HRA Rockwell Hardness Test: Technical Overview and Procedure

The HRA Rockwell hardness test is a widely utilized method for determining the hardness of cemented carbides, thin steel sheets, and surface-hardened layers. By utilizing a diamond cone indenter with a 60 kgf load, this testing standard provides precise measurements for materials requiring a lower load force than the standard HRC scale. This article explores the technical specifications, sample requirements, and procedural steps essential for conducting an accurate HRA Rockwell hardness experiment.

Technical Overview

The Rockwell hardness test is defined by the indentation depth produced under a specific load. The HRA scale specifically employs a diamond cone indenter, often referred to as a Brale indenter, with a total load (major load plus minor load) of 60 kgf. This scale is particularly suitable for harder materials that are too thin or brittle for the HRC scale, which uses a higher load of 150 kgf. The result is a dimensionless number derived from the permanent increase in depth of indentation caused by the major load.

Testing Samples

Cemented Carbides

Cemented carbides, such as tungsten carbide, are the most common materials tested using the HRA scale. These materials are extremely hard and brittle, making the lower load of the HRA scale ideal to prevent cracking or excessive fracture during the indentation process.

Thin Hardened Steel Strips

For hardened steel sheets that possess a thickness insufficient to support the deeper indentation of the HRC scale, the HRA test is mandated. If the indenter penetrates too deeply, the anvil effect may distort the reading, so the lighter load ensures the measurement reflects the material properties rather than the support structure.

Surface Hardened Layers

Components with shallow case-hardened surfaces require hardness testing that does not penetrate the hardened layer into the core material. The HRA scale provides a reliable assessment of these surface treatments without compromising the integrity of the test zone.

Testing Items

The primary objective of the HRA Rockwell hardness experiment is to determine the hardness value of the test specimen. This value is crucial for quality control, material selection, and verification of heat treatment processes. The test evaluates:

• Resistance to permanent deformation under a standard load.
• Consistency of material hardness across a specific batch.
• Effectiveness of surface hardening or carburizing treatments.

Testing Methods

The testing procedure must follow strict standardized protocols, such as ASTM E18 or ISO 6508, to ensure accuracy and repeatability. The step-by-step methodology is outlined below:

• Preparation: The test surface must be flat, smooth, and free from oxide scale or lubricants. The specimen must be rigidly supported on the anvil.
• Initial Contact: The indenter is brought into contact with the surface, and a minor load of 10 kgf is applied. This sets the zero point or "set" position.
• Major Load Application: The major load of 50 kgf is added, bringing the total force to 60 kgf. The indenter penetrates the material.
• Load Removal: The major load is removed, while the minor load remains. This allows for elastic recovery.
• Reading: The hardness number is read directly from the dial gauge or digital display, based on the final depth of indentation.

Multiple test points are usually taken to establish an average hardness value. The distance between indentations must be sufficient to prevent work-hardening effects from influencing adjacent readings.

Applications

The HRA Rockwell hardness test is indispensable in various high-tech and industrial manufacturing sectors.

Cutting Tool Manufacturing

Manufacturers of drill bits, end mills, and turning inserts rely on HRA testing to verify the hardness of carbide tools, ensuring they possess the necessary wear resistance for machining operations.

Automotive and Aerospace

In these industries, thin sheet metals and shallow case-hardened components are prevalent. The HRA test ensures that lightweight materials meet strict durability requirements without compromising structural integrity through destructive testing.

FAQ

Below are frequently asked questions regarding the HRA Rockwell hardness test to clarify common technical inquiries.

What is the main difference between HRA and HRC scales?

Both scales use a diamond cone indenter, but HRA uses a total load of 60 kgf, while HRC uses 150 kgf. HRA is generally used for thin hard materials or cemented carbides, whereas HRC is used for thicker, harder steels.

Why is the HRA scale used for cemented carbides?

Cemented carbides are extremely hard but can be brittle. The heavier load of the HRC scale might cause spalling or cracking of the carbide surface, leading to inaccurate readings or damage. The lighter HRA load minimizes this risk.

How should the sample surface be prepared?

The surface should be polished or ground flat to ensure accurate readings. Rough surfaces can cause the indenter to slip or settle unevenly, resulting in erroneous hardness values.