Rockwell Hardness Test on Metals

The Rockwell Hardness Test is based on the indentation of the hard tip, or indenter, into the test piece under the action of two consecutively applied loads - minor (initial) and major (final). In order to eliminate zero error and possible surface effects due to roughness or scale, the initial or minor load is first applied and produces an initial indentation.

A conical shaped diamond (called a brale) with 120* apex angle and 0.2 mm radius is used as the indenter or penetrator in the Rockwell test for hard materials. For softer materials, a hardened steel ball 1.5 mm in diameter is generally used.

A number of different scales are used, each scale being suitable for certain classes of materials. It should be understood that each scale is entirely arbitary, the hardness number obtained having relevance to that particular scale only. 

Rock well Hardness Scale ,

Scale - A  

Diamond cone Indenter, Major load 60 kg , Dialed number - black , For materials - Cemented Carbides, thin steel , case hardened surfaces.

Scale - B

Indenter -1.5 mm ball, Major load 100 kg , Dial number - Red, For materials - Copper , Aluminum, brass, malleable iron, Grey cast iron.

Scale - C 

Indenter - Diamond cone, Major load 150kg , Dial number - Black , For Materials - Hard Cast iron , Hardened Steel 

Scale - D 

Indenter - Diamond Cone , Major Load 100 Kg , Dial number - Black , For Materials - Thin Steel Specimens

Scale - E 

Indenter - 3mm ball, Major load -100 kg , Dial number - Red , For materials Soft Aluminum and alloys, magnesium alloys, bearing metals.

Scale - F 

Indenter - 1.5 mm ball , Major load - 60 kg , Dial number - Red , For materials Annealed copper alloys, bearing alloys.

After loading, The major load is removed . The Rockwell Hardness number is the difference in depths of the indentations made by applying the Major and 10 kg Minor load, measured after removing the major load. 

The load is applied and maintained for upto 15 secs and then released, the hardness number then being read off the scale graduated in hardness units to the nearest whole number.

Rockwell tests are widely applied in the industry due to the rapidity and simplicity with which they may be performed , high accuracy achieved and due to the small size of the impressions produced. The Rockwell Hardness number may be converted into Brinell number using special tables or charts.