Sheikh ShahirCold Working and RecrystallizationKEM120702TITLE

Cold Working and Recrystallization

ABSTRACT

The following experiment was conducted to apply cold working on pieces of copper metal followed to observe any changes in their microstructure. This was followed up by annealing the said copper pieces and determining the crystallization temperature. As a control, one piece of copper didnt undergo any cold working while the other three were cold rolled to 10%, 30% and 50% respectively. After that, we cut each piece into four samples and annealed them at 3 different temperatures: 200oC, 300oC and 600oC. The batch that was left over didnt receive any treatment and were used as the control .Having being annealed for a period of 30 minutes, we observed each sample using a microscope. This was followed up by testing their hardness using a Vickers Hardness testing machine. Microstructures of each sample were drawn out and the characteristics were observed. Also, a graph of hardness against cold-work degree for each annealing temperature was plotted to observe the trend of the results.

INTRODUCTION

Cold working can be referred to as altering the shape or size of a metal by plastic deformation. Processes include rolling, drawing, pressing, spinning, extruding and heading, it is carried out below the recrystallization point usually at room temperature. Hardness and tensile strength are increased with the degree of cold work whilst ductility and impact values are lowered. The cold rolling and cold drawing of steel significantly improves surface finish.

Whereas hot working refers to processes wheremetalsare plastically deformed above theirrecrystallizationtemperature. As the material is above the recrystallization temperature, it can recrystallize during deformation. The importance of this is that it keeps the material from strain hardening and thus keeping the yield strength and ductility low. This is in extreme contrast tocold working.

The cold working process thats taking place in this experiment is known as cold rolling. In this process, metal stock is passed through a single or more than one pairs of rolls in order to reduce its thickness and make the thickness uniform. When the temperature of the metal is below its recrystallization temperature this is known as cold rolling.

Fig 1: Example of Cold Rolling

Annealing, in metallurgy and materials science, is considered to be a process of heat treatment that can be used to alter the properties of a material and make it more ductile and workable. It requires heating a material above its critical temperature, maintaining a suitable temperature and then cooling it. Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogenous and improve cold working properties.For the metal, this process simply involves heating the material for a while until it glows and then slowly letting it cool down to room temperature. It can also be cooled by quenching in water, unlike ferrous metals such as steel which must be cooled slowly to anneal. Annealing prepares the metal for further work such as shaping, stamping or forming.

Hardness give a measure of how resistant a solid matter is to various kinds of permanent shape change when it is exposed to a force. Macroscopic hardness is generally characterized by strongintermolecular bonds, but the behavior of solid materials under force is complex; therefore, there are different measurements of hardness:scratch hardness,indentation hardness, andrebound hardness. In this experiment the pieces of copper metal undergo Vickers Hardness Test.

The Vickers test can be used for allmetalsand has one of the widest scales among hardness tests. The unit of hardness given by the test is known as theVickers Pyramid Number(HV). Vickers hardness test use a 136 pyramidal diamond indenter that creates a square indent.HV = Constant x test force / (indent diagonal)2The constant is a function of the indenter geometry and the units of force and diagonal. The Vickers number, which normally ranges from HV 100 to HV1000 for metals, will be increased as the sample gets harder. Tables are available to simplify the calculation, while all digital test equipment do it automatically. A typical Vickers hardness is specified as follows:212HV0.5Where 212 is the calculated hardness and 0.5 is the test force in kg.

The biggest advantage of Vickers is its scale, which comprises the smallest and the highest hardness values in one scale. It is thus very suitable for laboratory tests. Most of the disadvantages of Vickers are based on the long duration of the whole procedure because the indentation must be measured optically (with the help of a miscroscope or projector). This, of course, also is a source for measuring errors. However, modern, automatic image evaluation computer systems reduce this source of errors significantly. The surface must be well prepared and the penetrator must be applied evenly. Otherwise, the smallest inclination would cause irregularities in the indentation. Thus, the Vickers procedure is not suitable for routine tests. The indentation is not well readable on some materials because of the irregular distribution of the load (more load on the edges than on the sides of the pyramid).

OBJECTIVES

There are two objectives in this experiment:1) To do cold working on copper and observe any changes in microstructure.2) Followed by annealing the cold worked metal and find out crystallization temperature.

RESULTS

Table 1: Experimental ResultsTemperature(C)Percentage of Cold Working (%)Vickers Hardness Number(HV)

RoomTemperature10.00124

30.00130

50.00122

20010.00129

30.00135

50.00144

30010.0048.9

30.0045.7

50.0048.5

60010.0039.4

30.0046.1

50.0046.2

DISCUSSIONSCold working and hardness:

