clasification of steel (Materi): Cold Work

Objective: To study the effect of cold working on the hardness and microstructure of brass.

Introduction: Cold working promotes rapid change in the mechanical properties and in the appearance of copper alloys. The most rapid method of accomplishing it is by cold rolling as in rolling sheet and bar.

Equipment:

Rockwell Hardness Tester

Rolling Mill

Micrometer

Hacksaw

Belt Sander

Polishing and Etching Equipment

Metallograph

Material: One piece annealed cartridge brass 3/4 " x 4" x 1/8" thick.

Procedure:

1. Using the hacksaw, cut the annealed cartridge brass into 7 pieces of about 1/2 " length and mark them as 0, 10, 20, 30, 40, 50,60.

Measure the R_B or R_F (Rockwell B or Rockwell F Scale) of the piece marked "0". This means that sample "0" has no cold work. Take three readings and average them.
Using the rolling mill reduce the thickness of sample "10" by 10%. This means that the sample "10" represents 10% cold work. Measure the hardness and record.
Continue rolling and measuring the hardness numbers of samples 20, 30, 40 ,50 and 60 and record.
Examine the microstucture of the cold worked specimens, parallel to the rolling direction.

Experimental Data: Construct the table shown below and include your experimental results. If the % reduction you obtained in the laboratory varies from the nominal value, indicate what the actual % reduction is. For example, if you obtained 11% reduction (Actual value of cold work) instead of 10 % ( nominal value of cold work), use 11 % in the second column. If you measured the hardness in Rockwell B scale (R_B), convert each reading to Rockwell F scale (R_F) and include them in the last two columns of the table.

% Reduction Nominal	% Reduction Actual	Thickness After Rolling (in.)	Hardness (R_B)	Hardness (R_F)
0
10
20
30
40
50
60

Table 1

After the construction of the table plot a graph that shows the effect of cold work on the hardness of the material as shown below:

Figure 1

Report:

Prepare a report with the following format:

Objective

Equipment

Experimental Procedure

Laboratory Data

Discussion of Results

Present your experimental data as shown in Table 1.

Plot Hardness vs. % Cold Work as shown in Figure 1.

Label the microstructure photographs of the selected specimens. Discuss the structure differences and their relationship to hardness of the specimen and explain what has happened to the structure after 60 % cold working .

Answer the following questions:

What is the effect of cold working on the hardness of annealed brass? (Plot the graph before answering this question)

Time-Temperature-Transformation

(TTT ) Diagram

T (Time) T(Temperature) T(Transformation) diagram is a plot of temperature versus the logarithm of time for a steel alloy of definite composition. It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy. When austenite is cooled slowly to a temperature below LCT (Lower Critical Temperature), the structure that is formed is Pearlite. As the cooling rate increases, the pearlite transformation temperature gets lower. The microstructure of the material is significantly altered as the cooling rate increases. By heating and cooling a series of samples, the history of the austenite transformation may be recorded. TTT diagram indicates when a specific transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved.

Cooling rates in the order of increasing severity are achieved by quenching from elevated temperatures as follows: furnace cooling, air cooling, oil quenching, liquid salts, water quenching, and brine. If these cooling curves are superimposed on the TTT diagram, the end product structure and the time required to complete the transformation may be found.

In Figure 1 the area on the left of the transformation curve represents the austenite region. Austenite is stable at temperatures above LCT but unstable below LCT. Left curve indicates the start of a transformation and right curve represents the finish of a transformation. The area between the two curves indicates the transformation of austenite to different types of crystal structures. (Austenite to pearlite, austenite to martensite, austenite to bainite transformation.)

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Figure 1. TTT Diagram

Figure 2 represents the upper half of the TTT diagram. As indicated in Figure 2, when austenite is cooled to temperatures below LCT, it transforms to other crystal structures due to its unstable nature. A specific cooling rate may be chosen so that the transformation of austenite can be 50 %, 100 % etc. If the cooling rate is very slow such as annealing process, the cooling curve passes through the entire transformation area and the end product of this the cooling process becomes 100% Pearlite. In other words, when slow cooling is applied, all the Austenite will transform to Pearlite. If the cooling curve passes through the middle of the transformation area, the end product is 50 % Austenite and 50 % Pearlite, which means that at certain cooling rates we can retain part of the Austenite, without transforming it into Pearlite.

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Figure 2. Upper half of TTT Diagram(Austenite-Pearlite Transformation Area)

Figure 3 indicates the types of transformation that can be found at higher cooling rates. If a cooling rate is very high, the cooling curve will remain on the left hand side of the Transformation Start curve. In this case all Austenite will transform to Martensite. If there is no interruption in cooling the end product will be martensite.

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Figure 3. Lower half of TTT Diagram (Austenite-Martensite and Bainite Transformation Areas)

In Figure 4 the cooling rates A and B indicate two rapid cooling processes. In this case curve A will cause a higher distortion and a higher internal stresses than the cooling rate B. The end product of both cooling rates will be martensite. Cooling rate B is also known as the Critical Cooling Rate, which is represented by a cooling curve that is tangent to the nose of the TTT diagram. Critical Cooling Rate is defined as the lowest cooling rate which produces 100% Martensite while minimizing the internal stresses and distortions.

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Figure 4. Rapid Quench

In Figure 5, a rapid quenching process is interrupted (horizontal line represents the interruption) by immersing the material in a molten salt bath and soaking at a constant temperature followed by another cooling process that passes through Bainite region of TTT diagram. The end product is Bainite, which is not as hard as Martensite. As a result of cooling rate D; more dimensional stability, less distortion and less internal stresses are created.

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Figure 5. Interrupted Quench

In Figure 6 cooling curve C represents a slow cooling process, such as furnace cooling. An example for this type of cooling is annealing process where all the Austenite is allowed to transform to Pearlite as a result of slow cooling.

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Figure 6. Slow cooling process (Annealing)

Sometimes the cooling curve may pass through the middle of the Austenite-Pearlite transformation zone. In Figure 7, cooling curve E indicates a cooling rate which is not high enough to produce 100% martensite. This can be observed easily by looking at the TTT diagram. Since the cooling curve E is not tangent to the nose of the transformation diagram, austenite is transformed to 50% Pearlite (curve E is tangent to 50% curve). Since curve E leaves the transformation diagram at the Martensite zone, the remaining 50 % of the Austenite will be transformed to Martensite.

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