MatE25 -13 Chafey
Engineering Report 2
Mr. Mark Benjamin:
Experiments have been conducted to determine which of the following is in Picodyne’s best interest: to continue buying brass plates from our supplier, or to buy brass ingots instead at half the price and roll them ourselves. It was possible to roll the sample brass ingots down to a specified size with the Stanat Rolling Mill while at the same time keeping track of the hardness of the specimens with the Rockwell hardness tester. Also, a heat treatment was effective in reducing the rolled samples to original hardness, as desired. It is clear that we possess the capacity to roll and heat treat the brass ingots to our specifications, but further research must be done in order to determine the price involved in making this transition.
The experiment to determine if it was possible to perform cold rolling and annealing of brass was performed to determine the potential cost reduction of buying brass in ingot form instead of the pre-rolled and annealed plates. The manufactured plates cost twice as much as the ingots, and it is possible that we could produce the same quality of plates at a lower cost (depending on additional production costs). A few concepts that are important in understanding the experiment are discussed in the following three paragraphs.
Cold working refers to many methods of altering the shape of metal by plastic deformation. Rolling, drawing, extruding, and pressing, are a few examples of this. All cold working is conducted at a temperature below the recrystallization point, usually at room temperature. Hardness and tensile strength are increased with the degree of cold work, but ductility is lowered.
An important aspect of cold working is strain hardening. Strain hardening creates additional defects which increase the yield stress of a metal. Due to strain fields repelling one another, dislocation movement is restricted, increasing the hardness and yield strength of the metal and making it harder to deform.
The basis of annealing is the three-stage energy-releasing process consisting of recovery, recrystallization, and grain growth. Recovery occurs at the lowest temperature, mostly rearranging defects into multi-defect clusters. The dislocation density and grain structure are not affected much, leading to an actual slight increase in strength at this stage. The second phenomenon that occurs during annealing is recrystallization. Recrystallization causes major nucleation of strain free grains from the cold-worked metal. The strength and hardness decrease at this stage due to the new strain-free grains, and ductility is increased. If a high temperature is maintained or increased, the grain size will then start to increase. This final stage is called grain growth. Ductility will further increase, but the yield strength and yield stress will decrease further as well (yield stress is inversely proportional to the average grain diameter).
Since we wish to cold roll and anneal our samples, hot working could also be considered. Hot working mixes the two processes just mentioned, and causes recrystallization and plastic strain to occur simultaneously. The advantage of such a process is that larger deformations can be made without a risk of breaking or making the specimens too brittle.
Cold working is obviously important because it can harden metal and increase yield strength, both of which are extremely useful in designing materials. Annealing can essentially reverse this process, and if done correctly, a point can be achieved using both processes where the ductility, tensile strength, and hardness can all be maximized.
A sample ingot of 260 Brass (70% Cu and 30% Zn) with an initial hardness of 38.57 (RB) was passed through a Stanat Rolling Mill many times to reduce the thickness of the specimen from 10.4 cm to 3.0 cm. The hardness of the sample was tested ten times on a Rockwell hardness tester: once before rolling, and approximately every 8 mm of thickness reduction during rolling. The hardness was measured three times in each “test” and the result was averaged.
After the final run, the thickness of 3.0 cm was attained and a hardness of 91.8 (RB) was reached. The sample was cut and distributed into five furnaces at various temperatures: 100, 260, 350, 450, and 565 degrees C. The samples were left in the furnaces for 45 minutes before being removed and allowed to cool. At this time the hardness of all the samples was again measured, determining which temperature would provide the original hardness, as desired. The 450 degree chamber produced a sample that had a hardness of 40.6 (RB) which is approximately the equal to the original value.
Cold Rolling Results:
The % cold work was calculated using the formulas:
Annealing Temp (oC)
Discussion of Results
The results make perfect sense. As is evident from the tables and graphs, the results show that the hardness of the sample was increased from the cold rolling. The annealing process then lowered the hardness proportional to the increasing temperature of the furnaces.
Original hardness can be achieved after annealing, as in the 450 degree furnace sample. An even better annealing temperature to attain the original hardness would probably be slightly hotter, perhaps 455-460 degrees C.
Additional costs are based on the expense of labor and maintenance involved in a full-scale production of the brass plates. An estimation could accurately predict this cost with further research as to the number of workers and repairmen needed, as well as the amount of power needed to run the machines.
Some sources of error in this experiment include the following: the error of the hardness tester, the error in thickness and area measurements, and the error of the thermometers measuring the furnaces. The thickness measurement had the potential for the most error, so the area and thickness reduction values could be affected by this.
We can cold roll short brass ingots to a Rockwell hardness B value of 90 and proceed to heat treat to a value of 45.
I do not think cold working and annealing costs would negate material savings. The manufacturer we buy from obviously makes a profit from doubling the price of the brass ingots when sold in plate form, because they are still in business. We can potentially save that same profit they are getting from us by doing the process ourselves.