Carbon Content of Steel

Gurpreet Kalsi - [email protected]

Student ID: 081671072

Carbon Content Of Steel

1. SummaryThis report examines the effect of carbon content and alloying elements as well as oil quenching and tempering. Several different samples were tested with varying levels of carbon contents and it was found that increasing carbon content increases tolerances to stress and hardness but decreases ductility. Quenching and tempering also increase material strength further but also further decrease ductility. The microstructure of the steels is also affected by carbon, quenching and tempering. These aspects will be further elaborated on in the following report.

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2. IntroductionSteel is one of the most widely used materials within engineering, the scale of steel uses can range from any application such a suspension bridges, armour plating and locks. It can be applied in various different applications due to its wide range of mechanical properties which can be heavily modified with the addition of elements such as Carbon, Nickel and Chromium as well as heat-treatment in order to change the properties of the microstructure of steel. Iron is polymorphic which means at different temperatures it has different micro-structures, this is dependent on the temperature of the iron which in turn determines the phase of the iron and help shows what micro-structure the steel has. Below the melting point of 1538C iron has a body-centred cubic structure also known as delta ferrite. When the temperature falls again to 1394C the structure of the steel changes to face-cantered cubic also known as gamma iron or austenite. Further cooling will produce a microstructure which will change back to a body-cantered cubic structure which, after cooling to 912C and below is known as ferrite or alpha iron. Figure 1 Figure 2

Figure 1 is an example of a Face-Centred Cubic (FCC) microstructure and Figure 2 is an example of Body-Centred Cubic (BCC) microstructure. Other elements can also be added to iron to the heating process to form different kinds of steels for different applications. Low carbon steel (0.76%) steel microstructure is made up of pearlite and ferrite, steel containing exactly 0.76% carbon is 100% pearlite and above 0.76% consists of pearlite and cementite making the steel hard yet brittle. As the level of carbon varies, as does the microstructure of the steel vary, once examined under a microscope the results from the different carbon steels are displayed in Appendix 3. Quenching and tempering alters the structure of the steels and forms a more organised microstructure meaning the material maybe have increase ductility hardness or strength but at the cost of other aspects like toughness depending on the phase of steel, carbon content, and tempering time and tempering.

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3. Experimental ProcedureThe first step of the experiment was to obtain the dimensions of the sample being used during the test, the diameter was 5.125mm and the length was 26.75mm prior to the experiment. These values were taken using digital Vernier callipers and were also verified by a laboratory technician. After the dimensions were noted, the sample piece of 0.5% carbon, normalised steel and later on in the experiment, 0.15% Carbon Steel, 1000C, oil quenched and tempered (held at a certain temperature for a certain amount of time to alter the microstructure) at 500C, was placed in a Tinius Olsen Testing machine in order to obtain a force against extension graph. The machine constantly applied an increasing uniaxial load parallel to the length of the sample at a speed of 2mm/minute until the sample material failed. From the graph printed from the machine, the values in the results sections were extrapolated using equations 1 to 5. Upon calculating stress and strain values of the sample, it was then necessary to find the Vickers Hardness of the steel sample, this involved placing the sample in a testing machine and a 20Kg load was applied to a specific point on the sample, this load made an impression on the material which the dimension of which were measured and then added to equation 6 to calculated the HV20. This was repeated 3 times to get an average value of Vickers Hardness. Figure 4

Figure 4 shows an example of an impression made on a sample while obtaining the Vickers Hardness. The next test involved carrying out an industry used standardised high strain rate Charpy impact test also known as the Charpy v-notch test. This will help determine the materials toughness by the determining the amount of energy absorbed during a fracture. Tests are easily prepared and conducted which is a reason this test is widely applied in industry. The one major drawback is that the results are only comparative to other test on different material samples. These steps were repeated for normalised and tempered steel.

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Figure 5

The sample during the test was a standard size of 55mm x 10mm x 10mm with a 2mm deep v-notch at angle 45 and radius of 0.25mm at the base of the notch. The machine itself (Figure 5) comprises of a heavy pendulum axe (as shown on left) which strikes the sample on the side opposite to the notch. The difference between the height before and after striking the sample helps measure the energy absorbed and is measured in joules. Figure 6

Figure 6 shows a sample after a Charpy impact test, this image is very similar to the condition of the sample used in this experiment after the test. During the third session the microstructures of the different carbon steels were also examined, the results of which are in Appendix 3 In the final session of the experiment a CAL analysis was conducted. This involved using CAL software with different temperatures to predict which phases of steel would be present and takes into account cooling rates and heat treatments. A Time-Temperature-TransformationPage | 5




(TTT) diagram is often implemented to predict these different phases of steel after nonequilibrium states such as rapid cooling. The curves within the diagram are determined by quenching a sample to a specified temperature; maintain that temperature then cooling the sample at room temperature. The CAL package was then used to simulate different compositions of steels with varying levels of carbon, alloying elements, quench temperature and holding times in order to help determine the microstructure of the steel, the results of this are shown in Table 3.

