Stainless steel is an alloy
that is produced by combining a variety of different base metals to create a
new material. Like with other alloys, the final product is considerably stronger
than each of the source materials would be on their own.
The History Of Stainless
Steel
People have been experimenting with metal alloys for thousands
of years. While examples of iron smelting can be found as far back as 1800
B.C., the introduction of crucible steel goes as far back as 300 B.C.
Stainless steel is a
very versatile material. Through the years, engineers have experimented with
the production of alloys.
By adding and removing certain metals, they were able to produce different types of stainless steel with
different characteristics and applications. For example, it is used in
everything from the production of kitchen utensils to components for
construction projects.
Producing Different Types of Stainless Steel
When producing stainless steel, several different materials
can be used to achieve different results. These include:
1. Carbon
Stainless steel with high carbon content is much more durable and
longer-lasting than other alloys. This type of stainless steel is popular for
the production of food-grade utensils and cooking implements.
2. Chromium
Stainless steel made with chromium is much more resistant to corrosion, stains,
and tarnishing. One well-known example of this type of stainless in use is the
Gateway Arch in St. Louis.
3. Nickel
Making stainless steel with nickel improves the formability, weldability, and
ductility of the material. This also improves its corrosion-resistant
properties. Nickel is what allows for stainless steel to be such a versatile
alloy.
The Three Primary Types of Stainless Steel
While there are thousands of different types of stainless steel
alloy that can be produced, most alloys will fall into one of the following
three categories.
1.
Austenitic stainless steel
There are
two main characteristics of this type of stainless steel. First, there is a lot
of chromium included during the production of austenitic stainless steel. Items
produced with this material are more corrosion-resistant. Second, these items
tend to be non-magnetic, although they can become magnetic with a cold forming
process. These are the most common types of stainless steel alloys.
2.
Ferritic Stainless Steel
This type
of stainless steel is the second most common type of alloy and is also
magnetic. Items produced with this alloy can be hardened through cold forming
and also tend to be less expensive than other types due to the lower nickel
content.
3.
Martensitic Stainless Steel
This
material is the least common type of stainless steel alloy. Martensitic
stainless steel is used in applications where high tensile strength is needed
or a lot of impact resistance. In many cases, this material is also combined
with a protective polymer coating to improve corrosion-resistance
Different Grades of Stainless Steel Alloys
Within the three primary categories mentioned above, there are
also different grades of stainless steel.
Within the austenitic steel types, there are two main grades –
grade 304 and grade 316. Grade 304 is known for its high tensile strength of
roughly 621 MPa (90 ksi). Grade 316 stainless steel also has a tensile strength
of 579 MPs (84 ksi), making the two grades reasonably similar in this regard.
They also both have a high maximum operating temperature. Grade
304 has a maximum temperature of 870°C (1598°F) and Grade 316 has a maximum of
800°C (1472°F).
When comparing 304 vs. 316 stainless steel, one major difference
is resistance to chlorides such as salt. Grade 316 is more resistant to the
elements, making it a more desirable stainless steel for things like maritime
applications.
Ferritic stainless steel also has two main grades – grade 430
and grade 434. Grade 430 stainless steel isn’t as strong as either of the
austenitic steels but offers better resistance to nitric acid. It is also still
strong enough for most heavy-duty applications.
Grade 434 steel has high
tensile strength and maximum operating temperature and is tougher than the 430
grade, including better pitting resistance.
Martensitic stainless steel has only one grade, grade 420. This
grade offers slightly less resistance to chemicals than the austenitic and
ferritic steels, but it still has good resistance to things such as some alkalis,
mild acids, water, and food compounds. It also has great tensile strength and
impact resistance. This makes it ideal for producing items like cutlery.
