Meaning of stainless steel grade number:
What does stainless steel grade number represent?
Stainless steel grade reflects the properties and characteristics of various stainless steel materials, and is the “universal language” in the process of manufacturing, trading and using stainless steel materials.
According to the structural characteristics of steel, chemical composition characteristics of steel and the combination of the two, it is generally classified into martensitic stainless steel, ferritic stainless steel, austenitic stainless steel, duplex stainless steel and precipitation hardening stainless steel, or into chromium stainless steel and nickel stainless steel.
For example, the numerical meaning of the 304 stainless steel model and 06Cr19Ni10 stainless steel model, please refer to the following:
(Note: stainless steel grade is a classification of steel grades, which cannot be independent of other steel grades, and can only be accurately understood with comprehensive knowledge).
Related reading: Stainless Steel Grades: The Ultimate Guide
1. Classification description of China’s steel grade representation method
（1） Stainless steel and heat-resistant steel
1. The carbon content in the steel grade is expressed in thousandths.
For example, 2 in 2Cr13 indicates that the average carbon content of steel is 2% 0 (i.e. 0.2%).
If the carbon content in the steel is ≤ 0.03% or ≤ 0.08%, the steel grades of the new national standard are respectively preceded by “022” and “06”, such as 022Cr17Ni12Mo2 and 06Cr19Ni10.
(The steel grades of the old national standards are indicated by “00” and “0” respectively, for example:00Cr17Ni14Mo2,0Cr18Ni9）
2. The main alloy elements in the steel are expressed in percentage, while titanium, niobium, zirconium, ammonia, etc. are marked according to the following expression method for microalloyed elements in alloy structural steel.
（2） Alloy structural steel
1. The first two digits of the steel grade indicate the carbon content of the steel, expressed in tens of thousands of the average carbon content, such as 40C.
2. The main alloy elements in steel, except for individual micro-alloy elements, are generally expressed in percentage.
When the average alloy content is less than 1.5%, the steel grade is generally marked with element symbols instead of content.
However, if it is easy to be confused under special circumstances, the number “1” can also be marked after the element symbols, such as steel grade “12CrMoV” and “12Cr1MoV”.
The chromium content of the former is 0.4-0.6%, the latter is 0.9-1.2%, and the other components are all the same.
When the average content of alloy elements is ≥ 1.5%, 22.5%, ≥ 3.5%, the content shall be indicated after the element symbol, which can be represented as 2, 3, 4.
For example, 18Cr2Ni4WA.
3. Vanadium V, titanium Ti, aluminum AL, boron B, rare earth RE and other alloy elements in steel belong to microalloying elements.
Although the content is very low, they should still be marked in the steel grade.
For example, in 20MnVB steel.
Vanadium is 0.07-0.12% and boron is 0.001-0.005%.
4. “A” shall be added at the end of the steel grade of high-grade high-quality steel to distinguish it from ordinary high-quality steel.
5. For alloy structural steel with special purpose, the steel grade is preceded by (or suffix) a symbol representing the purpose of the steel grade.
For example, the steel grade of 30CMSi steel dedicated for riveting is ML30 CrMnSi.,
2. Designation of American steel products
There are many American steel product standards, mainly including ANSI American national standards;
- AISI American Iron and Steel Institute Standards;
- ASTM American Society for Testing and Materials;
- ASME American Society of Mechanical Engineers;
- AMS aerospace material specification (the most commonly used material specification in American aviation industry, formulated by SAE);
- API American Petroleum Institute Standard;
- AWS American Welding Society Standards;
- SAE Society of Automotive Engineers;
- ML American Military Standard;
- QQ US federal government standards
This article only introduces the widely used ANS to refer to ASTM, SAE and AISI standards.
（1） Designation of carbon steel and alloy steel in ASTM, SAE and AISI standards
In ASTM, SAE and AISI standards, the designation of carbon steel and alloy steel is basically the same.
Most of them are represented by four Arabic numerals, with letters added in the middle or at the end.
For example: 1005, 94B15, 3140, etc.
The first two digits of the four digits indicate the type of steel and its main alloy element content.
The last two figures indicate that the average carbon content of steel is several ten thousandths.
1. The first digit (or the first and second digits) represents the following category numbers:
- 1 – Carbon steel,
- 2 – nickel steel,
- 3 – nickel chromium steel,
- 4 – molybdenum steel,
- 5 – chromium steel,
- 61 – chromium vanadium steel,
- 8 – low nickel chromium steel,
- 92 – silicon manganese steel,
- 93, 94, 97, 98 – chromium nickel molybdenum steel.
2. The second digit (excluding the category number with two digits) indicates the content of the following steel grades or alloy elements:
Carbon steel: 0 – general carbon steel; 1 – free cutting steel; 3 – manganese structural steel.
Molybdenum steel: 1 – chromium molybdenum steel; 3 and 7 – nickel chromium molybdenum steel; 6 and 8 – nickel molybdenum steel; 0, 4 and 5 – molybdenum steel with different Mo content.
Nickel and nickel chromium steel: the average nickel content is expressed as a percentage.
Chromium steel: 0 – low chromium content; 1 – high chromium content.
Low nickel chromium steel: 6, 7, 8, 1 means that the nickel and chromium content is certain, but the molybdenum content is different.
6 represents molybdenum content 0.15~0.25; 7 represents molybdenum content 0.2~0.3; 8 represents molybdenum content 0.3~0.4; and 1 represents molybdenum content 0.08~0.15.
3. The third and fourth digits represent the average carbon content, expressed in tens of thousands.
B or L are inserted in the middle of some steel grades: B – boron steel; L – lead steel.
When “H” is added at the end, it indicates the steel grade with certain requirements for hardenability.
Some stiffeners are prefixed with the letter “M” or “MT”: M is mechanical grade, and MT is mechanical pipe.
（2） Designation of stainless and heat-resistant steels
For example, S31803321H321D, etc., the AIS standard numbering system is mainly used for this type of steel.
The grade is composed of three Arabic numerals, and the first digit represents the steel category.
The second and third digits represent the sequence number.
Steel category number: 1 – precipitation hardening stainless steel; 2 – Cr-M-Ni-N austenitic steel; 3 – CrNi austenitic steel; 4 – high chromium martensite and low carbon high chromium ferrite steel; 5 – low carbon martensite steel.
S31,803 is a medium alloy duplex stainless steel coded according to the UNS coding system, which is equivalent to the Chinese standard 00Cr22Ni5Mo3N stainless steel (GBT24,511-2017 stainless steel material grade standard).
The range of Cr, MO and N elements is relatively narrow, which makes it easy to achieve phase balance (i.e., the two phases account for about half of each other), improves the strength, corrosion resistance and welding performance of the steel, and is mostly used for materials with high performance requirements and need to be welded, such as oil and gas pipelines.
321 is a stainless steel standard of American Iron and Steel Institute, equivalent to 1Cr18Ni9T stainless steel in China.
321H is a steel grade with H added after 321, indicating that it has certain requirements for hardenability.