Annealing is the process of heating a sample to allow the dislocations to move past each other via enhanced atomic diffusion at the higher temperature and relieve the stored energy in the strain fields. This is termed recovery and allows the dislocations to relocate to sites producing low strain energies. Continued exposure to the higher temperature allows recrystallization to occur. During this process, new strain free and equiaxed crystals are formed into grains that have low dislocation densities and the mechanical properties like hardness and ductility revert to their pre-cold worked state.
Theory:
The main equation that will be used is to work out the tensile stress:
Tensile strength= [Maximum Load]/ [Cross sectional area] (1)
The cross-sectional area in (1) is calculated from the Thickness and width of the copper Block in its final state:
Cross-sectional Area= [Thickness] x [Width] (2)
Experimental arrangements:
The apparatus consists of a fully softened copper block with dimensions 50x25x5mm. Measure the copper block with a micrometer and a ruler to find its thickness (t), length (l) and its width.
On this initial sample carry out a hardness test using a Vickers Hardness testing machine with a 5 Kg load (HV5).
Roll this metal block to a ~54mm and then again measure its dimensions as above and carry out another Vickers Hardness test. Roll the block successively and carry out the same procedures as previous until the maximum length of ~600mm is rolled.
From this final length cut four lengths of ~150mm each. Test the tensile strength of the rolled section with the Hounsfield Tensometer. Heat up the other sections at 360oC and 160oC, on these heated samples carry out the Hounsfield Tensometer.
Tabulate the results as appropriate and plot graphs.
Results:
The percentage area reduction was calculated by:
- Calculate the cross sectional area of the original and rolled out block.
- Subtract the area of the rolled out block form the original area and divide by the original area.
- This will give the percentage area reduction when multiplied by 100.
Table 1: % area reduction for given thicknesses
The percentage increase in length:
- Measure the lengths of the original block and the rolled block.
- Subtract the rolled length from the original length and then divide it by the original length.
- This will give the percentage increase in length when multiplied by 100.
Table 2: % increases in length, for given length.
The Hounsfield Tensometer test is then carried out on the final sample that has been cut, two of which are annealed and another remained as rolled and no heat treatment.
When the Hounsfield Tensometer test is carried out the Maximum load is given from which you have to calculate the tensile strength with equation (1) and (2).
Table 3: Annealing temperature and tensile Strength
The data is analysed by plotting three graphs, % reduction in area against
Hardness, % increases in Length against hardness and Tensile strength Vs Annealing temperature.
The plot of data is shown in fig 1, fig 2 and fig 3.
Discussions:
From the graph of data shown in fig.1, it can be seen that the points are increasing consecutively until the maximum hardness is approached after this point the graph has an exponential affect. The hardness will not go above this maximum length and therefore is fully hard at this point. All the points on the graph have a smooth line through it except one, this could be due to the bending effect on the copper from the rollers, when the Hardness test is done on this sample it is not be completely flat and therefore giving a value which is inaccurate.
From the graph it can also be seen that when the area reduction is reaching its maximum the hardness increase is not as steep as the initial increases of the small reductions in area.
Fig.2 also shows the same relationship as fig.1, as the length increases the hardness increases. The first few increases in length have a rapid increase in hardness. The greater the increase in length the smaller the step increases in hardness.
Fig.3 shows that when final length of the copper, where it had full hardness, undergoes annealing then the properties of the same-dimensioned copper block is altered from what the rolled block without heat treatment has. When the temperature is increased to 160oC the hardness of the copper sample decreases, yet if the temperature is increased further to 360oC the hardness decreases to the fully softened value that was first started with. Subsequently the tensile strength decreases as the temperature increases.
The rolling machine is designed with vertically adjustable rollers that grab the samples and squeeze them through the opening between them, in the process deforming them by reducing their thickness (depth) to that of the rollers. The copper when pulled through the rollers gets pulled through with the velocity of the rollers this squashes the block causing friction, which heats the block up the block then gets pushed out with the velocity of the rollers, therefore having energy transferred from the rollers.
Overall the experiments show an increase in hardness as the % cold work is increased which supports the theory of strain hardening of metals, by increasing the dislocations in the crystal lattice, and the localized strain between the constituent atoms. The experiments also show that annealing causes the copper to become more ductile, (softer) and that cold working, when followed by annealing, offers an enhanced recovery of the pre-cold work material properties.
Conclusion:
The overall method used was quite accurate to get the relationship between the affect of rolling and annealing in a copper block. Although there were systematic errors in the experiment these did not affect the overall result of the experiment.
The graphs drawn are reasonably accurate and only have one point at which there was a slight inconsistency with the results.
Relationship between Cold Work and Hardness
The plot of above hardness versus % cold Work above shows that there is a relationship between the amount of cold work performed and the amount of “strain hardening” as evidenced by the steady increase in hardness as the sample is worked more.
Relationship between Annealing and Hardness
The above plot also shows that samples #1 and #2 had similar hardness, and hence, crystal states.
Note: after several passes, one group’s 60% sample was erroneously passed through the rollers transverse to the direction of the initial passes. This caused a general cupping of the sample and subsequently brittle fractures occurred. The effect on the sample is unclear, but it is likely that it has a different deformation of its crystal structure compared to the other two samples, which were rolled in the same orientation each time.
Hardness Testing:
Each group performed hardness tests using Rockwell Hardness Testers (by Wilson) in the Polishing Lab in Meuller Hall. Hardness is a measure of a material’s resistance to deformation by surface indentation or abrasion. According to the Callister text, the Rockwell Hardness tester works by pressing 1/16 inch steel ball under an initial minor load of 10kg into the sample, followed by a second major load on 100kg (B scale) into the test sample and comparing the depth of the indentation made by each load to derive a relative number indicating the “hardness”. Multiple readings are taken at various locations on each sample, which are then averaged to find and average hardness number for the specimen.
Figure 1.
Figure 2.
Figure 3.
Appendix:
Raw data showing intermediate calculations: