Steel Material Preparation

Any fabrication activity is preceded by the steel material preparation activity. Steel
plates and sections are received from the steel mills in as rolled condition. The very
process of production in steel mills leads to formation of residual stress and a hard
layer of oxides over the steel surface. Both this locked-in stress and the oxide layer,
known as mill scale, needs to be removed before taking the plates and sections for
further production operation.
The handling of materials from steel mill to shipyard steel stockyard may also lead to
surface deformation, hence straightening is required. Thus steel material preparation
involves, straightening, stress relieving and mill scale removal. The plates are fed from
the steel stockyard to the straightening machine and from there to the surface
dressing station. Surface dressing is necessary to remove mill scale present on the
plate surfaces.
Mill scale is a layer of ferric and ferrous oxides formed on the plate surface during the
hot rolling operation of the steel plates in steel rolling mills. There are various
methods of surface dressing, however shot blasting and chemical pickling processes
are the most efficient ones.
 Sequence of plate fabrication activities

Straightening and Stress Relieving

The straightening and stress relieving operations are done by passing the steel plates
through a multiple roller machine subjecting the plate to several reversal of plastic
bending as schematically shown in Fig above. For thicker plates (thickness
≥ 12 mm) generally a 5 roller machine is used. However for lesser thickness
plates, several bending are to be imparted to relieve the residual stresses and to
straighten the plates. For plates of 4–6 mm thick, often 21 roller machine becomes
necessary. Another compromise could be, 2 or 3 plates can be stacked together and
passed through a 5 roller machine for straightening and stress relieving.
Schematic diagram of plate straightening and stress relieving operation
Surface Dressing—Mill Scale Removal
Surface dressing is necessary to remove mill scale present on the plate surfaces.
Mill scale is a layer of ferric and ferrous oxides formed on the plate surface during
the hot rolling operation of the steel plates in steel rolling mills. It consists primarily
of Fe3O4, of characteristic blue-gray colour covered by an extremely thin outer film
of Fe2O3. The innermost layer contains fine metal grains and sometimes, residual
black FeO which contribute to the roughness of descaled metal.

There are various methods of surface dressing, e.g. natural, flame treatment,
manual wire brushing, sand blasting, shot blasting and chemical pickling. However
shot blasting and chemical pickling processes are the most efficient ones.
Shot Blasting
Shot blasting is an efficient method of surface dressing. It effectively and completely
removes mill scale from steel plates and profiles. The process involves removal of
mill scale using tiny metal shots which are fired onto the metal surface from both
sides as schematically as shown. The shots impinge on the metal surface and are
rebounded chipping off the mill scale with it exposing the bare steel plate. Next to
the shot blasting chamber is the priming chamber. As the plate comes out of the
shot blasting chamber, it is given a coat of primer paint, generally rich in zinc, to
immediately protect it from atmospheric corrosion forming rust. To speed up the
process of drying of the primer paint, the plate is moved to a drying chamber, where
the plate is subjected to hot air jet. At one end of the shot blasting operation the
straightened plate enters and at the other end a clean primed plate comes out,
ready for subsequent fabrication activities.
 The process is fully automated, plate feeding takes place using roller conveyor.
The shots are recycled. The various control parameters are,
• Velocity of the shots
• Angle of shot impingement
• Size and mass of Shots
• Speed of feeding the plate
Merits
(i) By setting the process parameters suitably the mill scale can be completely removed
(100 % removal).
(ii) The complete process is carried out in a noise and dust insulated chamber and hence
there will be no heavy dust and noise pollution which is produced as a result of the shots
striking the plate.
(iii) The efficiency of this process is generally very high about 100–200 m2/h.
Demerits
(i) Lesser thickness plates up to 6 mm may get deformed.
(ii) When the shots strike the plate, locally the stress may exceed the yield stress at that
point of the plate leading to work hardening. As a result the reserve plasticity decreases
and may lead to formation of cracks.
(iii) Depending on the process parameters the shots may chip off part of the metal along
with the mill scale (or) may not remove the mill scale completely. So to avoid these
problems, the parameters have to be adjusted suitably.
Acid Pickling
In this method of surface dressing the mill scale is removed through chemical reaction.
Hence the prime advantage of the process is surface hardening, crack formation or plate
deformation does not take place.

