Hardness Test

1
Hardness Test

• Appendix C

2
What are we going to learn today?

• Explain the basics of Brinell, Rockwell and
  microhardness tests
• Explain advantages and disadvantages of each type
  of test and select the best for each application
• Perform the test, determine hardness reading and
  convert into another scale

3
In his book on Hardness the scientist ONeill
states that

• The hardness of metals, like the storminess of
  seas, is easily appreciated but not readily
  measured for one would hope to express it in
  terms of fundamental units.

4
Hardness is

• the resistance of a material to deformation,
  abrasion, scratching, penetration, resistance to
  cutting, lack of malleability etc.

5
Accompanying factors

• Materials with higher hardness have higher
  Tensile Strength and Brittleness
• Softer material are more ductile

6
Scratch Hardness

• Introduced by Friedrich Mohs in 1822

7
Mohs hardness Scale
8
Brinell Test - Load over Area Test
Introduced by Dr. Jonathan Brinell in 1900.
9
Brinell Test Method ASTM Standard E10

• 1. Press a 10mm (3/8") diameter ball into
  material with a known amount of load.
• 2. Measure diameter of the indentation.
• 3. Calculate hardness according to the formula

10
(No Transcript)
11
How to calculate hardness ?
P load in kg D diameter of the ball in mm d
diameter of indentation in mm
12
P/D2 ratio for Brinell test
13
(No Transcript)
14
Limitations of the Brinell Hardness Test

• a) Sample must be ten times thicker than the
  indentation depth (sample usually should be at
  least 3/8" thick).
• b) Test is most accurate if the indentation depth
  is 2.5 - 5.0 mm. Adjust load to achieve this.
• c) Test is no good if BHN gt 650
• d) Sensitivity problem

15
Advantages of the Brinell Test

• Widely used and well accepted
• Large ball gives good average reading with a
  single test
• Accurate
• Easy to learn and use

16
4.) Disadvantages of the Brinell Testa.)
Destructiveb.) Non-portablec.) High initial
cost (5,000)Error due to operator reading
Brinell Microscope (10max) Disadvantages of
the Brinell Test

• Destructive
• Non-portable
• High initial cost (5,000)
• Error due to operator reading Brinell Microscope
  (10max)

17
Hardness and Tensile Strength

• Tensile Strength BHN x 500 psi

18
Microhardness Test another load over area test

• Vickers
• ASTM E384 micro force ranges
• 10g to 1kg
• ASTM E92 macro force ranges
• 1kg to 100kg
• Knoop - ASTM E384

19
Vickers

• developed in England is 1925 and was formally
  known as the Diamond Pyramid
• Range HV 100-1000

20
Vickers test
All Vickers ranges use a 136 pyramidal diamond
indenter that forms a square indentation.
21
Vickers Test Method

• The indenter is pressed into the sample by an
  accurately controlled test force.
• The force is maintained for a specific dwell
  time, normally 10 15 seconds.
• After the dwell time is complete, the indenter is
  removed leaving an indent in the sample that
  appears square shaped on the surface.
• The size of the indent is determined optically by
  measuring the two diagonals of the square indent.
  
• The Vickers hardness number is a function of the
  test force divided by the surface area of the
  indent. The average of the two diagonals is used
  in the following formula to calculate the Vickers
  hardness.

22

• HV Constant x test force / indent diagonal
  squared
• HV 1854.4 x P/d2

23
Knoop Test
Knoop testing is done with a rhombic-based
pyramidal diamond indenter that forms an
elongated diamond shaped indentation
24
Knoop Test Method

• The indenter is pressed into the sample by an
  accurately controlled test force.
• The force is maintained for a specific dwell
  time, normally 10 15 seconds.
• After the dwell time is complete, the indenter is
  removed leaving an indent in the sample that
  appears square shaped on the surface.
• The size of the indent is determined optically by
  measuring the longest diagonal of the diamond
  shaped indent.
• The Knoop hardness number is a function of the
  test force divided by the projected area of the
  indent. The diagonal is used in the following
  formula to calculate the Knoop hardness.

25

• HK Constant x test force / indent diagonal
  squared
• HK 14229 x P/d2

26
Advantages of the Vickers-Knoop Test

• One scale covers the entire hardness range.
• A wide range of test forces to suit every
  application.
• Nondestructive, sample can normally be used.
• The small diamond indenter and low test forces
  allows testing very small parts or material
  features not capable if being tested any other
  way

27
Disadvantages of the Vickers and Knoop Test

• The main drawback of the Vickers and Knoop test
  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.
• Slow. Testing can take 30 seconds not counting
  the sample preparation time.

28
Rockwell Hardness Test

• Introduced in 1919 by Stanley Rockwell
• Manufacturing started in 1920 by Charles Wilson
• Wilson revised the Rockwell method

29
Rockwell Test Procedure ASTM Standard E18

• Select Scale - load and indentor depending on the
  scale
• Press the indenter into material
• - Diamond Indenter (Brale)
• - 1/16" ball
• - 1/8" ball

30
Machine measures depth of penetration and
computes hardness
31
Rule of Thumb

• Keep indentations 2-3 diameters apart
• Select the scale to keep reading between 20-80
• For Steel
• If HRA gt 60, use HRC scale
• If HRA lt 60, use HRB scale

32
Limitations of the Rockwell Test

• 1) Sample must be ten times thicker than the
  indentation depth (sample usually should be at
  least 1/8" thick).
• 2) Need 3 tests (minimum) to avoid
  inaccuracies due to impurities, hard spots
• 3) The indenter travel is limited to 100
  Rockwell points or 0.2mm.

33
Advantages of the Rockwell Test

• Widely used and well accepted
• Little operator subjectivity direct reading
• Accurate
• Fast
• Large range of scales (plastics to steels)
• Regular surface preparation (polishing not needed)

34
Disadvantages of the Rockwell Test

• Destructive
• Non-Portable
• Initial cost (5,000)
• Compared to Brinell the device is not as rugged
  and need adjustments
• Small impressions not so representative as
  Brinell

35
Rockwell regular scales (10kg minor load)
36
Rockwell Superficial scales (3kg load)
37
Shore Scleroscope Hardness

• Is a very old hardness testing originating in the
  early 1900's
• Is a dynamic 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.

38

• Scleroscopes are no longer produced, however many
  are still in use.
• testers used a glass tube graduated from 0 to 140
  to measure the rebound. The operator would
  observe the height of the rebound on the
  graduated glass tube.

39
Some remarks on Scleroscopes

• Used on a wide variety of metallic parts but the
  size of the sample should be large enough to
  support the rebound.
• Large well-finished rolls are a good application
  for a Scleroscope.
• They do not leave an indent so the part can be
  used after testing without refinishing.

40
And finally,

• While fairly versatile and portable,
• Scleroscopes are difficult to use.

41
Hardness conversion chart
www.instron.us/wa/library/streamfile.aspx?doc250
42