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All kinds of elements in the role of stainless steel pipe
Jun 27, 2018

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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