How Do Alloying Elements Affect the Structure and Properties of Stainless Steel?

Classification of stainless steel

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 1

1. According to chemical composition, it can be divided into: chromium stainless steel, chromium nickel stainless steel, chromium manganese stainless steel, chromium nickel molybdenum stainless steel, ultra-low carbon stainless steel, high molybdenum stainless steel, high-purity stainless steel, etc.

2. According to the metallographic structure, it can be divided into: martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, austenitic ferritic stainless steel, etc.

3. According to the performance characteristics and uses of steel: such as nitric acid resistant (nitric acid grade) stainless steel, sulfuric acid resistant stainless steel, pitting corrosion resistant stainless steel, stress resistant stainless steel, high-strength stainless steel, etc.

4. According to the functional characteristics of steel: such as low-temperature stainless steel, non-magnetic stainless steel, free cutting stainless steel, superplastic stainless steel, etc.

The development process of stainless steel grades is shown in the figure below:

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 2

Effect of alloying elements on Microstructure and properties of stainless steel

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 3

Note:  – strong effect, ⚪—— Moderate action, ▲ – weak action

Ways to improve the corrosion resistance of stainless steel

(1) Make the stainless steel have a stable anodic polarization curve of the passivation zone for the specific medium.

(2) Increase the electrode potential of stainless steel substrate and reduce the electromotive force of corrosive galvanic cell.

(3) Make the steel have single-phase structure and reduce the number of micro batteries.

(4) A stable protective film is formed on the surface of steel, such as adding silicon, aluminum, chromium, etc. in steel, which can form a dense protective film in many corrosion and oxidation occasions and improve the corrosion resistance of steel.

(5) Reducing or eliminating various uneven phenomena in steel is also an important measure to improve the corrosion resistance of steel.

Adding alloy elements to steel is the main method to improve the corrosion resistance.

Adding different alloy elements can work in one way or several ways at the same time to improve the corrosion resistance of steel.

Effect of alloying elements on polarization and electrode potential of iron

The type and content of alloy elements directly affect the corrosion resistance of stainless steel. The role of alloy elements is first of all to affect the polarization performance of iron and electrode potential.

1. Effect of alloy elements on polarization properties of iron

The anodic polarization process of commonly used metals such as Fe, Cr, Ni and Ti has a unique polarization form.

After the anode passage, the anode potential increases, and the anode current (corrosion rate) changes with it, almost with the same law.

The typical form of polarization curve is shown in the figure below.

With the increase of anodic polarization potential, the corrosion current does not decrease uniformly, but first increases, then decreases to the minimum, and keeps this current through a certain potential increase stage, and then increases.

This kind of polarization curve is called anodic polarization curve with activation and passivation transition, and this kind of curve is divided into three regions: activation region (A), passivation region (B), and over passivation region (T).

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 4

Fig. anodic polarization curve of activated and passivated transition metals

Polarization is of great significance to improve the corrosion resistance of metals.

Factors that enhance anodic polarization or cathodic polarization can improve the corrosion resistance of metals;

The factors of depolarization or depolarization will reduce the corrosion resistance of metals.

Different alloy elements have different effects on the polarization properties of iron.

The elements that expand the passivation zone, that is, the elements that reduce the potential of ECP and P zone and increase the potential of Er point, all improve the corrosion resistance of steel;

All elements that enhance the passivation performance, that is, the position of ICP and I1 points moves to the left, will reduce the corrosion current and improve the corrosion resistance.

All elements that increase the Er point potential have the tendency to reduce the pitting corrosion, because when the Er point potential is low, when the potential fluctuates near the over passivation potential, it is easy to cause local breakdown of the passivation film and produce pitting corrosion.

Among the commonly used alloy elements in steel, Cr can strongly improve the passivation performance of pure iron, increase the potential of Ecp, Ep and Er points, and shift the position of Icp and I1 points to the left.

Cr is the most effective element to improve the corrosion resistance of iron.

Alloy elements Ni, Si, Mo, etc. can also expand the passivation zone to varying degrees and enhance the passivation performance.

Mo can not only enhance the passivation performance of iron, but also increase the potential of Er point, so as to improve the pitting corrosion resistance of iron.

2. Influence on electrode potential of iron

Generally, the electrode potential of metal solid solution is always lower than that of other compounds, so in the process of corrosion, metal solid solution is always corroded as an anode.

The corrosion resistance can be improved by increasing the electrode potential of iron.

Research shows that when Cr is added to iron to form a solid solution, the electrode potential of the iron solid solution can be significantly increased, as shown in the figure below.

