- 1 -1 Heat Treatment Engineering Materials Module 4 Heat Treatment Page 1 - 2 -2 Heat Treatment Engineering Materials 4.1 Equilibrium Diagrams 4.1.1 Exploration: Equilibrium Diagram There are various types of steel alloys and their specific carbon content differentiates all. A graph, or Equilibrium Diagram, represents the varying states of steel as related to its iron and carbon content percentages. These states represent liquid, solid, and states where both forms exist. Let’s see if you can mark where the different steel alloys belong on the iron-carbon equilibrium diagram. Student Exercise: Read Equilibrium Diagram 1. Obtain four (4) samples each of low carbon (example-1020), medium carbon (example-1040), and high carbon (example-1095) steel. 2. On the following iron-carbon equilibrium diagram draw a vertical line representing the actual steel alloys obtained in step 1. Figure 4.1.1 Page 2 - 3 -3 Heat Treatment Engineering Materials Questions: Equilibrium Diagram Exercise 1. At what point would the medium carbon steel begin to melt? _____________ ______________________________________________________________ ______________________________________________________________ 2. At what point would that same medium carbon steel be 100% liquid? _______ ______________________________________________________________ ______________________________________________________________ 3. How does that melting point of medium carbon steel compare to the melting point of 100% carbon? ___________________________________________ ______________________________________________________________ ______________________________________________________________ 4.1.2 Dialog: Equilibrium Diagrams Since ancient times heat treatment has played an important role in society. A highly secretive process was developed for producing superior steel for Damascus Swords during the era of Alexander the Great. Today, that process is no more than legend, but undoubtedly involved heating, quenching, and tempering. We will look at each of these processes in detail. The wide range of products produced today from microwaves, electronics, automobiles, aeronautics, to the space industry would not be possible without the modern methods of material processing including heat treatment. In order to understand the basics of heat treatment, we must first investigate the process of changing temperature, pressure, and alloying percentages. As these variables are changed, we will study the associated changes in the physical properties of metals. These changing variables are represented on equilibrium diagrams. Page 3 - 4 -4 Heat Treatment Engineering Materials Equilibrium diagrams are shown below in Figure 4.1.2 and 4.1.3 for Copper-Nickel. Figure 4.1.2 Figure 4.1.3 This shows the reaction to the changing alloy percentages versus temperature. A solid solution phase is shown on the bottom of the diagram with a pure liquid phase shown on the top portion of the diagram. In between there is a curved region or two-phase region of both liquid and solid that exists. On the left side of the vertical axis there is 100% copper that has a single melting point, on the right side, 100% nickel with a single melting point. In between, a copper-nickel alloy will go through a two-phase transition from a solid to a liquid. Page 4 - 5 -5 Heat Treatment Engineering Materials A following iron-carbon equilibrium diagram graphically depicts the addition of carbon as an alloying element for iron. From 0 to 2% carbon materials are called steels and greater than 2% carbon, the materials are called cast irons. We will study the effects of these changing variables in detail in this module. 4.1.3 Dialog: Understanding the Iron-Carbon Equilibrium Diagram Looking at a portion of the iron-carbon equilibrium diagram shown below that show the steel portion of the curve, we can see the effects of the heating of the alloy versus alloying percentage. At low temperature, iron is in a body centered cubic (bcc) crystal structure called ferrite. Ferrite or iron can hold little carbon atoms interstitially and is very weak. As the percentage of iron increases in the steel alloy, the iron and carbon combine to form cementite, a chemical compound called iron carbide (Fe 3 C). Cementite is a very hard and brittle structure with magnetic properties. Pearlite is a layered or lamellar structure of ferrite and cementite. When observed under a microscope, the white layers are the ferrite and the black ridges the cementite. At elevated temperatures, steel makes an allotropic conversion from body centered cubic (bcc) ferrite to a face centered cubic (fcc) austenite. Austentite is not magnetic and when slowly cooled will reverse its crystal structure to bcc ferrite. Austentite or iron has a high solubility for iron and can hold approximately 2% carbon interstitially. Knowing the percent of carbon in the steel you can determine the structure of the steel at any given temperature. As steel is heated from the ferrite and cementite region of the diagram you first reach the lower transformation curve labeled A1, which begins the transformation from BCC-ferrite to FCC-austenite. Upon reaching the upper transformation curve, labeled A# and Acm, there is complete transformation to austenite. The primary reason for changing the carbon content in steel is to achieve the maximum degree of hardness. The carbon