Hardness Testing Methods
How important and useful is material and hardness testing? Consider the information provided and its significance in structural, aerospace, automotive, quality control, failure analysis and many other forms of manufacturing and industry. Determining these material properties provides valuable insight to the durability, strength, flexibility and capabilities of a variety of component types, from raw materials to prepared specimens and finished goods. This article will summarize hardness testing methods of materials with a focus on metals testing, commonly referred to as indentation hardness testing.
Hardness testing is a widely used form of materials test. It’s relatively easy to perform, it’s typically nondestructive, and most of the instrumentation is inexpensive by comparison to other types of material verification equipment. In addition, it can usually be performed directly on the component without significant alteration.
While testing techniques and hardware have significantly improved as the electronics and computer age has advanced, earlier forms included simple scratch tests. These tests were based on a bar that increased in hardness end to end. The level at which the material being tested could form a scratch on the bar was a determining factor in the specimen’s hardness. Later hardness testing forms included scratching material surfaces with a diamond and measuring the width of the resultant line and, subsequently, indentation of the material using a steel ball under force.
With the increased manufacturing requirements global industrialization brought on and then a much more urgent demand during both World Wars, more refined machines and techniques were developed. Accurate, efficient forms of testing were needed in reaction to heavy manufacturing demands, structural failures and the need to design sufficient material integrity into the growing global infrastructure. Recently, significant advances in hardware, electronics and software has led to much more sophisticated hardness testing equipment that can quickly, reliably and with extreme precision provide useful and property-critical information.
Indent Hardness Testing
What precisely is indentation hardness testing? The most basic and commonly used definition is the resistance of a material to permanent, plastic deformation. While other forms of hardness testing such as rebound, electromagnetic and ultrasonic are used in a variety of applications and measure material hardness through other techniques, indentation hardness testing provides a reliable, straightforward and commonly understood test type. It is measured by loading an indenter of specified geometry and properties onto the material for a specified length of time and measuring either the depth of penetration or dimensions of the resulting indentation, or impression (Fig. 1). As the material being tested is softer, the depth of penetration or indent dimensions become larger.
Common hardness testing types include Rockwell (indentation depth or unrecovered indentation), Knoop/Vickers and Brinell (area of indentation). Rockwell testing is the most commonly used method by virtue of the quick results generated, and it is typically used on metals and alloys (Fig. 2). Knoop and Vickers testing are more suitable for thin materials, coatings and mounted metallographic components. Brinell testing applications generally include cast iron, large steel framework and aluminum.
Some hardness testing can be done within seconds with a handheld device. The indent made by the hardness test can either be ground out or can be so small as to not affect the performance or appearance of the component. Because the testing is done to the component itself, each product, or a spot check of products, can be tested before shipping to the customer.
How are these common types of hardness tests performed? The Rockwell hardness test is based on an inverse relationship to the measurement of the additional depth to which an indenter is forced by a heavy total (major) load beyond the depth result-ing from a previously applied preliminary (minor) load. Initially, a minor load is applied, and a zero datum position is estab-lished. The major load is then applied for a specified period and removed, leaving the minor load applied. The resulting Rock-well number represents the difference in depth from the zero datum position as a result of the application of the major load. The entire procedure requires as little as a few seconds up to 15 for plastics.
In the Rockwell test, results are quickly and directly obtained without the need for a secondary dimensional-measurement requirement. The most common indenter type is a diamond cone ground at 120 degrees for testing hardened steels and car-bides. Softer materials are typically tested using tungsten-carbide balls ranging in diameters from 1/16 inch up to 1/2 inch. The combination of indenter and test force make up the Rockwell scale. These combinations make up 30 different scales and are expressed as the actual hardness number followed by the letters HR and then the respective scale. A recorded hardness number of 63 HRC signifies a hardness of 63 on the Rockwell C scale. Higher values indicate harder materials such as hard-ened steel or tungsten carbide. These can have HRC values in excess of 70 HRC. Rockwell test forces can be applied by ei-ther closed-loop load cell or traditional deadweight systems.
Microhardness or macrohardness testing, commonly referred to as Knoop or Vickers testing, is also performed by pressing an indenter of specified geometry into the test surface. Unlike Rockwell testing, the Knoop or Vickers test applies only a single test force. The resultant impression or unrecovered area is then measured using a high-powered microscope in combination with filar measuring eyepieces or, more recently, automatically with image-analyzing software. The Knoop diamond produces an elongated rhombic-based, diamond-shaped indent with a ratio between long and short diagonals of about 7 to 1. Knoop tests are mainly performed at test forces from 10-1,000g, are commonly referred to as microhardness or microindentation tests, and are advantageous for use in small test areas or on brittle materials as minimal material deformation occurs on the short-diagonal area.
The Vickers diamond produces a square-based pyramidal shape with a depth of indentation of about 1/7th of the diagonal length. The Vickers test has two distinct force ranges, micro (10-1,000g) and macro (1-100kg), to cover a range of testing requirements. The indenter is the same for both force groups. Therefore, Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV1000).
