Describe as many as you can of the physical test supplied to metals in construction of
boilers.
The metal used for most of the main parts of a marine boiler, both multi-tubular and water tube
is mild steel but of varying quality.
The tests carried out in the metal used for various parts are:
• tensile tests for shell plates, drums, header, tubes, and stays
• bend test for end plates corrugated furnaces, rivets
• flattening test for rivets heads and boiler tubes
• hydraulic test for tubes, smoke tubes, and water tubes
For welded parts of pressure vessels the following added tests are required
1. Radiographic examination for the detection of faults in the metal
2. Micro examination for picturing the structure of the .....
TENSILE TEST:
This test is carried out to ascertain the strength and ductility of a material. To carry out this test
a specific testing machine is necessary. The specimen to be tested is held in self-aligning gripe
and is subjected to a gradually increasing tensile load. The beam must be maintained in a
floating condition by movement of the jockey weight as the oil pressure to the straining cylinder
is increased. An enteriameter fitted across the specimen gives extension readings as the load is
applied with respect to extension, the normal stress shear diagram is plotted for comparison
purpose on the same diagram.
The difference is due to the fact that the value of stress in the minimal diagram is calculated
using the rise sectional area of the specimen. The actual fact the cross sectional area if the
specimen is reducing as the specimen is extended. Specimens may round or rectangular in cross
section, the gauge length being found by reducing the cross section of the certain portion of the
specimen. This reduction must be gradually, rapid change of section can affect the results. In the
tensile test the specimen is broken. After breakage the broken ends are fitted together and the
distance between reference marks and the smallest diameter is measured. Maximum load and
load at yield are also determined. The tensile stress can be calculated by.
ULTIMATE TENSILE STRESS= MAXIMUN LOAD / ORIGINAL CROSS SECTIONAL AREA
BENDING TEST:
This is a test which is carried out on boiler plate materials and consists of bending a straight
specimen of plate through 180 degrees around a former. For the test to be satisfactory, no
cracks should occur at the outer surface of the plate.
FLATTENING TEST:
This test is used for testing rivets. The head of the rivet is hammered while hot until it is 2 1/2
times the diameter of the shank. The ends are then inspected for defects. The shank of the rivet
is bent cold and then hammered until the end meet. The curved part is then inspected for
defect.
To test the welds on a pressure vessel the following tests are carried out.
CADIOGRAPHY:
This can be used for the examination of welds, forgings and casting, that is x-rays, which
penetrate up to 180mm of steel pass through the metal and impinge up a photographic plate or
paper to give a negative. Due to the variation in density of the metal the absorption the rays is
non-uniform, hence giving a shadow picture of the material. It is like shining light through a
semitransparent material, x-rays produce in a Coolidge to give quick clear results and a clear
negative.
ULTRASONICS:
With ultrasonic we do have the limitations of metal thickness to consider so we have radio sonic
testing. High frequency sound waves reflect from internal interfaces of good metal and defects.
These reflected sound waves are then displayed onto a screen of cathode-ray oscilloscope. Size
and position of a defect can be ascertained. It can also be used for checking material thickness
that is a probe could be passed down a heat exchanger tube. a portable battery operated, hand
held cylindrical detector with cable to a set of headphones can be used to detect leakage in
vacuum, air lines, superheated steam, air conditioning etc. a recent application of ultrasonic is
testing concept. A generator placed inside the condenser floods it with ultrasound. By using a
head set and probe, tube leakage can be found. Where a pin holes exist, sound leaks through
and noise a tube is thinned vibrates like a diaphragm transmitting the sound through the tube
wall.
METHODS OF DETECTING SURFACE DEFECTS
1. A visual examination, including the use of a microscope or hand lens.
2. PENETRANT TESTING:
Penetrating liquids must have a low viscosity in order to find there way into fine cracks.
a. Oil and white wash. This is one of the oldest and simplest of the penetrate tests. The oil is first
applied to the metal then the surface is wipes clean. Whitewash or chalk is then painted or
dusted over the metal and oil remains in the cracks will discolor then whitewash or chalk.
Paraffin oil is often used because of its low viscosity and the component may be alternately
stressed and unload to assist in bringing oil to the surface.
b. Fluorescent penetrate wiped or sprayed over the metal surface which is then washed, dried
and inspected under near ultra-violet light. A developer may be used act as a blotter, to cause
the penetrate to re-emerge at the surface.
c. Red Dye Penetrate: This is probably the most popular of the penetrate methods because of its
convenience. The aerosol cans are supplied, red dye penetrate, cleaner, and developer.
Components must be thoroughly cleaned and degreased, and then the red dye is applied by
spraying on. Excess dye is removed by hosing with a jet of water or cleaner is sprayed on and
then wiped off with a dry cloth. Finally a thin developer is applied and when it is dry the
component is examined for defects. The red dye stains the developer almost immediately, but
further indication of defects can develop after 30min or more. Precautions that must be
observed are 1) use protective clothing 2) use aerosols well ventilated placed 3) no naked light,
the developer is inflammable.
MAGNETIC CRACK DETECTION
A magnetic field is applied to the component under tests... and where ever there is a surface or
subsurface defect flux leakage will occur. Metallic powder applied to the surface of the
component will accumulate at the defect to try and established continuity of the magnetic field.
This will also occur if there is a non-metallic in the metal or at just below the surface.
