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.0 Oxygen Decarburization(AOD) converter to remove excess

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Introduction

          

Figure 1: 316 stainless
steel marine anchor

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Figure 2: 316 stainless
steel marine anchor chain

316 stainless steel or known as marine grade stainless
steel is widely used in marine environment especially for the making of marine
anchor and anchor chain. This is due to 316 stainless steel contains molybdenum
which contributed for the superior corrosion resistance in the marine
environment. 316 stainless steel also have good strength at high temperature
and high toughness at extremely low temperature so it is widely used for
cryogenic applications. This is due to the presence of nickel which prevents
the problem of brittleness at low temperature. Figure 1 and 2 show the marine
anchor and anchor chain that made up of 316 stainless steel.

 

 

2.0 Alloying Element

List
of alloying elements present in the 316 Stainless Steel:

Element

Percent
by weight

Carbon

0.08

Manganese

2.00

Silicon

0.75

Chromium

16.00-18.00

Nickel

10.00-14.00

Molybdenum

2.00-3.00

Phosphorus

0.045

Sulfur

0.03

Nitrogen

0.1

Iron

Balance

 

3.0 Manufacturing of  316 Stainless Steel

First of all, the raw
materials to make 316 Stainless Steel are melted together in an electric arc
furnace. This process consumes around 8-12 hours of intense heat for all the
materials to melt to molten. Then, the molten metal is processing in an Argon
Oxygen Decarburization(AOD) converter to remove excess carbon. During this
process, the alloying elements to make 316 Stainless Steel like Molybdenum and
Chromium can be added in the AOD converter. After the removal of excess carbon
process is done, the molten metal is cast into desired semi-finished forms. In
order to improve the strength of the Stainless steel, the semi-finished form
stainless steel is heat-treated. During the annealing process, some scale is
formed on the steel, so the stainless steel is undergone the descaling process
to remove these scales using
pickling, which involves bathing the steel in nitric-hydrofluoric acid. Next,
the semi-finished, heat-treated and descaled stainless steel is cut into
desired size. Last but not least, the stainless steel is surface finished such
as polishing to provide better corrosion resistance and good appearance.

 

4.0 Hot Working

316
stainless steel will undergo hot working by using hot rolling method. Hot
working carried out at temperature more than 930°C (in between the range of
1150°C-1260°C). The stainless steel will be hot-deformed and recrystallize its
microstructure partially. After hot working, annealing process should be taken
in order to induce the maximum corrosion resistance.

5.0 Cold Working (Work
hardening)

316 stainless steel which is tough and ductile
strengthened by undergoing rapid cold working to enhance its mechanical properties
and increases its strength (fatigue strength, tensile strength) and hardness.

 

Cold rolling is most widely used for the work hardening
of the stainless steel as the stainless steel pass through pairs of rollers and
being compressed and the grains are deformed and produce dislocations. 316
stainless steel readily cold rolled into desired part. Cold rolling is a
plastic deformation occurs with
the metal below its recrystallization temperature, this process increases the
strength of 316 stainless steel by strain hardening up to 20%. It also enhances
the finishing of surface (smoother surface) and holds tighter tolerances.

 

Cold working decreases ductility
of the 316 stainless steel due to strain hardening which will lead to
brittleness. Besides that, some ‘strain induced’ martensite are presented in the stainless steel after
being cold- worked and this make the stainless steel become partially ferromagnetic
and eventually lead to corrosion. Hence, annealing from time
to time during rolling is needed to eliminate the undesirable effects of cold
working and to increase the ductility, formability, machinability and
workability of the 316 stainless steel. Annealing carried out after cold rolling in order to
recrystallize the microstructure of 316 stainless steel and enhances corrosion resistance. Cold-worked
316 stainless steel is nonmagnetic in the annealed condition hence results in
good corrosion resistance and formability.

