Infrastructure Material Science - Civil Engineering Materials - Exam 1 | CE 305, Exams of Civil Engineering

Download Infrastructure Material Science - Civil Engineering Materials - Exam 1 | CE 305 and more Exams Civil Engineering in PDF only on Docsity! 1 CE 305: Civil Engineering Materials Lab Rockwell & Brinell Hardness Tests -Vickers Microhardness Test1 Introduction Hardness is the ability of a material to resist localized plastic deformation, i.e., indentation. Hardness provides a measure of a material’s strength. It can easily be measured in a nondestructive manner. Hard materials are strong, offer more resistance to plastic deformation and resist wear during contact. Many measurement scales are based upon some type of standard. The freezing and boiling points of water determine temperature scales. In this case, the scale is an absolute scale. Unlike the previous examples, hardness has no absolute scale. The hardness of the material is determined by indenting its surface using an indenter that has the shape of a ball, a pyramid, or a cone. Hardness is then calculated based on the value of the applied load, cross-sectional area or the depth of indentation. The indenter is generally made from hardened steel, tungsten carbide, or diamond. Large hardness means: • Resistance to plastic deformation or cracking in compression. • Better wear properties. Several hardness-testing techniques have been developed such as Rockwell, Brinell, Vickers and Knoop, see Figure 1. In all these techniques, a small indenter is forced into the surface of a material to be tested, under controlled conditions of load and rate of application. The depth or size of the resulting indentation is measured, which in turn is related to a hardness number; the softer the material, the larger and deeper the indentation, and the lower the hardness index number. Measured hardnesses are only relative (rather than absolute), and care should be exercised when comparing values determined by different techniques. Figure 1. Hardness Testing Techniques [1]. 1 Original lab document by: Y. L. Shen, T. Khraishi and M. Al-Haik, UNM, Mechanical Engineering. Modified by M. Reda Taha CE 305: Civil Engineering Materials Lab Fall 2008 2 Lab Objectives • To determine the Rockwell Hardness numbers (HR) & Brinell Hardness number (HB) of A36 steel, Tool steel and Heat treated tool steel. • To determine Vickers Microhardness of A36 Steel, Tool Steel and Heat treated tool steel. A. Rockwell Hardness Test The Rockwell hardness test is an empirical indentation hardness test. Its worldwide adoption has likely resulted from the many advantages provided by the test method. The test is fast, inexpensive, and relatively non-destructive, leaving only a small indentation in the material. By correlation with other material properties, the Rockwell hardness test can provide important information about metallic materials, such as the tensile strength, wear resistance, and ductility. A.1. Rockwell Indentation Test Principle Most indentation hardness tests are a measure of the deformation that occurs when the material under test is penetrated with a specific type of indenter. In the case of the Rockwell hardness test, two levels of force are applied to the indenter at specified rates and with specified dwell times, as illustrated for the Rockwell C scale (HRC) test in Figure 2. Unlike the Brinell and Vickers hardness tests, where the size of the indentation is measured following the indentation process, the Rockwell hardness of the material is based on the difference in the depth of the indenter at two specific times during the testing cycle, indicated by the X marks in Figure 2. The value of hardness is calculated using a formula that was derived to yield a number falling within an arbitrarily defined range of numbers known as a Rockwell hardness scale. Because the hardness value is dependent on the definition of the test method, there are no alternative measurement systems to directly or independently measure Rockwell hardness, nor are there intrinsic artifacts to reference. The general Rockwell test procedure is the same regardless of the Rockwell scale or indenter being used. The indenter is brought into contact with the material to be tested, and a preliminary force (formally referred to as the minor load) is applied to the indenter. The preliminary force is usually held constant for a set period of time (dwell time), after which the depth of indentation is measured. After the measurement is made, an additional amount of force is applied at a set rate to increase the applied force to the total force level (formally referred to as the major load). The total force is held constant for a set time period, after which the additional force is