What is meant by alloy steel? Most commonly, the phrase “alloy steel” refers to low-alloy steels.Alloy steel is a type of steel alloyed with several elements such as molybdenum, manganese, nickel, chromium, vanadium, silicon, and boron. These alloying elements are added to increase strength, hardness, wear resistance, and toughness.The added alloying elements that are added to the base iron and carbon structure typically total no more than 5% of the alloy steel’s material composition.
Alloy Steel Advantages
Whether your project requires advanced corrosion resistance, machinability, strength, or another quality, there is an alloy steel that provides the features you need. With added heat treatment alloy steels can provide a wide range of beneficial qualities including:
- Enhanced corrosion resistance
- Increased hardenability
- Superior strength and hardness
High & Low Alloy Steel Differentiating Qualities
A high alloy steel has alloying elements (not including carbon or iron) that make up more than 8% of its composition. These alloys are less common, because most steel only dedicates a few percent to the additional elements. Stainless steel is the most popular high alloy, with at least 10.5% chromium by mass. This ratio gives stainless steel more corrosion resistance, with a coating of chromium oxide to slow down rusting.
Meanwhile, low alloy steel is only modified slightly with other elements, which provide subtle advantages in hardenability, strength, and free-machining. By lowering the carbon content to around 0.2%, the low alloy steel will retain its strength and boast improved formability.
Common Steel Alloying Elements
When it comes to steel, there are many different elements that can be added to the base material, allowing the purchaser to tweak variances until the right alloy is found. Common alloying elements include the following:
- Manganese: Used in tandem with small amounts of sulfur and phosphorus, the steel alloy becomes less brittle and easier to hammer.
- Chromium: A small percentage (0.5% – 2%) can help to harden the alloy; larger percentages (4% – 18%) have the added effect of preventing corrosion.
- Vanadium: With only .15%, this element can boost strength, heat resistance, and overall grain structure. Mixed together with chromium, the steel alloy becomes much harder, but still retains its formability.
- Nickel: Up to 5%, this alloying element will improve the steel’s strength. In excess of 12%, it provides impressive corrosion resistance.
- Tungsten: Boosts heat resistance, so the melting point is higher. Also improves the structural makeup of the steel.
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 . Both typically have superior mechanical properties in comparison to carbon steels .
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
The effects of adding various elements to steel, along with the typical amounts in weight fraction, is summarised in the table below .
| 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 | 2–512–20 | Increases toughnessImproves 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.
Production & processing
The alloying and processing methods for alloy steel depend on the desired result. The required combination of elements is first melted together in a furnace at over 1600°C for 8 to 12 hours. The steel is then annealed at over 500°C in order to remove impurities and to alter the physical and chemical properties .
Next, the mill scale (a mixture of iron oxides), which results from the annealing process, is removed from the surface of the steel with hydrofluoric acid before repeating the annealing and descaling process. Finally, the steel is melted and cast for rolling and shaping into the final form.
Sources
[1] R. Elliott, Cast Iron Technology. Butterworths, 1988, p. 1
[2] J. T. Black and R. A. Kohser, DeGarmo’s Materials and Processes in Manufacturing, 12th Edition. Wiley, 2017, p. 105
[3] “Difference between low alloy steel & high alloy steel,” Amardeep Steel Centre Blog, Dec. 27, 2017. [Online]. [Accessed: Oct. 10, 2018].
[4] B. Index, “The Alloy Steel Manufacturing Process,” Sciencing, Apr. 25, 2017. [Online]. Available: https://sciencing.com/alloy-steel-manufacturing-process-7267414.html. [Accessed: Oct. 17, 2018].