content in low carbon steel (0.05 to 0.30% carbon) is insufficient to produce substantial hardness changes in the steel. Medium carbon steels (0.30 to 0.80% carbon) are very responsive to the heattreating process. High carbon steel ( 0.80% carbon) is also very responsive to heat treatment, but really has too much carbon in the crystal structure. The maximum hardness value is reached as the carbon content in steel is increased to approximately 0.60%. Page 5 - 6 -6 Heat Treatment Figure 4.1.4 Page 6 Engineering Materials - 7 -7 Heat Treatment Engineering Materials 4.1.4 Exploration: Heat Treatment of Various Types of Steel Let’s look at the effect of heating and cooling on various types of steel. The steel samples will vary in the percentages of carbon. Student Exercise: Heating and Cooling 1. Obtain at least four (4) samples each of low carbon (ex:1020), medium carbon (ex:1040), and high carbon(ex:1095) steel. 2. Test each type of steel for its Rockwell (B or C) Hardness and record the value. Mark/stamp each specimen so that it can be identified after heat treatment. 3. Load all twelve (12) specimens in a heat-treatment furnace and heat to 1250 deg F. Hold at 1250 F for 20 minutes. Quench one (1) of each type of steel in cold water, perform a Rockwell Hardness test and record the value. 4. Repeat this process at 1400 F, 1550 F, and 1700 F. DATA SHEET Heat Treating Module Engineering Materials Exploration 4.1.2 Heat Treatment of Various Types of Steel Hardness Values - Steel Specimens Temp - F Low Carbon Medium Carbon Room 1250 1400 1550 1700 Page 7 High Carbon - 8 -8 Heat Treatment Engineering Materials Questions: Heating and Cooling 1. Were there any changes in hardness values due to the various temperatures?__________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ 2. Did the different types of steel react differently to the various temperatures? ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ 3. Which type of steel drastically changed hardness values first?____________ ______________________________________________________________ ______________________________________________________________ 4. In your opinion, do these results agree with the curve associated with the iron-carbon equilibrium diagram? ___________________________________ ______________________________________________________________ ______________________________________________________________ 4.1.5 Exploration: A Heat Treating Recipe Using your knowledge gained from the use of the equilibrium diagram, and the two (2) previous explorations, recommend a heat-treating temperature for the following two (2) types of steel. 1. Recommend a heat-treating temperature for 1060 steel using the iron-carbon equilibrium diagram to assure that the material will get into the austenite area of the diagram. _________________________________________________ ______________________________________________________________ ______________________________________________________________ 2. Recommend a heat-treating temperature for 1030 steel using the iron-carbon equilibrium diagram to assure that the material will get into the austenite area of the diagram. _________________________________________________ ______________________________________________________________ ______________________________________________________________ Page 8 - 9 -9 Heat Treatment Engineering Materials 3. Discuss what expectation you would have in terms of hardness for the heattreated and quenched metal. ______________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ 4.2 Quenching and Tempering 4.2.1 Exploration: Investigating Quenching Mediums Quenching is the rapid cooling of heated steel in a quenching medium for the purposes of hardening. Steel is heated into the austenite region and then quenched. The steel is quickly plunged into the quenching medium to rapidly cool the metal. The quenching medium may need to be agitated to allow for uniform cooling of the steel. The most common quenching mediums are air, oil, water, and brine. After heating and quenching, steel is generally hard and brittle. Quenched austenite is transformed into martensite. If left in this state, steel is subject to cracking. Reheating, or tempering, causes martensite to break down. The steel is provided a degree of stress reduction and an increase in ductility. In part 1 of this module, we looked at the effect of heat treatment on various percentages of carbon steel. In this exploration, we will vary the cooling rate of steel by changing the quenching mediums. 1. Obtain at least four (4) samples of medium carbon (1040) steel. 2. Test each sample of steel for its Rockwell (B or C) Hardness and record the value. Mark/stamp each specimen so that it can be identified after heat treatment. 3. Load all four (4) samples in a heat treatment furnace and heat to 1600F (for medium carbon steel). Hold at this temperature for one hour. 4. Remove the first sample and quickly quench the specimen in water at room temperature. Note: be sure to quickly close the furnace door after removing this and each sample to prevent the premature cooling of the other samples. 