1) It can be seen from the graph that, at room temperature, the hardness decreases slightly at first before rises, as the cold worked percentage increases. This in turn shows that the metal undergoes strain hardening when it is cold rolled.2) Whereas when annealing at a temperature of 200oC, the hardness is seen to increase with increased percentage of cold-rolling. Also, a large difference in hardness is seen between samples of 10% and 30% cold rolled. For the temperature of 300oC, the hardness is fairly constant and for 600oC, the hardness increases and becomes constant.3) When the copper metal is cold-worked far below its melting point, the dislocation density increases rapidly during plastic deformation. The dislocations together with other obstacles such as grain boundary and inter-metallic particles prevent further dislocation motion. This results in an increase of the hardness. The greater the amount of dislocation density, the greater the effect of work hardening.4) The annealing process at temperatures of 200 oC and 300 oC causes the internal stresses to be relieved by heating the metal. The crystal lattice is broken by plastic deformation which causes distortions and results in the cold worked metal in being thermodynamically unstable resulting in a decrease in hardness.5) If we allow the temperature to be sufficiently high, the metal will try to achieve equilibrium through three stages, namely recovery, recrystallization and grain growth. The process of recovery is the result of dislocations interacting with each other at high dislocation densities, by virtue of attractive and repulsive forces. The line of 200oC and 300oC in the theoretical graph shows the process of recovery. Recovery softens a material by lowering the dislocation density. Hence, the hardness number is decreased while the ductility goes up.

Annealing Temperature and Hardness The general trend is that the higher the annealing temperature, the lower the hardness after annealing. This result agrees with the theory. This is because, when further heating is applied to the metal, nucleation and growth of new crystals will replace all the deformed crystals of the cold worked metal. This is recrystallization process. The hardness and strength of the metal are greatly reduced while ductility increases.

Observations under microscope In general, grain patterns followed the theoretical predictions. For the rolled (but not annealed) grain, the directionality of the grains increases with increasing percentage cold work. The grains are equiaxed for 0% cold work, but gradually become more elongated as % cold work increases. For the 50% cold worked sample, grains appear as long, thin pieces longitudinal to rolling direction. This is because thickness of the grains decreases as the length increases. For the samples annealed at 200oC, a similar pattern can be observed. As % cold work increases, the grains show more elongation in rolling direction. However, it is also observed that for the 30% cold worked sample, there are many small grains at the grain boundary of larger grains. For the 50% cold worked sample, this is even more obvious as many small grains are formed. This is because increasing the amount of cold work prior to annealing increases the instability of the structure. This reduces recrystallization temperature and speeds up recrystallization rate. Hence, the higher the degree of cold work, the smaller, new, equiaxed grains are formed. For the samples annealed at 300oC, while it is visible that some degree of elongation occurs as % cold work increases, it is not very obvious. This is because at 300oC, recrystallization occurs quite fast resulting in the elongated grains being replaced by newly formed, equiaxed grains. For the 10% cold worked sample, several small equiaxed grains are already visible although the degree of cold work is low. For the 30% cold worked sample, some of the equiaxed grains are growing significantly. For the 50% cold worked sample, there are many small equiaxed grains and it is visible that many grains are non-directional they are the grains that are formed during heat treatment and subsequently grew in size to replace the directional grains. For the samples annealed at 600oC, grains no longer appear directional even if cold worked. Even for the 0% cold worked sample, there are signs of nucleation. For the 10%, 30% and 50% grains, there are very few grains that are elongated. They are mostly large equiaxed grains with some newly formed small grains at boundaries. The higher the % cold work, the smaller, equiaxed grains are formed. There are a few special observations worth extra mentioning. The first is the appearance of some very black spots under the microscope. Although we cannot be certain of the composition of the black spots, they are likely to be copper oxide regions, which are formed when copper was heated in the presence of oxygen. The second is that some grains appear slightly darker than the others. There are two reasons for this observation. Firstly, the amount of light reflected from the crystal grains are dependent on crystallographic orientation some orientations more favourably reflect more light. Secondly, consistent with our observation, highly deformed grains tend to be darker. This is because dislocations, when view with greater magnification, appear as dark lines. On a smaller magnification, such as the degree used during our experiment, dislocations cannot be seen but the effect remains grains that are more deformed (more elongated) have high dislocation concentration and hence appear darker due to the light-scattering effect of the distorted lattice caused by deformations. The same reasoning applied to the small, newly formed equiaxed grains they appear brighter because they are less plastically deformed. The inconsistencies of the results with the theory are caused by some possible reasons or errors that may occur during the experiment:1. The actual percentage of thickness reductions after cold-roll are not exactly the same with the one desired. 2. The surfaces of the copper pieces are not perfectly smooth due to oxide layer. This will affect the accuracy of readings measured in Vickers Hardness Test.Thus, in this experiment, some precaution steps have to be taken to minimize the discrepancies and to get more accurate results:1. The thickness of copper piece is reduced slightly for every pass when rolling process of the copper.2. All the copper pieces are cleaned by using sand papers before the Vickers hardness test is performed. 3. The crucibles are labeled first before putting them into the furnaces to avoid confusion.

CONCLUSIONSTo conclude, it can be seen that cold working causes microstructural change in copper alloys which is the elongation of copper grains. Generally, the higher the cold working percentage, the higher is the hardness. In addition to that, annealing softens the copper samples. The higher is the annealing temperature, the lower the hardness of the sample after annealing. Annealing causes smaller, equiaxed grains to replace the elongated grains formed during cold working. Unfortunately, some of the experimental results are inconsistent with the theory due to experimental errors. REFERENCES1) Wikipedia.2) Laboratory sheet.3) Material Science and Engineering : An Introduction by William D. Callister Jr.,David G. Rethwisch4) http://metals.about.com/library/bldef-Cold-Working.htm5) Howstuffworks.com