Effects of Flame Hardening on Steel and Iron

30 June 2017

As stated in our earlier articles, flame hardening technology selectively heats and hardens parts of a metal workpiece. Imagine the teeth of a spur gear being subjected to the flame, or perhaps it’s the leading edge of a chisel that experiences the focused flame. Looked at from the perspective of the subject steel or iron part, how do we quantify these hardening effects? Well, we begin by recruiting the Rockwell Scale.

What is The Rockwell Scale? 

Used to measure metal hardness, the Rockwell test is defined as an alloy’s ability to resist indentation. More accurately, the test uses a set load and a specially engineered indentation tool to measure how deeply the strike registers. Incidentally, Rockwell engineering data tends to go hand-in-hand with a hardness curve, a heat treatment signature that’s every bit as unique as a fingerprint.

Flame Hardening Effects: Ferrous Materials

Thermal gradients and Rockwell tables provide a great deal of information. The data illustrates temperature and hardenability relationships, it shows how many millimetres of material hardness have been applied to a subject steel or iron part, and it also depicts carbon content changes. As the oxy-acetylene torch closes on the workpiece, a thin, hard transformative layer is imparted. Importantly, this is not an additive process, so no dimensional expansion takes place. No, the process actually converts the microstructure of the existing surface into an austenite, then that processed grain changes again during the quench phase until a full martensite shell surrounds an untouched ferrous core.

Executing a Dramatic Phase Change 

The effects of the flame hardening work only tell half the story. Obviously, this is the stage of the process where the metal part is heated rapidly, but, if left to cool, that state change won’t produce a properly case hardened shell. In order to achieve the gradient curves and Rockwell numbers detailed above, the quench station must also rapidly cool the alloyed part so that the martensite phase is quickly reached. In other words, those data-illustrating thermal gradient curves tell a story, one that’s based on applied thermal energy, the period that temperature was maintained, and the length of the quenching interval.

The final component in this material-centric formula is the composition of the ferrous material. The amount of carbon in the alloy, the steel family used in the heat treatment process, even the use of nearly pure iron, all of these processing factors impact the hardening curves. In the latter instance, iron, the carbon-exempt metal gains fatigue resistance and localised strength when the flame hardening torch gets to work.

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