Introduction to the SAE-AISI Designation System

---

Understanding Steel Classification Systems

The SAE-AISI designation system serves as the foundation for steel identification in North American manufacturing and engineering applications. This standardized approach enables engineers, metallurgists, and materials specialists to quickly identify steel compositions and properties through a systematic numerical coding method.

The Four-Digit SAE-AISI Numbering System

The SAE designation system employs a straightforward four-digit numbering scheme, with certain alloy steels utilizing five digits (51XXX; 52XXX series). This steel classification system follows a logical structure where each digit position conveys specific information about the steel's chemical composition and characteristics.

The system's structure operates as follows: the first digit identifies the primary alloying element, the second digit indicates the concentration of major alloying elements, and the final two digits represent the carbon content in hundredths of a percent. This systematic approach ensures consistent steel identification across different manufacturers and applications.

Carbon Steel Classifications in the SAE-AISI System

Carbon steels represent the most fundamental category within the SAE-AISI designation system. These steel grades are organized into distinct series based on their manganese content and processing characteristics:

• The 10xx series encompasses plain carbon steels with a maximum manganese content of 1.00%. These steels form the backbone of many structural and manufacturing applications due to their versatility and cost-effectiveness.
• The 11xx series identifies resulfurized carbon steels, which contain added sulfur to improve machinability. This steel classification proves particularly valuable in high-volume machining operations where improved cutting characteristics are essential.
• The 12xx series designates resulfurized and rephosphorized carbon steels. These steels incorporate both sulfur and phosphorus additions to enhance machinability while maintaining adequate mechanical properties for specific applications.
• The 15xx series represents non-resulfurized high-manganese carbon steels containing up to 1.65% manganese. These steels offer improved hardenability and strength compared to standard carbon steels.

Alloy Steel Classifications and Designations

Alloy steels within the SAE-AISI designation system are categorized based on their primary alloying elements. The first digit of the steel grade immediately identifies the dominant alloying element, enabling quick recognition of the steel's fundamental characteristics.

• The 2xxx series identifies nickel steels, which offer enhanced toughness and low-temperature properties. Nickel additions improve the steel's impact resistance and provide better performance in challenging environmental conditions.
• The 3xxx series encompasses nickel-chromium steels, combining the toughness benefits of nickel with the hardenability and corrosion resistance of chromium. These alloy steels find extensive use in automotive and machinery applications.
• The 4xxx series represents molybdenum steels, where molybdenum additions enhance hardenability and high-temperature strength. These steel grades are particularly valuable in applications requiring elevated temperature performance.
• The 5xxx series designates chromium steels, which offer improved hardenability and wear resistance. Chromium content enhances the steel's ability to achieve higher hardness levels through heat treatment.
• The 6xxx series identifies chromium-vanadium steels, combining chromium's hardenability benefits with vanadium's grain refinement effects. These steels typically exhibit excellent strength and toughness combinations.
• The 7xxx series encompasses tungsten-chromium steels, historically important for tool steel applications requiring high hardness and wear resistance.
• The 9xxx series represents silicon-manganese steels, where silicon additions improve strength and spring properties while manganese enhances hardenability.

Special Letter Designations in Steel Classification

The SAE-AISI designation system incorporates additional letter designations to provide comprehensive steel identification beyond the basic numerical code. These letters convey important information about processing methods, special additions, or quality requirements:

• The "B" designation indicates boron content ranging from 0.0005% to 0.003%. Boron additions significantly improve hardenability even in small quantities, making these steels valuable for applications requiring deep hardening.
• The "L" designation signifies lead content between 0.15% and 0.35%. Lead additions improve machinability by acting as a chip breaker and lubricant during cutting operations.
• The "M" designation identifies merchant quality steel, indicating standard commercial quality with typical chemical composition ranges and mechanical properties.
• The "E" designation indicates electric-furnace steel production, which often provides better cleanliness and more precise chemical composition control compared to basic oxygen furnace production.
• The "H" designation specifies hardenability requirements, indicating that the steel must meet specific hardenability bands as defined by Jominy end-quench testing.

For example, SAE 5130 represents a chromium steel alloy containing approximately 1% chromium and 0.30% carbon, demonstrating how the numbering system conveys precise compositional information.

Table 1. The SAE/AISI steel numbering designation system table

Steel category	Designation	Description
Carbon steels	10XX	Plain carbon, Mn 1.00% max
	11XX	Resulfurized free machining
	12XX	Resulfurized/rephosphorized free machining
	15XX	Plain carbon, Mn 1.00-1.65%
Manganese steels	13XX	Mn 1.75%
Nickel steels	23XX	Ni 3.50%
	25XX	Ni 5.00%
Nickel-chromium steels	31XX	Ni 1.25%, Cr 0.65-0.80%
	32XX	Ni 1.75%, Cr 1.07%
	33XX	Ni 3.50%, Cr 1.50-1.57%
	34XX	Ni 3.00%, Cr 0.77%
Molybdenum steels	40XX	Mo 0.20-0.25%
	44XX	Mo 0.40-0.52%
Chromium-molybdenum steels	41XX	Cr 0.50-0.95%, Mo 0.12-0.30%
Nickel-chromium-molybdenum steels	43XX	Ni 1.82%, Cr 0.50-0.80%, Mo 0.25%
	47XX	Ni 1.05%, Cr 0.45%, Mo 0.20-0.35%
Nickel-molybdenum steels	46XX	Ni 0.85-1.82%, Mo 0.20-0.25%
	48XX	Ni 3.50%, Mo 0.25%
Chromium steels	50XX	Cr 0.27-0.65%
	51XX	Cr 0.80-1.05%
	50XXX	Cr 0.50%, C 1.00% min
	51XXX	Cr 1.02%, C 1.00% min
	52XXX	Cr 1.45%, C 1.00% min
Chromium-vanadium steels	61XX	Cr 0.60-0.95%, V 0.10-0.015%
Tungsten-chromium steels	72XX	W 1.75%, Cr 0.75%
Nickel-chromium-molybdenum steels	81XX	Ni 0.30%, Cr 0.40%, Mo 0.12%
	86XX	Ni 0.55%, Cr 0.50%, Mo 0.20%
	87XX	Ni 0.55%, Cr 0.50%, Mo 0.25%
	88XX	Ni 0.55%, Cr 0.50%, Mo 0.35%
Silicon-manganese steels	92XX	Si 1.40-2.00%, Mn 0.65-0.85%, Cr 0-0.65%
Nickel-chromium-molybdenum steels	93XX	Ni 3.25%, Cr 1.20%, Mo 0.12%
	94XX	Ni 0.45%, Cr 0.40%, Mo 0.12%
	97XX	Ni 0.55%, Cr 0.20%, Mo 0.20%
	98XX	Ni 1.00%, Cr 0.80%, Mo 0.25%

Applications and Industry Standards

The SAE-AISI designation system extends beyond basic steel identification to encompass various specialized categories. Structural and constructional steels follow specific standards tailored to building and infrastructure applications. Stainless and heat-resisting steels utilize modified designation systems to accommodate their unique alloying requirements. Steel castings employ related but distinct classification methods to address the specific needs of cast steel products.

This comprehensive steel classification system ensures consistent communication throughout the supply chain, from steel producers to end users, facilitating proper material selection and quality control in engineering applications.

Click on the link to see a list of AISI-SAE standards the following material categories

• Group of standards for structural and constructional steels
• Group of standards for stainless and heat resisting steels
• Group of standards for steel castings