hardness testing, or dynamic hardness testing, in which a scleroscope is used to measure hardness by tracking the height of rebound of a steel hammer after falling on a specimen from a fixed height.
The Brinell test is widely used on large parts such as castings and forgings of low to medium hardness. The Rockwell and Vickers tests are mainly used on small parts and those of low, medium or high hardness. The scleroscope, which leaves no depression on the test specimen, is used in quick, routine inspection and shop tests. Tests performed on a microscale, such as the Knoop and microcharacter, are used for measuring the hardness of very small parts, thin sections, thin cases and individual grains of particles.
When deciding on a hardness test, the thickness of the specimen tested must be considered. The thickness must be such that the backing material or anvil on which the specimen rests has no effect on the testing machine’s penetrator; an imprint on the specimen’s undersurface after a test is an indication that the specimen was too thin. When such an imprint occurs, a test using a lighter load or a larger indentor should be performed.
Some types of hardness testers are available in both manual and motorized models. Motorized models are well suited to production testing of simply shaped, small pieces that can be fed or placed into a test quickly. For heavier and more complicated pieces, a motorized tester can lessen operator fatigue.
What follows is an offering to quality professionals of today’s hardness testing equipment.
When used on a microindentation tester, the Minuteman ELT Series is an upgradeable system that allows the operator to start with a basic system and progress to a fully automatic version as needs and budget change. The entry-level ELT1 is a video filar system for measuring the indents on the computer’s monitor using a virtual joystick for easy, on-screen operation. The ELT1 has all of the report writing capability of the higher-level systems, such as multiple graphing, MS Excel export, statistics and case depth plotting.
The ELT2 option adds image analysis capabilities to the software to automatically measure the size of the indent, eliminating the need for manual measurement by the operator. The Minuteman’s image analysis indent software program provides high indent measurement accuracy on a range of surface conditions.
The fully automatic Minuteman ELT3 and ELT4 versions have a large motorized stage, which can accommodate multiple samples. The stage has an integrated controller that eliminates bulky hardware. Jominy tests and any other repetitive operations can be automated using this fully automatic system. All of the Minuteman ELT systems have complete computer control of the tester for hands-off operations.
Following the ASTM procedures for testing in Rockwell, the indicator is initialized then a minor load is established with a known hardness force. This eliminates surface variations before setting the tester to a zero point. Next, the major load is applied. After reaching the major load, it is removed to measure the difference between the major load point and the minor load point. This distance is calculated into a Rockwell number. The standard Ames tester includes everything needed for testing in the A, B, C, D, F and G Rockwell scales. Three certified and traceable test blocks are included. Additional balls and special anvils are available. The testers are available in 10 configurations.
The system offers a one-touch automated testing operation that requires minimal operator involvement. Hardness numbers are displayed from 1 to 1/100th point on an illuminated digital control. To comply with ASTM E-18 tolerances, the system provides preset dwell times and automatic cylindrical correction. Built-in electronic conversion charts for cross-referencing hardness scales and approximating tensile strength eliminate the need for mechanical or electronic adjustments.
The system includes closed-loop NTEP load cell technology that improves repeatability and efficiency, and removes the need to use the conventional weights method. The system also has a rigid, modular base assembly, RS232 output and a detachable clamping device for oversized or irregular parts. A self-contained head assembly is adaptable for special applications. Additional features include upper and lower limits alarms and a fitted tray for indentors and anvils. The system supports minor loads of 10 kilograms, superficial minor loads of 3 kilograms, major loads of 60/100/150 kilograms and superficial major loads of 15/30/45 kilograms. Maximum test height is 10 inches.
To raise and lower the test head automatically, the test stand has a motor drive with hand controls connected to the tester by cable, so the test head can be raised to provide more room for samples or lowered into position for testing. The clamping shield protects the indenter from impact against the test piece while being lowered onto the piece, so operators need not worry about positioning at an exact height and breaking diamond indenters. The test cycle can either be motorized or have the standard lever-pull activation.
The tester mounts to sound structures such as steel columns in a factory. The test pieces can be placed on the floor or on a raised base. Potential applications include testing large diameter pipe, large forgings and automotive parts that cannot be lifted by hand.
Precise control of the indentation process is ensured through the closed-loop force feedback system found in the IBIS Nano-indentation module. The module also offers exclusive multi-frequency indentation testing for those researching MEMS or visco-elastic deformation of plastic and polymer materials as a function of frequency. The scratch tester operates with the force range of 0.1 to 50 newtons and is suitable for coatings on a range of applications including plasma-processed layers, pharmacological applications and optical components.