Sunday, September 15, 2019
Examine the tensile strength of three specimen of low, medium and high carbon steels is examined
The aim of this laboratory experiment is to examine the tensile strength of three specimen of low, medium and high carbon steels is examined. The microstructure of the specimen is determined and calculations such as tensile strength, yield strength etc were clearly recorded. Also, the background theory was stated, the apparatus and procedure used to achieve the experiment was described. The main part of this lab report is the discussion on the results and how close they've been calculated to the original theoretical values by taking into consideration some external experimental errors. The last part of this report is the conclusion on the whole procedure. INTRODUCTION The main purpose of this lab report is by using a tensile testing machine (Hounsfield tensometer), to determine mechanical properties of three different plain carbon steel materials (low carbon steel, medium carbon steel and high carbon steel). Also, their grain structure is to be examined using a Metallurgical microscope. BACKGROUND The three different materials are the low-carbon, medium-carbon and high carbon steels. Their tensile strength is examined which by definition is explained as the amount of stress that a material can resist when a force pulls it along its length until a complete deformation takes place. A ductile material is a material that contains the properties of plasticity and tenacity and it's able to change its shape when a force acts on it and can keep that changed shape even after that force is removed. (Timings R. 2006) The tensile test is mainly used to specify the strength and ductility of a material. Also the tensile test involves: 1. Material showing a yield point which is the point that an extension takes place without any increase in load 2. Proof stress which is used to determine the amount of plastic deformation. 3. Secant modulus which is used to determine the elasticity of the material. (Timings R. 2006) Plain carbon steels: Ferrous metals are basically a metallic material (iron) and it means that iron is combined with carbon. Iron and carbon, the simplest of the ferrous metals (Latin ferrum=iron), are the main elements of plain carbon steels. Low-carbon steels have a carbon content 0,1-0,3% in addition to impurities. This kind of steels cannot be directly hardened by heat treatment, but they can be readily carburized and case hardened. The type of medium-carbon steels have a carbon content 0,3-0,5%. They can be toughened by heat treatment. All types of high carbon steels (carbon content 0,8-1,0%) are extremely strong and their response to heat treatment is better than the medium-carbon steels. However, because of the high carbon content they can be hardened to a high degree of hardness. (Timings R. 2006) The iron-carbon (Fe-C) diagram in figure1 helps to study and learn more about the microstructure of carbon steels as well as their heat treatment. Figure1. Ã¢â¬Å"The Fe-C phase diagram shows which phases are to be expected.Ã¢â¬ (1) Ã¢â¬Å"At the low-carbon end of the Fe-C phase diagram, we distinguish ferrite (alpha-iron), which can at most dissolve 0.028 wt. % C at 738 Ã ¯Ã ¿Ã ½C, and austenite (gamma-iron), which can dissolve 2.08 wt. % C at 1154 Ã ¯Ã ¿Ã ½C.Ã¢â¬ (1) EXPERIMENTAL PROCEDURE In order to complete this test, three tensile test specimens, each of different carbon content, are given. Also a tensometer machine is available in order to tense the specimens. The machine works as follows: Firstly, the specimen is placed on the machine and a force pulls it along its length. This force is measured (in kN) on a digital force meter which is connected to the machine. On the top there is a cylinder with a graph paper around it in order to sketch a graph of force against the extension of the specimen. This is done by moving the pointer on the graph paper by 0,5kN respectively and pointing on the graph each time the reading on the digital force meter increases by 0,5kN, for instance, if the reading reaches 1,0kN the pointer has to be pointing at 1,0kN and by the time that the reading is 1,0kN a point is sketched on the graph. (See figure 2 below) Figure 2. Furthermore, measurements of the length and cross-sectional area were taken before and after the test in order to determine the percentages of elongation and the reduction in area. The last part of the experiment is to examine the three micro-specimens given which is the exact same material and condition as the three materials used on the tensile machine and determine the percentage of the carbon content of their grain structure. This is done by using a Metallurgical Microscope. RESULTS The results of the experiment were calculated and recorded on a table as shown below: Test piece material % carbon content Yield strength (N/mm2) Ultimate tensile strength (N/mm2) % elongation % reduction in area Specimen A Low-carbon steel 0,1 315 430 37 66 Specimen D Medium-carbon steel 0,4 475 660 28 62 Specimen N High-carbon steel 0,8 932 960 13 30 All the specimens had normalized treatment conditions. Graphs were plotted for every specimen, which state clearly the points of force and extension. (See Tables below) The ultimate tension strength (uts) was calculated by the following formula: Ã¢â¬Å"Ã¢â¬ (3) The yield strength (ys) was calculated by the following formula: Ã¢â¬Å"Ã¢â¬ (4) The elongation percentage (elon.) was calculated by the following formulae: Ã¢â¬Å"Ã¢â¬ (5) The reduction in cross-sectional area (red.csa) was calculated by the following formula: Ã¢â¬Å"x 100Ã¢â¬ (6) Microstructure results The following specimens are the result of the experiment. The white region of each specimen is ferrite and the gray region is pearlite. The carbon content is determined using the iron-carbon (Fe-C) phase diagram. SPECIMEN A SPECIMEN D SPECIMEN N DISCUSSION The experiment is now completed and a discussion about the results is made. The values calculated in the experiment are going to meet the theoretical values of the three specimens used. In the table below all the results were recorded: CALCULATED THEORETICAL Low carbon steel UTS (N/mm2) 430 162-3200 YS (N/mm2) 315 140-2400 ELONGATION (%) 37 1-48 REDUCTION A. (%) 66 13-99 Medium carbon steel UTS (N/mm2) 660 450-2290 YS (N/mm2) 475 245-1940 ELONGATION (%) 28 0.6-34.2 REDUCTION A. (%) 62 0.2-71.4 High carbon steel UTS (N/mm2) 960 161-3200 YS (N/mm2) 932 275-2750 ELONGATION (%) 13 1.9-30 REDUCTION A. (%) 30 13.4-75.2 The table above show clearly that the calculated values are close to the theoretical values. This means that the experiment was successful and the calculation were correct. Although, there's always a small percentage error in every experiment. The most common error in every experiment is the human error and this is the main type of error that may took place in this experiment. Also, differences in temperature and the purity of the material used is an important issue. Furthermore, from the examination of determining the grain structure of each material under the microscope the difference between them was very clear. For instance, the different amount of ferrite and pearlite could be identified, high-carbon steel had darker color than low and medium carbon steels which means that the amount of pearlite is almost 100%. Also, from the tables plotted on the tensometer machine the load that every specimen could withstand, the elongation percentage and the reduction in cross-sectional area were different. By considering these values, low-carbon steels have the least amount of load before complete deformation and the most percentage on both reduction in cross-sectional and elongation of the three specimens. This means that low-carbon steels have the least amount of carbon. In addition, low carbon steels can be defined as ductile materials. Medium-carbon and high-carbon steels are less ductile have les percentage of elongation. This means that they are harder and they are applied more load in order for deformation to take place. Finally, the last part of the discussion is about the different yield point of the three specimens. If the graphs are considered, a sudden fall of the load appears to take place on the graphs of low and medium carbon steels during the procedure. This means that the two specimens faced a reduction in cross-sectional area (also known as necking). This doesn't seem to happen to the specimen of high-carbon steel which means that the deformation took place without having any noticeable reduction in cross-sectional area as the load was kept increasing. CONCLUSION In conclusion, the three specimens where tested and results were given. Since the calculated values meet the theoretical values, the experiment was successful. Discussion about the ductility and the main structure of the given specimens was made and also the differences between them were stated.