|
Materials of Construction - Ductile Iron
October 25, 2003
Introduction
Photomicrograph or ductile iron at
100X magnification showing carbon in the form of nodules. |
Ductile iron derives its
name from its ductility or bending ability whereas grey cast iron is more brittle. The difference between ductile and grey cast iron is their metallurgy. The presence of a closely controlled amount of
magnesium causes the graphite to form as nodules rather than flakes.
A finer iron grain
matrix is also formed in the surrounding ferritic structure. This change results in a material that is more
ductile, stronger and tougher than regular grey cast
iron.
Ductile iron exhibits
the same corrosion resistance as regular grey cast
iron but has the improved mechanical properties.
Virtually all cast iron used in sulphuric acid service today is ductile cast iron. In the past, grey cast iron was used but after
failures due to mechanical impact, ductile iron has become the industry standard.
Ring section cut from 12"
ductile iron pipe, which has been squeezed under hydraulic pressure to demonstrate that
the material bends. |
Back
to top
Manufacturing
Pipe
Most cast iron pipe is
centrifugally cast in water cooled metal moulds. The
casting machine consists of a cylindrical metal mould mounted on rollers in a water jacket
so it can be rotated at a relatively high speed. The
entire rotating assembly is mounted on wheels so that it can be moved by means of a
hydraulic cylinder. The hydraulic cylinder is
supplied with a regulated amount of water at a constant pressure resulting in uniform
longitudinal movement of the mould. Molten
metal is fed into the centre of the mould through a trough that extends down the entire
length of the mould. The trough ends in a
spout that is directed to the side wall of the mould.
Molten iron is supplied
by a casting ladle which is tilted at a uniform rate by an electrically operated tilting
mechanism which maintains a constant pouring rate. The
amount of iron in the casting ladle is sufficient to form one pipe only.
Pipe is cast by bringing
the mould up to speed and actuating the tilting mechanism to start the flow of molten iron
into the mould. The iron flows down the fixed
trough and flows onto the surface of the mould where it is held in place by centrifugal
force. The result is pipe with a uniform wall
thickness and perfectly cylindrical bore. The
mould is moved longitudinally by the hydraulic cylinder to form the entire length of pipe. The constant uniform pour rate, uniform
longitudinal movement and high rotating speed results in a cast pipe of high quality. Adjusting these factors allows the manufacturer to
closely control and vary pipe wall thickness.
After the pipe is
completely cast, the mould continues to rotate until the pipe has cooled to 815°C
(1500°F). The pipe is removed from the mould
and taken to a heat treating furnace where it is heated to 938°C (1720°F) and then
slowly cooled to 649°C (1200°F).
After a pipe is cast,
the mould is cleaned and is prepared for the next pipe.
The entire casting operation takes 1½ to 8 minutes depending on the pipe
diameter. Most cast iron pipe is made in
nominal 5.49 m (18 ft) lengths. After
trimming and the installation of flanges to the pipe ends, the finished length of the pipe
is a maximum of 5.33 m (17½ ft). Smaller pipe sizes (3” to 6”) may be
available in finished spool lengths up to 5.94 m (19½ ft) in length. The maximum available length will depend on the
manufacturer and size of the casting machine.
Back
to top
Fittings
Cast iron fittings are
static cast in sand moulds with cores to create the inside bore of the fitting. Casting is part art and part science. The mould must be designed to allow the molten iron
to flow uniformly into the mould and quickly distribute to all parts.
The density of the core
is less than the density of molten iron so it will have a tendency to ‘float’
inside the mould unless it is held firmly in position.
The traditional means of holding the core in place is to use chaplets which
look like two-headed nails. The chaplets are
positioned in key locations and will form a part of the fitting wall. Thus it is important that the chaplet be of the
same material as the casting. When the
molten metal is poured into the mould, the chaplet will melt partially and fuse with the
rest of the metal. If the molten metal is
too hot, the chaplet may melt completely causing the core to shift position. The result is a fitting where the bore is not
centred and one part of the wall will be thinner than the rest of the fitting. If the molten metal is too cold, incomplete fusion
of the chaplet and the metal will occur. The
result will be a leak path along the surface of the chaplet.