5. Remove the second sample and quickly quench the specimen in a commercial grade of motor oil. 6. Remove the third sample and allow it to air cool. Either place it in open air on a ceramic tray of place it in front of a fan. 7. Allow the fourth sample to oven cool. 8. Once all samples have cooled, perform a Rockwell Hardness test and record the values for each. Page 9 - 10 -10 Heat Treatment Engineering Materials DATA SHEET Heat Treating Module Engineering Materials Exploration 4.2.1 Investigating Quenching Mediums Hardness Values - Steel Specimens Temp - F Air Water Oil Oven 1600 Questions: 1. Were there any changes in hardness values due to the various quenching mediums? Explain ________________________________________________ ________________________________________________________________ ________________________________________________________________ 2. Which cooling medium resulted in the greatest gain in hardness of the samples? ________________________________________________________________ ________________________________________________________________ 3. Which cooling medium resulted in the least gain in hardness of the samples? ___________________________________________________________________ ___________________________________________________________________ 4. Rank the order of most gain to least gain of the quenching mediums? _______________________________________________________________________ ______________________________________________________________________ ________________________________________________________________ Note: This exploration may be performed with other types of steel. For low temperature steel, ex: 1018 steel, heat samples to 1700 F. When using high carbon steel, ex: 1095 steel, heat samples to 1500 F. Low carbon steel will be less effective than either medium or high carbon steel. 4.2.2 Dialog: Quenching The heat-treating process is continued after heating the metal it is suddenly plunged into a cooling medium until it is cool. Agitation may be required to assure rapid and uniform cooling. Page 10 - 11 -11 Heat Treatment Engineering Materials Quenching mediums are commonly liquids. Some examples of non-liquid quenching mediums are sand and air. The most common quenching mediums are water, brine, oil, and air. Water is the most common quenching medium. It provides a good quench due to its high heat of vaporization. Water has the advantage of being commonly available and is cheap. On of the major disadvantages of water is that vapor bubbles form next to the metal and produce soft spots due to uneven cooling. One method of minimizing this effect is to provide agitation during the quenching. Water can cause internal stresses, cracking and distortion. Water is a good quenching medium for low carbon steel. Brine quenching is a modification of water as a quenching medium. Salt is added to water to nucleate the bubbles that limit the effect of water. Brine cools the metal faster than water. However, the results of brine quenching are similar to the results of water quenching. Brine will increase corrosion problems unless the salt is completely removed from the metal. Oil is used as a quenching medium when a slower, gentler cooling rate is required. Oil is used for parts having thin cross sections of sharp edges. Oil has reduced internal stress, cracking and distortion. However the steel will not be quite as strong or as hard as water or brine quenched steel. Oil is most effective as a quenching medium when slightly heated (100 – 150 F). Heating the oil reduces the oil viscosity promoting better cooking. High flash point oils should be used for quenching. Air quenching is the least drastic to the metal that is being cooled. The heated metal is placed on a screen and air is blown over the metal. Compared to the other quenching mediums, air is the least likely to cause internal stress, distortion, or cracking. The resulting strength and hardness are not as high as the faster quenching mediums are. Air quenching is best used on high carbon steels that have the most potential for quench cracking, distortion, or internal stresses. The addition of alloying elements such as chromium will improve the hardening of steel using air quenching. The following figure shows the potential cooling rates of steel using water, brine, oil, and air quenching. There is a curve that shows the normalizing cooling rate for steel. Page 11 - 12 -12 Heat Treatment Engineering Materials Steel Cooling Rates 1800 1600 Temperature (F) 1400 1200 Normalizing (Still Air) Air Quench 1000 Oil Quench 800 Water Quench Brine Quench 600 400 200 0 0 5 10 15 20 25 30 Quench Time (Sec) 4.2.3 Application: Select A Quenching Medium Using your knowledge gained from the use of the exploration and dialog section that have developed the approach to quenching. Choose the quenching medium for the following 2 types of steel. 