Hardness, in materials science, is defined as a material’s resistance to indentation. It is a critical property that determines how well a material can withstand mechanical wear and deformation. The Rockwell hardness test is one of the most widely used methods for measuring hardness, especially for metals. This test measures the permanent depth of indentation on a metal surface by applying a fixed load using a specific type of indenter. The smaller the indentation, the harder the material.
The Rockwell hardness test is commonly applied to a variety of metals, including aluminum, thin steel, lead, iron, titanium, copper alloys, and cemented carbides, making it versatile and invaluable in industrial quality control.
Principle of the Rockwell Hardness Test
The Rockwell hardness test operates on the principle of the differential-depth method. During the test, an indenter is forced into the specimen’s surface to create a residual indentation. The depth of this indentation is carefully measured. To minimize inaccuracies caused by surface roughness or measurement errors, the test force is applied in two stages:
- An initial preload creates a reference indentation depth, overcoming surface imperfections.
- A secondary, larger load is then applied to increase the indentation, followed by a return to the preload for the final depth measurement.
This two-stage loading ensures precision and repeatability in the hardness measurement.
Rockwell Hardness Test Equipment
The equipment for the Rockwell test consists primarily of an indenter and a loading mechanism. The indenter can be either a diamond spheroconical tip or a steel ball, depending on the material and the scale used. The test begins by pressing the indenter into the specimen under a preliminary load, also known as the preload or minor load, which creates a baseline penetration depth (denoted as ho). This preload reduces the effect of surface finish variations on the test results.
Step-by-Step Test Procedure
Step 1: The indenter is pressed into the metal specimen with a preload to achieve the reference indentation depth (ho). This preload is held for a short dwell time, allowing the indenter to settle into the material surface and establish a stable starting point.
Step 2: A larger major load is then applied over the preload. This combined force is maintained for a specified dwell time to allow the metal to undergo elastic deformation and stabilize. After this period, the major load is removed, leaving the preload still applied. The new depth of indentation (h) is measured. The difference between the final depth (h) and the reference depth (ho) gives the net indentation depth (H), which is used to calculate hardness.
Calculation of Rockwell Hardness Number (RHN)
The Rockwell hardness number is calculated based on the net depth of indentation (H) and the type of indenter used. According to ASTM E18 standards:
- For a diamond spheroconical indenter:
RHN = 100 – (H / 0.002) - For a ball indenter:
RHN = 130 – (H / 0.002)
Where H is the indentation depth measured in millimeters. After calculating the hardness number, the preload and indenter are fully removed from the specimen.
Types of Rockwell Hardness Scales
There are several Rockwell scales available, each defined by specific combinations of indenter types and applied loads. These scales are selected based on the material’s characteristics, such as metal type, size, shape, and homogeneity.
The ASTM E18 (1984) standard specifies various Rockwell scales, including the commonly used A, B, and C scales, which differ in load magnitude and indenter type. A key guideline when selecting a scale is to use the largest indenter and highest load possible without exceeding the material or specimen’s limitations.
The table below summarizes the key aspects of some Rockwell scales:
| Scale | Indenter Type | Major Load (kgf) | Common Applications |
|---|---|---|---|
| A | Diamond spheroconical | 60 | Hard steels, thin materials |
| B | Steel ball (1/16 in) | 100 | Soft metals like copper alloys |
| C | Diamond spheroconical | 150 | Hard steels and cemented carbides |
Frequently Asked Questions (FAQs)
Q: What is the principle of the Rockwell hardness test?
A: It measures the permanent depth of an indentation created by an indenter under a fixed load. The test uses a differential-depth method, applying the load in two stages to ensure accurate measurement despite surface roughness.
Q: How is the Rockwell hardness number determined as per ASTM E18?
A: Using the formulas:
- For diamond indenters: RHN = 100 – (H / 0.002)
- For ball indenters: RHN = 130 – (H / 0.002), where H is the indentation depth in mm.
Q: On which metals can the Rockwell hardness test be conducted?
A: It can be performed on metals such as aluminum, thin steel, lead, iron, titanium, copper alloys, and cemented carbides.