When a discussion turns to heat treatment, aluminium isn’t the first metal that comes to mind. We think of its lightweight features, the uses of aluminium alloys as an aeronautics material and a commercial commodity. Be that as it may, there are thermally conditional aluminium alloys out there, forms of the abundant metal that can be heat treated. Starting with homogenizing, it’s time to investigate the heat treatment of aluminium alloys.
Aluminium Alloys: Heat Treatment Suitability
In its purest form, this is a soft metal. Composite variants alloy the pure metal so that it gains the capacity to achieve a transformative state when heat is applied. Interestingly, when alloyed with manganese or magnesium, the newly amalgamated alloy does not respond well to heat treatment. We need to turn to other metallic elements if we’re to accomplish this thermally transformative operation. These are the copper, magnesium silicide, and zinc alloying materials that enhance aluminium and gift the normally soft metal with greater strength.
Thermally Redistributing the Grain
Aluminium alloys undergo a physical change when they’re processed, but that material alteration tends to precipitate outwards. Imagine an aluminium casting, a product that’s cooling. Due to the innate characteristics of the material, grainy crystals form against the cool casting surface. Basically, the metal substructure is unevenly distributed. It’s soft on the outside and harder in the centre of the casting. The product is not ready for application. The heat homogenizing process cooks the aluminium just below its melting point. The thermal energy then redistributes the alloying elements until all of the soft precipitates are eliminated.
Annealing and Solution Heat Treatment
If the lightweight metal alloy becomes work hardened, it’s no longer workable. Annealing heats the workpiece beyond its upper critical temperature (300°C to 400°C), then it holds that temperature for a predetermined period. The resulting microcrystalline ‘reset’ reintroduces slip planes, an important malleability feature that’s lost if the workpiece is work hardened. As for solution heat treatment, there are similarities between this method and annealing. The major difference, a quenching phase, stops age hardening while promoting a homogenized material structure.
Time is also a strong ally here, which means the hardening process can continue after the solution heat treatment work. Stored in sheds for several days, the age hardening occurs at room temperature as the grains “lock” in place. In conclusion, though, there are heat-treatable aluminium alloys. They’re divided into several numerically labelled groups. Copper alloyed aluminium forms the core of the 2xxx series, zinc hosts the 7xxx series, and magnesium/silicon amalgams occupy the 6xxx series. Before considering one of the above heat treatment methods, the correct aluminium alloy series must be sourced.
Let’s navigate our way through a passivation process that earns its name by producing a blue-black protective layer around the subjected steel part. The heat treatment technique essentially forms a protective finish around the metal, which is why screws, cutting blades, and even hand weapons can procure that signature “bluing of steel” look. But, as with all heat treatment processes, there’s more to this technique than cosmetic appeal.
A Rust Resistant Finish
Not long after dipping into the whys and wherefores of steel bluing, we come across this primary benefit. Simply put, the passivation process makes the steel rust resistant. That’s because the heat treatment equipment produces magnetite, which is a Fe3O4 black iron oxide compound, not the Fe2O3 red oxide that’s commonly known as ‘Rust.’ As every owner of a ferrous-heavy product knows, rust is a corrosive oxide, one that flakes away and breaks down the metal. The black oxide variant holds firm, so it acts as a protective barrier, plus it delivers big in terms of a visually alluring metal finish.
The Bluing of Steel: How is it Accomplished?
In order to convert a virgin steel exterior surface into a bluish protective finish, we need to send it into a specially equipped heat treatment station. In here, the steel part is suspended in a furnace for a predetermined amount of time. But this isn’t an ordinary furnace. No, there’s a molten salt bath in here, and it’s a dip in that nitrate-rich compound, plus the thermal energy, that gives the steel its deep blue lustre. In effect, this is a controlled rusting procedure, but it’s not the nasty orange-red (Fe2O3) that breaks down ferrous metals. Rather, this is a controlled black oxide finish, a Fe3O4 membrane that uses passivation technology to form a shield material around a steel component, be it a knife blade or a screw, a gun or a hacksaw blade.
The molten salt is maintained at 300°C to 400°C, which is a high enough temperature to trigger the passivation process. Lowered into that bath, the steel part turns blue. Deeper shades of blue are reached if the component is held in the hot salt bath for longer periods. Alternatively, an air circulating furnace method can be employed. freed of messy salt compounds, this latter method uses an air circulating furnace and a predetermined quantity of steam to gain the same protective finish, one that again travels through different shades as long as the workpiece is suspended in that hot, steamy furnace interior. Importantly, the final product is rust resistant but not rustproof.
Today’s journey through heat treatment technology looks at Austempering. As always, we’re industriously seeking the best, most cost-effective way to improve the mechanical and physical characteristics of a workpiece. More than this, we’re using specific engineering methods to ensure the workpiece absolutely exhibits the traits that have been set in stone by a client. So what is austempering? Does this isothermal heat treatment procedure support our thermal processing ideal?
Austempering Process
Unlike conventional workpiece tempering, the austempering process does not use a traditional quench phase. That means there are no tanks of oil or water employed in this bath. No, the workpiece is instead bathed in molten salt. Raised to the austenitizing temperature, the carbon steel or cast iron workpiece is lowered into the hot salt. The temperature of the salt treatment varies between 240°C and 400°C, and that temperature range is governed by the type of iron or steel passing through this tempering phase. Basically, the red-hot transformative furnace is unchanged, but the tempering sequence here has undergone significant alterations. What’s the result of the thermally active tempering solution? Well, the austenitized microstructure does not cool rapidly and harden as a martensitic material. Instead of this rapid cooling result, the steel component transforms directly into a microcrystalline Bainite form.
