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Aircraft grade alloyed steel - or carbon steel - is indispensable in aircraft construction.  Unlike wood, aluminum and composites, it is not generally the main component of an aircraft because of its weight.  While it is very strong, it is also heavy.  Tube and fabric construction relies on steel to create a structure to cover with fabric.  But tube and fabric aircraft use only as much steel as necessary.   The wings will generally have wood or aluminum spars with aluminum leading edges, and wood or aluminum ribs, flaps and ailerons.  Cowlings may be made from aluminum or composites, all to save weight.  But every aircraft whether wood, composite or aluminum needs steel components.  Nuts and bolts are steel, engine mounts are steel, engine parts, wheel parts and control cables are steel.  So you will need steel no matter what. 

The addition of carbon to steel increases hardness and strength and improves hardenability.   Carbon steel is a steel with carbon content from about 0.05% up to 2.1% by weight.  The addition of other elements changes the steel to have many different qualities.  Below are the more common elements added to steel.

Aluminum (Al) retards grain growth and is used to control grain size. In nitriding steel it aids in producing a

uniformly hard and strong nitrided case.

Carbon (C) is the most important component of steel. It raises tensile strength, hardness and increases

resistance to wear and abrasion.


Chromium (Cr) increases tensile strength, hardness, hardenability. toughness, resistance to wear and

abrasion. resistance to corrosion and scaling at elevated temperatures.


Cobalt (Co) increases strength and hardness. It also enhances the individual effects of other major

elements in more complex steels.


Lead (Pb) is added to improve machining characteristics. It is almost completely insoluble in steel, and minute lead particles, well dispersed, reduces friction and chipping in cutting.


Manganese (Mn) increases tensile strength, hardness, hardenability and wear resistance. It decreases scaling and distortion. It increases the rate of carbon-penetration in carburizing.


Molybdenum (Mo) increases strength, hardness, hardenability and toughness.  It improves machinability and corrosion resistance and intensifies the effects of other alloying elements.


Nickel (Ni) increases strength and hardness without sacrificing ductility or toughness and increases corrosion resistance. 


Phosphorus (P) increases hardness, strength and improves machinability, but adds marked brittleness.


Silicon (Si) increases tensile and yield strength, hardness, forgeability and magnetic permeability.


Sulphur (S) improves machinability in free-cutting steels, but it decreases weldability, impact toughness and ductility.


Tungsten (W) increases strength, hardness and toughness. Tungsten steels have superior hot-working and greater cutting efficiency at elevated temperatures.


Vanadium (V) increases strength, hardness and resistance to shock impact. It retards grain growth, and enhances the individual effects of other elements.


These different alloys are identified with a four-number system that designates the steels chemical composition. The first two digits indicate the grades of carbon steel and are as follows:


10 - Nonresulphurized carbon steel grades.
11 - Resulphurized carbon steel grades.
12 - Rephosphorized and resulphurized carbon steel grades
I5 - Nonresulphurized high manganese carbon steels.

The last two numbers indicate the approximate middle of the carbon range in the steel. For example, in the grade designation 1125, the 25 represents a carbon range of 0.22 to 0.28 per cent.

Before you bog yourself down in the mind-boggling array of possibilities, let’s state that the most common steel for aviation construction is 4130.  4130 is a chromium-molybdenum alloy that is one of the most widely used aircraft steels because of its combination of weldability, ease of fabrication and mild hardenability. In relatively thin sections, it may be heat treated to high strength levels. In the normalized condition it has adequate strength for many applications. It may be nitrided for resistance to wear and abrasion.  It is readily available and proven. 

So, if you are not a steel nerd, or have no intention of becoming one, start at 4130 and wander out from there.

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