http://www.materialsengineer.com/E-Stainless-Steel.htm
Stainless Steels
Stainless Steels are
iron-base alloys containing Chromium. Stainless steels usually contain
less than 30% Cr and more than 50% Fe. They attain their stainless characteristics
because of the formation of an invisible and adherent chromium-rich oxide
surface film. This oxide establishes on the surface and heals itself in the
presence of oxygen. Some other alloying elements added to enhance
specific characteristics include nickel, molybdenum, copper, titanium,
aluminum, silicon, niobium, and nitrogen. Carbon is usually present in
amounts ranging from less than 0.03% to over 1.0% in certain martensitic
grades. Corrosion resistance and mechanical properties are commonly the
principal factors in selecting a grade of stainless steel for a given
application.
Stainless steels are
commonly divided into five groups:
Martensitic stainless
steels
Ferritic stainless
steels
Austenitic stainless
steels
Duplex (ferritic-austenitic)
stainless steels
Precipitation-hardening stainless steels.
Martensitic stainless steels are essentially alloys of
chromium and carbon that possess a martensitic crystal structure in the
hardened condition. They are ferromagnetic, hardenable by heat treatments, and
are usually less resistant to corrosion than some other grades of stainless
steel. Chromium content usually does not exceed 18%, while carbon content
may exceed 1.0 %. The chromium and carbon contents are adjusted to ensure
a martensitic structure after hardening. Excess carbides may be present to
enhance wear resistance or as in the case of knife blades, to maintain cutting
edges.
Ferritic stainless steels are chromium containing
alloys with Ferritic, body centered cubic (bcc) crystal structures. Chromium
content is typically less than 30%. The ferritic stainless steels are
ferromagnetic. They may have good ductility and formability, but
high-temperature mechanical properties are relatively inferior to the
austenitic stainless steels. Toughness is limited at low temperatures and
in heavy sections.
Austenitic stainless steels have a austenitic, face
centered cubic (fcc) crystal structure. Austenite is formed through the
generous use of austenitizing elements such as nickel, manganese, and nitrogen.
Austenitic stainless steels are effectively nonmagnetic in the annealed
condition and can be hardened only by cold working. Some ferromagnetism
may be noticed due to cold working or welding. They typically have
reasonable cryogenic and high temperature strength properties. Chromium content
typically is in the range of 16 to 26%; nickel content is commonly less than
35%.
Duplex stainless steels are a mixture of bcc
ferrite and fcc austenite crystal structures. The percentage each phase is a
dependent on the composition and heat treatment. Most Duplex stainless steels
are intended to contain around equal amounts of ferrite and austenite phases in
the annealed condition. The primary alloying elements are chromium and nickel.
Duplex stainless steels generally have similar corrosion resistance to
austenitic alloys except they typically have better stress corrosion cracking
resistance. Duplex stainless steels also generally have greater tensile
and yield strengths, but poorer toughness than austenitic stainless steels.
Precipitation hardening
stainless steels
are chromium-nickel alloys. Precipitation-hardening stainless steels may be
either austenitic or martensitic in the annealed condition. In most
cases, precipitation hardening stainless steels attain high strength by
precipitation hardening of the martensitic structure.
Selecting a
Stainless Steel
There are a large number of
stainless steels produced. Corrosion resistance, physical properties, and
mechanical properties are generally among the properties considered when
selecting stainless steel for an application. A more detailed list of
selection criteria is listed below:
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Corrosion resistance is
commonly the most significant characteristic of a stainless steel, but can also
be the most difficult to assess for a specific application. General corrosion
resistance is comparatively easy to determine, but real environments are
usually more complex. An evaluation of other pertinent variables such as
fluid velocity, stagnation, turbulence, galvanic couples, welds, crevices,
deposits, impurities, variation in temperature, and variation from planned
operating chemistry among others issues need to be factored in to selecting the
proper stainless steel for a specific environment.
AMC can provide engineering
services to determine how to optimize the selection of stainless steel for your
application. Our engineering analysis can reduce overall costs, minimize
service problems, and optimize fabrication of your structure.
