Dark black oxide coatings look attractive. Aside from that smoky appeal, there’s another benefit to this technique. Designed to act as a corrosion-resistant finish, black oxide treatments stop ferrous parts from oxidizing. If there’s one factor that sets the process apart, it’s the amount of chemical processing that occurs during the procedure. As to how the coating performs its duties, read on for a more detailed answer.
Chemical Conversion Basics
Dipped in a hot oxide bath, the workpiece doesn’t accumulate a rustproof coating. In actuality, the ferrous surface of that part is transformed. It’s oxidized by a controlled substance. So there are no layers deposited here, no specially precipitated coatings. Instead of a separate film, the workpiece surface is altered by the hot oxidizing salts. Then, produced in the heat treatment shop, a quality assurance system checks for colour variance issues and non-uniform coating problems. That latter matter includes spotty finishes and other process shortcomings. After all, a hint of surface-corrupting residue will compromise the coating.
Does Black Oxide Treatment Stop Material Corrosion?
In the first place, this chemical conversion technique adds a mild corrosion resistance feature to the workpiece. It’s not intended as a full rustproofing solution. The newly converted ferrous layer is Magnetite (Fe3O4), a stable compound that won’t oxidize any further. Submerged in that hot pool full of sodium hydroxide, the freshly formed chemical bonds won’t react when another oxidizing substance comes calling. The corrosion-resistant surface rejects further oxidization, stays dark black, and adds a non-reflective aspect to the metal. Having said all that, is it possible for the black oxide treatment technique to go further?
Supplementary Post-Finishing Choices
The low-reflectance looks are appealing and the dark metal certainly exudes eye-catching allure, but the fact that it offers a moderate amount of corrosion stopping power is somewhat worrying. The process can stop here, and the final product will repel most oxidizing substances. Still, what happens when the metal is really exposed to the elements? If that black oxide coating is to realize its full potential, we need some post-processing polish. An oily veneer is one answer. Applied until the post-processing medium fills the slightly porous finish, the oil or wax impregnation stage reinforces the black oxide until the metal surface can really withstand a determined material oxidizing threat.
Simply put, this supplementary heat treatment stage uses a hot alkaline bath to control the part’s oxidization. Held in check in this manner, the chemically transformed metal surface stabilizes until it becomes mildly corrosion-resistant. It also turns black, forms a low reflectance finish, and adds an eye-catching aesthetic to the metal part.
Numerous variables govern the heat treatment process. If one of those variables moves beyond an allowable error margin, the toughened and tempered workpiece will likely demonstrate a set of less than desirable physical characteristics. The dimensions of the part alter. It warps or develops a materially diminished structure. Other processing imperfections include uneven case hardening, structural brittleness, and more. Eliminate heat treatment problems by adhering to the following tips.
Avoid Common Preprocessing Pitfalls
A sequentially managed preheat cycle is critical. The subject component must be evenly preheated. Following on from that tip, alloy equiaxing performs a similar function. Used to prevent workpiece warping, this normalizing stage ensures grain homogeneity.
Set Heating Boundaries in Stone
Tough steel alloys withstand massive quantities of thermal energy. Still, they’re not invulnerable. Don’t overheat the alloy. Steel, the alloy we’ve chosen here as the industry mainstay, will incur damage at around 1200°C. That damage is reversible, but it takes time (lost productivity) to make the repair.
Use a Storage-Smart Production Plan
Specialty coatings work best here as a product equalizing asset. The steel billets rest free from corrosion. No electromechanical reactions take place when the pretreated components are in contact, nor can any unsolicited environmental condition impact the technically precise nature of the heat treatment operation.
Logistically Maintained Selection Criteria
Oil or water quenching stations are flushed and cleaned periodically. Meanwhile, within the furnace, a uniform thermal envelope is monitored and timed to ensure the fruition of a demanded set of mechanical and physical properties. Case hardened or structurally toughened, the output workpiece quenches and deposits in its allocated spot without any residual stress. Refer to the furnace heat charts when mapping this procedural operation.
Injecting Process Repeatability
The furnace charts mesh with the quench substance and processing times to guarantee a fully heat treated component. Overheating errors and decarburization events are banished, residual stresses are cancelled, and the requisite surface finish is there, but what about the next object? Closely monitor the treatment run to ensure process repeatability. If the heat treatment technique delivers uneven results, initiate a maintenance check.
What common factor is shared by all of the above tips? Well, simply put, there’s not a single unknown variable in play during a heat treatment operation. Upper and lower furnace tolerances are identified, then we go deeper. We establish a signature thermal envelope by mapping the furnace charts. Then the precise nature of the quenching fluid is determined. Every single control element must be identified. Then, if there are still processing errors, it’s time to maintain the equipment.