Stainless Steel Types: What Makes Stainless Steel Stainless

Stainless steel can be seen everywhere in our daily life and is widely used. Stainless steel has good heat resistance, corrosion resistance, polishing properties, and relatively high hardness. There are also many types of stainless steel, which play a very important role in hospitals or other sanitary conditions. Because the surface of stainless steel is very smooth, it is not easy to accumulate dirt, and its corrosion resistance is very strong. Therefore, stainless steel is also inaccessible material in the construction industry.

What is Stainless Steel?

Stainless steel refers to steel that is resistant to corrosion by weak media such as atmosphere, steam, and water, while acid-resistant steel refers to steel that is resistant to corrosion by chemically corrosive media such as acid, alkali, and salt. Stainless steel (also known as stainless acid-resistant steel) refers to steel that is resistant to corrosion by chemical media such as the atmosphere or acid. Stainless steel has been around for more than 90 years since it came out at the beginning of the last century. The invention of stainless steel is a major achievement in the history of world metallurgy. The development of stainless steel has laid an important material and technological foundation for the development of modern industry and scientific and technological progress.
Stainless steel is not rust-free, but the corrosion behavior in different media is different. Commonly used stainless steel can be divided into three types: martensitic stainless steel, ferritic stainless steel, and austenitic stainless steel according to their organizational characteristics.

Stainless Steel material

Stainless Steel Types:

①Martensitic Stainless Steel

The carbon content of commonly used martensitic stainless steel is 0.1~0.45%, and the chromium content is 12~14%. It belongs to chromium stainless steel, which is usually referred to as Cr13 stainless steel. Typical steel grades are 1Cr13, 2Cr13, 3Cr13, 4Cr13, etc. This type of steel is generally used to make various valves, pumps, and other parts that can withstand loads and require corrosion resistance, as well as some stainless tools.

In order to improve corrosion resistance, the carbon content of martensitic stainless steel is controlled in a very low content range, generally not more than 0.4%. The lower the carbon content, the better the corrosion resistance of the steel, and the higher the carbon content, the higher the carbon content in the substrate, the higher the strength and hardness of the steel; The higher the carbon content, the greater the number of carbides that form chromium, and the worse its corrosion resistance becomes. It is not difficult to see that the strength and hardness of 4Cr13 are better than 1Cr13, but its corrosion resistance is not as good as 1Cr13.

1Cr13 and 2Cr13 have the ability to resist the corrosion of atmosphere, steam, and other media, and are often used as corrosion-resistant structural steel. In order to obtain good comprehensive performance, quenching + high-temperature tempering (600~700℃) is often used to obtain tempered sorbate to manufacture steam turbine blades, boiler tube accessories, etc. And 3Cr13 and 4Cr13 steel, due to high carbon content, corrosion resistance is relatively poor, through quenching + low-temperature tempering (200~300℃), tempered martensite, with high strength and hardness (HRC up to 50), so often used as tool steel, manufacturing medical instruments, cutting tools, hot oil pump shaft.

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②Ferritic Stainless Steel

Commonly used ferritic stainless steel has a carbon content of less than 0.15% and a chromium content of 12-30%. It is also chromium stainless steel. Typical steel grades are 0Cr13, 1Cr17, 1Cr17Ti, 1Cr28, etc. Due to the corresponding reduction of carbon content and a corresponding increase of chromium content, the microstructure of steel is always single-phase ferrite when heated from room temperature to high temperature (960~1100℃). Its corrosion resistance, plasticity, and weldability are better than martensitic stainless steel. For high-chromium ferritic stainless steel, its ability to resist oxidative medium corrosion is strong, and with the increase of chromium content, the corrosion resistance is further improved.

The addition of titanium to steel can refine grains, stabilize carbon and nitrogen, and improve the toughness and weldability of steel. Ferritic stainless steels cannot be strengthened by heat treatment because they do not undergo phase transformation when heated and cooled. If the grains are coarsened during the heating process, only cold plastic deformation and recrystallization can be used to improve the structure and properties. If this type of steel stays at 450~550 ℃, it will cause embrittlement of the steel, which is called “475 ℃ brittleness”. Embrittlement can be eliminated by heating to about 600°C followed by rapid cooling. It should also be noted that the hard and brittle σ phase will be produced when the steel is heated for a long time at 600~800℃, resulting in σ phase brittleness of the material. In addition, when quenched at 250 ℃ or higher, intergranular corrosion tendency and brittleness caused by significant grain coarsening will occur. These phenomena are serious problems for welded joints. The former can be eliminated by short-term tempering at 650~815℃. This type of steel is obviously lower in strength than martensitic stainless steel and is mainly used in the manufacture of corrosion-resistant parts, and is widely used in the nitric acid and nitrogen fertilizer industries.

