Lab 6 Hardness

AIN SHAMS UNIVERSITY

FACULTY OF ENGINEERING

NEW PROGRAMS IN MATERIALS ENGINEERING

Materials TestingMATL 352

Lab. 5

Hardness Measurements

[A] Surface Preparation and Proper Use

- Prepare the surface by fine grinding. Coarse grinding will not produce results that are as reliable as smooth surface.

- Remove rough scale (rough oxide layer)- Remove decarburized layer (soft skin)- Remove cured surface from the spot of measurement- For cylindrical work pieces use a V-type centering anvil, Fig.1.- For tubes use an internal mandrel to avoid deformations of thin tubes during

testing- For long work pieces, such as shafts, use an external support (Jack test), Fig.2.- Keep the penetrator at least 1/8” away from the edge- Use the test block to check the calibration of the tester

Fig. 1 Basic anvils used with hardness tester. (a) Plane, (b) Shallow, (c) Spot, (d) Cylinderon

Fig. 2 The correct method of testing long, heavy work requires the use of a jack rest

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(B) Brinell Hardness Test

Fig.3. Brinell hardness Test

← It is important to choose the combination of load and ball size carefully, so that the indentations are free from any distortion and suitable for measurement.

← The load F (kgf) is related to the ball diameter D (mm) by the interrelation:

Where: K is a constant:

K = 30 for ferrous metals, BHN = 67 - 450K = 10 for copper and copper alloys, BHN = 22 - 315K = 5 for aluminum and aluminum alloys, BHN = 11 - 158K = 2.5 for soft metals, BHN = 06 - 78K = 1 for lead, tin and white-bearing metals, BHN = 03 - 39

For steel the ratio is 30:1, a 10 mm ball can be used with a 3000-kgf load or a 1mm ball with a 30-kgf load. For aluminum alloys the ratio is 5:1, the test is most frequently performed with a 500-kgf load and 10-mm ball.

The load is applied for a fixed length of time, usually 30 seconds.

The diameter is measured either by a low powered microscope, Fig. 4 or projection screen with micrometer adjustment.

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Fig. 4 Brinell microscope provides a fast, accurate means for measuring the diameter

of the impression for determining the Brinell

harshness number

The Brinell hardness number (BHN) is found by dividing the load by the surface area of the impression.

P in kgD, d in mm

To ensure consistent results, the following precautions should be observed:

The thickness of the specimen should be at least 7 times the depth of the indentation to allow unrestricted plastic flow below the indenter.The edge of the indentation should be at least 3 times the diameter of the indentation from the edge of the test piece.The test is unsuitable for materials where hardness exceeds 500 BHN as the ball indenter tends to flatten resulting in an inaccurate reading. To overcome this limitation a tungsten carbide ball is used instead of the hardened steel ball but there is also a hardness limit of 600 BHN with this indentor.

The testing procedure is as follows:

1- The desired load in kilograms is selected.2- The specimen is placed on the anvil. Make

sure the specimen is clean and free from

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burrs. It should be smooth enough so that an accurate measurement can be taken of the impression

3- The load is then applied by pulling out the plunger control. Maintain the load for 30 seconds for nonferrous metals and 15 seconds for steel. Release the load

4- Remove the specimen from the tester and measure the diameter of the impression, using a small microscope, Fig. 4

5- Determine the Brinell hardness number (BHN) by calculation or by using the table.

6- Check for hardness in two or three locations on each specimen.7- Record your results in the following table

Material Surface condition LoadReading: Diameter of impression

BHN1 2 3 average

(C) Vickers Hardness Test

Fig.5. Vickers hardness test

The Vickers hardness test operates on similar principles to the Brinell test, the major difference being the use of a square based pyramidal diamond indentor rather than a hardened steel ball.

Also, unlike the Brinell test, the depth of the impression does not affect the accuracy of the reading so the P/D2 ratio is not important.

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The diamond does not deform at high loads so the results on very hard materials are more reliable (Diamond is the hardest material known – approximately 6000HB).

It is necessary to state the load when specifying a Vickers hardness number.

The Vickers hardness is the quotient obtained by dividing the kgf load by the square mm area of indentation.

The expected small impression of Vickers means that the surface must be flat and perpendicular to the indentor and should have a better than 300 grit finish.

Fig. 6 shows the difference between Brinell and Vickers methods

Fig. 6 The difference between Brinell and Vickers methods

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The corresponding units of HV are then kilogram-force per square millimeter (kgf/mm²). To convert a Vickers hardness number in SI units (MPa or GPa) one needs to convert the force applied from kgf to Newton’s and the area from mm2 to m2

to give results in Pascal (1 kgf/mm² = 9.80665×106 Pa). A practical method to convert HV to SI units:

To convert HV to MPa multiply by 9.807To convert HV to GPa multiply by 0.009807

Vickers hardness numbers are reported as xxx HV yy, e.g. 440HV30, where:

440 is the hardness number, HV gives the hardness scale (Vickers), 30 indicates the load used in kg.