1. Recommend a quenching medium for 1060 steel that will maximize the strength and hardness of the steel but minimize the potential internal stress, distortion, of cracking. ___________________________________ ___________________________________________________________ ___________________________________________________________ 2. Recommend a quenching medium for 1030 steel that will maximize the strength and hardness of the steel but minimize the potential internal stress, distortion, or cracking. ___________________________________ ___________________________________________________________ ___________________________________________________________ 3. Discuss what expectation you would have in terms of hardness for the heat-treated and quenched metal. _______________________________ ___________________________________________________________ ___________________________________________________________ Page 12 - 13 -13 Heat Treatment Engineering Materials 4.2.4 Dialog: Tempering The as quenched state of steel consists of a material that is hard and brittle and has relatively high level of internal stress. If left in this condition the material will have a tendency to crack. Tempering is the immediate reheating of the steel after quenching. The steel is reheated to a temperature below the lower transformation temperature. The overall effect on the material is increased toughness, internal stress relaxation and an increase in ductility. During quenching the austenite in the steel was transformed to martensite. Martensite is hard and strong but very brittle. Martensite has little uses as an engineering material due to this brittleness. The crystal structure of the austenite transform from its face-centered cubic structure to a body-centered cubic type structure. However, due to the quick quenching, the carbon atoms held interstitially in the austenite are not diffused form the structure. The bodycentered structure is elongated, forming a body-centered tetragonal structure. When martensite is investigated under the microscope, it appears fine and needlelike. Tempering requires a temperature between 300F and approximately1275F. The upper limit of temperature should be just below the lower transformation temperature (approximately 1333F). A lower tempering temperature (i.e. 400F) produces high strength and hardness. High temperature (i.e. 1100F) produces increased toughness and ductility. Time at the tempering temperature is important to the material properties. The material should soak at the tempering temperature for approximately one (1) hour. This time may be adjusted upward for thicker material cross sections. The effect of tempering on engineering material properties is decreased hardness, strength, and brittleness. However, tempering causes an increase in toughness and ductility. 4.2.5 Application: Quenching and Tempering Objective: To gain an understanding of the methods and effects of heat treatment on the properties of ferrous materials. Specimens: Four (4) Steel – 1040 hot rolled steel impact specimens Apparatus: Two (2) heat –treating ovens Gloves and tongs Rams Rockford-Model 10A Rockwell Hardness Tester Water Model SI-1 Universal Impact Testing Machine Dial Calipers Page 13 - 14 -14 Heat Treatment Engineering Materials Reference: Modern Materials and Manufacturing Process; Bruce, Tomovic, Neely and Kibbe, Prentice Hall, Ch4, pp. 75-89, 2nd ed., 1998. Background Information: Steel is a highly useful alloy of iron containing approximately 1% or less carbon. The use of heat treatment can produce a variety of effects on properties such as ductility, strength and hardness. The amount of carbon in steel has an effect on the response of the steel to heat treatment. Low carbon steel (.2% and below) cannot be hardened greatly hardened, medium carbon steel (.4%) can be hardened with a rapid quench. High carbon steel (.8%) to 1%) can be easily hardened. With control of temperature, time and rates of processing, iron alloys can be engineered to meet many engineering needs. Carbon is an allotropic element, which means that it will undergo a crystal lattice structure change above its critical temperature. Critical temperature is the temperature at which iron changes its crystal lattice structure and will hold some carbon atoms interstitially. For most steel, the critical temperature is above 1450F. Quenching is a rapid cooling process, which lowers the material’s temperature form the critical temperature quickly enough to freeze the carbon in the crystalline structure. Drawing is a re-heating process following quenching performed to increase ductility while hardness decreases. Usually the material is heated to a predetermined temperature and allowed to air cool. Annealing is a heating and cooling cycle designed to reduce hardness, improve ductility, refine grain size and improve toughness. Annealing involves heating the steel above the critical temperature and allowing slow cooling. Procedure: Heat treat 3 of the 4 impact specimens as indicated below. Make an impact test of the specimens following the procedure for impact testing and record the results. Specimen #1: Specimen #2: Specimen #3: Specimen #4: Heat to 1600F and water quench, test as is. Heat to 1600F, water quench, draw at 400F. Heat to 1600F, water quench, draw at 600F. Heat to 1600F, water quench, draw at 1100F. Page 14 - 15 -15 Heat Treatment Engineering Materials Using the Rams Rockwell Hardness Tester, determine the hardness using the broken impact specimen as a specimen. Draw a file firmly across the specimen and observe its behavior. DATA SHEET Heat Treating Module Engineering Materials Application 4.2.3 Quenching and Tempering Sample # Type of Specimen Rockwell Hardness Absorbed Energy (ftlbs) Type of Surface at break 1 2 3 4 5 4.3 Metallography 4.3.1 Exploration: Microscopic Evaluation of Heat-Treated Steel Objective: To observe and interpret the microscopic effects of the heat treatment of steel. Specimens: Various carbon steel specimens. A low carbon steel, medium carbon steel and a high carbon steel specimen is recommended. Apparatus: 1. Metal cut-off saw 2. Specimen mounting press 3. Grinder/Polisher 4. Handimet II Roll Grinder 5. Rockwell Hardness Tester 6. Metallurgical microscopes Procedure: 1. Rockwell Hardness testing: Measure and record the Rockwell “C” hardness of each of the specimens using the Rams Rockford-Model 10A, Rockwell Hardness tester. Page 15 - 16 -16 Heat Treatment Engineering Materials 2. Section the sample: Sectioning (cutting) is necessary to expose the internal areas of the specimen for observation. Use the metallurgical cut-off saw with abrasive blades and coolant flow. Avoid overheating of the specimen. 