Recrystallization of Brass

Recrystallization of Brass T.J.F.Dy Echo, N.R.Preclaro Department of Mining, Metallurgical and Materials Engineering University of the Philippines, Diliman [email protected], [email protected] [email protected], [email protected] Abstract The recrystallization of metal works is a method commonly employed to produce coldworked materials with sufficient ductility and yield strength. Cold-worked materials usually have high levels of stored energy, exhibit high hardness, and high tendencies of cracking. This is commonly remedied via annealing. Recrystallization is one of the primary microstructural changes which occur during this process, and is the stage wherein the material significantly experiences a decrease in hardness and an increase in ductility. In this experiment, the recrystallization temperature of brass was determined using hardness value measurement at different annealing temperatures. The temperature said was found to be around 650°C, denoted by a noticeable hardness difference of 18.33 between two trials. Photomicrographs were also taken to provide visual images of the changes which occurred during the treatments, but failed to provide any valuable information due to having low magnification and poor metallographic preparation of the samples. Factors affecting recrystallization and the temperature at which it occurs were also discussed. Low metal purity, large initial grain  sizes, and low degree of cold-working experienced were found to be among those which  promote increase in recrystallization temperature.  Keywords: Cold-working, annealing, recrystallization, recrystallization temperature

1. Introduction By definition, the process of recrystallization is a  process of forming new strain-free grains from existing cold-worked structures. For background purposes, cold-working is a  process performed  below the concerned material’s recrystallization temperature in an attempt to induce  plastic deformation. This process is usually used to achieve a specific product shape. As a consequence, the materials involved experience an increase in overall yield strength, coupled with a decrease in ductility. 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. This  phenomenon i s further induced by the elongation of grains parallel to the direction of the cold-working  process, similar to the figure below.

Figure 1. Cold-working of metal via rolling ( Image  Image from Principles of the Heat Treatment of  Plain Carbon and Low Alloy Steels, 1996 )

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 yield strength, the material in concern also experiences an increase in tensile strength, and in hardness. As a result of the plastic deformations, however, residual stresses remain within the processed material. For a simpler view, residual stresses could  be seen as a form of an elastic spring-back which occurs when every external loading applied is removed. This is most common on processes involving mechanical methods. Once the surface layers are removed via further machining or through corrosion, subsurface layers may tend to expand and experience distortion. This could then lead to the occurrence of cracking, and eventually material failure. In order to reduce a material’s residual stress, annealing processes are often employed. These  processes are heat treatments aimed to reduce hardness, as well as to complement employed coldworking processes. 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, then

held in that condition for a certain amount of time. Usually, the holding time is directly proportional to the amount of residual stress removed from the material. The material is then slowly cooled so as to minimize further development of internal stresses. Recovery, however, does not produce any change with respect to the material’s mechan ical properties, and rather increases only its electrical conductivity. Further, the elongated grains then turn into more circular structures. The second microstructural change which occurs is recrystallization, which is also this paper’s subject of interest. As was previously said, 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. These will be further discussed in this paper, along with the effect of metal purity, and with observable changes on the grain structures during the said s tage. 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. In the proceeding discussion, the determination of the recrystallization temperature of a cold-worked  brass sample will be focused on. Also, the observable changes in the samples’ microstructures   will be discussed.

2. Methodology A brass rod with a diameter of eight millimeters was initially subjected to a hardness test, and the resulting value recorded. This was done to obtain a reference point for hardness comparison after all subsequent tests have been performed. A diameter alteration for the sample was then achieved using a swaging machine, with the final diameter measured and used to compute for the

resulting percent reduction. This step was to serve as the cold-working stage for the brass sample. The sample was then divided into nine 5-mm long samples, with a piece set aside and labeled as the untreated. This segment was considered the control sample, and was used as microstructural reference. The rest were then paired, and annealed in a tube furnace. Each pair was assigned a corresponding treatment temperature, with settings at 200, 300, 400, and 500°C. For each pair, one sample was treated at the assigned setting for five minutes, and the other for 20 minutes. After treatment, each sample was then molded in Bakelite polymer and labeled accordingly. The use of the Bakelite allows the sample to have an acting  base, which will assist and serve as a stand during metallographic preparation. This served as a  precautionary step due to the extremely small and thin dimensions of the brass samples. The samples were then prepared for metallographic examination. The exposed part of the brass samples were grinded using SiC sheets with grit values of 60, 80, 100, 220, 400, 600, 800, 1000, 1200. These sheets were used to provide a relatively good-finish surface on all samples. Afterwards, each specimen was  polished on wheels using 1μm and 0.5μm alumina solutions, respectively. After polishing, each sample was etched using ferric chloride. The use of an etchant allows for a low degree of surface corrosion on the brass samples, which consequently reveals several microstructural details. Photomicrographs for each sample were then taken. Hardness values were then recorded for each specimen.

3. Results and Discussion The actual experimentation performed in class only involved taking photomicrographs of the samples annealed at varying temperatures. The hardness values to be shown in the following discussion, as well as the resulting trends, are data acquired from a similar experiment previously performed by the instructor and his colleagues. Based on the data, the hardness of the as-received sample averaged at a value of 56.5. As was mentioned in the methodology, this will serve as a reference point for succeeding comparisons. Further, the provided data for hardness values were for samples coming from different annealing temperatures. These are all tabulated below.

