Classification of Steels

The Society of Automotive Engineers (SAE) has established standards for specific analysis of steels. In the 10XX series, the first digit indicates a plain carbon steel. The second digit indicates a modification in the alloys. 10XX means that it is a plain carbon steel where the second digit (zero ) indicates that there is no modification in the alloys. The last two digits denote the carbon content in points. For example SAE 1040 is a carbon steel where 40 points represent 0.40 % Carbon content. Alloy steels are indicated by 2XXX, 3XXX, 4XXX, etc.. The American Iron and Steel Institute (AISI) in cooperation with the Society of Automotive Engineers (SAE) revised the percentages of the alloys to be used in the making of steel, retained the numbering system, and added letter prefixes to indicate the method used in steel making. 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:

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 1. Classification of steels

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

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

Red Hardness: This property , also called hot-hardness, is related to the resistance of the steel to the softening effect of heat. It is reflected to some extent in the resistance of the material to tempering.

Hardenability: This property determines the depth and distribution of hardness induced by quenching.

Hot-shortness: Brittleness at high temperatures is called hot-shortness which is usually caused by sulfur. When sulfur is present, iron and sulfur form iron sulfide (FeS) that is usually concentrated at the grain boundaries and melts at temperatures below the melting point of steel. Due to the melting of iron sulfide, the cohesion between the grains is destroyed, allowing cracks to develop. This occurs when the steel is forged or rolled at elevated temperatures. In the presence of manganese, sulfur tends to form manganese sulfide (MnS) which prevents hot-shortness.

Cold-shortness: Large quantities of phosphorus (in excess of 0.12%P) reduces the ductility, thereby increasing the tendency of the steel to crack when cold worked. This brittle condition at temperatures below the recrystallization temperature is called cold-shortness.