## Flame hardening (surface-hardening-process)

Flame hardening belongs to the group of surface-hardening processes and is suitable for steels with a carbon content of 0.4 to 0.7% or for the surface hardening of cast iron and cast steel products.

The area of application is particularly limited to hardnesspenetration depths of 1 to 6mm and mainly extends to large workpieces with wall thicknesses of over 15mm and the partial hardening of surface layers of functional surfaces.

Flame hardening requires little technical effort and can be used for almost any structural casting design. The equipment used for flame hardening can range from simple hand torches to program-controlled special machinery. Heating is usually accomplished by means of gas torches using oxygen/natural gas or acetylene mixtures with an oxygen excess of 10 to 20%. Cooling is mostly done with a downstream water jet, the surface layers being hardened by the formation of martensite and ledeburite (Fig. 1). When hardening sensitive steels or castings, it is also possible to use emulsions with an oil-like quenching behavior instead of water.

Flame hardening techniques can be divided into the main groups shell hardening and line hardening, consisting of the following subgroups:

Shell hardening: 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: progressive hardening, slip hardening, progressive spin hardening
Heating and quenching is 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 AC3 temperature, the hardening temperature during flame hardening is approx. 50°C greater than during conventional hardening and can be approximated according to eq. 1:

Eq. 1:$T_H = A_C_3 + 100$

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

Additional references:
Torch hardening
Hardness
Hardenability
Hardness stress
Hardening
Degree of hardening

Literature references:
Spur G., Stöferle Th., Handbuch der Fertigungstechnik, Band 4, Carl Hanser Verlag, Munich Vienna, 1987.