Engineering Materials: Steel Hardenability & Brittle Fracture in Ductile Materials (Lab 3), Lab Reports of Civil Engineering

Download Engineering Materials: Steel Hardenability & Brittle Fracture in Ductile Materials (Lab 3) and more Lab Reports Civil Engineering in PDF only on Docsity! CE 314K Properties and Behavior of Engineering Materials Laboratory 3 Hardenability of Steel and Brittle Fracture of Normally Ductile Materials Fall 2009 2 HARDENABILITY OF STEEL OVERVIEW “Hardenability” refers to the ability of a steel alloy to transform to martensite during a prescribed quenching treatment. Martensite is the hardest phase in steel and is formed when steel is cooled very quickly (quenched). The presence of more martensite makes harder steel. In this laboratory the hardenability of three steels will be examined using a Jominy hardness specimen. The specimens will be heated in a furnace to austenize the steel, removed, and then quenched at one end with a water jet. The end of the specimen in contact with the water jet cools very quickly while the material further from the end cools at a slower rate. The hardness of the quenched specimen will be measured along its length. Steels with good hardenability will have high hardness further along the specimen than steel with poor hardenability. The compositions of the three steels we will be examining are listed in Table 1. The first two steels are alloy steels. These steels are often referred to as Chrome-Moly steels. The third steel is a plain high carbon steel with only manganese and silicon added. The other elements present in this steel (e.g., nickel) come from the scrap used to make the steel and are not intentionally added. Table 1. Composition of Steel Samples Steel %C %Mn %P %S %Si %Ni %Cr %Mo %Cu 4340 0.41 0.70 0.009 0.019 0.22 1.64 0.74 0.21 0.05 4140 0.42 0.73 0.015 0.039 0.18 0.16 0.73 0.15 0.18 1040 0.48 0.72 0.021 0.009 0.21 0.05 0.14 <0.01 0.10 In addition to the hardness tests of the Jominy specimens, a hardness test will also be performed on the same steel material used in the tensile test from the previous laboratory session. You will compare the results of the estimated tensile strength from this hardness test with the tensile strength measured in your test. DEFINITION OF HARDNESS Hardness is defined as the resistance of a material to penetration of its surface. Hardness is related to strength, and hardness measurements can be used to estimate the strength of a material without subjecting it to destructive testing. Hardness testing is particularly common in checking the strength of heat-treated products, which are typically machined or formed in the soft (annealed) condition and then hardened. Hardness is measured using a standardized indenter, which is pushed into the test specimen with a prescribed force. The hardness is then calculated based upon the depth or the area of the indentation. Various testing machines and corresponding hardness scales are used. The softer the material, the larger the indenter and the smaller the prescribed load applied to it. 5 GENERAL REMARKS CONCERNING HARDENABILITY The hardenability of steel is the ease with which it can be hardened by rapid cooling (quenching). The cooling rate of a location in an object decreases with the distance from the surface being cooled. Therefore, the center of a bar will cool more slowly than its surface. Steel with good hardenability will produce high hardness both at the surface and at the center of the bar. Hardenability is related to the production of martensite. Steels with good hardenability will produce martensite on quenching not only at the surface, but at the center as well. Steel with lower hardenability will have a martensite layer on the surface due to the rapid cooling there, and a ferrite-pearlite interior due to the slower cooling rate there. Steels with high hardenability do not have to cool as rapidly as steels with lower hardenability to form martensite. The hardness of the quenched steel can be used to determine if martensite has been formed. Figure 1 shows the relationship between the hardness of martensite and ferrite + pearlite versus carbon content. A hardness of 10 Rc on a steel with 0.40% C would indicate that the steel had a ferrite + pearlite microstructure. A hardness of about 58 Rc would indicate that the steel had a martensite structure. The hardness measured in the Jominy specimen gives us an indication of the microstructure of the steel after quenching. Figure 1. Carbon Composition vs. Hardness for Martensite and Ferrite + Pearlite JOMINY END QUENCH TEST The Jominy End Quench test is used to determine the hardenability of steel. A steel bar with a diameter of 1 in. and a length of 4 in. is heated into the austenite region. The bar is then removed from the furnace and placed above a water jet with a standard flow rate as depicted in Figure 2. The end of the bar is quenched (cooled) very rapidly; the cooling rate decreases with increasing distance from the end in contact with the water. After quenching the Jominy specimen, the hardness along the length of the specimen is measured and the results are used to judge the hardenability of the steel. 