In this lecture, you will learn about alloy steel.

When you add various metallic and non-metallic elements in a specific amount to carbon steel, it will change the properties of carbon steel. We can manipulate these percentages of alloying elements in steel to achieve better property than plain carbon steel.

Alloy steel can further classify as

  • Low alloy steels: in which the sum of total Alloying elements is < 5%
  • High alloy steels: in which sum of total Alloying elements is > 5%

Alloying Elements

Commonly used alloying elements and their effects are listed in the table given below.

Alloying ElementsEffect on the Properties
Chromium Increases Resistance to corrosion   and oxidation. Increases hardenability and wear resistance. Increases high   temperature strength.
Nickel Increases hardenability. Improves   toughness. Increases impact strength at low temperatures.
Molybdenum Increases hardenability, high   temperature hardness, and wear resistance. Enhances the effects of other   alloying elements. Eliminate temper brittleness in steels. Increases high   temperature strength.
Manganese Increases hardenability. Combines   with sulfur to reduce its adverse effects.
Vanadium Increases hardenability, high   temperature hardness, and wear resistance. Improves fatigue resistance.
Titanium Strongest carbide former. Added to   stainless steel to prevent precipitation of chromium carbide.
Silicon Removes oxygen in steel making.   Improves toughness. Increases hardness ability
Boron Increases hardenability. Produces   fine grain size.
Aluminum Forms nitride in nitriding steels.   Produces fine grain size in casting. Removes oxygen in steel melting.
Cobalt Increases heat and wear   resistance.
Tungsten Increases hardness at elevated   temperatures. Refines grain size.
Alloy Steel tube
Piping Courses

Role of alloying elements

Depending on the quantities of alloying elements following properties  of material get affected such as

  • Corrosion resistance
  • Hardenability
  • Machinability
  • High or low-temperature Stability
  • Ductility
  • Toughness
  • Better Wear resistance
  • Improved Weldability

Use of Alloy Steel

Alloy steel can be used in a process area where carbon steel has limitation such as

  • High-temperature services such as Heater tubes
  • Low-temperature services such as Cryogenic application
  • Very High presser service such as Steam Header

Here you can see the common alloy steel grade that you will come across.

  • For Pipes: ASTM A335 Gr P1, P5, P11, P9
  • For Wrought Fittings: ASTM A234 Gr.WP5, WP9, WP11
  • For Forged Fittings: ASTM A182 F5, F9, F11 etc.

These P5, WP5, and F5 are having similar chemistry so that they can weld together.

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