Moreover, 316 stainless steel is hardened by cold working
(work hardening) as it cannot be hardened by heat treatment because there is no
phase change from ?, austenite to ?, ferrite occurs during cooling or heating and its
microstructure remain unchanged and remains in austenitic form at room
temperature.

 

6.0 Heat Treatment

6.1 Solution Annealing

Austenitic stainless steels like type 316, their
microstructure does not change from the melting temperature through to ambient
temperature, they cannot hardened by heat treatment, but hardened by cold
working. After the semi-finished part of stainless steel is formed from cold
working, it then undergoes annealing process. 316
stainless steel  undergoes annealing or
solution treatment where heat to bright red hot in a temperature range of
between 1040°C to 1175°C (1900°F to 2150°F) for an hour and then cool rapidly.
Annealing temperature above 1070oC
gives better corrosion resistance. Thus, solution annealing carried out at
about 1090°C and hold at this temperature for 1 hour in order for the
homogenization of the chemical composition of the steel and complete
dissolution of carbides. After solution annealed, 316 stainless steel will be
water quenched, as water quenching used for thick section. Rapid cooling is needed
in order to prevent the precipitation of chromium carbides at grain boundaries
and ensure to give optimum corrosion resistance.

Annealing (solution
treatment) of stainless steel recovers and restores maximum ductility and
softness and also eliminate microstructural irregularities. Besides, solution
annealing also gives maximum corrosion resistance as it dissolves impurities
like carbides at grain boundaries which contributed in the intergranular
corrosion of the stainless steel. Thus annealing must carried out above temperature
range of 425°C to 900°C to avoid precipitation of carbide. Rapid cooling which
is water quenching is used after annealing in order to ensure the dissolved
carbide remain in the solution and prevent deterioration in the uniform
composition of the austenite due to precipitation of carbides and also avoid
separation of the carbides on the grain boundary which will lead to
deterioration.

Furthermore,
solution annealing helps to prevent stress
corrosion cracking, improve ductility at the grain boundaries and also
increases impact toughness. Heat
treatment required to remove the undesirable effects of cold-worked stainless
steel or to dissolve chromium carbides precipitated from thermal exposures.
Annealing (solution treatment) recrystallizes the work-hardened austenitic
stainless steels and dissolves chromium carbides precipitates into solution in
the work-hardened stainless steels. Moreover, it relieves internal stresses
developed during cold working and homogenizes stainless steel to achieve a
homogenous microstructure.

Before heat treatment carried out, surfaces of
contaminants on the stainless steel should be thoroughly cleaned first to
remove all residual like oil and grease from manufacturing process in order to
achieve a proper heat treatment. Solution annealing are carried out in a controlled atmosphere (nitrogen
or nitrogen + hydrogen) so that the steel can maintain shiny grey, prevent
oxidation in air or carburization effect.

6.2 Stress-relieving heat treatment

Stress-relieving
treatment for 316 stainless steel is carried out at low temperature below 400°C
which is heat up to about 375°C and hold for 2 hours then slow cooled to room temperature.
Higher temperatures will decrease the strength of steel and may induce
intergranular corrosion. Austenitic stainless steels like type 316
will be slightly magnetic after cold working as it contains some ‘strain
induced’ martensite and this also reduces corrosion resistance. Therefore, stress relieving enhances
the stress corrosion resistance by removing residual tensile stresses in
stainless steel during cold working. Stress relieving gives dimensional or
shape stability and reduces distortion during forming and machining.
Stress-relieving heat treatment will re-transform any martensite formed back to
austenite. Slow cooling avoids distortion and formation of residual thermal
tensile stresses. The temperature ranges used in stress relieving must prevent
the stainless steel sensitize to corrosion due to precipitation of carbides.

