If we’ve placed a great deal of emphasis on materials, there’s a good reason for this seemingly off-kilter approach. Materials must be heat treated if the mechanical and physical properties of a component are to comply with the rigorous requirements found in today’s industrial applications. Now, with that important fact clarified, let’s introduce our audience to one of the sophisticated dimensional variables encountered in today’s contemporary engineering scenarios.
It’s hard enough targeting the microcrystalline structure of a flattened profile, so imagine the ramifications incurred by a curving outline, a metallic shape that has an inner and outer surface area. Simple geometry says that the two radial tracks curve identically when they’re part of a concentric configuration, but the thickness or distance between the two curving surfaces will dictate overall surface area. In short, due to the outer diameter of the part having a slightly larger surface area, a location-sensitive heat treatment method is needed.
Bearings use the above profile. Two or more races cage a series of sliding elements, and these elements skate along the tracks when a rotating mechanism turns. The bearings use balls and flattened pins to keep the two races separate, which is just as well since heavy loads and high velocities generate friction, a loss factor that’s observed as heat. Fortunately, the heat treating of spherical radius items works its magic on both the inner and outer race surface areas. An induction hardening methodology is typically employed here, with the targeted hardening process specifically addressing the load-carrying characteristics of this essential friction-inhibiting mechanism.
As the focused induction heating method armours arcing ball races, thus preventing heat and load-induced fractures, the flame hardening technique finds its own way into processes that harden larger curving surfaces. Vehicle axles and turbine drive shafts gain hard-as-nails toughness by passing through oxy-fuel jets, ignited flames that reach unimaginable temperatures. The heat treatment trial-by-fire regime strengthens the entire cylindrical form by rotating the component, but a quench stage also adds immersion-derived toughness to specified sections of the part.
It’s true that statically mounted structural parts bear mechanically challenging design loads, but rotating parts are located at the crux of all moving machinery. They’re the power transmitters and heat mitigators of the industrial realm, so the heat treating of spherical radius items must be attended to with diligent engineering acumen, for if one of these drive shafts or bearings were to fail, everything would come to a halt.
The principal objective of this discussion is to determine the causes of distortion and residual stresses in heat treatment. That’s something of a mouthful to read out, but these attribute-skewing stress factors must be accounted for if an alloy-strengthened component is to retain dimensional and mechanical viability during its passage through a material-torturing heat treatment cycle.
Distortion on an alloy-hardened scale is caused by tensile and elastic deformation. A thermally active segment of the operation injects stress into one part of the object while releasing compressive tension on another section. A stress gradient forms as these competing forces fight for dominance. Stress relieving techniques neutralize such adverse effects, but, again, these methods clash with the inherited properties of the metal part as it expands and contracts.
Passage through a machining and forming station breeds uneven mechanical forces, but the shop eliminates such negative events by employing a series of stress relieving methodologies. Conversely, thermal duress occurs when the metal reaches its phase transformation point. The distortion is induced by non-uniform heating, perhaps due to a poorly configured furnace, but the structural properties of the metal also play a role. Impurities cause distortions, as do the disparate elements that form the alloy. Even the volume and geometrical complexity of the worked product influence this distortion quotient, with outer material surfaces cooling faster than the internal volume.
When these structure-weakening effects are left untended, cracks may develop over time. The product is potentially dimensionally out-of-tolerance, mechanically substandard, and not finished. A diligent shop offsets distortion and residual stresses in heat treatment processing by knowing how these competing forces are generated. The facility then uses post heat treating technology to refine the part and prevent these stress-induced forces from being locked inside the product. Again, and this is worth repeating, slight effects cannot be avoided when the part is subjected to such phase transformative temperature extremes. For example, even the phase conversion of an alloy’s austenite state to its martensite form incurs volumetric change due to the fact that the latter alloy form is incrementally larger than the initial phase-transforming form.
Preheat treatment and thermal uniformity throughout the furnace does reduce material stress, but the superior option is to always incorporate a strong finishing station, a section of the facility dedicated to freeing these residual forces and straightening the component until it conforms to any and all designated design specs.
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.
In documenting the essential characteristics of the black oxide coating process, we’ve illustrated the engineering science and the main advantages of the procedure. More than a smart finishing process, a material depositing technique; the metal surfaces that undergo this process are actually converted into a new form, one that shields the underlying metal while gifting it with a superb onyx-black finish. Now, on knowing that we’ve satisfied the basics, let’s take a closer look at one specific benefit, an anti-corrosion feature that benefits countless industrial domains.
Hot and cold blackening techniques deliver contrasting corrosion properties. For example, a standard room temperature procedure produces a black oxide coating with nominal anti-corrosion features, but the hot process delivers a more uniform finish, a coating that’s far more resistant to corrosion. Alternatively, these two techniques can be further reinforced. Post-processing stations apply supplementary coatings, oils that dictate just how rustproof the part will be once it has passed the after-finish stage.
Unlike electroplating technology and several of the other corrosion-resistant techniques on the market, the black oxide method partners with anti-corrosive aptitude with a bundle of other tempting features. First of all, and this feature makes a lot of sense if the parts are manufactured according to high-tolerance engineering standards, the process only converts the surface. It doesn’t add to that surface. In effect, dimensional stability is retained.
Metal components are intended for a whole host of applications, some of which will incur physical deformations. A blackened component won’t suffer from abrasions or scratches, mechanical intrusions that would remove a deposited finish and expose the underlying ferrous or non-ferrous surface. Instead, the black oxide layer is part of the original metal, which means it exhibits the same strength as the original alloy.
When a fastener, machine part, or other metal component oxidizes, its microcrystalline structure alters and becomes coarse. The screw thread of the fastener is next to impossible to tighten or loosen, the metal part deteriorates, and material is sacrificed to the oxidizing process. Black oxide coatings reverse this trend by adding a matte black lubricated finish, a layer of slip-enhanced mechanical fortitude.
It helps to remember that the word “oxide” features prominently in this coating procedure. The blackening salts are designed to purposefully control an oxidizing regimen, a finishing technique that shields the metal from corrosive influences. Additionally, the option to add a post-finish oil really enhances this water-resisting feature.