Processing variables are confounded when different alloys enter a heat treatment facility. Cooling temperatures and time periods adjust accordingly to metallurgical variances. Likewise, a worked hardened metal part is subjected to ingrained stress. The plastic deformation stress is removed by heat treating the part and making it workable again. These examples apply to steel, but non-ferrous metal treatment work emulates this standard iron-heavy processing configuration, although there are differences between the two metal families.
Cold Worked Parallelism
Non-ferrous metals use the same cold work techniques as ferrous products. Copper is bent in the workshop, then aluminium and zinc are sheared. They’re drawn and compressed just like a similarly profiled steel workpiece. Naturally, the metals we just mentioned are more ductile than iron, plus they’re characterised by different transformative temperatures, so the heat treatment work envelope has to adapt to these processing changes, but that cold working layout is relatively indistinguishable from the ferrous-sourced setup. However, there are always exceptions to any rule, and such is the case here, for some non-ferrous metals are not cold workable. Certain copper alloys, for example, resist cold workshop tooling methods.
Non-Ferrous Metal Heat Treatment
Although it’s true that these metallurgical groups are amazingly versatile, that versatility factor can hamper the heat treatment process. Essentially, iron-free metals don’t react as predictably as steel when they’re hardened or otherwise mechanically altered. They do subscribe to the annealing process, though, so a strain-hardened component can be softened by heat-releasing the grain’s internal stresses. Otherwise, using aluminium as an example here, a ferrous-centric hardening and quenching cycle is simply not an option. Instead, we call upon the services of a precipitation hardening process. It’s here that lower furnace temperatures age the alloys until precipitates are formed. Quenching does follow the hardening work, just like it does in a ferrous-oriented workshop, but this is a water quenching station, not an oil bath.
As proven by these changed processing variables, a non-ferrous metal heat treatment setup deploys its equipment in a different manner. There are temperature and time variables to account for, such as magnesium’s 300-320°C annealing temperature, then there’s the kinetics, the operability changes to incorporate into the process. Non-ferrous hardening technology takes a sharp turn into a contrasting methodology, a place where precipitation hardening and water quenching take charge. On top of this realisation, many grades and alloy forms sub-branch from the non-ferrous metal groups, and these groups require cold work (cold rolling) technology to accommodate their unique hardening requirements.
Material strength isn’t a straightforward property, not when we’re talking about structurally-capable alloys. Yes, a hardened workpiece is stiff, but it could also be brittle. In reality, strength is an amalgamated property, something that combines hardness and material tempering. In using metal solidification technology as our starting point, we’ll explain the importance of hardening, then we’ll introduce tempering, a process that counters the hardening work by adding ductility to the alloy amalgamation regime.
Determining Central Precepts
The purpose of the hardening stage is to ensure it won’t deform, no matter how heavy the applied load. If that load compresses the metal part won’t collapse in upon itself, won’t fracture, nor warp. Instead, it stands resolute, with its original shape locked in place. In essence, the mechanical backbone of the processed metal is stiffened. But wait, a structure that’s imbued with this lone property could crack and crumble, unless there’s a ductility feature in place, that is. Workpiece tempering assumes this role, for this important low-temperature work phase is purpose-built as a material brittleness alleviator.
Heat Treatment Balancing and Counterbalancing
If a super-heated environment raises the transformative temperature of a selected alloy past its specified critical transformation threshold then is cooled rapidly, the workpiece hardens. Technically speaking, all pearlite content has been converted into martensite, an allotrope whose needle-like microcrystalline structure is extremely hard. The problem here is the brittleness of the martensite, an issue that can introduce stress and material deformation, although these effects are mitigated somewhat by the length of time the part was held at its transformative temperature. Tempering is required to counteract the strengthening (hardening) phase, a stage that can add material weakness (brittleness). The workpiece has been hardened and rapidly cooled, via a quenching station, so now the tempering process enters the heat treatment line as a counterbalance. Again, heat is the brittleness mitigating agent, but this is a reduced thermal load, a heat source that increases material strength by adhering to a low temperature. Intelligently managed in this manner, the tempering temperature reduces alloy brittleness while augmenting the workpiece’s strength and overall ductility.
We’ve described quite a few hardening and tempering methods over the months. They create tough metal parts and superior finishes, parts that are as material strong as they are corrosion-resistant. Still, what we’re stressing today is an overall strategy, a meeting of different heat treatment principles and processes. Designed to produce a desired hardness-to-strength ration, the hardening and tempering equipment use time and fiery temperatures to imbue a chosen part with a requisite material strength rating.
Robust metals are often judged on hardness. It’s a term that’s closely related to the stiffness of an alloy, which is all well and good, but what about ductility? What of plastic deformation and tensile strength? A material that’s solely engineered to be stiff is likely to hold its shape, but without elastic deformation, it becomes brittle, prone to fracturing. Now, with that undesirable material state in mind, let’s talk about stress relieving and hydrogen de-embrittlement.
What is Hydrogen Embrittlement?
As the most common element in the universe, hydrogen is known to exist everywhere, from the cold vacuum of space to the hearts of the hottest stars. Somehow, that same element can undermine the elasticity factor we so carefully add to our toughest steel alloys. Titanium and aluminium alloys also suffer when diffused hydrogen atoms are introduced. The metal parts retain their formidable stiffness properties, but that diffused element corrupts the ductility of the part so that it becomes dangerously brittle. Released to a client, a brittle alloy could crack, fracture, and fail when stress is applied.
Stress Relief as an Embrittlement Countermeasure
That brittle state cannot be allowed, not when stress is part-and-parcel of structural engineering. Consider stress relieving and hydrogen de-embrittlement an absolute necessity, then, a process that releases the hydrogen by “baking” the part until it’s free of the gas. In essence, the components are placed in an oven for several hours, perhaps longer, until the diffused hydrogen has been ousted from the material. Interestingly, there are several hydrogen de-embrittlement verification tests in use in the heat treatment industry, including destructive torque tests and nondestructive X-Ray Diffraction Residual Stress tests.
