Industrial foundries manufacture formidable products, mammoth components and intricate parts that go into service in the harshest applications. The thing is, we can’t just melt iron and fill a cavity. The cooled metal part would hold its form and even look tough enough to handle the stresses of its coming role, but this isn’t the way of steel and cast iron. True steel alloys and toughened cast iron products pass through a trial by fire, a heat treatment process that imbues the disorganized microcrystalline structure of each newly solidified metal part with important properties. Strength and ductility number among the most important of these thermally injected features, but there’s more.
Quite a few cast iron products don’t require a heat treatment process, but, when added as part of the production work, the “grain” of the metal changes, and leading to unparalleled processing options. The addition of carbon to iron, for instance, alters the malleability of the metal and makes hard iron easier to work. Next, tempering and annealing work remove post-process stress from the material, which improves overall machinability and refines the metal until its mechanical properties match customer specifications. Steel and cast iron processing stages both subscribe to such grain-altering principles.
It’s next to impossible to pass a day without interacting with steel. Stainless steel cutlery fills our kitchen drawers. Carbon steel, another term used to describe cast iron, is hard but malleability is lost, resulting in brittleness and poor workability. The heat treatment process rules all of these attributes, balancing hardness or brittleness against ductility, malleability against weldability, and so on, right down the line to the surface finish. Indeed, we see the polished chromium-injected exterior of a stainless steel countertop used due to its impermeable surface finish, a look that’s as aesthetically desirable as it is easy to clean.
Our manufacturing sector scours the globe for raw materials, for iron and carbon and other alloy-oriented elements, at which point it’s melted and formed as ingots. The ingots are again melted and this time poured into moulds, but the product isn’t ready for the market. Fire and water, oil and chemical baths enter the scene, pushing the metal to its limits, adding new elements to the mix so that alloys can be formed, and generally shaping the mechanical and surface properties of iron so that the metal has the inbuilt aptitude to properly fulfill its application.
Although classed as chemistry, the nitriding process identifies closely with heat treatment technology. In describing the stages, we see the alloyed steel component enter an oven-hot realm, an environment where metal is heated until its microcrystalline structure is on the cusp of transformation. Gaseous ammonia is added to the atmosphere at this point, and a complex interplay of temperature-derived wizardry takes place on the surface of the hot metal. Filled with ammonia, the gas is a cloud of short-chain molecules, one that binds nitrogen to hydrogen. As the gaseous compound comes into contact with the hot surface, it breaks down, disassociating into pure nitrogen and hydrogen.
As the controlled atmosphere splits into its basic atomic constituents, a fine case hardened surface develops on the object. This isn’t a separate film, it’s a part of the metal, a case hardened layer that leapfrogs quenching. And, since quenching is oftentimes a source of structural distortion, the chemical stage has achieved hardening without the introduction of quench-induced stress. Nitriding furnaces add surface wear resistance, improve fatigue resistance, and the layer provides a substantial corrosion barrier. Meanwhile, all of this case hardened proficiency comes without a price. The temperatures used are below the transformative thermal barrier, so base mechanical properties remain unchanged.
Computer monitored atmospheric injection combines with a thermostatically managed workstation to ensure the workflow is optimally conducted from start to finish. The uniform application of the ammonia gas reaches into every obstructed corner, every bore hole and recessed geometrical outline to react with the hot metal of the alloy. Special tapes are available to mask sections of the part if this is a desirable feature. This intelligent masking practice blocks off areas that require further machining.
Finite control of the nitriding process strengthens steel alloys without adding stress. The highly desirable case hardened feature requires no quench work, is atmospherically controllable, and added diffusion stages can easily eliminate a chalky-white layering effect without affecting productivity. In addition to gaseous methodologies, there are also salt bath and plasma nitriding facilities to consider.
On the flip side of this phenomenon, oxidation can be regulated so as to add a protective coating to a metal. A black oxide coating, for example, is a potent corrosion defeating mechanism because the addition of a surface-oxidized layer prevents further interactions between the underlying metal and the oxygen in the atmosphere. And, since the air is predominantly full of oxygen, this is a vital procedure if a chemically reactive metal is to be protected from rust (a less fanciful term for oxidation). In addition, the controlled application of the coating lends an attractive finish to the surface of the metal, an aesthetically pleasing look that’s dark, meaning harsh light reflectance is attenuated while good looks are maximized.
