Austenitic stainless steel VS martensitic stainless steel

In mechanical design, we often use austenitic stainless steel and martensitic stainless steel, because they have good physical and mechanical properties.
For example, the commonly used austenitic stainless steels AISI 303 and AISI 304 have an elastic modulus of about 200GPa and a yield strength of 190Mpa-230Mpa.
The modulus of elasticity of commonly used martensitic stainless steels AISI 420 and aisi440c is 215gpa. The yield strength of 420 after quenching and tempering can reach 345Mpa-1420Mpa, and that of 440 can even reach 1900Mpa.

Quenching is the process that the workpiece is heated to 30-50 ℃ above the critical austenitizing temperature, then taken out and cooled rapidly in water or oil. In the past, ironmaking, sickle making, machete making, etc. all used quenching to make the tool hard and not easy to break (tempering is required).

For the convenience of understanding and memory, quenching can be understood as dipping, that is, dipping the red metal components into the water, just like dipping in chili sauce, dipping in metal, which tastes a bit heavy.

Tempering is the process of reheating the quenched workpiece to below 727 ℃, taking it out after heat preservation and cooling it in air, oil or water. The word “Hui” reflects the meaning of “again”. This is again after quenching. Generally, tempering is required after quenching to eliminate internal stress and stabilize the structure.

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Martensitic stainless steel system (Image from nickelinstitute.org)

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Austenitic stainless steel system

Difference between austenitic stainless steel and martensitic stainless steel?

As we know, austenitic stainless steel has no magnetism and good corrosion resistance, such as 303304 and 316202 mentioned just now.
Martensitic stainless steel has magnetism, but its corrosion resistance is not as good as austenite, such as 420, 440, 410, 403.
So the question is, what is austenitic stainless steel? What is martensitic stainless steel? Why are the two different in magnetism and corrosion resistance? What is the difference in application?
These questions often flash in my mind. Every time I look through them, but after a period of time, I can’t remember the difference between them. I often reverse the properties of martensite and austenite. Do you have them?
So these two days, I combed the difference between the two. Today, I’d like to share with you. If something is wrong, you are welcome to point out and make progress together.
Since it is austenitic stainless steel and martensitic stainless steel, first of all, what is austenite and what is martensite?
I think I have to start with pure iron.
Because both martensite and austenite are essentially formed on the basis of pure iron by adding different concentrations of carbon at a certain temperature.
We know that when pure iron is heated to 1538 ℃ above its melting point, it becomes a liquid.
When pure iron starts to cool in liquid state, it will crystallize into crystals with different structures in different temperature ranges.
Crystallization means that a liquid becomes a solid.
A crystal is an object whose atoms are regularly arranged in space.
As for crystal, there are several concepts that need to be explained.
In order to make it easier to understand, I use a metaphor to think of atoms as apples. Now we have to send a batch of apples to our customers. Instead of throwing apples on the truck, we have to pack them first, and each box of apples is placed according to a certain rule. A box of apples is a crystal cell.
After several boxes of apples are filled into a car, they are called grains. Cars of different sizes can hold different numbers of boxes to form grains of different sizes. When all cars are filled and transported to customers, they form crystals.
So the crystal is made up of grains, which are made up of boxes of apples piled on the car, and the cell is made up of atoms.
For example, from the melting point to 1394 ℃, iron crystallizes into a body centered cubic structure, called δ- Fe, between 1394 and 912 ℃, crystallizes into a face centered cubic structure, called γ- When the temperature drops below 912 ℃, Fe has a body centered cubic structure, which is called α- Fe.