The corrosion resistance of the material can be significantly improved by increasing the electrode potential of the material.

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 5

Fig. effect of chromium on electrode potential of Fe Cr alloy

Due to the good effect of chromium on iron passivation and electrode potential, chromium has become the main alloying element of various stainless steels.

Effect of alloying elements on corrosion resistance and matrix structure of stainless steel

The matrix structure of stainless steel is the guarantee of obtaining the required mechanical properties and process properties, and more importantly, the guarantee of good corrosion resistance.

Single phase ferritic steel and single phase austenitic steel are two kinds of stainless steels with better corrosion resistance.

The influence of alloy elements on the matrix structure first depends on whether the alloy elements are ferrite (α) stabilizer or austenite (γ) stabilizer.

A the element of stabilizer is dominant, and single phase can be obtained α stainless steel;

Otherwise, single-phase γ stainless steel is obtained.

1. Effect of alloy elements on corrosion resistance of stainless steel

1. Chromium

Chromium is the main element that determines the corrosion resistance of stainless steel.

When the chromium content (atomic ratio) reaches 1/8 and 2/8, the electrode potential of iron jumps, and the corrosion resistance is also improved.

Chromium is a stabilizing element.

Chromium oxide is relatively dense, which can form a corrosion-resistant protective film.

2. Carbon and nitrogen

Carbon can strongly stabilize austenite, and its ability to stabilize austenite is about 30 times that of Ni;

At the same time, it is the main element of stainless steel strengthening;

Carbon and chromium can form a series of carbides, which seriously affects the corrosion resistance of stainless steel;

At the same time, carbon will deteriorate the processing and welding properties of stainless steel and make ferritic stainless steel brittle.

Therefore, the application and control of carbon is an important work in the production and development of stainless steel.

The effect of the combination of carbon and chromium on the formation of stainless steel structure is shown in the figure below.

The figure shows that when the carbon content is low and the chromium content is high, ferrite structure will be obtained;

When the carbon content is high and the chromium content is low, martensite structure will be obtained.

In chromium stainless steel, when the chromium content is below 17%, with the increase of carbon content, stainless steel with martensite matrix can be obtained.

Ferritic stainless steel can be obtained when the carbon content is low and the chromium content is 13%.

When the chromium content increases from 13% to 27%, due to the increase of chromium content, the ability to stabilize ferrite increases, and the carbon content in steel increases correspondingly (from 0.05% to 0.2%), which can still maintain the ferrite matrix.

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 6

Fig. effect of carbon and chromium on Microstructure of stainless steel

3. Nickel

Nickel is one of the three important elements in stainless steel.

Nickel can improve the corrosion resistance of stainless steel;

Nickel or γ phase stabilizing element is the main element to obtain single-phase austenite and promote the formation of austenite in stainless steel.

Nickel can effectively reduce Ms point and keep austenite at a very low temperature (-50 ℃) without martensitic transformation.

The increase of nickel content will reduce the solubility of C and N in austenitic steel, thus increasing the tendency of carbon and nitrogen compounds to desolvate and precipitate.

With the increase of nickel content, the critical carbon content of intergranular corrosion decreases, that is, the sensitivity of steel to intergranular corrosion increases.

The effect of nickel on the pitting corrosion resistance and crevice corrosion resistance of austenitic stainless steel is not significant.

In addition, nickel can also improve the high-temperature oxidation resistance of austenitic stainless steel, which is mainly related to the improvement of the composition, structure and properties of chromium oxide film by nickel, but the existence of nickel will reduce the high-temperature vulcanization resistance of steel.

4. Manganese

Manganese is a relatively weak austenite forming element, but it has a strong role in stabilizing austenite structure.

Manganese partially replaces Ni in austenitic stainless steel, and 2% Mn is equivalent to 1% Ni.

Manganese can also improve the corrosion resistance of chromium stainless steel in organic acids such as acetic acid, formic acid and glycolic acid, and is more effective than nickel.

When the Cr content in steel is more than 14%, a single austenite structure cannot be obtained only by adding Mn.

Because stainless steel has better corrosion resistance when the Cr content is greater than 17%, the austenitic stainless steel with Mn instead of Ni that has been used in industry is mainly Fe-Cr-Mn-Ni-N steel, such as 12Cr18Mn9Ni5N, while the amount of nickel free Fe-Cr-Mn-N austenitic stainless steel is less.

5. Nitrogen

Nitrogen was mainly used in Cr-Mn-N and Cr-Mn-Ni-N austenitic stainless steels in the early stage to save Ni.

