一、 What kind of steel is stainless steel?
Stainless steel is a type of steel. Steel refers to the material with a carbon (C) content of less than 2%, while anything above 2% is iron. During the steel-making process, alloy elements such as chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), and molybdenum (Mo) are added to improve the performance of steel, making it resistant to corrosion (i.e., not prone to rust), which is what we commonly refer to as stainless steel.
What exactly are "steel" and "iron", what are their characteristics, and what is the relationship between them? And how did the terms 304, 304L, 316, and 316L come about? What are the differences among them?
Steel：A material mainly composed of iron, with a carbon content generally below 2%, and containing other elements. ——GB/T 13304-91 "Classification of Steel"
Iron：A metallic element with atomic number 26. Iron materials possess strong ferromagnetism and have excellent plasticity and thermal conductivity.
Stainless steel：Steel types that are resistant to weak corrosive media such as air, steam, and water, or have stainless properties. The commonly used steel types are 304, 304L, 316, and 316L, which belong to the 300 series of austenitic stainless steel.
二、Why are there different steel grades for stainless steel?
During the smelting process of stainless steel, due to the different types of alloy elements added and the different amounts of each type of element added, the properties of the steel also vary. To distinguish them, different steel grades are given. Below is a table showing the "alloy element" content of common decorative stainless steel grades for reference only:
Chemical composition (mass fraction, %)
Steel grade | Carbon | Silicon | Manganese | Phosphorus | Sulfur | Chromium | Nickel |
304 | ≤0.08 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.03 | 18-20 | 8-10 |
301 | ≤0.15 | ≤1.00 | ≤2.00 | ≤0.045 | ≤0.03 | 16-18 | 6-8 |
202 | ≤0.15 | ≤1.00 | 7.5-10 | ≤0.05 | ≤0.03 | 17-19 | 4-6 |
201 | ≤0.15 | ≤1.00 | 5.5-7.5 | ≤0.05 | ≤0.03 | 16-18 | 3.5-5.5 |
Common Categories:
It is usually classified according to the metallographic structure as follows:
Generally, based on the microstructure, ordinary stainless steels are classified into three categories: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. On the basis of these three basic microstructures, to meet specific requirements and purposes, duplex steel, precipitation hardening type stainless steel, and high-alloy steel with a iron content of less than 50% have been derived.
1、Austenitic stainless steel.
The matrix is mainly composed of austenite with face-centered cubic crystal structure (CY phase), without magnetic properties. It is a type of stainless steel that is strengthened through cold processing (and may potentially acquire some magnetic properties). The American Iron and Steel Institute marks these materials with numbers such as 200 and 300 series, for example, 304.
2、Ferritic type stainless steel.
The matrix is mainly composed of ferrite with body-centered cubic crystal structure ((a phase)), which is magnetic. It generally cannot be hardened through heat treatment, but cold processing can slightly strengthen it. This type of stainless steel is specified by the American Iron and Steel Institute as 430 and 446.
3、Marine-type stainless steel.
The matrix is a martensitic structure (body-centered cubic or cubic), magnetic in nature, and is a type of stainless steel whose mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute uses the numbers 410, 420, and 440 to denote it. Martensite has an austenitic structure at high temperatures. When cooled to room temperature at an appropriate rate, the austenitic structure can transform into martensite (i.e., harden).
4、 Austenitic-ferritic (dual-phase) type stainless steel.
The matrix contains both austenite and ferrite phases. The content of the less abundant phase in the matrix is generally more than 15%. This magnetic stainless steel that can be strengthened through cold processing is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has higher strength, and its resistance to intergranular corrosion, chloride stress corrosion and pitting corrosion has been significantly improved.
5、Precipitation-hardening stainless steel.
The base material is of austenitic or martensitic structure and can be hardened through precipitation hardening treatment. This type of stainless steel is designated by the 600 series number by the American Iron and Steel Institute, such as 630, which represents 17-4PH.
Generally speaking, except for alloys, the corrosion resistance of austenitic stainless steel is quite excellent. In environments with low corrosivity, ferritic stainless steel can be used. In mild corrosive environments, if the material is required to have high strength or high hardness, martensitic stainless steel and precipitation hardening stainless steel can be chosen.
Characteristics and Applications
Surface treatment
Thickness differentiation
1、During the rolling process in the steel mill, the rollers get slightly deformed due to heat, causing the thickness of the rolled plates to deviate. Generally, the middle part of the plate is thicker while the sides are thinner. When measuring the thickness of the plate, the national standard stipulates that the measurement should be taken at the middle part of the plate head.
2、The cause of tolerance is that it is determined based on market and customer demands, and it is generally classified into large tolerance and small tolerance: for example
- 304 stainless steel -
Performance Introduction
304 stainless steel is the most common type of steel. As a widely used steel, it possesses excellent corrosion resistance, heat resistance, low-temperature strength and mechanical properties; it has good heat processing properties such as stamping and bending, and there is no heat treatment hardening phenomenon (it is non-magnetic, and the usable temperature range is -196℃ to 800℃).
