In this heat treatment technique, a counterintuitive reaction is seemingly encountered as a metal workpiece is heated and held at a predetermined furnace temperature. Remarkably, instead of hardening the alloy, the procedure softens the part and makes it more ductile. That thought goes against the grain. Well, it seems to, but this is the intended result. Again, as proof of concept, annealed components are softened, not hardened, and here’s why.
Material Property Restoration
The annealing process performs like a heat treatment eraser. It rewinds the hands of the factory clock. To explain that colourful metaphor, picture the scene. Cold work stress has become locked inside a metal part’s microcrystalline structure. Further heat treatment operations have compounded the situation, and hours of work have been wasted. For this concept, there’s no way to actually travel back in time, but the annealing work can restore the material. It can recover the original grain structure, release stress, and make the part workable.
Subcategorizing the Different Annealing Methods
Here’s a quick look at three different ductility restoring techniques. First, let’s talk about Subcritical Annealing. In this process, the temperature of the cold-stressed part is pushed high, until it reaches its subcritical melting point. It’s here that the material softens and trapped stresses are released. In Solution Annealing, subcritical temperatures and hold/soak periods are critical. They’re used to reprecipitate alloy carbides so that they’re transformed into pearlite. Last, we come to Spheroidize Annealing, a process variation that converts high-carbon steels. In this ductility recovering method, the grain assumes a more globular form, which facilitates parts machinability.
Regarded As an Important Service
Fabrication sites and metalworking yards produce beautifully finished steel parts. They’re uniformly strong, hard all over but never brittle, and each item is structurally toughened. If it wasn’t for annealing, none of this would be possible. The grains in each workpiece would be disturbed and stressed. They’d be rejected and cast into some gigantic scrapheap. Recovery procedures, including the methods mentioned above, act like giant erasers. Think of the procedures as backwards moving clock hands if that metaphor works better. Cold worked stress fades away, grain deformations are dismissed, and over-worked components get a fresh start.
Keep in mind, though, annealing work requires professional attention. For each grain type and carbon-rich steel gauge, there are different subcritical temperatures and soak times, plus a cooling period that must be managed so that the alloy becomes ductile and machine workable once more. And that’s the key phrase, for the “once more” principle allows heat treatment technology to redevelop a metal part’s formerly workable and formable material characteristics.
A problem-solving guide for black oxide coatings, that’s the direction this article is taking. The finish should be oily black. Putting a tick in that checkbox, the coating should also be moderately corrosion resistant. Only, the inspector’s pen is wavering. There’s an issue with the ebony-hued conversion coating. Spotting problems are apparent, as are strange material discontinuities. Taking the problem head-on, let’s discuss common coating causes.
A Black Oxide Coatings Review
There’s no need to call up the information from another source, not when the principle is described right here. To keep the explanation short, think of the metal blackening technique as a surface conversion technique. A chemical reaction takes place when a salting bath converts the surface of a ferrous-rich metal into iron oxide. No, this isn’t corrosion, not exactly, it’s iron’s magnetite form. The iron oxide converts to Fe3O4, which isn’t rust. The post-treatment work converts the surface into a corrosion resistant blue-black finish. Different than paints and supplementary coatings, there’s no applied electroplating work or substrates involved.
Troubleshooting Coating Discontinuities
Before getting underway, the process team should check out the post-processing room for problems. Unlike a conventional heat treatment system, chemical reactions rule this zone. If those chemicals are impeded, then the process will produce substandard results. Dirt and oil are an issue, as are oxides and material scale. Sandblasting machines, handheld buffers, and chemical cleaners are responsible for cleaning grimy surfaces. Solving this issue, a competent team leader audits the parts cleansing stations.
