Fine manufacturing processes create sturdy cast iron items and robust steel-reinforced components that resonate with structural soundness, but such mechanical solidity doesn’t just happen, not without an incorporated processing stage that intelligently manipulates key metallic properties until they fit a specified application. The heat treating of steels & cast irons delivers finite mastery over these base mechanical attributes, thus altering the behavior of the metal, but why should we go through all of this trouble? Doesn’t iron come out of the ground hard and ready to serve numerous robust applications?
Iron and its many alloy-rich familial members may look strong and feel dense, but looks can deceive because these apparently tough metals are relatively disordered at the granular level. The composition of the material is hard in some areas and possessed of localized brittleness in others. It needs to be homogenized and then toughened. Like a battalion of untrained soldiers, the microcrystalline structure must be tamed by heat and taught to assemble in an orderly manner.
Of course, all of these processed alloys are bound for specific applications, so they also have to be modified to emphasize certain material characteristics while attenuating others, which means we’re talking about more than one structurally altering technique at this point. The process begins by normalizing the metal, by homogenizing the entire structure so that it can be declared uniformly treated. Hardening and ductility aren’t the goals here, but the elimination of localized mechanical irregularities is a definite objective.
The heat treating of steels & cast irons then moves on apace by imbuing the object with machined features, bent shapes and sectional cuts that bring the product ever closer to its final marketable form. Annealing is important during this fabrication stage because a slowly applied heating and cooling action relieve the metallic stress added by the machining work, thus making the part workable once more. Meanwhile, the trial by fire centers on a rigorous hardening phase, one that hardens the part by exposing it to extreme levels of heat and then immediately quenches the fire so that the required hardness rating is locked in place.
What’s been described here is a basic linear run, but the heat treating of steels & cast irons will typically use a cyclical chain, one that works with the machining phase to clarify a form-versus-function configuration. Form has been realized through tooling while material function has been established according to client specifications. And, finally, a necessary finishing stage adds polish and refinement to the overall form.
In the foundry, blazing hot furnaces melt metal and produce castings, parts that are imbued with crude properties. It is then down to special alloy-refining facilities to alter those material characteristics, to intelligently manipulate key microcrystalline properties within the alloy so that the metal suits its application. Of these newly injected features, hardness and durability rank very high, perhaps because these are the features we all associate with a toughened metal. Anyway, regardless of these musings, let’s take a look at two of the easily identifiable hardening techniques, the processes we know as flame hardening and induction heating.
Both of these fine engineering processes yield excellently finished metal components, parts that have been case hardened to enhance the mechanical properties of the product. Still, there are differences to be found, and we should clarify those contrasting features. First of all, the induction process is a whole-unit one, a technique that uniformly case hardens a subject part. Conversely, flame hardening relies on convection and conduction, two influences that are hard to associate with uniform distribution. Similarly, the reliability factor of the induction method favours automation and mass production. Of course, these comparisons aren’t all characterized by a one-sided battle. The flame hardening and induction heating contest sides with the flame when cost is a factor, plus a focused flame can selectively case harden complex geometrical profiles.
Singled out as a mass production champion, the induction hardening mechanism is typically preferred, but the flame still has its place as a cost-effective and versatile hardening solution.