Hardness

• Ability to resist deformation, penetration, wear, abrasion, and cutting.

• Surface, or uniform throughout the metal.

• Brinell and Rockwell Hardness.

Brinell Hardness • Test piece is placed on the screw jack & raised until it touches a 10 mm diameter tungsten carbide ball.

• The ball is pressed into the test piece by a 3000 kg load (for steel).

• The diameter of the indentation is measured, the surface area of the mark determined, and the hardness calculated.

Brinnel

Brinell Hardness

Brinell hardness test Brinell hardness test

Brinell Hardness

• The round penetrator deforms the samples and they are generally scrapped afterwards.

• Limited to softer metals.

• Very accurate measure.

• Not good for very hard materials.

Rockwell Hardness• A 10 kp load holds a 1.6 mm ball ball on the test

piece. A 60, 100 or 150kp load is then applied to make the impression. The instrument dial measures the depth of the mark and converts it to a hardness reading which is read off the dial.

• For hard materials, a diamond conecone is used and the total load is 150 kp.

• Test made with the ball are called Rockwell BRockwell B readings, and those with the cone are called Rockwell CRockwell C readings

Rockwell

Rockwell Hardness Testing

• Most commonly used test.

• Eliminates effects of small surface imperfections by applying preliminary load.

• Results are very accurate.

Micro-hardness Testing

• Good for small, thin brittle parts – especially if the part must be used after testing.

• Use smaller loads (5 – 100kg) & sharp pointed penetrators.

• Can also test glass & ceramics.• Can test areas smaller than the size of a

crystal or grain.• Found in research labs usually.

Scleroscope

• Involves raising a precision weight with specific end shape to a specified height inside a tube.

• Tube is placed over surface to be checked

• Weight is dropped, allowing the point to hit the surface of the material. 

• Weight bounces back up the tube

• Height it bounces to is translated into a hardness value.

Scleroscope

Scleroscope

• Good correlation to Brinell & Rockwell Hardness scales.

• Imperfections in surface affect results.

• Impression left is almost negligible.

• Machine is small and portable.

Sonodour

• Measures hardness dependant on the resonant frequency of a piece of metal.

• Small & portable.

• Very quick response.

• Does not damage specimen.

• Very accurate.

Impact strength

• Impact strength is the ability of a material to resist sudden impact without fracturing.

• Also referred to as "Toughness”

• Impact strength varies with temperature. Temperature goes down - so does impact strength.

• Izod and the Charpy tests.

Measuring Hardness

Penetration Hardness• Very accurate• Use a precision

machine• Penetrator presses

against metal, measure impression, convert to hardness reading

• expensive

Scratch Hardness• Fast & crude• Scratch metal sample

with edge of tool or object

• Metal is defined as either ‘hard’ or ‘soft’

File Hardness

• Scrape metal with edge of file.• If it does not scratch it is ‘file hard’• If it does scratch it is not file hard.

• Fast, simple, convenient• Inaccurate• Dependant on sensitivity of tester and

sharpness of file.

Impact strength

Charpy impact test Charpy impact test movie

Izod Impact Test

Charpy Impact Test

Impact testing

Tensile strength