Top 3 Discolouration of Metal Parts Often Encountered with Vacuum Furnaces

30 September 2016

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.

Identifying the Cause of Heat Treatment Green

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.

Troubleshooting Golden Yellow Colour Changes

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.

A Blue Hue Raises a Red Flag of Concern

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.

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