Mechanical Testing Laboratory (MM 362)
Experiment 1. Rolling and Hardness measurement
1. AIM
To conduct rolling and hardness measurement tests on Mild-steel and Brass samples.
2. EQUIPMENT USED
Rolling machine, Vickers hardness tester, Rockwell superficial hardness tester, Mild-Steel and Brass
specimens & vernier caliper.
3. THEORY
Cold rolling is a process by which the sheet metal or strip stock is introduced between rollers and
compressed and squeezed. Cold rolling is done below the recrystallization temperature. After cold
working, its grains are in distorted condition. Plastic deformation or cold working affects all the
properties of metal that are dependent on the lattice structure. The strain introduced determines the
finished product's hardness and other material properties. By cold rolling tensile strength, yield strength
and hardness are increased while ductility is decreased.
The reduction in thickness during rolling is quantified by the percentage reduction in thickness or draft
(d). The draft is given by:
Where ti is the initial thickness and tf is the final thickness.
Vickers hardness:
Vickers hardness testing is a widely used method to measure the hardness of materials. It
involves indenting a diamond pyramid-shaped indent into the material's surface and measuring
the size of the resulting indentation. The Vickers hardness number (HV) is determined based
on the diagonal lengths of the indentation.
Vickers Pyramid Indenter
The Vickers indenter is a diamond pyramid with a square base and an apex angle of 136
degrees. It creates a square-shaped indentation on the material's surface.
Test Procedure
The Vickers hardness test involves applying a controlled load on the indenter and holding it for
a specific time to ensure consistent results. After removing the load, the indentation is measured
using a microscope or an optical system.
Calculation of Vickers Hardness Number (HV)
where F is the applied force (in Newtons), and d is the average of the two diagonals of the
indentation (in millimetres).
Superficial Rockwell: A more surface-sensitive hardness measurement scale. This technique
is useful for samples with hardness gradients at the surface, to test small areas, and for thin
samples. Superficial Rockwell hardness scales are N and T for metals and W, X, and Y for
nonmetallic materials and coatings.
4. EXPERIMENTAL PROCEDURE
1. Bring the distance between the rollers in the rolling machine to zero by rotating the top
wheel.
2. Set the distance between the rollers equal to the current thickness of the sample. Place
the sample in the specimen passage and push it toward the rollers using a rod. Retrieve
the rolled sample from the other end using tongs.
3. Perform rolling by reducing the distance between the rollers to 0.5mm in 2 passes in
one set (0.25mm in each pass) and measure the thickness of the sample using vernier
after each set of rolling. Observe the sample after rolling.
4. Similarly, perform three more sets (a total of eight passes).
5. Measure hardness after each set of rolling, for Mild-Steel measure using Vickers
hardness tester, and for Brass sample measure using Rockwell Superficial hardness
tester.
OBSERVATIONS
Sample name:__________
No. of passes
2
4
6
8
Initial thickness
Initial hardness value=___________
Final thickness
True strain
Hardness
EXERCISE SHEET
1. Calculate the true strain for all sets of passes and PLOT Hardness vs True strain. Fit the
curve using suitable fitting parameters and comment on the trend.
2. Describe the differences between hot rolling and cold rolling. Provide examples of
products produced using each method.
3. Calculate the draft percentage for a material that undergoes a reduction in thickness
from 6.4 mm to 1.2 mm during rolling.
4. Were there any defects seen in the sample after rolling? If yes, name the rolling defect
and list out the possible reasons.
5. What did you observe when the Brass sample was rolled by 0.25mm reduction in one
pass and 0.1mm reduction in one pass?
6. Why is surface preparation required for the Mild-steel sample and not for the Brass
sample?
7. In Vickers hardness testing, the length of diagonals measured were 67.81µm and
66.83µm. and the load applied was 500gf. Calculate the Vickers hardness value.