In recent years, nitrogen has become an important alloying element of Cr Ni austenitic stainless steel.

Adding nitrogen to austenitic stainless steel can stabilize the austenitic structure, improve the strength and corrosion resistance, especially the local corrosion resistance, such as intergranular corrosion, pitting corrosion and crevice corrosion.

In ordinary low-carbon and ultra-low-carbon austenitic stainless steel, the intergranular corrosion resistance can be improved.

The reason is that nitrogen affects the precipitation process of chromium carbide during sensitization treatment and increases the chromium concentration at the grain boundary.

In high-purity austenitic stainless steel, there is no precipitation of chromium carbide.

At this time, the role of nitrogen is:

First, nitrogen increases the stability of passive film and reduces the average corrosion rate;

Second, although chromium nitride precipitates in steel with high nitrogen content, the precipitation rate of chromium nitride is very slow.

Sensitization treatment will not cause intergranular chromium deficiency, and has little effect on intergranular corrosion.

Nitrogen can inhibit the segregation of phosphorus at the grain boundary and improve the intergranular corrosion resistance of steel.

At present, nitrogen-containing austenitic stainless steel is mainly corrosion-resistant and has high strength;

It can be divided into three types: nitrogen control type, medium nitrogen type and high nitrogen type.

Nitrogen control type is to add 0.05%~0.10%N to ultra-low carbon (C ≤ 0.02%~0.03%) Cr Ni austenitic stainless steel to improve the strength of the steel and optimize the intergranular corrosion resistance and stress corrosion resistance of the steel;

Medium nitrogen type contains 0.10%~0.50%N, which is smelted and poured under normal atmospheric pressure;

The nitrogen content of high nitrogen type is more than 0.40%.

It is generally smelted and poured under the condition of increasing pressure.

It is mainly used in the solid solution state or semi cold working state. It has both high strength and corrosion resistance.

At present, high nitrogen austenitic steel with nitrogen content of 0.8%~1.0% has been applied in practice and started industrial production.

6. Titanium, niobium, molybdenum and rare earth elements

Titanium and niobium are strong carbide forming elements, which can preferentially form carbides with carbon than chromium to prevent intergranular corrosion and improve corrosion resistance.

The addition of titanium and niobium must maintain a certain proportion with the carbon in the steel.

Molybdenum can improve the passivation ability of stainless steel and expand the range of passivation media, such as in hot sulfuric acid, dilute hydrochloric acid, phosphoric acid and organic acids.

The passivation film containing molybdenum has high stability in many media and is not easy to dissolve.

It can prevent cl- from damaging the passive film, so molybdenum containing stainless steel has the ability to resist pitting corrosion.

When rare earth elements such as Ce, La, Y are added to stainless steel, they can be slightly dissolved in the matrix, purify the grain boundary, modify the inclusions, homogenize the structure, reduce the precipitation of precipitates and the segregation at the grain boundary, so as to improve the corrosion resistance and mechanical properties of the steel.

2. Effect of alloy elements on the microstructure of stainless steel

The influence of alloy elements on the matrix structure of stainless steel can be divided into two categories:

Ferrite forming elements, such as chromium, platinum, silicon, titanium, niobium and so on;

Austenite forming elements, such as carbon, nitrogen, nickel, manganese, copper, etc.

When these two elements with different functions are added to steel at the same time, the microstructure of stainless steel depends on their comprehensive effects.

For simple treatment, the effect of ferrite forming elements is converted into the effect of chromium, which is called chromium equivalent [Cr], while the effect of austenite forming elements into nickel is called nickel equivalent [Ni].

According to the chromium equivalent [Cr] and nickel equivalent [Ni], a diagram is made to represent the actual composition of the steel and the obtained structural state, as shown in the following figure.

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 7

Fig. stainless steel structure diagram

It can be seen from the figure that 12Cr18Ni9 steel is in phase a zone and is austenitic stainless steel;

Cr28 stainless steel is ferritic stainless steel, which is located in the ferritic phase zone;

30Cr13 stainless steel is in the martensitic phase zone, which is martensitic stainless steel.

In order to obtain single-phase austenite structure, these two kinds of alloy elements must reach a certain balance, otherwise a certain amount of ferrite structure will appear in steel, which will become multiphase structure.

Effect of alloy composition and microstructure on mechanical properties of stainless steel

1. Strengthening mechanism of stainless steel

The strengthening mechanism of stainless steel widely adopts solid solution strengthening, in addition to phase transformation strengthening, second phase strengthening, grain refinement strengthening, precipitation strengthening and substructure strengthening.