There are a handful of easily controllable methods used in the industry. Electrochemical methods use a dry technique where an anodizing reaction takes place as passive electrolytic films are produced. Just like the anode of a battery, a generated potential difference causes a substrate to form, and the surface that interacts with the air then discolours and oxidizes. More popular techniques abound in industry, with chemical acid baths and pools of alkaline compounds acting as hot liquid baths. Both methods can, of course, be combined to optimize the process.
A black oxide coating adds something special to a product in terms of visual appeal. Like burnished steel, the low reflectance factor adds a lustrous finish to weaponry. Surgical instruments benefit from reduced glare, allowing a sharp pair of eyes to focus on the job at hand. And the coating isn’t separate from the metal, which means it can never crack or peel like a standard finish would when the going gets tough.
Suitable for hard-working tools and industrial locales, the chemically strengthened coating is as decorative as it is tough, and the underlying metal is now quite immune to rust, even when the component experiences shock, vibration, or exposure to the elements.
Product conformity depends on control systems and an event-free setting born of a pollutant-less environment. All but eliminated by tightly maintained quality standards, the final pollutant of note in this scenario is air. Remember, ambient air changes when subjected to heat, meaning it creates convection currents. Such thermal events are unpredictable and likely to jeopardise the distribution of thermal energy, meaning processed parts will be compromised. This is unacceptable, especially when the unevenly distributed heat is applied to geometrically complex components.
If a super-hot industrial-grade furnace is to achieve true uniformity, then a vacuum is a highly desirable part of the process. Heat is released, it radiates, and there is no deviation of thermal activity to deal with. The vacuum favours a linear transmission of generated energy, which results in the kind of controlled environment that falls in line with computer regulating subsystems. In fact, the majority of modern vacuum heat treatment equipment is appointed with this kind of control circuitry, logical timing controls and compact housings that incorporate a modular series of heat treatment stations, with every station falling within the vacuum, thus removing the unpredictability associated with atmospheric convection.
Operating at a fiery temperature range that shifts between 1,300°C and 1,600°C, the vacuum heat treatment process remains stable and predictable, which equals a repeatable procedure, one that will consistently output identically case hardened components. Quenching is part of the internalised cycle, so hardness and wear resistance properties are equally open to computer-initiated regulation. Special thermocouple controls maintain the relationship between heating and cooling sequences, matching both against the quenching stage to create heat treated parts that meet or even exceed aeronautical standards and military assessed specifications.
Free of all gaseous contaminants, the process is now doubly open to material property manipulation through atmospheric injection. In short, the furnaces can add inert gases and catalysing agents, carbon and other specific gases that change how the metal absorbs alloying materials, thus broadening the functions of the furnaces to incorporate gases that fare well in a vacuum.
Hardening takes place in this controlled environment, as does residual stress relief and other practices that increase the workability of the component. The induction hardening and tempering process is a member of this elite heat treatment family, an innovative method of using state-of-the-art technology to localize contact-free alterations in the microcrystalline structure of a specific metal. Let’s take a closer look at the advantages of this disciplined heat treatment procedure.
Induction hardening and tempering uses high-frequency electrical currents and scalable electromagnetic fields to achieve precise control of heat patterns, meaning the active thermal stage can treat individual sections of a part to create a precise hardening profile.
Faster than traditional tempering, the equipment switches to a low-frequency electrical induction circuit to improve the hardness-to-toughness relationship of the product. Ductility and machinability is delivered in minutes instead of hours, making the overall job far more productive when it’s used as part of a manufacturing cycle.
Electrical induction technology utilizes electromagnetic fields to “induce” a thermal event, a selective high-temperature spike that is controlled by electronic circuitry. The contactless nature of the technique requires kilowatts of electrical energy, resulting in two contact-free methods. First, static (single-shot) hardening holds the metal piece in place. The electrical coils are located around the part and triggered to treat individual sections, thus hardening them to create an exact hardening profile. The transverse model turns this technique on its head by progressively passing the component through a series of charged coils. The latter technique is favoured when elongated metal parts required hardening. These include but are not limited to shafts, steel pins, drive linkages, and axles.
Every advantage highlighted thus far covers control and narrow definition targeting, for these factors mean everything when induction hardening and tempering is incorporated as a lead section of a production line. Temperature profiles bring all of the above benefits together. The induction hardening stage is open to a precision control model, a performance setup that ties with computerized systems to apply rigorously tested hardening curves and to reinforce this mechanically accurate heat treatment mechanism with an equally efficient tempering solution, one that leverages the properties of electronically derived electromagnetic fields.