A black oxide finish imbues a worked ferrous component with an attractive ebony coating. The process transforms a thin layer of surface metal into iron oxide, so no deposits are called for here, just a chemical reaction that enhances corrosion resistance. As advantageous as this supplementary feature undoubtedly is, black oxide failure does occur, so we need to stay abreast of the common problems incurred by this oxidizing process.
Assessing Process Variables
As with any high-end engineering process, numerous variables are at work during a black oxide coating sequence. They all subscribe to a disciplined control methodology, a tightly regulated series of organized steps, but it only takes a single unbalanced ingredient to undermine everything. Black oxide failure happens when an ingredient is poorly applied or the process isn’t performed according to an exactingly conferred operational doctrine, but chemical composition isn’t the only factor that’s causing trouble.
Black Oxide: Common Failure Causes
The conversion technique, as already determined, isn’t a depositing technique. Rather, it’s a means of oxidizing the surface of a metal part, of transforming a predetermined layer so that it exhibits a uniform deep black finish. When it fails, that finish is corrupted at a molecular level. That’s a slightly worrying prospect, especially when we’re attempting to control an oxidization process. Fortunately, we can troubleshoot the issue. Common problems in application processing stations include the following:
Calling All Blackening Solutions
This list contains the worst processing culprits, so, in offering a solution, we’ll also solve the bulk of all other common and atypical problems. First of all, this is a relatively complex technique, one that “cooks” the ingredients in a precise manner. Timing issues and temperature ambiguities will obviously affect the final result, so correct oxidizing timing when the component is dipped into the salt bath. Likewise, maintain the temperature of the oxidizing tank at a predetermined level. Finally, turn to the part when the coating won’t convert the surface properly. Clean the part, desmut it, and return it to the bath for a second attempt.
Black oxide failure submits to a coordinated troubleshooting strategy, so restore all processing systems to their default settings. If this doesn’t work, then assess the part and clean it until it’s in virginal condition and ready for its oxidizing bath.
Large-scale engineering projects use machined metal components to support structural loads. They accomplish this task by accessing the strength of a selected alloy. Unfortunately, strength isn’t the only attribute contained within the metal. The worked part also contains undesirable properties, those incurred by numerous machine stations. Residual stress is the issue. Worryingly, if left unchecked, ingrained stress will undermine structural integrity, so how do we defuse this situation?
The Mechanics of Unconditional Fabrication
Fortunately, stress relieving is used as insurance for engineering projects. The process works on the base steel material to eliminate stored kinetic energy and stop that energy from undermining the stability factor that’s mandated by structural engineering science. The disciplines referenced here include vessel design and civil engineering projects.
A Mechanical Insurance Policy
When residual stresses are removed from machined frames, potential energy is leeched out of the steel. It regains its anchor-like constitution and will never deform or flex. That’s an essential attribute, one that must be instituted within the foundations of a stable structure if it’s to stand stress-free. In underscoring this mandate, it’s absolutely imperative that we employ this heat treatment process when machining final precision surfaces on load-bearing frames.
Zeroing Incurred Stress
Structures are naturally designed to cope with compressible forces, loads that push downward. Energy is then added to this scenario by waves of shearing power. None of this energy causes concern because engineers and architects have built the metal frame to withstand these forces. Residual stresses throw off these calculations and push the building dangerously towards its design limitations, which is why this stored energy must be zeroed out and eliminated.
Heat Treatment and Bar Profiles
Formed flat bars readily weld, as do machined round bars. They imbue the structure and its associated frame components with resolute strength. Next, die blocks deliver a detailed profile, one that typically stores more stress, so stress relieving is required if the accumulated deformation energy is to be moderated after a strenuous long run hot die process. The heavy gauge metal alleviation technique then works on the die during its scheduled downtime period as a carefully calibrated maintenance operation.
Stress relieving does work as an insurance policy, one that guarantees structural strength, but the technique goes deeper. Rough cut movement is reduced in material blanks. Welds in rolled metal vessel sections become movement-free and deformation agnostic, so weldment embrittlement is a non-issue during tempering work. Finally, we’ve used steel as an exemplary subject metal, but stress relieving techniques also work proficiently in extruded brass, aluminium, and other non-ferrous alloys as a crack prevention mechanism and a stress reduction aid.
Hot die materials are burdened with a formidable task, which is an elaborate way of saying high temperatures and higher pressures are at work. The forging process applies compressive force and heat to the tools used in forging processes, so deformation stress is a major issue. Fortunately, heat treatment techniques extend the lifespan of the die by injecting its material profile with a much-needed hardness boost.
Hot Die Materials Endure
Two forces assail this material. Firstly, contact force takes a big chunk out of the metal’s working life. Heat extremes also cause fatigue in the die. But it’s the cyclical nature of this event that really takes its toll, for the work is conducted again and again until the die material is aged and no longer able to function. The resulting thermal wear factor, therefore, combines with mechanically-induced fatigue to cause die failure.
Solving Die Manufacturing Costs
When the forging process is cycling, the header and die configuration work the softened alloy perfectly, but the cycle does inevitably impose a cost. The workpiece conducts energy, obviously, so the die ages. Pressure extremes have a similar effect, so the forging tools are destined to fail, which is unless they’re properly heat treated and hardened. A wide range of services come to the rescue when hot die tools require heat treatment, but even the best toughening service is only as good as its source material.
