Title -WORK HARDENING LABORATORY

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Coursework Title -WORK HARDENING LABORATORY
Module Name: Engineering Practice 2
Module Code: 5107MECH
Level: 5
Credit Rating:
Lecturer: Dr Hiren Kotadia
Contact: If you have any issues with this coursework you may contact your
lecturer. Contact details are:
Email: H.R.Kotadia@ljmu.ac.uk
Room: 1.34 C (James Parson Building, Byrom Street)
Issue Date: As Laboratory Rota
Hand-in Date: TBA – Your coursework should be submitted to the Campus Centre on or
before the hand-in date.
Feedback: Feedback will be given when your coursework is returned to you.
Feedback will be both written and verbal.
Programmes: MEng/BEng (Hons) Mechanical Engineering, MEng/BEng (Hons)
Automotive Engineering, MEng/BEng (Hons) Mechanical & Marine
Engineering
Department of Maritime and
mechanical Engineering
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Work hardening Laboratory
Introduction
Work hardening, also known as strain hardening, is the strengthening of a material by plastic
deformation below its recrystallization temperature. This is found in many manufacturing
processes, such as wire drawing, forging and rolling, etc. It may be desirable, undesirable, or
inconsequential.
In this lab, A hand-rolling machine (Figure 1) is used to roll coupon samples at room temperature
in this experiment. The amount of cold work is accomplished by changing the sample’s thickness,
which is controlled by the gap between the rolls of the rolling machine.
Laboratory Objective
• To experience the process of work hardening
• To explain the effects of work hardening in metal
• To explain the property change affected by annealing
• To understand the significance of the results with regards to a material’s properties.
Theory
• Strain hardening
Metals deform by movement within their atomic lattice of defects, the main ones being
dislocations. The movement of the dislocations allows slip to occur. During the process the lattice
structure becomes distorted and the grains deformed, whilst the density of the dislocations
increases. This increase in dislocation density results in an increasing resistance to deformation,
a phenomenon known as strain hardening.
As a ductile metal is plastically deformed during cold working, it becomes harder and stronger
while its ductility decreases.
• Recrystallization
At a temperature TRec (where TRec is the recrystallization temperature), which occurs within the
range TM/3 to TM/2 (where TM = melting point in K) the metal will recrystallize and remove the
effects of work hardening. Therefore any deformation undertaken at a temperature less than TRec
will result in work hardening, this being known as cold work. In contrast to this, deformation
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undertaken at a temperature greater than TRec does not induce work hardening and is known as
hot work.
• Cold work calculation
The amount of cold work can be calculated in percent cold reduction in thickness (Equation (1))
or in percent reduction in cross sectional area (Equation (2)). The percent cold reduction in
thickness is typically used in rolling process while the percent reduction in area is used in drawing
or swaging process.
ℎ = 0−
0
× 100
= 0−
0
× 100
In equations (1) and (2), 0 and Ao are initial thickness and cross sectional area, respectively,
while and are instantaneous thickness and cross sectional area, respectively.
Figure 1 –A hand-rolling machine used for cold work in this experiment.
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Operating Procedure for Rolling Machine:
1. Measure the average thickness of the coupon sample (annealed brass (70% Cu, 30% Zn) bar).
2. Mark one end and part of a surface at the same end with a permanent marker for identification
of rolling direction. The rolling direction is controlled with the marked end entering first and the
marked face up in every rolling pass.
3. The gap between rolls can be increased by turning the handle bar counter clockwise, reduced
by turning the handle bar clockwise.
4. With a gap between rolls about the same as the sample thickness, try to push the sample
through the gap between rolls. If the sample goes through (gap is too large), turn the handle bar
clockwise slowly to reduce the gap until the rolls pinch the sample while it is being pulled out.
Otherwise, turn the handle bar counter clockwise to increase the gap until the sample can be
pushed through.
5. Take note of the number at the pointer on the index plate.
6. Reduce the gap by one numbered division and roll the sample once.
7. The sample thickness can be further reduced by rolling with reduced gap between rolls. The
reduction in gap setting should be no greater than two numbered divisions in each pass. A large
thickness reduction can be achieved through consecutive passes with decreased gap in each
pass.
8. Always take note of the gap setting before leaving the rolling machine for hardness
measurement. The correct gap for follow-up rolling can be easily regained by turning the handle
bar for the recorded setting.
9. The Rockwell hardness test device is used to measure the hardness of the specimen before
and after deformation.
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Results
Table 1 Work hardening/ Strain hardening Lab Results
Sr no. Width
w
(mm)
Thickness
t
(mm)
Cross
sectional
area,
A = w x t
(mm2)
Percent area
reduction
=
[(Ao-Ai)/Ao] X
100
Percentage
thickness
reduction
=
[(to-ti)/to] X 100
Change in roll mill
gap
Hardness
1
Hardness
2
Average
Hardness
0
1
2
3
4
5
6
7
8
9
10
11
12
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Discussions:
• Are the results in agreement with the expected results?
• Comment on experimental results
• Explain the significance of work hardening.
• Discuss possible experimental errors and the effects on the results.
• Give any constructive criticisms of the test equipment and methods used.
Conclusions:
Briefly summarise the results bearing the objectives in mind