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Pengelasan Baja Tahan Karat
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Baja Tahan Karat
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Apa itu baja tahan karat (stainless steels)
• SS defined as Iron‐base alloy containing > 10.5% Cr & < 1.5%C and they
are considered
high
alloy
• used for corrosion and heat resistant applications especially in saline solutions, under oxidizing
conditions.
• Corrosion resistance is imparted by the formation of a passivation layer characterized by:
• Insoluble chromium oxide film on the surface of the metal ‐ (Cr2O3) .
• Develops when
exposed
to
oxygen
and
impervious
to
water
and
air.
• Layer is too thin to be visible
• Quickly reforms when damaged
• Susceptible to sensitization, pitting, crevice corrosion and acidic environments.
• Passivation can be improved by adding nickel, molybdenum and vanadium.
• In general
a minimum
concentration
of
12%
Cr
is
required
to
obtain
a film
that
completely
covers the exposed surface of a sample.
• The Cr2O3 in the steel is very stable against attack by a number of chemicals and electrolytic
corrosion actions.
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• Over 150 grades of SS available, usually categorized into 5 series
containing alloys
w/
similar
properties.
• All SS types
• Weldable by virtually all welding processes
• Process selection often dictated by available equipment
• Simplest &
most
universal
welding
process• Manual SMAW with coated electrodes
• Applied to material > 1.2 mm
• Other very commonly used arc welding processes for SS
• GTAW, GMAW, SAW & FCAW
• Optimal
filler
metal
(FM)• Does not often closely match base metal composition
• Most successful procedures for one family
• Often markedly different for another family
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• SS base metal & welding FM chosen based on• Adequate corrosion resistance for intended use
• Welding FM
must
match/over
‐match
BM
content
w.r.t
• Alloying elements, e.g. Cr, Ni & Mo
• Avoidance of cracking
• Unifying theme in FM selection & procedure
development• Hot cracking
• At temperatures < bulk solidus temperature of alloy(s)
• Cold cracking
• At rather
low
temperatures, typically
< 150
ºC
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•Hot
cracking• As large Weld Metal (WM) cracks
• Usually
along
weld
centreline• As small, short cracks (microfissures) in WM/HAZ
• At fusion line & usually perpendicular to it• Main concern in Austenitic WMs
• Common remedy
• Use mostly austenitic FM with small amount of ferrite• Not suitable when requirement is for
• Low magnetic permeability
• High toughness at cryogenic temperatures
• Resistance to
media
that
selectively
attack ferrite
(e.g.
urea)
• PWHT that can embrittle ferrite
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• Cold
cracking• Due to interaction of
• High welding stresses
• High‐strength metal
• Diffusible hydrogen
• Commonly occurs in Martensitic WMs/HAZs
• Can occur in Ferritic SS weldments embrittled by
• Grain coarsening and/or second‐phase particles
• Remedy• Use of mostly austenitic FM (with
appropriate
corrosion
resistance)
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Castro & Cadenet, Welding Metallurgy of
Stainless and Heat-resisting Steels
Cambridge University Press, 1974
A=Martensitic Alloys
B=Semi-Ferritic
C=Ferritic
12% Cr raises the critical
temperatures and reduces the
austenite region.
With sufficient amounts of carbon,
these steels
can
be
heat
treated
to
a
martensitic structure.
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General Properties
of
Stainless
Steels
• Electrical
Resistivity
• Surface & bulk resistance is
higher than that for plain‐
carbon steels
• Thermal Conductivity
• About 40 to 50 percent that of plain‐carbon steel
• Melting
Temperature
• Plain‐carbon:1480‐1540 °C
• Martensitic: 1400‐1530 °C
• Ferritic: 1400‐1530 °C
• Austenitic: 1370‐1450 °C
• Coefficient
of
Thermal
Expansion
• Greater coefficient than plain‐
carbon steels
• High Strength
• Exhibit high strength at room
and elevated temperatures
• Surface
Preparation• Surface films must be
removed prior to welding
• Spot
Spacing
• Less shunting
is
observed
than
plain‐carbon steels
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Grades
of
Stainless
Steel
• To make
a steel
"stainless"
it
needs
to
contain
a minimum
of
12%
Chromium (Cr).
