Materials Ology and Projection - Term Paper Example

Contents Page Contents Page 1 1.Abstract 2 2. Introduction 2 2.1.Objective 2 3.Theory 3 3.1.Metallography 3 3.2.Precipitation hardening and solid solution hardening of aluminium alloys 4 3.3.How the hardness of alloy steels and plain carbon is affected by cooling rate 5 4.Procedure 6 4.1.Experiment A: Metallography 6 4.2.Experiment B: Precipitation hardening and solid solution hardening of aluminium alloys 6 4.3.Experiment C: How the hardness of alloy steels and plain carbon steel are affected by cooling rate 7 5.Results 7 5.1.Experiment A 7 5.2.Experiment B 7 5.3.Experiment C 8 6.Discussion 8 6.1.Experiment A 8 6.2. Experiment B 9 6.3.Experiment C 10 7.Conclusion 10 1. Abstract This laboratory session aims at examining metallography, how phase equilibrium diagrams are applied, and heat treatment. The laboratory session involved three experiments aimed at determining the effect that cooling rates have on grain structures, the effect that different alloy compositions have on solid phases, and the effect that age hardening has on various steel types and the effect that various cooling rates have on various types of steel. The experiment showed that composition ratios and cooling rates have significant effects on existing phases and grain shapes. In addition, age hardening was found to depend on the time of ageing, and it was also found to increase the hardness and strength of alloys provided that it is done well. Finally, the experiment showed that various types of steel react differently to different cooling rates. With quenching however, steel was found to become harder. 2. Introduction Different industries put the mechanical properties studied in this experiment into use. As the experiment will show, the various properties are achievable by the use of a range of processes. This laboratory experiment aims to study the aspects of heat treatment, metallography and phase equilibrium diagram applications. 2.1. Objective To determine how cooling rates affect metals’ and alloys’ grain structure. To explore how various alloy compositions have affect existing phases. To establish how alloys are affected by age hardening. To observe how the hardness of various steel types is affected by cooling rates. 3. Theory 3.1. Metallography Liquid or molten materials are turned into their solid forms inside a hollowed mould through a process known as casting. As a result of the high temperature gradient, the materials’ outer regions, which have direct contact with the mould, undergo rapid cooling. On the other hand, the materials’ inner region undergoes sow cooling because of the low temperature gradient it has. Consequently, many nucleation sites, which indicate the start of solidification, form on the outer surface of the material. The aforementioned high temperature gradient means that the outer region requires low energy levels. The numerous nucleation sites only have limited space to expand since their grains will come into contact with the grains neighbouring them quickly. Resultantly, the grains of the outer regions are normally very small; hence they are referred to as chill crystals. The region that neighbours the outer region has a lower gradient and therefore the region has less nucleation sites. Thus, the region will have fewer grains. As a result of the fewer grains, the grains can become bigger as compared to their chilled crystals counterparts before their boundaries come into contact. After they come into contact with neighbouring grains, their circumference growth stops and growth towards the centre starts as a result of the high energy/temperature. The grains therefore adopt a column shape that is oriented towards the centre leading to their name – columnar crystals. The core of the material is made up of equi-axed crystals, which are characteristically large because of few nucleation sites that result from its low temperature gradient. These grains however become smaller when rapid cooling is done on the mould. This is because the entire material will have a higher temperature gradient than the one it would have in room temperature. Thus, the rate of cooling of a material substantially affects the materials’ grain structure. Figure 1: General Grain Structure of Cast Material (Higgins 1961) A soluble alloy that is partially solid has phases that are affected by the composition and the temperature of the alloy. The diagram of brass’ equilibrium phase is as shown in figure 2 below. Figure 2: Phase Equilibrium Diagram of Brass 3.2. Precipitation hardening and solid solution hardening of aluminium alloys The hardness and strength of materials is enhanced in a process known as solid solution. This is achieved by using two distinct elements to make an alloy. One element is dissolved into the other element, resulting to a material that has incomplete solid solubility. This characteristic ensures that the material has two different solid phases at different temperatures. Age hardening, which is also known as precipitation hardening, refers to a heat treatment, which enhances the harness and strength of alloys. The treatment is achieved by heating the alloy from the temperature Ts, at which the alloy has double phase solid states (𝛼 and β) to a given temperature (Ta) that makes the material have