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Crystal structure: body center, face center, close packed hexagonal

We know that water can dissolve sugar, salt and other soluble substances, which is called liquid solubility.
Similarly, iron in the above three temperature ranges, δ- Fe, γ- Fe, α- Fe can also dissolve carbon, but its ability to dissolve carbon is different. This is called solid solution.
Carbon dissolves in water α- Fe is called ferrite, ferrite = f, or it keeps the body centered cubic structure, and carbon is dissolved in carbon γ- Fe is called austenite = Au, it still has face centered cubic structure, austenite has good plasticity and is easy to deform.
But because γ- Fe atom gap ratio α- Fe is larger, so it can dissolve more carbon than iron α- Fe is large.
The maximum dissolution of carbon in austenite is 2.11%, and that in ferrite is 0.0218%.
What happens if the mass fraction of carbon exceeds the solubility limit of both?
The compound Fe3C is called cementite, and its carbon content can reach 6.69%.
Well, here we have the concept of austenite.
However, the austenite mentioned above is between 912-1394 ℃ at high temperature. If it is below 912 ℃, γ- I’ll tell you α- Because of the transformation of Fe, the single austenite does not exist.
When the temperature is lower than 727 ℃, austenite will mix with other structures to form a new structure, while the stainless steel we usually use is mostly at room temperature.
At room temperature, the structure formed by different concentration of carbon dissolved in iron is different.

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Iron carbon phase diagram

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Iron carbon phase diagram microstructure

For example, when the carbon content is less than 0.0218%, the microstructure formed at room temperature is ferrite.
When the carbon content is 0.77%, the microstructure formed at room temperature is a mixture of ferrite and cementite, i.e. pearlite, expressed by P.
When the carbon content is 4.3%, the microstructure at room temperature is a mixture of austenite and cementite, namely ledeburite, which is expressed by LD.
But there is no separate austenite.

Where does austenitic stainless steel come from?

At this point, we have to talk about the transformation process of heating and cooling of carbon steel.
Carbon steel is an alloy with iron and carbon as the main components. Iron carbon alloy with carbon mass fraction of 0.0218% – 2.11% is called steel. Among them, carbon steel with carbon content less than 0.25% is called low carbon steel. Carbon steel with carbon content of 0.25% – 0.6% is also called medium carbon steel. When the carbon content is more than 0.6%, it is called high carbon steel.
Alloy refers to a metal element combined with other elements to form a material with metal characteristics. For example, aluminum alloy windows at home are made of aluminum, magnesium and silicon. The main body of kitchen faucet is generally made of copper alloy, mainly copper and zinc, with a small amount of lead.
For example, Al-li8090 and titanium alloys are often used in aircraft structures because of their high ratio of strength to density.
At room temperature, carbon steel with different mass fractions will form austenite after heating it above the critical temperature. This austenite has a characteristic that it will form different structures when it is isothermal in different temperature ranges or cooled at different cooling rates.
The critical temperature is the temperature corresponding to lines A3, ACM and A1 in the iron carbon phase diagram, which indicates the temperature at which carbon with different mass fraction begins to transform into austenite when heated. For example, for a carbon steel with pearlite structure at room temperature, austenite begins to form when heated to 727 ℃.
For example, for carbon steel with 0.77% carbon content (also known as eutectoid steel), pearlite will be formed when it is isothermal between 727 ℃ and 560 ℃, bainite will be formed when it is isothermal between 560 ℃ and MS, and martensite will be formed when it is isothermal between MS-MF.

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Isothermal transformation diagram of austenite in eutectoid steel

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Possible structure in Austenite Isothermal Transformation Diagram of eutectoid steel

When austenite is kept at 727-560 ℃, cementite will first form at austenite grain boundary (grain boundary). Cementite grows slowly, which makes the surrounding austenite lack of carbon, and then ferrite will form on both sides. In this way, a small pearlite unit is formed. Many small units are diffused and overlapped, and finally the whole austenite becomes pearlite, Therefore, the basic structure of pearlite is a mixture of ferrite and cementite.
When austenite is kept in the temperature range of 560 to MS, supersaturated ferrite is precipitated at austenite grain boundary, and then fine cementite is precipitated in ferrite. Therefore, bainite is a mixture of supersaturated ferrite and cementite.
MS is the starting temperature of martensite transformation, i.e. Martensite Start. Different mass fraction of carbon steel corresponds to different MS, which varies from 150 ℃ to 310 ℃. MF is the end temperature of martensite transformation, i.e. martensite finish, which also varies according to the mass fraction of carbon, ranging from – 100 to 50 ℃.
Because martensite transforms between MS and MF, the transformation temperature is low and the transformation speed is fast, only the transformation of ferrite crystal structure occurs, and the carbon atoms are retained in martensite without time to redistribute. The mass fraction of carbon in martensite is the same as that of parent austenite, so martensite is carbon in α- Supersaturated solid solution in Fe.
OK, here we finally have the concept of martensite. It is a structure formed by the transformation of austenite in the Ms-Mf temperature range. It is carbon in the α- supersaturated solid solution in Fe.
Of course, because the workpiece is often continuously cooled rather than kept warm during actual heat treatment, the cooling rate is generally used to estimate the final normal temperature structure.

