Meaning of Stainless Steel Grade Number:
What does a stainless steel grade number signify?
A stainless steel grade number reflects the properties and characteristics of different stainless steel materials. It serves as the “universal language” in the manufacturing, trading, and use of stainless steel materials.
Based on the steel’s structural characteristics, chemical composition, or a combination of both, stainless steel is generally classified into martensitic, ferritic, austenitic, duplex, and precipitation hardening stainless steel, or into chromium and nickel stainless steel.
For instance, the numerical significance of 304 stainless steel and 06Cr19Ni10 stainless steel can be referred to below:
(Note: The stainless steel grade is a classification of steel grades and cannot be understood independently of other steel grades. A comprehensive understanding is necessary for accurate interpretation.)
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
- The carbon content in the steel grade is expressed as a percentage to the thousandth.
For instance, the “2” in 2Cr13 indicates that the average carbon content of the steel is 2% (i.e. 0.2%).
If the carbon content in the steel is less than or equal to 0.03% or 0.08%, the new national standard steel grades are preceded by “022” and “06,” respectively, such as 022Cr17Ni12Mo2 and 06Cr19Ni10.
(The old national standard steel grades are indicated by “00” and “0,” respectively, such as 00Cr17Ni14Mo2 and 0Cr18Ni9).
- The main alloy elements in the steel are expressed as a percentage, while titanium, niobium, zirconium, and ammonia are marked according to the expression method for microalloyed elements in alloy structural steel.
(2) Alloy Structural Steel
- The first two digits of the steel grade indicate the carbon content of the steel, expressed as a percentage to the ten-thousandth. For example, 40C.
- The main alloy elements in steel, excluding individual microalloy elements, are usually expressed as a percentage.
When the average alloy content is less than 1.5%, the steel grade is usually marked with element symbols instead of the content. However, if there is a possibility of confusion in special circumstances, the number “1” may be added after the element symbols, such as in steel grades “12CrMoV” and “12Cr1MoV.” The former has a chromium content of 0.4-0.6%, while the latter has a chromium content of 0.9-1.2%, with all other components being the same.
When the average content of alloy elements is 1.5% or greater, 22.5% or greater, or 3.5% or greater, the content should be indicated after the element symbol, represented by 2, 3, or 4. For example, 18Cr2Ni4WA.
- Vanadium (V), titanium (Ti), aluminum (AL), boron (B), rare earth (RE), and other alloy elements in steel are considered microalloying elements. Despite their low content, they should still be marked in the steel grade. For example, 20MnVB steel where vanadium is 0.07-0.12% and boron is 0.001-0.005%.
- A suffix of “A” should be added to the end of the steel grade for high-grade, high-quality steel to distinguish it from ordinary high-quality steel.
- For alloy structural steel with special purposes, the steel grade is preceded (or suffixed) with a symbol representing the purpose of the steel grade. For example, the steel grade of the 30CMSi steel dedicated for riveting is ML30CrMnSi.
(1) Designation of American steel products
There are numerous American steel product standards, including:
- ANSI (American National Standards Institute)
- AISI (American Iron and Steel Institute)
- ASTM (American Society for Testing and Materials)
- ASME (American Society of Mechanical Engineers)
- AMS (Aerospace Material Specification) – the most commonly used material specification in the American aviation industry, formulated by SAE
- API (American Petroleum Institute)
- AWS (American Welding Society)
- SAE (Society of Automotive Engineers)
- ML (American Military Standard)
- QQ (US Federal Government Standards)
This article only focuses on the widely used ANS, which refers to ASTM, SAE, and AISI standards.
(1).Designation of Carbon Steel and Alloy Steel in ASTM, SAE, and AISI Standards
In the ASTM, SAE, and AISI standards, the designation of carbon steel and alloy steel is generally similar, with four Arabic numerals and letters added in the middle or at the end. For instance, 1005, 94B15, 3140, etc. The first two digits of the four numerals indicate the type of steel and its main alloy element content, while the last two digits indicate the average carbon content of the steel, expressed in tens of thousands.
- Category Numbers: The first digit (or the first two 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
- Steel Grades or Alloy Elements: 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
- Average Carbon Content: The third and fourth digits represent the average carbon content, expressed in tens of thousands. B or L may be inserted in the middle of some steel grades: B – boron steel; L – lead steel. If “H” is added at the end, it indicates the steel grade with specific requirements for hardenability. Some stiffeners are prefixed with the letters “M” or “MT”: M is mechanical grade, and MT is mechanical pipe.
(2) Designation of Stainless and Heat-Resistant Steels
For instance, the AIS standard numbering system is primarily used for designating stainless and heat-resistant steels, such as S31,803, 321, 321H, etc.
The grade of the steel is made up of three Arabic numerals, with the first digit representing the steel category. The second and third digits denote the sequence number. The steel categories are as follows: 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
For example, S31,803 is a medium alloy duplex stainless steel that is coded according to the UNS coding system. It is equivalent to the Chinese standard 00Cr22Ni5Mo3N stainless steel (GBT24,511-2017 stainless steel material grade standard). This steel has a narrow range of Cr, MO, and N elements, making it easier to achieve phase balance (i.e., both phases make up about half of each other), which improves the strength, corrosion resistance, and welding performance of the steel. It is mostly used for materials that require high performance and need to be welded, such as oil and gas pipelines.
321 is a stainless steel standard of the American Iron and Steel Institute, equivalent to 1Cr18Ni9T stainless steel in China. Meanwhile, 321H is a steel grade that has the letter “H” added after 321, indicating that it has certain requirements for hardenability.