Hardness Testing - Rockwell Hardness

Topics Covered

Background

Rockwell Hardness

Rockwell Hardness Scales

Measuring Rockwell Hardness

Interpreting Rockwell Hardness Values

Testing of Sheet-Type Materials

Summary

Background

The hardness of a material is defined as its resistance to another material penetrating its surface and is related to its wear resistance and strength. Higher hardness is related generally related to higher strength, which in turn is related to its structure.

Rockwell Hardness

The Rockwell test uses a small steel sphere for the indenter of a diamond cone for harder materials. It differs from the Brinnell hardness testing technique in that it measures depth of penetration by the indenter. Measurements do not require measurements and calculation to be made, rather, hardness measurements are read off a gauge, which is effectively a calibrated depth gauge.

Rockwell Hardness Scales

Several different Rockwell scales are used for various materials and hardnesses. The most common are the Rockwell B and Rockwell C scales. The Rockwell B system incorporates a 1/16^th inch diameter steel ball, with a load of 100kg for softer materials such as aluminium alloys and materials of similar hardness. Harder materials such as cast irons are measured using the Rockwell C scale, which uses a 120° diamond cone with a 150kg load.

Measuring Rockwell Hardness

Measurements involve preloading the sample with a load of 10kg, immediately prior to the application of the main load. Its purpose is to remove any irregular surface effects. The hardness value for a material is determined from the difference in depth between the two loads.

Interpreting Rockwell Hardness Values

Rockwell hardness values consist of three parts. A typical value might be 50 HRB, of which “HR” indicates a Rockwell hardness of “50” using the “B” scale.

Testing of Sheet-Type Materials

A similar Rockwell scale exists for testing of thin sheet-type materials or surfaces subject to thin surface treatments, or parts that would not otherwise be able to produce meaningful readings using the standard Rockwell scale. Known as the Rockwell superficial test, it utilises lower loads including a preload of just 3kg and full load usually of 15 or 45kg depending on the hardness of the material. Readings for the superficial test also consist of three parts e.g. 15T-22. “15” indicates a load of 15kg was used and a reading of “22” was produced using the steel ball indenter, indicated by the “T”. The “T” would be substituted by a “N” if the diamond cone indenter was used.

Summary

The Rockwell technique can be performed quickly making it ideal for quality control. It also leaves only a tiny impression on the specimen and is capable of being used on a wide variety of materials and geometries.

Primary author: AZoM.com