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③Austenitic Stainless Steel

Adding 8~11% Ni to the steel containing 18% Cr is the austenitic stainless steel. As 1Cr18Ni9 is the most typical steel grade. Due to the addition of nickel, this type of steel expands the austenite region, so that a metastable single-phase austenite structure can be obtained at room temperature. Due to the high content of chromium and nickel and the single-phase austenite structure, it has higher chemical stability and better corrosion resistance than chromium stainless steel and is currently the most widely used type of stainless steel.

The austenite + carbide structure of 18-8 stainless steel is presented in the annealing state. The existence of carbide has great damage to the corrosion resistance of steel, so the solution treatment method is usually used. That is, the steel is heated to 1100°C and then water-cooled so that the carbides are dissolved in the austenite obtained at high temperature, and then through rapid cooling, a single-phase austenite structure is obtained at room temperature. Commonly known as stainless iron refers to ferritic stainless steel and martensitic stainless steel. The most common stainless steel used to distinguish it from rust resistance is austenitic stainless steel.

Where Does the Steel Come From?

Iron

Steel is often thought of as a substance or a raw material, but steel and iron are two very different substances, and steel is made from iron. Iron is a hard, tough metal, very similar in color to chromium. It has an atomic number of 26, making it extremely stable. Iron is extracted from ore, and different extraction and purification processes can result in different types of iron. Like the most common pig iron, and cast iron derived from pig iron, wrought iron. The difference between the three is the percentage of carbon content, of which wrought iron is the purest.

Steel

Steel is an important metallurgical material, steel itself has a strong hardness, and can bear higher pressure. Due to its stable chemical properties, steel can be used in various industries after deep processing, such as pots and pans in the field of livelihood and stainless steel thermos cups. Due to the addition of chromium in the iron, stainless steel has anti-rust and anti-fouling abilities. In the current metallurgical production, the carbon content of steel is about 2.14%, so steel has better strength and durability than iron.

Stainless Steel

Stainless steel is not a single material, stainless steel contains at least 11 percent chromium, due to the addition of this composition, stainless steel not only will not rust, and has strong heat resistance. Of course, according to the type and use of stainless steel, other elements can also be added, such as carbon-nitrogen silicon sulfur, and molybdenum.

The Main Difference Between Iron and Stainless Steel

Different Definition

Stainless steel is the abbreviation of stainless steel and acid-resistant steel. Stainless steel that is resistant to weak corrosive media such as air, steam, and water is called stainless steel.

Steel grades that are resistant to chemically corrosive media (acids, alkalis, salts, and other chemical corrosion) are called acid-resistant steels.

Iron is a general term for alloys mainly composed of iron, carbon, and silicon.

In these alloys, the carbon content exceeds the amount remaining in the austenite solid solution at the eutectic temperature.

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Different Chemical Contain

The corrosion resistance of stainless steel decreases with increasing carbon content.

Therefore, most stainless steels have a carbon content below 1.2%, and some steels are even below 0.03%.

The main alloying element in stainless steel is chromium.

Only when the chromium content reaches a certain value, the steel has corrosion resistance.

Iron-carbon alloys contain more than 2% iron and carbon. Industrial cast iron generally contains 2.5% ~ 3.5% carbon.

The carbon in cast iron mainly exists in the form of graphite, sometimes in the form of cementite.

In addition to carbon, iron also contains 1% ~ 3% silicon, manganese, phosphorus, sulfur, and other elements.

Alloy cast iron also contains nickel, chromium, molybdenum, aluminum, copper, boron, vanadium, and other elements.

Carbon and silicon are the main elements that affect the microstructure and properties of cast iron.

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Different Performance

The properties of stainless steel include weldability, corrosion resistance, polishability, and heat resistance.

When the content of chromium atoms in the steel is not less than 12.5%, the electrode potential of the steel will suddenly change from a negative potential to a positive electrode potential, so that electrochemical corrosion can be prevented.

Iron is characterized by high hardness and brittleness and cannot withstand shock loads.

For malleable cast iron blanks and wear parts.

Iron has the characteristics of heat resistance, wear resistance, corrosion resistance, low-temperature resistance, or non-magnetic properties, and is often used in the manufacture of mining, chemical machinery and instruments, meters, and other parts.

Iron does not contain carbon and has no hardness, and the carbon content of steel is at least about 0.06%, and it can reach 1.03% at most. Stainless steel contains chromium and nickel, the color is lighter, gray. Stainless steel containing chromium and nickel will not rust when it touches water. Some stainless steel will rust when exposed to water due to insufficient chromium and nickel content.

