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.