Knowing the Enemy
Hydrogen embrittlement can be assessed by analysing crystal lattices in a metal part. There are bending tests that measure ductility and the elastic modulus. Still, the best solution has to be the proactive technique, the stress relieving methodology that eliminates the atomic remnant by baking the part. Better still, avoid processes that introduce hydrogen, the finishing and pre-processing techniques that electroplate or acid pickle the component. If those procedures are mandated, though, the de-embrittlement phase must be followed so that the element is removed from the alloy.
What has been diffused by one industrial process, can be ejected by another process. The heating solution counteracts gaseous diffusion. Finally, as high-grade steel, titanium, and aluminium alloys are vulnerable to this condition, the engine and aeronautics industry are mentioned in the same breath as hydrogen embrittlement. A diligent heat treatment facility is well aware of this fact and always prepared to counteract the presence of hydrogen molecules by applying a rigorously tested hydrogen de-embrittlement solution.
If heat treatment technology has ever been perceived as an afterthought, well, nothing could be further from the truth. This is an essential process regimen, a series of techniques that equips metal parts with tailored material properties. Hardness is usually regarded as the primary reason for altering these already robust alloys, but that’s not always the case, especially in large engineering projects.
How Important is Heat Treatment Technology?
Critically important, that would be the short answer. The construction techniques employed on large-scale building projects may use the latest construction techniques, the best welders and mechanical fasteners, but these framework assets don’t mean much without a properly heat-treated structural alloy. That metal cage is hard, workable, load capable, and entirely weather-resistant. Granted, the expert services of a top-notch machine shop are responsible for creating the high-tolerance profiles of the beams and support components, but it’s the heat treatment stage that ensures the base material is perfectly suited for the project.
Setting Process Priorities
Just because the heat treatment phase is positioned at the end of the machining work, that doesn’t mean this result-critical stage should ever be postponed. We reschedule meetings and dental appointments, not a process stage that ingrains principal mechanical components with their fundamental physical characteristics. The project absolutely requires load-bearing structural parts that can endure, that can withstand projected stress levels. That’s a cardinal rule in this industrial domain, one that’s infringed when the project director doesn’t realise the importance of material altering technology. Basically, heat treatment determines the entire backbone of the structure, not just its finish or weathering properties.
Preventing Budget Blow-Outs
If a project falls behind schedule, there are budgetary concerns to consider. Penalties may be incurred, so executives look for shortcuts. Oftentimes, the cutting budgetary blade slices the heat treatment work in two. The annealing and normalising stations are swiftly navigated, the quenching is incautiously concluded, and final tests are rushed. Certainly, this cut-back processing methodology cannot safely be employed within the framework of a large engineering venture, so the metal components are finished as ordered, but only by accommodating ballooning budgetary demands.
The resulting heat treated parts satisfy all project demands, so the alloy is mechanically and chemically acceptable. Still, the budget blow-out, the time constraint issues, perhaps even a question over the metal’s quality, all of these factors make the source heat treatment company look inefficient. In point of fact, this circumstance occurs through no fault of the heat treatment facility. The fault is in not comprehending the essential role of this processing stage, its place as a project investment, not an added expense.
This fully contained heat treatment procedure is usually tendered by Alpha Detroit when a workpiece requires a mechanically enhanced, metallurgically consistent finish. Therefore, to answer the initial question, it is possible to oil quench materials from a vacuum furnace, but this technique should only be called upon when superior mechanical properties and a perfectly clean end-line product is specified.
Superior Tensile Strength
Raised to its transformative temperature in the vacuum furnace, the workpiece remains in the vacuum until it’s transferred to the quenching oil. Back at the airless furnace, there are no convection currents or oxidation effects to corrupt the uniform hardening process. Likewise, as the component cools rapidly in the oil quenching chamber, its uniform cooling curve evenly tempers the hardened alloy so that it’s free of weak spots.
A Multi-chambered Furnace Architecture
A furnace is an isolated heat treatment asset. A quenching station is similarly self-contained. In multi-chambered vacuum furnace setups, though, the furnace occupies one chamber, the quench enclosure occupies another, and both chambers are contained inside a single overreaching housing. In effect, the superheated component is discharged from its sealed oven, conveyed to the oil chamber, rapidly cooled in that oil, and ejected so that the cycle can begin anew. Meanwhile, the vacuum in the two adjacent but sealed chambers prevents carburization or decarburization, an undesirable process event that causes carbon molecules on the surface of the worked component to react with atmospheric gases. Essentially, if there are no atmospheric gases present, there can be no carburization, so the part leaves the furnace and cooling chamber clean.
Vacuum Oil Quenching Advantages
If the atmosphere in a furnace creates problems, imagine those problems when a vaporised oil complicates matters. An incautiously conducted quenching cycle can cause parts distortion, or it can even leave residual stresses intact inside the component piece. Nucleate boiling and vapour phased oils complicate this situation. Fortunately, a vacuum oil quenching chamber sidesteps these issues. It once again removes gaseous convection currents so that the component cools evenly. Granted, that cooling effect is rapid, but it still delivers uniformly hardened and tempered workpieces because of this atmosphere free design.
As the only vacuum furnace and vacuum oil quench facility in all of Australia, Alpha Detroit is uniquely equipped to be your high-end parts provider. Our heat treatment units are contaminant free, atmosphere-less, and built to process superior products. These products potentially include aircraft landing struts, structurally capable alloys, and end-cycle metals that are guaranteed to furnish uniformly toughened mechanical attributes.