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.
Metal fabricators employ numerous tooling stations, which is as it should be because complex geometrical profiles are processed within these expert workshops. Some tools bend, others drill, then, waiting in the wings, there are bandsaw cutters and shop presses waiting. In essence, the workpieces are exposed to enormous stresses. These cold work stresses steal away part’s ductility. With that drawback in mind, let’s talk about workpiece recovery.
Annealing Equipment: Resetting Workpiece Ductility
It may take an arsenal of metal fabrication tools to produce a certain geometrical profile, but the effects of this cold-worked processing environment needn’t be permanent. Intransigent microcrystalline damage, a sort of metal memory, is deletable. To eliminate this effect, annealing equipment enters the workshop tooling chain at a predetermined point, then the physically inflexible part is slowly heated until its microstructure softens. As it softens, the part regains its former pre-cold worked properties.
A Metal Fabrication Facilitator
As we can see, without annealing equipment, a machined part may as well have a countdown clock stamped into its hardened material. A bend here, a cut there, each additional processing stage slices form-manipulating seconds off of the figurative timepiece. When that clock is in the red, the partially finished part becomes rigid, perhaps even brittle. Any further bends or cuts stand a good chance of sending fractures through the part. Even if fractures don’t develop, invisible weaknesses proliferate while ugly ripples grow along the inner turn of a bend. In short, for lack of an annealing station, the plastic deformation feature attached to the part has taken a leave of absence. Again, that absence is temporary, but it’s only temporary if this heat treatment process is properly plugged into the metal fabrication chain.
The Importance of Normalising
This heat treatment solution varies the functions of annealing equipment ever so slightly. Primarily, the normalising variant is geared towards ferrous alloys, so steel parts are subjected to this ductility resetting process. As for procedural differences, normalised parts are typically cooled by air or under a protective gas. In this way, parts decarburization is circumvented while grain uniformity and plasticity is regenerated.
When metal fabrication shops are responsible for forming complicated workpieces, some degree of cold worked inflexibility is inevitable. In order to sidestep this potentially expensive issue, time has to be turned backwards within the material. Annealing equipment acts as the time machine, a heat treatment solution that heats the part slowly until all of its workable mechanical properties are reset.