H2SO4 pickling
The acid generally used is either HCL or H2SO4. The pickling rate gets nearly doubled with
only 15 °C increase in temperature from 70 to 85 °C. Typical pickling condition of hot rolled
low carbon steel would be for acid concentration of 10 % by volume at 70 °C for 15 min. The
acid bath is replenished to maintain a concentration level of about 10 % by volume. As iron
build up approaches about 5 % and acid concentration drops to about 5 % increasing
pickling time, the acid bath is then discarded. Before discharging the waste liquid, it is
necessary to neutralize the residual acid in the liquid waste.
HCl pickling
Chloride salts have better solubility in water compared to sulphate salts, therefore
salt removal is easier. However HCl is more volatile and more so at elevated temperature
compared to H2SO4, therefore acid consumption becomes more in case of HCl at elevated
temperature pickling. Hence it is preferable to use HCl at room temperature with a dilution
of industrial grade concentrated acid with 1–3 parts of water. Hydrochloric acid has less
pickling time compared to that of sulfuric acid.
Variation of pickling time with temperature and concentration (by weight) of H2SO4 for
hot rolled low carbon steel
The merits of this process are:
• No work hardening takes place (as in shot blasting)
• Thin plates are not deformed.
• The process is noise free.
The demerits are:
• Operations involving acids and alkalis are hazardous.
• It has a high water requirement.
• The acidic fumes which develop in the pickling hall can corrode the cranes
and
other equipment.
Plate Cutting

In shipbuilding, cutting of various plate parts of varying thicknesses with accepted

dimensional accuracy is required. Plate cutting is done either by mechanical means or
by thermal processes. In mechanical process, cutting is done by applying a shearing
force either using guillotine shear or high pressure water jet. Whereas in thermal
processes, cutting is done either through oxidation, i.e. oxy-flame cutting or through
fusion, i.e. plasma arc cutting or through sublimation i.e. laser cutting.

Various methods of plate cutting

 Mechanical Shearing

Mechanical cutting by shearing is done by the action of two blades, one fixed at the bottom
and one moving vertically above. The moving blade has an inclined edge. The angle of
inclination is generally ½ to 2½°. It is called the rake of the blade. The upper blade is usually
angled. As it comes down, it progressively meets the plate placed on the lower blade from one
end to the other. The cutting pressure thus remains concentrated exactly at the junction of the
two blades achieving a cut parallel to the blades. The cut progresses from one end to the other,
thus reducing the required force. A gap of approximately 5–10 % of plate thickness is
maintained between the two blades. This clearance and the rake are dependent on the
thickness of the material to be cut. The fixed bed holding the bottom blade also has a series of
hold-down pins to hold the plate in position at the time of cutting. The blades typically have a
square edge and are made of high-carbon steel. The shearing machines are often referred to as
guillotine shear.
 Water Jet
In this method of plate cutting, water is forced through a
small orifice, under extremely high pressure. Thus it
creates a water jet which has a tremendous concentration
of energy. The flow through the tiny orifice creates high
pressure and a high velocity jet. The water is pressurized
between 1300 and 6200 bar. This is forced through the
orifice, which is typically 0.18–0.4 mm in diameter This
creates a very high velocity, very thin beam of water jet
having a speed almost equal to that of sound. As the thin
stream of water leaves the orifice, fine abrasive material is
added to the water jet. The high velocity water exiting the
orifice creates a vacuum which sucks in abrasive particles
from a line feeding abrasive particles. It then mixes with
the water in the mixing tube. The abrasive particles are
then accelerated through the orifice by the water jet and
cuts through the metals.
Waterjet cutting of steel plate
Comparison of waterjet with laser cutting
In laser cutting, it is focused on the material to melt, burn, or vaporize the material.
To make the laser move over the material being cut, additional optics are required
as the distance from the emitting end of the laser changes. Here the cutting speed
being very fast, it is highly productive and also leaves behind very small heat affected
zone. Waterjet can easily cut steel plates of 50 mm thickness, whereas laser has a
practical limitation of about 12–19 mm thickness. Material cut by laser tends to have a
rougher, scaly edge, which may require additional machining operations to clean up.