Duplex Stainless Steel
Duplex stainless steel is a family of stainless steel that is
made of austenitic steel and ferritic steel in nearly equal proportions. This
type of stainless steel is both stronger and more resistant to corrosion than
your typical types of stainless steel. Duplex stainless steel is also more
ductile than ferritic grades, however, it is not quite as ductile as the
austenitic grades.
One of the main differences in the composition is that duplex
stainless steel is made with a higher chromium content and a lower nickel
content. This not only makes the material stronger, but it also provides cost
benefits. For these reasons, duplex stainless steel is often used in
applications like pipes and risers in offshore oil rigs.
Grades of Duplex Stainless Steel
Just as with the other types of stainless steel, duplex
stainless steel is also broken down into different grades or groups. The
three primary groups are based on the pitting and the corrosion resistance of
the material.
- Standard Duplex Steel
Standard
duplex steel has a PREN range of 28-38. This is the most typical type of duplex
steel used today. It is also sometimes called Grade EN 1.4462 or 2205.
- Super Duplex Steel
With a PREN
range of 38-45, super duplex steel was created for use in the oil & gas
industry and the chemical industry. This grade of steel is stronger and far
more corrosive resistant but is more difficult and more costly to process than
standard duplex steel. This is typically grade EN 1.4410.
- Duplex 2507 Stainless
Steel
Duplex 2507
stainless steel is another form of super duplex steel. This material is made
with 25% chromium, 4% molybdenum, and 7% nickel. This type of stainless steel
is known for its resistance to chloride stress corrosion cracking, high thermal
conductivity, and a low coefficient of thermal expansion.
- Zeron 100 Stainless Steel
Zeron
stainless steel is a super duplex stainless steel that was developed by Rolled
Alloys. It contains 25% chromium, 7% nickel, 3.6% molybdenum, along with copper
and tungsten additions.
- Lean Duplex Grades
Lean duplex
steel has a PREN range of 22-27 and was created for use in less demanding
applications such as general building and construction. While the corrosion
resistance is going to be similar to standard austenitic steel, it has better
strength and mechanical properties.
Applications of Duplex Stainless Steel
Duplex stainless steels are very commonly used in more demanding
applications due to their corrosion resistance properties, high mechanical
properties, and how strong the material is. This makes the material very
popular in a wide variety of industries.
You can find duplex stainless steel used in the oil and gas
industry, chemical engineering industry, water and energy industries,
architecture industry, and in the construction of things like bridges,
seawalls, tunnels, and piers.
18/10 Stainless Steel and Other Types of Stainless Steel
When discussing stainless steel alloys, you’ll typically hear
numbers such as 18/10, 18/8, and 18/0 used. These numbers identify the
percentage of chromium and nickel that are present in a particular alloy.
For example, 18/8 stainless steel is 18% chromium and 8% nickel;
18/0 stainless steel is 18% chromium and contains roughly .75% nickel, and so
on. The chromium-nickel mixture is important to know when deciding on the right
alloy for your needs.
For example, if you’re a professional cook, you may have
specific requirements for the strength, durability, or heat resistance of
various tools and appliances in your kitchen. Understanding the composition of
the alloys used in the production of equipment will help choose stronger,
longer-lasting products for each application.
The Melting Points for Different Grades of Stainless Steel
One of the more important considerations when choosing the right type of stainless steel for
your component or application is the melting point of the specific grade.
One of the reasons stainless steel is chosen over other types of
materials, for example, plastic polymers, is its ability to stand up to
high-temperatures and still retain its strength and durability.
When comparing the melting points of steel alloys, the number is
usually given in a range due to the variations used in the metals used in the
production. That number also relates to the metals’ resistance to stress,
corrosion, cracking, and other attributes.
As a general rule, metals that have been heat-treated have the
following melting points:
- Grade 304: 1400-1450°C (2552-2642°F)
- Grade 316: 1375-1400°C (2507-2552°F)
- Grade 430: 1425-1510°C (2597-2750°F)
- Grade 434: 1426-1510°C (2600-2750°F)
- Grade 410: 1480-1530°C (2696-2786°F)
- Grade 420: 1450-1510°C (2642-2750°F)
It is also important to remember that the maximum use
temperatures recommended are usually lower than the maximum number listed in
the above chart.