From NACE2005 Paper 05278
(Corrosion center paper)
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Table 2. Chemical analysis
of the carbon steels used in the experiments |
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Element |
1020 Composition (%) |
X65 Composition (%) |
API 5L X65 Standard (%) |
AISI 1020 Standard (%) |
|
C |
0.19 |
0.13 |
< 0.26 |
0.13-0.23 |
|
Mn |
0.8 |
1.16 |
<1.40 |
0.30-0.60 |
|
P |
0.01 |
0.009 |
< 0.03 |
< 0.04 |
|
S |
0.023 |
0.009 |
< 0.03 |
< 0.05 |
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Table 3. Hardness (HRB) results |
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1020 |
X65 longitudinal cut |
X65 transversal cut |
|
1 |
83.5 |
81.3 |
60.3 |
|
2 |
84.5 |
94.4 |
68.7 |
|
3 |
82.1 |
98.7 |
63.3 |
|
4 |
89.1 |
87.9 |
78.0 |
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5 |
83.2 |
95.4 |
59.1 |
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6 |
86.8 |
89.3 |
51.1 |
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7 |
80.9 |
88.7 |
66.5 |
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8 |
80.2 |
92.9 |
75.0 |
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9 |
89.1 |
93.3 |
58.5 |
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10 |
83.2 |
85.1 |
67.7 |
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Average |
84.3 |
90.7 |
64.8 |
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Approx.Tensile Strength |
79,000psi for 85HRB |
90,000 psi for 90.7HRB |
56,000 psi for 65.7HRB |
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Tensile requirements |
58,000 psi (min) |
77,000psi (min) |
77,000psi (min) |
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Yield Strength |
36,000 psi (min) |
65,000psi (min) |
65,000psi (min) |
Chemical composition and mechanical properties of welded
carbon steel pipes according:
API-5L and DIN standards
|
Standard And Steel Grade |
Chemical composition, % |
Tensile strength, MPa |
Yield strength, MPa |
Elongation, % |
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C |
Mn |
P |
S |
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MAX |
MIN |
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DIN |
API |
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St37 |
|
0.19 |
|
0.05 |
0.05 |
350-480 |
235 |
25 |
|
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A |
0.21 |
0.90 |
0.04 |
0.05 |
331 |
207 |
|
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St44 |
|
0.23 |
|
0.05 |
0.05 |
430-550 |
275 |
21 |
|
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B |
0.26 |
1.15 |
0.04 |
0.05 |
413 |
241 |
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X42 |
0.28 |
1.25 |
0.04 |
0.05 |
413 |
289 |
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St52 |
|
0.22 |
1.7 |
0.05 |
0.05 |
500-650 |
355 |
21 |
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X52 |
0.28 |
1.25 |
0.04 |
0.05 |
455 |
358 |
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Ste385.7tm |
|
0.14 |
1-1.6 |
0.035 |
0.025 |
530-680 |
385 |
19 |
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X56 |
0.26 |
1.35 |
0.04 |
0.05 |
489 |
386 |
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Ste445.7tm |
|
0.16 |
1-1.6 |
0.035 |
0.025 |
560-710 |
445 |
18 |
|
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X65 |
0.26 |
1.4 |
0.04 |
0.05 |
530 |
448 |
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Strength
Standard linepipe is still commonly designated by its ‘X’ grade.
This designation comes from the API5L specification for linepipe, which is
still the most commonly used specification worldwide. ‘X7’ grade refers to the specified minimum yield
strength (SMYS) of the linepipe steel measured in kilopounds per square inch
(ksi). Hence X56 pipe has a yield
strength of 56 ksi, or 56,000 psi.
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1. Figures without parentheses are for API5L; those in
parentheses are for EN10208.
2. API5L elongation figures
vary with specimen dimensions. Those quoted are for 0.2 sq in specimen.
Instructions: The UNS number (for
"Unified Numbering System for Metals and Alloys") is a systematic
scheme in which each metal is designated by a letter followed by five numbers.
It is a composition-based system of commercial materials and does not guarantee
any performance specifications or exact composition with impurity limits. Older
nomenclature systems have been incorporated into the UNS numbering system to
minimize confusion. For example, Aluminum 6061 (AA6061) becomes UNS A96061.
Overview of the UNS system
This is an overview of the UNS system, with special emphasis on
common commercial alloys. As with any system, there are ambiguities such as the
distinction between a nickel-based superalloy and a high-nickel stainless
steel.
- Axxxxx
- Aluminum Alloys
- Cxxxxx
- Copper Alloys, including Brass and Bronze
- Fxxxxx
- Iron, including Ductile Irons and Cast Irons
- Gxxxxx
- Carbon and Alloy Steels
- Hxxxxx
- Steels - AISI H Steels
- Jxxxxx
- Steels - Cast
- Kxxxxx
- Steels, including Maraging, Stainless, HSLA, Iron-Base Superalloys
- L5xxxx
- Lead Alloys, including Babbit Alloys and Solders
- M1xxxx
- Magnesium Alloys
- Nxxxxx
- Nickel Alloys
- Rxxxxx
- Refractory Alloys
- R03xxx-
Molybdenum Alloys
- R04xxx-
Niobium (Columbium) Alloys
- R05xxx-
Tantalum Alloys
- R3xxxx-
Cobalt Alloys
- R5xxxx-
Titanium Alloys
- R6xxxx-
Zirconium Alloys
- Sxxxxx
- Stainless Steels, including Precipitation Hardening and Iron-Based
Superalloys
- Txxxxx
- Tool Steels
- Zxxxxx
- Zinc Alloys
About 75% of the metal data sheets in MatWeb have a UNS number
that has been tied to its particular MatWeb entry. You may easily find such
metals by typing the UNS number in the Quick Search box in the upper right of
this page. You may also make your selection from inside the boxes below and
click on the appropriate 'Find' button. You can then follow the links to
the complete technical data on MatWeb's extensive list of aluminum, copper,
lead, magnesium, tin, titanium, nickel, stainless steel, and other metal
alloys.
Precipitation
hardening stainless steels are chromium and nickel containing steels that
provide an optimum combination of the properties of martensitic and austenitic
grades. Like martensitic grades, they are known for their ability to gain high
strength through heat treatment and they also have the corrosion resistance of
austenitic stainless steel.
The high
tensile strengths of precipitation hardening stainless steels come after a heat
treatment process that leads to precipitation hardening of a martensitic or
austenitic matrix. Hardening is achieved through the addition of one or more of
the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum.
The most well
known precipitation hardening steel is 17-4 PH. The name
comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper
and 0.3% Niobium. 17-4 PH is also known as stainless steel grade 630.
The advantage
of precipitation hardening steels is that they can be supplied in a
“solution treated” condition, which is readily machinable. After
machining or another fabrication method, a single, low temperature heat
treatment can be applied to increase the strength of the steel. This is known
as ageing or age-hardening. As it is carried out at low temperature, the
component undergoes no distortion.
Characterisation
Precipitation
hardening steels are characterised into one of three groups based on their
final microstructures after heat treatment. The three types are: martensitic
(e.g. 17-4 PH), semi-austenitic (e.g. 17-7 PH) and austenitic (e.g. A-286).