The Metal Treating Institute (MTI) hosted a special meeting for members at the Embassy Suites by Hilton in Downtown Pittsburgh, PA, on Monday, October 17, to review key Nadcap and AMEC topics. During the meeting, members addressed challenges that heat treaters face in Nadcap/audit compliance, how to navigate audits more effectively, and what suggestions to present to the Nadcap committee so that heat treaters would be better equipped for audits.
MTI’s Technical Standards Committee Co-Chairs Bob Ferry, VP of Engineering and Quality at FPM Heat Treat, and Edward (Ed) Engelhard, VP of Corporate Quality at Solar Atmospheres, facilitated the meeting. It was hosted by Tom Morrison, CEO MTI Management, and Jim Orr, president of Penna Flame Industries and current president of MTI. Several attendees who made particularly significant contributions to the discussion were; Doug Shuler, lead auditor at Pyro Consulting, LLC; and Roy Adkins, director of Corporate Quality at Braddock Metallurgical and recipient of the 2022 MTI Award of Industry Merit.
A Room Full of MTI members Including (l-r): Doug Glenn, Ed Engelhard, Bob Ferry, and Doug Shuler
At the meeting, attendees identified the number one challenge in Nadcap/audit compliance is understanding and implementing new Nadcap revisions; a close second was the challenge of ensuring quality when auditors give different feedback. These challenges were addressed in the meeting, especially when discussing two specific topics: first, Auditee Advisories – Type P (Potential Product Impact) and Type C (Confirmed Product Impact) as well as Audit Observations.
Several key points that came out of these discussions were to (1) always read up on the most recent revisions in order to be confident in your compliance with quality standards; (2) be sure to reference objective evidence on the Nadcap Checklist questions to help facilitate the audit; (3) let the Nadcap auditor do their job but address any clarifications/follow-ups to the staff engineer immediately; (4) investigate immediately when receiving a Type P write-up so that you can ask the auditor to add a comment on the limits of that product impact; and finally (5) always push-back on findings that are clearly not valid so that they are “voided” by the Performance Review Institute (PRI).
Another main point of the meeting was to address AMS2750H, an update consisting of editorial and language updates for added clarity.
Lastly, the facilitators of the meeting addressed aerospace standard AS13100: AESQ Quality Management System Requirements for Aero Engine Design and Production Organizations. The standard seeks to harmonize and simplify supplier quality requirements among the major aero engine manufacturers, supplemental to standard AS9100. This standard is in the process of being flowed down to the supply chain and compliance is required January 1 of 2023, meaning that heat treaters have a couple months to get up to standard.
This special meeting happens each year during the October Nadcap meeting in Pittsburgh, PA. MTI encourages heat treaters to attend the Nadcap meetings to share their invaluable voice to guide industry standards.
Photo caption for main image: Jim Orr speaks to members of MTI.
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Today's episode delves into the term "austempering". What is it? Why do heat treaters need to use it? For what applications is it necessary? Join Doug Glenn, publisher of Heat Treat Today and host of this podcast, as he talks with "The Heat Treat Doctor", Dan Herring, about all things austempering.
Below, you can watch the video, listen to the podcast by clicking on the audio play button, or read an edited transcript.
The following transcript has been edited for your reading enjoyment.
Earlier Episode of Lunch & Learn
Doug Glenn (DG): Alright, welcome everyone. We’re here with another Lunch & Learn with Dan Herring. Today, we’re going to be talking about the principles of austempering. We do these Lunch & Learns really for the benefit of our Heat TreatToday team and we knew that learning from Dan would also be educational for the entire industry. We are just really happy to be able to have Dan Herring with us once again to educate us a bit. We’re going to try to spend about 30 minutes or so learning about some of the very basic principles of austempering. So, the ball is over the fence to you, Dan.
Dan Herring (DH): Well, welcome everyone. It’s my pleasure to discuss the heat treat topic that we call austempering. One of the things we’re going to do today is we’re going to recall from a previous Lunch & Learn the definition of heat treating. We called it the controlled application of time, temperature, and atmosphere to produce a predictable change in the internal structure of what metallurgists call the microstructure of a material. So, we’re going to introduce various words that are related to different types of microstructures today or these internal structures.
But before we do, I’ve put on the screen a brief definition of austempering. It’s certainly a heat treat process. It’s used in medium to high carbon, both plain and alloy steels, as well as cast irons (an example being ductile iron) and we’re trying to produce a microstructure called bainite which is probably a foreign word to most of you and I’ll endeavor to explain it in a moment.
But to give you just a view from about 30,000-feet, you might be asking yourself, “Well, what types of products are austempered and why?” So, I put a couple of examples here. I’ve put an example of a lawnmower blade, seat belt components like the tongue and receptacle, and some tractor parts, as well.