Vickers tests are mainly known as macro-indentation tests and are used on a wider variety of materials, including case-hardened and steel components. Vickers indents are also less sensitive to surface conditions than the Knoop test. In both test types, the measured area is used in a formula that includes applied force to determine a hardness value. Look-up tables, digital filar measurement or automatic imaging measurements are a more common and convenient way to generate Knoop and Vickers hardness results. More recently, automatic testing combined with video indentation analysis is becoming an effective and efficient way to perform these tests (Fig. 3).
Another common hardness test type, the Brinell test, consists of applying a constant load or force, usually between 500 and 3,000 Kgf, for a specified time (10-30 seconds), typically using a 5- or 10-mm-diameter tungsten carbide ball. The load time period is required to ensure that plastic flow of the metal has ceased. Lower forces and smaller-diameter balls are sometimes used in specific applications. Similar to Knoop and Vickers testing, the Brinell test applies only a single test force. After removal of the load, the resultant recovered round impression is measured in millimeters using a low-power microscope or an automatic measuring device. Brinell testing is typically used in testing aluminum and copper alloys at lower forces and steels and cast irons at the higher force ranges.
Highly hardened steel or other materials are usually not tested by the Brinell method, but the Brinell test is particularly useful in certain material finishes because it is more tolerant of surface conditions due to the indenter size and heavy applied force. Brinell testers are often manufactured to accommodate large parts such as engine castings and large-diameter piping.
Hardness testing plays an important role in materials testing, quality control and acceptance of components. We depend on the data to verify the heat treatment, structural integrity and quality of components to determine if a material has the properties necessary for its intended use. Establishing a correlation between the hardness result and the desired material property allows this, making hardness tests very useful in industrial and R&D applications. Hardness ensures that the materials utilized in the various items and components we use every day contribute to a well-engineered, efficient and safe world. IH
For more information, contact Bill O’Neill, sales manager, Wilson Instruments, 825 University Ave., Norwood, MA 02062; tel: 781-5757-5873; fax: 781-575-5770; e-mail: firstname.lastname@example.org; web: www.instron.com
SIDEBAR: Technology Advancements
Some of the more recent methods and advancements that show promise in raising efficiency, speed and accuracy of hardness testing involve automatic systems, which are particularly useful on Knoop and Vickers testing.
Two of the most common hardness tests are Knoop and Vickers, which are used in micro and macro testing to determine material hardness based on measuring the size of a diamond-shaped impression left from an application of a specified force. The nature of the test typically dictates a relatively light force, resulting in extremely small impressions that must be measured at the micron level.
Traditional techniques involve microscopes, with objectives of varying resolution integral to the hardness tester, to manually measure through an eyepiece. Predictably, this is a time-consuming, subjective and potentially error-filled process. It is not uncommon for a technician to produce and measure by eye many hundreds of indentations during a day, with fatigue likely compromising the measurement process as the indents increase in quantity. Add to this the need to produce a full analysis of a hardness traverse often consisting of more than 15 indents each, usually several times on a single sample, and the need for advanced techniques becomes evident.
Over the past several years and increasingly in the future, these manual processes are rapidly giving way to automation in every aspect of the process. New techniques are being developed in material preparation, stage movement, results analysis and even reporting. One such technology being implemented into many labs around the world is automatic stage traversing and image analysis of Knoop and Vickers indentations.
An automatic Knoop or Vickers system typically consists of a fully controllable tester, including an auto-rotating turret and actuation in the Z axis for both applying the indentation and focusing on the specimen. Add to this a computer loaded with dedicated software, an automatic XY traversing motorized stage and a USB video camera, and you have a fully automated hardness testing system. After initial setup with samples and a stored program, it can be left alone to automatically make, measure and report on an almost unlimited number of indentation traverses.
The capabilities of current and developing cameras, coupled with the processing capacity of today’s PCs and continually improving software packages, have significantly improved the accuracy, repeatability and dependability of automatic indentation reading.
Past limitations in regard to surface finish, lighting, preset calibration, threshold and pixel sizing have been mitigated and are continually undergoing improvement. The result is increased ability and dependence on “letting the instrument do the work,” contributing to substantial increases in throughput and consistency while freeing up the operator for other responsibilities.
Expanding productivity even further is the ability to utilize larger size XY stages capable of holding two, four or even six samples at a time in an array of fixturing types. Pre-programmed and saved traverses are opened, and samples are aligned in holders. With a single click, the indentation, reading and reporting of a multitude of traverses on each sample is initiated. Autofocus mitigates any issues of indent clarity due to “Z” position variation. Newer software even allows different scales, forces and microscope objectives within and between traverses.
Automated testing is also increasingly beneficial for Rockwell hardness testing, particularly in repetitive pattern requirements such as Jominy testing, where a number of bars can be fully tested and reported unmanned after one click of the mouse.