3. Mounting the specimen: This step provides a mounting of the metal specimen in plastic using the Simplimet Mounting Press. This mounting provides a safe and efficient means of handling small and irregular samples. Compression mounting resins cure at 3000 to 4200 psi pressure and 150C (302F) temperature. a. Spray the mold cavity and the mold closure thoroughly with silicone mold release spray. b. Place the specimen on the ram face of the mounting press. c. Lower the ram ¼ of the way and pour in enough resin to cover the sample. d. Lower the ram to its furthest point and add enough phenolic powder to complete the mount. e. Replace the mold closure and tighten in a clockwise direction. Place the thermometer in the top of the mold closure to monitor the temperature for the curing process. f. Turn on the heater coil to heat the resin. g. Apply the pressure of 4200 psi for the 1 ¼” mold and allow curing for 7 minutes. No further operation is needed until the end of the curing period. h. After the recommended curing time, release the pressure for an instant, closing the valve quickly. Using gloves, carefully unscrew the closure until the threads disengage the mold. Operate the pump handle to elevate the ram raising the closure out of the mold cavity. Carefully remove the hot closure and the mold specimen. 4. Rough Grinding: This is a corrective technique needed to remove gross surface irregularities. Additionally, sharp edges produced by the molding process are removed by grinding. Coarse grinding employs grinding using a 120 or 180 grit abrasive disk on the grinder/polder. 5. Fine Grinding: This step progressively removes coarse scratches and deformations utilizing a series of decreasing grit size (240, 320, 400, & 600) abrasive papers. Between steps in the grinding process, the specimen should be thoroughly washed to prevent abrasive particles from being carried to finer Page 16 - 17 -17 Heat Treatment Engineering Materials grit papers. Fine grinding should be performed on the Handimet 2 Roll Grinder using a continuous water flow for lubrication. A valve located on the front of the grinder controls water flow. Carbimet Silicon papers are waterproof and may be used in either a wet or dry configuration. (We will grind the specimens wet.) To perform fine grinding, manually draw the specimen back to front (one direction only) across the paper. Each step should take approximately two (2) minutes. After proper washing and drying, move to the next grid paper. Rotate the specimen 90 degrees for this next step. When the visible scratches from the previous step have been removed, continue grinding additional 6-8 strokes. 6. Rough Polishing: The rough polishing step has the greatest potential for success or failure of the entire process. This step should completely remove the remaining scratches from the fine grinding step. Use METADI II Diamond compound, which is tough and removes material faster and cleaner than other abrasives. Use a nylon polishing cloth on the Leco grinder/polisher. Apply the METADI II diamond abrasive using the applicator syringes and distribute uniformly over the polishing cloth. A liquid extender is then applied, to aid in the distribution of diamond particles and provide lubrication. Rough polishing should take approximately two (2) minutes. 7. Final Polishing: The final polishing step should provide a scratch free surface. The material removal at this step is extremely low. This step is just a final polish, it will not correct errors committed in the previous steps. Final polishing should also be accomplished using the Leco Grinder/Polisher with a microcloth on the 8” wheel. Moisten the cloth with deionized water. Use micropolish B, alumina suspension and apply in a circular bead. For best results, do not overwet the cloth. Rotate the sample in a direction opposite the wheel rotation. Wash the sample in alcohol immediately or the polished surface will oxidize. 8. Etching: Use an etchant of 98 ml ethyl alcohol and 2 ml Nitric acid for a time of 45 seconds. Perform this step using an approved hood in the chemistry lab. Placing the completed specimens under a stream of water should stop the etching action. Clean the specimen with alcohol and use a hair dryer to Page 17 - 18 -18 Heat Treatment Engineering Materials dry the sample. Do not touch the polished/etched specimen. This will alter the metal surface condition. 9. Microscopic Examination: Determine the grain size of the specimen and graphite type and distribution. 