Conducting a hardness test using the Rockwell hardness method can be difficult, especially if you misunderstand its principles and application. Not to overstate it, but without an understanding of the Rockwell hardness method, you might end up going round in circles or, worse still, end up with an inaccurate material hardness value. That is not what we want. This post helps you understand the underlying principles and the various use cases where the Rockwell hardness test method is suitable. Without further ado, let’s dive in.
I. What is the Principle of the Rockwell Method?
You can think of the Rockwell Hardness method as an impact-dependent hardness measuring method. The reason is that it requires an indenter with a known diameter and a specified load to impact the surface of the test material. Indenters in a Rockwell hardness test are either hardened steel balls or a conical diamond. Two sets of loads cause an impact on the material surface, which include an initial minor load and a final major load. Additionally, the impact often causes an indentation on the material surface, and measuring the depth of this indentation helps to calculate the material hardness.
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II. What Distinguishes the Rockwell Method?
The distinguishing factors of a Rockwell method from other methods include
- Type of Indenter – The Rockwell method indenters include a hardened steel ball with four different diameters and a conical diamond indenter. These indenters are unique to the Rockwell hardness test alone.
- Test Load – The range of loads and the combination of initial minor loads and final major loads also distinguishes the Rockwell method from other test methods. The applied load ranges from 15kgf to 150kgf, where a 3kgf is usually the minor load before applying the major loads.
- Scale Division – A combination of the load applied, the indenter type, and the diameter all contribute to form unique scale divisions. These combinations produce up to 30 scales which also distinguishes the Rockwell method from other hardness testing methods.
III. What are the Applications of the Rockwell Method?
ROCKWELL STANDARD TEST METHOD | |||
Test Method | Load | Indenter Type | Application |
A | 60 kgf | Diamond cone | Cemented carbides, thin steel, and shallow case-hardened steel |
B | 100 kgf | 1/16″ WC Ball | Copper alloys, soft steel, aluminum alloys, malleable iron, etc. |
C | 150 kgf | Diamond cone | Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened steel |
D | 100 kgf | Diamond cone | Thin steel and medium case-hardened steel, and pearlitic malleable iron |
E | 100 kgf | 1/8″ WC Ball | Cast iron, aluminum, and magnesium alloys bearing metal |
F | 60 kgf | 1/16″ WC Ball | Annealed copper alloys, thin, soft sheet metals |
G | 150 kgf | 1/16″ WC Ball | Phosphor bronze, beryllium copper, malleable irons |
H | 60 kgf | 1/8″ WC Ball | Aluminum, zinc, lead |
K | 150 kgf | 1/8″ WC Ball |
Bearing metals and other very soft or thin materials, including plastics |
L | 60 kgf | WC Ball | |
M | 100 kgf | WC Ball | |
P | 150 kgf | WC Ball | |
R | 60 kgf | 1/2″ WC Ball | |
S | 100 kgf | 1/2″ WC Ball | |
V | 150 kgf | 1/2″ WC Ball |
IV. What are the Applications of the Rockwell Superficial Method?
ROCKWELL SUPERFICIAL TEST METHOD | |||
Scale Symbol | Penetrator | Major | Typical |
15N | Brale | 15 kgf (3kgf) |
Rockwell superficial scales use lower loads than a regular Rockwell test and are used to test materials too thin or small for the regular scales or outside the normal ranges. Generally, diamond indenters are used for hard materials, while Ball indenters are used for soft materials. These materials often include:
|
30N | Brale | 30 kgf (3kgf) | |
45N | Brale | 45 kgf (3kgf) | |
15T | 1/16″ Ball | 15 kgf (3kgf) | |
30T | 1/16″ Ball | 30 kgf (3kgf) | |
45T | 1/16″ Ball | 45 kgf (3kgf) | |
15W | 1/8″ Ball | 15 kgf (3kgf) | |
30W | 1/8″ Ball | 30 kgf (3kgf) | |
45W | 1/8″ Ball | 45 kgf (3kgf) | |
15X | 1/4″ Ball | 15 kgf (3kgf) | |
30X | 1/4″ Ball | 30 kgf (3kgf) | |
45X | 1/4″ Ball | 45 kgf (3kgf) | |
15Y | 1/2″ Ball | 15 kgf (3kgf) | |
30Y | 1/2″ Ball | 30 kgf (3kgf) | |
45Y | 1/2″Ball | 45 kgf (3kgf) |
V. Factors for the Accuracy of the Rockwell Hardness Test
While the Rockwell hardness test has its uniqueness, its accuracy is also guided by some critical factors you need to consider before engaging in this test method.
- Rockwell Testing Machine – The machine itself is one critical consideration for an accurate Rockwell test result. And though the Rockwell test is often seen as very easy and quick to carry out, still, it requires that the machine set up, specimen settings, and the machine components are all working appropriately.
- Test Specimens – Another critical concern for an accurate test result is the specimen which must be clean and void of any surface impurities that might interfere with the reading. Though the Rockwell test is relatively independent of surface preparation, the presence of debris and loose materials can cause a misalignment of the specimen on the anvil, resulting in errors.
- Material Type – When no hardness scale is specified for a material type, it is often critical to identify the material type so that you can verify the applicable hardness scale among various hardness tables. Generally, the largest load possible is often used to generate the deepest indentation possible. Also, the hardened steel and very hard materials often require diamond scales where softer metals and materials use ball scales.
- Material Thickness – There are scale thickness requirements often provided by applicable standards like ASTM that ensure the result of the Rockwell hardness test is accurate..
- Surface Finish – While this factor is generally not very important, the significance of a material’s surface finish becomes increasingly critical as the load applied reduces. Hence, a polished surface will be ideal for a low hardness load like 15 Kgf.
- Indenter Type – The type of indenter you select for the test can also influence the accuracy of the result. Selecting a ball indenter where a diamond indenter is more suitable can lead to errors in the indentation and measurement processes.
Conclusion
In conclusion, getting the accurate hardness value for material using the Rockwell hardness value is highly dependent on understanding the working principle of this test method. Knowing its part, which includes the indenter types, the scales, and applicable loads, will help you identify its most suitable application. Still, your understanding of the critical consideration for an accurate result, including material type and thickness, surface finish, indenter type, test specimen, and Rockwell machine itself, will count.
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