Examples

Some HV values

Stainless Steels 140-180HV30 (316L & 347L stainless respectively) Carbon Steel 55-120HV5 (Note: load is different to that of stainless) Iron 30-80HV5


Fig. 7 Hardness testing, (a) Vickers hardness machine, (b) measuring screen showing magnified image of Vickers impression

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http://www.bookrags.com/Kilogram-force





1- Put the prepared sample so that the upper surface is horizontal;2- Look through the eyepiece of the microscope and adjust the sample position

until you see the surface of the work piece;3- Select the load according to the material, if the indentation is too small, use a

higher load, if it is too large, and use a smaller load;4- Apply the load for 15 seconds then remove the load;5- Look through the eyepiece to find the resulting indentation;6- Measure the diagonals of the indentation;7- Take the average of these diagonals;8- Compute the hardness.

Material Surface condition LoadReading: Diagonals of impression

VHN1 2 average

(D) Rockwell Hardness Test

The Rockwell test method measures a permanent depth of indentation produced by the preliminary and total test forces.

The purpose of the preliminary loading is to exclude inaccuracy of surface depth measurements.

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

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Table 1 Rockwell Hardness Scales

Scale IndenterMinor Load

F0 (kgf)Major Load

F1 (kgf)Total Load

F (kgf)

A Diamond cone 10 50 60

B 1/16 "steel ball 10 90 100

C Diamond cone 10 140 150

D Diamond cone 10 90 100

E 1/8 "steel ball 10 90 100

F 1/16 "steel ball 10 50 60

G 1/16 "steel ball 10 140 150

H 1/8 "steel ball 10 50 60

K 1/8 "steel ball 10 140 150

L 1/4 "steel ball 10 50 60

M 1/4 "steel ball 10 90 100

P 1/4 "steel ball 10 140 150

R 1/2 "steel ball 10 50 60

S 1/2 "steel ball 10 90 100

V 1/2 "steel ball 10 140 150

Typical Application of Rockwell Hardness Scales

HRA Cemented carbides, thin steel and shallow case hardened steelHRB Copper alloys, soft steels, aluminum alloys, malleable irons, etc.HRC Steel, hard cast irons, case hardened steel and other materials harder

than 100 HRD Thin steel and medium case hardened steel and pearlitic malleable ironHRE Cast iron, aluminum and magnesium alloys, bearing metalsHRF Annealed copper alloys, thin soft sheet metalsHRG Phosphor bronze, beryllium copper, malleable ironsHRH Aluminum, zinc, leadHRK HRL HRM HRP HRR HRS HRV Soft bearing metals, plastics and other very soft materials.

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1- Before starting, see that the crank handle is forward2- Select the proper penetrator and insert it in the plunger rod3- Place the proper anvil on the elevating screw.4- Select the proper weights.5- Place the specimen on the anvil.6- Raise the specimen into contact with the penetrator by turning the capstan hand

wheel clockwise. Continue motion until the small pointer is near the dot. Continue until the larger pointer is in a vertical position. The minor load is now applied.

7- Turn the bezel of the dial gage until “SET” line is directly behind the large pointer8- Release the weights (major load) by tripping the crank handle clockwise. Do not

force the crank. Allow it to come to rest9- When the large pointer has come to rest, return the crank handle to the starting

position. This removes the major load; the minor load is still applied10- Read the scale letter and Rockwell hardness number from the dial gauge11- Remove the minor load by turning the capstan hand wheel counterclockwise to

lower the elevating screw and specimen.12- Remove the specimen. Repeat the test procedure in one or more locations on the

test piece13- Record your results in the following table

Material Load Indentor scale Hardness No.

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How to Read a Hardness Number

Scale Name Example Explanation

Rockwell 60 HRC Hardness Rockwell "C" scale with a "60" test value

80.5 HR15TW

Hardness value in Rockwell "15T" scale with a "80.5" test value using a "W" or Tungsten carbide ball indenter. (Rockwell test results in scales using a ball indenter must indicate either "W" for carbide or "S" for steel indenter on all results (Steel balls are no longer permitted as of June 2007

Brinell 200 HB10/3000/15"200" test value using a 10 mm diameter carbide ball, a 300 kg load and a 15 second dwell

500 HBS 1/30/20An old report of a "500" test value using a no-longer-valid steel (HBS) ball of 1mm diameter with 30 kg load and 20 second dwell

HB 200An informal report assuming the most common parameters - 10 mm carbide ball, 3000 kg load and "200" test result value

Microhardness 200 HV 500/15"200" test value with Vickers 500 g load, 15 seconds duration

200 HK 500/15"200" test value with Knoop 500 g load, 15 seconds duration

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Designation conforming to standards of Brinell hardness

(According to DIN EN ISO 6506-1 conditions: 03/2006)

Besides the hardness value also the used method, the ball diameter and the test load must be indicated.

Example: 345 HBW 10/3000

With:

345 = hardness value

HBW = method of testing*

10 = ball diameter D in mm

3000 = load (force F) in kilopond kp

During a load, lasting longer than 15 s, the load period must also be indicated. Ex.: 210 HBW 5/750/60

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Documents

Lab 6 Hardness