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.