Alloy steel

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Alloy steel is steel that is alloyed with a variety of elements in total amounts between 1.0% and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. The difference between the two is disputed. Smith and Hashemi define the difference at 4.0%, while Degarmo, et al., define it at 8.0%. [1] [2] Most commonly, the phrase "alloy steel" refers to low-alloy steels.

Contents

Strictly speaking, every steel is an alloy, but not all steels are called "alloy steels". The simplest steels are iron (Fe) alloyed with carbon (C) (about 0.1% to 1%, depending on type). However, the term "alloy steel" is the standard term referring to steels with other alloying elements added deliberately in addition to the carbon. Common alloyants include manganese (the most common one), nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common alloyants include aluminium, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium.

The following is a range of improved properties in alloy steels (as compared to carbon steels): strength, hardness, toughness, wear resistance, corrosion resistance, hardenability, and hot hardness. To achieve some of these improved properties the metal may require heat treating.

Some of these find uses in exotic and highly-demanding applications, such as in the turbine blades of jet engines, and in nuclear reactors. Because of the ferromagnetic properties of iron, some steel alloys find important applications where their responses to magnetism are very important, including in electric motors and in transformers.

Low-alloy steels

A few common low alloy steels are:

Principal low-alloy steels [3]
SAE designationComposition
13xxMn 1.75%
40xxMo 0.20% or 0.25% or 0.25% Mo & 0.042% S
41xx Cr 0.50% or 0.80% or 0.95%, Mo 0.12% or 0.20% or 0.25% or 0.30%
43xxNi 1.82%, Cr 0.50% to 0.80%, Mo 0.25%
44xxMo 0.40% or 0.52%
46xxNi 0.85% or 1.82%, Mo 0.20% or 0.25%
47xxNi 1.05%, Cr 0.45%, Mo 0.20% or 0.35%
48xxNi 3.50%, Mo 0.25%
50xxCr 0.27% or 0.40% or 0.50% or 0.65%
50xxxCr 0.50%, C 1.00% min
50BxxCr 0.28% or 0.50%, and added boron
51xxCr 0.80% or 0.87% or 0.92% or 1.00% or 1.05%
51xxxCr 1.02%, C 1.00% min
51BxxCr 0.80%, and added boron
52xxxCr 1.45%, C 1.00% min
61xxCr 0.60% or 0.80% or 0.95%, V 0.10% or 0.15% min
86xxNi 0.55%, Cr 0.50%, Mo 0.20%
87xxNi 0.55%, Cr 0.50%, Mo 0.25%
88xxNi 0.55%, Cr 0.50%, Mo 0.35%
92xxSi 1.40% or 2.00%, Mn 0.65% or 0.82% or 0.85%, Cr 0.00% or 0.65%
94BxxNi 0.45%, Cr 0.40%, Mo 0.12%, and added boron
ES-1 Ni 5%, Cr 2%, Si 1.25%, W 1%, Mn 0.85%, Mo 0.55%, Cu 0.5%, Cr 0.40%, C 0.2%, V 0.1%

Material science

Alloying elements are added to achieve certain properties in the material. The alloying elements can change and personalize properties — their flexibility, strength, formability, and hardenability. [4] As a guideline, alloying elements are added in lower percentages (less than 5%) to increase strength or hardenability, or in larger percentages (over 5%) to achieve special properties, such as corrosion resistance or extreme temperature stability. [2] Manganese, silicon, or aluminium are added during 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 solid solutions in ferrite. Chromium, vanadium, molybdenum, and tungsten increase strength by forming second-phase carbides. Nickel and copper improve corrosion resistance in small quantities. Molybdenum helps to resist embrittlement. Zirconium, cerium, and calcium increase toughness by controlling the shape of inclusions. Sulfur (in the form of manganese sulfide), lead, bismuth, selenium, and tellurium increase machinability. [5] The alloying elements tend to form either solid solutions or compounds or carbides. Nickel is very soluble in ferrite; therefore, it forms compounds, usually Ni3Al. Aluminium dissolves in the ferrite and forms the compounds Al2O3 and AlN. Silicon is also very soluble and usually forms the compound SiO2•MxOy. Manganese mostly dissolves in ferrite forming the compounds MnS, MnO•SiO2, but will also form carbides in the form of (Fe,Mn)3C. Chromium forms partitions between the ferrite and carbide phases in steel, forming (Fe,Cr3)C, Cr7C3, and Cr23C6. The type of carbide that chromium forms depends on the amount of carbon and other types of alloying elements present. Tungsten and molybdenum form carbides if there is enough carbon and an absence of stronger carbide forming elements (i.e., titanium & niobium), they form the carbides W2C and Mo2C, respectively. Vanadium, titanium, and niobium are strong carbide forming elements, forming vanadium carbide, titanium carbide, and niobium carbide, respectively. [6] Alloying elements also have an effect on the eutectoid temperature of the steel. Manganese and nickel lower the eutectoid temperature and are known as austenite stabilizing elements. With enough of these elements the austenitic structure may be obtained at room temperature. Carbide-forming elements raise the eutectoid temperature; these elements are known as ferrite stabilizing elements. [7]

