Hardness Test on Low Carbon Steel and Aluminium

2.0 TITLE: HARDNESS TEST ON LOW CARBON STEEL AND ALUMINUM 2.1 - ABSTRACT This study is to figure out how Hardness test is carried out by Rockwell hardness testing to measure the bulk hardness of metallic materials. In this study, aluminum and low carbon steel are tested on their hardness. Hardness of a materials correlates directly with its strength, wear resistance and other properties. ASTM E 18-07 Standard Test Methods for Rockwell Hardness of Metallic Materials is the active and current standard by which Rockwell Hardness Testing is performed, and is also the most frequently used hardness testing method. This report will provide valuable information regarding the hardness test with Rockwell hardness test and the properties of the materials.

2.2 - INTRODUCTION Hardness is the property of a metal that enables it to resist plastic deformation, usually by penetration. However, the term may also refer to stiffness or temper, or to resistance to scratching, abrasion, or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken, or have its shape changed), when a load is applied. The greater the hardness of the metal, the greater resistance it has to deformation. Hardness is not an intrinsic material property dictated by precise definitions in terms of fundamental units of mass, length and time. A hardness property value is the result of a defined measurement procedure. The usual method to achieve a hardness value is to measure the depth or area of an indentation left by an indenter of a specific shape, with a specific force applied for a specific time. There are three types of tests used with accuracy by the metals industry; they are the Brinell hardness test, the Rockwell hardness test, and the Vickers hardness test. ASTM E 18-07 Standard Test Methods for Rockwell Hardness of Metallic Materials is the active and current standard by which Rockwell Hardness Testing is performed, and is also the most frequently used hardness testing method. In this test method, the measurement of the permanent depth of an indentation produced when a standardized tool is acted upon by a known predetermined force is compared to a scale to arrive at a hardness number. This test can be performed on all metals unless the size, shape, or surface conditions of the specimens are prohibitive. The Rockwell Hardness Test is considered to be more accurate and easier to perform than other hardness test.

Figure 1: Example of Rockwell Hardness machine

2.3 - MATERIALS 1. 2 piece of metals – Aluminum and Low Carbon Steel 2. Rockwell hardness machine

2.4 - PROCEDURE 1. The location/area for the test on the low carbon steel piece is chosen. The area were made sure that there was no oil, dirt or rust. The steel piece does not need to be polished. 2. The test piece is put on the working platform 3. The indenter is lowered onto the test piece surface

Figure 2

4. A minor load is applied and a zero reference position is established

igure 3 5. The major load is applied for a specific time period (dwell time) beyond zero

Figure 4

6. The major load is released leaving the minor load applied when the indicator stops moving 7. The reading of the HRB was taken.

Figure 5

8. The experiment was repeated using Aluminum and the data was recorded.

2.5 - RESULT Metals

Brinnel Hardness (HB)

Rockwell Hardness (HRB)

105.3

85.11

160.6

83.55

163.6

84.54

315.8

108.48

231.4

98.57

185.3

90.06

Aluminium

Low carbon Steel

Brinell Hardness (HB) 350 300 250 200 150 100 50 0

1st Reading

2nd Reading Aluminium

3rd Reading

Column1

The average reading for aluminium is 143.17 while the average reading for mild steel is 244.17. Brinell hardness test is used to test the hardness of materials that have structures that are too coarse. As we can see from the bar chart in the comparison above,mild steel records a higher reading on the Brinell hardness test compared to aluminium. With an average reading that is 1.7 times bigger than aluminium,mild steel can be classified as having a higher hardness than aluminium.

Rockwell Hardness (HRB) 120 100 80 60 40 20 0

1st Reading

2nd Reading Aluminium

3rd Reading

Column1

Rockwell Hardness test is also used to test hardness of materials. The test involves determining the hardness of materials by measuring the depth of penetration of an indenter under a large load. The average reading for aluminium is 84.4 while the average reading for mild steel is 99.03.This shows that mild steel have a larger hardness compared to aluminium. In general, solids have 3 responses to force,depending on the amount of force and the type of material.In metals, they exhibit the properties of plasticity which is the ability of a material to permanently change shape in response to the force,but still remain in one piece. The yield strength is the point where elastic deformation give way to plastic deformation. This can be described by a stress-strain curve. In both Brinell and Rockwell hardness tests, mild steel shows that it have a higher degree of plasticity compared to aluminium. Based on the calculations made, it requires a far larger force to deform the structure of mild steel. Hardness of a material to deformation is dependent on its microdurability or small-scale shear modulus in any direction, not to any rigidity or stiffness properties such as its bulk modulus or Young's modulus.

The key to understanding the mechanism behind hardness is understanding the metallic microstructure .At the atomic level, the atoms in metals are arranged in an orderly threedimensional array called a crystal lattice. There are two types of irregularities at the grain level of the microstructure that are responsible for the hardness of the material. These irregularities are point defects and line defects. A point defect is an irregularity located at a single lattice site inside of the overall three-dimensional lattice of the grain. There are three main point defects. If there is an atom missing from the array, a vacancy defect is formed. If there is a different type of atom at the lattice site that should normally be occupied by a metal atom, a substitutional defect is formed. If there exists an atom in a site where there should normally not be, an interstitial defect is formed. This is possible because space exists between atoms in a crystal lattice. While point defects are irregularities at a single site in the crystal lattice, line defects are irregularities on a plane of atoms. Dislocations are a type of line defect involving the misalignment of these planes. In the case of an edge dislocation, a half plane of atoms is wedged between two planes of atoms. In the case of a screw dislocation two planes of atoms are offset with a helical array running between them.

Rockwell Hardness Test 160 140 120 100 80 60 40 20 0

Experimental Value Aluminium

Theoretical Value Column1

Brinell Hardness Test 300 250 200 150 100 50 0

Experimental Value

Theoretical Value Series 1

Column1

The difference in the theoretical value and experimental values from both hardness tests can be explained due to several factors such as presence of foreign materials on the surface of the materials during tests are made such as oxides (rust) due to exposure to air and water. The point defect and line defect in the crystal lattice structure in metals also plays a part in the discrepencies in values from both tests.

2.6 - CONCLUSION Based on the experiment, it is found that as a material, mild steel possesses a greater hardness level compared to aluminium. The difference in the theoretical value and experimental values from both hardness tests can be explained due to several factors such as presence of foreign substance on the surface of the materials and the irregularities that occurs inside the structure of the material.

2.7 - REFERENCES ASTM E 18-07. (n.d.). Retrieved from Westemoreland Mechanical Testing and Research: http://www.wmtr.com/en.rockwellhardness.html Jeffus, L. (2011). Hardness Test. In L. Jeffus, Welding and Metal Fabrication (p. 722). New York: Cengage Learning. Material Hardness. (n.d.). Retrieved from calce umd: http://www.calce.umd.edu/TSFA/Hardness_ad_.htm Rockwell Hardness Testing. (n.d.). Retrieved from Materials Evaluation and Engineering Inc: http://www.mee-inc.com/hamm/rockwell-hardness-testing/ Tabor, D. (2000). Hardness Measurement with Conical and Pyramid Identers . In D. Tabor, The Hardness of Metals (pp. 107-178). United States : Oxford University Press Inc .

2.8 - APPENDIX

Figure 7 Diamond-Cone Brale Indenter used in Rockwell Hardness Testing

Figure 6 : Table of typical applications of Rockwell Hardness Test