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Classification of Steels

Effect of alloying elements

The Society of Automotive Engineers (SAE) has established standards for specific analysis of steels.  The letter prefixes are:

A = alloy, basic open hearth

B = carbon, acid Bessemer

C = carbon, basic open hearth

D = carbon, acid open hearth

E = electric furnace

If the prefix is omitted, the steel is assumed to be open hearth. Example: AISI C1050 indicates a plain carbon, basic-open hearth steel that has 0.50 % Carbon content.

Another letter is the hardenability or H-value. Example: 4340H

General representation of steels:

Table 1. Classification of steels

SAE - AISI Number

Classification

1XXX

Carbon steels

Low carbon steels: 0 to 0.25 % C

Medium carbon steels: 0.25 to 0.55 % C

High carbon steels: Above 0.55 % Carbon

2XXX

Nickel steels

5 % Nickel increases the tensile strength without reducing ductility.

8 to 12 % Nickel increases the resistance to low temperature impact

15 to 25 % Nickel (along with Al, Cu and Co) develop high magnetic properties. (Alnicometals)

25 to 35 % Nickel create resistance to corrosion at elevated temperatures.

3XXX

Nickel-chromium steels

These steels are tough and ductile and exhibit high wear resistance , hardenability and high resistance to corrosion.

4XXX

Molybdenum steels

Molybdenum is a strong carbide former. It has a strong effect on hardenability and high temperature hardness. Molybdenum also increases the tensile strength of low carbon steels.

5XXX

Chromium steels

Chromium is a ferrite strengthener in low carbon steels. It increases the core toughness and the wear resistnace of the case in carburized steels.

86XX

87XX

93XX

94XX

97XX

98XX

Triple Alloy steels which include Nickel (Ni), Chromium (Cr), and Molybdenum (Mo).

These steels exhibit high strength and also high strength to weight ratio, good corrosion resistance.

 

Table 2. The effect of alloying elements on the properties of steel

Element

Effect

Aluminum

Ferrite hardener

Graphite former

Deoxidizer

Chromium

Mild ferrite hardener

Moderate effect on hardenability

Graphite former

Resists corrosion

Resists abrasion

Cobalt

High effect on ferrite as a hardener

High red hardness

Molybdenum

Strong effect on hardenability

Strong carbide former

High red hardness

Increases abrasion resistance

Manganese

Strong ferrite hardener

Nickel

Ferrite strengthener

Increases toughness of the hypoeutectoid steel

With chromium, retains austenite

Graphite former

Copper

Austenite stabilizer

Improves resistance to corrosion

Silicon

Ferrite hardener

Increases magnetic properties in steel

Phosphorus

Ferrite hardener

Improves machinability

Increases hardenability