a single solid phase (𝛼). After the single solid phase is achieved, material is rapidly cooled (quenched) back to its initial temperature. However, because of the quenching the material will not create another phase because it will lack the time to do so. Only phase 𝛼 will exist at temperature Ts. The aforementioned single phased alloy will therefore not be in an equilibrium state, and thus it will create an additional phase (β) over a given period by diffusing atoms. This process is referred to as a return into its equilibrium state, and can be sped up if the alloy is heated to a lower temperature than Ta. The temperature ensures that there is increased diffusion rate of atoms. Resultantly, the alloy precipitates small phase (β) micro-sites and penetrates phase (𝛼). The crystal structure of the material is effectively distorted by the many small sites, which then decreases movement of dislocations. This increases the hardness and strength of the alloy. Over-ageing or overheating of the material leads to bigger and fewer sites of phase (β) that implies less alloy hardness. The age hardening process is as shown in figure 3 below. Figure 3: Age Hardening Process (Higgins 1961). 3.3. How the hardness of alloy steels and plain carbon is affected by cooling rate Iron-carbon or steel alloys are capable of existing in various crystal structures - B.C.C and F.C.C, at different temperatures. The alloys in F.C.C structure are known as austenites while those with the B.C.C structure are known as ferrites. Upon quenching, carbon atoms in austenites lack sufficient time to leave the F.C.C structure and form ferrites. This results in trapping of carbon atoms in iron atoms leading to the B.C.T structure, which has more hardness in comparison to ferrites and austenites. B.C.T is an acronym for Body Cantered Tetragonal, and alloys that have the B.C.T structure are known as martensites. The relatively high hardness is because of the limitation of movement of dislocations as a result of a distorted crystal structure. The ability of alloy materials to form depths/thickness of martensites with various cooling rates is referred to as hardenability. When the procedure is performed on a single specimen of a material that is hardenable, martensites are formed throughout the whole material. On the other hand, non-hardenable materials result in a thickness of martensites that is limited. The hardenability of various steel alloys is as shown in figure 4 below. Figure 4: Hardenability curves of steel alloys (Callister 2007) 4. Procedure 4.1. Experiment A: Metallography The experiment used four samples including 70/30 brass, 60/40 brass, rapid cooled cast aluminium and slow cooled cast aluminium to observe grain structure. After observing the cast aluminium using naked eyes, magnification 1x, they were found to have two round and smooth/flat faces. The specimens’ grain structure was indicated in a drawing. An electronic microscope was used to observe the structure of the sample of 60/40 Brass under a 100x magnification, and the observation was drawn. A microscope was used to observe the structure of the sample of 60/40 Brass under a 100.8x magnification, and the observed structure was drawn. 4.2. Experiment B: Precipitation hardening and solid solution hardening of aluminium alloys The experiment used Aluminium Alloy 2011in fifteen different specimens, 5.5% CU, which were age hardened. The specimens were quenched after being heated to 525 C. After that, they were reheated for the different periods, which are shown in Appendix A, to 180 C. All samples subsequently had Rockwell A tests performed on them in line with set standards. Three tests were conducted on each of the samples and then an average was computed. The samples subsequently had Rockwell A Tests performed away from their surfaces’ indentation with a view to minimize measurement errors. 4.3. Experiment C: How the hardness of alloy steels and plain carbon steel are affected by cooling rate This experiment used alloy steel (AISI 4340) and plain carbon steel (AISI 1040). The specimens then had Jominy Bar Test done on them. Standard bars for the test were made out of the specimens – 100mm long and 25 mm diameter. The bars were heated up to 900 C. The bars were placed in a fixture then the bottom phase got quenched by having water sprayed on it as shown in figure 5. The bars were turned into rectangular prisms using a machine. Curves were formed from thin lines (extremely small curves) on the face of the bar to show distances from the end that was quenched for taking measurement. One Rockwell C (hardness) test was done on each of the aforementioned lines and then measurements were taken. Bottom Phase Figure 5: Jominy Bar Test (MRL 2012) 5. Results 5.1. Experiment A Figure 6: Rapid Cooled Cast Aluminium Figure 7: Slow Cooled Cast Aluminium Figure 8: 70/30 Brass Figure 9: 60/40 Brass 5.2. Experiment B Figure 10: Age Hardening Curve for Aluminium Alloy 2011 5.3. Experiment C Figure 11: Hardenability Curves for Plain Carbon Steel & Alloy Steel 6. Discussion 6.1. Experiment A The cast aluminium’s’ grain structure could easily be compared because of the two sketches in figure 6 and 7. The cast aluminium that had been slow cooled had big central grains and other thin grains that pointed towards the centre. Conversely, the