Example: Oven calibration – We have an oven with OMEGA temperature controller. We have had a calibration tolerance of -/+ 5 Deg F on it the temp controller range is from 0-1000F; and for another controller 0 – 2400 F calibration tolerance is -/+ 25 Deg F.
Robin
Hi Robin,
Most ovens have temperature gradients. They can be electric or gas, and depending upon the configuration of the heating source relative to the chamber, the gradients can be even larger than the +/- 25 F. Most controllers base their control on just one thermocouple. Errors within the chamber not only include the thermocouple errors and linearity, but reference junction (electronic or physical), type of leads used from the thermocouple junction to the controller, material composition of any plugs and jacks, and temperature environment of the controller itself. You should use thermocouple wire and connectors that are specifically matched to the type of thermocouple that you are using. Also note that various thermocouple have stated temperature ranges. With a calibrated thermocouple, you can also obtain a temperature versus emf compensation graph or equation.
The controller will require both a delta temperature range and a time response factor setting. If you set the delta too small, the controller will be constantly turning on the heating source and turning it off (cycling). For some furnaces, there may be not only an on or off condition, but a proportional response, depending upon how far the temperature has exceeded the limits. The time response factor also affects the cycling. It effectly smooths the response to temperature changes. Setting too long a time will cause the measured temperature to drift too far before a response. Setting it too short can also cause excessive cycling.
The digital controllers can be set to respond to a specific numeric value, e.g. 1136 F, while the older analog controllers, you are doing well to set them to the nearest 25 F value, e.g. 1125 F.
Now lets talk about chamber size. Do you have a 1 inch by 1 inch by 1 inch chamber, 6 inch by 6 inch by 6 inch chamber, or one that is 20 feet by 20 feet by 20 feet? I have seen all of these and several sizes in between.
If the oven is electric, do your parts see the wires directly, or are they shielded? The difference is that heating occurs by both radiation and convection. With radiation, surfaces can develop hot spots if they are near the radiant source. With convection, you may require fans to circulate the internal atmosphere. In the chamber do you have air or a controlled atmosphere, for example an inert gas or even a reducing atmosphere?
If you create a temperature profile of the chamber, you will find that the gradients increase with increasing temperatures. If you have 20 F difference from highest to lowest point at 1,000 F, you may have 50 F difference at 2,000 F.
Now let's talk about the parts themselves. Just because the chamber temperature is at 1,500 F, does not mean that the parts are at that temperature. Within the parts there can be significant temperature gradients as well. The reason for "soak time" in heat treating is to allow the center of the parts to reach the required temperature. The larger the part, the longer a soak time is required. One part may require 1 hour at 1,500 F, while another part may require 6 hours at 1,500 F, and both are to get the center to the 1,500 F temperature. The same applies on the cooling after removal from the oven. The exterior cools the fastest. While theoretical calculations give estimates of the temperature-time profile, most are based on trial and history. The rate of heat transfer depends on the thermal conductivity of the parts, and surface boundary. The time to reach a given temperature within the part also depends on the temperature difference and the geometry.
So how can you overcome some of these issues? One way is to put multiple calibrated thermocouples on and in your part. By "in your part" I mean actually drilling holes, inserting the thermocouples, then sealing the holes. Running a multi-channel digital recorder you can determine the temperatures of your parts throughout the heat treating process. You will have to deal with the holes later, or perhaps machine them off, if possible.
The digital controller output can be sent to a computer for later analysis and records to demonstrate compliance.
The appropriate degree (get it?) of instrumentation depends greatly on the criticality of the parts, number of parts, and size of parts.
Depending on any changes that occur within the part as a function of temperature, having a +/- 25 F fluctuation around the desired average temperature may not be an issue for some materials. For other materials, exceeding a maximum temperature by + 3 F, may cause a phase change in the material that cannot be reversed. In metals there are critical temperatures for phase changes, based on composition. If you are less than the critical temperature, the desired phase change does not occur, and you cannot achieve a required hardness with subsequent quenching and tempering.
The tolerances you need to set is more than just the controller, but is on the process for temperature, time and ramp rate.
When we sintered tungsten carbide-cobalt powders, in a 27 cubic foot vacuum furnace, with about a 3,000 pound combined part weight, the complete cycle was 3 days. The maximum temperature was about 1,500 C. Yes, C. That is well above the melting temperature of steel. The interior of the chamber was graphite. Portions of the cycle required a maximum ramp rate of 1 C per minute. If you tried to go faster, the parts blew apart. We also had multiple hold an soak points at various temperatures, where changes were occuring in the parts. After the hold at the peak temperature, the parts were slowly cooled to avoid cracking. The temperature was digitally controlled and manually monitored optically, with emissivity correction. We knew there were gradients within the chamber, so after sintering, we took samples from the corners and edges, as well as the center, and from various part sizes.
Wes R.