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Austenite Continuous Cooling diagram of eutectoid steel

For example, annealing is equivalent to furnace cooling. The cooling rate is very slow, usually 105-103k / s. The microstructure obtained is coarse flake pearlite, because the microstructure will grow slowly in the process of slow cooling.
Another example is normalizing, which is cooled in the air, and the cooling rate is faster, resulting in fine flake pearlite, also known as sorbite, and very fine pearlite is called troostite.
Finally, quenching in water, rapid cooling, get martensite structure, so the purpose of quenching, is to get martensite.
As mentioned above, martensite is carbon α- Because of the supersaturated solid solution in Fe, it maintains the body centered cubic structure of ferrite. However, because there are a large number of supersaturated carbon atoms inside, the atoms are crowded, resulting in larger internal stress. Therefore, martensite has higher strength and hardness. If the carbon content increases, the strength and hardness will also increase, but it will become very brittle. Therefore, internal stress must be eliminated by tempering, To use.
At this point, I think it is necessary to talk about the meaning of annealing and normalizing heat treatment.
Annealing is the process of heating the workpiece to the critical point, that is, above the A1, A3, ACM lines in the phase diagram, or holding a certain temperature below the critical point for a certain period of time, then cooling very slowly, such as furnace cooling, pit cooling, etc. The purpose is to improve the structure, refine the grain, reduce the hardness, improve the processing performance, reduce the stress, etc.
Annealing can be understood as removing the “fire” inside the workpiece. Metal has fire as well as human. For example, the internal thermal stress is a kind of fire. Annealing can not be too hasty, must be slowly, in order to work, just like people on fire, you can slow down the fire by drinking tea.
Normalizing is a little similar to annealing. The difference is that normalizing is cooled in air, and the cooling speed is faster. The purpose is to refine the structure, properly improve the hardness and strength, and machinability.
Normalizing, evolved from the word normalize, can be understood as normalization. What is normalization? Cooling in the air is normalization, because cooling in the furnace or in the water is controlled artificially. Cooling in the air does not need to be controlled artificially and can be regarded as normal cooling.
So normalizing is cheaper than annealing.
In application, low carbon steel and low carbon alloy steel are usually treated by normalizing, while high carbon steel is usually treated by annealing. Because of high carbon content and high hardness, it is not easy to process. Annealing can reduce hardness and improve processing performance.
OK, here, we finally understand the origin of martensite and austenite, but what are austenitic stainless steel and martensitic stainless steel?
One more step is needed to obtain austenitic stainless steel and martensitic stainless steel from austenite and martensite.

Which step?

As mentioned above, at room temperature, austenite does not exist alone, and its composition in steel is not high, so it cannot be called austenitic steel.
However, when enough alloying elements are added to the steel, the austenite phase zone will be enlarged. For example, adding 9% nickel or 13% manganese can make the A3 line drop, make the austenite stable at room temperature, and form austenite steel.
So austenitic steel is actually a kind of alloy steel.

Why should alloying elements be added to carbon steel?

