Automation Drives Vickers Hardness Testing Into the 21st Century
The Vickers test is well suited to characterization of materials with extremely hard surfaces.
As applications continue to demand more of engineering materials and as the stakes for life expectancy of finished products continue to rise, testing is anticipated to play an ever more important role in product development and quality control processes.
Heat treatment is widely recognized as a parameter that can influence material performance and through the application of novel hardness testing techniques, development engineers and quality lab managers are able to evaluate material properties with greater efficiency. Novel imaging capabilities support assessments over a large area, simplifying analyses of welds and complete weld joints.
Modern Vickers test systems have found acceptance in the most demanding quality control applications.
The most advanced Vickers testing solutions available today apply automated control and specialized software to streamline the testing process.
These systems can survey large areas by stitching together individual test results and coordinates.
Among the most useful and accurate tests for determining hardness in advanced alloys is the Vickers test. The test was developed by the Vickers Company, UK, in the 1920s to test armor plates.
The Vickers test is well suited to characterization of materials with extremely hard surfaces. As such it is also utilized to evaluate welded joints and areas known as the heat-affected zones (HAZ).
In a Vickers test, the surface of the material is subjected to a standard pressure by a four-sided pyramidal diamond indenter for a standard length of time. The diagonal of the resulting indentation is measured under a microscope and the Vickers Hardness value read from a conversion table.
Modern Vickers Test Systems
Modern Vickers test systems have found acceptance in the most demanding quality control applications including the inspection of welds on submarine hulls and heat treatments on heavy duty factory machinery. The most advanced Vickers testing solutions available today apply automated control and specialized software to streamline the testing process. These systems can survey large areas by stitching together individual test results and coordinates.
An example of such an application is found in the design and manufacture of containment structures for nuclear submarines. The containment structures or pressure hulls are composed of welded high strength Naval Quality 1 steel and are expected to withstand significant pressure changes throughout the 30 year life of the vessel. Robust characterization of the steel and weld joints enables engineers to project life expectancy of these structures. The inspection of submarine hulls requires a comprehensive set of destructive and nondestructive tests on parent metal, heat affected zones and welds. Changes in materials, joint design and welding technique can have significant effects on joint strength. The verification of weld quality must be continually assured throughout the production of the vessel. Three parallel runs of hardness tests must be conducted on each side of the welds.
Carrying out these tests manually is time consuming and costly. Limiting aspects of the test using traditional equipment include image capacity and physical motion of the mechanical stage of the equipment.
The obvious answer is the application of advanced automation and testing solutions that not only reduce testing time but also maintain the highest levels of quality control.
A modern Vickers micro/macro, 0.2 through to 30Kgf, hardness testing system incorporating automatic indentation, motorized XYZ stage, multiple lenses and sample scanning software combines automation with accuracy.
Most important to this specific application, automated test systems can incorporate the ability to automatically stitch together images of multiple test points spaced about 250 micrometers apart. Templates included with the software facilitate the process.
Following preparation of samples, the breadth of the surveys can be increased in size from tens of points including heat-affected zones to more than 25,000 points covering the entire weld. Each test point is created with a hardness force of 1Kgf.
The loading and the spacing were selected to maximize the density while still retaining sufficient resolution of the results to differentiate subtle microstructural differences within the fabricated joint.
Automated Vickers hardness testing can also be used as a research and development tool to select better materials and to design and build better pressure hulls. Surveys can be undertaken to compare submerged arc tee-butt-weld-joints manufactured using different types of welds or weld materials. One aim of such studies is the identification of any potential effects of parent plate dilution into the weld material resulting from the weld preparation geometry. Comparisons emphasizing the root run and the proceeding weld runs in their order of sequence yield insightful results.
In these studies, the peak hardness associated with each weld run was clearly shown to associate with the highest hardness regions in the heat affected zone. Small differences in the HAZ width and hardness range are associated with different welding parameters from automated welding practices.
The analysis of the welds identified regions of greater hardness associated with the cap weld bead and the toe weld bead runs. Under 2-D and 3-D imaging the survey can also reveal hardness contouring. Scanning electron microscopy may also be performed using the hardness indent coordinates as a guide.
This level of capability in hardness testing delivers a comprehensive picture of the variations associated with the complete welded joint. Enabling more precise targeting of the fatigue cracks associated with the CTOD test piece to locations of interest within welds is a development that offers crucial insights, particularly where changes in weld technique, joint design and welding consumables are proposed.
A much more comprehensive database of the fracture toughness characteristics of the weld can then be generated which will enhance the safety case for any new design.
Automated Vickers systems support a wide range of testing protocols on heat treatment and surface engineering processes, including vacuum hardening and plasma nitriding, for the aerospace, motorsport, petroleum, medical, tool-making and general engineering sectors.
Quality of process and delivery performance are key to providing customers with the best service possible. ISO 9001 and AS 9100 accreditation assure quality objectives meet or exceed customer expectations.
These standards can be used for processing a wide range of material such as super-alloys, stainless steel and the whole range of tool steels, carbon and alloy steels, copper and copper alloys. In such a demanding environment, hardness testing plays a vital role in the production process.
The modern testing lab must be able to cope with the extensive variety of hardness requirements, from 50HV 0.2 up to 69HRC.
Service, support and equipment calibration, coupled with application experience, must also be factored into the decision to equip a lab with highly automated hardness test machines.