Understand the material hardness and effect of carbon content and heat treatment on hardness

 Aim

“Understand the material hardness and effect of carbon content and heat treatment on hardness”

Aim of this lab work is to understand the material hardness of different materials and effect of carbon content present in material on material hardness and effect of heat treatment of material on material hardness.


Objectives

In order to achieve the aim of this lab work the following mention objectives have to be completed in said sequence

1. Develop a comprehensive understanding of methods used for measuring material hardness

2. Develop a comprehensive understanding of effect of carbon percentage on material hardness

3. Develop a comprehensive understanding of effect of heat treatment on material hardness

4. Perform hardness test to measure material hardness

5. Perform hardness test after increasing carbon content

6. Perform hardness test after heat treatment of material

7. Develop a comprehensive conclusion about work

Theory

Hardness

Material hardness can be defined as the material ability to resist indentation. In more details it can be define as the material ability to resist the localized plastic deformation which is in the shape of indention and scratch. Material hardness is also a great way to understand the material wear resistance as it is observed that greater the material hardness greater is the material wear resistance. Another important relation of hardness with material is that the material hardness is roughly proportional material strength. 

Hardness testing

Material hardness tests are very simple, easy and straight forward to perform as the only thing needed to be done in material hardness test is to produce a dent in material and then the force or load needed to product dent in material is used to measure the material, hardness. Hardness measure during the experiment is usually a dimension less number only that defines the level of hardness of material means hardness is a unit less quantity. Hardness test are destructive test by their nature of testing material but in some case these tests can be considered as non-destructive test as they only create a small dent on material surface and material can be used in any way possible after the test. 

There are three different hardness testing methods namely as Brinell hardness test, Rockwell hardness test, and Vicker hardness test. Difference in type of hardness test tis based on the type indenter used in the experiments, load applied during the experiments and manner in which load is applied during the experiment. There are three different types of indenters used for the hardness test. First is the ball indenter made from the steel and has the diameter of 10 mm in most of the cases. Second the diamond cone and third is diamond pyramid.

Material Sample

In order to check the effect of carbon content and heat treatment on the material, different samples of material are provided as per below mention details

For checking carbon content


 

Plain Carbon Steel Sample No

Carbon content

1

0.18

2

0.35

3

0.60

4

0.90

5

1.20

 

For heat treatment

·         As-received sample

·         Sample hardened at 1300oC and quenched

·         Samples quenched and tempered for 1 hour at various temperatures from 200oC to 750oC.

 

Sample No for Tempering

Tempering

Temperature, oC

01

 

As-received

02

As-hardened

(Room temp. tempering, 25oC)

03

200

04

400

05

450

06

500

07

550

08

600

09

700

Procedure
Following are the steps needed to perform a hardness test on material
First step is the setting up of the apparatus for the experiment and this includes ensuring that apparatus is placed on horizontal surface, no initial load is applied on the machine, related indenter is installed properly 
Second step is the preparing the sample for the test and that includes making a clean horizontal surface with dimensions as per standard provided.
Third step is to place the sample material on the anvil of the machine in the manner that it is directly below the indenter
Fourth step is to move the elevation screw of the machine to move the anvil up so that indenter and work piece almost touches each other
Fifth step is to select the load as per type of indenter and then apply the load for few second
In the case of Brinell hardness and Vicker hardness manual reading of the dent diameter and diagonal end will be taken respectively and they will be used to calculate the respective hardness number. In Rockwell case hardness number will be available directly on the screen of the machine
Repeat the process for different sample of material provided for the test and record data of each material in the table provided

Results

Table 1 Carbon content and Material Hardness

 

Rockwell Hardness

Vickers Hardness

% Carbon

Hardness Readings, HRC

Ocular Readings (mm)

Hardness

 

1

2

Mean

X

Y

Mean

HV

0.18

21.7

24.1

22.9

0.464

0.47

0.467

255

0.35

30.5

31

30.75

0.4195

0.4165

0.418

321

0.6

36.7

36.1

36.4

0.375

0.363

0.369

387

0.9

36.5

36.6

36.55

0.373

0.377

0.375

396

1.2

31

27.4

29.2

0.399

0.402

0.4005

346

 

Table 2 Tempering Temperature and Material Hardness

 

Rockwell Hardness

Vickers Hardness

Tempering

Temperature, oC

Hardness Readings, HRC

Ocular Readings (mm)

Hardness

 

1

2

Mean

X

Y

Mean

HV

As-received

66.6

66

66.3

0.239

0.24

0.24

966

(for reference only)

As-hardened

67.1

67

67.05

0.234

0.244

0.239

990

(Room temp. tempering, 25oC)