Because although carbon steel has good mechanical and processing properties, and its price is cheap, it is not easy to completely harden, its strength is not high enough, and it does not have special properties such as corrosion resistance, high temperature resistance, wear resistance, etc.
The addition of alloy elements can just make up for these shortcomings, so alloy steel is widely used in practical engineering.
Of course, not every alloying element makes the austenite region expand. The addition of some alloying elements will reduce the austenite region, or even make the austenite region disappear.
For example, with the addition of Si, Cr, Al, Ti, etc., when the added chromium reaches 17% – 28%, the austenite area disappears at room temperature, and the steel presents a single-phase ferrite structure at room temperature, which is called ferritic steel.
So what is austenitic stainless steel? Why not rust?
Austenitic stainless steel is based on low carbon steel, adding 17% – 25% chromium and 8% – 29% nickel. For example, typical 18-8 austenitic stainless steel is alloy steel with chromium ≥ 18% and nickel ≥ 8%.
The addition of nickel makes the steel present single-phase austenite structure at room temperature, which reduces the number of micro cells formed due to different microstructure in the metal, thus improving the ability of electrochemical corrosion resistance.

What is electrochemical corrosion?

For example, pearlite in steel is ferrite α The microstructure interphase with cementite F3C lamella constitutes numerous micro cells in nitric acid alcohol solution. α When the potential is low, the anode of the micro cell is formed, and iron ions are continuously separated out, that is, it is corroded. When the F3C potential is high, the cathode of the micro cell is formed, and electrons are transferred to hydrogen ions in the solution to form hydrogen.
The higher the potential is, the less likely it is to be corroded. For example, brass used to make radiators is a copper zinc alloy, which is easy to dezincate in use. Because the electrode potential of copper is higher than that of zinc, aluminum, silicon, nickel and other trace elements are generally added to prevent dezincification.
At the same time, the addition of chromium improves the electrode potential of the matrix, and forms a dense oxide film Cr2O3 on the surface of the steel, so that the steel is not easy to rust in a certain medium, so it is called austenitic stainless steel.
Similarly, martensitic stainless steel can be formed by adding 12% – 18% chromium to carbon steel with 0.1% – 1% carbon content and air cooling.
Because of the single alloy element, martensitic stainless steel has good corrosion resistance only in non oxidizing medium, such as atmosphere and water vapor, but in non oxidizing medium, such as hydrochloric acid solution, the corrosion resistance becomes very low.
Therefore, the corrosion resistance of austenitic stainless steel is higher than that of martensitic stainless steel. If the corrosion resistance is required, austenitic stainless steel is the best choice.
So far, we finally know the concept of austenitic and martensitic stainless steel.
But back to our original question, why is austenitic stainless steel not magnetic? And martensitic stainless steel has magnetism?
According to the principle of magnetic iron absorption, martensite and ferrite can be magnetized, but austenite can not be magnetized.
But more recently, why? I have consulted a lot of materials. So far, I haven’t seen a good explanation.
In any case, the result is that martensite and ferrite have magnetism, but austenite has no magnetism or only weak magnetism.
If you have a good explanation, you are also welcome to leave a message below.
Sometimes austenite is magnetic for two reasons.
First, due to component segregation or improper heat treatment during smelting, there will be a small amount of martensite or ferrite in austenitic stainless steel.
In addition, the microstructure of austenitic stainless steel will transform to martensite after cold working. The larger the cold working deformation, the more martensite transformation, the greater the magnetic properties of the steel.
For the application of stainless steel, we use 303 and 304 most, but because 304 is inferior to 303 in machinability, because 304 sticks to the tool, so we use 303 more.
In addition, our sheet metal parts are generally bent with 304 steel plate, and the thicknesses used most are 1 mm, 1.5 mm, 2 mm and 3 mm. Of course, sometimes when it is only used for covering, the aluminum plate is bent and blackened to prevent rust.
420 is also widely used, because sometimes the workpiece is too large, and 303 and 304 raw materials are not so large, so it is replaced by 420 processing, but the surface treatment, such as zinc plating and chromium plating, should be done to prevent rust.
420 and 440C have high yield strength after quenching and tempering (quenching and tempering at 500-650 ℃), so they are often used in the design of high strength requirements, such as the flexible positioning pin in the robot quick change device I mentioned earlier.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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