High tolerances rule hot and cold precision straightening work. The factory floor is filled with high-tonnage presses and industrial grinding stations, the larger-than-life tools that enable expert material engineers to meet any deadline. But why is a straightening service essential in a heat treatment shop? In order to answer this question, we need to know why treated parts manifest physical deformations.
Quantifying Mechanical Deformations
There’s no way around the fact that some heat treatment work does cause an observable deformation effect. This is due to the stress placed on the alloyed part during its passage through the tempering and quenching process. Machining and cold working stations also introduce distortion zones, but a recovery annealing procedure offsets this undesirable result. Otherwise, a warping component could crack or split while clamped. This abrupt deformation becomes even more likely when complex profiles are part of the part’s design.
Rectifying Warpage by Employing Cold Straightening Processes
If the microcrystalline grain of the metal part hasn’t assumed an overly hardened form, then it can be loaded into a powerful mechanical press and straightened. High tolerance straightening is further refined by passing the part through a grinding machine. The plasticity quotient of the metal’s structure is key here, for a low elasticity limit locks the metal in its twisted form unless further thermal processing is applied. Any attempt to cold straighten a part that’s this hard will likely result in cracking and stress fracturing, thus rendering the part useless.
Using Controlled Thermal Straightening
If cold type precision straightening is voided, a thermal technique is called upon, which makes sense since we’re working under the constraints of a heat treatment facility. Thermal methodologies clamp the part and restore its profile to acceptable tolerances by controlling the direction of the linear-restorative vector, all so that the final part is ready for application. Of course, further annealing may be required to offset the strain introduced by any straightening, but this is an executive decision, one best left to the head engineer.
Linear corrections are as simple as loading the part into a 20-Tonee press until it’s dimensionally stable, but new straightening techniques use proprietary technologies to assure a dead-level profile. These computer-aided scanners use mathematical analysis to correct flaws that are invisible to the eye. Hot and cold precision work is currently meshing the finest attributes of mechanical processing and electronic scanning to create a perfectly repeatable corrective mechanism, one that straightens based on material hardness and any deformations introduced by the heat treatment workflow.
When troubleshooting heat treatment imperfections in vacuum ovens, the undesirable results that stymy production engineers don’t always fall into convenient pigeonholes. Fortunately, the discolouration of metal parts that have undergone this property-manipulating process is a readily identifiable issue. And, once identified, such processing defects and their associated side effects can be solved. Take a look at the top 3 reasons for these discolouration defects in vacuum ovens and learn how to quickly resolve colour casts.
The green tinge in a just-finished metal part is likely the result of atmospheric contamination. A negative atmosphere eliminates such cross-contaminating defects. Likewise, excess quantities of evaporated water in the furnace can cause an unwanted surface reaction. Check the furnace sealing mechanism for potential leaks and keep water out of this super-heated environment. Finally, if these solutions don’t yield results, lower the furnace’s temperature, as unbound chromium may be causing a verdigris colour cast to flourish.
Engineering acuity ensures all heat treated components are quickly and uniformly raised to their critical transformative temperature, at which point they’re cooled with contrasting processing methods. Argon is used in one of these cooling techniques, for this gaseous substance is inert but it makes for an invaluable tool when used in tandem with vacuum furnaces. Unfortunately, argon becomes slightly active if air is added to the mix. A leak in the vacuum seal provides the air, the mix reaches its critical density, and the cooling part gains a yellow tint. The resulting saturation and hue of yellow illustrate the severity of the leak.
Gas quenching problems are the suspected culprit, in this case. Again, inert argon is likely acting as a proficient cooling mechanism, but it’s combining with air to transform the colour of the metal part. If argon is proven to be a red herring, in this instance, then a serious air leak is the next suspect. Our atmosphere contains more nitrogen (seventy-eight percent) than oxygen (twenty-one percent), so the nitrogen content is what’s causing the metal to shift from its sparkling clear finish to an undesirable blue tint.
Heat treatment work is applied near the end of a manufacturing cycle, yet it’s quite possibly the most important stage in the whole process. Avoid discolouration problems in vacuum furnaces by identifying their root causes and setting a solution in motion that will return the aesthetic appeal of the treated components.
Flame hardening operations extend the operating life of alloy-rich components by adding rigid strength to the microcrystalline backbone of the heat-treated part. The superheated torch produces predictable results, but calculable hardness quotients are obviously contingent on the material build of the product. The differences between low and high carbon steel in flame hardening represent an ideal case study here, so let’s observe the variances that occur when heat treating these two carbon-mutable forms of steel.
Steel is an alloy that is rich in iron. After this primary metallic component, carbon is the next influential element in the mix, plus there are a number of other additives that contribute to the robust characteristics of this structurally adept alloy. Added as the principal alloying constituent, carbon content affects the quality of the metal and its behavior during heat treatment work. A low carbon steel component, for example, adds approximately 0.3 percent of carbon to the mix. In contrast, high carbon steel parts have a higher carbon content rating, a range that varies from 0.3 percent to 1.7 percent. Now, with a figure attached to these two types of carbon steel, we can begin to understand how they behave when flame hardening occurs.
