Do It Right – Temperature Uniformity Surveys
The importance of successful Temperature Uniformity Surveys for heat treaters with Nadcap accreditation can not be overstated. A record of conforming surveys completed within the frequency dictated by AMS2750D shows a heat treatment shop in control of their processes. Incomplete records, out of date or failed surveys and the Heat Treater is facing sanctions from the auditor that can have far reaching impacts on their business. So, what steps can the Heat Treater take to ensure TUS tranquillity?
Be On Time, Every Time
Ensure that you have a clear and disciplined planning process. Tables 8 and 9 in AMS2750D give the required TUS frequency of for all classes of furnace and for each Instrumentation Type. These frequencies can be programmed into graphical chart recorders, such as the 6180AeroDAQ from Eurotherm, enabling automatic monitoring with warnings and alarms as the next TUS date approaches. Equally important is the need to ensure that the Field Test Instrument used to carry out the TUS is calibrated in accordance with Table 3.
Get into Good Condition
Prior to each TUS, run a series of checks on the furnace to ensure that it is the same condition as the last time it was surveyed. Checks to carry out include:
· Furnace insulation integrity
· Door seal integrity
· Heating element validation
· Burner validation
· Fan speed validation
· Thermocouple seal integrity
· PID values
If the parameters have not changed since the last survey, there is no reason that the uniformity will have changed. Recent generations of furnace instrumentation are sufficiently accurate to meet the requirements of a Class 1 furnace.
Do what you are told
Produce a TUS report that meets the requirements of AMS2750D section 3.5.21.1. This section lays out clearly the information required in the TUS report, and yet failure to produce a compliant report was the largest single cause of audit non-compliance in the last quarter of 2006. Proprietary software packages are available to take tamper-resistant information from the furnace survey recorder, such as the Eurotherm 6100A, and convert this into a compliant report
Stay in Control
AMS2750D requires that there is no overshoot beyond upper tolerance of the temperature uniformity band during the TUS. This includes the ramp to temperature and settling, as well as the period that the uniformity is under survey. Good furnace control enables the Heat Treater to ensure that this is achieved.
Tuning the furnace at commissioning and after major maintenance ensures optimal furnace control. Advanced temperature controllers, such as the Eurotherm 2000 and 3000 series, include auto-tuning algorithms which enable the Heat Treater. Advanced controllers also include features such as overshoot inhibition and ramp/dwell transition control to ensure a smooth ramp to set point with no overshoot to meet process and survey requirements
Size Matters
To conform to AMS2750D, the entire working zone must be surveyed and have the uniformity required by the Furnace Class. In practice, this means that the fixture or sample load used to survey the furnace must have thermocouples at or outside the extremities of any Nadcap work put through the furnace. If this is not achieved, then the furnace will be deemed non-compliant for any loads treated in the furnace whose dimensions exceed those of the survey load/fixture.
Know your furnace
Monitoring uniformity trends of previous surveys can show if a furnace has a tendency to drift. AMS2750D demands a minimum survey frequency, and the Heat Treater has the licence to run surveys more frequently. The costs of taking a furnace out of production to run a TUS is high, but the costs of a TUS failure can be far higher.
Do-It-Right
Uniform furnaces are a critical tool for a Heat Treat workshop. Nadcap accreditation is essential for suppliers to the aerospace industry, and AMS2750D is becoming the de facto pyrometry standard. Following these simple rules can take the fear out of a TUS, and enable the Heat Treater to have confidence in maintaining compliant furnaces.
The AeroDAQ recorder is AMS2750 rev D compliant:
