mml2014_gsreekar_rephardness

8/11/2019 MML2014_GSreekar_REPHardness

1/6

MACRO AND MICRO

HARDNESS TESTING

A . R . G S r e e k a r ,

M M 1 2 B 0 0 2 .


2/6

1

Aim:

To measure the hardness of Tool steel, Fractured Mild steel, SS304L-Stealite welded samples.

Apparatus:

Brinell hardness machine, Vickers micro hardness machine, Rockwell hardness machine.

Theory:

Hardness is the measure of a materials resistance to localized plastic deformation (e.g., a small

dent or scratch). The depth or size of the resulting indentations is measure, which in turn is

related to a hardness number; the softer is the material, the larger and deeper is the

indentation, and lower the hardness index number. Measured hardness is only relative and

care should be taken when comparing value determined by various techniques. Some of the

techniques to find macro hardness are Brinell test, Rockwell test; and for very low loads Vickersmicro hardness test is used.

Macro Hardness Tests:

In Brinell tests, as in Rockwell measurements, a hard, spherical indenter is forced into the

surface of the metal to be tested. The diameter of the hardened

steel indenter is 10.00mm. Standard loads range between 500

and 3000 kg in 500-kg increments; during a test, the load is

maintained constant for specified time. The brinell hardness

number, HB, is a function of both the magnitude of the load andthe diameter of the resulting indentation. This diameter is

measured with a special low-power microscope, utilizing a scale

that is etched on the eyepiece. The measured diameter is then

converted to appropriate HB number using a chart; only one

scale is employed with this technique. For this technique, surface

finish should be smooth and flat where the indentation is to be

made.

Steel sphere is pressed against a metal surface for a

specified period of time (10 to 15 s, according to the ASTM), and

the surface of the indentation is measured. The load (in kgf)

divided bythe area (in mm2) of the curved surface gives the

hardness HB. Figure 1: Brinells Machine.

P 2PHB = =

D Depth D2(D(D

2-d

2))


3/6

2

Where D and d are the diameters of the sphere and impression, respectively. The parameters

are indicated. Since d = Dsin ,we have

2P

HB =

D2 (1cos )

Rockwell Hardness Test:

Rockwell test constitute the most common method used to measure the hardness because they

are simple to perform and it also fetch results in a very short time. Different indenters are used

based on the material to which the hardness has to be found out. Indenters include spherical

and hardened steel balls having diameters 1/16, 1/8, , inch, and a conical diamond (BRALE)

indenter, which is used for the hardest materials. In this test hardness is determined by

difference in the depth of penetration resulting from the application of an initial minor load

followed by a larger major load; utilization of a minor load enhances test accuracy. For Rockwell

the minor load is load 10 kg , whereas major load is 150 kg. Each is represented by a letter of

the alphabet.

When specifying Rockwell hardness, both hardness number and scale symbol must be

indicated. The scale is designated by the symbol HR followed by the appropriate scale

identifications. For each scale, hardness may range up to

130; however, as hardness values rise above 100 or dropbelow 20 on any scale, they become inaccurate; and

because the scales , have some overlap, in such a situation

it is best to utilize the next harder or softer scale.

Inaccuracies also result if the test specimen is too thin, if

an indentation is made too near a specimen edge, or if two

indentations are made too close to one another.

Specimens thickness should be at least 10 times the

indentation depth. Furthermore, testing of specimens

stacked one on top of another is not recommended. Also,accuracy is dependent on the indentation made into a

smooth flat surface. Each measurement takes only few

seconds.Usually, the C scale is used for harder steels an

the B scale for softer steels; the A scale covers a wider

range of hardness.

Figure 2: Rockwell Hardness machine.


4/6

3

Rockwell Hardness Scales:

Scale Symbol Indenter Major Load (Kg)

A Diamond 60B 1/16 inch ball 100

C Diamond 150

D Diamond 100

E 1/8 inch ball 100

The following precautions are recommended for reproducible results in Rockwell testing.

1. The indenter and anvil should be clean and well seated.

2.

The surface to be tested should be clean, dry, smooth, and free from oxide. A rough-ground surface is usually adequate for the Rockwell test.

3. The surface should be flat and perpendicular to the indenter.

4. Tests on cylindrical surfaces will give low readings, the error depending on the

curvature, load, indenter, and hardness of the material.

5. The thickness of the specimen should be such that a mark or bulge is not produced on

the reverse side of the piece. It is recommended that the thickness be at least 10 times

the depth of the indentation. Tests should be made on only a single thickness of

material.

6.

The spacing between indentations should be three to five times the diameter of theindentation.

7. The speed of application of the load should be standardized. This is done by adjusting

the dashpot on the Rockwell tester. Variations in hardness can be appreciable in very

soft materials, unless the rate of application of the load is carefully controlled. For such

materials, the operating handle of the Rockwell tester should be brought back as soon

as the major load has been fully applied.

Figure 3: Image of the tool sample. Figure 4: Image of the fractured mild steel.


5/6

4

Vickers Micro-Indentation Hardness:

Vickers micro hardness test uses 136 angled diamond pyramid. For each test a very small

diamond indenter having a pyramidal geometry is forced into the surface of the specimen.

Applied loads are very small; they are of the order 1 gram to 1Kg. The resulting indentation is

observed under a microscope and measured; this measurement is automatically converted into

a hardness number. Careful specimen surface preparation may be necessary to ensure a well-

defined indentation that may be accurately measured. They are referred to as micro

indentation-testing methods based on the indenter size.

Modern micro indentation hardness-testing equipment has been automated by

coupling the indenter apparatus to an image analyzer that incorporates a computer and

software package. The software controls important system functions to include indent location,

indent spacing, computation of hardness values, and plotting of data.

Figure 5: Image of the SS304L-Stealite welded sample. Figure 6: Image of Vickers Micro-hardness machine.

Results:

Table showing Hardness vs Distance of SS304L-Stealite welded sample.

Distance (mm) Hardness(HV)

0 235

1 216

2 225

4 249


6/6

5

Figure 7: Hardness vs Distance graph of the welded sample.

From the above figure, hardness is first decreasing up to a distance of 1mm, and increasing

after with distance. Hardness is low at the weld joint because of the brittle weld metal, as it is a

micro indenter the indentation might have happened at the area where Heat affected zone is

present, due to the presence of coarse grains; and the hardness again increases with increase in

distance from the HAZ due to fine grains according to Hall-Petch relationship which states that

hardness increases with decrease in grain size but up to a critical grain size.

The hardness of the tool steel measured by Rockwell hardness equipment is 55 HRC,

very close to real value 60 HRC from [3] by using a load of 150 Kg.

The hardness of the Fractured Mild steel is around 85 HRB by using a load of 31.25 Kg

obtained from Rockwell test.

References:

1.

mml2014_gsreekar_rephardness

Documents