Metallurgy

  

Alloying Agents of Carbon Steel

The addition of small amounts of carbon to harden iron, making steel, was discovered around 1000 A.D. When methods of making steel of relatively high purity were developed during the 19th century, the addition of other elements was found to produce desirable properties.

Society of Automotive Engineers Designations for Alloy Steels

The Society of Automotive Engineers (SAE) designates steel alloys as follows:

• 1xxx Carbon
• 2xxx Nickel
• 3xxx Nickel-Chromium
• 4xxx Molybdenum
• 5xxx Chromium
• 6xxx Chromium-Vanadium
• 7xxx Tungsten
• 8xxx Nickel-Chromium-Molybdenum
• 9xxx Silico-Manganese

Effects of Alloying Elements

Alloying elements and the properties they impart are listed in the following table.

 

Effects of Alloying Agents on Carbon Steel

Element	Comments
Aluminum	Used as a grain refinier and deoxidizer. Up to 1.5 % is used to provide surface hardness through reaction with nitrogen.
Chromium	Provides corrosion, heat, and abrasion resistance, and hardenability properties. The "classic" composition for stainless steel includes 18% chromium and 8% nickel (18-8). Chromium specifically adds resistance to oxidizing agents, such as nitric acid.
Copper	About 0.25% improves corrosion resistance and is widely used in structural steels. 4% copper acts as an effective hardening agent, and improves tensile and yield strengths with only a slight loss of ductility.
Manganese	Acts as a deoxidizing agent during the refining process. Hardening agent; greatly increases wear resistance. Combines with sulfur, forming manganese sulfide, MnS, which improves the machinability of the steel.
Molybdenum	Provides high-temperature hardness and tensile strength, and hardenability in the annealed state. Note: The barrel for the M1 Garand was made from SAE 4150, a molybdenum-alloy steel.
Nickel	Increases strength and fatigue resistance. The "classic" composition for stainless steel includes 18% chromium and 8% nickel (18-8). Nickel specifically adds resistance to reducing agents, such as hydrochloric acid.
Phosphorus	In small amounts, slightly increases strength of low-carbon steels. In amounts up to .20% and in the presence of copper, phosphorus improves corrosion resistance.
Silicon	Increases strength of low-alloy steels. Increases hardenability. Acts as a deoxidizer during refining, releasing trapped gas.
Sulfur	Increases machinability. Used to make "free-cutting" steels.
Tungsten	Add stability at high temperatures. Used to make high-speed steel.
Vanadium	Toughening agent and grain refiner. Higher percentages act as a hardener.