If surface discontinuities are still causing trouble, the technician heads on over to the salting bath. Contaminants have found their way into the pool, as evidenced by a scum-like surface film. The oxidizing salts, thanks to the contamination, need to be replaced before the operation can get underway once more. Cranking the process back to full speed, the oxidizing temperature is uniformly applied so that dangerously thinner coatings don’t develop along a part’s flank. Nesting issues are next. If this is a batch processing operation, every item and every item surface requires space so that the chemical reactions can develop properly.
A sharp pair of eyes and a seasoned mind are essential. The spots and discontinuities are easily detected and corrected when a systematic approach is adopted. Pin up a troubleshooting guide. Memorize common problems, as found in this uniquely chemical-centric heat treatment process. For instance, grey finishes are commonly produced by oxidizing bath temperature glitches. Brown “rub-off,” another frustrating processing snag, can often be traced to high-temperature extremes or to unusually high quantities of colloidal iron, which have perhaps been left behind in the oxidizing bath.
There’s no end to the number of metals out there, all of which serve different roles in countless applications. Selecting the materials, it’s up to a metallurgically talented sourcing expert to choose the material classifications that satisfy a customer’s demands. Heat treatment technology then further processes the metal so that it has all of the physical and mechanical characteristics it’ll need to serve its application.
A Blindingly Daunting Task
How many properties can a metal sport when it is first sourced? To answer that question, think about the hundreds of ferrous and non-ferrous materials on the market. Some of them are more workable than others. Some alloys accept certain heat treatment processes, but other classifications refuse thermally active treatment, so the workpiece needs case hardening carburization or nitrocarburizing work. Elsewhere, a mild steel doesn’t possess enough carbon, chromium oxidization problems are plaguing the process, or its melting point doesn’t conform to a furnace’s current workpiece baking configuration. And that’s just the ferrous alloys. For non-ferrous metals, the issues multiply.
The Importance of Material Classification Expertise
A capable material selection professional knows every aluminium series and every steel gauge. Metric or imperial, SAE standard or AISI, alloy connoisseurs know the features to highlight and the factors that’ll facilitate an expertly executed heat treatment run. Stainless steel or aluminium, tool steel or carbon strengthened alloy, even the many non-ferrous alloys, every metal is known to the sourcing professional. And not just by name, either. The composition, austenitic or martensitic microstructure, the forged characteristics and datasheet properties, they’re all intimately recognized by sourcing professionals.
Selected and Classified Alloys: It’s Half the Battle
Correct, by buying in an alloy that serves a customer’s application specs, the heat treatment process is already halfway over. What remains is the engagement of a furnace/tempering procedure that will take the metal workpiece the rest of the way, all the way to the point that its operational parameters absolutely assure it’ll function in its eventual application. In service of this duty, coefficients of thermal expansion are assessed, melting points predetermined, and thermal conductivities recorded. At the end of the day, the primary goal here is to make sure the alloy selection and classification phase picks out an alloy that will be strong but not brittle, hard but not rigid, ductile, but not overly elastic, and environmentally capable but in no way chemically unstable.
To satisfy the above processing parameters, engineer expertise operates as a two-way street. On the one hand, the mechanical and physical specs are supplied. Meanwhile, the shortlisted selection candidates are picked out, but they can only be processed after the heat treatment facility delivers the right equipment. Given the go-ahead, the furnaces and tempering gear convert the raw workpieces into their final form.
Sintered components are produced when fine metallic powders are compacted inside special cavities. As high pressures cause densification, heat is added. Therefore, as a process precursor, this operation employs a finite amount of heat treatment toughening right at the end of the product forming procedure. However, if the tightly packed, materially diffused particles still don’t exhibit sufficient tensile strength and pure hardenability, additional heat treatment work may be deemed necessary.
Powder Metallurgy: Post-Processing Heat Treatment
The atomized powder has its binding agents and lubricants. A slightly porous metal construct has taken shape inside the cavity compaction mechanism, and the presence of material diffusing heat has even introduced a measure of heat treated hardness. Still, the project engineer knows where this product batch is heading. He knows the present fatigue resistance and hardness rating ingrained within each part just won’t cut-the-grade. To maximize hardness, to really address this hardenability issue, the components require the services of a heat treatment furnace.