Comparison of waterjet with plasma cutting

In plasma cutting, an extremely hot (about 15,000 °C) stream of plasma is created
which melts the material and the molten material is blown away by high seed plasma
jet.
Comparison of waterjets with flame cutting
In oxy-acetylene cutting of steel, the cutting location is heated up to about 1000 °C
and then additional oxygen is supplied to start the oxidation of the material. At that
temperature the oxide remains in molten state and is thrown off the plate under the
action of the oxy-acetylene jet. Oxy-acetylene cutting can be used only for iron and
steel.
Thermal Process
Cutting through oxidation
The material is oxidised (burnt) and the oxide in its molten state is blown off the cutting
location by the oxygen jet. Here the melting temperature of the material must be higher
compared to that of its oxide. Example: Oxy-acetylene flame cutting.
Cutting through fusion
The intense heat melts the material and is blown out by the high energy gas jet. Here
oxidation of the material does not take place, however if there is a oxide layer already
existing as in case of aluminium alloys, the oxide layer along with the metal gets melted
instantly and is blown off the cutting location. Hence in this method an intense heat
source is required. Example: Plasma arc cutting.
Sublimation cutting
In this method of thermal cutting, the heat source is so intense that the material
evaporates and the cutting is done. The metal vapour expands under its own vapour
pressure and also additional gas jet helps to transport out the metallic vapour from the
cutting zone. In extreme cold conditions, it is necessary to bring the steel plates to a
temperature level of about +20 °C before cutting. The warm-up time depends on the
plate thickness.
The warm-up time from −20 to +17 °C are as
follows:
• about 8 h for 12 mm thick steel plate,
• about 12 h for 21 mm thick steel plate,
• about 17 h for 40 mm thick steel plate.

Oxy-Fuel Flame Cutting

It is a process of cutting through oxidation (burning). A fuel gas is burnt in presence of
oxygen to generate the required heat. For industrial applications and in shipyards in
particular, acetylene gas is used as the fuel. Acetylene has higher calorific value of
18,890 kJ/m3 compared to other fuel gases, e.g. it is 10,433 kJ/m3 for propane.
Therefore it produces the highest flame temperature in oxy-fuel process. The maximum
flame temperature with acetylene in oxygen environment is about 3,160 °C, whereas with
propane it is about 2,828 °C. The hotter flame leads to rapid piercing at the start of a
cutting process. The piercing time is typically about one third that of with propane. This
intense flame at the metal surface also reduces the width of the Heat Affected Zone (HAZ)
and heat induced distortion. In this process, the material is brought to the ignition
temperature, about 1000 °C for steel, by an oxy-acetylene flame and is then burnt by
supplying additional oxygen (cutting oxygen) to the heated zone. The oxide in the molten
state is blown off by the gas jet.
To achieve successful oxy-acetylene cutting, the following conditions need to be
satisfied:
• The ignition temperature (oxidation) of the material has to be lower than its melting
temperature, otherwise the metal will melt and cutting cannot be done.
• The melting temperature of the oxides has to be lower than the melting temperature
of the material itself such that the molten oxide can be blown off by the gas jet.
• As the cutting progresses, the ignition temperature has to be continuously
maintained. This implies that a positive heat balance has to be maintained balancing
the heat supplied by the gas flame and the heat losses by way of conduction and
convection.
• The oxidation reaction between the oxygen jet and the metal must be an exothermic
one to maintain a positive heat balance
Plasma Arc Cutting
In this thermal process plate cutting is done through fusion. In this process a high
temperature, high velocity ionised gas jet (plasma) is formed which causes through
thickness melting of the plate and blows off the molten metal achieving the cut. This
plasma is often referred to as the 4th state of matter. The three states of matter are solid,
liquid and gas. The difference between these states relates to their energy levels. Matter
changes from one state to the other through the introduction of energy, such as heat.