Clarifying the Benefits of Austempering
Martensitic steel and iron parts are incredibly hard. Unfortunately, the traditional tempering path adds stress and distortion to the workpiece because the abrupt cooling method causes the carbon steel to flex. This non-uniform flex cannot be allowed in certain components, especially the parts that rely on a uniform metallic structure. Car springs, for example, are manufactured so that they uniformly deliver material strength and physical elasticity throughout the product. Here, then, is one of a host of applications that benefits from a shock-less tempering phase, as delivered by the molten salt bath used in the austempering process. Engine components, crankshafts, transmission parts, suspension springs and more, all of these stress-heavy parts gain tempered effectiveness when they’re exposed to a high-temp heat treatment stage that toughens carbon steel and cast iron components without adding cooling-incurred mechanical distortion to the final product.
Intended as a shock less workpiece toughening phase, this heat treatment process produces distortion-free springs and other dimensionally critical components. The austempering salt bath holds the treated part for minutes, perhaps even hours, at which point the now austempered workpiece is uniformly hardened, ductile and entirely wear resistant. That Bainite steel part or ausferrite cast iron component is then ready to serve in the most mechanically torturous work conditions.
It’s tough to case harden low carbon steels. Simply put, more carbon equals more hardness, a feature that even the casual observer can appreciate. One solution to this processing puzzle is to introduce more carbon. In essence, the low carbon surface is infused with a supplementary carbon source. Consequently, as the additional carbon is artificially introduced, the super-heated workpiece is slowly cooled. Let’s examine the intricacies of this process.
Carburization Facilitated Case Hardening
Just to underscore the issue, it’s hard to case harden a low carbon steel when carbon is the required process instigator. Logically, we need more carbon. We need to add carbon-rich iron shavings, carbon powder, or some other sooty medium to the heat treatment procedure. Cycled several times, the carbon medium binds itself to the surface of the carbon-low alloy until it’s case hardened. Referred to as a cementing or carburizing stage, the operation basically enhances the technique by introducing the case hardening element.
Furnishing the Carbon Medium
Depending on the furnace configuration, several carbon supplementing techniques are on hand at this stage of the project. A gas carburization process injects carbon monoxide into a heated furnace. Alternatively, there are molten liquid carbons and salted baths, which diffuse the element into the surface of the workpiece. Deposited and cemented, the slow cooling surface assumes its case hardened characteristics. Finally, we’ll use pack carburizing as a work-in-motion example, a method that illustrates the finer points of the carbon supplementation cycle.
Pack Carburization Proceduralism
Uniformly heated until the steel part glows red-orange, the component is immersed in its carburizing medium. This time, instead of a salted bath or a gaseous environment, it’s a bone charcoal mix that covers the processing part. Still furnace-hot, the workpiece is run through this cycle several times. The charcoal is binding itself to the low steel component, cementing the carbon to the part’s surface. We’re not quite done just yet. The charcoal detritus needs to be rinsed and the part hardened via a water cooling station. Finally, the annealing stage takes place, which means the subtly altered microcrystalline surface should be left to slowly cool. Further tempering work is typically carried out before the case hardening project is considered complete. Typically, the additional stages call for reheating, cooling, and water or oil quenching.
Here’s a process that can’t run on heat alone. A secondary element is required to facilitate low carbon case hardening, a literal second element. Supplied as a supplementary reservoir of sooty carbon, as a gaseous carbon monoxide compound, or simply as wood charcoal, the subsidiary medium increases surface carbon content, then ultimately cements that element to the low carbon alloy until it’s properly case hardened.
Vacuum heat furnaces perform expeditiously as atmosphere controlled heat treatment chambers. Drained of process-influencing oxygen and filled instead with an inert gas, the furnace stops scaling, stops parts discolouration, and generally improves the mechanical and physical characteristics of any processed workpiece. What happens if this sealed chamber develops a leak? The vacuum is tainted and the heat treatment process is compromised. We need to find that leak, and fast.
Fundamental Leak Management Tips
Sudden leaks are often easier to diagnose, for they’re often preceded by some other equipment defect. However, a chamber breach that develops slowly is one that will likely go undetected for some time. An unpredictability factor, therefore, exists unless a proactive maintenance strategy is adopted. In short, a scheduled leak testing check and repair program should be monitoring vacuum integrity.
A Deeper Investigation
A niggling thought occurs: if the leak is small enough, can it be ignored? The short answer is always a firm, “No.” The workpiece isn’t experiencing the full vacuum. That inert gas isn’t uniformly distributed, the reactive heating elements are showing signs of attack, and the pumping mechanism is running continuously. The leak, even if it’s a pinprick, must be found and sealed. At this point, a background history is handy. Is this a newer or older model? Has it been in service for years? Perhaps the seals have failed before? Consult past maintenance documentation, and look for a pattern.
Active Leak Detection Methods
If you remember the old method of finding a hole in the walls of a bicycle inner tube, then you’ll know what’s coming next. In this case, however, the technology takes the principle to a whole other level. Advanced solvent test mediums are sprayed on suspect surfaces. Sealed once more, the vacuum is applied to the chamber. If the pressure gauge drops and the solvent indicates a leak, then a temporary putty seal is created. Tested again, the leak is said to be found if the gauge now holds steady. Alternatively, and this technique works best for those pinhole leaks mentioned earlier, a qualified operator sets up a helium mass spectrometer, a test instrument that traces small quantities of free-floating helium.
Solvent tests and inert gas spectrometer checks are effective vacuum heat furnace leak detection solutions. However, a predictive maintenance program can be just as effective, especially when the documentation held in that program contains the history of the chamber. Used as a tool to mark areas where wear and fatigue are occurring, the pattern detected by a scheduled maintenance plan isn’t quite as active as a spectrometer, but it can provide valuable clues, signs that point to certain seals and surfaces.