ENSC 3313 Oklahoma Ductile Metals & Alloys Deformed Below Meting Point 0.5 Lab When ductile metals and alloys are plastically deformed below about 0.5 of t

ENSC 3313 Oklahoma Ductile Metals & Alloys Deformed Below Meting Point 0.5 Lab When ductile metals and alloys are plastically deformed below about 0.5 of the melting temperature (Tm), they strain harden. The process is commonly called cold work. On heating to above about 0.5 Tm subsequently, the material softens to its orig ENSC3313
COLD WORK AND RECRYSTALLIZATION
(Bring Safety Glasses)
Lab Experiment #3
Introduction
When ductile metals and alloys are plastically deformed below about 0.5 of the melting temperature
(Tm), they strain harden. The process is commonly called cold work. On heating to above about 0.5 Tm
subsequently, the material softens to its original state by recrystallizing. This involves the replacement of the
distorted cold worked grains of high dislocation density (up to 1011cm-2) by equi-axial grains of lower
dislocation density (~ 106cm-2). It occurs by the growth of grain nuclei, from grain boundaries or heavy slip
bands, into the deformed grains. After recrystallization is complete, the fine grain size coarsens if the material
is left at temperatures above 0.5Tm. Since strength decreases as grain size increases (less obstacles to
dislocation motion), grain growth leads to some further softening. The fully soft state of a material is called
the annealed state. A clear illustration is given in the blue ASM Handbook, 8th edn. Volume 7, pages 283287. Alternately, note 9th edn. Volume 9, pages 180-181, 411.
The theme of this experiment is to familiarize you with the cold work-recrystallization sequence, by
verifying the process using a copper base alloy. The grain changes can be observed under the microscope, the
strength changes can be monitored by hardness measurements. Appreciate that a material can be used with any
desired degree of cold work present. For example, steel and aluminum cans are formed by plastic deformation.
The cold work maximizes the strength, so minimizes the wall thickness (hence material) needed.
Procedure
1. Sign up, for a 2.5 hour time slot, online using link found on D2L course site, no more than 6/session.
2. Review cold work in your textbook.
3. Measure the hardness of the as received cylindrical samples, HRB or HRF scale, as appropriate.
4. Cold work the samples by compression, remeasure the diameter and hardness. (see table)
5. Soak your samples at 575°C for 10 minutes.** Remeasure the hardness. (see table)
6. Complete the table below.
7. Polish, etch, and examine the microstructure.
8. Answer the questions on the reverse side.
Observations
Sample Number
_________________
Initial Diameter, d0 (in.)
_________________
Initial Hardness (HRB)
_________________
Annealed Hardness (HRB)
_________________
Hardness, HRB
Sample* Load** Diameter,
(lbs)
do (in.)
Diameter, df
(in.)
Worked
Reheated
True Stress
True Strain
a
7,000
0,502
0.58
21.9
20.7
26494.3
-0.297
b
9,000
0,503
0.61
27.8
24.2
30795.9
-0.398
c
11,000
0,501
0.64
33.9
29.5
34193.4
-0.494
d
13,000
0,502
0.69
37.1
32.6
34766.0
-0.644
e
15,000
0,504
0.73
42.9
39.5
35829.0
-0.757
f
17,000
0,501
0.78
49
44.7
35577.0
-0.889
References
1. Callister text 6th edn., pp. 131-137,177-187, 370-376; Smith text 4th edn., pp. 225-257,403-410
*1-2 samples per participant.
**…or as stated by TA.
Note: True Strain = 2ln(d0/df)
True Stress = Load/Actual Area
NAMES: ______________________, ______________________, ______________________, ______________________
Note: You may attach additional sheets to fully respond to the following questions.
Sample #: ______________________
Report
5 Points
Q1. Do (and attach) a log true stress – log true strain plot; report the strength coefficient and the strain
hardening exponent, if feasible.
Q2. Plot (a) the cold worked and (b) the recrystallized hardness versus % cold work. Compare the curves.
Inferences? Be sure to include data for no cold work! (2 graphs!)
Q3. Show the microstructure of the sample after coldworking and after recrystallization. Compare the two
microstructures. (Images already given! Please comment by looking at them)
Q4. What, approximately, was the % cold work in your as received sample? State your reasoning.
Q5. Identify your material from C10200, C11000, C17200, C21000, C26000, C36000, C46400. The alloys
differ in color (copper or brass), microstructure, (1 or 2 phase) and properties. Justify your choice! The
ASM handbook (vol. 2) and the SAE handbook (vol. 1) provide data.
LAB 3 – COLD WORK AND RECRYSTALLIZATION
1
6 samples. Cut from same copper
alloy rod. Therefore they can be
considered as the same sample
initially. Initial hardness is considered
to be same as it is given in your lab
assignment
2
3
All samples placed in furnace.
Annealed, process takes about 10
minutes @575 ºC for the copper
alloy we have
All samples are
compressed under
loads specified in the
lab report respectively.
This is how cold work is
performed.
Hardness testing is
performed
4
Hardness testing is performed on the
cold worked and annealed samples
as the final step
Magnification: 10 x
Channel Count: 3
50 um
P
Magnification: 10 x
Channel Count: 3
50 ?m

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