When discussing furnace compliance, we might ask ourselves the following question:

“If my thermocouples and control instruments are calibrated, and my SATs are performed successfully, does that mean the temperatures inside my furnaces are correct?”

Your immediate response might be “Yes”, however the correct answer is “Not necessarily”. A successful calibration and SAT only tells us that our furnace’s temperature system is accurate at one location: the tip of our sensor. Unfortunately, we don’t yet know the temperatures of any other locations inside our furnace.

This is why we must evaluate temperature uniformity, meaning the temperature variation of several locations within the furnace work zone (expressed in ±degrees) with respect to the temperature set point. To do this we must perform Temperature Uniformity Surveys (TUS)

A TUS is a test or series of tests where calibrated instrumentation and sensors measure temperature variations inside the furnace. By identifying where in the work zone these temperature variations exist, heat treaters can answer important questions such as:

  • Where are the cold and/or hot spots in my furnace?
  • Why are there cold and/or hot spots, and how do I fix them?

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Repeatability, repeatability, repeatability!!!!!!!

The TUS test helps ensure the entire work zone produces a repeatable metallurgical result. It also encourages proper maintenance of thermal processing equipment by identifying where non-homogeneous temperatures exist within your work zone. Above all else, performing TUSs tests reduces the likelihood of scrap and/or quality issues that could potentially lead to product RECALLS.

Since TUSs are a proactive measure (not reactive), the true costs related to performing TUSs can only be measured by NOT performing the TUSs and waiting for the bad news.  Unfortunately, far too many companies have “paid the price” for NOT performing TUSs. This decision has oftentimes resulted in recalls measured in the many millions of dollars!

In conclusion, if TUSs are not performed, the heat treater needs to quantify what potential liabilities might he/she inherit if metallurgical related recalls were to take place as a result of poor temperature uniformity.

 

Victor Zacarias

Heat treat management expertise: CQI-9, AMS2750

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Written by Victor Zacarias

Heat treat management expertise: CQI-9, AMS2750

This article has 2 comments

  1. Gerardo Velazquez Reply

    In my previous job they have a Quality Control department(QC) to do temperature surveys with tighter tolerances(+ or – 5’F when below 1000’F and +/- 10’F above 1000’F) in order to meet audits and certification from comercial, aerospace and defence contractors. The apparatus used for such surveys are sized to its working zone using a series of thermocouples(TC) located equially spaced across the working zone. This scope has to be placed to the closest working zone limits set by QC when furnaces installed, tested and previously aproved to be used, such designs are closer to your sketch in your report, the TC’s are individually connected to a data adquisition system to monitor, record and store such information for proof as monthly surveys perfomed within time.
    Individual TC has to be readed every five minutes, the data adquisition system has to be calibrated and certified by an independient testing laboratory, the surveys must include time stamp, heat-up process, temperature set point, overheating and availability when requested.
    Furnaces used for Aluminum Alloys weren’t used avobe 1000’F, Titanium alloys were preferably heat treated on Electric Furnaces, when temperatures beyond 1850’F have to use S type TC,
    Some vendors and/or specifications need attach a TC to any actual part in the lot being processed, and/or a series of TC’s at Top, Bottom and corners, or embeded to any actual part.
    When TC is embeded to an actual part or a similar part TC has to be placed to the thicker area by drilling a hole to reach its center and cover it with mollycoat.
    Pre-heating, over-temperature, right after reach actual temperature always differ from actual to the furnaces instruments, so the soaking time starts when TC’s on actual parts reach minimun temperature requirement, always have to be within tolerance while heat treating.
    When daily audits performed need to check specifications to see that job is performed in accordance, actual parts are marked in order to identify them in the whole process, furnaces calibrations are not expired, furnace temperature is within limit, actual part temerature is within limit when applicable, TC’s correction factor, data adquisition log, time stamp, vendor, operator, etc., time from furnace to quenching medium,.
    Hardness tests are performed for a quick heat treatment response.

    Usually cold spots are located at the bottom furnace, and tested parts are placed above furnace floor by a metal pallet which crates a gap within furnace floor and actual part. So the working zone is spaced from walls, floor and/or ceiling.

    Hot spots are mostly located at top and/or close to heat sources, to avoid them furnaces have installed a fan located at ceiling and working while furnace is on(used on temperatures below 1000’F).

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