Scope of Application
Household items (1st and 2nd category tableware, cabinets, indoor pipelines, water heaters, boilers, bathtubs)
Automobile parts (windshield wipers, mufflers, molded products)
Medical equipment, building materials, chemicals, food industry, agriculture, ship components
- 304L stainless steel -
（L represents low carbon.）
Performance Introduction
As a low-carbon 304 steel, in its normal state, its corrosion resistance is similar to that of 304 steel. However, after welding or stress relief treatment, its anti-groove corrosion ability is excellent. Even without undergoing heat treatment, it can still maintain good corrosion resistance, with a usage temperature range of -196℃ to 800℃.
Scope of Application
It is applied to outdoor machinery in the chemical, coal and petroleum industries with high requirements for anti-growth corrosion, as well as to heat-resistant parts of building materials and parts with difficulties in heat treatment.
- 316 stainless steel -
Performance Introduction
316 stainless steel, due to the addition of molybdenum, has particularly excellent corrosion resistance, atmospheric corrosion resistance and high-temperature strength, and can be used under harsh conditions; it has excellent work hardening property (without magnetism).
Scope of Application
Equipment for seawater use includes chemical, dye, papermaking, oxalic acid, fertilizer production facilities; photography, food industry, coastal area facilities, ropes, CD poles, bolts, nuts.
- 316LStainless steel -
(L represents low carbon)
Performance Introduction
As a low-carbon series of the 316 steel type, in addition to sharing the same characteristics with 316 steel, it also has excellent resistance to intergranular corrosion.
Scope of Application
Products with special requirements for resisting the corrosive effects of grain boundaries.
What kind of stainless steel is less prone to rust?
The main factors influencing the corrosion of stainless steel include three points:
1、The content of alloying elements.
Generally speaking, steel with a chromium content of 10.5% or higher is less prone to rusting. The higher the content of chromium and nickel, the better the anti-corrosion property. For example, in 304 material, the nickel content should be between 8-10%, and the chromium content should reach 18-20%. Under normal circumstances, this type of stainless steel will not rust.
2、The smelting process of the manufacturing enterprise also affects the corrosion resistance of stainless steel.
Large stainless steel plants with excellent smelting technology, advanced equipment and advanced processes can ensure the control of alloy elements, the removal of impurities, and the control of the cooling temperature of the steel billets. Therefore, the product quality is stable and reliable, with good internal quality and low susceptibility to rust. On the contrary, some small steel mills have outdated equipment and processes. During the smelting process, impurities cannot be removed, and the products produced are inevitably prone to rust.
3、 The external environment, with dry and well-ventilated conditions, is less prone to rusting.
When the air humidity is high, in areas with continuous rainy weather or in regions with high acidity or alkalinity in the air, rusting is more likely to occur. For 304 stainless steel, even if the surrounding environment is poor, it can still rust.
What should be done to deal with rust spots on stainless steel?
1、Chemical method
Use acid wash cream or spray to re-passivate the corroded areas and form an oxide chromium film to restore the corrosion resistance. After the acid wash, it is very important to rinse thoroughly with clean water to remove all contaminants and acid residues. After all the treatments, re-polish with a polishing device and seal with polishing wax. For areas with minor rust spots, you can also use a 1:1 mixture of gasoline and engine oil to wipe off the rust spots with a clean cloth.
2、Mechanical method
Sandblasting cleaning, using glass or ceramic micro-particles for blasting, immersion, scrubbing and polishing. It is possible to remove the contamination caused by previously removed materials, polishing materials or obscured materials through mechanical methods. All kinds of contamination, especially foreign iron particles, can potentially become sources of corrosion, especially in humid environments. Therefore, it is best to perform formal cleaning of the surface under dry conditions when mechanically cleaning the surface. Mechanical cleaning can only clean the surface and cannot change the material's own corrosion resistance. Therefore, it is recommended to re-polish after mechanical cleaning using polishing equipment and seal with polishing wax.
Why does stainless steel rust? When brown rust spots (or stains) appear on the surface of stainless steel pipes, people are astonished: they think "Stainless steel doesn't rust. Rusting means it's no longer stainless steel. Maybe there's something wrong with the steel quality." In fact, this is a one-sided and incorrect view of stainless steel that lacks understanding. Stainless steel can also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation - that is, its resistance to corrosion - and also the ability to resist corrosion in acidic, alkaline and salt-containing media - that is, its corrosion resistance. However, the extent of its corrosion resistance varies depending on the chemical composition of the steel itself, its protective state, the usage conditions and the type of environmental medium. For example, 304 steel pipe has an absolutely excellent anti-corrosion ability in a dry and clean atmosphere, but when it is moved to a coastal area and exposed to sea fog containing a large amount of salt, it will soon rust; while 316 steel pipe performs well. Therefore, not any type of stainless steel can resist corrosion and not rust in any environment.
