Controlled Atmosphere Carburizing Furnace Maintenance FAQs

Vertical radiant Tube Allcase Furnace

As the leading supplier of controlled atmosphere equipment for heat treating, Surface Combustion is your resource for preventative maintenance and furnace operation.

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R. Miller: Hello, and welcome to this first podcast sponsored by Surface Combustion. I’m your host Reed Miller. Today we will be addressing frequently asked questions about controlled atmosphere carburizing furnace maintenance. Surface Combustion is a leading supplier of controlled atmosphere equipment for heat treating. Surface invented the Allcase® Batch Integral Quench Furnace and RX® Endothermic Atmosphere Gas Generator. With the largest installed base in North America, their dedicated customer service engineers support customers every day. Our questions will be answered today by Mike McConkey. Mike is a dedicated customer service engineer who began working at Surface Combustion in 1981. He spent years starting up equipment and he then managed furnace projects and field service engineering. Mike is an expert on controlled atmosphere furnace operation, construction, safety, and maintenance. Welcome Mike, thanks for joining us. We’re going to start right into our questions if that’s okay with you.

M. McConkey: That’s good. Thank you.

R. Miller: Good. So, what are the main areas for preventive maintenance in an Allcase® Batch Integral Quench Furnace?

M. McConkey: The main areas that we typically get maintenance problems with are the combustion system, the atmosphere system, the quenching, and then also the mechanical motions. The combustion system is probably one of the biggest ones we get questions about. And then the mechanical motions if a load gets hung up, why it gets hung up and how we can prevent that from getting hung up.

R. Miller: Okay. So, if a customer needs to contact you for service, what kind of information should they prepare or what should they have ready for a visit or ready when they contact you? What do you need to know from them?

M. McConkey: I think one of the biggest things that will help all of us a lot is the serial number of the unit. Make sure that you have a good serial number that will allow me to pull up drawings so I can look at the drawings and correlate with the customer what he’s going on. I guess also the second thing is if they do have trouble, what kind of troubles have they had? What have they done? How have they tried to fix it? What changes have they made in the last few months? Because a lot of times when customers get into trouble, they had a problem and they fixed the problem. Well, the next problem is a result of that fixing. It may not always be correct, it may have had something wrong so that’s the biggest problem they can have. Then the equipment they’ve used, any type of equipment available, three gas analyzer due pointer and what they’ve got available for us to troubleshoot with.

R. Miller: Right. Okay. Atmospheric carbon control is vital to carburizing heat treatments. Can you share some common carbon control issues and maybe some maintenance tips associated with that.

M. McConkey: Sure, sure. Some …well, most problems that come from carbon control is that the furnace is sooted up. The atmosphere in the furnace is a high carbon potential atmosphere and the furnace typically will get sooted up. The best way to get rid of that is to ‘burn the furnace out,’ where you actually lower the temperature a little bit and you induce air into the furnace for a period of time. Then you also analyze the atmosphere in the furnace after the burnout. Typically, a burnout would be a very, very low oxygen level when you first start putting the air in. And then after you’ve put the air in for a period of time, the O2 would start to come up and it will get up to 17, 18, 19% oxygen, knowing that we’re putting in 20% oxygen into it with the air. That’s usually one of the biggest problems that the customer sees is the problem with sooting in the furnace.

M. McConkey: I guess the other type of problems are the means that we analyze the atmosphere. The carbon probe in particular is a very good way to analyze what the atmosphere is and that also gets sooted up over time. When it does get sooted up, what happens is the carbon coming in the probe reads low. Therefore you put more enriching in, you soot the furnace more. Then the carbon probe becomes more sooted, reads low, you put more carbon in. So, it’s kind of a snowball effect. And you can really get a lot of extra carbon, just residual carbon lane and the floor on the tubes any place in the furnace and that will cause a very large carbon problem.

M. McConkey: I guess one of the other things is air infiltration and / or water. A lot of our fans are water cooled. If you get a crack in the water-cooled member, you get water in the furnace, the carbon potential goes significantly down. The probe reads a low carbon and it tries to overcome that. Same with air…if you get a tube that leaks packing glands around a joint, leaks air in the furnace, the carbon potential will be very low and you’ll try to overcome that with more enriching. And then I guess the last thing is a tube leak in a gas fired furnace. If you get a crack in the tube or a hole in the tube every time the burner goes to high fire. The tube puts in products in combustion into the atmosphere, which will then drive the carbon probe to read very, very low and it’ll continue. You can get around this by looking to see once you put the burners on high fire. See if the carbon potential changes significantly, and then once you turn the burners off, does the carbon potential come back.

R. Miller: Okay, so maybe you mentioned it there, but if I’m doing a burnout cycle, how do I know that it’s complete?

M. McConkey: The best way to know if a burnout session is done is by looking at the O2. Once you put atmosphere or oxygen into the furnace and the carbon potential is very low, you look at the O2 and the O2 will probably be down around two, three, 4%…the oxygen. Then as the carbon is burned up, it, the oxygen, will start getting extra oxygen because it’s no longer maintaining a burn. So, the oxygen will start to increase, and you’ll end up with an oxygen of around 17 or 18% in the furnace when you’re putting straight air in. That means the burnout has been complete and you’re no longer burning carbon in the furnace.

