Cold Working

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					   Cold Working &
Solidification Behavior
      MATE-202L-02
       Orlando Lopez
         11/11/09
Abstract:

        The purpose of this lab was to learn about the effects of cold working on metal and to
learn how metals solidify in different types of molds. In our cold working experiment, we cold
worked two samples of Copper 110 and mounted them along with a strip of “as received” and
annealed Cu110. The sample was then ground, polished, and etched to make it possible to find
the microhardness and take micrographs. Our results showed that the grains become more
elongated and the material gets harder with increased cold working. In the solidification
experiment, molten tin was poured into molds made of three different materials; steel, copper,
and graphite. The temperature of the metal was recorded every five seconds in order to plot a
cooling curve. The solidified tin was then sectioned to show the grains and the solidification
behavior. The cooling curves showed that copper cooled the fastest, then steel, and graphite
cooled the slowest. These same results could be drawn from the sectioned samples, where the
copper and steel molds showed much more columnar grain growth when compared to the
graphite mold. The graphite mold allowed the tin to solidify slowly, allowing equiaxed grains to
form.

Introduction:

        Solidification of metal consists of two different steps; nucleation and growth.
Nucleation is the formation of stable solid particles of certain size, called nuclei, within the
molten metal. These nuclei are the places where growth of the metal crystals occurs. Growth is
when nuclei grow into grains. Growth includes the formation of a solid within the molten metal.
                                                                            Critical radius is the
                                                                            minimum size of the
                                                                            solid particle formed that
                                                                            is stableand grows further
                                                                            in size. There are two
                                                                            types of growth, planar
                                                                            and dendritic. Planar
                                                                            growth is growth under
                                                                            equilibrium conditions.
                                                                            Dendritic growth is when
                                                                            the liquid is undercooled
                                                                            and does not properly
                                                                            nucleate. Dendrites form
                                                                            on the region between
                                                                            liquid and solid as a
                                                                            result of dendritic
cooling. Solidification can be broken down into homogenous and heterogeneous. Homogeneous
solidification is when the solidification starts within the liquid. As a result of homogeneous
solidification, the structure is very uniform throughout the solid. Heterogeneous solidification
occurs at mold walls and impurities. With heterogeneous solidification, the solid is not uniform,
instead having three distinct regions within the solid. These regions are the chill zone, columnar
zone, and equiaxed zone. The chill zone is nearest to the mold wall and contains a band of
randomly oriented grains. The columnar zone consists of long columnar grains that grow
opposite to the direction of heat flow. The equiaxed zone is in the center of the mold and has
uniform equiaxed grains and controlled nucleation. Solidification rate and microstructure are
very important because undercooling and columnar grains seriously affect strength of the
casting. For this reason, industry takes care to follow strict cooling processes and add nucleation
sites into the molten material. This makes a more equiaxed microstructure, making the casting
stronger and tougher. In our solidification lab, we used tin. Tin is a gray metal used in many
alloys, such as bronze, pewter, electrical solder, and die casting metal. Tin has good corrosive
properties, making it a good material for coating corrosive metals. An example of this is tin
cans, which are actually steel coated in tin. Tin is not very ductile nor does it have a high
strength.

         Cold working is the process of shaping a material by deformation without the addition of
heat. Cold working increases internal stresses and makes the grains more elongated. This
hardens the material and is often referred to as work hardening. There is a limit to how much
cold working can be done to a material without heat treatment. Without heat treatment, cold
working will cause the material to eventually crack. One type of heat treatment is called
annealing. Annealing relieves all internal stresses from cold working by adding heat to the
structure, which recrystallizes the grains. Hot working is the forming of a metal with the
addition of heat. The metal is not hardened, but grains stay equiaxed and the material remains
ductile. Also, no additional heat treatment is necessary after hot working. Copper 110 was used
for our experiments. Cu 110 consists of 99.9% copper and is used for plumbing tubing and
fittings.

Procedure:

        For the solidification lab, we used molten tin that was heated in a pot device. Melting the
tin made it able to pour into molds. The tin was poured into three molds made of graphite, steel
and copper. Each mold was filled with the tin and the temperature of a thermometer within the
metal was taken every five seconds. These temperatures were then used to plot a cooling curve.
The tin samples were then sectioned with a band saw, ground and micrographs were taken of the
surface. This allowed us to examine the three zones inside the solid.

        The cold working lab involved four pieces of copper 110, one annealed and three
untreated. We cold worked two of the pieces with a roller and calculated the cold work
percentages. After that, the four samples were mounted in Bakelite. Next, the specimen was
ground, polished, etched, and microhardness tested.
Results:

Our results seemed reasonable, as the cold working went up, so did the hardness.

Sample               VHN trial 1   VHN trial 2
annealed                   76.8          63.0
22% CW                   105.0         100.0
47% CW                   113.0         120.0
untreated                  95.9          94.9

Sample               VHN trial 1   VHN trial 2
45% CW                   122.0         120.0
Annealed not CW            59.0          66.3
As Is                      44.5          53.3
36% CW                   110.0         104.0
Annealed          22% cold worked




47% cold worked   Untreated
              Steel mold                                Copper mold




                                     Graphite mold




Notice how the grains in the steel and graphite are more columnar than those in the graphite
mold. This is due to rapid cooling rates.
                                     Cooling Curves
 250



 200



 150
                                                                                        Steel
                                                                                        Graphite
 100                                                                                    Copper


  50



   0
       0       200       400       600        800       1000     1200       1400


Shown above is the cooling curve for our three molds.



Discussion:

        Our data from the cooling curves is what I would expect from the different materials used
for the mold material. Graphite is an insulator while steel and copper are good thermal
conductors. Differences in grain appearance can easily be seen in the pictures of the sectioned
samples. The tin that solidified in the graphite mold had equiaxed grains, while the metal molds
had columnar grains. It was apparent that the grain microstucture became more uniform as the
cooling rate decreased. The micrographs of the cold worked copper specimens look very
different with increasing amount of cold working. It is apparent that the grains become
elongated when they are cold worked. From the analysis of the micrograph and the
microhardness, you can determine that the as received copper had a little cold working. I would
say it had somewhere between 10 and 20 percent cold working. Although there are differences
between each set of data, the results are consistent with each other. In both sets of data, the
hardness increases with increased cold working.
Conclusion:

        The cooling curves and sectioned samples of solidified tin demonstrate that the graphite
had the slowest cooling rate, resulting in more uniform zones. There were less defined chill,
columnar, and equiaxed zones. The cooling curve showed that the steel took the shortest amount
of time to cool. This rapid cooling made the solidification of the tin non-uniform and had the
three common zones associated with heterogeneous solidification. Copper was somewhere
between steel and graphite, containing some columnar grains, but overall more uniform grain
structure. For the copper cold working it can be said that the amount of cold working on a
material definitely affects hardness of a material. Annealing also affects the amounts of stress in
a material and decrease in strength and increase in ductility. The cold worked specimens had
two different hardness numbers and with the more cold working the material had undergone, the
specimen became harder. The grain structures of a cold worked and annealed material are
different, depending in the extent of each step. It was also shown that annealing relieves all of
the internal stresses left by cold working. Cold working results in columnar grains and annealing
has an opposite result and leaves the material with equiaxed grains.
References:

http://www.matweb.com/

http://geology.com/usgs/uses-of-copper/

http://www.webelements.com/tin/uses.html

				
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