6 Figure 2. Jominy End Quench Test (from Herman W.W. Pollack, Materials Science and Metallurgy, 4 th Edition, Prentice Hall, 1998) The cooling rate along the length of the Jominy bar is given in Figure 3. The measured hardness can be used to determine the cooling rate required to form martensite in an alloy. For example, if a 75 mm bolt (Figure 4) is to be quenched to form martensite, the suitability of an alloy for this bolt can be determined by comparing the hardening of the Jominy bar for the expected cooling rates when the bolt is quenched. The difference between the bolt and the Jominy specimen is that cooling rate for the bolt is the fastest on the surface and decreases toward the center, while in the Jominy specimen the cooling rate is fastest at the quenched end and decreases with increasing distance from the quenched end. The cooling rates at the surface, ¾ radius, mid-radius and center of the bolt are given in Table 4 for two different quenching media. The approximate distance from the quenched end of the Jominy specimen at which the same cooling rate exists is also listed. This information can be used to specify the requirements of the Jominy test. For example, if the product is to be oil-quenched and it is desired that martensite form in the center of a 75 mm bolt, the material must form martensite at a cooling rate of 5.5°C/sec, which is the cooling rate for the center of the bolt. This cooling rate occurs at approximately 25 mm from the end of the Jominy bar (see last column of Table 4). Therefore, the hardness at 25 mm from the end of the Jominy bar should equal the hardness of the martensite for the alloy used to ensure the transformation of the center of the bolt to martensite in an oil quench. 7 Figure 3. Cooling Rate vs. Distance from Quenched End R/2 R 3R/4 Figure 4. Diagram of a 75-mm Bolt Table 4. Jominy Results for a 75-mm Bolt Dqe - distance from quenched end of Jominy bar that has the same cooling rate at 700°C Agitated water quench Agitated oil quench Position Degrees C/sec Dqe, mm Degrees C/sec Dqe, mm Surface ® 100 3 20 11 ¾-radius 27 9 9.5 18 Mid-radius 14 14 7.5 21 Center 11 17 5.5 25 10 0 10 20 30 40 50 60 70 80 0 Temperature A b s o rb e d E n e rg y Dynamic Loading Static Loading Loading Rate Temperature Shift Figure 7. Influence of Strain Rate on Steel Toughness Notch toughness or notch sensitivity, the tendency of a ductile material to behave in a brittle manner, can be obtained from the Charpy impact test shown in Figure 8. In this test a notched specimen is struck with a hammer and the total energy absorbed in breaking the specimen is determined. Standard impact specimens are shown in Figure 9. Pendulum-type Hammer Notched Sample Figure 8. Charpy Impact Test 11 (a) Charpy impact specimen 45° Specimen 10 x 10 x 55 mm (0.394 x 0.394 x 2.165) Striking edge Direction of blow 40 mm (1.574 in.) Cross-section at notch 8 mm (0.315 in.) (b) Izod impact specimen Direction of blow Notch (same as Charpy) 22 mm (0.866 in.) Striking edge 28 mm (1.102 in.) Specimen 10 x 10 x 75 mm (0.394 x 0.394 x 2.952 in.) Vise Figure 9. Standard Impact Specimens Materials like carbon steels, plastics, and ceramics are notch-sensitive. Materials that are normally brittle are often not notch-sensitive although their strength is seriously impaired by the stress concentration at the notch. Also, temperature plays an important role in this behavior as shown in Figures 6 and 7; yield stresses decrease with increasing temperatures. 12 JOMINY END QUENCH TEST AND ROCKWELL HARDNESS TEST PROCEDURES The hardenability of three steel specimens will be tested. Your lab group will watch demonstrations of the Jominy End Quench procedure described below and perform hardness testing of Jominy specimens that have already been heat treated and prepared. Your lab group will also perform hardness testing on a sample of each of the three materials that has not been heat treated. The three materials being tested are 4340, 4140 and 1045 steel. In addition to hardness testing of these materials, you will perform hardness testing on a sample of the steel specimen you previously tested in tension. JOMINY END QUENCH TEST 1. The furnace has been preheated to austenizing temperature, approximately 1650-1700°F. 2. Note type of steel, and place Jominy specimen into furnace. 3. Before removing the specimens from the furnace, adjust the water jet so that the stream of water rises to a free height of 2.5 in. above the 0.50 in. orifice. Remove the specimen from the furnace, and within 5 seconds place it in the stand above the water jet. Continue quenching the specimen for 10 minutes; and then remove it. Continue to cool it under running water from the tap. During quenching, observe how the end of the specimen changes color rapidly while further up the specimen the color remains red longer. This is an indication of the difference in the cooling rates between the end of the specimen and the locations further up, away from the water quench. 4. The specimen will be cleaned by the laboratory technician to remove the mill scale and create a flat surface for hardness testing. ROCKWELL HARDNESS TEST PROCEDURE 1. Obtain hardness values using the Rockwell B scale for the steel (1018) that was tested in tension. Take the average of three readings. 