1. Three Families of Alloy Steel
Alloy steel is divided into three sub-categories: alloy structural steel, used for machine parts and structures that need strength and toughness; alloy tool steel, used for cutting tools, dies and measuring tools that need hardness and wear resistance; and special performance steel, which covers stainless steel, heat-resistant steel, wear-resistant steel and other grades with special chemical or physical properties. Understanding which family a grade belongs to is the first step in reading its name.
2. Alloy Structural Steel Designation
In the traditional Chinese system, alloy structural steel grades are written as "digit + chemical element symbols + digit". The leading number gives the average carbon content in ten-thousandths: for example, 40Cr means an average carbon content of about 0.40% (40 per ten thousand) with chromium as the main alloying element. Alloying elements are shown by their chemical symbols. When an element content is below 1.5%, only the element symbol is written, without a number; when the average content reaches 1.5%, 2.5%, 3.5% and so on, it is shown as 2, 3, 4 and so forth. This is why 40Cr and 40CrNiMoA are read differently: the letter at the end can carry further meaning such as a quality class.
3. Alloy Tool Steel Designation
Alloy tool steel follows similar element rules but different carbon rules. When the average carbon content is 1.0% or more, the carbon figure is omitted; below 1.0%, it is expressed in thousandths. High-speed steel is an exception: its average carbon content is not marked even when it is below 1.0%. This reflects the different design priorities of tool steel, where high carbon and carbide-forming elements such as tungsten, molybdenum, vanadium and chromium determine hardness and wear resistance.
4. Special Performance Steels
Special performance steel includes stainless, heat-resistant and wear-resistant steels. Common chromium stainless grades in the traditional system include 1Cr13, 2Cr13, 3Cr13, 4Cr13 and 1Cr17; chromium-nickel stainless grades include the 18-8 type such as 0Cr18Ni9 and 1Cr18Ni9Ti, named for roughly 18% chromium and 8% nickel. Heat-resistant steel is divided into anti-oxidation steel and heat-strength steel; typical grades are pearlitic types such as 15CrMo and 12Cr1MoV, martensitic types such as 1Cr11MoV and 4Cr9Si2, bainitic types and austenitic types such as 1Cr18Ni9Ti. Wear-resistant steel in this context refers mainly to high-manganese steel with about 1.0–1.3% carbon and 11–14% manganese, usually produced by casting and designated as ZGMn13.
5. Reading Grades in Practice
When comparing specifications, note that grade designation systems differ between standards families: the traditional Chinese system described above, the newer Chinese system introduced in current editions, and the systems used in ASTM, EN, JIS and other standards cannot be converted by rule of thumb. Always check the governing standard and the mill certificate, and confirm the meaning of each symbol in the grade before substitution.
Frequently Asked Questions
What does 40Cr mean? In the traditional Chinese system it is an alloy structural steel with an average carbon content of about 0.40% and chromium as the main alloying element below the 1.5% marking threshold.
What are the three families of alloy steel? alloy structural steel, alloy tool steel, and special performance steel (stainless, heat-resistant and wear-resistant steels).
Why is the carbon number omitted in some tool steel grades? Because tool steel with 1.0% or more average carbon omits the carbon figure, while grades below 1.0% show it in thousandths; high-speed steel never shows it.
What is 18-8 stainless steel? n austenitic chromium-nickel stainless family with roughly 18% chromium and 8% nickel, traditionally designated in China as grades such as 0Cr18Ni9 and 1Cr18Ni9Ti.
What is ZGMn13? cast high-manganese wear-resistant steel containing about 1.0–1.3% carbon and 11–14% manganese, used for parts under severe impact and abrasion.