 

 

7.0 Heat Treatment Profile

7.1 Solution Annealing

Hold for 1 hour at 1090°C

 

Temperature (°C)

 

 

 

 

 

 

Figure 3: Heat treatment profile of solution annealing of 316 stainless
steel

 

7.2 Stress-relieving treatment

Temperature (°C)

 

 

 

 

 

 

Figure 4: Heat treatment profile of stress-relieving treatment of 316
stainless steel

 

8.0 Surface Treatment

8.1 Low Temperature Plasma Ion Nitriding

316 stainless steel
are nitrided at a temperature of 400°C at a constant pressure of 1000Pa for 5
hours by using a mixture of 80vol% of nitrogen gas and 20vol% of hydrogen gas
in a total flow rate of 300 sccm (standard cubic centimeter per minute).

316 stainless steel can be surface hardened
by low temperature plasma ion nitriding without reduce its corrosion resistance
as the formation of chromium or
iron nitride precipitates are prevented. Plasma ion nitriding at low
temperature introduces nitrogen atoms into the stainless steel as a solid
solution and thus increases the strength of the stainless steel. This
process super-saturates the surface of the stainless steel and expands its
lattice. This expanded layer is known as the “S-Phase” or called
supersaturated phase where is metastable and also known as expanded austenite ?N
which have a crystalline face-centered cubic lattice (FCC). S-phase is
a very hard thin layer with thickness of 10-30 microns (0.0004-0.0012in) and hardness of 900-1300 HV
that gives corrosion resistance, chemical resistance, wear or abrasion
resistance, fretting resistance, fatigue resistance and also other useful and
demanding life limiting properties. Good wear resistance is important criteria
for marine anchor and anchor chain as they always lies on the sea floor with
surface waves and tides pulling and releasing it and the sea may moves the
anchor chain back and forth over rocks on the sea floor. Moreover, 316
stainless steel undergoes low temperature plasma ion nitriding to improve its
mechanical and tribological properties due to formation of S-phase, a hard
nitrogen diffusion zone which supersaturated with nitrogen interstitials at the
surface. This is because S-phase further increases its strength and surface hardness
for heavy application, also wear and corrosion resistance of the annealed 316
stainless steel.

 

 

 

 

9.0 Microstructure

9.1 Cold working effect on the
microstructure of 316 stainless steel

Figure
5: Microstructure of 316 stainless steel before cold working

Figure
6: Microstructure of 316 stainless steel after cold working

Plastic deformation caused the shrinkage of the g phase in the 316 stainless steel. The more
the thickness was reduced, the greater the amount of ?’, and the superior the
hardening of g . This hardening is
responsible for modifying its mechanical properties, increasing its resistance,
flow stress, hardness and fragility, reducing the malleability, ductility, as
well as making the material very vulnerable to corrosion due to the formation
of pit particles. For the 316 stainless steel, the higher the level of
deformation, the more easily the deformed structure of the g  phase
could be seen.

9.2 Solution annealing effect on the
microstructure of 316 stainless steel

Figure
7: Microstructure of 316 stainless steel after solution annealing

Microstructure of 316
stainless steel after solution annealing is composed of polyhedral austenitic
grains with twinning typical for fcc structure. The average austenitic grain
size of this state is about 40±8 m. The small amount of ferrite was also
recorded. No precipitates were observed at the grain boundaries of the solution
annealed steels.

9.3 Low temperature plasma ion
nitriding effect on the microstructure of 316 stainless steel

Figure
8: Microstructure of 316 stainless steel before nitriding

Figure
9: Microstructure of 316 stainless steel after nitriding

From figure 6, the large
plastic deformation is obviously observed on the worn region of untreated steel.
The morphology of the worn region also showed the deep plow with plate-like
wear debris. The deep plow is explains the wear severely occurred on the sample by abrasive mechanism. From
figure 7, it appears to be less worn and the shallow plow built up on the
surface. This indicates that abrasive wear also experienced on the sample but
not severely compared to untreated sample. Wear resistance and surface hardness
of nitrided 316 stainless steel is significantly improved through the low
temperature gas nitriding treatment.