2. Obtain hardness values using the appropriate Rockwell scale for the three specimens (1045, 4140 and 4340) prior to heat treatment. 3. Before placing the Jominy specimen into the fixture, clean any debris from the magnetic supports that would prevent the specimen from seating in the supports. 4. Place the specimen into the holder with the ground flat portion facing up and perpendicular to the axis of the hardness tester. 5. Use the carriage screw to align the quenched end of the Jominy specimen directly under the tip of the hardness penetrator. 6. Turn the carriage screw one revolution. This advances the specimen 1/16 inch. Perform a Rockwell hardness test at this location and record the results in the table below. 15 Data Sheet: Charpy Impact Testing Date ______________________________ Name: _____________________________ Group Members:____________________ ___________________________________ Specimen Fracture Energy (ft-lb) Description Temperature Steel Aluminum Cooled in Dry Ice -60 °C 3 6 Cooled in Solution of Alcohol and Dry Ice Room Temperature Heated in Water Bath Heated in Oven 125 °C 70 8 Heated in Oven 150 °C 73 9 16 ASSIGNMENT - LABORATORY 3, HARDENABILITY OF STEEL AND BRITTLE FRACTURE OF NORMALLY DUCTILE MATERIALS For this calculation assignment you must provide the requested data and answer all questions. Follow the guidelines for calculation assignments in the CE 314K Technical Writing Guide. This assignment will be graded by the technical TAs only. Submit one copy of each of the following: • Cover sheet • Calculations o Include at least example calculations for each step o Hand calculations and sketches should be done in pencil, not ink o Always show the formula before inserting numerical values and define notations o Reference all equations and formulas following the format discussed in the CE 314K Technical Writing Guide (you may reference the textbook, this lab manual, or texts from other classes, the library, etc.) o Keep track of significant figures o State any assumptions o Make note of any errors in testing • References (only include documents referenced within your calculations) • Appendix A – Measured Data (a neat copy of the data sheet is adequate) Assignment 1. Plot estimated tensile strength after heat treatment versus distance from the quenched end for each of the three materials tested. Document your calculations. Round to the nearest whole number when interpolating the ultimate strength from Table 3. Plot the results from all three steels on one graph. Plot the unconnected data points and fit trendlines to each data set using Excel. 2. Plot the ratio of estimated tensile strength after heat treatment to the estimated tensile strength before heat treatment as a function of distance from the quenched end for each material tested. Document your calculations. Plot the results from the three steels on one graph. Which steel is the most hardenable? What does “hardenable” mean and why does it matter? 3. For the 4140 steel, plot your actual data (showing hardness versus distance from quenched end) and the typical data for 4140 steel, as shown in Figure 5, on the same graph. Comment on how close your actual data is to the “typical” data. Be sure to clearly label this graph, showing your data and the typical data. 4. Using the ASTM table (Table 3), estimate the tensile strength of the 1018 specimen tested last week in direct tension. Compare this estimated tensile strength to the measured tensile strength from last week’s laboratory (Lab 2). How well does the estimated tensile strength match the measured tensile strength? In your opinion, is this “nondestructive” hardness test a good substitute for direct tensile testing? 17 5. Assume that the three steels tested in lab were used to produce 75 mm diameter bolts that were heated to the austenizing range and quenched in an agitated oil bath. Estimate the tensile strength for each bolt by using the distances in the last column of Table 3 to give the location along the Jominy specimen that corresponds to 4 radial locations in your bolt (surface, ¾ radius, mid radius, and center). Then estimate the tensile strength of the locations using the correlation between hardness and tensile strength given Table 2. Calculate the tensile strength of the bolts using a weighted average of the strengths. Calculate the weighted average by summing the strengths of the 3 areas divided by the total area. Estimated tensile strength = A AFAFAF 3u32u21u1 ++ R/2 R 3R/4 Where A= A1+A2+A3, and Fui= the estimated tensile strength of each area, which is approximated by the estimated strength at the edge of each area. 6. Plot the Charpy curves (fracture energy vs. temperature) for the steel and aluminum specimens tested. Plot the results from both steel and aluminum on one graph; indicate the location of the ductile-to-brittle transition (if applicable). Include the data that are given to you on the data sheet for the additional temperature conditions. Put the y-axis at the left of the plot, rather than at the x=0 intersection. What is the expected behavior and why is this behavior expected? Do your results meet expectation?