MENG 302L Lab 1: Hardness Testing

Transcription

1 Introduction: A MENG 302L Lab 1: Hardness Testing Hardness Testing Hardness is measured in a variety of ways. The simplest is scratch testing, in which one material scratches or is scratched by another. This allows materials to be ranked by comparative hardness. The Mohs Scale 1 is such a test and is commonly used in minerals science. Static Indentation tests involve slowly pressing a hard, pointy object (the indenter) into the material being tested and measuring the resulting dent. Rockwell 2, Brinell 3, and Vickers 4 tests are commonly used for metals. Lab 1 will deal with Rockwell and Brinell testing. Durometer 5 hardness testing is another static indentation test. It is used for plastics and rubbery materials. Dynamic, or Rebound hardness tests measure the bounce of a calibrated hammer dropped onto a surface. Scleroscope 6 testing is an example of such tests. B Rockwell Testing 7 Rockwell hardness tests are conducted in four steps: 1) The minor load (usually 10 kg) is applied to the indenter. This tends to push it through any surface films and finish marks. 2) The major load (usually 60, 100 or 150 kg) is applied. This causes the indenter to (surprise!) dent the material surface. 3) The major load is removed, leaving the minor load on. Doing this relaxes the elastic deformation, leaving the indenter resting in the plastically deformed dent. 4) The difference in indenter height between steps 1 and 3 is measured. Call this distance h mm. The Rockwell hardness is then computed as Rockwell Hardness 100. (for diamond indenters) Rockwell Hardness 130. (for ball indenters) There are a large number of Rockwell Hardness scales. Each scale represents a specific combination of indenter and major load. The different scales make it possible to measure the Rockwell hardness of a huge variety of materials. There are also Regular and Superficial Rockwell scales. The discussion that follows refers to Regular Rockwell scales. Superficial scales are used for measuring thin and soft materials. The A, C and D scales use a diamond indenter and 60, 150, and 100 kg major loads, respectively. Other Rockwell scales use hard steel balls of various diameters and major loads of 60, 100 or 150 kg. Rockwell Hardness values are expressed as HR(scale) (value). A very hard steel might read HRC 62. HRA 55 might be an annealed mild steel. The ABS plastic impact pieces we will destroy in Lab 8 are rated HRR 105. (Rockwell R scale uses a ½ ball with a 60 kg major force.) This is indicative of a soft material and a nasty mixing of imperial and metric units. Advantages of Rockwell tests: They re fast, easy, versatile, and inexpensive. Disadvantages: A permanent dent is created in the sample. The large number of scales that make Rockwell testing versatile can also make it hard to select the appropriate scale for a particular material. Page 1 of 8

2 C Brinell Testing 8 Brinell Hardness tests are conducted by pressing a hard ball of diameter D (mm) into a test piece under a known weight P (kg). The weight is removed after a specified length of time. The diameter of the resulting dent d (mm) is then measured, and the hardness is computed as follows: HB π 2 If the hardness test was conducted with a 10 mm ball, a 3000 kg weight, and a delay time of 10 to 15 seconds, Brinell hardness is written as HB (value). For other test conditions, the hardness designation is supplemented as HB (value) (dia)/(load)/(time) where (dia) is the indenter diameter in mm, (load) is the load applied in kg, and (time) is the loaded time in seconds. For example, a Brinell hardness test conducted with a ½ ball and 60 kg load applied for 30 seconds would be written as HB (value) 12.7/60/ T6 aluminum has a hardness of HB 95 10/500/ T0 aluminum has a hardness of HB 30 10/500/30 10 M1020 HR steel has a hardness of HB Brinell hardness can be used to estimate tensile strength of steel 12 : S (psi) HB * 515 (for HB < 175) S (psi) HB * 490 (for HB > 175) Advantages of Brinell tests: - One scale for virtually everything. - Yield strength estimation. Disadvantages: - Permanent denting. - Not useable for very hard materials. - Slower and trickier to perform than Rockwell. - Different indenter/weight combinations yield different values for the same material. D Brinell Testing with the QV-700 & 2.5x Eyepiece Here are some facts you need to know for doing Brinell tests with the QV-700: - After indenting your sample, lower it 7 full turns of the crank to bring it (approximately) into focus under the eyepiece. - The stadia marks in the eyepiece are mm apart. - The micrometer wheel moves the crosshair mm per revolution. - So, to measure dent diameters o Maneuver the dent so it is visible in the eyepiece. o Use the micrometer wheel to position the crosshair on the right edge of the dent. Record the reading in the data table. o Use the micrometer to move the cross hair to the left edge of the dent. o Record the reading in the data table. o Subtract the second reading from the first and multiply the result by 0.4 mm. That s the dent diameter. Page 2 of 8

