Brass Annealing

BRASS ANNEALING

Student Name: Christos Kalavrytinos

Module: Materials Science D1

Module Coordinator: Adrian James

Assignment: Brass Annealing

Materials Science D1 2008

Christos Kalavrytinos 1

Abstract

This report aims to determine in which ways the mechanical properties of 70/30 brass are affected by heat treatment. Tensile testing and Vicker’s Hardness Testing were the methods used to determine the above. Analysing the results and findings showed that the re-crystallisation temperature is affected by the alloy, the percentage of cold work, the original grain size and the time of heat treatment. The re-crystallisation temperature increases with increased alloying but decreases with increased cold work, finer grain size and longer heat treatment times. The re-crystallisation temperature was estimated around 300°C.



Contents

Abstract........................................................................................... 1

Contents..........................................................................................2

1.0 Introduction..............................................................................3

1.1 Aims & Objectives.............................................................3

2.0 Background Theory...................................................................3

2.1 Tensile Testing..................................................................3

2.2 Vicker’s Hardness Test......................................................3

2.3Brass Phase Diagram................................ .........................4

2.4 Hot and Cold Working.......................................................4

2.5 Elastic Recovery after Plastic Deformation........................6

3.0 Apparatus.................................................................................6

4.0 Experimental Procedure............................................................6

5.0 Results & Findings.....................................................................7

6.0 Discussion.................................................................................8

7.0 Conclusion................................................................................9

List of References & Bibliography.....................................................10



1.0 Introduction

1.1 Aims & Objectives

This report aims to determine the effect of heat treatment on the properties of 70/30 brass. This was done by tensile testing using a manually operated Hounsfield tensometer and by Vicker’s Hardness Testing. Finally, by analysing the results, we will be able to understand how and why the mechanical properties and the brass change as well as to determine the re-crystallisation temperature.

2.0 Background theory

2.1 Tensile Testing

In order to determine some key mechanical properties of materials, tensile testing is used. During this procedure, a standard specimen is gradually elongated under tension, in this case using a manual operated Hounsfield machine. By continuously recording the load and elongation, we end up with a plotted curve identical to a graph of stress over strain. This stress over strain relationship depends on the chemical composition, heat treatment and manufacturing process and differs for most materials.

Depending on the material, the yield stress, maximum load and breaking point, the ways in which a material behaves vary. For example, in brittle materials necking does not occur and failure occurs after limited extension. Ductile materials exhibit large strains before failure but the stress at fracture is lower than the tensile strength (although the actual stress in the neck is higher). By performing the tensile test, we get information such as:

Ultimate Tensile Strength: The maximum stress the metal can take, the high point on a stress–strain curve. This shows up at the top of the stress–strain curve. The material still can take more stress, but it's a relatively small amount before the material fractures.

Elongation Percentage: The percentage of elongation of the specimen after the tensile test.

2.2 Vicker’s Hardness Test

The Vickers hardness tester uses a square-based diamond pyramid indenter, and the hardness number is equal to the load divided by the product of the lengths of the diagonals of the square impression. Vickers hardness is the most accurate for very hard materials and can be used on thin sheets. It employs a pyramid shaped diamond with an included angle of 136o which is impressed into the specimen using loads of 5 to 120 kg making a small square impression. This test is used for finished or polished components because the impression can be very small. The diamond pyramid hardness number is obtained from a calculation based on measuring the diagonals of the impressions in the steel. Figure 1 illustrates a schematic of the diamond indenter and figure 2 shows an indentation on hardened steel.



Fig 1, diamond indenter. Fig 2, diamond indentation on hardened steel.

Images from: http://www.inspecttest.com/hardness-test.html accessed on 5/2/09

2.3 Brass Phase Diagram

The eutectic copper-silver and lead-tin phase diagrams have only two solid phases, α and β which are sometimes termed terminal solid solutions, because they exist over composition ranges near the concentration extremes of the phase diagram. In the case of copper-zinc system, intermediate solid solutions may be found at other than the two composition extremes.

