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.

In this first post, Ken talks about three customer categories for large industrial oven purchases:

One of my college professors started each day’s lecture with the words “today we’re going to talk about…” So, everyone, today we are going to talk about selecting and specifying an oven, a subject near and dear to every oven builder’s heart.

This is a challenging industry, especially where large industrial ovens are concerned. If we’re talking small lab units or other small standard catalog ovens, the process is easy–unless the customer’s requirements are misinterpreted. For larger units that must be designed for the application I tend to group customers in three general categories. That’s not to say some don’t slop over from one category to another, but in general they fit into distinct categories:

Category #1: The primary contact is inquiring on the behalf of others and has very little information on the application. He or she can try to get more information for you, but they may or may not repeat the requirements accurately and additional questions may or may not yield helpful answers.

Category #2: Primary contact is inquiring on behalf of others and is passing on a spec that makes it sound like they are constructing the next great missile system. Every detail is spelled out in exact terms. There are paragraphs and sub-paragraphs, ranging from those that are straightforward to those that ask for features either extremely complex or plain impractical.

Category #3: Primary contact is the end user and can tell you anything you want to know.

Of these three the second is received with mixed reactions by most builders. Certainly you have an apparently unyielding spec which should make it easy for the customer to evaluate all bids on an apples-to-apples basis, but most often the requirements are seen to be over the top or unrealistic, meaning many builders will need to take exception to a number of the requirements. That’s where things get murky. The builder wonders if he has a chance at getting the order. He may feel that if he quotes exactly to the spec the unit is going to be so expensive that the customer is likely to buy from another bidder who had the sense to take exception to the requirements he feels pile on the costs unnecessarily. In addition, the customer has a lot of technical exceptions to wade through. He may or may not have the expertise to evaluate the exceptions properly and may end up tossing the quotes that are too difficult to wade through.

So – let’s guess at which one is my favorite. The third of course. Easiest to understand and respond to. This article is addressed to those of you who need an oven and want to get a good comprehensive bid on your requirements. We are going to concentrate on forced convection ovens. Here’s what you should be thinking about when you talk to prospective bidders.

Next up: Ken Klein talks about the characteristics that matter when choosing an oven.

Question 7: My Cress furnace is set up for the 2400F higher temperature. Can I heat treat high speed steels and powdered steels?

Yes, a Cress furnace set up for 2400F can heat treat high speed steels and powder metal tooling. But first, I generally don’t recommend heat treating these metals if you only do them occasionally. The reason I take this route is because high speed steels have extremely short austenizing soak times. Most all of them need to be soaked for 5 minutes maximum per inch of thickness once they reach austenization temperature. So, over soaking them in the length of time they are in the furnace is extremely critical, and that soak time literally comes down to seconds. In fact, every second in excess time takes away longevity of the tool’s life. For that reason, I generally suggest sending the tools to a good commercial house that works on high speed tools everyday.

But, if you have more than just an occasional piece to do and your operator is able to do some scrap pieces and record how he does them, then by all means, you can heat treat these steels very successfully.

One further word of caution. When the parts are removed from the austenizing temperature, if you are using the oil quench method, they must be quenched in the oil immediately, but the door to the furnace should also be closed as fast as possible. A furnace at 2400oF receiving an extended incoming flow of room temperature air receives a heavy thermal shock, which over time will effect the longevity of the ceramic and fire brick liner.

Question 8: Cress offers a Draw furnace but I can’t afford to spend the money. How important is it to have a draw furnace?

When a piece of metal comes out of the furnace and goes through the quench, whether by air, oil or water quenching, and the temperature drops below 150oF it should be placed in a pre-heated tempering oven or the grain structure will be effected. That will cause premature tool failure. If the furnace is at 1750oF, it takes a long time for the heat to dissipate and to get it down to 400oF for tempering. Opening the furnace and force cooling will cut short the life of the fire brick and ceramic plus, may have an affect on the elements. Thus it makes sense to have a draw furnace that can be pre-heated and ready to complete the process.

DISCLAIMER OF LIABILITY
The material presented in this article is intended for general educational information only. It should not be used for a specific application without careful analysis and study of the intended use. Anyone using this information or relying on it assumes all risk and any liability arising from their applications and use.

COPYRIGHT © April 2007, by Advisor In Metals

The author, Bill Bryson, Advisor In Metals has had numerous years and extensive experience in the heat treating of tool steels. He has conducted over 250 seminars to leading companies in the U.S. to train their tool makers and engineers on proper steel selection and heat treatment practices. He is also the author of the book called “HEAT TREATMENT, SELECTION AND APPLICATION OF TOOL STEELS” published by Hanser-Gardner Publications.

If you want practical information on the heat treatment process in understandable everyday language, inquiry to: Advisor In Metals or via e-mail at thegateway@metrocast.net. Information about the book or seminars is available on line at: Advisor In Metals