Coursework Title-WORK HARDENINGLABORATORY

Module Name: Engineering Practice 2
Module Code: 5118ENG
Level: 5
Credit Rating:

Lecturer: Dr Lisa Li; John Carrier
Contact: If you have any issues with this coursework you may contact your lecturer. Contact details are:

Email: [email protected]
Tel: 0151 231 2099
Room: Rm 127B
Email: [email protected]
Tel. 0151 231 2006
Room: 1.23 (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
Work hardening Laboratory
Introduction
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 density results in an increasing resistance to deformation, a phenomenon known as work hardening, or strain hardening.
At a temperature TRec (where TRec is the recrystallisation temperature), which occurs within the range TM/3 to TM/2 (where TM = melting point in K) the metal will recrystallise 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 undertaken at a temperature greater than TRec does not induce work hardening and is known as hot work.

Laboratory Objective
• To assess the capacity of an annealed sample of metal to be deformed at room temperature by cold work.
• To understand how dislocations affect the mechanical properties of the metal.
• To be able to undertake a hardness test and understand the significance of the results with regards to a material’s properties.

Apparatus
1. Rockwell hardening tester.
2. Tensometer.
3. Measuring device.
4.
5.

Experimental Procedure
An annealed brass (70% Cu, 30% Zn) bar is stretched through a tensile tester several times by applying the load starting from 4000Nuntil it breaks. The increasing scale in the load of the bar is 2000N for each run.
Ensure that you have marked the gauge length on the specimen and record the readings indicated on the results table.

Part 1:
Prior to commencing the experiment measure the following:
i. Initial thickness, width of the bar using a digital caliper,
ii. Initial gauge length,
iii. Hardness of the bar using the Rockwell B scale,

For each stretch of the bar through the tension measure the following:
i. Thickness, width, and gauge length using a digital caliper,
ii. The hardness of the bar using the Rockwell B scale.
iii. The tensile load (KN)
Part 2:
A piece of 70%/30% brass that has been used in a previous identical experiment has been annealed by heating to a temperature of about (TRec + 100)°C for a minimum of 30 minutes. Measure the hardness, using the Rockwell B scale.

Results
Table 1 Work hardening Lab Results
Data for strain hardening of 70%/30% Brass
Gauge Length (mm) Hardness (HRB) Load (KN) Elongation (%)
Engineering stress (N/mm2) True stress (N/mm2) **Yield stress σy (N/mm2)
Thickness (mm) Width (mm) Area (mm2) %CW* 1 2 Av
Datum readings 5.92 8.09 47.89 0 50 17 17 17 0.00 0 0 0
1 5.87 8.08 47.43 0.96 50.02 16 21 18.5 4000 0.04 84.33 83.52
2 5.82 7.98 46.44 3.03 50.54 18.5 31 34.8 6000 1.08 129.19 125.29
3 5.73 7.88 45.15 5.72 53.02 50 45 47.8 8000 6.04 177.19 167.05
4 5.62 7.68 42.99 10.75 55.48 55.5 55.5 55.5 10000 10.96 232.61 208.81
5 5.29 7.35 35.88 18.81 60.63 65.5 64.5 65 12000 26.98 308.64 250.57
6 5.13 7.02 36.01 24.81 66.63 70 72 71 14000 33.26 338.78 292.34
7 4.64 6.47 30.02 37.31 81.92 80 79.5 79.5 15000 63.88 532.98 313.21
8
9

Annealed bar 10 13 11.5
*CW: percent of cold work;**Yield stress σy (N/mm2): this column must be completed after Lab session with the data saved

Comments
Sample calculations:
 
Complete this part during the Lab session
• Calculate the area of the cross-section.
• Calculate the percent of the cold work
• Calculate the elongation
• Calculate the engineering stress
• Calculate the true stress
• Plot the load chart vs %CW (%CW is to be plotted on the horizontal axis)
• Plot the hardness chart vs %CW (%CW is to be plotted on the horizontal axis)
Expected Laboratory Outcomes

Questions:
1. Find the melting point of brass (70%Cu, 30%Zn), then work out the recrystallization temperature of this alloy.
2. What is Recrystallization? Compare the mechanical properties of cold worked materials and Recrystallized materials.
3. In this experiment (tensile test), plastic deformation makes strain hardening occur, is there any other operation which will make strain hardening occur? Give an example/examples.
4. Briefly explain why some metals (i.e. lead and tin) do not strain harden when deformed at room temperature.
5. Please explain the difference between HRA,HRB, and HRC.

Assessment

During the laboratory experiment all notes, results/Graphs/Charts or any information should be recorded in your laboratory note book. At the end of the session your book will be signed by the tutor (inside the front cover) to indicate that you have attended, competently completed the laboratory and recorded suitable information to enable you to complete a formal report at a later date. On completion of the yearlong laboratory rota you will be requested to complete a full “Formal Laboratory Report” on one of the eight activities completed throughout the year.

The mark awarded for the laboratory section of the module 5118ENG Engineering Practice 2 is worth 50% to the final module mark.