R. Miller: Gotcha. Okay. One of the things you mentioned too is conveying parts. So, the Allcase automatically moves your hot, heavy loads under a combustible atmosphere. Can you briefly talk about the safety interlocks for internal load transfers?

M. McConkey: Sure, sure. One of the big things is that we want to make sure that there are several things before you transfer a load from the furnace in the quench tank, or even from the quench tank in the furnace. There are a few interlocks that we want to make sure that are happening. The first one and the most important one is that the atmosphere is on high flow. If a lot of furnaces have high – low flow, and we want to make sure that we’re putting in an extra amount of atmosphere into the furnace and the vestibule during a transfer. The other thing that we want to make sure is the elevator needs to be either up or down, depending on whether or not you’re charging or discharging a load. The agitators themselves need to be on so that we don’t overheat the oil on the outside, in the quench tank.

M. McConkey: The oil level…we want to make sure that the oil level is not too low. We don’t indicate high oil levels. We do indicate low. And also oil temperature. We want to make sure the oil temperature is not high because if we do quench a load with high oil temperature, we’ll end up with a problem. The other couple of things is the vestibule door. We want to make sure the vestibule door, when you’re transferring from the furnace to the quench tank, is not open. So, we’ll make sure that the vestibule door down limits which is made. Those are kind of the big things that we want to make sure when we’re transferring a load.

R. Miller: Okay. So, if the inner door, if it’s locked, locked out, like we can’t get it open or that we’re having problems with the inner door. What might cause that then?

M. McConkey: There is a newer furnace, there is a, what’s called a bypass switch on the inner door. It will allow the operator to go ahead and move the load, provided he knows what he’s doing. The alarm will be going off the whole time. We’re relying on the operator to know that the inner door problem is something he knows, he knows where the load is. He knows the furnace. He knows the interlocks are going to be bypassed during that time. But the inner door and the outer door both have a lockout and those same type of things are not allowed to happen, not allowed to open the door. We’re not allowed to quench a load and not allowed to charge a load. If the elevator’s not up or down, atmospheres got to be on high flow, oil levels ok, temperatures ok, and also the vestibule door is down.

R. Miller: I know one of the problems that people have is occasionally their parts, loads get hung up. So, can you share some easy checks to review why maybe a load of parts might get hung up during a transfer?

M. McConkey: Sure. One of the biggest things that the operators need to check is there’s a grid that the parts are loaded on. And the grid, over time starts to warp. The grid will warp down the handler head in the middle where we pick up the load. There’s a bar in the front that will also warp down and it will get hung up on either the rollers or the handler head. And also the baskets could have shifted when they get old, if they start warping the load could shift from side to side. You want to make sure that the trays and the baskets are in good shape so that you don’t drag them on the rollers as they’re coming out.

M. McConkey: The other thing that happens is the misalignment. Over time, the chain guide is where the handler head moves in and out. And the roller is on the outside of that. The chain guide can become misaligned. The chain guide is in two separate pieces from the front to the back of a furnace. And if the chain guide becomes misaligned, the chain itself could get hung up. The other thing that can happen is the rollers themselves could bow out and / or sag down. And you want to make sure that the rollers are all on the same plane and straight with the tray. Make sure that they’re in good shape. If you get an old tray, you need to replace it. If you get a tray that’s warped or if you have roller rails, you need to look at the rollers. The only way you can check the rollers properly is to shut the furnace down. You can look in through them and you can kind of see a wobble, which sometimes is a good indication that you need to shut the furnace down and do some maintenance.

R. Miller: Yeah, I’ve seen some old trays and baskets that we’ve “just a few more loads, let’s just get a few more loads out of them.” And that sometimes causes problems.

R. Miller: So also temperature control and burner maintenance, I’m sure are pretty common questions that you experience. So, what kinds of things are you asked about temperature control and burner maintenance, and what should people be looking for?

M. McConkey: A lot of times that you get in burner type maintenance with the ignition. A faulty ignition of the flame rod is in the burner. It actually sparks when you first light the burner. It sparks against the side of the air cone. And if the air and gas mixture are okay, and you don’t have ignition, you won’t have a burner light anyway. So, the ignition flame rod, I’m sorry….the spark rod will light off the side of the cone, and it will ignite the air gas combustible mixture they’re coming in. The other big thing we have is the burner won’t stay lit. When a burner is on low fire, the excess air typically will be somewhere in the neighborhood of 16, 17% excess O2. When we go to high fire, which is where the burner will spend a lot of its life, the excess air will go to around 2 to 4% excess air. So, in that transition point you’ve got to make sure that the low fire is a good, hardy flame. And then when you bring the air control valve up, it back loads to the natural gas. And it will bring the air and the gas on together so that the transition between a load of high fire will be smooth. We want to make sure that that’s a good point too, and we want to make sure that you get the rated input. Typically, Surface’s rated input on the Allcase furnace in particular is…there’s a main gas orifice, and it has a three inch Delta P drop across the main gas orifice, which is rated flow. And then you tune the high fire and low fire by adjusting the amount of air going through to the recuperator.