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Why Does Metal Rust?

The main reasons for metal rusting are electrochemical corrosion and air oxidation. Pure metal or active metal is mainly due to the oxidation reaction between the metal surface and the oxygen in the air, and metal oxides, that is, rust, are formed on the metal surface. If the metal contains more impurities, these impurities will form a chemical primary battery with the metal, and electrochemical corrosion will occur, thereby oxidizing the metal to form a metal oxide.

Some metals will produce a dense oxide film after rusting to prevent further oxidation of the metal, such as aluminum oxide. However, the oxide film produced by some metals after rusting is loosened, which accelerates the oxidation of metals, such as rust.

Rust will cause deterioration of steel products or structures. The volume of rust is larger than that of iron of the same quality, so the rust will squeeze the adjacent non-rusted parts, thereby causing damage; Rust is also one of the reasons for the current global consumption of steel resources. Therefore, we need to remove rust from steel and other metal products in time to reduce the occurrence of steel corrosion. We can also add a large number of other alloys to steel to make stainless steel.

Why Does Metal Rust?

 

What Makes Stainless Steel Stainless?

Metal rust is a natural phenomenon, so What Makes Stainless Steel Stainless? In fact, although stainless steel is the product of iron extraction because it contains chromium, and it is more than 11% chromium, stainless steel will form a thin protective film on the surface. This protective film has a name in the industry: oxide layer. The oxide layer can effectively block oxygen so that the metal will not rust.

What Kind of Stainless Steel is Not Easy to Rust?

There are three main factors that affect stainless steel corrosion:

①Content of alloying elements

Generally speaking, the content of chromium is 10.5%, and the steel is not easy to rust. The higher the content of chromium and nickel, the better the corrosion resistance. For example, the content of nickel in 304 material is 8-10%, and the content of chromium is 18-20%. Such stainless steel will not rust under normal circumstances.

②The smelting process of the manufacturer will also affect the corrosion resistance of stainless steel

A large stainless steel plant with good smelting technology, advanced equipment, and advanced technology can ensure the control of alloying elements, the removal of impurities, and the control of the cooling temperature of the billet, so the product quality is stable and reliable, the internal quality is good, and it is not easy to rust. On the contrary, some small steel mills have backward equipment and backward technology. During the smelting process, impurities cannot be removed, and the stainless steel produced will inevitably rust.

③The external environment, the climate is dry and ventilated, and the environment is not easy to rust

However, stainless steel in areas with high air humidity, continuous rainy weather, or environments with high pH in the air is prone to rust. 304 stainless steel, if the surrounding environment is too bad, it will rust.

How to Deal With Rust Spots on Stainless Steel?

Chemical Process

Use pickling paste or spray to assist the re-passivation of the rusted part to form a chromium oxide film to restore the corrosion resistance. After pickling, in order to remove all contaminants and acid residues, it is very important to rinse properly with clean water. After all processing, re-polish with polishing equipment, and seal with polishing wax. For local slight rust spots, you can also use a 1:1 mixture of gasoline and engine oil to wipe off the rust spots with a clean rag.

Mechanical Method

Sandblast cleaning, blast cleaning with glass or ceramic particles, annihilation, brushing, and polishing. It is possible to mechanically wipe away contamination from previously removed material, polishing material, or submerged material. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should preferably be properly cleaned in dry conditions. The use of mechanical methods can only clean the surface, and cannot change the corrosion resistance of the material itself. Therefore, it is recommended to re-polish with polishing equipment after mechanical cleaning and seal with polishing wax.

Stainless Steel Application Scope

Stainless steel has good stability and oxidation resistance, making it suitable for use in the field of people’s livelihood. At present, the application of stainless steel can cover the following aspects.

Kitchen Utensils: Stainless steel has a strong metallic color, effectively resists corrosion, and can be used in tableware and cookware.

Armarium: Stainless steel is antibacterial and will not react with cleaning solutions. Therefore, it is convenient for disinfection and pretreatment. Stainless steel’s anti-rust and anti-fouling properties make it suitable for the manufacture of medical devices.

Vacuum Cup: Stainless steel is heat resistant and will not rust due to immersion in water. 304 stainless steel, this model is food-grade stainless steel, the surface will present a fine drawing state.

Automobile: Stainless steel can also be used in automotive aircraft precision parts.

Conclusion

Stainless steel materials have many excellent properties, specifically: high strength, excellent weldability, ease of processing and gloss, etc., and have been widely used in aerospace, machining, energy, and other fields. The use of stainless steel is constantly expanding, and the demand for stainless steel fasteners in the future must not be underestimated.

 

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