cast aluminium that had been rapid cooled had smaller but more grains in comparison to the aluminium that had been slow cooled. The experiment’s results regarding grain shape was similar to the theoretical diagram (figure 1). This therefore shows that grain shape can be significantly altered by varying the cooling rates. The interpretation of the results was that if higher cooling rates were used, then the grains will grow increasingly smaller. It was however noticed that the sketch of cast aluminium grains, which had been developed through slow cooling, lacked chill crystals – outer small grains (figure 7). This may have resulted from cast aluminium’s shiny surface and the small size of the grains. A microscope was used to observe the 60/40 and 70/30 brass samples, which implies smaller grains in comparison to the ones in cast aluminium. The experiment showed irregularly shaped grains on the 70/30 brass, mostly with the same colour, which showed that they result from a single phase. The experiment also showed few dark patches, which cannot be interpreted as another phase because they are few. Grain boundaries were also easily visible in the experiment. In the same way, the grains on 60/40 brass sample were irregularly shaped. It was however noted that black areas that were thin were spread over the sample. The aforementioned areas cannot be interpreted as grain boundaries since their size is not constant. As a result of their colour and sizes, they represented a separate phase – that is multi-phases, which exist in 60/40 brass’s structure. Figure 2, the theoretic diagram showed that, at room temperature, 70/30 brass will have one phase (𝛼). At the same temperature, 60/40 brass has (𝛼 and β). The dark patches seen on 70/30 brass were not explained. Speculatively, the patches were a result of dust/dirt or reflection of the microscope’s light, thus they can be considered as experimental error. Most of the experiment’s observations were consistent with what was represented in the theoretical diagram (figure 2). The results verify grain formation on cast structures is affected by cooling rates, and that phase equilibrium is affected by various alloy compositions. 6.2. Experiment B Aluminium Alloy 2011’s hardness increased steadily as shown in figure 10 up to the maximal 40.3 HRa, which was achieved after it was heated for a period of 32 minutes. The hardness then reduced to an approximate 32 HRa, which resulted after the material was heated for a period of 80 minutes. This remained up to 240 minutes as shown in figure 10. Theoretically, the alloy’s hardness increases until it reaches a maximum. Continued heating after the maximum hardness amounts to over-ageing, and it will result to a decrease in the hardness of the alloy. Ageing also continues until an equilibrium phase is reached by the material. Experiment results are in line with theory. Aluminium Alloy 2011 show maximum hardness and strength after it is heated for a period of more than 32 minutes. Over-ageing results if the material is continually heated after this period leading to a reduction of the hardness and strength. After heating the alloy for 80 minutes, it reaches its equilibrium phase. As a result of insufficient data between 80 and 240 minutes, the equilibrium cannot be confirmed. All in all, the theory was confirmed according to the available data. 6.3. Experiment C Both samples alloy steel (AISI 4340) and plain-carbon steel (AISI 1040) begin with high hardness, which is observed close to the end that is quenched. The results showed that the alloy steel is uniformly hard throughout the specimen. On the other hand, the hardness of plain-carbon steel reduces significantly until a distance of about 10mm from the sample end that is quenched. From that point, the hardness of the alloy reduces gradually until the bar’s end. The curves that resulted from the experimental data (Figure 11) are in line with theory (Figure 4), which confirms the theory. The theory states that in alloys, martensites form throughout the specimen. In plain-carbon steel however, martensites are only numerous in the area that is close to the end of the bar that has been quenched. Their numbers decrease with increased distance from the end that is quenched. Therefore alloy steel is more hardenable than plain-carbon steel. 7. Conclusion The aim of the experiment was achieved. The results and data verified the available theoretical information. Composition ratios and cooling rates influence existing phases and grain shapes. The use of high cooling rates leads to a high number of grains that are small in size. In addition, ageing time determines the effectiveness of age hardening, which increases the hardness and strength of materials if done correctly. Various steel types were found to react differently to varying cooling rates, with their harness increasing after they are quenched. The dark patches in experiment A were explained by experimental errors. References Callister, William D. 2007. Materials Science and Engineering: An Introduction. Engineering Materials 100 Lecture Notes. 2012. Curtin University. Higgins, Raymond A. 1961. Engineering Metallurgy part one: Applied Physical Metallurgy. Hoboken, New Jersy: John Wiley & Sons. Materials Research Laboratory. 2012. http://www.mrl.ucsb.edu/ Read More