R. Miller: Okay. So, one of the things you mentioned, I think it was in a previous question, you talked about being able to evaluate the rails or whatever, and you have to shut the furnace off. Can you talk about how and when to idle a furnace, and why it’s not a good idea to shut them down cold in general?

M. McConkey: Yeah. Typically, the furnace is designed to stay at a temperature. Most of the time the customers would like to idle the furnace down over a weekend. We typically don’t recommend idling it down, unless you’re going to be down in the neighborhood of four to five days. Don’t shut it off. When you go to an idle condition, the furnace typically wants to be above 1400, which is the auto ignition point for fuel. Around 1400, 1450 is the idle temperature we’d like to go down to. The reason that we don’t want to have the furnace shut on and off every weekend or every time you’re going to be down for a day or so is because of alloy. The alloy and the brick work will take a lot of abuse as you cycle it up and down and up and down. The alloy will have a permanent growth and the brick will what’s called “spall off” because of its going up and down in temperature so much.

R. Miller: So, there are, though, times that we do need to shut down the furnace for preventative maintenance. Which maintenance items require the actual shutdown?

M. McConkey: The big items that require it be shut down is a roller rail and handler system malfunctions. Usually those happen on a load-by-load basis. The brick work will require a shutdown also, but the brick work usually deteriorates over time. But you can get an alloy that has actually failed on a couple loads. You ran yesterday three loads were fine and then you came in today and the alloy was just about in a position where it was going to start to come apart or warp or bend, and it will hang up a load. Usually, you’ll know that the load itself is hung up by, there’s a clutch in the handler system that will slip. So every time the torque in the furnace requires a higher torque than what the setting on the clutch is used to, the clutch will slip. And at that point, you know that either the handler’s hung up or the chain guide got hung up, or the load itself got hung up. And a lot of times it will slip once and come back in and it automatically resets itself. You’ll get an alarm, but it’ll automatically reset itself, and it will continue to load out. But you need to be aware that if that did slip, and you did get an alarm, that the alloy could be a suspect and it may need some maintenance here in a short period of time.

R. Miller: If for those items that you mentioned, if we do need to cool down the Allcase and heat it back up, how long does that take?

M. McConkey: Yeah, typically a cool-down for someone to get in the furnace will probably take around 2 to 3 days. We recommend that you bring the furnace down. And then after you get it below about 500, 4- 500 degrees, you can open the furnace door. But then after you get it down in around the 300s, it’s still got a lot of heat in there. So, a lot of customers put a man fan at the vestibule door with both maintenance bars in and try to cool it down so that you can go in there and inspect the rollers, inspect the alloy, inspect the fan in the furnace.

M. McConkey: On the flip side, heating the furnace up, we recommend a new brick takes around anywhere from 24 to 36 hours. That’s a conservative number I agree. But you want to be with, especially with new brick, you want to be more conservative than you are aggressive. Because you want to have several holds on the way up. You’ll want to heat the furnace to 300 degrees, with no atmosphere and obviously down that temperature. And hold it for 4 or 5 hours. Then go up to around 8 or 900, have another hold. And then, on the way up until you get the operating temperature, you should have several holds on the way up so that the brick has a chance to react to the environment. It won’t spall off. If you’ve just cooled the furnace down, and didn’t do any brick work, you don’t have to be quite that conservative, but you do want to make sure that the brick work has a chance to expand.

M. McConkey: Every time the furnace is cooled down, the brick itself will contract a little. The roof is a sprung arch. So, it will also sag down a little, and you’ll see some cracks in the brick work, which are pretty normal. They’ll seal back up because the brick has contracted when it comes down cold. But when you heat it back up, after existing brick, you don’t have to be quite that conservative. It’ll take about 18 hours. Sometimes you can go a little faster than that, but if you have the chance to be more conservative, when you heat up and cool down, the better off you’re going to be in the long run.

R. Miller: We’re always in a hurry. But then, you know, we’re going to end up spending a lot more money on our brick or something like that in the future, because we just didn’t take the extra few hours to do it right.

M. McConkey: Yeah. You’re definitely going to damage it down the road. You might not see it initially, but you’re exactly right. You’ll see that overall down the road, you’re going to spend a lot more money than a little bit of time you did for heating up or cooling down.

R. Miller: Absolutely. All right, well, good. Do you have any other thoughts before we sign off for today? Do you have any other thoughts that you’d like to share with the maintenance crews that are listening, working on controlled atmosphere furnaces. Any other tidbits of information from your many years of experience?

M. McConkey: Yeah, I guess one thing is we would be happy to help with any problems that you may have. We have a rebuild group that will provide rebuild for your furnaces on whatever you need done. And, we have a very fine field service group. There’s 12 people in our field service group that travel throughout the United States and Canada and Mexico and we’ll be happy to help in any way we can.

R. Miller: Great. All right. Well, thanks Mike, for this really informative discussion, I really enjoyed it and I hope that our listeners did as well. We look forward to our next podcast sponsored by Surface Combustion. To everyone out there, have a great day.

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