The role of carbon (C) in stainless steel
Carbon in austenitic and ferritic stainless steel with interstitial elements present in the
solid solution, is the most effective austenitic stainless steel solid solution strengthening
elements; in high-carbon martensitic stainless steel, there will be eutectic carbide and other
Carbide generation, the hardness and wear resistance is very favorable, suitable for the
production of various types of tools.
In the austenitic stainless steel, the carbon exists in the solid solution, and when C> 0.03%
if the steel is placed in the temperature range of 538 ~ 815 ℃, the carbon may precipitate in
the form of carbide at the grain boundary, (Cr) area (the most prone to welding when the
phenomenon), this phenomenon becomes sensible precipitation (Susceptibility), so that
stainless steel increased the sensitivity of grain boundary corrosion; ferritic stainless steel
will produce chromium carbide Causing grain boundary corrosion phenomenon. The current technical
means to avoid sensitization and to prevent the presence of carbon monoxide in the austenitic
stainless steel by adding the carbon element content below 0.03% or by adding titanium (Ti) or Nb
(niobium) elements to form stable carbides Chromium (Cr) area (poor - Cr Zone) and to avoid
welding when the knife-like corrosion.
The role of chromium (Cr) in stainless steel
Chromium is an indispensable element in stainless steel, corrosion resistance and oxidation
resistance of stainless steel are increased with the increase of Cr content; because Cr in the
stainless steel surface to form a thin oxide film, hinder or prevent further oxidation of
stainless steel And corrosion, in the oxidation of this layer of film has been strengthened.
In the Fe-Cr system, at all temperatures when the Cr content of more than 12%, are reflected in
the ferrite; but at high temperatures may produce austenitic tissue, the reason is because it
contains a certain amount of C and N elements reason. Glossary ---- gap phase: When the
non-metallic atomic radius and metal atomic radius ratio of less than 0.59, the formation of a
simple lattice of the gap compound, known as the gap phase; when the ratio is greater than 0.59,
the formation of complex structure of the gap compound The In the case of two groups of elements
A and B, it is possible to form a crystal structure which is different from that of A and B in
addition to the solid solution (end-solid solution) with A or B as a matrix The new phase Air
quenching effect: steel under certain conditions to obtain hardened hardened layer (martensite)
depth. It is one of the important indexes to measure the quenching capacity of different steel
grades. The hardenability depends mainly on the critical cooling rate, while the critical
cooling rate depends mainly on the stability of the undercooled austenite.
The role of nickel (Ni) in stainless steel
Nickel prices have been rising recently, affecting the volatility of stainless steel prices.
But what impact does nickel have on stainless steel? Nickel is an important alloying element in
stainless steel after chromium. In order to resist the corrosion of the reducing acid and the
alkali medium, only chromium is contained in the steel, and chromium must be added to the nickel.
Nickel promotes the stability of stainless steel passivation film and improves the thermodynamic
stability of stainless steel. Therefore, stainless steel in the coexistence of chromium and
nickel, can significantly strengthen the stainless steel stainless steel and corrosion resistance
. Nickel is useful for high temperature antioxidant properties of stainless steel, but is
detrimental to high temperature resistance to vulcanization. Because nickel and sulfur action is
easy to form low melting point sulfide. While the formation of low melting point sulfide will
significantly reduce the hot workability of steel. The combination of nickel and chromium can
significantly improve the corrosion resistance of austenitic stainless steel in caustic media
(such as NaOH), and nickel also improves the resistance to corrosion resistance of 18-8 stainless
steel. Although the PRE value (Cr + 3.3Mo + 16N, the greater the value, the stronger the pitting
resistance, the stronger the crevice corrosion resistance) in the pitting resistance and crevice
corrosion resistance is not in the role of nickel, Molybdenum general chromium nickel austenitic
stainless steel, the role of nickel is still useful. Nickel is a stable element formed by
austenite. If the Ni content is about 8%, the Fe-Ni system has austenite structure at room
temperature, has good formability, better weldability, and excellent toughness. Ni has improved
the high temperature performance, especially the strength, the metallurgical stability and the
stability of the protective oxide film. In ferritic stainless steel, adding Ni can improve
toughness and bending properties, welding performance and corrosion resistance. In precipitation
hardening stainless steel, Ni is an important element. In the dual phase steel, by adjusting the
Cr and Ni content changes, can change the percentage of ferrite, Ni can also improve the
dual-phase steel resistance to comprehensive corrosion and stress corrosion resistance. Ni in
austenitic stainless steel will reduce its melting point, an average increase of 1% nickel
content, can be reduced by 4.4 ℃. But the thermal processability, low temperature plasticity and
toughness can be improved, its molding performance (deep drawing performance, work hardening
performance is reduced, but also increase in the sulfuric acid in the passivation (passivation)
Nickel can significantly improve the plasticity and toughness of stainless steel, can be brittle
transition temperature of some stainless steel brittle temperature down.Nickel can improve the
cold forming of some stainless steel and weldability, reduce the austenitic stainless steel cold
work hardening tendency. Nickel can improve the cold formability and weldability of some
stainless steels and reduce the cold work hardening tendency of austenitic stainless steels.