• The problem with 12% Cr is that it is fairly brittle and only provides the
minimum corrosion resistance. Increasing the Chromium content to 17%
improves corrosion resistance but increases brittleness. Adding 8% Nickel
makes the
steel
ductile
again.
Thus
18/8
stainless
was
born
(304).
316
/ 316L has additional Molybdenum and higher Nickel which provides greater
corrosion resistance.
• With stainless when you see two numbers they always refer to the
Chromium and Nickel content ‐ 18/8 is 18%Cr and 8%Ni. If you see three
numbers like 19/12/3
they
refer
to
the
Chromium,
Nickel
and
Molybdenum
content. 316L is 19%Cr, 12%Ni and 3%Mo.
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Jenis baja tahan karat
In general, there are five types of stainless steels based on their crystal
structure and strengthening mechanisms. They are (AISI classes for SS ):
1. Austenitic stainless steels
– 200 series = chromium, nickel, manganese (austenitic)
– 300 series = chromium, nickel (austenitic)
2. Ferritic stainless steels
• 400 series = chromium only (ferritic)
3. Martensitic stainless steels
– 500 series = low chromium <12% (martensitic)
4. Precipitation-hardened stainless steels – 600 series = Precipitation hardened series (17-7PH, 17-7 PH, 15-5PH)
5. Duplex stainless steels
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Aplikasi
• Food industry
(cookware,
flatware,
food
transport
and
storage
tankers) due to its corrosion resistance and antibacterial properties.
• Surgical equipment
• Aerospace
• High end automotive, industrial, etc.
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Austenitic SS
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Fe
18 Cr
8 Ni)
C Phase
Diagram
Baja
Tahan Karat
Austenitik)
• Nickel stabilizes the austenite, –phase instainless steels (SS).
• When 8% Ni is added to an 18% Cr steel –
18/8 SS – the -phase is stable down to room
temperature at very low C – the three phase (+ + carbide) eutectoid region is at lower
temperatures.
• The high temperature -ferrite is also very
restricted.( + + carbide) eutectoid
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Characteristics of
Austenitic
Stainless
steels
• Chrome-nickel or chrome -nickel- manganese alloys Austenitic, non magnetic and donot harden by heat treatment.
• Total content of nickel and chromium is at least 23%• Difficult to machine. Can be improved by Selenium of sulfur additions.• Best high temperature strength and reistance to scaling. Hence the best corrosion
resistance.
• Cold working causes work hardening.• Can be hot worked easily.• Type 302 stainless steel is more used.(austenitic). Modified into 22 different alloys.• Lowering the carbon to 0.08% gives stainless steel type 304 with improved weldability.
Used for most fabrication that needs welding.
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The
200/300
Series
of
Austenitic
Stainless
Steels
• These alloys are based on a minimum of 18% Cr – 8% Ni with a maximum of 0.15C. Most common SS (roughly
70% of total SS production).Contain between 16 and 25 percent chromium, plus sufficient amount of nickel,
manganese and/or nitrogen.
• Have a face-centered-cubic (fcc) structure, Nonmagnetic, Good toughness, Spot weldable, Strengthening can be
accomplished by cold work or by solid-solution strengthening
• General use where corrosion resistance is needed. Used for flatware, cookware, architecture, automotive,
etc. Typical alloy 18% Cr and 10% Ni = commonly known as 18/10 stainless. Also have low carbon version of
Austenitic SS (316L or 304L) used to avoid corrosion problem caused by welding, L = carbon content < 0.03%
• 20% Cr – 10% Ni have better properties for higher specifications such as very low carbon grade (L), eg., < 0.03% C is
prevents the formation of (CrFe)4C at grain boundaries, which depletes the Cr below 12% in the bulk.
• Addition of 2-3% Mo enhances corrosion protection in neutral salt solutions. As well, very low carbon grade < 0.03% C
is required for welded components
• Addition of Ti (5xC) or Nb (10 x C), enables carbon to be increased to 0.08% for welded products by forming TiC or
NbC instead of (FeCr)4C.