The merits of plasma arc cutting can be summarised as:

• It can cut all metals.
• Higher cutting speed compared to oxy-acetylene cutting, especially on steels
less than 25 mm thick.
• Heat affected zone and heat induced deformation are minimised.
• Carbonisation of cut edge as in oxy-acetylene cutting does not take place.
• Hazardous or explosive gases are not used.
• Electric power is required unlike oxy-acetylene process.
• Metal fumes and UV radiation can pose a health hazard.
• Capital cost of equipment is substantially higher compared to oxy-acetylene
process.
Plasma cutting process can have the following variations:

• Plasma cutting with transferred arc

In this process, the arc is between the plasma torch electrode (cathode) and the
transferred to the workpiece (anode). Thus only electrically conductive materials can be
cut by this mechanism. Additional gas is supplied to the plasma arc torch for shielding
the plasma jet as well as to cool the torch. This secondary gas flow also helps in
constricting the plasma jet.
• Plasma cutting with non-transferred arc
Here the arc is established inside the nozzle between the electrode and the nozzle body.
The hot stream of plasma jet comes out of the nozzle orifice in the form of flame. It can be
used to cut electrically non-conducting materials.
 schematic of
schematic of a transferred arc plasma cutting non-transferred arc plasma
cutting
A schematic of plasma arc circuitry
Laser Cutting

Laser as a heat source is also used to cut steel and aluminum plates

• A high intensity laser beam is generated.

• The beam is focused onto a very small area on the surface of the workpiece by
means of a lens.
• The focused beam almost instantaneously heats up and melts the small (generally
less than 0.5 mm diameter) area through and through along the plate
thickness.
• The pressurised gas jet acting coaxially with the laser beam ejects the molten
material from the plate.
• In some cases, the gas that is used reacts with the material exothermically and
greatly enhances the cutting process, e.g. oxygen is used in cutting of carbon
and/or mild steels. In presence of the laser heat the material burns in the oxygen
jet and the molten oxide is blown off under the force of the oxygen jet. Thus it
improves the efficiency of the process.
• The laser beam is moved over the plate surface or the plate is moved by a CNC
X-Y table and the cut is achieved
Merits of laser cutting

• Laser cutting is done at a substantially higher speed, e.g. A 5 kW CO2 laser can cut a 10 mm
thick mild steel plate at a speed of about 2.35 m/min and a 25 mm thick mild steel plate at
0.80 m/min.
• The actual heated area in laser cutting is very small and most of this heated material is
removed during cutting. Also the speed of cutting being high, the overall heat input to the plates
is very low. Thus the heat affected zones are minimized as well as the heat induced distortion.
• Laser cutting being a non-contact process, minimum of fixturing and clamping of plates is
required. The plate can be just positioned under the beam.
• The kerf width is extremely narrow (typically 0.1–1.0 mm).
• The process can be fully CNC controlled.
• The kerf width being very small and the process being fully CNC operated, close nesting of
piece parts can be done such that scrap or wastage of material is minimized.
• In most cases the laser cut components can be used for subsequent operation without any
edge cleaning operation.
• The capital cost of a laser-cutting machine is quite high, however, the running costs are
generally low. Hence break even can be faster.
• The laser cutting process is extremely quiet compared to other techniques, e.g. plasma cutting.
It is important from the point of view of improving working environment and thus the efficiency
of the shop floor workers.
A schematic of
laser beam combustion
cutting
Application of laser cutting in shipyards