In addition, there are six different solid solutions, two terminal (α and η) and four intermediate (β, γ, δ, and ε). Some phase boundary lines near the bottom of the diagram are dashed to indicate that their positions have not been exactly determined. The reason for this is that at low temperatures, diffusion rates are very slow and inordinately long times are required for the attainment of equilibrium. Again, only single and two-phased regions are found on the diagram. The commercial brasses are copper-rich copper-zinc alloys; for example, cartridge brass has a composition of 70 wt% Cu / 30 wt% Zn and a microstructure consisting of a single α phase. (William D. Callister)

2.4 Hot and Cold Working

An increase in the temperature at which a metal is worked means less energy is required to work the metal since it is more malleable at higher temperatures. Higher temperatures can mean, however, surface scaling or damage occurring. The initial cast material has coarse grains; hot working breaks the grains down to give a finer structure and thus better mechanical properties. The main hot working processes are rolling, forging and extrusion.



During cold working, the crystal structure becomes broken up and distorted, leading to an increase in mechanical strength and hardness and a decrease in ductility, this being termed work hardening. The more the material is worked, the harder and more brittle it becomes.

When a cold-worked metal is heated the events that occur depend on the temperature to which it is heated. These events can be broken down into three phases:

Recovery:

When a cold-worked metal is heated to temperatures up to 0.3 Tm (where Tm is the melting point on the Kelvin temp. scale of the metal concerned) then the internal stresses resulting from the working start to become relieved. There are no changes in grain structure during this but some slight rearrangement of atoms in order that the stressed become relieved. This process is known as recovery.

Re-crystallisation:

If the reheating is continued to a temperature of about 0.3 to 0.5 Tm there is a very large decrease in hardness, decrease in strength and increase in elongation. The grain structure of the metal changes and crystals begin to grow from nuclei in the most heavily deformed parts of the metal.

Grain growth:

As the temperature is further increased from re-crystallisation temperature, so the crystals grow until they have completely replaced the original distorted cold-worked structure. The hardness, tensile strength and percentage elongation change little during this phase, the only change being that the grains grow.

The term annealing is used for the heat treatment of changing the properties of an alloy by re-crystallisation. The factors affecting re-crystallisation are:

1) A minimum amount of deformation on necessary before re-crystallisation can occur. The amount of permanent deformation necessary depends on the metal concerned.

2) The greater the amount of cold work the lower the re-crystallisation temperature for a particular metal.

3) Alloying increases the re-crystallisation temperature.

4) No re-crystallisation takes place bellow the re-crystallisation temperature. The higher the temperature above the re-crystallisation temperature the shorter the time needed at that temperature for a given crystal condition to be attained.



5) The resulting grain size depends on the temperature, the higher the temperature the larger the grain size.

6) The amount of cold work prior to the heat treatment affects the size of the grains. The greater the amount of cold work, the smaller the resulting grain size and the more centers are produced for crystal growth to be initiated. (W. Bolton 1998)

2.5 Elastic Recovery after Plastic Deformation

Upon release of the load during the course of a strain test, some fraction of the total deformation is recovered as elastic strain. In a stress-strain plot, during the unloading cycle, the curve traces a near straight line path from the point of unloading, and its slope is virtually identical to the modulus of elasticity, or parallel to the initial elastic portion of the curve. The magnitude of this elastic strain, which is regained during unloading, corresponds to the strain recovery. If the load is reapplied the curve will traverse essentially the same linear portion in the direction opposite to unloading; yielding will again occur at the unloading stress level where the unloading began. There will also be an elastic strain recovery associated with fracture. (W. D. Callister 2003)

3.0 Apparatus

The following apparatus were used to carry out the experiment:

Hounsfield Tensometer

Micrometer

Metallurgical Microscope

Vicker’s Hardness Testing Machine

4.0 Experimental Procedure

First, the width and thickness of each sample, used for the tensile test, are measured and recorder. They were used to calculate the cross-sectional area of the sample and thus helped determine the Ultimate Tensile Strength.

The tensile test was performed in order to acquire values for Max Load, Fracture Load, Elongation, and decrease in area. The Tensometer is shown in Figure 3.

Then the Vicker’s Hardness Test Machine was used to test hardness on two samples annealed at different temperatures. A schematic of the Vicker’s Testing Machine is illustrated in Figure 4.

In the last part, the microstructures of each sample were examined using a metallurgical microscope and sketched.



Fig. 3 Hounsfield Tensometer Fig. 4 Vicker’s Hardness Test Machine.

(http://www.auscal.com.au/images/systems/hounsfield_4(691x372).jpg)

(http://www.twi.co.uk/content/jk74.html)

5.0 Results & Findings

The following Figure 5 illustrates the three different specimens.

