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.

Contents

Type of Alloy Steel

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.

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) and nothing else (excepting negligible traces via slight impurities); these are called carbon steels. 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 (Mn) (the most common one), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), silicon (Si), and boron (B). Less common alloyants include aluminium (Al), cobalt (Co), copper (Cu), cerium (Ce), niobium (Nb), titanium (Ti), tungsten (W), tin (Sn), zinc (Zn), lead (Pb), and zirconium (Zr).

Properties

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.

Although alloy steels have been made for centuries, their metallurgy was not well understood until the advancing chemical science of the nineteenth century revealed their compositions. Alloy steels from earlier times were expensive luxuries made on the model of "secret recipes" and forged into such tools as knives and swords. Modern alloy steels of the machine age were developed as improved tool steels and as newly available stainless steels. Today alloy steels find uses in a wide array of applications, from everyday hand tools and flatware to 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:

  1. D6AC
  2. 300M
  3. 256A
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]

The alloying elements tend to form either solid solutions or compounds or carbides.

Alloying elements also have an effect on the eutectoid temperature of the steel.

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

<span class="mw-page-title-main">Alloy</span> Mixture or metallic solid solution composed of two or more elements

An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, opacity, and luster, but may have properties that differ from those of the pure metals, such as increased strength or hardness. In some cases, an alloy may reduce the overall cost of the material while preserving important properties. In other cases, the mixture imparts synergistic properties to the constituent metal elements such as corrosion resistance or mechanical strength.

<span class="mw-page-title-main">Steel</span> Metal alloy of iron with other elements

Steel is an alloy of iron and carbon with improved strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels, which are resistant to corrosion and oxidation, typically need an additional 11% chromium. Because of its high tensile strength and low cost, steel is used in buildings, infrastructure, tools, ships, trains, cars, bicycles, machines, electrical appliances, furniture, and weapons.

<span class="mw-page-title-main">Cast iron</span> Iron-carbon alloys with a carbon content more than 2% and silicon content between 1 to 3%

Cast iron is a class of iron–carbon alloys with a carbon content more than 2% and silicon content around 1-3%. Its usefulness derives from its relatively low melting temperature. The alloying elements determine the form in which its carbon appears: white cast iron has its carbon combined into an iron carbide named cementite, which is very hard, but brittle, as it allows 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.

A period 4 element is one of the chemical elements in the fourth row of the periodic table of the chemical elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The fourth period contains 18 elements beginning with potassium and ending with krypton – one element for each of the eighteen groups. It sees the first appearance of d-block in the table.

<span class="mw-page-title-main">Austenite</span> 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.

<span class="mw-page-title-main">High-strength low-alloy steel</span> Type of 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.

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.

<span class="mw-page-title-main">Carbon steel</span> Steel in which the main interstitial alloying constituent is carbon

Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:

<span class="mw-page-title-main">Tool steel</span> Any of various steels that are particularly well-suited to be made into tools and tooling

Tool steel is any of various carbon steels and alloy steels that are particularly well-suited to be made into tools and tooling, including cutting tools, dies, hand tools, knives, and others. 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, as for example in cutting, machining, stamping, or forging.

<span class="mw-page-title-main">High-speed steel</span> Subset of tool steels

High-speed steel is a subset of tool steels, commonly used as cutting tool material.

<span class="mw-page-title-main">Maraging steel</span> Steel known for strength and toughness

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 very-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.

Ferroalloy refers to various alloys of iron with a high proportion of one or more other elements such as manganese (Mn), aluminium (Al), or silicon (Si). They are used in the production of steels and alloys. The alloys impart distinctive qualities to steel and cast iron or serve important functions during production and are, therefore, closely associated with the iron and steel industry, the leading consumer of ferroalloys. The leading producers of ferroalloys in 2014 were China, South Africa, India, Russia and Kazakhstan, which accounted for 84% of the world production. World production of ferroalloys was estimated as 52.8 million tonnes in 2015.

<span class="mw-page-title-main">Staballoy</span> Depleted uranium alloy

Staballoy is the name of two different classes of metal alloys, one class typically used for munitions and a different class developed for drilling rods.

<span class="mw-page-title-main">Chromium(II) carbide</span> Chemical compound

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.

<span class="mw-page-title-main">Intergranular corrosion</span> When crystallite boundaries are more corrosive than their interiors

In materials science, 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.

<span class="mw-page-title-main">Austenitic stainless steel</span> One of the 5 crystalline structures of stainless steel

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 such as nickel, manganese and nitrogen. The Incoloy family of alloys belong to the category of super austenitic stainless steels.

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