With all the high cutting quality, still the basic cost of laser cutting equipment remains as
the prime deterrent for its wider application. However some of the shipyards have
implemented CO2 laser cutting systems of 4–6 kW power. The practical limit of these
machines is 25 mm thick steel plates. To increase the capacity to cut even thicker plates, so-
called LASOX processes are being adopted. In this system, oxygen stream provides the
cutting force while the laser only preheats the plate surface. Using such machines, it offers
the advantage of a single machine that can cut a very wide range of plate thickness from
few mm to thick plates of 50 mm and even more.
The limitation of thickness was overcome by a process which combined laser with a jet of
oxygen. At the cutting location, the steel plate is heated up focusing laser on to it. As it
attains the ignition (oxidation) temperature of about 1000 °C, a supersonic stream of oxygen
is injected at the spot to initiate the process of oxidation and cutting. This is in essence same
as that of oxy-fuel cutting, where the fuel gas or acetylene is replaced by laser. In principal it
is also same as that of laser beam combustion cutting. The exothermic reaction adds to the
heat energy in the cutting process. Unlike in any of the other oxygen assisted cutting
process, here an extremely high speed jet of oxygen is used. This is a patented method
named as LASOX. In this method steel plates of 50 mm thick can be cut with much less laser
power of about 2 kW. Whereas any other laser cutting process can effectively
cut only about 25 mm thick steel plates
Plate and Section Forming
In a ship structure the percentage of curved plates is generally not more than 15 % of the
total plates used in the hull structure. Here each individual plate has a different
curvature. The stiffeners are also required to be bent matching with the curvature of the
plates at the respective sections. The plates in the forward and the aft end have compound
curvature. Mathematically these curvatures are referred to as Non-Gaussian curvature.
These are non-developable curvatures.

Plate and section forming is carried out either by mechanical means or through a thermal
process using line heating technique. The concept of matched die has been used in
devising universal press. Here the die is made flexible, such that it can easily take the
required curvature. Controlled heating is applied in line heating method along
predetermined line segments. This causes differential shrinkage along the plate thickness,
resulting in angular bending of the plate. By applying this bending at the required
location, the target shape of the plate can be achieved. This method of line heating can be
gainfully utilized using a numerically controlled heating torch to apply the required
thermal load to achieve the desired target shape.
Mechanical Methods

Roller Bending and Hydraulic Press

Plate Bending
Plate bending through mechanical means is carried out using 3 or 4 roller bending
machines and hydraulic press. The bilge plates generally have cylindrical, i.e. single
curvature. These are developable surfaces. This bending is done by roller bending
machines. Prior to bending of the plates, the frame positions are marked on the
plate. As the bending progresses, tame plates at the frame positions are used to
check the correctness of the shape being obtained. This process of bending is fully
manual and highly worker’s skill dependent.

Stiffener Bending
The stiffener bending is carried out by 3-point frame bending machines. The
stiffener is progressively fed to the machine from one end and through the strokes of
the side rams of the frame bender the required curved shape is obtained.
Schematic of a typical frame bending machine

A typical frame (concave) bending operation

Universal Press for Plate Bending

The concept of matched die has been used in devising universal press. In
shipbuilding, there is no scope of large scale mass production. Hence it is not feasible
to keep large number of expensive dies for plate forming. However if the die can be
made flexible, such that it can easily take the required curvature. Then plates having
different curvatures can easily be formed using this pair of flexible die. The universal
press essentially consists of a pair of flexible die, which are computer controlled.

Difficulties in This Process of Automatic Plate Bending

When the jacks are retracted after bending, the bent shape deviates from the shape
under pressure. It is known as spring back action. The extent of spring back that
may take place depends on mechanical properties of the given plate, i.e. yield stress
and modulus of elasticity.
Schematic presentation of universal press
Line Heating
Line heating is a method of applying thermal load for shaping a plate or section.
Through this process, compound curved surfaces conforming to hull surface
definition of a ship can be fabricated. As the name suggests, controlled heating is
applied along predetermined line segments. A heat source is moved over a plate
following these lines. This causes differential shrinkage along the plate thickness,
resulting in angular bending of the plate. By applying this bending at the required
location, the target shape of the plate can be achieved. Hence once the required
heating pattern for a given target shape is generated, this method of line heating can
be gainfully utilized using a numerically controlled heating torch to apply the
required thermal load to achieve the desired target shape.
Mechanism of frame bending by line heating

Line heating pattern for frame bending

Manual line heating with oxy-acetylene gas flame