University of New Mexico

Civil Engineering Department

Civil Engineering Materials Laboratory, CE 305L

ROCKWELL HARDNESS OF METALLIC MATERIALS

ASTM E18

Scope

This test method covers the determination of the Rockwell hardness of metallic materials.

Terminology

Rockwell hardness test – an indentation hardness test using a verified machine to force a diamond spheroconical indenter (diamond indenter), or hard steel ball indenter under specified conditions, into the surface of the material under test in two operations, and to measure the difference in depth of the indentation under the specified conditions of preliminary and total test forces (minor and major loads, respectively).

Rockwell hardness number, HR – a number derived from the net increase in depth of indentation as the force on an indenter is increased from a specified preliminary test force to a specified total test force and then returned to the preliminary test force. Rockwell hardness numbers are always quoted with a scale symbol representing the indenter and forces used. The hardness number is followed by the symbol HR and the scale designation (for example, 64 HRC = Rockwell hardness of 64 on the Rockwell C scale).

The Rockwell hardness test is an empirical indentation hardness test and provides useful information about metallic materials. This information may correlate to tensile strength, wear resistance, ductility, toughness, and other physical characteristics of metallic materials. It may be useful in quality control and selection of materials. Note that there is no general method of accurately converting the Rockwell hardness numbers on one scale to Rockwell hardness numbers on another scale, or to other types of hardness numbers or to tensile strength values. Such conversions are approximations, at best.

Apparatus

The general principle of the Rockwell hardness test is illustrated in Figure 1. The tester for making the Rockwell hardness determination is a machine that measures the difference in penetration depths of an indenter under two specified forces, called the preliminary (minor) and total force (major). The minor force is 10 kg. The major force is 150 kg for the Rockwell C scale with the diamond “Brale” indenter. The major force is 100 kg for the Rockwell B scale with a 1/16 inch ball indenter.

Procedure

Click here to download the Rockwell hardness procedure

1) While rigidly supporting the specimen, bring the indenter in contact with the specimen applying the minor load of 10 kg.

2) Zero the Rockwell dial indicator and apply the major force over a period of 1 to 8 seconds.

3) Remove the major load as follows:

a) For materials exhibiting no time-dependent plasticity, remove the load within 3 seconds.

b) For materials exhibiting time-dependent plasticity, remove the load within 5 to 6 seconds when using the diamond indenter, and within 6 to 8 seconds using the ball indenter.

c) For materials showing considerable time-dependent plasticity, remove the load within 20 to 25 seconds after the major force is applied

4) After removal of the major load with the minor load still applied, read the Rockwell hardness directly from the appropriate scale on the Rockwell dial indicator.

Click here to view a Quicktime movie of a Rockwell hardness test

Report

1) Report the Rockwell harness number for the specimens tested.

2) The time of major force application if greater than 3 seconds.

1) Determine the hardness values (Rockwell C), for the following specimens:

a) AISI O1 Tool Steel; un-heat treated (~ 6 HRC)

b) AISI O1 Tool Steel; hardened by heat treating to 800°C and oil quenching (~60 HRC)

c) ASTM A36 Structural Steel.

2) After hardness testing, determine the Charpy impact fracture toughness of same test specimens (ASTM E23).

Figure 1. Principle of “Rockwell” Test (ACCO, Wilson Mechanical Instrument Division).

Muffle furnace

Quenching Oil

Rockwell hardness tester