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.
The successful utilization of a vacuum heat furnace hangs on several important factors, most importantly of which would be the integrity of this vacant space. Contingent on the purity of the vacuum, the efficacy of the equipment seal, and the condition of the integrated quenching tank, furnace downtime is a manageable productivity obstacle. With that said, let’s sound out the quenching section. Is that a crack?
Quench Tank Micro-Fractures
Heavy heat loads torture this fluid station. Oil or water, that liquid absorbs thermal energy. The metal assembly containing the quench medium then acts as a heat sink. It distributes this energy, but material conduction effects are combining with the circulating hotness in the quench pool. The result is component expansion, plus a parts-fatiguing contraction effect. Micro-fractures form in the vacuum heat furnace. They propagate, then water seeps into the cracks, widening them. Seal continuity fails as component corrosion impacts the area.
Seal Continuity Defects
This complex piece of equipment operates most effectively when its vacuum is fully intact. There are no unpredictable air currents disturbing the heat treatment cycle, so the treated workpiece is processed as a distortion-free, contaminant-less product, one that satisfies the most stringent processing guidelines. However, seal flaws allow air and moisture into the void. Hot spots form, the operation does not yield a high-quality resolution, and the equipment pumps fail to pump down. If the leak continues, the vacuum won’t form, so the pumping stage operates continually. Like falling dominoes, that one near invisible leak undermines the void, allows water into the heat treatment enclosure, and causes the pumps to age then fail. Furnace downtime is an inevitability when a damaged seal fails. Check that seal with a helium leak detector.
Weighing Extraneous Causative Factors
Depending on unique design influences, certain vacuum heat furnaces can incur undesirable periods of system downtime when a subsystem component fails. For example, jacketed cooling systems are often employed in these equipment stations. They add an element of augmented thermal control to the intricate architecture of a vacuum heat treatment furnace. That cooling jacket must be maintained. If it’s compromised, perhaps by dirt or corrosion, the process is jeopardized. Gasket damage, thermocouple failure, instrumentation miscalibration, any one of these peripheral system flaws can and will incur equipment downtime.
It’s a fast-paced site, this vacuum-enabled heat treatment facility. Downtime is the one event that simply cannot be allowed here, not when workpiece integrity is the most important attribute invested by the vacuum heat furnace. Helium leak detectors are all very efficient, but this test must be conducted cold, which means more lost processing time. Check for seal nicks and gasket tears before they leak, and avoid that expensive work suspension period.
Metal embrittlement occurs during numerous heat treatment processes. Furthermore, certain electrochemical techniques are known to promote this undesirable effect. The result is a finished metal product that’s drastically weakened. Fracture-prone and flawed, the embedded microcrystalline defects cast a blemish-ridden shadow on what should be a universally toughened alloy part. If there’s an increased likelihood of embrittlement failure, what de-embrittlement operations can we call upon from our heat treatment arsenal?
Favoured De-Embrittlement Procedures
A steel core that’s laden with fracturable microcrystalline intersections equals a potential disaster. Brittle fasteners snap when this issue is neglected. The same goes for a structural steel beam, although the chance of a catastrophe is greatly increased at this point. There must be a way to counteract these tiny structural flaws, one that addresses the fault by calling upon an existing equipment station. Known as de-embrittlement, the process is applied as a carefully controlled bake operation. In effect, the flawed metal part is heated in a furnace. Raised to between 200°C and 220°C, this thermal peak is maintained for approximately 8 hours. Of course, dependent on the type of hardened steel held in the stress relief bake, that interval can increase significantly. Bake relief cycles can last as long as twenty-two hours because of this metallurgically-sensitive proviso.
Why Is The Steel Part Brittle?
Electroplating processes cause a phenomenon known as hydrogen embrittlement. The simple molecules accumulate at the plating terminal, they’re diffused into the metal, and the formerly hardened metal is transformed into a rigid but non-deformable mass. If stress is applied to that rigid workpiece, it fractures. Acid cleaning procedures are also known hydrogen generators, as are several other cleaning and electrochemical pre-treatment operations. If that acid pickling or electroplating stage must be applied, it should be intelligently managed so that the embrittlement effect is minimized. For example, a mechanically induced cleaning agent, such as sand, won’t produce a hydrogen byproduct, which is a smart move because the excess hydrogen facilitates material embrittlement.
In conclusion, if a de-embrittlement phase is added to the heat treatment operation, more time in the furnace is on the books. That time is spent subjecting the steel workpiece to a stress relieving bake, a thermally active cycle that skirts between 200°C and 220°C. Held at this temperature for a predetermined length of time, the mandatory bake ejects the fracturable structure. Incidentally, this is a time-sensitive process. It must be conducted within 3 hours of the original causative incident, be it a coating stage or a chemical cleaning procedure.