4.3.2 Dialog: Metallography Metallography is the science of interpreting the microstructure of engineering materials. Sample preparation involves sectioning, mounting, grinding, and polishing. The sample is then investigated under a microscope for crystal structure and grain size. Steel is an alloy having important mechanical properties. Heat treatment is one of the most common and widely used manufacturing processes used to alter the mechanical properties of steel. The size of grain affects the mechanical properties of metals. During heat treatment the process including soak time, temperature, and alloying elements all affect grain growth and size. A small grain is preferable to a large grain structure. Small grain metals have greater hardness and tensile strength and distort less during the quenching process. In steels, large grain materials have increased hardenability and are better for the cold working process. Grain size is determined by the count of grains per square inch under 100x microscopic magnification. Steel is considered fine-grained if it is between 5 and 8 and coarse-grained if it is between 1 and 5. Acceptable grain structure has approximately 70% of the grain within a given range. The metallurgical microscope is used to determine the structure of metals. Grain size and the effect of heat treatment may be determined by microscopic evaluation. Approximate carbon content may be evaluated by the pearlite, which appears as dark areas in the steel. The specimen is sectioned using a metal cut-off saw. In the process of cutting, overheating of the specimen should be avoided. Overheating may subject the material to additional tempering, thus modifying the steel. Grinding is the gross removal of surface metal using abrasive materials. The next step is polishing of the surface, which is the slight cutting of this surface. Etching of the specimen surface aids in the visual examination of the surface. 4.3.3 Application: Metallography of Heat-Treated Specimens Objective: To gain a further understanding of the methods and effects of heat treatment on the properties of ferrous materials. Specimens: Four (4) Steel – 1040 hot rolled steel specimens Apparatus: Page 18 - 19 -19 Heat Treatment Engineering Materials 1. Three (3) heat treating ovens 2. Gloves, goggles, and tongs 3. Rams Rockford - Model 10A Rockwell Hardness Tester 4. Water 5. Metal cut-off saw 6. Specimen mounting press – Simplimet II 7. Ecomet 3 – Automet 2 – Grinder/Polisher 8. Neomet Microscope 9. Pixera Digital Camera for use with Neomet Microscope 10. Heat Gun 11. Nital Etchant 12. Various consumables for Metallography Process Procedure: 1. Heat Treatment as follows: a. Specimen #1 – leave as is b. Specimen #2 – heat to 1600F and water quench c. Specimen #3 – heat to 1600F, water quench, draw at 400F d. Specimen #4 – heat to 1600F, water quench, draw at 800F 2. Rockwell Hardness Testing: Measure and record the Rockwell Hardness ‘C’ hardness of each of the specimens using the Rams Rockford – Model 10A, Rockwell Hardness tester. 3. Section the sample: Sectioning (cutting) is necessary to expose the internal areas of the specimen for observation. Use the metallurgical cut-off saw with abrasive blades and coolant flow. Avoid overheating of the specimen. 4. Mounting the specimen: This step provides a mounting of the metal specimen in plastic using the Simplimet II Mounting Press. This mounting provides a safe and efficient means of handling small and irregular samples. Compression mounting resins cure at 3000 to 4200 psi pressure and 150C (302F) temperature. a. Spray the mold cavity and the mold closure thoroughly with silicone mold release spray. b. Place the specimen on the ram face of the mounting press. c. Lower the ram ¼ of the way and pour in enough resin to cover the sample. Page 19 - 20 -20 Heat Treatment Engineering Materials d. Lower the ram to its furthest point and add enough phenolic powder to complete the mount. e. Replace the mold closure and tighten in a clockwise direction. Place the thermometer in the top of the mold closure to monitor the temperature for the curing process. f. Turn on the heater coil to heat the resin. g. Apply the pressure of 2000 psi, quickly release and reapply a pressure of 4200 psi for the 1 ¼” mold and allow curing for 7 minutes. No further operation is needed until the end of the curing period. h. After the recommended curing time, release the pressure for an instant, closing the valve quickly. Using gloves, carefully unscrew the closure until the threads disengage the mold. Operate the pump handle to elevate the ram raising the closure out of the mold cavity. Carefully remove the hot closure and the mold specimen. Using the Ecomet 3 and Automet 2 Power head, perform steps 5-8. 5. Place three (3) mounted specimens in the three (3)-specimen barrel holder using the plate to align the specimens properly. Use a dummy mount if three (3) specimens are not available. 6. Planar Grinding Stage: Cover the bimetallic plate wit release agent, attach a 180-grit Buehler-Met II disk and carefully place on the magnetic disk on the Ecomet 3. Set the speed, force per sample, relative rotation and time accordingly. Once the controls are set, attach the barrel specimen holder to the Automet 2 power head and move the head so that it is located to the outside of the disk and lock into place. Pressing the green buttons on each side of the power head will lower the barrel holder down onto the plate and start the grinding sequence. Note: Only use 180-grit disk for one grinding operation. 