In addition, the other side of Ni is an irreplaceable "king" in its stainless steel and nickel
alloy index; A kind of "industrial interest chain" whose financial properties are far beyond its
real value, which is enough for the stainless steel and nickel metal practitioners to "HIGH" to
the extreme "ecstasy".
The role of manganese (Mn) in stainless steel
Manganese element can form an infinite solid solution, has a strong stability of the structure
of austenitic stainless steel structure; and for ferritic and austenitic stainless steel have a
strong solid solution strengthening effect, improve the hardness and strength of stainless steel.
Mn is an important alloying element produced by stainless steel. As a deoxidizing element in CrNi
stainless steel production, 1.5% Mn is added as an important alloying element in Cr-Mn-Ni-N and
Cr-Mn-N stainless steel. Add 6-20%. Compared with Cr-Ni austenitic stainless steel, the biggest
difference between Cr-Mn-Ni-N austenitic stainless steel is the addition of alloying elements Mn
and N, resulting in a series of performance changes.
In austenite, manganese is generally in the form of alloy, and the content of less than 2%;
this content for the stainless steel structure will not cause significant impact. But the
manganese element is treated as a residual element in the stainless steel production process as a
deoxidizing effect. Manganese at 120 ℃ above the temperature, with the temperature rise will
produce a certain degree of evaporation.
1, Mn in the beneficial effects of stainless steel
(1) N solubility in stainless steel Formula: N = 0.021 (Cr + 0.9Mn) -0.204 wt%. Therefore,
in order to increase the solubility of N, the Mn element is added in large amounts. In the
nickel-austenitic stainless steel, Mn is a very important alloying element, its main role is to
improve the solubility of ammonia in steel and improve the strength and toughness of steel, is an
essential element of nickel austenite. (2) Mn is a relatively weak austenite forming element,
but has a strong effect of stabilizing austenite. Cr-Ni austenitic stainless steel, with the
increase in Mn content increased strength. In the Ni-free Cr-Mn-N austenitic stainless steel at
low temperatures will appear ductile-brittle transition phenomenon.
(3) Another useful effect of Mn in stainless steel is to form MnS to inhibit the harmful
effects of sulfur in steel, improve the heat sensitivity of steel, add more than 2% manganese in
the welding material, improve the resistance of austenitic stainless steel Thermal cracking
sensitivity.
2, the adverse effects of Mn
(1) Mn on the stainless steel corrosion resistance of stainless steel, are basically negative.
With the increase in the amount of manganese, steel pitting corrosion resistance, crevice
corrosion resistance decreased. This is the formation of MnS with manganese and sulfur, or with
the increase in the amount of manganese in the steel, MnS in the reduction of chromium caused by
MnS inclusions dissolved in the corrosive medium, often become pitting, crevice corrosion source.
Experiments show that when the amount of manganese in 18-8 stainless steel is reduced to about
0.1%, the pitting resistance of this steel will reach a level of 316 with 2% Mo.
(2) Mn also promotes the precipitation of σ-fragile phase in stainless steel, and reduces
the plasticity and toughness of steel, which adversely affects the application of manganese in
high chromium and molybdenum stainless steel. The corrosion resistance of Ni, Ni, Mo, N and other
elements is mainly due to the effect of Mn on Ni, Ni and Ni-free austenitic stainless steel.
At present, the development of mature steel products are mainly 200 series and Arlnco's Nitronic
series. There are some steel is also widely studied.
The role of phosphorus (P) in stainless steel
The presence of phosphorus (P) in stainless steel does not form carbides with carbon elements,
but it is liable to cause severe segregation in stainless steels. Generally, phosphorus is
considered to be harmful in the smelting and subsequent processing of stainless steels, especially
during welding , Will produce hot cracking weld phenomenon. In the austenitic stainless steel,
phosphorus is allowed to exist in the content of about 0.03% to 0.035%; In addition, if the
stainless steel due to welding in the weld at the ferrite, the phosphorus element is allowed to
the maximum content of about 0.03%. Phosphorus has a strong effect on improving the strength and
cold hardening of stainless steel, but it increases the brittleness of stainless steel
(especially low temperature brittleness). If the phosphorus and copper elements, can greatly
improve the low alloy steel resistance to atmospheric corrosion.
Glossary: segregation phenomenon: the composition of the alloy elements in the distribution of
the phenomenon of uneven distribution is called segregation. Low-temperature brittleness: The
impact absorption of the material decreases with decreasing temperature. When the test
temperature is lower than Tk (ductile critical transition temperature), the impact absorption
power decreases significantly, and the material changes from ductile state to brittle state.
This phenomenon is called Low temperature brittleness. The low temperature brittleness of the
metal is due to the increase in the yield strength of the metal as the temperature decreases.
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