To avoid the problems
associated with chaplets, the design of the mould must be changed so the core is held in
place without using chaplets. The moulds used
for standard water works cast iron fittings are not suitable and new moulds must be
created where the core is held in place from the ends in order to produce chaplet free
fittings. This sort of investment in new
moulds will only be undertaken by a supplier who does a large part of their business in
the sulphuric acid industry.
Back
to top
Quality Assurance
All casting should be
inspected to ensure that they have been manufactured correctly and to specification. The manufacturer must have a quality assurance
program in place indicated tests and procedures to assure quality fittings. The purchaser should request that the appropriate
documentation is available upon request if required.
All castings, whether
pipe or fittings, shall be good quality close-grained cast iron free from flaws, blow
holes, sand inclusions, and other defects.
Wall thickness should be
checked using ultrasonic thickness testing to ensure that the pipe and fittings meet
specifications. The measured wall thickness
should be within the specified tolerances. Checking
the wall thicknesses of fitting is also a good way to check to see if the core shifted
during the casting process due to problems with chaplets.
In some cases it may be
necessary to check the metallurgy or physical properties of a casting. A sample tab can be included as part of the
casting which can be ground or broken off for analysis.
Since the sample tab was part of the casting it will have the same
metallurgy and properties as the rest of the casting.
Photomicrographs will confirm that the casting is either grey or ductile
cast iron.
Back
to top
Flanges
Flanged cast iron pipe
is made by threading plain end pipe, screwing on specially designed long hub flanges and
power tightening. The flange and pipe end are
then re-faced together to form the gasket seating surface.
The sealing gasket actually seats over the machined end of the pipe as well
as the flange itself. This prevents exposure
of the pipe threads to the process fluid and the line pressure.
Fittings have integrally
cast flanges that must be machined and drilled.
The flange face is
machined with a phonographic finish to ensure that the grooves bit into the gasket to form
a tight seal. The specification of the finish
varies from supplier to supplier. A 250 RMS
flange finish is a generally accepted standard.
Back
to top
Fittings
In North America, cast
iron fittings have identical face-to-face and centre-to-face dimensions and the same
flange drillings as ANSI B16.1 fittings. Flanges
are integrally cast with the body of the fitting. The
flange face is machined to provide the gasket seating surface and drilled to the
appropriate standard.
Back
to top
Wall Thickness
Cast iron will corrode
when placed in sulphuric acid service. The
life expectancy of the pipe or fitting is a direct function of the wall thickness. The thicker the wall, the longer the operating
life of the pipe or fitting. Standard cast
iron pipe wall thickness for water service is not adequate for sulphuric acid service. In some cases, wall thickness may be double those
typically used in water service.
In cast iron pipe,
classes are used to specify the pipe wall thickness.
Classes can be used when specifying pipe and fittings but it is better to
specify the minimum wall thickness for each pipe diameter.
The following table indicates the typical range of pipe wall thicknesses to
be used for sulphuric acid service.
| Nominal Pipe Size |
Wall
Thickness |
Class |
| (inches) |
(inches) |
(mm) |
| 3 |
0.34 |
8.64 |
54 |
| 4 |
0.41 |
10.41 |
56 |
| 6 |
0.49 |
12.45 |
58 |
| 8 |
0.51 |
12.95 |
58 |
| 10 |
0.56 |
14.22 |
59 |
| 12 |
0.61 |
15.49 |
60 |
| 14 |
0.66 |
16.76 |
61 |
| 16 |
0.70 |
17.78 |
62 |
| 18 |
0.74 |
18.79 |
63 |
| 20 |
0.81 |
20.57 |
65 |
| 24 |
0.89 |
22.61 |
67 |
| 30 |
1.15 |
29.21 |
69 |
Back
to top
| 
