Alloy Steel versus Carbon Steel: the differences

2020年07月08日 星期三 | Sangjoo Kim | 专门知识, 专门技术知识

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There are about 36 million different kinds of steel grades in the world. However, it is hard to categorize each and one of them. Therefore, in this article, we will introduce two main types of steel: Alloy steel and carbon steel. Carbon steel is iron with carbon added (including a trace of other elements), while alloy steel also includes other elements.

Alloy Steel:

Alloy steels have a high percentage of other elements apart from iron and carbon. Other elements, such as manganese, silicon, nickel, titanium, copper, and chromium, are also called alloy elements because they form an alloy. Alloy elements are added to improve the hardness and durability of the steel. Also, it improves corrosion resistance due to the high amount of other elements like chromium. Depending on each component’s proportion, alloy steel’s property changes.

Commonly, alloy steel has comparatively low strength, high weldability, high melting points, high ductility, and high corrosion resistance.

In addition, there are common alloy elements and traits: 

      Manganese:

  • Added to fine-tune the heat-treating requirements.
  • Requires fast quench from high temperature to a very low temperature to harden. However, fast quench has a high risk of cracking.
  • Slower cooling rate. It can be quenched in warm oil, water, room-temperature air. Example of air quenching steel: A4 tool steel, which has 1.8% to 2.2% manganese.

      Chromium:

  • Over 11% chromium, you get stainless steel, which reduces corrosion dramatically.
  • Dramatically affects strength, hardness, and heat treatment.
  • Combination of cobalt and chromium gives very high wear resistance.
  • Commonly used for cutting dies, forming, tire shearing blades, and punches.

      Molybdenum:

  • Increase corrosion resistance. Works with manganese to lower the required quench rate.
  • Increase toughness and tensile strength. Heavy load application.
  • 4140 Steel is the most common of molybdenum and chromium combinations. Also referred to as Chromoly steel.
  • Applied in heavy gears, large shafts, workhorse of the steel world.

      Vanadium:

  • During the heat treating, it helps to control the grain size of the metal. Harder and stronger.
  • Steels such as O1 and D2.

      Nickel:

  • See it in stainless steel such as stainless 304.
  • When 18% or more chromium and 8% or more nickel, you get austenitic stainless.
  • This boosts corrosion resistance, which increases toughness and impact strength.

Carbon Steel:

Carbon steels have a high percentage of other elements apart from iron and carbon. Other small amounts of elements include silicon, manganese, sulfur, and phosphorous. Commonly, carbon steel has high strength, low weldability, low melting points, low ductility, and low corrosion resistance.

Carbon steel is also divided into high carbon steel, medium carbon steel, and low carbon steel. However, unlike alloy steel, this is the main distinction between carbon steel types. Here is detailed information on each type:

      Low carbon steel:

  • 0.05% to 0.25% carbon content with maximum manganese content of 0.4%
  • Relatively Cheaper
  • Most common type of steel that does not require any particular requirements.
  • Very weldable and machinable (Relatively). Easy to work with.
  • Only way to harden it is through case hardening (heat treating). This adds carbon to the surface – harder outer layer and a softer core.

      Medium carbon steel:

  • 0.29% to 0.54% carbon content with manganese content of 0.6% to 1.65%
  • Stronger steel with good wear resistance, but thicker to form, weld, and cut.
  • Can be heat treated and tempered.

      High carbon Steel:

  • 0.55% to 0.95% carbon content with manganese content of 0.3% to 0.9%
  • Usually specialized. Not a common material used.
  • Very strong, common steel for springs and wires. A lot of compressions to get plastic deformation
  • Heat treatable but hard to machine and weld. Need annealing before cutting mechanically.

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