Work Hardening and Annealing of Brass
Abstract
In this experiment, the effect of cold working and the annealing process on the a material’s mechanical properties were analyzed. Alloy samples were measured and tested to specifically examine their hardness and strength. To measure the dimensions and hardness of each sample, the materials used included four disk-shaped brass samples, calipers and Rockwell tester. For the cold-working and annealing processes a hand roller and a hot tank were used. The results of this study indicated that as the amount of cold-working increases, so does the hardness of a material. It was observed that an increase in temperature led to a decrease in hardness in a material.
The recrystallized grain size increased with temperature, implying that grain growth was induced in the process. It was also noted that the more a material becomes cold-worked, the lower its recrystallization temperature.
Introduction
The objective of this experiment was to determine the effects of cold working and annealing of brass disk samples. The features of significance lie within the material’s microstructure. The microstructures of the samples have a significant effect on the strength and hardness of the material. When the grain boundary area increases, the grain size decreases and amount of resistance of dislocation motion will increase as well. This will produce an increase in both the hardness and strength of the sample. This will be controlled through the use of cold working and annealing application (of increasing temperature).
Cold-working is a process performed below the concerned material’s recrystallization temperature in an attempt to induce plastic deformation. As a consequence, the materials involved experience an increase in strength, coupled with a decrease in ductility (Askeland, 270). These effects on the said mechanical properties are brought about by the occurrence of strain hardening. Strain hardening is caused by the intersection and of multiple dislocations within the material (Askeland, 273). This phenomenon is further induced by the elongation of grains parallel to the direction of the cold-working process.
To calculate the percent cold work, the formula is defined as
As the degree of cold-working increases, so does the grain elongation within the material. In turn, the dislocation density also increases, eventually giving the material a consequent increase in resistance to plastic deformation. Aside from strength, the material in concern also experiences an increase in hardness. As a result of the plastic deformations, however, residual stresses remain within the processed material. In order to reduce a material’s residual stress, annealing processes are often employed. These processes are heat treatments aimed to reduce hardness. As the material is gradually softened, there are three main microstructural changes which occur within the concerned specimen. The first of these three is recovery. To induce the progress of this stage, a material is heated over a temperature range specific for its composition (typically low), then held in that condition for a certain amount of time. Recovery, however, does not produce any change with respect to the material’s mechanical properties, and rather increases only its electrical conductivity.
Further, the elongated grains then turn into more circular structures (Askeland, 285). The second microstructural change which occurs is recrystallization. This stage involves the formation of new strain-free grain structures. These new grains tend to initially appear on regions of existing grains which experienced severe deformation prior to the annealing treatment, such as grain boundaries. It is in this state of change wherein the alteration of a material’s mechanical properties begins to occur, and is usually exhibited by an increase in ductility along with decreasing overall strength. It is also worth mentioning that recrystallization is affected by more than a couple of factors.
Most evident perhaps, would be the recrystallization temperature, and the corresponding holding time(Askeland, 286). The third microstructural change that occurs during annealing is grain growth. This occurrence normally takes place when a material is annealed at a relatively very high temperature, or if retained at a high temperature for very long periods of time. During this stage, recrystallized grains begin to tend to absorb one another and so further promote grain coarsening. In general, the occurrence of grain growth is not preferred given that larger grains tend to mean a lower toughness for a given material (Askeland, 286). In the following discussion, the determination of the recrystallization temperature of a cold-worked brass sample will be focused on.
The annealing a cold worked specimen decreases its hardness which is thought to be due to the reformation of the grains to their normal shape from their compressed longitudinal shape. The higher the annealing temperature; the more grains reform to their normal shape and the more the hardness decreases. As can be seen in Figure 3, the more a sample was cold worked, the lower the Recrystallization Temperature. Discussion In order to fully interpret the significance of the results, some discussion should be explored about cold working and annealing. With increasing percent cold working, the yield hardness of metals are increased, whereas the ductility is decreased. Due to formation of new dislocations, the dislocation density in metals also increases. Moreover, the resistance to dislocation motion by other dislocations is increased as well. The strain fields around the dislocations most often repel one another, limiting dislocation movement.
Therefore, the stress needed to deform a metal increases as a result of cold working. When the samples were rolled, the widths were increased by a very small amounts, which was neglect in this experiment for the calculation of percent cold work. According to equation, the volume of the bars would expect to be decreased, since the dislocation density increased, and the weight of the bars remained the same. When stress is applied on a material, dislocations form inside the material.
Dislocation motion leads to plastic deformation, as in the case of the sample of Brass used in this study, which was cold-worked. As the work hardening increases, more energy is inputted to plastically deform the material, hence more dislocations are formed. In other words, the dislocation density increases. One major reason to support this trend is that as the dislocation density in the material increases with percent reduction, it is much harder for dislocation motion to occur, as each dislocation makes it harder for another dislocation to occur. Since hardness measures the strength of a material, characterized by how resistant a material is to dislocation motion, the more a material is cold-worked, the higher its hardness, explaining why the hardness is highest in the sample that was cold worked the most.
There is always a temperature at which recrystallization occurs, because there is a threshold level of energy that is required for the new grains to nucleate. The more work-hardened a material, the more the energy that has already been transferred into the material, meaning the lower the amount of energy that needs to be provided by the increase in temperature, explaining why the more work-hardened samples would have the lower recrystallization temperature.
After recrystallization begins, and as the temperature increases, so does the grain size. Since it was observed that hardness decreased as the temperature increased, it can be correlated that the same happens when grain size increases. A logical explanation for this would be that as the recrystallized grain size increases, there are larger grains that are defect-less, hence the hardness would be lower. This corresponds to what was explained earlier, that hardness increases with the number of dislocations – defect-less crystals have no dislocations, hence a lower hardness.
The literature value states that recrystallization for Brass that is 70% Copper and 30% Zinc begins at approximately 240,8 which is almost 100 different from the calculated recrystallization temperature for the sample through experiment. However, this is plausible, as the recrystallization temperature would definitely be lower for a more work-hardened sample. In other words, this comparison with the literature value is not accurate, for the sample used in this experiment was work-hardened, while the sample used to obtain the literature value might not have been work-hardened, or might not have been cold-worked as much as the samples used in experiment.
Conclusions
Cold working a metal will increase its strength proportionally to the amount of cold work put into the metal. Furthermore, after cold work is completed, annealing will recrystallize the metal and change the mechanical properties of the metal on a time and temperature dependent basis; including lowering the hardness of a metal. ReferencesAskeland, Donald R, and Wendelin J Wright. The Science and Engineering of Materials. Lab Handout
