An alloy steel is a type of steel alloyed with more than one element (alloying elements) and these are added to increase strength, hardness, wear resistance and toughness. The added alloying elements that are added to the base iron and carbon structure typically total no more than 5% of the alloy steel’s material composition.

Alloy Steel Advantages

Whether your project requires advanced corrosion resistance, machinability, strength, or another quality, there is an alloy steel that provides the features you need. With added heat treatment alloy steels can provide a wide range of beneficial qualities including:

  • Enhanced corrosion resistance
  • Increased hardenability
  • Superior strength and hardness

High & Low Alloy Steel Differentiating Qualities

A high alloy steel has alloying elements (not including carbon or iron) that make up more than 8% of its composition. These alloys are less common, because most steel only dedicates a few percent to the additional elements. Stainless steel is the most popular high alloy, with at least 10.5% chromium by mass. This ratio gives stainless steel more corrosion resistance, with a coating of chromium oxide to slow down rusting. Meanwhile, low alloy steel is only modified slightly with other elements, which provide subtle advantages in hardenability, strength, and free-machining. By lowering the carbon content to around 0.2%, the low alloy steel will retain its strength and boast improved formability.

Common Steel Alloying Elements

When it comes to steel, there are many different elements that can be added to the base material, allowing the purchaser to tweak variances until the right alloy is found. Common alloying elements include the following:

  • Manganese: Used in tandem with small amounts of sulfur and phosphorus, the steel alloy becomes less brittle and easier to hammer.
  • Chromium: A small percentage (0.5% - 2%) can help to harden the alloy; larger percentages (4% - 18%) have the added effect of preventing corrosion.
  • Vanadium: With only .15%, this element can boost strength, heat resistance, and overall grain structure. Mixed together with chromium, the steel alloy becomes much harder, but still retains its formability.
  • Nickel: Up to 5%, this alloying element will improve the steel’s strength. In excess of 12%, it provides impressive corrosion resistance.
  • Tungsten: Boosts heat resistance, so the melting point is higher. Also improves the structural makeup of the steel.

Grading of AISI Materials

A basic four-digit system is used by the SAE system to designate the chemical composition of alloy steels and carbon steels. There is a possibility that AISI grades are quoted as SAE grades and very often the same steel identification number is present in AISI/SAE standards. A four-digit number is usually given to the 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 further explained in the figure given below through a schematic representation of the AISI/SAE steel designation system.

Schematic Representation of AISI/SAE Steel Designation System

Stainless steels are also included in the AISI steel specifications range. The stainless steels are provided with three-digit numbers starting with 2, 3, 4, or 5. The 300 series austenitic stainless grades and the 400 series martensitic grades are the popular stainless steel specifications.
Given below is a table of AISI material grades explaining the steel type indicated by each four-digit number along with its specifications.

AISI Material Grades

Carbon Steel 10XX Plain carbon steel , Mn 1.00% max
11XX Resulfurized free cutting
12XX Resulfurized - Rephosphorized free cutting
15XX Resulfurized - Plain carbon steel, Mn 1.00-1.65%
Manganese Steel 13XX Mn 1.75%
Nickel Steel 23XX Ni 3.50%
25XX Ni 5.00%
Nickel Chromium Steel 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 Steel 40XX Mo 0.20-0.25%
44XX Mo 0.40-0.52%
Chromium Molybdenum Steel 41XX Cr 0.50-0.95%, Mo 0.12-0.30%
Nickel Chromium Molybdenum Steel 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 Steel 46XX Ni 0.85-1.82%, Mo 0.20-0.25%
48XX Ni 3.50%, Mo 0.25%
Chromium Steel 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 Steel 61XX Cr 0.60-0.95%, V 0.10-0.15%
Tungsten Chromium Steel 72XX W 1.75%, Cr 0.75%
Nickel Chromium Molybdenum Steel 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 Steel 92XX Si 1.40-2.00%, Mn 0.65-0.85% Cr 0.65%
Nickel Chromium Molybdenum Steel 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%

Understanding of the SAE/AISI Steel Numbering Designation System

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

Four-Digit Index Classification of Alloy Steels
SAE designation Type
1xxx Carbon steels
2xxx Nickel steels
3xxx Nickel-chromium steels
4xxx Molybdenum steels
5xxx Chromium steels
6xxx Chromium-vanadium steels
7xxx Tungsten steels
8xxx Nickel-chromium-vanadium steels
9xxx Silicon-manganese steels

The first digit of AISI/SAE Steel Designation represents a general category grouping of steels. This means that 1xxx groups within the SAE-AISI system represent carbon steels. Because of the variations in some of the fundamental properties among the carbon steels, they are further divided into four classes. Thus the plain carbon steels are represented within the 10xx series containing a maximum of 1.00% Mn, resulfurized carbon steels are represented within the 11xx series, resulfurized and rephosphorized carbon steels are represented within the 12xx series, and non-resulfurized high-manganese (up-to 1.65%) carbon steels are represented in 15xx series. The non-resulfurized high-manganese carbon steels are developed for ensuring better machinability.

The second digit of the series indicates the presence of major elements, which may affect the properties of the steel. For example in 1018 steel, the zero in the 10XX series depicts that major secondary elements such as sulfur, are not present. Sulfur in the steel ensures better machinability, but all free machining agents such as sulfur, lead, calcium, etc., are not clean or directly taken from the earth. These free machining elements do not completely homogenize during the steel making process and can also cause stringers, pockets or other faults which can affect some of the properties of the steel.

The last two digits indicate that the carbon concentration is 0.01%. For instance, SAE 1018 indicates non-modified carbon steel containing 0.18% of carbon. SAE 5130 indicates a chromium alloy steel containing 1% of chromium and 0.30% of carbon.