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Flame Hardening

Last updated: July 21, 2020

What Does Flame Hardening Mean?

Flame hardening is a surface-hardening method that involves heating a metal with a high-temperature flame, followed by quenching. It is used on medium carbon, mild or alloy steels or cast iron to produce a hard, wear-resistant surface.

Flame hardening uses direct impingement of an oxy-gas flame onto a defined surface area. The result of the hardening process is controlled by four factors:

  • Design of the flame head
  • Duration of heating
  • Target temperature to be reached
  • Composition of the metal being treated

Typical flame-hardening applications include:

  • Blades
  • Gears
  • Rolls
  • Cams
  • Automotive components

Corrosionpedia Explains Flame Hardening

Flame hardening is a rapid, economical method for selectively hardening specific areas on the surface of a part. This process is applied to selected metal surfaces of carbon and alloy steels, cast and ductile irons and some stainless steels, followed by an appropriate quenching method.

Flame hardening in its simplest form is the heating up of steel to its hardening temperature by a flame and then quenching in water or oil. It is used to produce a hard case on the surface of a wide range of mechanical components.

There are four types of flame hardening:

  • Stationary flame hardening – requires that the specified area be heated
  • Progressive flame hardening – involves the use of a flame head with integrated quench capability
  • Spin flame hardening – requires the specified area being treated to be spun in front of the flame head(s)
  • Combination flame hardening – couples the progressive and spinning methods

The gas used for the heating process is a mixture of oxygen and acetylene, although occasionally propane is employed. When the surface reaches the austenizing temperature, the part is immediately quenched to produce a locally hardened surface. Typical surface hardness range is 55-60 HRC in medium carbon steels. It is used on parts such as:

  • Crane wheels
  • Cable sheaves
  • Rollers

The benefits of flame-hardening include:

  • Increased wear resistance
  • Less distortion
  • Reduced processing time
  • Ability to use low to medium carbon steels
  • Reduces cost by hardening only selective areas
  • Achieves high hardness for increased life
  • Less machining and grinding than other methods

Flame hardening can be employed in various applications. Various shapes-curved/straight, varying lengths, widths, and thickness, can be flame hardened in selective areas that are subject to excessive wear.


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