• Austenitic, High strength, best corrosion resistance. High temp capability up to 1200 F. non-magnetic,
good ductility and toughness, not hardenable by heat treatment, but they can be strengthened via cold
working, best corrosion resistance but most expensive, corrosive in hydrochloric acid.
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Typical Microstructure of 300 series of Austenitic Stainless Steels
Microstructures of 302 Stainless Steel containing 18Cr – 8Ni – 0.11C
Quenched from 985 oC
Austenite + annealing twins
(boundaries are lines) plus
undissolved carbides
(mag – 1000x)
Quenched from 1205 oC
Course Austenite + annealing
twins and no undissolved carbides
(mag – 1000x)
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200/300 Series SS (Austenitic):
• Most common
SS
(roughly
70%
of
total
SS
production)
• Used for flatware, cookware, architecture, automotive, etc.
• 0.15% C (max), 16% Cr (min) and Ni or Manganese
• Austenitic, High strength, best corrosion resistance. High temp capability up to
1200 F. non‐magnetic, good ductility and toughness, not hardenable by heat treatment,
but
they
can
be
strengthened
via
cold
working,
best
corrosion
resistance but most expensive, corrosive in hydrochloric acid.
• General use where corrosion resistance is needed.
• Typical alloy 18% Cr and 10% Ni = commonly known as 18/10 stainless
• Also have
low
carbon
version
of
Austenitic
SS
(316L
or
304L)
used
to
avoid
corrosion problem caused by welding, L = carbon content < 0.03%
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•300 Series—austenitic chromium‐nickel alloys
Type 301—highly ductile, for formed products. Also hardens rapidly during mechanical working. Good weldability.
Better wear resistance and fatigue strength than 304.
Type 302—same
corrosion
resistance
as
304,
with
slightly
higher
strength
due
to
additional
carbon.
Type 303—free machining version of 304 via addition of sulfur and phosphorus. Also referred to as "A1" in accordance
with ISO 3506.[10]
Type 304—the most common grade; the classic 18/8 stainless steel. Also referred to as "A2" in accordance with ISO
3506.[10]
Type 304L— same as the 304 grade but contains less carbon to increase weldability. Is slightly weaker than 304.
Type 304LN—same as 304L, but also nitrogen is added to obtain a much higher yield and tensile strength than 304L.
Type 308—used
as
the
filler
metal
when
welding
304
Type 309—better temperature resistance than 304, also sometimes used as filler metal when welding dissimilar steels,
along with inconel.
Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of
molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased
resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants. 316L
is an extra low carbon grade of 316, generally used in stainless steel watches and marine applications due to its high
resistance to corrosion. Also referred to as "A4" in accordance with ISO 3506.[10] 316Ti includes titanium for heat
resistance, therefore it is used in flexible chimney liners.
Type 321—similar to 304 but lower risk of weld decay due to addition of titanium. See also 347 with addition of
niobium for desensitization during welding.
Common 300 series grades of SS:
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Carbide
Phases
in
Stainless
Steels
• There are three Fe-Cr carbides phases
formed in slowly cooled stainless steels as a
function of carbon and chromium content.1. Up to 15%, Cr can enter cementite without
changing its structure, to form (FeCr)3C, which
is the carbide present in low alloy steels.
2. The next carbide is (FeCr)7C3, which contains a
minimum of 35% Cr. This is the carbide formedin high-carbon high-chromium tool steels.
3. (FeCr)4C, which contains > 70% Cr, is the
carbide normally found in stainless steels.
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Carbide
Precipitation
at
Grain
Boundaries
• The precipitation of (CrFe)4C, which contains 70 % Cr, at grain boundaries
causes the concentration of Cr in the adjacent austenite to fall below 12%,which degrades the corrosion resistance properties of the steel.
• The optimum temperature for precipitation of (CrFe)4C is around 650 oC,
which is attained in the heat affected zone adjacent to a fusion weld.
• Stainless steels with carbon as low as 0.15% can thus suffer “weld decay”.
• It can be eliminated by
1) lowering carbon to 0.03%, or
2) use Ti or Nb to remove the carbon as TiC or NbC, without lowering the Cr content of
the austenite.