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4. ResultsThe following section will outline the results obtained over the course of the experiment. The focus of the first section of the results is respective to a sample of steel 0.15% Carbon, normalised, with diameter 26.75mm and diameter 5.125mm. The following equations are for calculating the different stated values of the 0.15% normalised steel. Values are extrapolated from graph in Appendix 1. Equation 1 calculates the cross-sectional area of the sample piece of steel.

[1]

Equation 2 calculates the percentage elongation from the original length of the sample i.e. how much longer the piece is when it fails.

= 36.4% Elongation

[2]

Equation 3 calculates the Yield Stress, this is the point where the material stops behaving linearly elastic after the yield stress is applied to the sample. [3]

Equation 4 calculate the proof stress which is also known as the offset yield point, for the case in this experiment a line was drawn parallel to the elastic region with a value of an extension of 0.5% of the of the initial gauge length. [4]

Equation 5 calculates the ultimate tensile stress, this is the highest stress subjected to the sample until it irrevocably fails. [5]

The next step of this experiment involved calculating the Vickers Hardness using equation 6. A load of 20kg was applied, hence HV20, for a moment of time and then the lengths of d1 and d2 were measured giving an average of d1 = 5.46mm and d2 = 5.39mm, these values were then calculated using equation 6 as well as added to a computer programme which calculated the HV20 to be 157 for a 0.15% Carbon Normalised sample of steel. [6] Equation 6 was used to verify and manually calculate Vickers hardness where P is the Load (Kg) and d2 is the diameter of the imprint.

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Table 1 Carbon Content of Steel (wt. %) 0.15 0.4 0.54 0.8 1.15 low alloy Yield Stress (MPa) 280 523 448 773 1199 0.5 % Proof Stress (MPa) 279 500 448 858 390 1164 Ultimate Tensile Strength (MPa) 409 756 786 984 992 1199 Vickers Elongation Hardness (%) (HV20) 36.4 22.1 15.5 7.6 6.2 12.4 157 228 202.18 333 323 351.13

Team 1 2 3 4 5 6

Table 1 shows the results of different steel samples with varying levels of carbon content as well as a low alloy steel. Graph 11200 1000 800 MPa 600 400 200 0 0 0.2 0.4 0.6 Carbon Content (%) Yield Stress (MPa) 0.5% Proof Stress (MPa) Ultimate Tensile Strength (MPa) 0.8 1 1.2

Graph 1 shows Yield Strength, Proof Stress & Ultimate Tensile Strength against Carbon Content.

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Graph 240 35 30 Elongation (%) 25 20 15 10 5 0 0 0.2 0.4 0.6 Carbon Content (%) 0.8 1 1.2 7.6 6.2 22.1 15.5 36.4

Graph 2 shows elongation percentage against carbon content in percentage. Graph 3400 Vickers Hardness (HV20) 350 300 250 200 150 100 50 0 0 0.2 0.4 0.6 Carbon Content (%) 0.8 1 1.2 157 228 202.18 333 323

Graph 3 Shows Vickers Hardness (HV20) against Carbon Content.

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Table 2Carbon Content of Steel (wt, %) Heat Treatment* N N+Q N+Q+T300 N+Q+T500 N N+Q N+Q+T300 N+Q+T500 N N+Q N+Q+T300 N+Q+T500 N N+Q N+Q+T300 N+Q+T500 N N+Q N+Q+T300 N+Q+T500 N N+Q N+Q+T300 N+Q+T500 Yield Stress (MPa) 280 302 277 250 523 587 1132 448 1190 1237 975 773 748 1325 563 1325 1199 1623 1150 0.5 % Proof Stress (MPa) 279 302 277 250 600 1395 1132 448 1190 1237 1032 858 790 1987 390 798 1987 1164 1470 1112 Ultimate Tensile Strength (MPa) 409 388 356 317 756 1628 1202 786 1381 1410 1090 984 790 2284 992 780 2284 1199 1623 1150 Elongation (%) 36.4 26 24 22.1 22.1 5.5 11.4 15.5 2 3.44 6.74 7.6 0.5 5 6.2 780 5 12.4 5.7 8 Vickers Hardness (HV20) 157 146 157 139.53 228 587 333 202.18