Table 1. Hardness values for annealed samples at varying temperatures Temperature (°C) Hardness 25 56.5 200 61.67 250 60.83 300 58.83 350 55.83 450 53.17 550 50.5 650 32.17

To provide a more visual representation of the above data, a graph representing the hardness values with respect to their annealing temperatures was  plotted. With the plot shown below, an occurring trend is now more readily observable.

strain-free grains within the material, and the resulting annihilation of dislocation densities that  previously built up within the material matrix during the cold-working process. The minimal hardness decrease observed in the temperatures ranging from 200-600°C is inferred to  be within the boundaries of the recovery stage of the annealing process. Even though there is no hardness decrease to be expected within this stage, it is  possible that the occurrence of stress relief allowed for a small degree of dislocation mobility within the material and so the observed change. As a supplementary visual material, the  photomicrographs of the untreated and treated samples are shown below, with their corresponding received treatment.

Hardness vs Annealing Temperature 70 60    e    u 50     l    a    V    s 40    s    e    n 30     d    r    a 20    H

10 0 0

200

400

600

800

Figure 3. Untreated sample

Annealing Temperature (°C)

Figure 2. Hardness vs Annealing Temperature

From the plot above, there is an increase in hardness that could be observed from the 20°C data  point going to that of 200°C. This increase is attributed to the fact that before annealing, the brass sample underwent a swaging process. This process is considered as a form of cold-working, and consequently will increase the material’s hardness  prior to the annealing treatments. Again, this increase in hardness is brought about by the resulting limitations in dislocation movements within the material. The plot above also shows a general decrease in hardness with respect to increasing annealing temperature, as seen in the data points starting from the 200°C mark which signifies the beginning of the treatments. This is in accordance with the expected results, given what annealing treatments should do to a given material. Further, it is inferred from the above plot that the sample’s recrystallization temperature is around 650°C. This is supported by the noted substantial decrease in hardness from 50.5 to 32.17. This decrease is brought about by the formation of new

Figure 4. Annealed at 200°C

Figure 5. Annealed at 300°C

Figure 6. Annealed at 400°C

First off would be the degree of cold-working experienced by the material. There is a minimum amount of deformation that must be induced on the material which will allow the formation of a nucleus vital for recrystallization. Further, the amount of cold-work done serves as the driving force which sustains the growth of recrystallized grain structures. Figure 7. Annealed at 500°C

With the magnification used during microscopy, the resulting photomicrographs offer no observable microstructural changes which could be used for discussion. Similar experiments used as further reading material for this paper made use of magnifications at least ten times stronger than what was used in this particular experiment.

The amount of cold-work done on the material also affects the temperature in which recrystallization occurs. It was observed that as the amount of strain induced is increased, there is a decrease in the required temperature. This is due to the higher amount of stored energy available within the material, which promotes nucleation and grain growth.

Theoretically, there should be a noticeable decrease in grain distortion during the recovery phase. Further, as the material nears it recrystallization temperature, signs of nucleation must begin to appear. These theoretical observations are shown in the image  below.

Also, the required recrystallization temperature could be set to a lower degree provided that the annealing time is increased. This is governed by microscopic mechanisms following the Arrhenius Equation, correlating recrystallization rate and temperature.

Figure 8. Theoretical microstructural changes ( Image from Annealing, 2013)

After recrystallization, the new strain-free grains should have been sufficiently formed with evident  boundaries. Another visual representation of this  process is shown below.

Recrystallization temperature of a material also tends to increase when the initial existing grains are relatively large in size. This is due to the reduction of available grain boundaries, which serve as favored sites for nucleation. The same is observed when the deformation process on the material is performed at a higher temperature. This is brought about by the occurrence of dynamic recovery, which effectively increases the degree of recovery experienced by the material. As a result of this phenomenon, there is lower stored energy that becomes available to drive nucleation. Lastly, highly pure metals tend to exhibit lower recrystallization temperatures. The presence of impurities and other alloying elements tend to inhibit nucleation and grain growth within the material due to their tendency to move towards dislocations and grain boundaries. This motion deprives nucleation of favored sites in which it could initiate.

4. Conclusion Figure 9. Recrystallization ( Image from Constitutive Modeling Computational Materials Science)

and

The occurrence of nucleation along the boundaries of existing grains is due the high degree of deformation and dislocation density found in these regions. This preference is an attempt to form a more stable matrix within the material. As was mentioned, recrystallization and recrystallization temperature is governed by several factors.

The experiment determined recrystallization temperature for a given brass sample, and found it to  be at 650°C. The experiment, however, provided no means for the derived temperature’s comparison, given that the brass composition is unknown. The experiment also determined a general decreasing trend in hardness with respect to the annealing temperature, as shown in the provided graph. This conclusion is in accordance with the expected results. Based on the photomicrographs acquired from the experiment alone, no determinate microstructural change has been observed and concluded. Further

reading on the subject, however, affirmed that there is nucleation and formation of new grains which occur as the material nears the recrystallization stage of annealing. It is thus recommended to take  photomicrographs at higher po wers of magnification. It is also recommended that precautionary measures  be taken during metallographic preparation, given that the images above showed signs of scratches and over-etching. Further reading also affirmed that recrystallization and recrystallization temperature is affected by different parameters, such as metal purity, degree of cold-working, and annealing temperature. It was found that the recrystallization temperature tends to increase when the material of concern contains alloying elements and other impurities. The same goes when the material experienced a relatively low degree of cold-working, or if the deformation process was done at a higher temperature. It was also observed to increase if the material of concern initially contained large grains.

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