A good example of this might be the seatbelt components. We’ve learned to put on seatbelts (in my day, we didn’t have them, but now we do) and we all learned to buckle up. And, if you get into an accident, you discover why your seatbelt is really your friend. We want something that’s strong, that if we get into an accident, it will not shatter and break. But, at the same time, we want something that’s tough and slightly ductile so it will bend and not break.
Austempering is a process that’s used to produce all seatbelt components, that I’m aware of. Similarly, with lawnmower blades- we don’t want a blade, if it hits a rock as we’re mowing the lawn, (I don’t expect most of the people on the call to have mowed the lawn), but if we hit a rock or a hard object as we’re mowing the lawn, we might want that lawnmower blade to get a ding in it, but we don’t want it to shatter. So, those are some typical examples.
You might ask yourself, why do you austemper? What we’re seeing here is that if you need increased ductility, toughness, and strength at a given hardness level, austempering is right for you. We’re typically talking about parts that are in the range of, maybe, 35-55 Rockwell C. We are developing, as I said, a bainitic structure as opposed to a martensitic structure, which is what’s produced when we harden a steel and quench into something like oil or water.
So, we get improved toughness. And we get some superior properties related to that, as well. And some of the properties don’t change very much but they’re equal to what we get when we harden the steel, when we get this martensitic structure.
The bottom line is we typically get less distortion, we get better wear resistance, we don’t suffer from cracking as some of the high carbon steels are prone to do, and, interestingly enough, with cast irons, we get some, what are called "improved dampening characteristics" -- noise and vibration. So, wire is an important like, for example, in an automotive engine to have dampening characteristics because we want the engine to run quietly.
What types of materials can be austempered? This is just a partial list, but mostly it’s medium carbon steels. That’s carbon steels with anywhere from .5 carbon to .95 carbon or, in other words, an AISI 1050 to 1095 grade. We can also do medium alloy steels -- the 4130’s, the 4140’s, the 5140’s, the 5160’s, etc. Certain stainless steels can be austempered although not many of them. And, as I said, cast irons, the example being ductile iron, can also be austempered.
And I wanted to give you some idea of the mechanical and different properties of steel. We talked in an earlier Lunch & Learn about the fact that steel is an alloy of iron and carbon and manganese. And we add other elements to the mix in order to get various either mechanical properties, chemical properties, electrical or magnetic properties, and certain other advantages.
So, an example of mechanical properties that we’re typically interested in is hardness and strength, brittleness, ductility, elongation, wear, and shock resistance. Now, strength is measured a number of ways. There are things called "fatigue strength" and "flexure" and "impact strength" and "sheer strength" and "tensile strength" and "torsion strength" and "yield strength."
This is a metallurgist’s rendition of a teeter totter in a schoolyard. Now, don’t laugh. This is what defines the difference between a metallurgist and a mechanical engineer. For all the mechanical engineers out there, metallurgists draw cartoons -- that’s the easiest way to say it. Howsoever, at one point in all of our lives, we’ve probably been on a teeter totter. We know that, in this particular teeter totter, we have strength properties on one side of the teeter totter and ductility properties on the other. We know that as the strength goes up, the ductility will go down and as the ductility will go up, the strength will go down. As a result of this, we decide what we want for properties and we realize that there’s a compromise going on. If we make them extremely strong, they’ll be brittle because they’ll have very, very low ductility. If we make them extremely ductile, they’ll have very low strength. So, this balancing act is what we’re trying to do when we look at the properties we’re trying to achieve. And, if you remember, the microstructure is what gives us these properties.
Now, this is something that is not intended to confuse, but I thought I’d add a little metallurgy into the mix because we are going to talk about several microstructures. This is what metallurgists call a "time temperature transformation" or "TTT diagram." This is really an artist’s rendition of one. There is a lot more information typically contained in one of these diagrams. But for our purposes, it isn’t too important. We can use this artist’s rendition to get the essence of what we’re trying to do.
We start off by heating steel to austenitizing temperature. And that’s above the dotted line shown in this particular diagram, so, at the very top of those turquoise lines and temperature. And then what we do is we make sure that the component part is uniformly up to temperature and now we get ready to harden it. We get ready to quench it. What we’re dealing with is we’re rapidly cooling, and under normal hardening, you’ll notice that there are two lines there- one called MS and one called MF. MS is the martensite start line and MF is the martensite finish line.
Typically, in hardening, our goal is to produce martensite. In order to do so, we want to cool rapidly enough to miss what we call "the nose of the curve" because if you look at this type of diagram, you’ll see that it, on profile, looks like somebody’s nose and the turquoise lines are missing the "nose" of the curve. As a result of that, we’re cooling rapidly. But the difference between hardening and austempering is that we don’t cross the MS point, we don’t cross into the martensite range. We don’t transform to martensite, instead what we do is we put the brakes on, we stop, and then we introduce a long soak or hold period and we cross into the banitic range of the curve.