Principal effects of major alloying elements for steel [8]
ElementPercentagePrimary function
Aluminium 0.95–1.30Alloying element in nitriding steels
Bismuth -Improves machinability
Boron 0.001–0.003(Boron steel) A powerful hardenability agent
Chromium 0.5–2Increases hardenability
4–18Increases corrosion resistance
Copper 0.1–0.4Corrosion resistance
Lead -Improved machinability
Manganese 0.25–0.40Combines with sulfur and with phosphorus to reduce the brittleness. Also helps to remove excess oxygen from molten steel.
>1Increases hardenability by lowering transformation points and causing transformations to be sluggish
Molybdenum 0.2–5Stable 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 of turbojet engines. Also used in rocket motors.
Nickel 2–5Toughener
12–20Increases corrosion resistance
Silicon 0.2–0.7Increases strength
2.0Spring steels
Higher percentagesImproves magnetic properties
Sulfur 0.08–0.15Free-machining properties
Titanium -Fixes carbon in inert particles; reduces martensitic hardness in chromium steels
Tungsten -Also increases the melting point.
Vanadium 0.15Stable carbides; increases strength while retaining ductility; promotes fine grain structure. Increases the toughness at high temperatures

See also

Related Research Articles

Alloy Mixture or metallic solid solution composed of two or more elements

An alloy is an admixture of metals, or a metal combined with one or more other elements. For example, combining the metallic elements gold and copper produces red gold, gold and silver becomes white gold, and silver combined with copper produces sterling silver. Combining iron with non-metallic carbon or silicon produces alloys called steel or silicon steel. The resulting mixture forms a substance with properties that often differ from those of the pure metals, such as increased strength or hardness. Unlike other substances that may contain metallic bases but do not behave as metals, such as aluminium oxide (sapphire), beryllium aluminium silicate (emerald) or sodium chloride (salt), an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opacity, and luster. Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic titanium alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. In some cases, a combination of metals may reduce the overall cost of the material while preserving important properties. In other cases, the combination of metals imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength. Examples of alloys are steel, solder, brass, pewter, duralumin, bronze, and amalgams.

Steel Metal alloy made by combining iron with other elements

Steel is an alloy of iron with typically a few percent of carbon to improve its strength and fracture resistance compared to iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant need typically an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, machines, electrical appliances, and weapons. Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties.

Cast iron Iron-carbon alloys with a carbon content more than 2%.

Cast iron is a group of iron-carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its colour when fractured: white cast iron has carbide impurities which allow cracks to pass straight through, grey cast iron has graphite flakes which deflect a passing crack and initiate countless new cracks as the material breaks, and ductile cast iron has spherical graphite "nodules" which stop the crack from further progressing.

Austenite Metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element

Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron, with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902); it exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.

Brazing High-temperature soldering; metal-joining technique by high-temperature molten metal filling

Brazing is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, with the filler metal having a lower melting point than the adjoining metal.

High-strength low-alloy steel

High-strength low-alloy steel (HSLA) is a type of alloy steel that provides better mechanical properties or greater resistance to corrosion than carbon steel. HSLA steels vary from other steels in that they are not made to meet a specific chemical composition but rather specific mechanical properties. They have a carbon content between 0.05 and 0.25% to retain formability and weldability. Other alloying elements include up to 2.0% manganese and small quantities of copper, nickel, niobium, nitrogen, vanadium, chromium, molybdenum, titanium, calcium, rare earth elements, or zirconium. Copper, titanium, vanadium, and niobium are added for strengthening purposes. These elements are intended to alter the microstructure of carbon steels, which is usually a ferrite-pearlite aggregate, to produce a very fine dispersion of alloy carbides in an almost pure ferrite matrix. This eliminates the toughness-reducing effect of a pearlitic volume fraction yet maintains and increases the material's strength by refining the grain size, which in the case of ferrite increases yield strength by 50% for every halving of the mean grain diameter. Precipitation strengthening plays a minor role, too. Their yield strengths can be anywhere between 250–590 megapascals (36,000–86,000 psi). Because of their higher strength and toughness HSLA steels usually require 25 to 30% more power to form, as compared to carbon steels.

