Flame Hardening

flame hardening
Tribology Wikipedia > Flame Hardening

What is flame hardening?

By putting the solid red-hot steel into water ( producing martensite), and tempering the quench-hardened steel at a moderate temperature, have been known and used for thousands of years. The extensively used material for heat treatment is medium and high carbon steel. Steel has been regularly subjected to heat treatment of one form or another starting from its discovery.

Flame hardening is a surface hardening process to produce a hard wear-resistant surface on the applied part. It is generally used on medium carbon, mild or alloy steels or cast irons. This hardening process involves rapid heating of metal and then subsequently cooling it. The aim is to harden the metal while maintaining malleability for subsequent working. It requires the direct impingement of a high-temperature flame from a well-positioned burner with quenching capabilities. This hardening process is often used on the tooth flank of a gear wheel and also on the surface of a rail. The key parameter in flame hardening is achieving a very high & controlled temperature. As a fuel gas, Acetylene (ethyne) has proven to have the best heating properties for this process. When mixed with oxygen, the acetylene flame can reach a temperature of 3160°C.


Flame hardening is one of the parts of the family of heat treatment processes. Steels having a carbon content of 0.4-0.7% can be processed by flame hardening. Other materials like cast iron and cast steel products can also be processed by this process. One thing to note is that sensitive materials can undergo flame hardening, given the condition that instead of water, emulsions with an oil-like quenching behaviour should be used.

Flame hardening is performed on parts made of mild steel, alloy steels, medium carbon steels and cast iron. As the name suggests, flame hardening uses direct heat from oxy-fuel flames. The metals are heated to their austenitization temperature, which causes the surface to change, but the core remains soft and unchanged. After the material reaches austenitization temperature, quenching is done immediately. The quick quenching process develops a harder surface that is less prone to wear and corrosion. The steel surface, often composed of austenites and/or ferrites before hardening forms martensites. Flame hardening can be applied selectively or over the whole surface of a workpiece. The final results of flame hardening rely on the heat of the flame, the heating time, and speed, quenching temperature, as well as material properties of Job material.

In most common cases, it involves oxygen and acetylene, as well as propane in some cases. The flame enables it to reach high and stable temperatures. Flame Hardening reaches a surface hardness of around 55-60 HRC, depending on the job material. Case depth can be anywhere between 0.127 mm to 6.35 mm, depending on the intensity of flame.

About 3000ºC temperature level of gas flame is exposed to workpiece material and brought out into a red hot stage or recrystallization temperature and suddenly quenched by water immediately. These drastic microstructural changes lead to the increase of surface microhardness. It is mainly influenced by surface temperature, stand-off distance and quenching time.

Benefits of flame hardening steels include:

  • Improves wear resistance
  • It leads to short processing time (in comparison to other case hardening processes, such as nitriding and carburizing)
  • It is cost-effective
  • Produces less distortion
  • Consists of a few processing steps
  • Higher hardness

Flame hardening is a good choice for:

  • Large gears and machined components where whole-part quench is impractical to perform.
  • Cases of very tight dimensional tolerances, as the smaller heated area, will result in less distortion.
  • Cases when hardness is required on a small segment or on the particular surface of the gear.

Disadvantages of flame hardening include:

  • Possibilities of fire hazard: Since flame hardening requires working with an open gas-fueled flame, strict caution is advised.
  • Martensites are hard in nature, but at the same time can be very brittle when overheated. it may become more susceptible to cracking and flaking.
  • Flame hardening can not be applied as precisely as other case hardening processes, such as induction hardening.
  • Oxidation and decarburization may occur on the material during the process.

Flame hardening vs Induction Hardening

Flame hardening is similar to induction hardening in many ways. The process includes heating the material to austenitization temperature and then quenching to form hard martensites. The point to note is that induction hardening does not work with an open flame. It uses the principle of electromagnetic induction to heat materials within a coil. Alternating magnetic fields surround the material while electric currents flow into the surface of the part.

Induction hardening is more precise than flame hardening because the latter is controlled by an open flame, while the former can be directed more precisely. Another hardening method that can be applied more precisely like induction hardening is boronizing. It is a heat treatment in which hard borides are diffused into the surface layer of material. Hence, in turn, increases the surface hardness, also protecting the metal from corrosion and wear while maintaining the core softness.

Flame hardening requires less effort and can be used for almost any type of structural casting design. The equipment used for the process can range from simple hand torches to sophisticated program-controlled machinery. Heating is accomplished by means of gas torches using oxygen/natural gas or acetylene mixtures with oxygen in excess of 10-20%. The cooling is mostly done with a downstream water jet, the surface layers being hardened by the formation of martensite and ledeburite (Figure 1). When hardening sensitive steels or castings, emulsions with an oil-like quenching behaviour can be used instead of water.

Cooling during flame hardening
Figure 1 Cooling during flame hardening

Flame hardening techniques can be divided into two main groups: shell hardening and line hardening, consisting of the below-mentioned subgroups:

Shell hardening: It includes Stationary hardening, Cycle hardening, Spin hardening

Here, the process involves two steps: first, the casting is heated up completely, then the entire casting is quenched.

Line hardening: It includes Progressive hardening, Slip hardening, Progressive spin hardening

In this type of hardening process, Heating and quenching are done in one step, a narrow zone is heated and quenched by a targeted water jet. Hardening is therefore performed in the form of a line or linear pattern.

Due to the very high heating rate and its influence on the transformation temperature (Ferrite to Austenite), the hardening temperature during flame hardening is approx. 50°C greater than during conventional hardening and can be laid out according to eq. 1:

Eq. 1:

Here, Ac3 is the temperature at which the ferrite metal is completely transformed into austenite by the heating process.

The hardness penetration depth results from the combination of component cross-section, torch settings and heating duration. The hardness distortion occurring during flame hardening is generally slightly lower than with normally hardened components.


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  5. HEATON, J., BRISTOW, J., WHITTINGHAM, G. et al. Frictional Properties of Bearing Metals. Nature 150, 520–521 (1942). https://doi.org/10.1038/150520a0

Ankit works in the Mechanical Maintenance Division of Hot Strip Mill, Jindal Stainless in India. He has keen interest in HVAC , Hot Rolling Machinery & Equipment, and Industrial Hydraulics.