The Flame Hardening of Low Carbon Steel
Soft but ductile, the metal is easy to form, but it requires significant quantities of flame-distributed energy to heat treat the surface and harden the material. The carbon absorbs thermal energy, but, due to the lower content rating, greater temperatures are needed if the required case hardening depth is to be realized. Advantageously, this low carbon candidate is less prone to cracking and distortion than its carbon-rich relative, which is a desirable feature, considering the localized nature of an oxy-acetylene flame hardening torch.
Logic informs us that a higher carbon content equals a greater hardness rating, but this isn’t always a desirable property, at least not when the feature turns brittle and increases the likelihood of a parts-weakening surface crack. Strong by design, toughness must be maximized while avoiding brittleness. Fortunately, as long as the carbon content sits around the 0.4 percent mark, the heat treatment work will likely proceed efficiently and yield a high-carbon martensitic coating.
Different quantities of thermal energy and carbon-rich atmospheres are used to compensate for carbon content, so the same results can be achieved in both forms of steel. The case hardening process also offsets carbon content by adjusting flame pitch and intensity, and by varying the medium used in the quench stage.
Heat treatment techniques use a two-step processing methodology. Logically, it begins with the workpiece entering a fiery environment, perhaps a gas-fired furnace. Here, the metal component is subjected to enough thermal energy to modify the metal’s base properties. This elevated temperature is intelligently maintained, but the time comes for the part to be cooled, which is when the metal quenching stage enters the treatment work. Lowered into a bath full of oil or water, the part cools rapidly.
Much effort is put into precisely heating a ferrous or non-ferrous component. The thermal treatment normalizes the part, hardens it, and removes work-incurred stress. Naturally, these astutely interred properties should remain intact and unaffected as the part cools. Unfortunately, the cooling stage represents yet another journey into the material modification realm. This is why we use metal quenching, as this accelerated cooling technique is regarded as a way of quickly cooling the hot part while retaining all heat-tailored properties.
Rapid cooling in water equals a climactic lockdown of the microcrystalline structure within the workpiece, but what if some material variations are desired? Quenching is used to harden metal parts, including steel. In stainless steel, for example, the austenite-induced state is rapidly cooled until it transforms into its martensite form, which is a brittle configuration, one that will require further tempering. Meanwhile, other material properties can be achieved by switching the cooling medium. Water is commonly used in these tanks, but oil and brine water are also employed, as is high-pressure nitrogen quenching.
Although it may sound counterintuitive, the controlled cooling of a hot metal part is every bit as important as the furnace stage, for this heat treatment method controls phase changes on the downward arc of the thermal cycle. Hardening is managed in this manner, as is the deadlocking of heated metal states. Of course, the quenching bath must be carefully used, for a substandard cooling station can introduce material cracks and parts deformation. Furthermore, a superior quenching station is a highly adaptable heat treatment asset, with oil and salt baths transforming the metal part into other, more desirable grains. These are the bainite and pearlite forms, the grains that emphasize certain carbon-rich lattices while minimizing others.
A well-supervised metal quenching station universally hardens a component and prepares it for tempering. Further fluid control elements, including oil and brine, enhance mastery over this rapid cooling phase, thus breathing a versatile state-altering mechanism into the process.
Work hardened metals are difficult to form. The machining and shaping work has impacted the material in such a way as to cause the metal’s grain to compact and shrink. The processed alloy is now tougher but it’s also brittle, next to impossible to deform without incurring a material weakening fracture. Fortunately, we can reset a metal’s ductile properties by annealing it, thus phase transforming the alloy and returning lost workability to its internal structure.
Every alloy suffers in some way from the stresses encountered in a machine workshop, but alloys of copper are particularly prone to these hardening effects. The grain of brass and bronze becomes smaller when it’s hammered and cut, which means the once malleable material no longer bends easily. Pure copper acts in the same manner, and so does a number of the most popular aluminium alloys. In fact, these non-ferrous metals are all renowned for their ductile properties, but those selfsame properties are lost or simply drastically attenuated unless we recruit an annealing stage.
We see examples of bronze parts cut into intricate shapes. Meanwhile, brass tubing angles around sharp corners, angles that mirror the dexterous twists and turns of a bronze product. Many engineering processes are used to create these curvilinear outlines, plus individual components require exact cutting in order to satisfy the scale of the project. A work hardened part is a likely result of this machining, which means no more bending or complex shaping can take place. The component requires annealing, a passage through a hot oven. The heat treatment process softens the metal, which restores ductility and much-needed malleability to the part.
Localized and complete heat treatment work anneals aluminium so that it can be formed into beautifully formed fences and geometrically detailed outlines. The lightweight metal is tough and more than a match for mild steel, but it requires this heat treatment stage to ensure its ductile characteristics are retained throughout the machining stage. Conversely, stainless steel favours a toughened material profile, but work hardening does reinforce an already formidable resistance factor. This heat treatment stage once again restores the robust alloy to its state of equilibrium.
An annealing station softens copper alloys. The result is a restoration of the metal’s predominant state, that of a supremely malleable alloy. Similarly, stainless metals are stress relieved and stabilized.