10.0 Operating
Environment

The anchor and its chain of a ship are always soaked in the seawater
when the ship is kept stationary in the middle of the sea. Therefore, the
anchor and its chain will subject to pitting and crevice corrosion in warm
chloride environments. This may cause lost of anchor due to breakage of anchor
chain or breakage of the anchor itself. The lost anchor under the seabed will
continue to corrode and will cause severe pollution of the sea environment.  Under the severe weather, the environmental
forces such as strong wind, fast current and big wave will accelerate the
corrosion rate or stress corrosion cracking of the anchor and anchor chain.
Therefore, the corrosion resistance from chloride environment is indispensable
properties during materials selection to manufacture both anchor and its chain.
The surface finishes of the materials also may play a main role in corrosion
resistance.

On the other hand, the anchor and its chain also have to withstand
cryogenic temperature especially for the ship operation at the Arctic and
Antarctic. During materials selection, we also have to prevent to choose a
material which tend to become brittle and crack or break without significant
plastic deformation at low temperature. Apart from this, the other environment
should be concerned is the type of the seabed. When there is uncontrolled
dropping of anchor on a rocky seabed, impact of the anchor with the rocky
seabed might cause the damage and breakage of the anchor. The dragging of the anchor
with the seabed when the ship is sailing will cause wear resistant of the
anchor and break. In addition, the anchor chain may break due to overstress or
overload. So, both anchor and chain should manufacture with the materials with
high toughness.

 

 

11.0 Background information of the application

There is several
experiment done in the pass for the material used. Mild steel which has been
hot-dipped galvanized with zinc was used before. However, zinc is soft metal. When
anchor rubs and scrapes, the coating is damaged and the unprocted steel surface
will be oxidized. The another material is aluminium. The aluminium is weaker
but lighter

and less corrosive
than steel. It is than withdrew since many users value on the strength of
anchor and chain but not the weight of the chain. Hence the stainless steel is
the best option as the material used in anchor and chain. Lets talk about the
roles of elements in the stainless steel 316.

The carbon element is
present in every steel, same as in stainless steel. In 316 stainless steel, the
carbon contain is kept very low, which is below 0.1%. The carbon in stainless steel
the role to obtain high hardness and strenght which has martensitic structure.
Beside the carbon also provide corrosion resistance when combine with chromium
to form chrome carbides, it have the ability to form unreactive layer on the
surface of stainless steel.

Since the chromium
have disscussed above, the element will be disscussed is chromium. Chromium
metal is reactive element in nature but it have a passive oxide layer on the surface.
When it is added into 316 stainless steel, it also form le oxides layer to
protect the steel. However, when the concentration is below 10.5%, the layer is
too thin that the oxygen atoms can pass through the oxides layer and oxidize
the iron.

The manganese element
in 316 stainless steel act as a assistant to de-oxidation. During melting
process, manganese prevent sulfer element which could not be eliminated completely
combine with iron to form iron sulfide which is blamed on hot cracking problems.

The silicon is
usually added into 316 stainless steel in small amount with molybdenum to improve
corrosion resistance to suffuric acid. It also stabilize the ferrite structure
and also improve resistant of oxidation.

The 300 series
stainless steel must have nickel element as an essential element. It improve
strenght, ductility and toughness, even at cryogenic temperatures by the formation
of austenitic structure. Since iron is magnetic, it can make the material into non-magnetic.

Molybdenum has proved
that it can add resistance to localized pitting attack when it is added into
chromium-iron- nickel matrix and provide better resistance to the crevice corrosion.
It also prevent the de-icing salts situation. Nitrogen is also proven to
increase the resistance to localized pitting attack and inter-granular
corrosion. Besides, nitrogen is also improved yeild strenght of low carbon
steel back to grades of high carbon steel.

Sulfur need to be
kept as low as possible, however, present of a little sulfer improve machinability.

Phosphorus is believe
to improve strenght and hardness. But high content of phosphorus increase the
tendency to crack. It is controlled below 0.05%

x

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