Alloy steel is steel, which is alloyed with various elements in the total amount of 1.0% to 50% by weight to improve the mechanical properties.
Alloy steels are divided into two groups:
Each become truly alloy, but not all are called "alloy steels." Even the most simple steels are iron (Fe) (about 99%), doped with carbon (C) (from 0.1% to 1%, depending on the type). However, the term "stainless steel" is the standard term applied to steel with other alloying elements in addition to carbon. General alloyants include manganese, nickel, chromium, molybdenum, vanadium, silicon and boron. Less common alloyants include aluminum, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, zirconium.
Improved properties of alloy steels (compared to carbon steel): strength, hardness, wear resistance, hardenability, and hot hardness. To achieve some of these improved properties of the metal may require heat treatment.
Some of them are used in exotic and highly demanding applications, such as turbine blades of jet engines, space vehicles, and in nuclear reactors. Due to the ferromagnetic properties of iron, some steel alloys find important applications where their responses to magnetism are important, including electric motors and transformers.
Low-alloy steels used to achieve better hardenability, which in turn improves its mechanical properties. Also used to increase the corrosion resistance in certain environments.
Middle-and high-carbon low-alloy steel is difficult to weld. Reducing the carbon content in the range of 0.10% to 0.30%, along with some reduction of the alloying elements, improves weldability and ductility, while maintaining its strength. This metal is classified as a high-strength low-alloy steel.
Alloying elements are added to achieve certain properties of the material. As a guideline, alloying elements are added to a low percentage (less than 5%) to increase the strength or hardenability, or large percent (5%) to achieve special properties such as corrosion resistance and high temperature stability.
Manganese, silicon or aluminum is added to the steelmaking process to remove dissolved oxygen, sulfur and phosphorus from the melt.
Manganese, silicon, nickel and copper are added to increase strength by forming a solid solution of ferrite. Chromium, vanadium, molybdenum and tungsten increase strength by forming the second phase of the carbides. Nickel and copper improve corrosion resistance in small quantities. Molybdenum helps to resist embrittlement. Zirconium, cerium and calcium, increasing the strength by controlling the shape of inclusions. Manganese sulfide, lead, bismuth, selenium, tellurium, and increase workability.
Alloying elements tend to form either compounds or carbides. Nickel is very soluble in iron, so it forms a connection. Aluminum is dissolved in the ferrite. Silicon is also very soluble. Manganese is mostly dissolved in the ferrite. Chromium forms a partition between the ferrite and carbide phases in the steel. Type chromium carbide forms depends on the amount of carbon and other alloying elements present. Tungsten and molybdenum form carbides, if there is enough carbon and the absence of strong carbide forming elements (for example, titanium, and niobium). Vanadium, titanium, niobium and strong carbide forming elements to form vanadium carbide, titanium carbide and niobium carbide, respectively.
Alloying elements also have an effect on the temperature of eutectoid steel. Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements austenitic structure can be obtained at room temperature. Carbide-forming elements enhance the eutectoid temperature, these elements are known as ferrite stabilizing elements.
|Effects of alloying elements for steel|
|Aluminium||0.95–1.30||Alloying element in nitriding steels|
|Boron||0.001–0.003||A powerful hardenability agent|
|4–18||Increases corrosion resistance|
|Manganese||0.25–0.40||Combines with sulfur and with phosphorus to reduce the brittleness. Also helps to remove excess oxygen from molten steel.|
|>1||Increases hardenability by lowering transformation points and causing transformations to be sluggish|
|Molybdenum||0.2–5||Stable carbides; inhibits grain growth. Increases the toughness of steel, thus making molybdenum a very valuable alloy metal for making the cutting parts of machine tools and also the turbine blades ofturbojet engines. Also used in rocket motors.|
|12–20||Increases corrosion resistance|
|Higher percentages||Improves magnetic properties|
|Titanium||-||Fixes carbon in inert particles; reduces martensitic hardness in chromium steels|
|Tungsten||-||Also increases the melting point.|
|Vanadium||0.15||Stable carbides; increases strength while retaining ductility; promotes fine grain structure. Increases the toughness at high temperatures|