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Quenched from 1150 oCReheated 24 h at 650 oC
Carbides at grain boudaries
(low mag – 240x)
Quenched from 1150 oCReheated 24 h at 650 oC
Carbides at grain boundaries
(high mag – 1000x)
Precipitation
of
CrFe)
4
C
at
Grain
Boundaries
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Precipitation
of
CrFe)
4
C at
Grain
Boundaries
• The concentration profile of Cr in the matrix adjacent to a precipitate
of (CrFe)4C is given below.
• The Cr level falls from 18% to 7-8%, which is well below the 12%
limit for effective corrosion protection.
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Oxidation
Resistant
Stainless
Steels
• In order to maintain stability of the austenite phase the Cr was increased to 22 –
26% Cr with Ni of 12 – 22%. The addition of Ni gives increased resistance to
oxidation at high temperatures. These steels are very expensive and only used for
special applications.
Micrograph of a welded joint in 20Cr – 12Ni
Stainless Steel, x50
• The structure of the original metal is
shown on the left.
• The fine-grained dark structure on the
right is the weld material (filler).• In the centre where the metal has been
heated close to its melting point the
structure is largely austenitic with some
darker alloyed ferrite.
• In the heat affected zone, the austeniteshows pronounced grain growth and is
thus weaker than the original fine grained
structure.
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Ferritic SS
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Fe
Cr
Phase
Diagram
Baja
Tahan Karat
Jenis Feritik)
Cr is a ferritic stabilizer.
The austenite phase is thus condensed into a
small “ loop”, which extends out to 16% Cr
over the range of temperature 900 – 1400 oC.
At concentrations greater than 16% Cr, the –
Fe and -Fe phases are not distinguishable and
a common –phase extends all the way to100% Cr.
The 50/50 composition orders at temperatures
below ~900o
C to form the –phase, whichcauses embrittlement in stainless steels.
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Pseudo
binary
Fe
– 12 Cr)
C
Phase
Diagram
• Carbon is soluble in Fe-Cr austenite and increases the Cr limit of the –loop.
• Hardenable cutlery steels, which contain the minimum 12% Cr, are described in terms of a
pseudo-binary (Fe + 12%Cr)-C phase diagram.
• The -field is severely constricted compared to the Fe-C diagram.
– The maximum solubility of C is 0.7% and the eutectoid is at 0.35% C. – In addition, the eutectoid temperature (range) is raised to >800 oC.
Two forms of carbide are inequilibrium with the –phase, ie., the
(CrFe)4C and (CrFe)7C3, depending
on the carbon content. eutectoid temperature (range)
Note 12%Cr
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Characteristics of
Ferritic Stainless
steels
• 14 to 27% Cr. Low in carbon but high in Cr compared to martensitic steels.
• Not hardened by heat treatment. Only moderately hardened by cold working• Can be cold or hot worked. Achieves maximum softness in annealed condition.• As annealed, their strength is 50% higher than plain carbon steels and corrosion
resistance and machinability is better than martensitic steels.• Annealing is done to relieve stresses due to welding or cold working.• Susceptible to embrittlement during slow cooling during annealing.
• Since martensite is not formed and since there is embrittlement possibility, thesesteels are not tempered.
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•400 common alloys
Type 405— ferritic for welding applications
Type 408—heat‐resistant; poor corrosion resistance; 11% chromium, 8% nickel.
Type 409—cheapest
type;
used
for
automobile exhausts;
ferritic (iron/chromium
only).
Type 410—martensitic (high‐strength iron/chromium). Wear‐resistant, but less corrosion‐resistant.
Type 416—easy to machine due to additional sulfur
Type 420—Cutlery Grade martensitic; similar to the Brearley's original rustless steel. Excellent
polishability.
Type 430—decorative, e.g., for automotive trim; ferritic. Good formability, but with reduced
temperature and
corrosion
resistance.
Type 440—a higher grade of cutlery steel, with more carbon, allowing for much better edge
retention when properly heat‐treated. It can be hardened to approximately Rockwell 58 hardness,
making it one of the hardest stainless steels. Due to its toughness and relatively low cost, most
display‐only and replica swords or knives are made of 440 stainless. Also known as razor blade
steel. Available in four grades: 440A, 440B, 440C, and the uncommon 440F (free machinable). 440A,
having the
least
amount
of
carbon
in
it,
is
the
most
stain
‐resistant;
440C,
having
the
most,
is
the
strongest and is usually considered more desirable in knifemaking than 440A, except for diving or
other salt‐water applications.
