Plain carbon steels, alloy steels, pearlitic cast irons and martensitic stainless steels may be flame hardened. The most common plain carbon steel we flame harden is K1045 and the most common low alloy steel is 4140. Tool steels may be locally flame hardened to improve the hardness of shut-off faces and localised areas subject to wear.
This is because typically flame hardening is used to harden the surface or selected areas of a component rather than the whole component. Examples are bearing journals, gear or sprocket teeth, the surface of a steel roll or wheel or the edge of a guide rail. If you really want the entire part hardened case carburizing or through hardening may be the best option.
In many cases the material is pre-hardened and the mould cavity requires no further treatment – only the shut off faces require hardening – an ideal application for flame hardening. Where the cavity does require hardening, say in a tool made from K1045, this is done by hardening high wear areas and selective strips and produces very acceptable results.
The exact carbon content of a material is critical in determining the final hardness level. For example 4140 ranges from 0.38%C to 0.43%C and produces hardness levels from 54-59 HRC when fully hardened. Component design and section thickness also have a significant effect on results obtained.
A soft section of material typically 2-5 mm in width between adjacent hardened zones or between the start and finish of an overlapping progressive hardening run. This may result in a flat spot during use.
Progressively hardened wheel treads, circular cams and rollers always have a “slip” which in most cases has no effect on the functionality of the component. As the diameter decreases the impact of a flat spot increases.
If this is undesirable the component should be designed for spin or progressive circular (sometimes called combined or ring) flame hardening.