7. Sample integrity stage: This stage will involve two (2) steps. a. Using Ultrapad surface and Metdi Supreme Diamond Suspension, 9 m set the controls accordingly and follow the procedure similar to step 6. The surface should be charged prior to operation and should be maintained wet during the entire stage (application approximately every 30 seconds). Note: These cloths may be used over and over again by placing in a protective plastic bag and sealing. Page 20 - 21 -21 Heat Treatment Engineering Materials b. Using Trident, Nylon or Texmet surface and Metadi Supreme diamond suspension, 3 m set the controls accordingly, follow the procedure to step 6. The surface should be charged prior to operation and should be maintained wet during the entire stage (application approximately every 30 seconds). Note: These cloths may be used over and over again by placing in a protective plastic bag and sealing. 8. Final Polishing: Using the Chemet or Microcloth surface and Masterprep Alumina Suspension, follow the procedure similar to step 7 above. 9. Etching: Use an etchant of 98 ml ethyl alcohol and 2 ml Nitric acid for a time of 45 seconds. Perform this step using an approved hood in the chemistry lab. Placing the completed specimens under a stream of water should stop the etching action. Clean the specimen with alcohol and use a heat gun to dry the sample. Do not touch the polished/etched specimen. This will alter the metal surface condition. 10. Microscopic Examination: Determine the grain size of the specimen and graphite type and distribution. 4.4 Transformation Diagrams 4.4.1 Exploration: Isothermal Transformation Diagrams Using figure 4.4-1, Isothermal Transformation diagram for a generic ironcarbon alloy, answer the following questions: 1. What variable missing on the iron-carbon equilibrium diagram is included on the isothermal transformation diagram?_________________________ ___________________________________________________________ ___________________________________________________________ 2. What type of microstructure exits above the upper transformation temperature line?_____________________________________________ ___________________________________________________________ ___________________________________________________________ Page 21 - 22 -22 Heat Treatment Engineering Materials 3. What type of microstructure exits below the lower transformation temperature line, above the Ms temperature line, and to the left of the Ccurve?_____________________________________________________ ___________________________________________________________ ___________________________________________________________ 4. What type of microstructure exits to the right of the C-curve?___________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ 5. What does the dashed line between the C-curve and the right C-curve represent?__________________________________________________ ___________________________________________________________ ___________________________________________________________ 6. If a sample is quickly quenched to 1100F and held at a constant temperature of 1100F, how long does it take for the austenite to pearlite reaction to go to 50% completion? How long to 100% completion? ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ ___________________________________________________________ Insert Isothermal Transformation Diagram for Generic Iron-Carbon Alloy Page 22 - 23 -23 Heat Treatment Engineering Materials Figure 4.4.1 4.4.2 Dialog: Transformation Diagrams Equilibrium diagrams were used in part one to show the carbon content and temperature on the phase changes of iron-carbon alloys. However, this diagram accurately depicts the effect of temperature and carbon content only for slow cooling. This slow cooling results in a microstructure of ferrite, cementite, or pearlite. Fast cooling or quenching results in martensite. The iron-carbon equilibrium does not distinguish between different cooling rates. The missing variable is time. Another type of diagram is required to depict the effects of varying cooling rates. The isothermal transformation (I-T) diagram was developed to include the variable of time and the transformation of austenite. Another name for the I-T diagram is a time-temperature-transformation (T-T-T) diagram. Plotting temperature versus time creates these diagrams. Each diagram is created for a unique alloy of carbon and iron. The upper and lower transformation temperatures to austenite are first shown as horizontal lines on the upper portion of the diagram. To create the transformation information many like samples are heated above the upper transformation temperature to achieve 100% austenite. The samples are then quickly transferred to a heated molten salt bath and held at this temperature (isothermal condition). At incremental times, the samples are removed and quenched in a iced brine. These samples are the investigated using metallography techniques to determine the microstructure of the samples. Microstructure, time, and temperature are recorded for each sample. This procedure is repeated with decreasing temperature to determine the Page 23 - 24 -24 Heat Treatment Engineering Materials transformation information. An I-T diagram is the prepared for this particular iron-carbon alloy. Transformation at upper temperatures is the combination of ferrite and cementite, forming fine and coarse pearlite. Below the nose of the transformation curve, the