Heat Treat Radio #97: Lunch & Learn, Ovens vs. Atmosphere Furnaces

Doug Glenn: Welcome everybody. This is another Lunch & Learn event with the staff of Heat Treat Today and the illustrious Dan Herring, The Heat Treat Doctor®. Dan, we’re always very happy to spend some time with you.

We are here to learn a little bit about some basics about heat treat equipment, mostly ovens, air and atmosphere furnaces, and possibly vacuum furnaces.

Dan Herring: It’s always a pleasure, Doug, and hello everybody.

It is an exciting topic for me because I happen to love heat treat equipment. Let’s start with industrial ovens.

All About Ovens (01:42)

Years ago, industrial ovens were very easy to differentiate from furnaces. I’m going to give you my understanding of the differences between ovens and furnaces, and then talk a little bit about some general characteristics of all types of heat-treating equipment.

Ovens are typically designed for low-temperature operation. When I talk about low-temperature operation, years ago the definition was “under 1,000° F.” That definition has changed over the years. We now usually say either under 1250°F or under 1400°F. All of that being said, there are some ovens that run all the way up to 1750°F. But what we’re going to concentrate on are, what I call, “the classic temperature designations for ovens.”

First of all, ovens are typically rated at 500°F, 750°F, 1000°F, or 1250°F. If you see a heat treat operation that’s running — certainly under 1450°F — but even under 1250°F, it may be being done in either an oven or a furnace.

Let’s talk about some of the distinguishing characteristics of ovens, so everyone gets a feel for it.

Ovens always have a circulating fan. If you see a piece of equipment without a circulating fan, it can’t be an oven. At these low temperatures, the heat transfer — in other words, how you heat a part — is done with hot air or circulating hot air. So, ovens always have fans.

In most cases — and years ago in all cases, but today in most cases — ovens are metal lined. If you were to open the door of an oven and look in, and you see a metal-lined chamber, that would typically be an oven.

The fan and the type of insulation or lining that’s used is very characteristic for distinguishing features of ovens.

Today, however, there are ovens that use fiber insulation and even some ovens that have refractory-insulated firebricks, refractory in them. The lines are a little bit blurred, but typically you can distinguish them by the fact that they have fans and are metal lined.

Ovens come in either “batch” or “continuous” styles. If the workload inside the unit, the piece of equipment, is not moving, we call that a batch style furnace. If the workload is somehow being transferred through the unit, we call that a continuous furnace. Ovens and furnaces can be both batch and continuous.

Ovens and furnaces can both be either electrically heated or gas fired.

One of the distinguishing characteristics of ovens is that if they are gas fired, they are, what we call, “indirectly heated.” This means your burner, your combustion burner, is firing into a closed-ended tube, a radiant tube, as we call it, so that the products of combustion do not “intermix.” They do not create an atmosphere that’s used inside the oven. In fact, the majority of ovens run with an air atmosphere – that’s another distinguishing feature.

However, there are ovens that can run inert gases. Those ovens typically have continuously welded shells. Again, that’s an exception rather than a rule, but there are ovens of that type.

There are also vacuum ovens out there. We actually have an oven chamber on which we can pull a vacuum. They are less common than their cousins, the air ovens, but they are out there in industry.

We have the method of heating and type of movement of the hearth or movement of the load that typically is consistent between ovens and furnaces.

What I’d like to do is just show everybody a couple of pictures of some very typical, what I’m going to call, “batch ovens.”

Doug Glenn: Because ovens are typically low temperature, you’re able to have metal on the inside, right? If it was higher temperature, you’d start experiencing warping. Is that the primary reason why you tend to see metal in an oven and not in a furnace?

Dan Herring: That’s correct, Doug.

The lining can be made of steel: it can be made of “aluminized’ steel,” it can be made of zinc-gripped steel (those are just coatings), it can be just steel, and they can be made of stainless steel (a 300 series stainless steel). That’s why you have the different temperature ratings and the different types of materials that this metal interior can be made from.

If you open the door of a metal-lined oven or an oven that had a metal lining, you would typically see what’s pictured here.

Ovens can be very small or they can be very, very large. What you’re seeing on the screen is a “double door shelf” oven.

It is very similar to your ovens at home. You open the door, there are shelves, and you can put trays on the various shelves. These can be small, to the point where, sometimes, they can sit on a benchtop. Sometimes they can be very, very large and be floor-mounted, as this one is.

