Alloy Steel: Properties, Processing and Applications

Alloy steel is a class of steel that, in addition to carbon, is alloyed with other elements, ranging from 1 wt.% to 50 wt.%, which are used to enhance the material’s various properties [1].

These elements commonly include manganese, nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common elements include aluminium, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium.

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 [2]. Both typically have superior mechanical properties in comparison to carbon steels [3].

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.

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 [4].

Next, 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 into for rolling and shaping into the final form.

 

Applications & examples

As the term alloy steel encompasses numerous types of steel, its application area is broad.

Low alloy steels are used in a wide range of industries due to their extreme strength, machinability, cost-effectiveness and availability. They are found in military vehicles, construction equipment, ships, pipelines, pressure vessels oil drilling platforms and in structural components. Examples include HY80 and HY100.

High-alloy steels can be expensive to manufacture and difficult to process. Nevertheless, their superior hardness, toughness and corrosion resistance make them ideal for structural components, automotive applications, chemical processing and power generating equipment. Examples of high-alloy steels include the grades HE, HF, HH, HI, HK, and HL.

Alloy Steel.jpeg

 

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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].