What is Flame Hardening? – Process, Benefits

Flame hardening is a common surface hardening method. Metal surfaces, such as steel, are heated with a high-temperature flame and then quenched. The result of flame hardening is a hard surface that is more resistant against wear and corrosion.

The Process of Flame Hardening

Flame hardening is performed on parts made of mild steels, alloy steels, medium carbon steels and cast iron. As its name suggests, flame hardening uses direct heat from oxy-gas flames. The metals are heated to temperatures to their austenization temperature, causing the surface to change, while the core remains unchanged and soft. Once the steel surface reaches austenization temperature, quenching is to take place immediately. By quickly quenching the material afterwards, it develops a harder surface that is less susceptible to wear and corrosion. The steel surface, often composed of austenites and/or ferrites before hardening, form martensites.

Flame hardening can be applied differentially or on the whole surface of a workpiece. The final results of flame hardening are determined by the heat of the flame, the duration of the heating, and the speed, temperature of the quenching process, as well as the elemental composition of the target material.

The flame is based on gases that enable it to reach high and stable temperatures. In most common cases, it involves oxygen and acetylene, as well as propane in some cases.

The Benefits of Flame Hardening

Flame Hardening reaches surface hardness of around 55-60 HRC, depending on the base material. Case depth can be anywhere between 0.050″ and 0.250″ (0.127 mm to 6.35 mm), depending on the intensity of flame hardening.

Benefits of flame hardening steels include:

  • Improved wear resistance
  • Short processing time (in comparison to other case hardening processes, such as nitriding and carburizing)
  • Cost-effective
  • Less Distortion
  • Few processing steps
  • Higher hardness

Disadvantages of flame hardening include:

  • Increased fire hazard: Since flame hardening requires working with an open gas-fueled flame, caution is advised.
  • Martensites are known to be hard, but at the same time can be very brittle when overheated. While the surface might display higher hardness, it might also become more susceptible to cracking and flaking.
  • Flame hardening can not be applied as precisely as other case hardening processes, such as induction hardening or boronizing.
  • Oxidation and/or decarburization can occur on the material during the process.

The Difference Between Flame Hardening and Induction Hardening

In many ways, flame hardening is similar to induction hardening. They share similar steps: First heating the material to austenization temperature and then quenching to form hard martensites.

However, induction hardening does not work with an open flame. It uses the principle of electromagnetic induction to heat materials in a coil. The material is placed into alternating magnetic fields while electric currents flow into the surface of the part.

Induction hardening is more precise than flame hardening because flame hardening is controlled by an open flame, in contrast to induction hardening, which can be directed more precisely.

Another hardening treatment that can be applied more precisely is boronizing. Boronizing is a treatment in which hard borides are diffused into the surface layer of parts. This increases the surface hardness, protecting the metal from corrosion and wear, while maintaining core softness. Here at BorTec, we developed a special boronizing technique. Contact us to find out more about BoroCoat®.