Type 446—For elevated temperature service
Common 400 series grades of SS:
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400 Series SS (Ferritic):
• Ferritic,
Automotive
trim,
chemical
processing,
blades,
knives,
springs,
ball
bearings,
surgical instruments. Can be heat treated!
• Contain between 10.5% and 27% Cr, little Ni and usually molybdenum.
• Common grades: 18Cr‐2Mo, 26Cr‐1Mo, 29Cr‐4Mo, and 29Cr‐4Mo‐2Ni
• Magnetic (high in Fe content) and may rust due to iron content.
• Lower strength vs 300 series austenitic grades
• Cheap
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The
400
Series
of
Heat
Treatable
Stainless
Steels
Martensitic Stainless Steels)
• These steels are based on Martensite, 12-16% Cr with various amounts of Carbon.
• Low carbon grades containing up to 0.2 C containing up to 12-13% Cr are hardenable by air
quenching to form a low-carbon martensite (lath type) and are used for cutlery.• High carbon grades contain 0.6-1.2 C and 16-18 Cr form much harder high-carbon martensite
(lenticular type) on quenching and are used for surgical instruments.
Air cooled from 955 oC. Low
carbon martensite x1000
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The
400
Series
of
Heat
Treatable
Stainless
Steels
erritic Stainless Steels)
• Low carbon grades with up to 0.2 C and 14-18% Cr are ferritic and
can only be hardened by 1) cold work or 2) precipitation of carbide.
Air cooled from 790 oC ferrite plus carbide x1000
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Martensitic SS
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Characteristics of Martensitic Stainless steels• Straight chromium steels with 11.5 to 18% Cr. C 0.15 Mn 1.25 Si 1• For turbine blades and corrosion resistant applications• Magnetic
• Can be machined (poorer machinability than plain carbon steels. Machinability can beimproved by adding small amounts of Selenium or Sulphur.)
• Hot working possible.• Can be hardened (by air cooling or oil quenching itself)
• Full hardness on air-cooling from ~ 1000 ºC• Softened by tempering at 500–750 ºC
• Maximum tempering temperature reduced If Ni content is significant• On high-temperature tempering at 650–750 ºC
• Hardness generally drops to < ~ RC 30• Useful for softening martensitic SS before welding for
• Sufficient bulk material ductility
• Accommodating shrinkage stresses due to welding• Coarse Cr-carbides produced
• Damages corrosion resistance of metal• To restore corrosion resistance after welding necessary to
• Austenitise + air cool to RT + temper at < 450 ºC
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500 Series SS (Martensitic):
• Not as corrosion resistant as the other classes but extremely strong and tough as well as
machineable and
can
be
hardened
via
heat
treat.
• High strength structural applications (Su up to 300 ksi) – nuclear plants, ships, steel turbine blades, tools, etc.
• Magnetic
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PH SS
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Precipitation
Hardened
Ferritic Stainless
Steels
• Ferritic stainless steels with ~17% Cr have very low carbon of 0.04 – 0.07 C, which give
good corrosion resistance and high strength.
• The 17 – 4 PH* (with 4% Ni) alloy is transformed to low carbon martensite (lath
martensite) on cooling from austenite and is hardened by ageing at 480
oC due to the
precipitation of Al‐Ti and a Nb‐Cu compound.
• The 17‐7PH ( with 7% Ni) alloy is semi‐austenitic and requires a more complicated series of
treatments to produce a precipitation‐hardened martensite.
• 5% ‐ 20% d‐ferrite is present after this steel is quenched from the solution annealing
temperature of
1065
oC as Al
is
a strong
ferrite
former.
• It is easily worked in this condition but it rapidly “work hardens”* because of its low Ni
content.
• It is also hardened by ageing at 565 oC when an Al‐based compound is precipitated.
• An
ageing
treatment
at
510 o
C gives
a
higher
strength
at
the
expense
of
lower
ductility.
