Effects of Heat Treatment on Plain Carbon Steel - Term Paper Example

Effects of Heat Treatment on Plain Carbon Steel Abstract The purpose of the study is to investigate the effects of heat treatment on plain carbon steel and its microstructure. The study will help answer a question such as: Is there a clear correlation between the impact energy absorbed and the fracture surface character? The main objective is to establish the ductile to brittle transition temperature in plain carbon steel. Another objective is to observe the features of fracture surface and the changes that are linked to the transition from ductile to brittle failure. The study will require an experiment to be done on the plain carbon steels. They were subjected to different temperatures to help identify the changes in the steel alloys. The metals retained the brittle fracture at low temperatures or very high strain rates. Ductile fracture was maintained at low temperatures. The ductile to brittle transition temperature was identified to -40°C. The study was able to determine the fracture of the carbon steel and their impact energy at different temperatures. The experiment helped to identify that transition temperature could be reduced if carbon content minimized in the microstructure, and subjected to same conditions. Introduction V-notch tests were performed on medium carbon steel which was heat-treated for 1 hour at 1100°C then cooled at room temperature. The medium carbon steel contains 0.045wt% C and is used in many structural components in industries such as automotive, shipbuilding, oil pipeline and aerospace. The higher the content of carbon in the carbon steel metals, the lower the ductility and brittle fracture. Ductile to brittle transition temperature is the temperature at which failure of a metal shifts from ductile fracture to brittle fracture mode. The DBTT of many steel alloys is mostly below the room temperature. The ductility of a metal might be acceptable at room temperature but it fails in service when there is a decrease in temperature. This means that before any application such as designing ships and automobiles the DBTT ought to be considered if the steel is to be subjected to low temperatures. This would help prevent the brittle failures experienced. The Charpy V-notch test was used to determine the DBTT as it is the most common method that is used. The ductile to brittle transition temperature was observed by analyzing the energy absorbed by the metal sample in relation to temperature. It can also be obtained by analyzing the relative fractions of ductile and brittle fracture surface as a function of temperature. The impact testing offers a screening method that is brilliant and this is due to the fact that its easy to fabricate the metal samples. Background The study requires a laboratory experiment. Students in groups were handed medium carbon steel. They were needed to carry out a heat treatment experiment on the metal sample. A Charpy v-notch impact test will need the carbon steel samples to undergo through tests to determine the impact energy in the given material. The test will also identify the Ductile to Brittle transitional temperature. The students subjected the metal samples to different temperatures for a given period of time. The sources of the different temperatures were: hot water on a hot plate or a furnace or oven if available for temperatures up to 99°C. Ice water and a mixture of liquid nitrogen ethanol also provided cold temperatures of up to -150°C. The impact energy of the materials will be recorded and plotted so as to determine the ductile to brittle transition. The students then used the stereo microscopes to observe and sketch the fracture surface of the metal samples. After recording the impact energy of the metal sample, they shared the data by compiling the impact energy vs. temperature. Materials and Temperatures A Charpy V-notch and impact tester 1045 steel -196 °C, liquid nitrogen -78 °C, dry ice 0 °C, ice 75 °C, oven 150 °C, furnace Methods A Charpy V-notch test was performed on medium carbon steel (1045 steel alloy). The pendulum will be the source that will offer the impact energy while swinging back and forth. The test was to help determine the impact strength of 1045 steel alloy under different temperature ranges. The students were divided into different groups that would carry out the test on the alloy samples using Charpy impact tester. The experiment will determine the fracture behavior of the carbon steels and their impact energy. The test would also help to determine the toughness reduction in the materials when there was reduced temperature. The V-notch tester performed impact tests on square v-notch samples of 10mm X 55mm with 45-degree notches .25mm deep cut. This will be performed on the middle section equal to the specimen’s length. The samples that were under the test were heated under the temperatures of up to 99 °C on a hot plate in hot water. The samples were then cooled in ice water. A mixture of liquid nitrogen ethanol was also used to cool for temperatures of down to -150°C. The samples were then taken to the testing device, and placed flush, notch positioned at the center of the holder after equilibrate. The test was required to run for 5 seconds out of the temperature controlling liquid to help develop a DBTT profile. The gage was reset after performing each test. Results Test Temp (°C) Fracture Energy (ft. lbs.) % Brittle Fracture -120 4.5 100 -95 5 100 -80 7 99 -50 12.5 98 -30 13.5 97 -20 42.5 96 -16 34 95 -7 48.5 94 23 59 85 29 56 82 