Specimen A Specimen B Specimen C

Specimen D Specimen E

Fig. 5 (Photographs taken using digital microscope)



Specimens Max Load (kN)

UTS Elongation %

Reduction in Area %

Hardness Index

A 80% CW 6.25 852 1-2 10 226

B 80% CW+ 200°C 5.6 763 2 - 231

C 80% CW+ 300°C 4.12 561 25 - 135

D 80% CW+ 400°C 3.5 477 48 20 106

E 80% CW+ 500°C 3.17 432 54 - 76

Table 1, results from tensile and hardness test.

Specimen A: There was no change in the microstructure as it went through recovery phase.

Specimen B: The material went through to the re-crystallisation phase. A small dislocation and new crystals started to show up on the microstructure.

Specimen C: The re-crystallisation was more obvious than Specimen B and more crystals appeared in the microstructure.

Specimen D: After annealing at 400°C, re-crystallisation was almost complete and partial grain growth was observed.

Specimen E: There was total re-crystallisation and a new formed grain was visible.

6.0 Discussion

The mechanical properties of the material are mainly affected by its composition and in this case the heat treatment. Annealing had the following effects:

Elongation was proportional to the temperature each specimen was annealed to. The higher the temperature, the more the elongation.

Hardness stays rather constant with the temperature until the re-crystallisation temperature is reached. There is a sudden decrease after the re-crystallisation temperature, which is around 300 °C, and the metal becomes more ductile.

The annealing also had a major effect on the strength of the material. The higher the temperature, the less Ultimate Tensile Strength was exhibited by the specimens. The difference between the UTS of 100°C and 500°C annealed brass was almost double.

The temperature required for re-crystallisation varies with the metal. It is approximately one third to one half of the melting temperature of a pure metal. Therefore steel is annealed at red heat, whereas



aluminiun would be liquid in this temperature. On the other hand, lead can only be cold worked at room temperature because its re-crystallisation temperature is so low. Furthermore, re-crystallisation temperature is really a range, not a sharp point. Also, the greater the amount of cold work, the lower the re-crystallisation temperature, because of the stored energy. The sudden drop in hardness of 70/30 brass of magnitude of the drop in re-crystallisation temperature that is to be expected with increased cold work.

The variables that influence the re-crystallisation temperature, which is not fixed, follow. It depends on the alloy, the percentage of cold work, the original grain size and the time of heat treatment. The re-crystallisation temperature increases with increased alloying but decreases with increased cold work, finer grain size and longer heat treatment times.

The effects of cold work can and grain size can be explained by the fact that the new grains can be more readily nucleated at sites of high dislocation density. The alloy and time effects depend on diffusion. However, it is reasonable to expect that atom motion is required in the re-crystallisation phase and that the presence of foreign atoms (alloying elements) makes atom motion difficult. Similarly, longer holding times give atoms more time to move, so re-crystallisation can occur at a lower temperature. (Flinn, Trojan 1990)

Consequently, it is assumed that many dislocations remain in every crystal and that increase in hardness results from their mutual interference and the building up of a transcrystalline ‘traffic jam’. Increase in hardness and strength is due to the greater difficulty in moving new dislocations against the jammed ones, whilst a ‘pile up’ of jammed dislocations may propagate a fracture. (R. A. Higgins 1993)

7.0 Conclusion

Concluding, this lab report determines the effects of annealing on the mechanical properties of 70/30 brass using tensile testing, microscopic examination and Vicker’s Hardness test. Analysing the results and findings showed that the re-crystallisation temperature is affected by the alloy, the percentage of cold work, the original grain size and the time of heat treatment. The re-crystallisation temperature increases with increased alloying but decreases with increased cold work, finer grain size and longer heat treatment times. The re-crystallisation temperature was estimated around 300°C.



List of References & Bibliography:

Engineering Metallurgy, Applied Physical Metallurgy, R. A. Higgins, 6th edition, 1993

Engineering materials technology, W. Bolton, 2nd edition, 1993

Engineering materials and their applications, Richard A. Flinn, Paul K Trojan, 4th edition, 1990

www.twi.co.uk/j32k/getfile/jk74.html

Images from: http://www.inspecttest.com/hardness-test.html

http://www.inspecttest.com/hardness-test.html

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Brass Annealing