Hot Die Materials: Selecting Alloys
In general, a methodical tooling approach will select an alloy that retains its hardness rating at high temperatures, and this toughness factor won’t be affected by temperature variances. This baseline requirement provides a material backbone for the heat treatment process, so existing properties are intelligently targeted and empathized while any undesirable mechanical characteristics are minimized.
Heat Treatment for Tools Used in Forging Processes
Now that the alloy has been reinforced with tungsten or vanadium, chromium or molybdenum, it’s time for the hardening and tempering stage. Elevated heat-to-strength retention factors harden the material, but that hardness rating isn’t merely skin deep. It goes deep, deep into the cross-sectional area of the tool block so that uniform mechanical resistance is assured throughout the die material’s structure.
Forging processes use dense metal alloys that are further exposed to advanced heat treatment techniques. The baseline properties of the selected metal are deeply transformed by the process, so material conductivity is also a major characteristic, one that requires proper assessment if the hardening technique is to toughen the entire cross-sectional area of the tool.
As we discuss carbonitriding, expect to be carried through a heat treatment process that relies heavily on the control of local atmospheric variables. In particular, as the label implies, carbon and nitrogen atoms are injected into the controlled environment. Undoubtedly, this is an essential technique, one that surface hardens base steel, but how does it work?
Carbonitriding Fundamentals
When a soft metal part is heated, its mechanical properties become accessible and readily manipulable. Imagine doubling down on this engineering science by controlling the atmosphere around the part. That’s what this technique does. It uses a branch of engineering science known as gaseous diffusion to incite surface absorption. Essentially, the air around the steel becomes saturated with carbon. The diffusion process takes over, resulting in an interstitial layering effect. Finally, the part is removed from its gas-filled furnace. Case hardened, the newly heat treated surface sports 0.07mm to 0.75mm of wear-resistant martensite.
Nitrogen Infused Hardness
The fundamentals go a long way towards describing the basics, but this heat treatment technology is far from simple when it’s applied in practice. The source of the two gas mediums represents an initial hurdle. Decayed methane or propane provide the carbon component. Meanwhile, nitrogen is also introduced as an ammonia-based gas. The nitrogen increases the hardness quotient and deepens the case beyond what can be achieved by the nitrogen-less carburizing technique. Additionally, only material hardness is affected. No additional layers are deposited, so dimensional stability is not altered in any way.
Two Roads to Success
A furnace with a gas-infused atmosphere has been used as our case study, but one other method exists in a fully-fledged carbonitriding arsenal. The second version dispenses of gasses so that liquids can take on the burden of the case hardening task. Here, metal components are lowered into a special salt bath. The cyanide-heavy salt then acts as the liquid diffusion mechanism. Again, carbon is released with nitrogen. This mode of operation delivers uniform coverage, but it is best suited for small-scale batch processing work.
Relatively lower furnace temperatures are employed when carbonitriding commences. A purely carburized case requires upward of 900°C of heat, for example, whereas the addition of nitrogen lowers this temperature to approximately 800°C. In exchange for lower energy expenditure and the addition of ammonia, the process grants the low carbon steel a much harder and thicker surface layer, one that doesn’t change the dimensions of the worked part. Commonly worked products include gears, pistons, and shafts.
Gears and wheel sprockets are low profile components, but that understated physical form belies an innate knack for handling near unendurable loads. Geometrically detailed, these essential mechanical parts are machined from the finest materials. Manufactured, machined, and balanced, the parts are then heat treated so that they can tolerate high loading factors. Flame hardening is the preferred metal-toughening service on this occasion, but what are the advantages of this technique?
Directional Flame Hardening
Tight engineering tolerances manufacture dimensionally accurate single-piece gears and wheel sprockets. Teeth mesh, power is transferred, and torque is governed. A quick application of engineering principles is enough to determine these meshing teeth as the site of all mechanical loading, so this outlying area is where the heat treatment process is required. Subsequently, the hardening flame directs case hardening energy straight to the teeth, the site of known wear. Additionally, dimensional tolerances are maintained here, for the flame hardening process does not distort the geometry of the balanced gear.
Highlighting the Effects of Wear
The guilty culprit in most wear and tear studies is the point where physical contact is made. Gears mostly brush off contact caused wear by using fine lubricants, but these oily substances are mostly there to mitigate frictional influences. Consequently, flame hardening is used to comprehensively reinforce the triangular pitched rim, thus creating a case hardened ring of beveled and pitched gear contacts. Hardened in this manner, the triangular outer rim is now fully capable of enduring the abrasions and impacts that take place when gears dynamically interact.
Targeted Tempering and Highlighted Hardening
There’s a vast range of gear types available on the engineering market. Some products work in tandem with each other, so a quantifiable stress factor is a burden, but it’s manageable. But then there are worm gears and rack and pinion parts, plus a whole range of helical gears. The latter product stretches the meshing interface and adds an angular component to the mix. In order to make these different outer tooth configurations wear-resistant, flame hardening techniques change the fundamental structure of the triangular edges, from each peak to each trough, so that the wear resistance of each heat treated gear wheel is supplemented.
The meshing outer edges of these gears and wheel sprockets represent the harshest focal point in any mechanical power transmission system, so they require protection in the form of a flame hardening heat treatment procedure. Meanwhile, the rest of the wheel, its main body, remains ductile and workable.