Introduction: Hardness is a characteristic of a solid material expressing its resistance to permanent or plastic deformation. There are three general types of hardness measurements: (1) scratch hardness, (2) Indentation hardness, and (3) rebound or dynamic hardness. Among three only indentation hardness is of major engineering interest for metals. Some of the scales used for indentation hardness in engineering - Rockwell, Vickers, Brinell, and Knoop - can be compared using practical conversion tables for a particular material. The different techniques are shown in Figure 2.1. Figure 2.1. Different hardness test techniques. When testing metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk materials. There are several mechanisms of hardening of a material system. Some are listed below 1. Strain Hardening (Brass) 2. Precipitation Hardening (Al alloy) 3. Martensitic Transformation (Rapid Quenching) (Steel) 4. Refining Grain Size 5. Solid Solution Strengthening In this lab, we will evaluate first three mechanisms through hardness measurement (Rockwell hardness Test) for different material systems. 1 January 9, 2010
Rockwell Hardness: This hardness test uses a direct reading instrument based on the principle of differential depth measurement. Initially a minor load is applied, and a zero datum position is established. The major load is then applied for a specific period and removed, leaving the minor load applied. The resulting Rockwell number represents the difference in depth from zero datum position as a result of the application of major load. The conical diamond (Brale) indenter is used mainly for testing hard materials such as hardened steels and cemented carbides. Hardened steel ball indenters with diameters 1/16, 1/8, 1/4, 1/2 in. are used for testing softer materials such as fully annealed steels, softer grades of cast irons, and a wide variety of nonferrous metals. Generally, in Rockwell testing, the minor load is 10 kgf, and the major load is 60, 100 or 150 kgf. The indenter used may be either a diamond cone (Brale) or steel ball, depending principally on the characteristics of the material being tested. Common Rockwell hardness scales are provided in Table 2.1. Table 2.1. Common Rockwell hardness scales Objective: 1. To understand the importance of hardness test in engineering. 2. To learn how to use Rockwell hardness tester. 3. To understand the different hardening mechanisms in different material systems and effect on hardness values. 4. To understand the significance of Engineering Standards. Equipment and Materials: 1. Rockwell Hardness tester 2. Material systems a. Steel (Martensitic 440a vs. 440c ) b. Aluminum alloy (6061-T6 vs. 6061-0) c. Brass (Preannealed vs. Postannealed) Experimental Procedure: Three different material systems (6 samples) will be tested using Rockwell hardness tester. The test parameters and conditions will be selected according to the materials. For each sample a total of 3 measurements are required for statistical analysis. In order to obtain uniform and accurate results, it is important that all tests have to be conducted under standard conditions. 2 January 9, 2010
The American Standard for Testing and Materials (ASTM) has set up standard methods of hardness tests, which are included in the reference. Proceed with the following steps: a) Identify the material of each specimen used. b) Clean the surface of the specimen using an emery cloth, if needed. c) Position the specimen under the indenter of the instrument. d) Apply a specific load depending on the material and indenter. e) The student will need to insert the correct tip and change the amount of load applied for the Rockwell tests. (e.g. The Rockwell B scale uses a 1/16-inch diameter hard steel ball and a 100-kg load. The C scale uses a conical diamond point and a 150-kg load. g) Take other hardness measurements at different locations. In all cases, be sure to write your observations in your notebook for each test. You need to include these observations in your report. Lab Deliverables: 1. Report the Rockwell hardness number for each test. Report the values for all tested specimens in a table format. One column should indicate the average values, and an adjacent column should show the standard deviations of the results. (Include raw data) 2. Predict the tensile strength from the Rockwell hardness numbers for steel and brass specimens using Figure 2.2. 3. Comment on the usability of the hardness experiment (specify few examples if possible). 4. Discuss the comparisons of different material systems by comparing the hardness values obtained. (e.g. compare within a group: Martensitic Steel 440a vs. 440c etc.) 5. What is the limitation on the thickness of specimens for a hardness test? Explain. Calculate the minimum thickness for a specimen for the Rockwell test. 6. What are the limitations for distance from specimen edge to indentation and distance between indentations? Explain why these limitations exist in both cases. 7. How would you measure the hardness of 1 a) Unmovable part of a large machine which is very heavy to transport b) How do you differentiate between these pair of materials: i. Martensitic vs. Austenitic Steel ii. Steel vs. Al iii. Brass vs. Al 8. Use your observations from the lab to support your discussion. References: 1. W.D. Callister, Jr., Materials science and Engineering an introduction, 6 th edition (2003). 2. M.P. Groover, Fundamentals of modern manufacturing, 3 rd edition, (2007). 3. Standards: 1 Question 7: Don t think only from hardness perspective. Use lateral thinking. Carries extra credits. 3 January 9, 2010
a. Conversion : ASTM E 140: 1956 b. Rockwell: DIN50103, ASTM E l8-74, BS 4175: Part 1: 1967, TS 14O Figure 2.2. Hardness and tensile strength co-relation. 4 January 9, 2010
Figure 2.3. Comparison of different hardness scales. 5 January 9, 2010
IE 337: Materials and Manufacturing Processes Lab # 2: Sample Material Rockwell hardness Reading Average Std. Deviation 1 2 3 1 Al 6061-0 (Annealed) 2 Al 6061-T6 (Precipitation strengthened) 3 Stainless Steel 440a 4 Stainless Steel 440c 5 Brass (Half hardened) 6 Brass (Annealed) Note: 6 January 9, 2010