45 60 75 70 64 70 89 63.5 99 Diagram above: Fracture surface of mild carbon steel at different temperatures 1045 steel alloy brittle fracture fails at temperature of below -50°C. As the temperature increases, the fracture energy decreases. Ductile- brittle transition temperature was determined to be at -40°C. Discussion Charpy impact test helps to determine the ductile to brittle transition temperature (DBTT) of steel. This is mostly due to the fact that steels with similar properties in tension when subjected to slow strain rates can show different brittle fracture when they are tested using the notch impact. High strain rates, states of triaxial stress and temperature that is below the DBTT. It is evident that temperature affects the fracture mode of steel. The brittle fracture of 1045 steel alloy fails when under the temperatures of below -50°C. This is due to the irregular arrangement of tiny bright facets on the fracture surface of the samples. These facets are formed when inter-granular fracture expose cleaved crystals surface. When nearly 100% brittle fracture is obtained the effect of temperature on fracture energy absorbed exhibits a leveling effect. At high temperatures the samples exhibited up to 60% ductile fracture. As temperature increases, the rate of energy absorbed decrease to a steady level. As temperature is increased above the range tested the energy absorption will show very little change. The impact test fracture surface obtained from the sample tested at 89°C, exhibited 60% brittle fracture. The dimensions used to determine the percent shear fracture are labeled on the drawing. In this region the energy absorption changes drastically with temperature. As temperature decreases, the energy absorbed goes from 59.5ft-lbs at 23°C to 12.5ft-lbs at –50°C. This is equivalent to an average decreasing rate of .65 lbs per degree Celsius. This extremely high rate of change is very poor for structural applications that may be subjected to temperatures in this region. There is reasonable correlation between % cleavage fracture and energy expended by breaking the sample. Over the full temperature range tested as the impact strength decreases the percent brittle fracture increases to100% at –95°C. The 15ft-lb DBTT was determined to be –40°C. The medium steel is also known as plain carbon steel. It’s common because it’s cheap and used to make many items. The carbon has around 0.16- 0.29 percent of steel. Medium carbon steel is not hardened by use of heat treatment. Low carbon steel has less carbon compared to other steel. Low carbon steel has 0.05- 0.15 percent of carbon. Steel gains heat when put inside fire. When heat is above 900°C, carbon steel turns to a visible color. The steel heats to a point where it appears to hang. At this juncture, the metal is undergoing transformation called austenite. The point where the steel seems to hang is known as decalescence whereby the temperature is usually critical around 1335f in carbon steel. The heat depends on the amount on carbon content on the metal. Increase in temperature will lead to austenite process to occur in all parts of the steel. The process continues up to 10- 15 degrees and ferrite is consumed (Don Fogg Custom Knives). Low carbon steel have strength and high tensile. The steel is used to make automobile engines and aircraft engine. The steel is used as heat resistant, valve and as stainless steel. Medium carbon steel possesses toughness, hardenability, ductility and has strength compared to low carbon steel. The metal responds well to heat treatment. When the steel has gone under well heat treatment, is resistant to wear and shock. Most of the medium carbon steels include studs and bolts, high duty engines and oil refining and steam installations. The steels are heated to gain superior mechanical properties (Rajan, Sharma and Sharma, pp. 223- 232). The low carbon steels are easily formed. Wear resistance and hardness can be increased by carburizing them. The low carbon steels are used to make nuts, bolts, nails and screws. They are mainly used in swaging, bending and riveting. They do not respond well to heat treatment. High carbon steel contain 0.60-1.70% C. This type of plain carbon steels are hardest and have the best tensile strength than low and medium carbon steels. They also respond well to heat treatment. They are mostly used to make hand tools for instance files, chisels, wrenches and punches. They are also used to make cutting tools like drills, rails, rail road wheels and wood working tools among others (Rao, p.28). The following are types of heat levels applied on medium steel carbon; a. Hardening heat is between 845 to 870 Celsius degree b. Normalizing heat is usually between 870 to 925 Celsius degree and this is done to decrease strength c. Annealing heat is between 830 to 870 Celsius degree d. Tempering heat is between 200 to 700 Celsius degree. The heat temperature depends on hardness of the steel e. Spheroidizing heat is between 760 to 775 Celsius degree. When hardening the stainless steel some parts should be preheated. These parts include; sharp corners, heavy sections, hardened parts, thick and thin sections, straightened parts and heavily ground sections (Davis, pp. 336- 338). High temperatures on medium and low carbon steel leads to steel overheating. This is as a result to choice of wrong temperatures. Steel overheating leads to excessive retention of austenite, grain growth and coarse martensite. When overheating occurs, it causes component brittleness (Prabhudev, pp. 120- 121). Conclusion Read More