200

65

64.2

64.6

0.244

0.244

0.244

934

400

63.3

63.8

63.55

0.244

0.244

0.244

934

450

63.7

64.1

63.9

0.238

0.241

0.24

960

500

64.8

5.2

35

0.225

0.23

0.228

1070

550

66.6

66.7

66.65

0.214

0.215

0.215

1203

600

61.4

60.7

61.05

0.258

0.259

0.259

829

700

46.6

46.6

46.6

0.329

0.336

0.333

502




Rockwell hardness test was performed using the basic method and procedure mention above in procedure section and results were recorded in Rockwell section of provided table. Two different values were taken and an average of these values was calculated and then that averaged value was used a hardness value of the material. Graph one shows the comparison of carbon content in material with the material hardness with carbon content on x axis and hardness on y axis. Graph show that by increase in carbon content in material the hardness of the material increase but this increase is up to a certain limit and after that material hardness start to decrease. This is due to the fact that steels that has more than 0.8 percent Carbon, has a combination of cementite and pearlite in it. When more carbon is added steels, cementite which is formed and it is brittle but hard, so it increases hardness of material. After a certain limit addition of carbon start to make material brittle enough to decrease its hardness.


 
Figure 2 Vicker hardness and carbon content
Vicker hardness test was performed using the basic method and procedure mention above in procedure section and results were recorded in Vicker section of provided table. X and Y values were taken and an average of these values was calculated and then that averaged value was used to calculate the hardness of the material. Graph two shows the comparison of carbon content in material with the material hardness with carbon content on x axis and Vicker hardness on y axis. Graph show that by increase in carbon content in material, the Vicker hardness of the material increase but this increase is up to a certain limit and after that material Vicker hardness start to decrease. This is due to the fact that steels that has more than 0.8 percent Carbon, has a combination of cementite and pearlite in it. When more carbon is added steels, cementite which is formed and it is brittle but hard, so it increases hardness of material. After a certain limit addition of carbon start to make material brittle enough to decrease its hardness.


 
Figure 3 Rockwell hardness and tempering temperature

In order to check the effect of tempering temperature on material Rockwell hardness, Different material samples were prepared at different tempering temperatures. Value of each sample was recorded and noted again the respective temperature column present in table provided. Graph three was generated for the effect of temperature on Rockwell hardness with temperature on x axis and Rockwell hardness on y axis. Graph show that increase in tempering temperature has very small effect on hardness initially where hardness increase very little. This small increase in hardness is up to 550 degree temperature and after that for 600 and 700 degree temperature hardness start to decrease steadily but continuously. Increase in material hardness is due to the fact that temperature removes the internal stress and allows material to have stronger bond between atoms but this is up to a certain limit after that increase in temperature makes material soft which reduce hardness.

 

Figure 4 Vicker hardness and tempering temperature
Different material samples were prepared at different tempering temperatures, in order to check the effect of tempering temperature on material Vicker hardness. Hardness Value of each sample was recorded and noted again the respective temperature column present in table provided. Graph four was generated for the effect of temperature on Vicker hardness with temperature on x axis and Vicker hardness on y axis. Graph show that increase in tempering temperature has very sharp effect on hardness lately where hardness increase very sharply. This sharp increase in hardness is at 500 and 550 degree temperature and after that for 600 and 700 degree temperature hardness start to decrease steadily and continuously. Increase in material hardness is due to the fact that temperature removes the internal stress and allows material to have stronger bond between atoms but this is up to a certain limit after that increase in temperature makes material soft which reduce hardness.

Conclusion
Aim of this lab work was to understand the material hardness of different materials and effect of carbon content present in material on material hardness and effect of heat treatment of material on material hardness. In order to check the effect of carbon content and heat treatment on the material, different samples of material were provided with the following carbon contents (in weight %): 0.18, 0.35, 0.60, 0.90, 1.20 and sample hardened and quenched  at 1300 C and set of samples tempered and quenched for one hr at temperatures range from 200 C to 700 C. Graph show that by increase in carbon content in material the hardness of the material increase but this increase is up to a certain limit and after that material hardness start to decrease. This is due to the fact that steels that has more than 0.8 percent Carbon, has a combination of cementite and pearlite in it. When more carbon is added steels, cementite which is formed and it is brittle but hard, so it increases hardness of material. Graph show that increase in tempering temperature has very sharp effect on hardness lately where hardness increase very sharply. This sharp increase in hardness is at 500 and 550 degree temperature and after that for 600 and 700 degree temperature hardness start to decrease steadily and continuously. Increase in material hardness is due to the fact that temperature removes the internal stress and allows material to have stronger bond between atoms but this is up to a certain limit after that increase in temperature makes material soft which reduce hardness.

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