The Rockwell hardness test is a widely used method to determine the hardness of materials, based on the measurement of the depth of permanent indentation.
How the Rockwell Hardness Test Works
The fundamental principle of the Rockwell hardness test is based on the measurement of the depth to which an indenter is forced by a heavy (major) load beyond the depth resulting from a previously applied preliminary (minor) load. This differential depth measurement provides a direct and rapid hardness reading. The test is non-optical, meaning it doesn't require measuring the indentation's size with a microscope, which contributes to its speed and ease of use.
Step-by-Step Rockwell Hardness Test Procedure
The Rockwell hardness test follows a precise sequence of load applications and depth measurements to ensure accurate and repeatable results. Here's a breakdown of the procedure:
1. Preparation and Setup
Before testing, ensure the specimen is clean, free of scale and foreign material, and has a smooth, flat surface. The specimen must be thick enough to prevent the indentation from affecting the opposite surface. The appropriate Rockwell scale (which dictates the indenter type and major load) is selected based on the material's expected hardness and thickness.
- Sample preparation: Ensure the test piece is stable, clean, and has a suitable surface finish.
- Indenter selection: Choose between a diamond cone (for hard materials) or a hardened steel/carbide ball (for softer materials).
- Load selection: Select the minor and major loads corresponding to the chosen Rockwell scale.
2. Application of Minor (Preliminary) Load
The testing process begins with the indenter being brought into contact with the test specimen. A preliminary, or minor, load is then applied. This initial load serves several crucial purposes:
- It seats the indenter firmly into the material.
- It removes any slack from the testing machine's loading system.
- It provides a stable reference point (zero depth) from which subsequent depth changes will be measured.
3. Application of Major (Full) Load
Immediately after the minor load is applied and the zero reference is established, the additional, major, load is applied. This significantly increases the force on the indenter, pushing it deeper into the material. The major load is typically maintained for a specified dwell time (usually 2 to 8 seconds, depending on the material and standard) to allow for the full plastic deformation to occur.
4. Removal of Major Load
After the prescribed dwell time, the major load is carefully removed. Crucially, the minor load remains applied throughout this stage. This allows for the elastic recovery of the material while still keeping the indenter seated and the reference point maintained. Measuring the depth while the minor load is still applied minimizes errors caused by spring-back or recoil of the material.
5. Measurement and Calculation
With the major load removed but the minor load still on, the machine measures the permanent increase in penetration depth. This measurement represents the difference between the depth of indentation caused by the minor load and the depth of indentation after the major load was applied and then removed (with the minor load still present).
The measured depth is then automatically converted into a Rockwell hardness number by the testing machine. The formula for this conversion depends on the specific Rockwell scale used. For regular Rockwell scales, the conversion often follows a formula like:
$HR = 100 - \left( \frac{\text{depth}}{0.002 \text{ mm}} \right)$
For superficial scales, it's typically:
$HR = 130 - \left( \frac{\text{depth}}{0.002 \text{ mm}} \right)$
where 'depth' is the increase in penetration in millimeters.
Key Components and Scales
The accuracy and versatility of the Rockwell test come from its various scales, each defined by a specific combination of indenter and loads.
Indenters
- Diamond Cone Indenter: A conical indenter with a 120-degree angle and a rounded tip (0.2 mm radius). Used for harder materials like hardened steels, carbides, and ceramics.
- Hardened Steel or Tungsten Carbide Ball Indenters: Spherical indenters with varying diameters (e.g., 1/16", 1/8", 1/4", 1/2"). Used for softer materials like aluminum, brass, and unhardened steels.
Loads
The test utilizes two loads:
- Minor (Preliminary) Load: Typically 3 kgf for superficial scales and 10 kgf for regular scales.
- Major (Full) Load: Varies significantly depending on the scale, from 15 kgf to 150 kgf.
Rockwell Scales
There are numerous Rockwell scales, categorized into Regular and Superficial series.
- Regular Rockwell Scales: Use a 10 kgf minor load. Major loads are 60 kgf, 100 kgf, or 150 kgf. Common scales include HRC (Rockwell C), HRB (Rockwell B), and HRA (Rockwell A).
- Superficial Rockwell Scales: Use a 3 kgf minor load. Major loads are 15 kgf, 30 kgf, or 45 kgf. These are used for thin materials, case-hardened surfaces, or when a shallower impression is desired. Examples include HR15N, HR30T.
Here's a table of common Rockwell scales:
| Scale | Indenter | Minor Load (kgf) | Major Load (kgf) | Typical Application |
|---|---|---|---|---|
| HRC | Diamond Cone | 10 | 150 | Hard steels, carbides, titanium alloys |
| HRA | Diamond Cone | 10 | 60 | Very hard materials, thin hard materials |
| HRB | 1/16" Steel Ball | 10 | 100 | Aluminum, brass, soft steels |
| HRN | Diamond Cone | 3 | 15, 30, 45 | Nitrided, carburized, very thin hard layers |
| HRT | 1/16", 1/8", 1/4", 1/2" Steel Ball | 3 | 15, 30, 45 | Thin, soft sheet materials |
Practical Insights and Best Practices
- Specimen Thickness: Ensure the specimen is at least 10 times the depth of the indentation to avoid anvil effect errors.
- Surface Finish: A smooth surface is preferred. Rough surfaces can lead to inconsistent readings.
- Test Location: Avoid testing near edges or previous indentations. Maintain a minimum distance of 2.5 times the indenter diameter from edges and other indents.
- Standards: Adhere to international standards such as ASTM E18 for Rockwell Hardness of Metallic Materials to ensure accuracy and comparability of results.
By following this detailed procedure, the Rockwell hardness test provides a quick, reliable, and widely accepted method for material characterization in various industrial and scientific applications.
