Hardness is a property of steel that describes its resistance to permanent deformation, indentation, or scratching. This characteristic is a measure of how well the material can withstand surface wear and abrasion. Testing for this property is a fundamental step in materials engineering and quality control. Determining the hardness of steel is necessary for predicting a component’s wear resistance and verifying that the material’s structural quality meets design specifications. The measured hardness value often correlates directly with other mechanical properties, such as the material’s tensile strength, providing a simple, quick check on overall material quality.
The Dominant Method: Rockwell Hardness Testing
The Rockwell test is the most common industry standard for measuring steel hardness due to its speed and the direct reading it provides. This method measures hardness based on the depth of permanent indentation left by a specific indenter under a defined load. The process begins with applying a minor load, or preload, which seats the indenter and eliminates the effect of any surface imperfections. A major load is then applied for a specified duration, causing further penetration into the material.
The final Rockwell hardness number is calculated from the difference between the indentation depth under the minor load and the depth after the major load is removed. This differential depth measurement makes the Rockwell method less sensitive to small variations in surface finish compared to other techniques.
A variety of scales exist for Rockwell testing, depending on the material’s anticipated hardness. For hardened steels and alloys, the Rockwell C scale (HRC) uses a diamond cone indenter and a 150 kgf major load. Softer metals, like aluminum or brass, utilize the Rockwell B scale (HRB), which uses a 1/16-inch steel ball indenter and a 100 kgf load.
Indentation Area Analysis: Brinell and Vickers Testing
Unlike the Rockwell method, the Brinell and Vickers tests are optical methods that determine hardness by measuring the surface area of the resulting impression. The Brinell test uses a large, spherical tungsten carbide ball (typically 10 mm) forced into the material under a high load, often 3000 kgf for steel and cast iron. This substantial indentation provides a mechanical average of the material’s hardness over a larger volume.
Brinell testing is particularly suited for materials with coarse surfaces, such as castings and forgings, where smaller indentations would yield inconsistent results. After the load is removed, the diameter of the circular indentation is measured using a microscope. The Brinell Hardness Number (BHN) is calculated by dividing the applied load by the surface area of the impression.
The Vickers test also relies on optical measurement but uses a diamond indenter shaped like a square-based pyramid with a 136° angle. The resulting Vickers Hardness (HV) is calculated from the average length of the two diagonals of the square impression.
A significant advantage of the Vickers method is its ability to test across the widest range of hardnesses, from very soft to extremely hard materials, because the indentation shape remains geometrically similar regardless of the applied load. The Vickers test requires a smoother surface preparation and mandatory calculation, unlike the direct readout of the Rockwell method.
Testing Thin Materials and Field Applications
When standard macrohardness tests are unsuitable, specialized techniques are necessary for thin materials or for testing components in the field. Microhardness testing methods, such as the Knoop test, are designed for applications involving very thin layers, plated coatings, or individual microstructural features.
The Knoop test employs an elongated diamond pyramid indenter, creating a shallow, asymmetrical impression under very light loads (10 to 1000 gram-force). This shallow indentation minimizes damage to the specimen and is useful for analyzing the hardness profile of a case-hardened layer or a weld zone. The hardness value is determined by measuring the long diagonal of the impression, making it highly sensitive for precise measurements over a small area.
For testing large, installed components that cannot be brought to a laboratory, portable dynamic methods like the Leeb rebound test are used. The Leeb method is a non-destructive technique that measures the energy lost when an impact body rebounds off the material’s surface. A defined impactor is propelled against the material, and the ratio of the rebound velocity to the impact velocity is measured electronically. A higher rebound velocity indicates a harder material, making the Leeb test suitable for on-site quality control of large structures, such as pipelines or large castings.
Choosing the Right Hardness Test
Selecting the appropriate hardness test method requires evaluating several factors related to the material and the testing environment. The material’s thickness is a primary constraint, as the test impression must not be influenced by the supporting anvil. Surface condition is another consideration, with the Brinell test being the most tolerant of rough surfaces, while Vickers and Knoop require a polished surface for accurate optical measurement.
The desired precision and testing speed also influence the choice. Rockwell is favored in high-volume production settings for its speed and direct numerical readout. The Vickers method is chosen when the widest possible range of hardness needs to be covered with high precision. Ultimately, the test must be chosen to match the material’s characteristics, the component’s geometry, and the application’s specific quality control needs.