This is an example of a batch oven, something that you would load, and the load stays stationary within the oven. Then, when you’re ready, you unload it.

Ovens can come in slightly larger sizes.

That’s a picture of a larger, horizontal oven. The door on this particular oven is closed shut, but you can see the fan system — that’s that yellow arrangement that’s sitting in back of this particular oven.

There is another style of oven.

We call this a “walk-in” oven — very creative, because you can walk into it. I’ve seen batch ovens that are very, very small and very, very large — ones that will fit on a benchtop and ones that are a hundred feet long.

You can see the heat source on the right hand side. Remember, whether it’s electrically heated with sheathed elements or if it’s gas-fired with, typically, an atmospheric-type burner, again, you have circulating air past either the electric elements or circulating air past the tube into which the burner is firing. You’re relying on convection — or moving hot air — to transfer that heat energy to your load.

These are just some different styles of different types of ovens, so everyone can see them. I don’t want to take too long, but I’ll show you another picture of one.

This is an industrial oven. You can see the fan; it has a yellow safety cover on it. You can see the fan mounted on top, and this is a typical oven that you’d find at a typical heat treat shop.

Ovens have the characteristics that I pointed out. I’ll bring up one more picture which you might find interesting.

Since there are a variety of oven shapes and sizes, this happens to be a monorail conveyer oven. What you’re looking at is the inside of the oven. You’ll notice that in the ceiling there are hooks. The loads are actually placed on the hooks and sent through or pulled through the oven. This happens to be a gas-fired unit, and you can see that it has U-shaped radiant tubes into which you’re firing.

This oven is fiber-lined and not metallic-lined. You’ll also notice that because you see different colors of the tubes, this particular shot was taken and you destroyed the uniformity of temperature within the oven. Usually, they’re very tight.

Ovens are typically in the ±10°F range for temperature uniformity, sometimes in the ±5°F range.

Those are basically some pictures of ovens, whether they be batch or continuous, for everyone to see and think about, from that standpoint.

Q&A on Ovens (16:58)

Bethany Leone: What is the reason for the increase in temperature range for what classifies an oven?

Dan Herring: The main reason is the materials of construction have gotten better, so we’re able to withstand higher temperatures. But going to some of these temperature ratings, one of the things that heat treaters look at is if I have a process that runs at 1,000°F or 970°F (let’s take an aluminum heat treat example where a process is running at 970°F), I could run that in an oven rated at 1,000°F but I’m right at the upper limit of my temperature.

It's much better to buy an oven rated at 1250°F and then run a process such as 970°F where I have a margin of safety of the construction of the oven, so the oven will last longer.

However, industrial ovens tend to last forever. I’m the only person on this call old enough to have seen some of these ovens retired. It’s not unusual that an oven lasts 40 or 50, or sometimes 60 years.

Ovens are used in the heat treating industry for processes such as tempering, stress relief, for aluminum solution heat treatment, aluminum aging operations, and to do some precipitation hardening operations that run in these temperature ranges. Ovens are also commonly found in plating houses where you’re doing a hydrogen bake-out operation after plating. You also do various curing of epoxies and rubbers and things of this nature in ovens.

There are a variety of applications. Ovens are used also for drying of components. Ovens are used for drying of workloads, these days, prior to putting in your heat treating furnace. Many times, our washers are inefficient when it comes to drying. You take a wet load out of a washer and put it into a low-temperature oven, maybe running between 300°F and 750°F. Consequently, you both dry the washing solution off the parts and you even preheat the load prior to putting it into the furnace.

Doug Glenn: One of the things I’ve always distinguished ovens by is the term “panel construction” opposed to “beam construction.”

If you can imagine a sheet of metal, some insulation, and another sheet of metal – that’s a panel. It’s got enough insulation in it because the temperatures are not excessively high, but you really only need those three layers. You take those panels, you put them in a square or whatever, put a lid on it, put a bottom on it, and you basically have an oven, right?

Where furnaces are not typically constructed that way; they are constructed more where you have a support structure on the outside and then a heavy metal plate and then you build insulation on the inside of that. It doesn’t even need to have metal on the inside — it can be brick or another type of insulation.

Many people claim — and I’m sure there are some very strong ovens — that the oven construction is not as hardy, not as rugged. That’s one other minor distinction, but the main distinction is ovens tend to be lower temperature.

