Introduction to Alloys

Steel is essentially iron and carbon alloyed with specific additional elements that serve a purposse. The process of alloying is used to change the chemical composition of the steel and to improve its properties over carbon steel or adjust them to better meet the requirements of a particular application.Take Stainless steel which is alloyed with chromium, well known for its tendancy to exhibiut less corrosion - a key property of Stainless Steel, what about its non-magnetic properties?

The common language we use in understanding the purpose of alloys being added to the base metal are, malleabl[ty, ductility, conductivity (of heat and electricity), breakability, reliability, consistency, predictability, workability.

Steel Alloying Agents at the Micro Structure Level

Different alloying elements—or additives—each affect the properties of steel differently. Some of the properties that can be improved through alloying include:

• The addition of certain alloying elements, such as manganese and nickel, can stabilize the austenitic structure, facilitating heat-treatment of low-alloy steels.
• Stabilizing austenite: Elements such as nickel, manganese, cobalt, and copper increase the temperatures range in which austenite exists.
• Stabilizing ferrite: Chromium, tungsten, molybdenum, vanadium, aluminum, and silicon can help lower carbon's solubility in austenite. This results in an increase in the number of carbides in the steel and decreases the temperature range in which austenite exists.
• Carbide forming: Many minor metals, including chromium, tungsten, molybdenum, titanium, niobium, tantalum and zirconium, create strong carbides that—in steel—increase hardness and strength. Such steels are often used to make high-speed steel and hot work tool steel.
• Graphitizing: Silicon, nickel, cobalt, and aluminum can decrease the stability of carbides in steel, promoting their breakdown and the formation of free graphite.

In applications where a decrease of eutectoid concentration is required, titanium, molybdenum, tungsten, silicon, chromium, and nickel are added. These elements all lower the eutectoid concentration of carbon in the steel. Many steel applications require increased corrosion resistance. To achieve this result, aluminum, silicon, and chromium are alloyed. They form a protective oxide layer on the surface of the steel, thereby protecting the metal from further deterioration in certain environments.

Common Steel Alloying Agents

• Aluminum (0.95-1.30%): A deoxidizer. Used to limit the growth of austenite grains.
• Boron (0.001-0.003%): A hardenability agent that improves deformability and machinability. Boron is added to fully killed steel and only needs to be added in very small quantities to have a hardening effect. Additions of boron are most effective in low carbon steels.
• Chromium (0.5-18%): A key component of stainless steels. At over 12 percent content, chromium significantly improves corrosion resistance. The metal also improves hardenability, strength, response to heat treatment and wear resistance.
• Cobalt: Improves strength at high temperatures and magnetic permeability.
• Copper (0.1-0.4%): Most often found as a residual agent in steels, copper is also added to produce precipitation hardening properties and increase corrosion resistance.
• Lead: Although virtually insoluble in liquid or solid steel, lead is sometimes added to carbon steels via mechanical dispersion during pouring in order to improve machinability.
• Manganese (0.25-13%): Increases strength at high temperatures by eliminating the formation of iron sulfides. Manganese also improves hardenability, ductility and wear resistance. Like nickel, manganese is an austenite forming element and can be used in the AISI 200 Series of Austenitic stainless steels as a substitute for nickel.
• Molybdenum (0.2-5.0%): Found in small quantities in stainless steels, molybdenum increases hardenability and strength, particularly at high temperatures. Often used in chromium-nickel austenitic steels, molybdenum protects against pitting corrosion caused by chlorides and sulfur chemicals.
• Nickel (2-20%): Another alloying element critical to stainless steels, nickel is added at over 8% content to high chromium stainless steel. Nickel increases strength, impact strength and toughness, while also improving resistance to oxidization and corrosion. It also increases toughness at low temperatures when added in small amounts.
• Niobium: Has the benefit of stabilizing carbon by forming hard carbides and is often found in high-temperature steels. In small amounts, niobium can significantly increase the yield strength and, to a lesser degree, the tensile strength of steels as well as have moderate precipitation strengthening the effect.
• Nitrogen: Increases the austenitic stability of stainless steels and improves yield strength in such steels.
• Phosphorus: Phosphorus is often added with sulfur to improve machinability in low alloy steels. It also adds strength and increases corrosion resistance.
• Selenium: Increases machinability.
• Silicon (0.2-2.0%): This metalloid improves strength, elasticity, acid resistance and results in larger grain sizes, thereby, leading to greater magnetic permeability. Because silicon is used in a deoxidizing agent in the production of steel, it is almost always found in some percentage in all grades of steel.
• Sulfur (0.08-0.15%): Added in small amounts, sulfur improves machinability without resulting in hot shortness. With the addition of manganese hot shortness is further reduced due to the fact that manganese sulfide has a higher melting point than iron sulfide.
• Titanium: Improves both strength and corrosion resistance while limiting austenite grain size. At 0.25-0.60 percent titanium content, carbon combines with the titanium, allowing chromium to remain at grain boundaries and resist oxidization.
• Tungsten: Produces stable carbides and refines grain size so as to increase hardness, particularly at high temperatures.
• Vanadium (0.15%): Like titanium and niobium, vanadium can produce stable carbides that increase strength at high temperatures. By promoting a fine grain structure, ductility can be retained.
• Zirconium (0.1%): Increases strength and limits grains sizes. Strength can be notably increased at very low temperatures (below freezing). Steel's that include zirconium up to about 0.1% content will have smaller grains sizes and resist fracture.

Where Alloys contribute to mechanical properties of your metal

Hardenability	Strength	Toughness	Machinability
Boron	Carbon	Calcium	Lead
Carbon	Cobalt	Cerium	Manganese
Chromium	Chromium	Chromium	Phosphorus
Manganese	Copper	Magnesium	Selenium
Molybdenum	Manganese	Molybdenum	Sulfur
Phosphorus	Molybdenum	Nickel	Tellurium
Titanium	Nickel	Niobium
	Niobium	Tantaium
	Phosphorus	Tellurium
	Silicon	Vanadium
	Tantalum	Zirconium
	Tungsten	Vanadium
	Vanadium