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Tuesday, February 22, 2011

Classification of steels by composition


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. American Iron and Steel Institute (AISI) together with Society of Automotive Engineers (SAE) have established four-digit (with additional letter prefixes) designation system:

First digit 1 indicates carbon steel (2-9 are used for alloy steels);
Second digit indicates modification of the steel.
0 - Plain carbon, non-modified
1 - Resulfurized
2 - Resulfurized and rephosphorized
5 - Non-resulfurized, Mn over 1.0%
Last two digits indicate carbon concentration in 0.01%.
Example: SAE 1030 means non modified carbon steel, containing 0.30% of carbon.

A letter prefix before the four-digit number indicates the steel making technology:
A - Alloy, basic open hearth
B - Carbon, acid Bessemer
C - Carbon, basic open hearth
D - Carbon, acid open hearth
E - Electric furnace
Example: AISI B1020 means non modified carbon steel, produced in acid Bessemer and containing 0.20% of carbon. 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


Low alloy steels (alloying elements <= 8%);
High alloy steels (alloying elements > 8%).

According to the four-digit classification SAE-AISI system:

First digit indicates the class of the alloy steel:
2- Nickel steels;
3- Nickel-chromium steels;
4- Molybdenum steels;
5- Chromium steels;
6- Chromium-vanadium steels;
7- Tungsten-chromium steels;
9- Silicon-manganese steels.

Second digit indicates concentration of the major element in percents (1 means 1%).
Last two digits indicate carbon concentration in 0,01%.
Example: SAE 5130 means alloy chromium steel, containing 1% of chromium and 0.30% of carbon.

General representation of steels:

Table 1. Classification of steels
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
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.
Nickel-chromium steels
These steels are tough and ductile and exhibit high wear resistance , hardenability and high resistance to corrosion.
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.
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.
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
Ferrite hardener
Graphite former
Mild ferrite hardener
Moderate effect on hardenability
Graphite former
Resists corrosion
Resists abrasion
High effect on ferrite as a hardener
High red hardness
Strong effect on hardenability
Strong carbide former
High red hardness
Increases abrasion resistance
Strong ferrite hardener
Ferrite strengthener
Increases toughness of the hypoeutectoid steel
With chromium, retains austenite
Graphite former
Austenite stabilizer
Improves resistance to corrosion
Ferrite hardener
Increases magnetic properties in steel
Ferrite hardener
Improves machinability
Increases hardenability

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.

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