Stellite is a range of cobalt-chromium alloys designed for wear resistance. The alloys may also contain tungsten or molybdenum and a small but important amount of carbon.

Refractory metals are a class of metals that are extraordinarily resistant to heat and wear. The expression is mostly used in the context of materials science, metallurgy and engineering. The definition of which elements belong to this group differs. The most common definition includes five elements: two of the fifth period and three of the sixth period. They all share some properties, including a melting point above 2000 °C and high hardness at room temperature. They are chemically inert and have a relatively high density. Their high melting points make powder metallurgy the method of choice for fabricating components from these metals. Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, and chemical reaction vessels in corrosive environments. Partly due to the high melting point, refractory metals are stable against creep deformation to very high temperatures.

Carbon steel Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05% up to 3.8% by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

Tool steel

Tool steel refers to a variety of carbon steel and alloy steel that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion and deformation, and their ability to hold a cutting edge at elevated temperatures. As a result, tool steels are suited for use in the shaping of other materials. With a carbon content between 0.5% and 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The presence of carbides in their matrix plays the dominant role in the qualities of tool steel. The four major alloying elements that form carbides in tool steel are: tungsten, chromium, vanadium and molybdenum. The rate of dissolution of the different carbides into the austenite form of the iron determines the high-temperature performance of steel. Proper heat treatment of these steels is important for adequate performance. The manganese content is often kept low to minimize the possibility of cracking during water quenching.

Maraging steel

Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates. Original development was carried out on 20 and 25 wt% Ni steels to which small additions of aluminium, titanium, and niobium were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels.

Titanium alloys are alloys that contain a mixture of titanium and other chemical elements. Such alloys have very high tensile strength and toughness. They are light in weight, have extraordinary corrosion resistance and the ability to withstand extreme temperatures. However, the high cost of both raw materials and processing limit their use to military applications, aircraft, spacecraft, bicycles, medical devices, jewelry, highly stressed components such as connecting rods on expensive sports cars and some premium sports equipment and consumer electronics.

Chromium(II) carbide

Chromium(II) carbide is a ceramic compound that exists in several chemical compositions: Cr3C2, Cr7C3, and Cr23C6. At standard conditions it exists as a gray solid. It is extremely hard and corrosion resistant. It is also a refractory compound, which means that it retains its strength at high temperatures as well. These properties make it useful as an additive to metal alloys. When chromium carbide crystals are integrated into the surface of a metal it improves the wear resistance and corrosion resistance of the metal, and maintains these properties at elevated temperatures. The hardest and most commonly used composition for this purpose is Cr3C2.

Intergranular corrosion

Intergranular corrosion (IGC), also known as intergranular attack (IGA), is a form of corrosion where the boundaries of crystallites of the material are more susceptible to corrosion than their insides.

Austenitic stainless steel is one of the five classes of stainless steel by crystalline structure. Its primary crystalline structure is austenite and it prevents steels from being hardenable by heat treatment and makes them essentially non-magnetic. This structure is achieved by adding enough austenite stabilizing elements nickel, manganese and nitrogen.

Microalloyed steel is a type of alloy steel that contains small amounts of alloying elements, including niobium, vanadium, titanium, molybdenum, zirconium, boron, and rare-earth metals. They are used to refine the grain microstructure or facilitate precipitation hardening.

Native element mineral

Native element minerals are those elements that occur in nature in uncombined form with a distinct mineral structure. The elemental class includes metals and intermetallic elements, semi-metals and non-metals. The Nickel–Strunz classification system also includes the naturally occurring phosphides, silicides, nitrides and carbides and arsenides.

USAF-96 is a high-strength, high-performance, low-alloy, low-cost steel, developed for new generation of bunker buster type bombs, e.g. the Massive Ordnance Penetrator and the improved version of the GBU-28 bomb known as EGBU-28. It was developed by the US Air Force at the Eglin Air Force Munitions Directorate. It uses only materials domestic to the USA. In particular it requires no tungsten.

References

  1. Smith, p. 393.
  2. 1 2 Degarmo, p. 112.
  3. Smith, p. 394.
  4. "What Are the Different Types of Steel? | Metal Exponents Blog". Metal Exponents. 2020-08-18. Retrieved 2021-01-29.
  5. Degarmo, p. 113.
  6. Smith, pp. 394-395.
  7. Smith, pp. 395-396
  8. Degarmo, p. 144.

Bibliography