SAE / AISI Carbon Steel Naming Conventions

Jul 12, 2012

Topics covered

introduction
Origin of the coding system
AISI material classification
Schematic representation of the AISI / SAE steel designation system
Understanding of the SAE / AISI steel numbering designation system

introduction

Origin of the coding system

The early 1940s saw the development of a coding system for classifying different types of steels. Designers, thermal processors, draftsmen and engineers were able to obtain specific information on the types of steel as well as its grades. The coding system is known for its efficient classification of steels because it uses American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) standards.

For a number of years, alloy steels and carbon steels have been designated with specific grades by a four-digit SAE / AISE numerical index system. The grades of alloy steels and carbon steels were identified by this system on the basis of standard chemical compositions. A discontinuity in the relationship between grade designations and AISI has occurred as a result of the decision by AISI to stop writing material specifications.

AISI material classification

A basic four-digit system is used by the SAE system to denote the chemical composition of alloy steels and carbon steels. It is possible that AISI grades are referred to as SAE grades and very often the same steel identification number is present in AISI / SAE standards. A four-digit number is usually assigned to AISI alloy steels and carbon steels. The alloying element in the AISI specification is indicated by the first two digits and the amount of carbon is indicated by the last two digits.

The chemical composition of alloy steels and carbon steels is explained in more detail in the figure below through a schematic representation of the designation system for AISI / SAE steels.

Schematic representation of the AISI / SAE steel designation system

Stainless steels are also included in the AISI steel specification range. Stainless steels are supplied with three digit numbers starting with 2, 3, 4 or 5. Austenitic 300 series stainless steel grades and 400 series martensitic grades are the most common stainless steel specifications .

Below is an AISI material grade table explaining the type of steel indicated by each four-digit number and its specifications.

Table 1. AISI material qualities

Understanding of the SAE / AISI steel numbering designation system

Below is a table illustrating the four-digit classification of alloy steels by the SAE-AISI system.

Table 2. Classification of the four-digit index of alloy steels

The first digit of the AISI / SAE steel designation represents a general grade of steels. This means that the 1xxx groups of the SAE-AISI system represent carbon steels. Due to variations in some of the fundamental properties among carbon steels, they are further divided into four classes. Thus, simple carbon steels are represented within the 10xx series containing a maximum of 1.00% of Mn, resulfurized carbon steels are represented within the 11xx series, resulfurized and rephosphorus carbon steels are represented within of the 12xx series, and the high-manganese unresulfurized (up to -1.65%) carbon steels are shown in the 15xx series. Non-resulfurized high manganese carbon steels are developed to ensure better machinability.

The second number in the series indicates the presence of major elements, which can affect the properties of the steel. For example, in 1018 steel, the zero in the 10XX series indicates that major secondary elements such as sulfur are not present. Sulfur in steel provides better machinability, but not all free machining agents such as sulfur, lead, calcium etc. are clean or directly extracted from the earth. These loose machining elements do not homogenize completely during the steelmaking process and can also cause spars, pockets or other defects that can affect some of the properties of the steel.

The last two digits indicate that the carbon concentration is 0.01%. For example,

SAE 1018 indicates unmodified carbon steel containing 0.18% carbon.

SAE 5130 indicates a chromium alloy steel containing 1% chromium and 0.30% carbon.

Sometimes an additional letter is added between the second and third digits of code groups such as 11L41, 12L14 or 50B40. The letter L indicates the addition of lead (between 0.15% and 0.35%) to improve the machinability of the steel. The letter B indicates the addition of boron (between 0.0005% and 0.003%) to low carbon steels to improve the hardness of the steel. In addition, commercial grade steels used as hot-rolled steel bars in the production of non-critical parts of machines and structures are designated with the prefix M. Alloy steels with the prefix E indicate electric furnace steel and the suffix H indicates that the steel has been produced to the required hardenability limits.