Much has been covered in past posts. The basics of heat treatment technology formed a foundation, then that groundwork received new and interesting additional levels. They dealt with surface hardening issues, quenching anomalies, and all manner of tricky alloy manipulating techniques. Now we’re reinforcing the groundwork and reviewing the fundamentals of heat treating science, plus many of the physical traits that characterize this essential post-processing phase.
Property Manipulating Essentials
Heat treatment engineers master many alloy altering disciplines, yet they never lose track of one basic truth. Qualified to wield all sorts of advanced engineering equipment, the goal is still to develop a raw metal workpiece until it exhibits the desired material profile. On balance, pure metals don’t slide into a furnace. No, it’s a specially amalgamated alloy that undergoes the process. Put it this way, thanks to added carbon, to property-altering trace metals, that selected material comes to the heat treatment area with a unique fingerprint. Faced with this specially tailored alloy, it’s the job of the engineering team to process that workpiece until it’s imbued with a nominated set of mechanical and physical properties.
In-Process Stress and Desirable End Properties
After the attached ASME or SAE-AISA labelling codifiers are interpreted, the project gets underway. The goal is to convert the microcrystalline structure of the alloy, to change it so that it becomes corrosion-resistant and harder. Other essential material properties include ductility and tensile strength. Take heed, processing environments and machine shops can also inadvertently alter the structure of a worked metal part. They cut and bend, drill and mill components until cold-worked stress is trapped inside the part. Therefore, not only must the heat treatment equipment add the desired end-process material characteristics, it must also dissolve in-process work stress.
The Different Heat Treatment Options
At the most fundamental level, there are furnaces and quenching pools. The furnace hardens or softens the part. Alternatively, the outer surface is subjected to a case hardening procedure. Next, the alloy part is rapidly cooled. The quench operation locks in the desired material grain, and thus the sought after metallurgical traits are achieved. Of course, a hard component isn’t much use if it’s going to fracture. To avoid this brittle condition, the alloy needs to be tempered. As the furnace imparts an element of toughness to the tempered material, it ends up malleable and workable.
Reviews of fundamental processes are always helpful. Still, that groundwork can’t hide the complex issues that come into play when the furnaces are lit. There’s numerous alloy grain types, heat treatment systems, localized flames, all-covering induction ovens, and vacuum or atmospheric gases employed during a working heat treatment operation. Driving away below these complex elements, however, the fundamentals keep the process in check.
Black oxide is a pleasing surface conversion coating. As stated in past articles, the process dresses steel and other select alloys with a corrosion resistant skin. Although mild in nature, that rust impeding property binds equitably with the latest oil impregnating methods to produce a superior oxidization barrier. Better yet, the formation of that blackened barrier doesn’t impact a part’s dimensions, which is an especially desirable feature for smaller components.
Benefiting Smaller Metal Components
Intricate geometrical profiles dictate the outlines of essential steel parts as they exit a machining station. Over at the next metal cutting and pressing equipment line, zinc alloyed components are gaining similarly accurate dimensional shapes. Essentially, these undersized workpieces are being shaped according to a high-tolerance design methodology. Exiting the profiling equipment, the post-processing operation is approaching. What if that stage changes the dimensions of the components and alters the tightly imbued geometry to the point the small part no longer satisfies those dimensional constraints? Black oxide coatings solve this tricky issue by simply converting the existing surface. No spatial alterations take place, so the component slots into place as it’s added to an assembling frame.
A Compact Metal Parts Finish
Oiled and equipped with a corrosion resistance feature that acts as a non-dimensional finish, black oxide post-processing technology has a penchant for compact workpiece processing. In terms of productivity gains, costly alternative finishes have trouble treating the large batches that must run down production lines at speed. The post-treatment stage becomes a bottleneck, the dozens of compact fasteners, discrete equipment components, or batch-processed parts slow to a crawl on the line. And where’s the company bottom line going? It’s narrowing because of a simple issue with the parts finishing stage. In black oxide coating, the small parts are dipped in oxidizing salts, and the entire process is over in minutes, not hours. That’s a tidy little benefit when thousands of screws or drill bits are the subjects of a fast-operating finishing stage.
Imagine a heavier coating. It’s tested on blade edges, on drill bits, and it’s finally used to protect screws. The results of the experiment are disastrous. The threads of the screws are messed up, the drill bits are dimensionally altered, and those blade edges are dulled. Black oxide coatings prevent such issues from occurring by sidestepping the additive approach that’s normally associated with post-processing finishes. Instead of an added coating, the surface metal is converted into black oxide, a hardened and corrosion resistant finish that doesn’t alter thread profiles or drill bit edges in any way whatsoever.