Unlike Other Product Structures
Let’s say this is a batch of bushings or bearings. They’re heading for a shafting mechanism, where they’ll be placed under great stress. Loaded with their self-lubricating abilities and capillary action, the bushings address their duties with unmitigated ease. But the loads are heavy, the dynamic forces extreme, and the porous metal is under pressure. Using in-house precipitation hardening technology, the friction-mitigating components gain cross-sectional strength and uniform mass hardness. Alternatively, there are all the usual surface hardening techniques, which include nitrocarbonizing, carbonizing, and plasma nitriding. Internal stresses are less likely here, probably because the bearings avoid the machine shop. Unless the parts are taken through a post-production shape refinement phase, there’s not likely to be any tool-produced stress in sintered parts, after all. If cold work stress does rear its ugly head, however, the P/M processed parts can be annealed until those stresses are removed.
Two complex issues hamper heat treatment work, as carried out on sintered components. First of all, several soft materials are used in this sector, including bronze. Powder metallurgy work is also used to control structural porosity. The heat treatment process must account for such unique variables. There could also be a lubricating fluid stored inside those metal pores, which is common enough when the components incorporate a self-lubricating property. So, to answer the title’s question, yes, sintered components should be subjected to heat treatment if their structures lack strength and hardness. For those bearings and bushings alone, the service does supplement and optimize the metal, leaving the friction-handling components ready for the most challenging rotational duties.
A newly arrived batch of mild steel indicates this is a low carbon steel. The carbon quotient is noted, then the load is transported onwards. Putting this scenario on pause for a minute or two, we ask the following question: can low carbon steels be heat treated? Still frozen in time, the engineers ponder a workable solution. Problematically, there’s just not enough carbon in the alloy to facilitate a conventional alloy hardening process.
Sourcing an Affordable Alloy
So why would a client ever select low carbon steel in the first place? For the simple answer, think about the alloy from a financial perspective. It’s a remarkably affordable metal, and it’s not exactly hard to find. From here, though, problems rear their ugly heads. The alloy is let down by its low tensile strength. An unremarkable 0.05 to 0.25% carbon content label came attached to the stack of workpieces, and that value seems like it’s ingrained. Granted, the alloy is workable and weldable, but it’s also relatively weak, possibly even corrosion-prone. Is this really the best a heat treatment facility can do with this load of mild steel? No, there is a solution, but it’ll require some innovative processing.
Can Low Carbon Steel Be Heat Treated?
At first, the answer is a dubious yes. The workpieces take on a small degree of case hardened strength, but the inner core is still soft, so those hardened surfaces are only skin deep. Cold worked hardening uses mechanical stresses to further toughen the material, but such stresses can be unpredictable. To turn that lowercase “yes” into a big uppercase affirmative, the low carbon steel workpieces require further processing. Now, instead of transporting the parts through an atmospherically neutral furnace, the engineers recruit the services of a carburization station. This heat treatment technique adds carbon to the furnace atmosphere, which then diffuses into the low carbon alloy. The result is a corresponding rise in mechanical strength.
But low carbon steels aren’t porous. There’s not a single alloy that can lay claim to that property, not unless it’s sintered. Therefore, even by diffusing atmospheric carbon during a heat treatment cycle, the workpieces can only expect additional case-hardened depth. A 0.1 – 1.0mm depth-hardened casing is achievable economically by taking this approach. Expect low-carbon steel to be cut and formed in a machine shop, for these processes are commonly applied on mild steels. Beyond machinability, however, limited case hardening is possible in a conventional furnace. Equipped with a carburization station, that depth can reach down as far as 0.1 – 1.0 millimeters economically, but the core remains untouched and thus not structurally sound. Greater case depths can be achieved but extremely long furnace soak times are required, which negates the cheap cost of the mild steel material.