transformation will be to a mixture of ferrite and bainite. At lower temperatures and at times missing the nose of the curve, the transformation process will be from austenite to martensite. These structures will be defined in the following paragraphs. I-T diagrams are useful in determining the type of microstructure and information on the process of transformation. Martensite Martensite transformation occurs with rapidly quenched steel alloys with the transformation occurring nearly instantaneously at or near room temperature. This rapid quenching prevents carbon transformation with the carbon atoms trapped in iron microstructure. The carbon atoms that are trapped interstitially in the microstructure elongate the body-centered-cubic microstructure. This microstructure is called body-centered-tetragonal (BCT). Martensite appears as a needle-like microstructure under the microscope. Martensite is a very hard, strong and brittle structure that has little engineering application. The structure is susceptible to cracking and retained internal residual stress. Martensite is very difficult to machine due to its high hardness values. Martensite is in a nonequilibrium condition with this trapped carbon transforming if reheated or tempered. Martensite is not included on the iron-carbon equilibrium diagram but is included on the I-T diagram. Martensite transformation is shown with a horizontal line indicating Ms of martensite start, M50 – 50% transformation, and M90 – 90% transformation to martensite. Bainite Bainite is a microstructure of transformation that exists between the fine pearlite and the martensite region of the I-T diagram. Named for E.C. Bain who discovered the existence of this type of microstructure. Bainite is not as hard or strong as martensite and has good ductility and toughness. The range of hardness of bainite is 50-55 Rockwell C. An electron microscope is needed to investigate the microstructure of bainite. Example I-T Diagram For an example of the I-T diagram see Figure 4.4.1, a diagram for generic iron-carbon alloy. The upper dashed line indicates the lower transformation temperature. Above this line, the carbon steel transforms to a face-centered cubic (FCC) microstructure with some carbon atoms held interstitially in the FCC Page 24 - 25 -25 Heat Treatment Engineering Materials microstructure. As the steel is cooled, there are two (2) C-shaped curves. The left hand C-curve is the beginning of the transformation from austenite to coarse pearlite, fine pearlite, or bainite. The right hand C-curve represents the end of this transformation process. The Ms horizontal line shown on the bottom of the diagram indicated the start of the transformation to martensite. 4.4.3 Application: Isothermal Transformation Diagram Problem: Using the isothermal transformation diagram for a generic iron-carbon alloy (Fig. 4.4.1), specify the final microstructure for a specimen that is subjected to the following heat treatment: 1. The specimen is heated to 1700F and held for 1 hour to achieve a 100% austenite microstructure. 2. Rapidly cool the specimen to 1100F. 3. Hold at 1100F for 100 seconds at isothermal conditions. 4. Quench the specimen at room temperature. Solution: The following figure (Fig. 4.4.2) graphically depicts the above conditions. The initial quench to 1100F is rapid enough to prevent any transformation. At 1100F the austenite completely transforms to pearlite. Since the specimen is completely transformed prior to passing through the martensite region, the final microstructure is 1005 pearlite. Insert Isothermal Transformation Diagram for Generic Iron-Carbon Alloy Figure 4.4.2 dialog example problem Page 25 - 26 -26 Heat Treatment Engineering Materials 4.4.4 Application: Using the Isothermal Transformation Diagram Using the isothermal transformation diagram for a generic iron-carbon alloy (Fig. 4.4.1), specify the final microstructure for specimens that are subjected to the following heat treatments: The specimen is heated to 1700F and held for 1 hour to achieve a 100% austenite microstructure. 1. Rapidly cool to 1100F, hold for 10 seconds, quench to 600F, and hold for 10,000 seconds, and quench to room temperature. 2. Quench to 500F, hold for 10 seconds, and quench to room temperature. 3. Quench to 1250F, hold for 100 seconds, rapidly cool to 800F, hold for 1000 seconds, and quench to room temperature. 4.4.5 Dialog: Tempering Martensite The as quenched martensite is not a suitable material for many if any engineering applications. The material is extremely hard and brittle and is often weakened by internal stresses. Tempering will relieve internal stress, and increase both ductility and toughness. While tempered martensite will possess increased ductility and toughness it will lose hardness and strength. Page 26 - 27 -27 Heat Treatment Engineering Materials The tempering range of martensite is between 400-1200F. The unstable body-centered-tetragonal (BCT) microstructure of martensite will transform to a stable mix of cementite in a matrix of ferrite. Spheriodite Tempering martensite of bainite to a temperature just below the lower transformation temperature (approximately 1300F) and holding for a period of 18 to 24 hours creates spherodite. The cementite structure is changed to sphere shaped particles in a ferrite matrix. Spherodite transformation is not shown on the I-T diagram. Page 27