Properties of Metals

Hardness

Prepared By: John Cawley

Presentation Objectives

Explain what hardness is.

Describe how the hardness of metal is

found.

Compare different hardness testing

methods.

Examine how each hardness testing method

works.

Convert between hardness scales.

What is Hardness

Defined as a measure of resistance to

deformation or its resistance to another

material penetrating its surface .

Hardness is proportional to the size of

penetration. The harder the material, the

smaller the penetration.

Hardness Testing Methods

Brinell Hardness

Vickers

Microhardness

Knoop Microhardness

Rockwell Hardness

Rockwell Superficial

Hardness

Shore Scleroscope

Hardness

Sonodur Hardness

Mohs Hardness

File Hardness

Brinell Hardness Test

Dr. J. A. Brinell invented the Brinell test in Sweden in 1900. The oldest of the hardness test methods in common use today, the Brinell test is frequently used to determine the hardness of forgings and castings that have a grain structure too course for Rockwell or Vickers testing. Therefore, Brinell tests are frequently done on large parts. By varying the test force and ball size, nearly all metals can be tested using a Brinell test. Brinell values are considered test force independent as long as the ball size/test force relationship is the same.

Brinell Hardness Testing Uses a 10mm diameter

tungsten carbide sphere.

The ball is pressed into the test piece under loads ranging from 500kg to 3000kg.

The load is applied for 10 to 15 seconds.

A microscope with a calibrated lens is used to measure the diameter of the dent.

Brinell Test Strengths

One scale covers the entire hardness range, although comparable results can only be obtained if the ball size and test force relationship is the same.

A wide range of test forces and ball sizes to suit every application.

Nondestructive,

sample can normally

be reused.

Used on softer alloyed

material such as

Aluminum or Iron and

Steel castings.

Vickers Hardness Test

The Vickers (HV) test was developed in England is 1925 and was formally known as the Diamond Pyramid Hardness (DPH) test. The Vickers test has two distinct force ranges, micro (10g to 1000g) and macro (1kg to 100kg), to cover all testing requirements. The indenter is the same for both ranges therefore Vickers hardness values are continuous over the total range of hardness for metals (typically HV100 to HV1000).

Vickers Microhardness Testing Uses a 136 degree square-

based diamond cone penetrator.

A 50kg maximum force is applied to the test piece.

The length of the indentation diagonal can be measured using a microscope, calibrated micrometer barrel, digital readout, or computer software which can display the image.

Vickers Test Strengths

One scale covers the entire hardness range.

A wide range of test forces to suit every application.

Nondestructive, sample can normally be used.

Test can be used on harder materials because the pointed penetrator can probe into a hard surface more easily than a ball penetrator.

Used on smaller areas of a part.

Requires a smaller load.

Knoop Hardness Test

Knoop (HK) hardness was developed by at the National Bureau of Standards (now NIST) in 1939. The indenter used is a rhombic-based pyramidal diamond that produces an elongated diamond shaped indent. Knoop tests are mainly done at test forces from 10g to 1000g, so a high powered microscope is necessary to measure the indent size. Because of this, Knoop tests have mainly been known as microhardness tests. The newer standards more accurately use the term microindentation tests.

Knoop Microhardness Testing

Uses a rhombic -shaped penetrator having angles of 130 and 172.5 degrees.

A load of less than 4kg is applied to the test piece. A load of 25g can be used for extremely small areas.

The length of the indentation diagonal can be measured using a microscope, calibrated micrometer barrel, digital readout, or computer software which can display the image.

Knoop Test Strengths

The elongated

diamond indenter and

low test forces allows

testing very small

parts or material

features not capable if

being tested any other

way.

One scale covers the

entire hardness range.

Test essentially does

no damage to the

specimen.

Can be used on very

thin material.

Can be used on very

small surface areas.

A wide range of test

forces to suit every

application.

Brinell, Vickers & Knoop Test

Weaknesses

The main drawback of the Knoop test ingeneral , is the need to optically measure the indent size. This requires that the test point be highly polished to be able to see the indent well enough to make an accurate measurement.

Slower Process -Testing can take 30 seconds not counting the sample preparation time.

Rockwell Hardness Test

Stanley P. Rockwell invented the Rockwell hardness test. He was a metallurgist for a large ball bearing company and he wanted a fast non-destructive way to determine if the heat treatment process they were doing on the bearing races was successful. The only hardness tests he had available at time were Vickers, Brinell and Scleroscope. The Vickers test was too time consuming, Brinell indents were too big for his parts and the Scleroscope was difficult to use, especially on his small parts.

Rockwell Hardness Testing

Uses either a 1/16” or

1/8” diameter tungsten

carbide ball otherwise it

uses a conical-shaped

diamond point penetrator.

Loads of 60kg, 100kg, or

150kg are used.

Hardness values are read

directly from a dial gauge

or digital readout.

Rockwell Superficial Hardness

Testing Uses a 1/16” diameter

tungsten carbide ball

otherwise it uses a

conical-shaped diamond

point penetrator.

Loads of 15kg, 30kg, or

45kg are used.

Hardness values are read

directly from a dial gauge

or digital readout.

Rockwell Test Strengths &

Weaknesses

Rapid test, usually less than 10 seconds

Direct readout, no questionable optical measurements required.

Non-destructive, part normally can be used.

Thin materials can be tested.

Multiple test scales (30) needed to cover the full range of metal hardness.

Conversions between scales can be material dependant.

Samples must be clean and have a smooth test point to get good results.

Scleroscope Hardness Test

The Scleroscope is a very old hardness testing instrument, originating in the early 1900's. It is a dynamic indentation hardness test that drops a diamond tipped hammer vertically from a fixed height onto the surface of the material under test. The height of the rebound of the hammer is a measure of the hardness of the material. Scleroscopes are no longer produced in this country, however many are still in use.

Shore Scleroscope Hardness

Testing Uses a small metal ball or

diamond tipped hammer.

The 3g hammer ball is dropped in a graduated glass tube from approximately 10 inches.

The hammer rebounds off the test piece and a number is read on the glass tube, the higher the bounce the harder the material.

Sonodur Hardness Testing

Uses a diamond tipped magnetostrictive rod 0.75mm in diameter.

The rod is pressed against the test piece and an electrical coil is used to vibrate the rod.

The frequency at which it vibrates is measured.

The harder the material, the higher the frequency.

Mohs Hardness Test

The Mohs hardness scale was introduced in 1822 by the German mineralogist Friedrich Mohs, and is still used as a classification for some minerals. It simply consists of 10 minerals arranged in order from 1 to 10. Diamond is rated the highest and is indexed as 10. Talc is the softest with an index number of 1. Each mineral in the scale will scratch those with a lower number. A completely arbitrary scale, the Mohs hardness has little value to an engineer. Copper would be in the 2 to 3 range, iron would be between 3 to 4 and hardened steels are in the 7 to 8 range.

Mohs Scale Hardness Testing

Uses ten stones of varying hardness.

The hardest stone has a value of ten while the softest stone has a value of one.

If the test sample is scratched by the stone it is softer than the stone.

No longer used except in the field of geology.

The difference

between #9 and #10

is about as great as

the difference

between #1 and #9.

Basic Tools for Scratch Hardness

Mohs Testing

Finger Nail 2.5

Penny (old) 3.5

Knife Blade 5.5

Steel File 6.5

Conversion Scales

Since hardness can be measured in many different

units, using different machines and scales. They

must be able to be converted from one method to

the next. This is accomplished through the use of

conversion charts. Use your textbook as a guide

to cross-reference readings between methods.