Alloy steelissteel 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.
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).
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.
A few common low alloy steels are:
| SAE designation | Composition |
|---|---|
| 13xx | Mn 1.75% |
| 40xx | Mo 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% |
| 43xx | Ni 1.82%, Cr 0.50% to 0.80%, Mo 0.25% |
| 44xx | Mo 0.40% or 0.52% |
| 46xx | Ni 0.85% or 1.82%, Mo 0.20% or 0.25% |
| 47xx | Ni 1.05%, Cr 0.45%, Mo 0.20% or 0.35% |
| 48xx | Ni 3.50%, Mo 0.25% |
| 50xx | Cr 0.27% or 0.40% or 0.50% or 0.65% |
| 50xxx | Cr 0.50%, C 1.00% min |
| 50Bxx | Cr 0.28% or 0.50%, and added boron |
| 51xx | Cr 0.80% or 0.87% or 0.92% or 1.00% or 1.05% |
| 51xxx | Cr 1.02%, C 1.00% min |
| 51Bxx | Cr 0.80%, and added boron |
| 52xxx | Cr 1.45%, C 1.00% min |
| 61xx | Cr 0.60% or 0.80% or 0.95%, V 0.10% or 0.15% min |
| 86xx | Ni 0.55%, Cr 0.50%, Mo 0.20% |
| 87xx | Ni 0.55%, Cr 0.50%, Mo 0.25% |
| 88xx | Ni 0.55%, Cr 0.50%, Mo 0.35% |
| 92xx | Si 1.40% or 2.00%, Mn 0.65% or 0.82% or 0.85%, Cr 0.00% or 0.65% |
| 94Bxx | Ni 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% |
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.
| Element | Percentage | Primary function |
|---|---|---|
| Aluminium | 0.95–1.30 | Alloying element in nitriding steels |
| Bismuth | — | Improves machinability |
| Boron | 0.001–0.003 | (Boron steel) A powerful hardenability agent |
| Chromium | 0.5–2 | Increases hardenability |
| 4–18 | Increases corrosion resistance | |
| Copper | 0.1–0.4 | Corrosion resistance |
| Lead | — | Improved machinability |
| 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 of turbojet engines. Also used in rocket motors. |
| Nickel | 2–5 | Toughener |
| 12–20 | Increases corrosion resistance | |
| Silicon | 0.2–0.7 | Increases strength |
| 2.0 | Spring steels | |
| Higher percentages | Improves magnetic properties | |
| Sulfur | 0.08–0.15 | Free-machining 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 |