The following figure shows the contribution of various strengthening mechanisms to yield strength in 8%~10%Ni austenitic stainless steel.

Surface in the figure: chromium, silicon and carbon provide solid solution strengthening of the matrix, which increases the yield stress of the austenitic matrix by several times;

The second is the existence of the second phase of α ferrite, the refinement of grain size and the precipitation of precipitates, which greatly strengthen austenite.

The figure shows that in austenitic stainless steel, solid solution strengthening is an important strengthening mechanism, and grain refinement contributes the most to the strength.

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 8

Fig. factors affecting the strength of austenitic stainless steel

2. Strength and plasticity of various stainless steels

The properties of various stainless steels are also different due to different compositions and organizational states.

The comparison of strength and plasticity of various stainless steels is shown in the figure below.

How Do Alloying Elements Affect the Structure and Properties of Stainless Steel? 9

Fig.  strength and plasticity comparison of various stainless steels and pure iron

Among all stainless steels, austenitic stainless steel has the best plasticity and precipitation hardening stainless steel has the highest strength.

Martensitic stainless steel has good comprehensive mechanical properties, that is, it has high strength and certain ductility.

Ferritic + austenitic duplex stainless steel has higher strength and better ductility;

Ferritic stainless steel and austenitic stainless steel have similar strength properties, but the ductility of the latter is much higher than other types of stainless steel. (the curve of pure iron is also listed in the figure for comparison).

Effect of corrosive medium on corrosion resistance of stainless steel

The corrosion resistance of metal is not only related to the metal material itself, but also to the type, concentration, temperature and pressure of corrosive medium and other corrosive environment conditions.

In practical application, the oxidation capacity of corrosive medium has the greatest impact, so the steel grade of stainless steel should be correctly selected according to the characteristics of working medium.

In weak corrosive media such as atmosphere, water and steam, the corrosion resistance of stainless steel can be guaranteed as long as the Cr content of solid solution of stainless steel matrix is greater than 13%.

Such as water compressor valves, steam generator turbine blades, steam pipes and other components.

In oxidizing media, such as nitric acid, NO3- of nitric acid has strong oxidizability, and the oxide film on the surface of stainless steel is easy to form with short passivation time.

However, H+ in acid is a cathodic depolarizer.

With the increase of H+ concentration, the depolarization of cathode is strengthened, and the chromium content required for passivation is also increased.

Therefore, only the oxide film containing high chromium has good stability in nitric acid.

In boiling nitric acid, 12Cr13 stainless steel is not corrosion resistant, and Cr17 and Cr30 steel with chromium content of 17% – 30% are corrosion resistant in nitric acid with concentration of 0% – 65%.

In non oxidizing media, such as dilute sulfuric acid, hydrochloric acid and organic acid, the oxygen content of such corrosive media is low, and the passivation time needs to be extended.

When the oxygen content in the medium is low to a certain extent, stainless steel cannot be passivated.

For example, in dilute sulfuric acid, because SO42- in the medium is not an oxidant, and the oxygen content dissolved in the medium is relatively low, it basically has no ability to passivate steel, and the corrosion rate of chromium stainless steel is even faster than that of carbon steel.

Therefore, general Cr stainless steel or Cr Ni stainless steel is difficult to achieve the passivation state, so it is not corrosion resistant.

Therefore, stainless steel working in this kind of medium needs to add elements to improve the passivation ability of steel, such as molybdenum, copper and other elements.

Hydrochloric acid is also a non oxidizing acid, in which stainless steel is not corrosion resistant.

Generally, Ni Mo alloy is required to form a stable protective film on the alloy surface to ensure that the alloy is not corroded.

In strong organic acids, due to the low oxygen content in the medium and the presence of H+, chromium and chromium nickel stainless steel are generally difficult to passivate.

Mo, Cu, Mn and other elements must be added to the steel to improve the passivation ability of stainless steel.

Therefore, Cr Mn stainless steel is better.

On this basis, a certain amount of Mo and Cu are added to make the steel easy to passivate and corrosion resistant.

In the medium containing Cl-,Cl- is easy to destroy the oxide film on the surface of stainless steel, pass through the oxide film and act with the steel surface, causing pitting corrosion of steel.

Therefore, seawater is very corrosive to stainless steel.

In fact, no stainless steel can resist the corrosion of all media.

Therefore, stainless steel must be selected according to the specific corrosion environment and the characteristics of all kinds of stainless steel.

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