removed, returning to the preliminary force level. After holding the preliminary force constant for a set time period, the depth of indentation is measured for a second time, followed by removal of the indenter from the test material. The measured difference between the first and second indentation depth measurements, h, (see Figure 2) is then used to calculate the Rockwell hardness number. The Rockwell Hardness number is based upon the difference in the depth to which an indenter is driven by a definite light or “minor” load and a definite heavy or “Major” load. CE 305: Civil Engineering Materials Lab Fall 2008 5 Table 1. Rockwell hardness scales with the corresponding indenter type, applied forces and typical applications [2]. CE 305: Civil Engineering Materials Lab Fall 2008 6 A.4. Rockwell Hardness Apparatus Figure 4. Rockwell Hardness Tester. Rockwell Hardness systems uses a direct readout machine determining the hardness number based upon the depth of penetration of either a diamond point or a steel ball. Deep penetration indicated a material having a low Rockwell Hardness number. However, a low penetration indicates a material having a high Rockwell Hardness number. The ball indenters are chucks that are made to hold 1/16” or 1/8” diameter hardened steel balls. Also available are ¼” and ½” ball indenters for the testing of softer materials. There are two types of anvils that are used on the Rockwell hardness testers. The flat faceplate models are used for flat specimens. The “V” type anvils hold round specimens firmly. Circular dial and pointers to indicate the Rockwell Scale Indenter Base Base elevation handle Loading Trigger Zero adjuster ring Load assignment knob Regular or Superficial Indicator. CE 305: Civil Engineering Materials Lab Fall 2008 7 Test blocks or calibration blocks are flat steel or brass blocks, which have been tested and marked with the scale and Rockwell number. They should be used to check the accuracy and calibration of the hardness tester frequently. A.5. Rockwell Test Procedure A. Machine Calibration 1- Select the test block, which corresponds to the scale being calibrated. Our three samples, would be tested using Scale D. 2- Place the test block on the anvil beneath the indenter. Care should be taken so that the test block is clean, smooth, and is properly supported by the anvil. 3- Using the table on the m/c (shown in the image below) determine : Figure 5. Rockwell Hardness test blocks CE 305: Civil Engineering Materials Lab Fall 2008 10 B. Brinell Hardness Test There are several methods used to determine hardness. Rockwell and Brinell are two of the most common forms of hardness testing. Rockwell Hardness testing is generally used on harder steels and samples where the Brinell hardness test leaves too large an impression on the specimen to be practical. The Brinell testing machine is relatively simple. The large indenter averages out load variations in the specimen, and it can be used on relatively rough surfaces. However, it does leave rather large indentations in the test specimens. The large Brinell indenter will not make a significant impression in hard materials, so the test is not useful beyond the Rockwell C-60 range. The thickness of the specimen being tested should be about 10 times the depth of the indentation for best accuracy. This prevents the test from being conducted on thin materials. In practice, good values may be obtained if there is no visible effect on the back of the specimen. A close correlation between the Rockwell and Brinell hardness numbers has been developed for steel, and conversion charts are available for changing from one to the other. A close correlation has been found to exist between the Brinell number and the tensile strength of steel. This is extremely important, for a fast hardness test may often be used in place of a tensile strength test. In Brinell tests, a hard (hardened steel or tungsten carbide), spherical indenter is forced into the surface of the metal to be tested, see Figure 6. The diameter of indenter (D) is 10.00 mm (or 5.00 mm). Standard loads (P) range between 500 and 3000 kg in 500-kg increments; during a test, the load is maintained constant for a specified time (between 10 and 30 s). Harder materials require greater applied loads. The Brinell hardness number, HB, is a function of both the magnitude of the load and the diameter of the resulting indentation [ ]22 2 dDDD PBHN −−π = This diameter is measured with a special low-power microscope, utilizing a scale that is etched on the eyepiece. The measured diameter is then converted to the appropriate HB number using a chart; only one scale is employed with this technique. Maximum specimen thickness as well as indentation position (relative to