The Concept of Work Hardening of Metals - Lab Report Example

Summary Metals deform at room temperature and are usually accompanied by layers of atoms sliding over one another within the crystal grains of the metal. The experiment studied the concept of work hardening of metals (work hardening normally escalates metal hardness on deformation). Cold and hot workings were also studied to determine their difference. In the first part of the experiment, the concept of cold working was demonstrated. Original measurement of copper annealed at 800 OC for 120 minutes were taken that is, width, thickness and hardness. A cold rolling machine was then used to deform the copper sample to specified percentages (5%, 10%, 20%, 40%, 60% and 80%) of its original dimension. The respective cross sectional area and cold work index were then calculated for each percentage deformation. The hardness of the copper sample was also calculated at each reduction. A graphical presentation of hardness versus cold work index was plotted which clearly showed a sharp increase of hardness with an initial increase in cold work index. Experiment two examined how annealing times and annealing temperature affected the hardness of copper. Each of the six specimens of commercially pure copper was subjected to different amounts of annealing subsequent to cold rolling. For a copper sample annealed at 0 C for 0 minutes, the measured hardness was 156.6HRB (Bendrich, pp178-256). The hardness decreased as time and temperature were increased. The hardness of a sample annealed at 800C for 120 minutes was 69.7HRB, which in essence approved that cold working a metal, increases its hardness. Again the more the metal was annealed and the higher the temperature, the more the grain growth continued, and the softer the metal became. The observation was made in that, the crystallization temperature of a metal affect how much the grain growth will be. Results Rockwell Hardness Tester was used to determine the hardness of a specimen of commercially pure copper that has been annealed at 800 C for 120 minutes. A rolling machine was used to reduce the thickness of the sample by 5%, 10%, 20%, 40%, 60% and 80%. The percentage cold work was calculated by the bellow formula; %CW=%CW= x 100% % CW: percentage cold work A: original cross sectional area A: final cross sectional area The work table 1 shows the calculated values for the dimensions of sample copper at each percentage reduction in width. It also includes the cross-sectional area, the percentage cold work as well as the hardness for each percentage reduction. . Table 1: Measurement of cold worked copper Predicted reduction S, (mm) W, (mm) A, (mm) % CW Hardness measurement Scale: HRB 1 2 3 Avg. Original 6.35 12.70 80.65 0 68.9 69.0 69.5 69.1 5% 6.10 12.80 78.08 3.2 97.0 96.3 97.3 96.9 10% 5.75 12.90 74.18 8.0 112.5 111.5 115.1 113.0 20% 4.80 13.20 63.36 21.4 133.5 138.5 132.3 134.8 40% 3.80 13.30 50.54 37.3 146.2 145.2 140.1 143.8 60% 2.50 13.50 33.75 58.1 157.2 162.0 153.9 157.7 80% 1.30 14.80 19.24 76.1 164.5 161.0 164.1 163.2 Individual specimens of commercially pure copper were subjected to different amounts of annealing subsequent to cold-rolling. Work table 2 shows the results, Table 2: Measurement of copper annealed at different times (t) and temperature (T) Specimen t, (min) T, (C) Hardness measurement Scale: HRB 1 2 3 Avg. 1 -- -- 158.2 155.2 156.3 156.6 2 10 400 142.2 143.4 143.2 142.9 3 20 400 83.9 85.8 79.1 82.9 4 60 400 75.1 73.5 71.4 73.3 5 120 400 71.6 74.9 70.5 72.3 6 120 800 72.8 67.2 69.1 69.7 Discussion Experiment 1 Hardness by definition is ‘a measure of a material’s resistance to localized plastic deformation’ (Callister pp234-318). Deformation of a metal at room temperature makes it harder and its strength rises while the ductility drops. This essential property is referred as cold working. From graphical presentation of hardness versus cold work index, an observation was made in that, as cold work index increases, the hardness of a sample increases a confirmation of the fact that cold work working strengthens materials by causing deformations. Figure 1 shows he graphical presentation of hardness versus cold work index. Experiment 2 Re-crystallisation temperature is the temperature above which a metal is heated in order for it to crystallize, that s form new-free grains. This temperature varies widely from metal to metal. Work hardening does not take place when a metal s formed above its crystallisation temperature, and it is said to be cold worked. Thus, ‘hot’ and ‘cold’ are purely relative. The second part of the experimental results clearly agrees with the re-crystallisation theory. It was observed that hardness decreases as the annealing time and temperature increases. Conversely, when time and temperature are decreased, hardness of sample increases. Annealing copper sample for 10 minutes 400 OC, the hardness decreased by Annealing negates the effect of cold work forming new-free grains. Hence, the copper sample got softer with time and an increase in temperature. Copper crystallizes at about 120C and the sample was originally annealed at temperatures 400C and 800C which would definitely heat it enough to cause crystallization (Callister, pp178-215). Annealing clears defects and the internal stress it causes, and results in re-crystallisation. It was observed from the experimental results, that the longer the specimens were allowed to anneal, the softer the material became and hence, the larger the grain size became. This resulted from the amount of time given for grain growth in the re-crystallization stage. The grains merge faster, and deformations are fixed quicker as the increased heat allows for easier movement among the atoms. If a lead sample is similarly cold worked and annealed in same conditions at room temperature, the grains will be bigger n size and drops in hardness will be much faster as compared to the copper sample (Bendrich, pp178-256). The lead sample ought to be cold worked bellow -4C because -4C is the re-crystallization temperature lead. The expected are due to the temperatures used to anneal the lead sample (400 oC and 800 oC) would be significantly greater than its re-crystallization temperature, hence causing faster grain growth and faster recovery periods (Callister, pp145-276). Alternatively, when a nickel sample is treated in the same way, there will be a decrease in grain size and a slower drop in hardness because its re-crystallization temperature, 370C, is closer to the annealing temperatures 400 C and 800 C. There is no change in the grain size or hardness occurs at the annealing temperature of 0 C. Conclusion The experiment above explored the concept of work hardening as well as the relationship between hardness, cold working and hot working by using copper sample. From the results, the more a sample was cold worked, the more its hardness increased although after a while the hardness slowed down as cold work continued largely due to the amount of deformation already caused by cold working. The deformations create stress in the crystal grain, making it difficult for any further deformations to occur. From the results, the longer a sample is annealed, and the higher the temperature, the larger its grain growth and the softer the material becomes. Time is of essential for metals to recover from their deformations to enhance the creation of new stress-free grains from scratch (Bendrich, pp178-256). The higher the temperature is above the re-crystallization temperature, the easier it is for recovery, as atoms move easer and diffusion are easier. With more time provided, more and more grains will merge which leads to grain growth leading to a rise in ductility and a decrease in strength. Recommendation The experiment was a good practice for concepts that were not yet taught in class. It would have been a noble idea and more beneficial if the experimental setup was conducted soon after the concepts were taught in class. Works Cited Callister, William D. Materials Science and Engineering an Introduction 7th Ed. New York: John Wiley & Sons Inc, 2007. Bendrich, G. Materials Science in a Nutshell: GD Publishing, 2010. Bendrich, G. (n.d.). ChE 2506 -Materials Science Laboratory. Retrieved October 30, 2009, from Blackboard Learning System: https://learning.unb.ca Read More