Stainless Steel Description
|
201 |
Chromium nickel manganese
steel was developed as a satisfactory alternate for Type 301 for many
applications. |
|
301 |
Chromium nickel
steel capable of attaining high tensile strength and ductility by moderate or
severe cold working. |
|
302 |
General purpose chromium
nicke stainless steel. Its corrosion resistance is superior to that of Type
301. It can be cold worked to high tensile strengths but with slightly lower
ductility than Type 301. |
|
Low carbon chromium
nickel stainless and heat resisting
steel somewhat superior to Type 302 in corrosion
resistance. |
|
|
Very low carbon chromium
nickel steel with general corrosion resistance similar to Type 304 but with
superior resistance to intergranular corrosion following welding or stress
relieving. It is recommended for use in parts which are fabricated by welding
and which cannot be subsequently annealed. |
|
|
A high
corrosion-resistant alloy with low rate of work hardening, designed for extra
deep drawing and spinning. |
|
|
High corrosion-resistant,
chromium nickel grade with carbon limited to .08 to reduce carbon
precipitation during welding. |
|
|
Similar to 309 with
higher resistance to corrosion and oxidation at elevated temperatures. |
|
|
Chromium nickel stainless
and heat resisting steel with superior corrosion resistance to other chromium
nickel steels when exposed to many types of chemical corrodents; superior
creep strength at elevated temperatures. |
|
|
Low carbon chromium
nickel stainless steel with general corrosion resistance similar to Type 316
but with superior resistance to intergranular corrosion following welding or
relieving. It is recommended for use in parts which are fabricated by welding
and cannot be subsequently annealed. |
|
|
Ti stabalized version of
316 with resistance to sensitization. (The formation of grain boundary
chromium carbides at evaluated temperatures.) |
|
|
Similar to 316L but with
additional molybdenum to improve corrosion resistance. |
|
|
Chromium nickel steel
containing titanium.Recommended for parts fabricated by welding which cannot
be subsequently annealed. Also recommended for parts to be used at
temperatures between 800°F and 1850°F. |
|
|
Chromium nickel steel
containing columbium and tantalum which is recommended for parts fabricated
by welding which cannot be subsequently annealed. Also recommended for parts
to be used at temperatures between 800°F and 1850°F. |
|
|
General purpose corrosion
and heat resisting chromium steel. Good corrosion resistance and fair
machining properties. Can be treated to RC35/45. |
|
|
Chromium steel capable of
hardening to a maximum of approximately RC53/58. |
|
|
General purpose grade,
corrosion-resistant, straight chromium grade, non-heat-treatable. |
|
|
430LI |
Similar to straight 430
in corrosion and mechanical properties. The low interstitials provide
improved transverse bending over straight 430. |
|
434 |
Modification of Grade 430
designed for automotive trim and resistance to atmospheric corrosion. |
|
440A |
High carbon grade, high
chromium, capable of being heat treated to a hardness range of RC51/62. |
|
444 |
A low carbon, low
nitrogen, ferritic that provides pitting and crevice corrosion resistance
superior to most ferritic grades. |
|
A2864 |
An iron, nickel chromium
alloy designed for service up to 1300°F where high strength and corrosion
resistance are required. |
|
AM 3508 |
Similar to 17-7PH. alloy
and PH15-7Mo. alloy, but with slightly higher elevated temperature
capability. |
|
CARPENTER |
A highly
corrosion-resistant alloy used in the chemical industry for applications
where corrosion resistance is extremely critical,superior to the general run
of 300 type stainless. |
|
PH15-7Mo®4 |
Similar to 17-7PH. alloy,
but with molybdenum added for higher strength with heat treatment. |
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