Black oxide is a conversion coating that is suitable for ferrous metals along with copper-based alloys, copper, zinc, stainless steel, silver solder and powdered metals. It accomplishes this through a chemical reaction since it is made of caustic oxidisers and additives. Also, it gives items a lustrous, black appearance and provides some corrosion protection. On top of all this, it offers other advantages such as anti-galling properties, smut-free finishes and dimensional stability.
Examples of Items That Benefit From a Black Oxide Coating
A wide assortment of items can benefit from being coated with black oxide, including such ones as:
• Assorted components ranging from parts to nuts and bolts
• Architectural pieces
• Tools
• Surgical instruments
Types of Processes to Apply Black Oxide
While you can achieve a black oxide coating through the hot-temperature, mid-temperature and cold-temperature processes, the first two are the most popular since they bond better to the various metals and offer improved abrasion-resistance. For the rest of this information, we will be discussing only the first two processes.
How the Hot-Temperature and Mid-Temperature Black Oxide Processes Differ
The hot-temperature method uses a temperature of 140.556-degrees C to produce a finish with black oxide iron magnetite or Fe3O4, and it will blacken a wide range of metals. The mid-temperature method accomplishes its task at temperatures ranging from 107.222-degrees C to 123.889-degrees C to coat various items with Fe3O4. The following are some other differences between these two processes:
• Hot process takes about 10 to 20 minutes while the mid-temperature one requires 30 to 45 minutes to blacken metals
• A glossy blue-black colour is achieved with the hot process, and the mid-temperature method produces a glossy black finish
• The hot method produces less sludge than the mid-temperature method does
• Mid-temperature process produces fewer fumes than the hot-temperature method
• The hot method produces a finish that is longer lasting than that of a mid-temperature method, even when the components are exposed to flexing
It is important to note that both methods create finishes that resist corrosion and abrasion. Matte finishes are possible upon request.
Which Process Is Right for Your Purposes?
Select the process that provide your workpieces the ideal finish for the conditions that they will face. Also, consider the alloy, steel or other metal in your components or other elements that need the black oxide coating.
Benefits of the Black Oxide Coating
To finish, read about the advantages of this coating in the following:
• More economical than other types of protective coatings
• All methods for applying black oxide can be performed in large batches of small components or items
• No significant dimensional changes
• Black oxide can conform to military standards
The process of induction hardening is for the purpose of hardening the surface of components made from steel or other carbon-alloy metals. Components or parts include a wide assortment of axles, pins, rollers and shafts. As much as 15-mm hardness depths can be achieved. Parts are placed in a special furnace that utilizes a coil to perform its function. When this coil is energised, it induces the parts with thermal energy via a strong magnetic field. While this process enhances the fatigue-resistance and toughness of various types of parts, the following issues do exist with this method.
Material Limitations
Typically, induction hardening is utilised on a wide variety of steels and carbon alloys. However, it is important to note that medium-carbon steels ensure the best results in hardening. Issues can arise in metals with a high carbon content, which will be discussed further later. Decarburized surfaces and surfaces with a low-carbon content will not harden properly with this process.
The Shapes of the Components Can Negate the Utilisation of Induction Hardening
Since the components must fit into the furnace without interfering with the induction coil, certain shapes may not be compatible with the shape of the coil or for that fact, the tooling of the furnace. Even though, there is a range of sizes and shapes of coils readily available for this use, some projects may require custom coils, which may cost more than the project merits. As a result, the shape and size of the components must be fully analysed to discover their compatibility with the available equipment for induction hardening.
Cracking Can Occur during the Induction Hardening Process
Another common issue that can happen with the induction hardening process is cracking. This can happen immediately during the process or as a delayed reaction days after the process. Top reasons for cracking include:
• Overheating during the process
• A high-carbon content in the steel or other carbon alloy
• Rapid heating and quenching also increases the risk for cracking in comparison to other heat-treating methods
• Geometric irregularities may interfere the proper operation of the induction coil
Distortions May Occur
The risk of distortion is higher with induction heating that with gas or ion nitriding, but it can be lower than the traditional heat treatment if it is applied to only a specific area.
Luckily, all these issues are preventable. Companies must analyse the materials closely to ensure that they contain the right contents for the ideal results. Also, the components need to be compatible with the shape and size of the induction furnace, coil and tooling. Proper monitoring is another necessity to keep problems from occurring due to overheating. When the process is performed correctly in all these areas, the results will be satisfactory.