Dan Herring: Yes, that’s very correct, Doug. In panel-type construction, there is typically mineral wool insulation in between the two panel sheets; and it’s rated for obviously very low temperature.

There are, what we call, “light duty” and “heavy duty” ovens. Heavy duty ovens have that plate and support structure — those I-beams or channels — supporting the external structure.

Doug Glenn: You reminded me of something, Dan: We talk about ratings – oven ratings, furnace ratings, and that type of stuff. That’s pretty important and we haven’t really discussed that much. But if a furnace is rated at a certain temperature, you do not want to take that furnace beyond that temperature because there are real safety issues here.

There was one picture that Dan showed where you could see the metal interior, and there was like a gasket, if you will, around the whole opening. That gasket is only rated to go up so high in temperature. If you go over that temperature, you’d end up deteriorating that gasket, if you will. It could cause a fire, it could cause a leak, it could cause all kinds of issues. And that’s only one example.

One other one he mentioned was fans. There is almost always a fan in an oven, and if you take the temperature of that oven over its rated temperature, all of sudden the bearings in that fan start . . . well, who knows what’s going to happen.

You always want to know the rating of your oven and furnace, and don’t push the rating.

Dan Herring: Yes, if you exceed temperature in an oven, typically the fan starts to make a lot of noise and you know you’re in trouble. You only do that once. But those are excellent points, Doug, absolutely.

So, the world of ovens -- although it’s they’re an integral part of heat treating -- are a “beast unto themselves,” as I like to say. Construction is a factor, and other things.

All About Atmosphere Furnaces (24:50)

Furnaces, interestingly enough, can be rated both to very, very low temperatures all the way up to very, very high temperatures. In other words, you can see industrial furnaces running at 250° or 300°F or 500°F or 1000°F, — at typical temperatures that you would associate with oven construction — but you can also see furnaces running at 1700°F, 1800°F, 2400, 2500, 3200°F. There are some very interesting furnaces out there.

But furnaces, although they can run in air — and there are a number of furnaces that do — they typically run some type of either inert or combustible atmosphere inside them. Furnaces typically have an atmosphere, and they do not always have a fan. The rule is the higher you go up in temperature, the more any moving part inside your furnace becomes a maintenance issue. Many times, furnaces do not have fans in them.

They can be electrically heated. They could also be gas-fired. In this particular case, they can either be direct-fired or the burners are actually firing into the chamber; and the products of combustion become your atmosphere. They could be indirect-fired — like we discussed with ovens — into a radiant tube as a source of heat or energy.

Furnaces typically have plate construction. It’s typically continuous welded, they have channels or I-beams surrounding the structure to make it rigid, insulation is put on the inside. Traditionally it’s been insulating firebrick, but in what I’ll call recent years (20 years or so) fiber insulations have come about, and they perform very, very well.

Fiber insulations reduce the overall weight. They have advantages and disadvantages. A refractory-lined unit can have a great thermal mass due to the storage of heat inside the insulation, so when you put a cold load into a brick-lined furnace, the heat from the lining will help heat the load up quickly.

You don’t have quite the same heat storage in a fiber insulation. At the same time, when you go to cool a furnace, a fiber-lined furnace will cool very quickly as opposed to a refractory furnace which cools a lot slower.

Again, furnaces can be batch style, they can be continuous style, they can be fairly small in size. The smallest ones that I’ve seen, typically, are about the size of a loaf of bread. Conversely, you have furnaces that are so large you can drive several vehicles or other things inside of them.

As a result of that, what distinguishes them are typically their temperature rating and the fact that they use an atmosphere. Some of the atmospheres are: air, nitrogen, argon. I’ve seen them run endothermic gas and exothermic gas which are combustible atmospheres, or methanol or nitrogen-methanol which are also combustible atmospheres; they can run steam as an atmosphere. I’ve seen furnaces running sulfur dioxide or carbon monoxide or carbon dioxide as atmospheres. The type of atmosphere that is used in an industrial furnace can be quite varied.

We have several different furnace categories that typically are talked about: Batch style furnaces are configured as box furnaces. They are very similar in shape to the ovens that we looked at. Pit style furnaces are where you have a cylindrical furnace that actually is quite tall and fits down, usually, into a pit that’s dug in the factory floor.

You also have mechanized box furnaces. Those, typically, today, would be called integral quench furnaces or sometimes batch quench furnaces or “IQs.” There are belt style furnaces, gantry, tip-up, and car-bottom furnaces. There is a wide variety of batch style furnaces, all of which have the characteristic that once you put the load into the chamber, it sits there until it’s been processed and until it's time for you to remove it.

