Hardness Testing
The Test Methodology Guide, Part 2

This brief guide is intended to be a refresher for those a ready involved in hardness testing as well as a reference for beginning users of hardness testers.

Hardness is a characteristic of a material, not a fundamental physical property.  It is defined simply as the resistance to indentation, and it is determined by measuring the permanent depth or width of a testindentation.  When using a fixed force (load)* and a given indenter, the smaller the indentation, the harder the material.  While the the concept is ectremely simple, the indentation hardness value is obtained by using one of over 12 different test methods.

Also see our recent article in ASM's Heat Treat Progress Magazine: Common Problems in Hardness Testing as well as the follow up: Common Problems in Microhardness Testing

Section 2: Test Method Principles

or navigate to: Part 3, or Part 1

ROCKWELL HARDNESS

The Rockwell test method measures a permanent depth of indentation produced by the preliminary and total test forces. First, a preliminary test force (pre-load or minor load) is applied. This is the zero or reference position. Then, an additional test force (or major load) is applied to reach the total required test force. This additional force is held for a predetermined amount of time and then released, but with the preliminary test force still applied. The indenter reaches the final position at the preliminary force and the distance traveled from the major load position is measured and converted to a value into one of the many scales for Rockwell hardness. Preliminary test forces range from 3 (superficial Rockwell) to 10 kilograms (regular Rockwell) to 200 kilograms (macro Rockwell scale). Total test forces range from 500 grams (micro) to 15 through 150 kilograms (superficial & regular) to 500 through 3000 kilograms (macro).

To see a more complete description of the Test method for Rockwell Hardness.

A. Depth reached by indenter after application of preliminary test force (minor load).

B. Position of indenter under total test force.

C. Final position reached by indenter after elastic recovery of the material.

D. Position at which measurement is taken.

BRINELL HARDNESS

Widely used on castings and forgings, the Brinell method applies a predetermined test force (F) to a carbide ball of fixed diameter (D) which is held for a predetermined time and then removed. The resulting indentation is measured across at at least two diameters - usually at right angles to each other and averaged (d). A chart is then used to convert the averaged diameter measurements to a Brinell hardness number. Test forces range from 500 to 3000 kilograms.

For a more complete description of the Brinell test method and related Newage Brinell testers see Test method for Brinell Hardness.

VICKERS HARDNESS (Micro and Macrohardness) and Knoop

Mostly used for small parts, thin sections, or case depth work, Vickers and Knoop methods are based on an optical measurement system. The new Computer Assisted Measurement System (C.A.M.S.), now available from Newage Testing Instruments, Inc., has improved productivity, accuracy and repeatability of these labor intensive methods. (See pages 14 and 15). To perform a test, a predetermined test force is applied with a pyramidal shaped diamond indenter. After a dwelt time, the force is removed. Then, in the Vickers method, the indentation length of vertical and horizontal axis is measured and averaged. In the Knoop method, only the tong axis is measured (Figure 5). A chart is used to convert the measurements to corresponding Vickers or Knoop hardness numbers, Test forces range from 1 to 2000 grams, Vickers does offer higher force capabilities - up to 150 kgs - but are not used frequently in North America.

Link to a more complete description of the Test method for Vickers Hardness.

DUROMETER & IRHD HARDNESS

Widely used in the plastics and rubber industries, the Durometer method (sometimes erroneously referred to as the Shore® method after the company that originally developed the test.) has evolved from a coarse handheld measurement to today's repeatable test method. Bench model testers can now read to a tenth of a point with good repeatability. The Durometer method applies a predetermined test force to a spherical or conical shaped indenter. The depth of indentation is translated into a hardness value by means of a dial gage or electronically. Test forces range from 822 grams (A scale) to 4550 grams (D scale). Non-standardized "micro" scales are also available from many manufacturers. These scales permit testing on thinner and more narrow specimens.

The use of IRHD, or International Rubber Hardness Degrees, has increased considerably in North America since its origin in Europe. It provides a very repeatable result on rubber parts of all shapes and sizes. It is especially important in the determination of the hardness of rubber O-rings. The method employs a preliminary test force that is applied to the specimen through the indenter. The test is zeroed at this position, then the total test force is applied. The distance between the these two forces is then measured and converted to an IRHD hardness value. Preliminary test forces are 8.46 grams for micro scales and 295.74 grams for regular scales. Total test forces are 15.7g for micro and 597 for regular scales.

Shore® is a registered trademark of Instron Corporation

How to Read a Hardness Number

Go to: Basic Training for Hardness Testing - Section 3

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