Figure 1: General Grain Structure of Cast Material (Higgins 1961) A soluble alloy that is partially solid has phases that are affected by the composition and the temperature of the alloy. The diagram of brass’ equilibrium phase is as shown in figure 2 below. Figure 2: Phase Equilibrium Diagram of Brass 3.2. Precipitation hardening and solid solution hardening of aluminium alloys The hardness and strength of materials is enhanced in a process known as solid solution. This is achieved by using two distinct elements to make an alloy.

One element is dissolved into the other element, resulting to a material that has incomplete solid solubility. This characteristic ensures that the material has two different solid phases at different temperatures. Age hardening, which is also known as precipitation hardening, refers to a heat treatment, which enhances the harness and strength of alloys. The treatment is achieved by heating the alloy from the temperature Ts, at which the alloy has double phase solid states (𝛼 and β) to a given temperature (Ta) that makes the material have a single solid phase (𝛼).

After the single solid phase is achieved, material is rapidly cooled (quenched) back to its initial temperature. However, because of the quenching the material will not create another phase because it will lack the time to do so. Only phase 𝛼 will exist at temperature Ts. The aforementioned single phased alloy will therefore not be in an equilibrium state, and thus it will create an additional phase (β) over a given period by diffusing atoms. This process is referred to as a return into its equilibrium state, and can be sped up if the alloy is heated to a lower temperature than Ta.

The temperature ensures that there is increased diffusion rate of atoms. Resultantly, the alloy precipitates small phase (β) micro-sites and penetrates phase (𝛼). The crystal structure of the material is effectively distorted by the many small sites, which then decreases movement of dislocations. This increases the hardness and strength of the alloy. Over-ageing or overheating of the material leads to bigger and fewer sites of phase (β) that implies less alloy hardness. The age hardening process is as shown in figure 3 below.

Figure 3: Age Hardening Process (Higgins 1961). 3.3. How the hardness of alloy steels and plain carbon is affected by cooling rate Iron-carbon or steel alloys are capable of existing in various crystal structures - B.C.C and F.C.C, at different temperatures. The alloys in F.C.C structure are known as austenites while those with the B.C.C structure are known as ferrites. Upon quenching, carbon atoms in austenites lack sufficient time to leave the F.C.C structure and form ferrites. This results in trapping of carbon atoms in iron atoms leading to the B.C.T structure, which has more hardness in comparison to ferrites and austenites. B.C.

T is an acronym for Body Cantered Tetragonal, and alloys that have the B.C.T structure are known as martensites. The relatively high hardness is because of the limitation of movement of dislocations as a result of a distorted crystal structure. The ability of alloy materials to form depths/thickness of martensites with various cooling rates is referred to as hardenability. When the procedure is performed on a single specimen of a material that is hardenable, martensites are formed throughout the whole material.

On the other hand, non-hardenable materials result in a thickness of martensites that is limited. The hardenability of various steel alloys is as shown in figure 4 below. Figure 4: Hardenability curves of steel alloys (Callister 2007) 4. Procedure 4.1. Experiment A: Metallography The experiment used four samples including 70/30 brass, 60/40 brass, rapid cooled cast aluminium and slow cooled cast aluminium to observe grain structure. After observing the cast aluminium using naked eyes, magnification 1x, they were found to have two round and smooth/flat faces.

The specimens’ grain structure was indicated in a drawing.

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