The exception is in an integral quench furnace. You push the load typically either directly into the heating chamber or into a quench vestibule and then into a heating chamber; you heat it in one chamber, you transfer it out, and you quench it into another chamber.

Those are some of the distinguishing features of batch style equipment. I’ve got a couple of pictures here that you might find interesting.

Here is a “box furnace.” You might say, “Oh, my gosh, it looks like an oven!” I see a fan on top, and it’s a box style. From the outside, it’s hard to tell whether it’s an oven or a furnace.

When you look at this unit, you might see that it’s made of plate construction. It would be difficult to tell if this unit were a heavy-duty oven or furnace unless you, of course, opened the door and looked inside. You would typically see either fiber insulation or insulating firebrick in these types of units.

Sometimes, just by sight alone, it’s very difficult to tell if it’s an oven or a furnace. But there are other telltale signs.

Now, this is a box furnace with a retort inside it. The workload is placed, in this case, into a metal container that’s physically moved on a dolly into the furnace itself. This is what we call a box furnace with a retort.

The process takes place inside the retort. You’ll notice that there’s a flow-meter panel there, of different gases, that are introduced directly into the retort. This style of furnace is very interesting because the furnace itself, outside the retort, is simply heated in air. It’s a relatively inexpensive construction. Also, when the time comes that the process is finished, usually you can remove the retort and introduce or put a second retort into the furnace while the first retort is cooling outside the furnace. It lends to increased production, from that standpoint.

But this is typically a box furnace; it looks like a big box. The shell does not have to be continuously welded because the process takes place inside the retort. You might be able to see, just past the dolly, there is a dark color and that is the blackish retort that’s actually being put in.

Doug Glenn: I think the reasoning of the retort is to protect the airtight atmosphere, right?

Dan Herring: That’s correct, Doug. The idea is the fact that it’s an effective use of your atmosphere.

The other thing you can do with a box furnace with a retort is you can pull a vacuum on the retort. As a result of this, you can actually have a “hot wall” vacuum furnace. That is what is defined as a hot wall vacuum.

The next type of atmosphere furnace we’re going to look at is pretty distinct or pretty unique: This is a pit style furnace.

What you’re seeing here is only that portion of the furnace that is above the floor. There is probably four times as much furnace below the floor as there is above. OSHA has certain requirements: there must be 42 inches above the floor not to have a railing or a security system around the pit furnace, because you don’t want to accidentally trip and fall into a furnace at 1800°F. We don’t want to say, “Doug was a great guy, but the last time I saw him . . .”

In this particular case, there is a fan which is mounted in the cover of this pit style furnace. Most pit furnaces are cylindrical in design; however, I have seen them rectangular in design. Some of them have a retort inside them; unlike the picture of the box furnace with the retort, the retort is typically not removable, in this case. Of course, there are exceptions. There are nitriding furnaces that have removable retorts.

I think this is a very distinctive design. If you walked into a heat treat shop, you’d say, “You know, that’s either a box furnace or an oven.” Or, if you looked at this style of furnace, you can clearly see it’s a pit furnace, or what we call a pit furnace.

Two other examples, one of which is just to give you an idea of what we call an “integral quench furnace.” I think this is a good example of one:

They’re made by a number of manufacturers. The integral quench furnace is probably one of the more common furnaces you’re able to see. It has, in this case, an oil quench tank in front and a heating chamber behind.

This would be an “in-out” furnace; the workload goes in the front door and comes out the front door. But once the workload is loaded into an area over the quench tank (which we call the vestibule), an inner door will open. The load will transfer into the heating chamber in back. That inner door will close, the workload will be heated and either brought up to austenitizing temperature, carburized or carbonitrided, the inner door will then open, the load will be transferred onto an elevator and either lowered down into a quench tank (typically oil) or, if the unit is equipped with a top cool, the load is brought up into the top cool chamber to slowly cool.

These styles of furnaces do processes like hardening, carburizing, carbonitriding, annealing, and normalizing. You typically don’t do stress relief in them, but I’m sure people have. These furnaces have a wide variety of uses and are quite popular. Again, the style is very distinctive.

They typically run a combustible atmosphere, and you can see some of that atmosphere burning out at the front door area.

