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Cold Working, Recrystallization,and Grain Growth of Brass

4.1 Background on Recrystallization and Grain Growth

To release the stored energy and restore the metal to an energy level closer to its original state, the

cold worked metal can be heated, either continuously from a lower to a higher temperature, or

isothermally, i.e., at a constant temperature. Energy will be released in three identifiable stages:

recovery, recrystallization, and grain growth. The process of releasing stored energy is called

annealing. When a cold worked metal is heated, there are some rather gradual property changes

that occur without appreciable microstructural alterations and other property changes that are more

pronounced and accompany marked alterations in the microstructure. See Callister ig. !."".

Recoveryis a phenomenon of the first kind that occurs at relatively lower temperatures than are

re#uired to produce ma$or grain restructuring. %uring recovery there may be a slight hardening whichaccompanies a rearrangement of dislocations into a more stable configuration than e&isted in the

cold'worked state. (owever, the density of dislocations is only slightly affected, and the basic grain

structure is not at all affected. )earrangement of point defects *such as vacancies+ into multi'defect

clusters may actually result in a strengthening during recovery.

Recrystallization is a phenomenon of the second kind. n contrast to recovery, ma$or structural

changes occur. These involve nucleation of strain free grains from the cold'worked metal matri&.

When all of the cold'worked metal structure has been replaced by strain free grains, there is a

marked decrease in strength and hardness and a corresponding increase in ductility.

The main variables that affect recrystalli-ation are + time/ "+ temperature/ 0+ amount of prior

deformation/ 1+ composition/ 2+ initial grain si-e/ and 3+ amount of recovery prior to recrystalli-ation.

The recrystalli-ation temperature is usually defined as the temperature at which the highly cold'

worked alloy completely recrystalli-es in about hour.

4fter a metal has completely recrystalli-ed, if the high temperature is maintained *or especially if it is

increased+, the grains will grow in si-e. This is the third identifiable stage of energy release during

annealing: namely, grain growth. The driving force for this growth is in the surface energy of thegrain boundaries. The process will measurably decrease the yield strength of the material as the

yield stress is inversely proportional to the mean grain diameter. %uctility, on the other hand,

increases. (ot working allows recrystalli-ation to occur simultaneously with plastic strain because of

the higher temperature. The advantage of hot working over cold'working is that much larger

deformations may be made without the danger of over'hardening or cracking the material.

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#.$ Cold Working %&'eri(ent

#.1 %)ui'(ent and *aterials

Stanat )olling 5ill

)ockwell hardness tester

Electrical resistance furnaces

6ernier caliper or micrometer

7."28 thick bar of ree'Cutting 9rass *"37 9rass: !7 Cu ' 07 n in the dead'soft condition+

+%- The sample that you will be rolling is to be left for the class that follows yours.

The sample that you will be using for heat treating will be the sample that was coldrolled by the class before yours

#. /afety 0recautions /2%3 0R%C5+/

5.3.A. Stanat Rolling Mill

. The Stanat )olling 5ill is to be operated by four students at each time: one to feed, one toreceive, one to pass the sample back to the feeder, and the fourth to start and stop the rollingmill. The rolling mill is to be turned off every time the students stop to take hardness andthickness measurements.

". ;se the wooden implement provided to push the specimen through the rolling mill. Do notuse fingers or hands.

5.3.C. Rockwell Hardness ester

%o not use )ockwell hardness tester until you have received the necessary instructions. %o not

test a specimen when it is warm. 5ake sure that the bottom of the specimen directly under theindenter is firmly seated on the anvil before applying the ma$or load.

#.6 Cold Working 0rocedure

1. Read the safety 'recautions in /ection #.6.

". 5easure the thickness and width of the metal specimen with a micrometer.

0. Carefully read the )ockwell (ardness directions provided by your instructor. Take )ockwellhardness readings on the as'received *annealed+ specimen.

1. )oll the specimen in stages using the Stanat )olling 5ill. )educe the thickness by rolling

from appro&imately 7."28 to 7."28. ;se the se#uence indicated in the )olling Scheduleinstructions provided in Section 2.1.

2. The thickness and hardness of the cold'rolled sample is to be measured at the beginning, andafter, each stage.

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>. 4fter the prescribed time remove the samples and place them on an aluminum block for rapidcooling to room temperature.

?. Take )ockwell hardness measurements. ;se the @9A Scale.

#.4 Rolling /chedule

)oll in stages from 7."28 to 7."28. Take initial hardness, thickness, and width. Each stage

consists of 1 passes as follows: *8-ero8 rolls to let sample $ust slip through/ then begin rolling in

the following manner+

Bne complete revolution e#uals a pass)eduction per pass 7.7728)eduction per stage 7.7"783'! stages should be involved to reach 7."28

R%*%*B%R-

' Bne direction through mill' )ecord final width

#.# nnealing 5nstructions

. ;pon completion of the cold'rolling, the following annealing schedule should beperformed. ive post heating temperatures will be used, with a heat treat time of 12minutes. C5+- 4lways use tongs to place and remove specimens from the furnace.

(ot

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!.$ Re'ort

Hour written report should include the following sections:

. Title

2. 7ata, 7ata nalysis: nclude your raw data in tables and the calculations and plots listed inSection 2.3. Think carefully about how to present the data before beginning to design yourtables and graphs. Combine data into single tables wherever possible and appropriate. 9esure to clearly label all your tables and figures and refer to them in the te&t.

3. 7iscussion of ResultsI E&plain what the data mean in terms of the purposes of thise&periment. %etermine whether the results of your data are appropriate, i.e. whether thevalues make sense compared to published values andJor e&pectations based on readings.

!. /u((ary< ConclusionsI Summari-e what you did and the outcomes of the study. %o notinclude 4KHT(KL in this section that has not been already presented in the report.Comment on the importance and relevance of the e&periment.

>. Referencesuse 4

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Lrading Luidelines for Maboratory )eport

Cold Working of Brass

Student Kame this grading guideline is only a guide

Score *3+Weak

*!+ *>+ *?+ *7+Effective

Writing Style N Structure Total: J27

Sentence structure

Spelling NKeatness

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Bluesheet 1 ?ardness vs Cold"Work 7ata

%ate Mab Section

Lroup Meader

Safety E&pert

)ecorder

5aterials 5anager

Bther Lroup 5embers

5aterial

Briginal Thickness *in+

Briginal Width *in+

inal Width *in+

Rockwell B ?ardness

Rolling/tage

hickness:in;

1 6 @G. 8 ColdWork

0

1

2

3

4

5

6

7

8

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Bluesheet - ?ardness vs nnealing e('erature 7ata a=le

%ate Mab Section

Lroup Meader

Safety E&pert

)ecorder

5aterials 5anager

Bther Lroup 5embers

5aterial

Time at temperature *minutes+

?ardness as a function of e('ering e('erature

Temp*oC+

Q( " 0 46L.

Kormali-ingactor

4verageCorrected

9

4

9

4

9

4

9

4

9

4

9

4

9

4

9

4

+%/- Q: ( (eat treatment/ 9 9efore Tempering/ 4 4fter Tempering

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