EPS Heat Treat Headlines

President of EPS, Ken Klein, has been sharing tips about what to look for when considering a large industrial oven purchase. Last week he told us about the importance of air flow and rate of rise. This week, we’ll hear from him about temperature uniformity and the challenge of temperature control:

Uniformity – What temperature uniformity do you need? If there isn’t a spec we will assume it’s not so critical we need to do something special to our normal design. Many spec writers confuse uniformity with accuracy. Accuracy refers to the capability of a control instrument to achieve desired temperature that is stable and repeatable to within a certain tolerance. Uniformity, on the other hand, is largely a function of airflow. If airflow is managed properly, the air temperature uniformity throughout the chamber will be tight, assuming you don’t block the airflow completely with your load.

Most builders will state the oven zone in which uniformity can be expected, i.e. to within what distance of the walls, ceiling and floor. This defines the “uniform zone”. Your builder should advise you that if he has to test the uniformity before the unit leaves his shop he will state all uniformity statements and certifications apply to an empty oven at steady state conditions.

One more note on uniformity. If you specify, for example, + 10F at 300F, can you expect the readings you get during a survey to be 290F to 310F throughout the “uniform zone”? Only if you have done one of two things:

1) By experimentation you have been able to physically locate the sensor (the control thermocouple) at a point that represents the mean of all temperature readings in the chamber, or…

2) You have taken all readings, found the control thermocouple to be closer to one end of the range (say 305F when readings range from 290F to 310F) and put an “offset” into the controller, which “adjusts the reading of the sensor so that it appears to read the mean of the range.

Control – this often proves the biggest challenge for a spec writer, but it has pretty simple rules. The simpler your operation the simpler the controls should be. Let’s go from one extreme to another. Say you are leaving the temperature in the unit at one point all day, and taking your parts in and out several times a shift. A single set point controller will suffice, teamed up with a door switch that shuts down the heat and circulation when the door is opened for loading and unloading. At the other end of the spectrum is the application where perhaps you’re composite curing and you need to:

• ramp up and down at a controlled rate
• acquire data from vacuum transducers and part thermocouples
• Make data available to your computer system for archiving and print out. This most often requires a graphical human-machine interface (HMI) and a PLC (programmable logic controller).

In between these two extremes are the programmable controllers that will allow the cycle to run automatically, ramping up, holding, cooling down and shutting off.

Next up: Ken Klein concludes the Oven Selection and Specifications series.

Last week, EPS President Ken Klein shared with us three categories of customers who purchase large industrial ovens. In this week’s post Ken shares with us what to look for when making the purchase:

Airflow – In real estate, the three primary considerations are location, location and location. For a large forced convection oven the primary considerations, as I see it, are airflow, airflow and airflow. Sounds pretty basic, doesn’t it? But airflow is going to determine a number of things, like how efficiently you will heat your load, how fast you will recover after a door open if you will be taking parts in and out, and the temperature uniformity you can expect. And yes – I have chatted with prospective customers who are entertaining quotes from other builders for their standard horizontal-in-from-the-side-vertical-return-to-the-ceiling airflow patterns, when as it turns out their load would block vertical airflow. Likewise I’ve seen instances of a customer looking at horizontal side-to-side quotes when they will stand large panels on end that will block the flow. Give this some thought up front—it will save you a lot of nightmares downstream.

Rate of Rise – How fast do you want to get to temperature? It’s one thing to say you want to get there in 45 minutes if you are heating a load that doesn’t weigh much. Heating up a 40,000 lb. weldment is another story. Be realistic, and depending on the weight and nature of your load the air temperature may get there long before the core of your product.

Next up: Ken talks about the difference between accuracy and uniformity and the challenges of control.

Stress relief heat treatment is used to remove stress induced in metals from various manufacturing methods. Some of these methods include: milling, turning, welding, bending, heating, cooling, shearing, forging, sawing, grinding, not to mention the steel making processes that leave the metal full of residual stresses. These stresses can cause harmful distortion, brittle fracture, and stress corrosion cracking near welds and within some grades of metal.

Removing residual stress is a time/temperature related event with a very controlled cooling cycle using your Cress Furnace. If not carried out correctly, new residual stresses can be produced that will result in greater stresses than the part had originally. To remove stresses it is recommended that you consult the mill literature for the grade of metal to determine what the Ac1 temperature is for your application. There are several methods to remove stresses successfully, but the most commonly used method is heat treat stress relief. The main criteria to use for choosing the correct temperature is to heat below the lower austenizing temperature (Ac1). Decarburization will take place above 960oF, so protect the surfaces if the surface is not going to be removed by machining.

Thus, if we have a 4140 steel part, (4140 has a 1380oF Ac1) that we want to stress relieve, we could place it in our Cress furnace, take it up to 1100oF and soak it for 6 hours, followed by controlled cooling at a rate of lowering it at 50 to 75oF per hour, in a closed furnace, to below 400oF, at which point it can be removed from the furnace, and it would be stress relieved. However, if you heated it to 1300oF, you can soak it for just an hour, followed by the controlled cooling and also stress relieve the part.