0.15

0.4

0.54

0.8

333 563 420 323 563 420 351 595 613 408

1.15

Low Alloy

*N = Normalised at 30 minutes at 1000C, Air Cooled. *N+Q = Normalised 30 minutes at 1000C and oil quenched. *N+Q+T = Normalised 30 min at 1000C and oil quenched and tempered for 30 minutes at indicated temperature. Table 2 shows the properties of each different sample tested, Appendix 2.

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Table 3% Carbon 0.3 0.3 0.3 0.3 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.8 0.8 0.8 Alloying Element None None 3% Ni 3% Ni None 1% Cr 3% Cr 1% Ni 3% Ni 0.1% Mo 0.3% Mo None 0.1% Mo 0.3% Mo Quench Temperature (K) 800 800 800 800 1000 700 700 700 700 700 700 900 900 900 Hold Time 24 Seconds 24 Minutes 25 Minutes 24 Hours 24 Minutes 24 Minutes 24 Minutes 24 Minutes 24 Minutes 24 Minutes 24 Minutes 24 Minutes 24 Minutes 24 Minutes Microstructure 92% Pearlite 8% Martensite 100% Pearlite 81% Pearlite 19% Martensite 100% Pearlite 89% Martensite 11% Ferrite 99% Bainite 1% Martensite 99% Bainite 1% Martensite 54% Bainite 46% Martensite 97% Martensite 3% Bainite 99% Bainite 1% Bainite 99% Bainite 1% Martensite 100% Pearlite 76% Pearlite 22% Martensite 84% Martensite 5% Pearl 1% Austenite

Table 3 shows the results from using the CAL computer programme in order to obtain different microstructures of steel through varying carbon levels, alloying elements, quench temperatures and holding times.

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5. DiscussionFrom the analysis of the results gained in the previous section, several relationships regarding carbon content. From graph 1, it was noticed that ultimate tensile stress, proof stress and yield stress all increase as the carbon content increases, however on the graph, the final point on the graph is anomalous as it does not follow the relationship of the line. The Vickers hardness also increases as carbon content increases meaning are higher weight percentage of carbon means higher hardness levels as shown in graph 3. However adding carbon to steel does also have negative impacts on certain other properties of the steel, as shown in graph 2. Increasing the carbon content means ductility in decreased which also means the steel is more brittle, this was observed when the steel samples began to neck, higher carbon contents did not neck as much as they were less ductile and failed more violently. It was also noticed that alloy steels where more favourable as they had highest strengths and hardness but in the real world are more expensive that pure carbon steels. The next set of results analysed where for quenched and tempered steels, quenching and tempering in the different samples increased everything except ductility compared to normalised steel, the best performing samples, despite having different levels of carbon contents, all where normalised, quenched and tempered at 300C as these sample had the highest stress tolerances and hardness respective to other samples in the same carbon weight percent group, however these samples were the least ductile in each group.Quenching and tempering also rearranges the microstructure forming a more organised structure meaning the molecules are better aligned to deal with forces placed on the samples hence why the quenched and tempered outperform the normalised steel samples, this is reflected in appendix 3 and is clearly visible. The concentration of pearlite is respectively higher which is shown as the white areas and the sheets of Bainite can be faintly seen in a layered formation within the pearlite.

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6. ConclusionFrom this experiment it has been possible to obtain certain relationships relative to the sample of steel being tested, higher carbon content usually results in a stronger steel as the ultimate tensile strength, proof stress, yield stress and Vickers hardness are relatively higher than with a lower carbon content. However, the addition of carbon also has negative effects as it reduces ductility which is shown in graph 2 and the general relationship is that the higher the carbon content is the less ductile a material is. The addition of carbon also decreases the grain size meaning the material is harder as it there more evenly formed grain sizes, the amount of pearlite also decreases as carbon content increases. Oil quenching and tempering also increase strength of the sample as there is a more organised molecular structure of the steel thus reducing the change for dislocations to propagate.

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Carbon Content of Steel - Report

Documents

steel sample

microstructure of steel

steel microstructure

low carbon steel

phase of steel

normalised steel

tempered steel