laboratory experiment

The primary goal of the lab experiment was to look into certain important processes in material science, such as recovery, recrystallization, and growth. The investigation was restricted to dislocation defects, or more precisely, line defects and grain boundaries. Heat Treatment, Metallographic Polishing, Metallographic Etching, Hardness Indentation Test, and Optical Microscopy were the five experimental procedures that were taken into consideration. Findings included the existence of a linear relationship between the material's hardness and the percentage reduction of the thickness of the material, as well as the fact that the hardness was consistent with the microstructure that was observed. Above all, the results compounded the understanding of the experiment’s main objectives that focused on measuring and comparing the hardness of the brass sample, the importance of thermal treatments in the restorative processing of materials that have been shaped by mechanical deformation, and a clear concise understanding of the detailed microstructural changes occurring during the three stages of annealing.

Introduction (183)

Material science delves into the processes of recovery, recrystallization, and grain growth of materials. The recovery process involves softening of the material at a lower temperature. Recrystallization, on the other hand, involves softening of the material at higher temperatures which eliminate most of the dislocations through nucleation of new and dislocation-free grains at the grain boundaries whereas growth of grains happens under higher temperatures and longer time. Engineering students find it imperative to understand the characteristics and properties of materials for purposes of advanced use observed in the ever-advancing engineering field that values and appreciates applications of technology. Within the context of this laboratory experiment, considerations are made of a metallic material that is to be work hardened followed by an investigation of the aforementioned processes at the center of material science. As such, this laboratory reports on an understating of the detailed microstructure changes that occur during the three stages of annealing, demonstrate the use of thermal treatments in the restorative processing of materials shaped by mechanical changes such as deformation, and the measure as well as a comparison of brass hardness.

Experimental Procedures (383)

Part I- Heat Treatment

A brass metal prepared as shown in the figure below was subjected to a thermal treatment inducing a temperature of the gradient along its length.



The brass sample was attached, aligned, and positioned as shown in the figure above taking into account the orientation. The brass was allowed to clear the rectangular slot in the bottom of the furnace to extend into the water bath and was quenched rapidly at the end of the annealing treatment.

When brass sample was in position, power to the furnace was turned on beginning at full power ramping up to the target annealing temperature of 900°C. Once the temperature reaches the data acquisition program was started and data recorded. Decreasing water level was adjusted with a squirt bottle to avoid wetting the furnace.

The temperature was held at 900°C for 15 minutes while adjusting the power source to compensate for the smaller variations.

After 15 minutes, the sample was quenched by loosening the retention screw which allowed the brass to be submerged in water.

The power was turned off, the furnace cooled and recording of data was stopped, and the brass sample recovered from the water bath.

Part II – Metallographic Polishing

The brass sample was mounted in the holding clamp and pressed lightly against the grinder in which a flat surface was obtained. The flat surface was then inspected for contours and abrasions through metallographic polishing.

Part III – Metallographic Etching

The sample was subjected to electro-polishing where a fine finish was achieved. The sample was then immediately washed using running water, rinsed with alcohol and then dried under hot air blower.

Part IV – Hardness Indentation Test

A series of Rockwell “A” hardness measurements were made at ⅛-inch intervals along the most severely worked edge of the brass sample and along the edge submerged in the water. The results were then recorded in the datasheet.

Part V – Optical Microscopy

The brass sample was mounted on a glass slide with clay using the specimen to level the surface. Data recorded in the datasheet for part IV, was then used in selecting five points on the edge to detect where hardness was different. A photograph of the microstructures at each of the identified points was sketched and their descriptions detailed.

Results and Discussion (530)

The laboratory experiment is limited to investigating dislocations and grain boundaries, the observed hardness increases as a percentage reduction of the metal’s thickness, which in turn increases the number of dislocations within the brass. Cold work provides an opportunity for the migration of the line defects or rather dislocations into configurations that orient the atoms to allow for plastic deformation. The more the brass sample is deformed, the more the number of dislocations which leads to an increase in energy. Since the dislocations occur randomly in different orientations, they meet while blocking each other’s movement, hence the metal becomes harder. The configurations that encourage acquisition of lower energy strains thus enable the metal sample to increase in hardness contrary to its former state.

From the microstructure sketches, it is observable that the recrystallized grain size increases with increasing temperature. The explanation is that at a higher temperature, the dislocations move out of grains into the grain boundaries. It is at higher temperatures that recrystallization of new grains begins. The implication is that, at higher temperatures, the recrystallized grain sizes increase with an increase in temperatures since new grains start to grow larger compared to cold-worked edge. Nevertheless, recrystallization aims at increasing the grain sizes which in the end eliminates most of the dislocations by nucleating the strain-free grains. In this view, the hardness of the brass metal decreases with increasing grain sizes since a number of dislocations are made few. Fewer dislocations imply few different orientations whose movements if blocked makes the metal brass harder, thus, the hardness observed the decrease in hardness. If it is assumed that the temperature gradient is linear, then there is a temperature below which no recrystallization is observed. Recrystallization begins at higher temperatures that must be attained before the grain sizes begin to increase. In comparison to the tabulated recrystallization temperature of brass, the temperature should not be above 900°C since at 920°C the metal brass would melt.

A plot of hardness against the length of the material which depicts the annealing temperature along the most deformed edge of brass shows that hardness readings are consistent with the observed microstructure. The microstructure is subdivided into five areas namely still worked area, recovery area, recrystallization area, the growth area, and cold-worked edge area. The area marked as the still cold-worked area is harder compared to other areas and is characterized with many dislocations that can readily undergo configuration and orientations upon deformation. In the recovery area, an annihilation of the brass metal allows the line defects to move. The movement is great where migration is out of the grains into the grain boundaries. The recovery area is observed to be softer. In the area marked recrystallization, small, new grains are observed to appear at the grain boundaries. In the growth area, grains that appear purple in color completely overcome the old grains that first appeared in the recrystallization area. In this area, it is the soft new grains that eat up the old grains facilitates by the presence of high thermal energy and enlarge to develop into the observed bigger grains. The fifth area is the cold worked edge. This area appears to be super-soft.

Conclusion (137)

In summary, the three objectives including: (1) to measure as well as compare the hardness of metallic sample, (2) to investigate the significance of the use of thermal treatments in the restorative processing of materials that have been shaped by mechanical deformation, and (3) to clearly understand the detailed microstructural changes occurring during the three stages of annealing have been achieved. Through the laboratory experiment, there has been insightful understanding as well as interpretation of the three main processes involved in material science that includes the recovery, recrystallization, and growth processes. From the experimental findings, the manner in which material hardening occurs after deformation can now be been well explained. The relationship between grain size and material strength has also been perfectly understood. Finally, various points characterizing the heat treatment process have been adequately observed and marked.