specimen edges) and minimum indentation spacing requirements are the same as for Rockwell tests. In addition, a well-defined indentation is required; this necessitates a smooth flat surface in which the indentation is made. Figure 6. Brinell Testing Technique Surface D Indenter Load P d CE 305: Civil Engineering Materials Lab Fall 2008 11 B.1. Brinell Hardness Apparatus The Brinell hardness test apparatus is shown in Figure 7. Figure 7. DynaBrinell hardness testing system. Test head cover Dial indicator Load wheel latch Load wheel gear Brass capstan wheel Indenter Anvil CE 305: Civil Engineering Materials Lab Fall 2008 12 B.2. Brinell Test Procedure: 1. The needle on the dial indicator should rest on zero. If adjustment is needed turn the faceplate of the dial indicator until the needle is zeroed. 2. Place the specimen to be tested onto the anvil. 3. To bring the indenter into contact with the workpiece, release the load wheel latch and disengage the load wheel gear by swinging it away from the brass capstan wheel. 4. Turn the brass capstan wheel to set the indenter ball lightly into the metal of the specimen. 5. Re-engage the load wheel and close the latch. If the gears do not mesh, simply turn the load wheel a bit until they do fit. 6. Turn the load wheel clockwise until the needle on the dial indicator registers that the desired load (500kg – 4803 N) has been applied. This position should be maintained for 10-15 seconds. 7. Remove the load and back the indenter away from the work piece. CE 305: Civil Engineering Materials Lab Fall 2008 15 C.1. Vickers Hardness Test The Vickers hardness test operates on similar principles to the Brinell test, the major difference being the use of a square based pyramidal diamond indenter rather than a hardened steel ball. The test was developed because the Brinell test, using a spherical hardened steel indenter, could not test hard steels. Also, unlike the Brinell test, the depth of the impression does not affect the accuracy of the reading so the P/D 2 ratio is not important. The diamond does not deform at high loads so the results on very hard materials are more reliable. The load may range from 1 to 120kgf and is applied for between 10 and 15 seconds. The pyramidal shape with an angle of 136° between opposite faces was chosen in order to obtain hardness numbers that would be as close as possible to Brinell hardness numbers for the same specimens. This made the Vickers test easy to adopt, and it rapidly gained acceptance. Unlike Rockwell tests, the Vickers test has the great advantage of using one hardness scale to test all materials. In this test, the force is applied smoothly, without impact, and held in contact for 10 to 15 s. After the force is removed, diagonals, d 1 and d 2 (illustrated in Figure 10) are measured and the average (d) is used to calculate the surface area. Figure 10. Vickers hardness test. Dividing then applied load by the calculated area will give the Vickers hardness number (HV) according to ( ) 22 4.18542sin2000 d P d P HV = α = where d is the mean diagonal in mm, P is the applied load in gf, and α is the face angle (136°). CE 305: Civil Engineering Materials Lab Fall 2008 16 C.2. Vickers hardness Tester C.3. Vickers Hardness Procedure Indentation 1- Turn the power switch on (toggle switch on the left side of the instrument). The power indicator will light will illuminate on the front panel. Stage elevating handle Right Filar Adjustment knob Left Filar Adjustment knob Eye piece Vise Power switch. Dial A –Weight selector knob CE 305: Civil Engineering Materials Lab Fall 2008 17 2- Rotate the Objective-Indenter Turret to the 10X scanning objective. The Turret is equipped with click stops to assist with proper alignment. 3- Place the specimen to be tested in the vise so that the surface to be tested is perpendicular to the diamond indenter. 4- The specimen can be brought into focus by the Stage Elevating Handle located on the right side of the tester. 5- The illuminator intensity control knob, located on the front panel, can be used to vary the light brightness. 6- Select the area of the sample to be indented. This should be the center of the eyepiece view. 7- Rotate the Objective-Indenter Turret to the 40X scanning objective. Bring into focus again, with fine-tuning, if there is a need. 8- Select the weight for the load application. The Dial A –Weight selector knob is located on the right side of the tester. 9- Set the load timer for the length of time that the load is to be applied (usually 10 sec). 10- Rotate the Objective-Indenter Turret to the indenter and press the START Button. The START button will remain illuminated for the duration of the testing cycle. When the start button light goes off the test is done.