Alloy steels contain varying amounts of different metals and materials to specialize their properties. Some of the most common materials added include chromium, molybdenum, nickel, and silicon.
Chromium is added in smaller amounts (0.5-2%) to increase hardenability and larger amounts (4-18%) to increase corrosion resistance.
Molybdenum is added in amounts of 0.25-0.40% to increase the toughness of the steel.
Nickel is added in smaller amounts (2-5%) to increase toughness and in larger amounts (12-20%) to increase corrosion resistance.
Silicon is added to steel in smaller amounts (0.2-0.7%) to increase strength and in larger amounts (>2%) to improve its magnetic properties.
Types of alloy steel
There are multiple subcategories of alloy steel. These include:
- Low-alloy steel
- High-strength low alloy (HSLA) steel
- High-alloy steel
- Stainless steel
- Microalloyed steel
- Advanced high-strength steel (AHSS)
- Maraging steel
- Tool steel
Low alloy steels generally contain less than 8 wt.% non-iron elements, whereas high-alloy steels contain more than 8 wt.% non-iron elements
Properties of alloy steel
Alloy steels can contain a wide variety of elements, each of which can enhance various properties of the material, such as mechanical thermal and corrosion resistance. Elements added in low quantities of less than around 5 wt.% tend to improve mechanical properties, for example increasing hardenability and strength, whereas larger additions of up to 20 wt.% increase corrosion resistance and stability at high or low temperatures [2].
The effects of adding various elements to steel, along with the typical amounts in weight fraction, is summarised in the table below [2].
| Element | Symbol | wt. % | Function |
| Aluminium | Al | 0.95–1.30 | Alloying element in nitriding steels |
| Bismuth | Bi | – | Improves machinability |
| Boron | B | 0.001–0.003 | Improves hardenability |
| Chromium | Cr | 0.5–2.0 | Improves hardenability |
| 4–18 | Corrosion resistance | ||
| Copper | Cu | 0.1–0.4 | Corrosion resistance |
| Lead | Pb | – | Improves machinability |
| Manganese | Mn | 0.25–0.40 | Prevents brittleness in combination with sulfur |
| >1 | Increases hardenability | ||
| Molybdenum | Mo | 0.2–0.5 | Inhibits grain growth |
| Nickel | Ni | 2–5
12–20 |
Increases toughness
Improves corrosion resistance |
| Silicon | Si | 0.2–0.7 | Increases strength and hardenability |
| 2 | Increases yield strength (spring steel) | ||
| Higher % | Increases magnetic properties | ||
| Sulfur | S | 0.08–0.15 | Improves machinability (free-machining steel properties) |
| Titanium | Ti | – | Reduces martensitic hardness in Cr steels |
| Tungsten | W | – | Increases hardness at high temperatures |
| Vanadium | V | 0.15 | Increases strength while maintaining ductility, promotes fine grain structure |
Overall, in comparison to carbon steels, alloy steels can exhibit increased strength, ductility and toughness. The disadvantages, however, are that alloy steels usually have lower machinability, weldability and formability.