The
so-called duplex stainless steel is that the ferrite phase and the austenite
phase in its solid-quenched structure each account for half, and generally the
minimum phase content must reach 30%.
Due
to the characteristics of the two-phase structure, DSS has the advantages of
both ferritic stainless steel and austenitic stainless steel through correct
control of chemical composition and heat treatment process.
Compared
with austenitic stainless steel, the advantages of duplex stainless steel are as
follows:
(1)
The yield strength is more than twice that of ordinary austenitic stainless
steel, and it has sufficient plastic toughness for forming. The wall thickness
of storage tanks or pressure vessels made of duplex stainless steel is 30-50%
less than that of commonly used austenite, which is beneficial to reduce
costs.
(2)
It has excellent resistance to stress corrosion cracking. Even the duplex
stainless steel with the lowest alloy content has higher resistance to stress
corrosion cracking than austenitic stainless steel, especially in an environment
containing chloride ions. Stress corrosion is a prominent problem that is
difficult to solve for ordinary austenitic stainless steel.
(3)
The corrosion resistance of 2205 duplex stainless steel, which is most commonly
used in many media, is better than that of ordinary 316L austenitic stainless
steel, while super duplex stainless steel has extremely high corrosion
resistance. In some media, such as acetic acid, Formic acid can even replace
high-alloy austenitic stainless steels and even corrosion-resistant
alloys.
(4)
It has good local corrosion resistance. Compared with austenitic stainless steel
with the same alloy content, its wear corrosion resistance and fatigue corrosion
resistance are better than austenitic stainless steel.
(5)
The coefficient of linear expansion is lower than that of austenitic stainless
steel, which is close to
that of carbon steel. It is suitable for connection with carbon steel and has
important engineering significance, such as the production of composite plates
or linings.
(6)
Whether under dynamic or static load conditions, it has a higher energy
absorption capacity than austenitic stainless steel, which is suitable for
structural parts to cope with sudden accidents such as collisions and
explosions. Duplex stainless steel has obvious advantages and has practical
application value. .
Compared
with austenitic stainless steel, the disadvantages of duplex stainless steel are
as follows:
(1)
The universality and versatility of the application are not as good as the
austenitic stainless steel, for example, the use temperature must be controlled
below 250 degrees Celsius.
(2)
Its plastic toughness is lower than that of austenitic stainless steel, and its
cold and hot processing technology and forming performance are not as good as
austenitic stainless steel.
(3)
There is a medium-temperature brittle zone, and the process system of heat
treatment and welding needs to be strictly controlled to avoid the appearance of
harmful phases and damage performance.
Compared
with ferritic stainless steel, the advantages of duplex stainless steel are as
follows:
(1)
The comprehensive mechanical properties are better than ferritic stainless
steel, especially the plastic toughness, which is not as sensitive to
brittleness as ferritic stainless steel.
(2)
Except for stress corrosion resistance, other local corrosion resistance is
better than ferritic stainless steel.
(3)
Cold working process performance and cold forming performance are far better
than ferritic stainless steel.
(4)
The welding performance is also much better than that of ferritic stainless
steel. Generally, preheating is not required before welding and no heat
treatment is required after welding.
(5)
The application range is wider than ferritic stainless steel.
Compared
with ferritic stainless steel, the disadvantages of duplex stainless steel are
as follows:
The
content of alloying elements is high and the price is relatively high.
Generally, ferrite does not contain nickel.
In
summary, it can be seen in a general way that DSS has an overview of its use
performance and process performance. It has won the favor of users with its
superior mechanics and corrosion resistance. It has become an excellent
resistance to both weight and investment. Eclipse engineering
materials.
304
steel will also have magnetism. Whether it is magnetism or not can not judge the
quality of stainless steel.
People
often think that magnets absorb stainless steel materials to verify their
quality and authenticity. They are not attractive and non-magnetic. They are
considered to be good and genuine; if they are magnetic, they are considered to
be counterfeit products. In fact, this is an extremely one-sided and impractical
method of identifying errors.
There
are many types of stainless steel, which can be divided into several categories
according to the organizational structure at room temperature:
1.
Austenite type: such as 304, 321, 316, 310, etc.;
2.
Martensite or ferrite type: such as 430, 420, 410, etc.;
Austenite
is non-magnetic or weakly magnetic, and martensite or ferrite is
magnetic.
The
stainless steel usually used as decorative tube sheet is mostly austenitic 304
material. Generally speaking, it is non-magnetic or weak magnetic. However, due
to chemical composition fluctuations or different processing conditions caused
by smelting, magnetism may also appear, but this cannot be considered as What is
the reason for counterfeit or unqualified?
As
mentioned above, austenite is non-magnetic or weakly magnetic, while martensite
or ferrite is magnetic. Due to component segregation or improper heat treatment
during smelting, a small amount of martensite or ferrite in austenitic 304
stainless steel will be caused. Body tissue. In this way, 304 stainless steel
will have weak magnetic properties.
In
addition, after 304 stainless steel is cold worked, the structure will also be
transformed into martensite. The greater the degree of cold working deformation,
the more martensite transformation and the greater the magnetic properties of
the steel. Like a batch of steel belts, Φ76 pipes are produced without obvious
magnetic induction, and Φ9.5 pipes are produced. Because the bending deformation
is larger, the magnetic induction is more obvious. The deformation of the square
rectangular tube is larger than that of the round tube, especially the corner
part, the deformation is more intense and the magnetism is more
obvious.
In
order to completely eliminate the magnetic properties of 304 steel caused by the
above-mentioned reasons, the austenite structure can be restored and stabilized
by high-temperature solution treatment, thereby eliminating the magnetic
properties.
In
particular, the magnetism of 304 stainless steel caused by the above reasons is
not at the same level as the magnetism of other materials, such as 430 and
carbon steel. That is to say, the magnetism of 304 steel always shows weak
magnetism.
This
tells us that if the stainless steel is weakly magnetic or not magnetic at all,
it should be judged as 304 or 316 material; if it is the same as carbon steel,
it shows strong magnetism, because it is judged as not 304 material.