3 Equipment: Qualitest QV-700 Universal Hardness Test Machine, O1 tool steel and 1018 steel, 6061-T6511 and 2024-T4 aluminum, 385-H02 brass, etc. Pre-Lab: a) Look up Brinell and/or Rockwell hardness values for the following materials. If you can find one but not the other, use a conversion table. There s a conversion table posted in the lab, and they re easy to find online. Don t forget to include the scale for Rockwell hardness! Material Brinell Rockwell Source O1 tool steel, annealed O1 tool steel, hardened CF 1018 flat 6061-T6 rect 2024-T4 rect 385 H02 brass (UNS C38500) b) Determine Brinell hardness given the following: Indenter Dia Major load Load time Dent Dia Brinell Hardness Value* 5 mm 150 kg 30 sec 2.02 mm * Don t forget to include ball diameter, load and test duration! Page 3 of 8

4 Figure 1: QV-700 Hardness Tester Figure 2: QV-700 Display Screen Page 4 of 8

5 Objective: Determine and compare the hardness of two materials using Rockwell and Brinell scales. Caution: It s easy to confuse Brinell hardness (HB) with Rockwell B-Scale hardness (HRB). Don t. Procedure: 1. Select two samples to test. (You may bring in your own material to test, but get the lab instructor s OK first.) Measure & record in the worksheet. 2. For all tests, do the following: (See Figures 1 & 2) a. Make sure the indenter, material, and platen are clean. b. Pick a spot for your test at least 3 indenter diameters (HB), or 3 mm (HR), from edges and dents from previous tests. c. Start with the loading handle in the minor load (forward) position. d. Center the fine adjustment knob. e. Turn the crank CW to bring the material into contact with the indenter. Keep turning until the SET line is near the horizontal mark on the screen. f. Use the fine adjustment knob to line up the Horizontal and SET marks precisely. g. Push the loading handle away from yourself to apply the major load. h. After the screen display stops moving, return the loading handle to the minor load position. 3. Install the 1/16 indenter. Set the load dial to 100 kg (980 N). Set the test dial to HR. Measure HRB once for both of your samples as follows: a. Perform steps 2a through 2h above. b. After the screen display stops moving, read HRB from the right side scale and record it in the worksheet. c. Turn the crank CCW to unclamp the test piece. 4. Install the diamond indenter and test both samples three times using HRA or HRC. If the sample is HRB 100 or softer, set the load dial to 60 kg (588 N) to measure HRA. If the sample is harder than HRB 100, set the load dial to 150 kg (1470 N), and measure HRC: a. Perform steps 2a through 2h above. b. After the screen display stops moving, read HRA or HRC from the left side scale and record it in the worksheet. c. Turn the crank CCW to unclamp the test piece. 5. Install the 5 mm indenter. Set the load dial to 150 kg (1470 N). Set the test dial to HB. Measure HB for one of your samples as follows: a. Perform steps 2a through 2h above. b. Turn the crank CCW ~ 7 turns. c. Slide the platen & sample over to the microscope. d. Measure the dent diameter as described in Section D above and record it in the worksheet. Page 5 of 8

6 Write-up: (Worksheet) - The executive summary should include a table showing published and measured hardnesses of your materials. - The Results consist of the completed handout. - In the Discussion, talk about repeatability of the multiple HRA or HRC measurements. Compare published to measured hardness values of your materials. Name likely causes of any discrepancies. Mention anything else you deem worthwhile. - For the Conclusion, recap everything in 50 words or less. Date: Team Members: Data Table/Worksheet Material Thickness x Width (in x in) HRB (1 test each) HRA or HRC (3 tests each. Remember to specify HRA or HRC) Sample 1 Sample 2 1) 2) 3) HB Indenter Dia 5 mm 5 mm HB Major Load 150 kg 150 kg HB Right Edge Reading: HB Left Edge Reading: HB Dent Dia: (Calculate per Sect. D, pg 2) HB* (Calculate per Sect. C, pg 2) * Don t forget to include ball diameter, load and test duration! Page 6 of 8

7 References: 1 Mohs scale: 2 ASTM E l8: Standard Test Methods for Rockwell Hardness of Metallic Materials 3 ASTM E 10: Standard Test Method for Brinell Hardness of Metallic Materials 4 ASTM E 92: Standard Test Method for Vickers Hardness of Metallic Materials 5 Shore Durometer: Matweb > Search 6061-T6 > 2 nd choice 0be5b&ckck=1 10 Matweb > Search > 4 th choice 11 Matweb > Search M1020 > 7 th choice 12 Efunda > Search hardness tensile strength > choose convert approximate tensile strength Page 7 of 8

8 Notes: Page 8 of 8