• - PH stands for “precipitation hardened”.
• Recall the concept of work hardening in bcc steels by dislocation pinning by carbon.
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600 Series
Precipitation
Hardening
Martensitic SS:• Have corrosion resistance comporable to 300 series austentic grades but can be
precipitation hardened
for
increased
strength!
• Key: High strength + corrosion resistance BOTH.
• Why? Aerospace industry – defense budgets determined 2% of GDP spent dealing with
corrosion so developed high strength corrosion resistant steel to replace alloy steels.
• Lockheed‐Martin Joint Striker Fighter – 1st aircraft to use PH SS for entire airframe.
• Common Grades:
• 630 grade = 17‐4 PH (17% Cr, 4% Ni),
• 17‐4 PH,
• 15‐
5
PH
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SSINA Stainless Steel Design Guidelines
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SSINA Stainless Steel Design Guidelines
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SSINA Stainless Steel Design Guidelines
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DUPLEX SS
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Characteristics of Duplex stainless steels
• Excellent resistance to stress corrosion cracking
• Very high mechanical strength
• Excellent resistance to pitting and crevice corrosion
• High resistance to general corrosion in a variety of environments
• Low thermal expansion
• High resistance to erosion corrosion and corrosion
fatigue
• Good weldability
• Lower
life
cycle
cost
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Duplex microstructure
• The austenite islands
(light) are embedded in
a continuous
ferrite
(dark) matrix.
• The duplex
microstructure typically
contains 45
‐65%
austenite and 35‐55%
ferrite.
Austenite Ferrite
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Yield Strength 0,2% Austenitic vs Duplex Stainless Steel
0
400
500
600
200
300
100
316L
SAF
2304
904L
SAF
2205
6Mo+N
SAF2507
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Solidif ication mechanism of aDuplex Stainless Steel
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Stress
strain
curves
Austenite,
ferrite
and
duplex
0,0 0,2 0,4 0,6 0,80
200
400
600
800
1000
austeniteduplex (2205)
ferrite
S t r e s s [ M P a ]
Strain
ferrite
duplex
austenite
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Conclusions
Key
Areas
• Good Weldability
• Uses Conventional Welding Processes
• Joint Design
• Role of Nitrogen
• Heat Input Important
• Interpass Temperature
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Welding
Stainless
• There are 2 common grades of stainless: 304L (welded using 308L filler), and 316L which is
welded using
316L
filler.
• Why is 308L filler used for 304L? Basically there are a number of grades that do similar jobs, 302L, 303L and 304L (they are 17/7, 18/8 and 19/9 respectively). 308L is 20/10 so can be used to weld
all 3 grades.
• Stainless is easy to weld but very difficult to keep flat, the coefficient of linear expansion is 1.7
times that of mild steel. There isn’t much you can do about that except to weld it quickly and by
doing so minimise the heat input.
• 304 and 316 (as opposed to the L low carbon versions) suffer from weld
decay. When heated to
welding temperatures the Chromium combines with the Carbon leaving the steel short of Chromium and therefore unable to self repair itself.
• This was virtually eliminated by introducing stabilised stainless steels 347 and 321 which contain
Niobium
or Titanium which sacrifices itself to save the Chromium, however, when lower carbon
versions 304L and 316L were introduced the problem of weld decay was eliminated. These days
the
higher
(in
fact,
normal)
carbon
versions
are
only
used
for
applications
where
heat
resistance
is
needed.
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Stainless
Steel
Filler
Metal
Choice* depends on environment ‐ if Sulphurous it must be 410
** preheat of 150°C required
304L 316L 310 347 321 410 430 Mild
Steel
308L 308L 310 308L 308L 309L 309L 309L 304L
308L 316L 310 316L 316L 309L 309L 309L 316L
310 310 310 310 310 309L 310 310 310
308L 316L 310 347 347 309L 309L 309L 347308L 326L 310 347 318 309L 309L 309L 321
309L 309L 309L 309L 309L 410/309L* 309L 309L 410
309L 309L 310 309L 309L 309L 309L** 309L 430
309L 309L 310 309L 309L 309L 309L Mild
Steel
Mild
Steel