General guideline for writing the Hardness Testing lab report:
1. Introduction ..... a brief description of what the lab is about, and define aim and objectives
2. Background/Literature research ..... brief background information of the theories and measurement techniques involved in the assignment, like hardness testing, steels and heat treatment (You need to do some research/reading)
3. Experimental procedure ..... describe the experiments exactly happened during the lab session. You need to provide details, not just copy from the lab sheets
4. Results ..... present the results in table and graph formats, and describe the results and your observations in details
5. Discussion/Analysis .... analyse the results, interpret the results scientifically, identify trends with explanations and compare with theories, error analysis specific to the experiment
6. Conclusions .... draw conclusions to sum up the results and observations.
7. References .... provide a list of references you used in writing the report. These references must be cited in the main body of the report.

1. Objectives
After successfully completing this laboratory experiment, including the assigned reading, the lab problems, and any required report, the students will be able to:
• Familiarize with the techniques used to measure the hardness of engineering materials.
• Identify the effect of carbon content on the hardness of steels
• Examine the influence of heat treatment on the hardness of steels.

2. Theory
Hardness is a measure of a material’s resistance to localised plastic deformation by indentation or scratch. It also gives a good indication of a material resistance to wear. Generally, the harder a material is the more wear resistant it is. For metals, there is also a roughly proportional relationship between hardness and tensile strength.

Hardness testing is relatively straightforward, simple, and in many cases non-destructive and can be made on real components. Special machines, called hardness testers, are commonly placed in engineering organizations.

The test involves making an indentation on the surface of a material with a known indenter and under a known load. The depth and dimensions of the indentation is measured and this is converted from the knowledge of the load applied to a hardness number (no units).

A number of hardness testing techniques are available for testing different materials. The differences are normally in the shape of the indenter. Typical indenters include hardened steel balls, conical diamonds, and diamond pyramids. The selection of a proper technique and indenter for the test depends on the nature, strength, and size of the material to be tested.

The three most widely used techniques are (1) Rockwell hardness test; (2) Brinell hardness test; and (3) Vickers hardness test.

(1) Rockwell hardness tests: constitute the most common method of hardness measurements of virtually all metals and some polymers. Several different scales may be utilised from possible combinations of various indenters and different loads, depending on how hard or how soft the material is. Indenters include spherical hardened steel balls having standardised diameters for testing a wide range of not-so hard materials, and a conical diamond for testing hard materials. The load applied ranges from 15 kg to 150 kg depending on the test scale.

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 identification. For example, 45 HRC represents a Rockwell hardness of 45 on the C scale.

This technique is not suitable for testing brittle materials such as ceramics because (a) the hardened steel balls are not hard enough or (b) the high load applied would induce cracking in the brittle material.

(2) Brinell hardness tests: use a 10.00 mm diameter hardened steel (or tungsten carbide) ball as the indenter. Standard loads range between 500 kg and 3000 kg. Harder materials require greater applied load. The Brinell hardness is specified by the measured number together with the symbol, HB. For example, 500 HB represents a Brinell hardness of 500.

(3) Vickers hardness tests: are widely used to measure the hardness of hard materials, such as hardened steels and ceramics. For each test, a small diamond indenter having pyramidal geometry is forced into the surface of the material. The resulting impression is observed under a microscope and measured; this measurement is then converted into a hardness number. Careful specimen surface preparation (grinding and polishing) may be necessary to ensure a well-defined indentation that may be accurately measured, because the indentation may be quite small due to the small load applied.

Two kinds of Vickers hardness test methods are available depending on the applied load range. In Vickers Microhardness testing, the applied load range is between 5 g and 1000g; while in common Vickers hardness testing, the applied load can be up to 50 kg.

A new type of hardness measurement instrument, called nanohardness tester or nanoindenter, has been commercially available since the mid of 1980’s. The applied load can be as small as 0.01 g. This has facilitated hardness measurements in the nanometer scale, e.g. for nanomaterials and thin films.

3. Apparatus
• Rockwell Hardness Tester with a conical diamond indenter
• Vickers Hardness Tester with a square base pyramid diamond indenter

4. Materials
(1) Plain carbon steels in the as received condition, with the following carbon contents (in weight %):
0.18, 0.35, 0.60, 0.90, 1.20
The only difference is that the carbon content varies from sample to sample.

(2) High speed steel (1.2C + many alloy elements, for making cutting tools):

Samples of this material have been heated-treated in the laboratory, detailed below.

• As-received sample
• Sample hardened at 1300oC and quenched
• Samples quenched and tempered for 1 hour at various temperatures from 200oC to 750oC.

5. Experimental Procedure

• Instruction on the use of the equipment is given during briefing/demonstration by technicians in the laboratory.
• Measure the hardness of all specimens by the Rockwell and Vickers methods.
• In the case of the Rockwell method, the hardness values can be read directly from the scale on the equipment. Two separate readings must be taken and their mean calculated for each specimen. These must agree to within 5%. If they do not agree, take another reading.
• With the Vickers method, the size of the indentation (lengths of diagonals) must be measured by using the attached microscope. Record the ocular readings (mm) in the X and Y directions and calculated their mean. Please note that the two readings should agree to within 0.005 mm. If the values do not fall within this limit, then re-measure or make another indentation and the test again. The hardness values are obtained from the tables provided.

NOTE:
• Use a 150 kg load on the Rockwell tester, Scale C (HRC)
• Use a 30 kg load on the Vickers tester

6. Results

The results should contain

(1) Tables recording the readings and hardness number of all the specimens (refer to the Log Sheet)
(2) Plot of Hardness v Carbon Content for the Plain Carbon Steel for each hardness test.
(3) Plot of Hardness v Tempering Temperature for the High Speed Steel for each hardness test.

7. Report

Your report must contain a brief introduction/background of the theories involved in the experiment, aim and objectives, a description of the experiment conducted, the results in the forms of tables and graphs, a detailed description of the results, a discussion on the results and the conclusions you can draw from this experiment. In your discussion and analysis, you must make as much use of your measured data as possible and examine the following;
• Relationship between hardness and carbon content in steels for the two hardness methods. Is there a maximum or minimum present? Offer your explanations.
• Relationship between hardness and tempering temperature for the high speed steel. Is there a peak or a drop? Offer your explanations.
• Possible errors involved in making the measurements.

You are encouraged to conduct wider reading and research in order to provide scientific analysis of the results.

General guideline for writing the Hardness Testing lab report: 1....