There are also, what we call, continuous furnaces or continuous atmosphere furnaces. They are furnaces where you have a workload and somehow the workload is moving through the furnace. A good example of that is a mesh belt conveyor furnace.

There are also what we call incline conveyor, or humpback-style furnaces. The mesh belts are sometimes replaced, if the loads are very heavy, with a cast belt: a cast link belt furnace. The furnaces can sometimes look like a donut, or cylindrical, where the hearth rotates around. We put the workload in, it rotates around, and either comes out the same door or comes out a second door.

A lot of times, rotary hearth furnaces have a press quench associated with them. You’re heating a part, or reheating a part in some cases, getting it up to temperature, removing it, and putting it into a press that comes down and tries to quench it by holding it so that you reduce the distortion.

There are other styles of furnaces typical of the “faster” industry which are rotary drums. Those furnaces you would load parts into, and you have an incline drum (typically, they’re inclined) with flights inside it. The parts tumble from flight to flight as they go through the furnace, and then usually dump at the end of the furnace into a quench tank.

For very heavy loads, there are what we call walking beam furnaces where you put a workload into the furnace. A beam lifts it, moves it forward, and drops it back down. Walking beam furnaces can handle tremendous weights; 10,000 to 100,000 lbs in a walking beam is not unusual. Any of the other furnaces we’re looking at wouldn’t have nearly that type of capacity.

There are some other fun furnaces: shaker furnaces. How would you like to work in a plant where the furnace floor is continuously vibrating, usually with a pneumatic cylinder so it makes a tremendous rattle, all 8 or 10 hours of your shift? That and a bottle of Excedrin will help you in the evening.

As a last example, the monorail type furnaces where we saw that you hang parts on hooks. The hooks go through the furnace and heat the parts.

I’ll show you just a couple of examples of those. These are not designed to cover all the styles of furnaces but this one you might find interesting.        

This is a typical continuous furnace. This would be a humpback style furnace where the parts actually go up an incline to a horizontal chamber and then go down the other side and come out the other end. These furnaces typically use atmospheres like hydrogen, which is lighter than air and takes advantage of the fact that hydrogen will stay up inside the chamber and not migrate (or at least not a lot of it) to floor level.

Atmosphere Furnaces Q&A (47:30)

Evelyn Thompson: Are the inclined sections of the furnace heated? Why do the parts need to go up an incline? Just to get to the heated part of the furnace?

Dan Herring: If you’re using an atmosphere such as hydrogen, it’s much lighter than air. If you had a horizontal furnace just at, let’s say, 42 inches in height running through horizontally, the hydrogen inside the furnace would tend to wind up being at the top of the chamber or the top of the furnace, whereas the parts are running beneath it! So, the benefit of hydrogen is lost because the parts are down here, and the hydrogen tends to be up here.

By using an incline conveyor, once you go up the incline, the hydrogen covers the entire chamber and therefore the parts are exposed to the atmosphere.

I did a study a few years ago: About 5–6% of the types of mesh belt furnaces in industry are actually this incline conveyor type.

Another good example is the fact that people like to run stainless steel cookware. I’ve seen pots, pans, sinks, etc. Sometimes you need a door opening of 20 or 24 inches high to allow a sink body to pass into it. Well, if that were a conventional, horizontal furnace, you’re limited to, perhaps, 9 to maybe, at most, 12 inches of height.

Typically you never want to go that high, if you can help it. 4–6 inches would be typical. So, there would be a tremendous safety hazard, among other things, to try to run a door opening that’s 24 inches high. But in an incline furnace, the height of the door can be 20, 24, 36 inches high. The chamber is at an 11° angle, and you must get up to the heat zone, but they run very safely at that.

Karen Gantzer: Could you explain what a retort is?

Dan Herring: Think of a retort — there are two types — but think of one as a sealed can, a can with a lid you can open, put parts in and then put the lid back on. The retort we saw in that box style furnace is that type. It is a sealed container. We typically call that a retort.

Now, in that pit furnace we saw, there could be a retort inside that one and they could be sealed containers, but typically they’re just open sides, that are made of alloy. Sometimes we call those “retorts” as opposed to “muffles” or “shrouds,” in another case. Muffles don’t have to be a sealed container, but they typically are. That’s the way to think of them.

Karen Gantzer: Thank you, Dan, I appreciate that.

Bethany Leone: Dan, thank you for joining us. It was really a valuable time.

Dan Herring: Well thank you, my pleasure.

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