And, so, austempering is typically performed about 25-50 degrees Fahrenheit above the martensite start temperature of steel. Now, there are some exceptions, but that’s a very typical range. If we’re not controlling the process properly, we might get a microstructure that’s both bainite and martensite. But if we do our job right, we’ll get a fully bainitic structure, which is often what we desire.
Read More in Dan Herring's Books
Now (and I realize this has words that some people may be unfamiliar with) but we’ve heated the part up until we’re austenitic- we’re in the austenite range, and there are three various methods of cooling that can be employed. On the far righthand side, if we rapidly quench a part into oil or into water, we might produce a microstructure that’s called martensite. It’s a body-centered tetragonal microstructure. We get something that’s very hard, but brittle. That’s why we have to reheat it and perform a process called ‘tempering’ in order to take some of the brittleness away and add some ductility back in.
Now, on the far lefthand side, we may slow cool the part rather than rapidly quench it and we produce a microstructure that is both ferrite and pearlite, the result of slow cooling. So, instead of getting something that’s very hard, we get something that is very soft. You might say, “My gosh, why do we want to do that?” Well, we like to do that sometimes because we like to take a steel and, for example, machine it into a final form before we go back in and reharden it. So, as a result of that, we form a ferrite/pearlite microstructure, we’re able to machine the part, then we can go back in and reharden it.
So, slow cooling gives us a ferrite/pearlite microstructure, rapid quenching gives us a martensitic microstructure, and a moderate cooling rate (the one shown in the center) gives us a bainitic microstructure. Bainite is a mixture of ferrite and cementite. Again, words that you’re perhaps not familiar with. But the way I like to say it is martensite gives us a microstructure that is not as hard as martensite but tougher, in general, than martensite, and we’ll explain that as we move forward.
But I thought before we do, you might want to see some typical type of heat-treating equipment that is used to austemper parts. A lot of parts are done in a mesh belt conveyer line. The one that is shown on the left, where parts are loaded onto a table, sent through the furnace, and dropped at the end of the furnace into a salt quench which is located in the floor, in this particular drawing. Salt is the primary medium that we quench parts that will be austempered in because salt gives us the temperature-range we need to be above the martensite start point.
Now, a number of people have asked me in the past: Can I use oil rather than molten salt to perform this operation? There are certain oils that can be used at extremely high temperature, but there are fire hazards and other hazards associated with them so the typical answer is ‘no’; molten salt is typically used to perform the quenching.
So, you have a mesh belt conveyer system for high volume, shown on the left. On the right, you’re showing a typical Shaker Hearth furnace where what happens is you load parts onto a pan that vibrates and the parts are moved down the length of the furnace and then drop into a salt quench at the back end.
I thought you might want to see some pictures of some stampings and things that are going into one of these mesh belt conveyer furnaces. You see the endothermic gas in this particular picture burning out the front of the furnace and the stampings moving on a conveyor belt, a mesh belt, in through the furnace. All sorts of different types, shapes, and sizes of stampings. One thing you’ll notice is that these parts are, typically, not single layer loaded; they’re loaded, perhaps, one to three to five parts thick, somewhere between anywhere from a half inch to about two or three inches thick as they’re moving through this conveyor belt.
And to complete the metallurgy aspect of it, you might say, “Hey, what type of microstructure am I actually seeing?” The picture on the left is a primarily bainitic microstructure with some martensite and its hardness is 44 Rockwell C. The microstructure on the righthand side is a combination of bainite and ferrite. The ferrite in this microstructure shows up as white or very light in color, exactly. This hardness, because you have ferrite present, is about 36 HRC. So, depending on the hardness you’re trying to achieve, you will get different types of microstructures- that’s the purpose of this slide.
Now, as far as molten salt goes, a typical austempering bath consists of either a sodium nitrate or a potassium nitrate salt, typically in a 50/50 mixture, and this salt is operating somewhere between 300 degrees Fahrenheit and 650 degrees Fahrenheit, depending on, again, the desired, not only the composition in the salt, but the desired temperature that we would want to hold to.
Let me back up for a second, Doug. So, to kind of summarize this: What we’re trying to put the brakes on as we’re rapidly cooling down, missing the nose of the transformation curve, we want to fall into this bainitic region and, in order to do so, we need to stay above that martensite start temperature which for many steels is in the 400–450-degree Fahrenheit range. So, our molten salts will typically run at 475, 500, even 550 degrees, all the way up to 650 degrees. So, we pick our salt temperature, not only depending on the salt, but also depending on the temperature that we want to hold the bath in.
Some of the reasons for selecting a salt quench are that the temperature of the salt bath dictates the ultimate hardness that we’re going to achieve. You might find this interesting: If I didn’t mention it in a previous Lunch & Learn, but I did, it’s that when we quench into the martensite range or field, martensite is the instantaneous sheer transformation. It really progresses at the speed of sound. So, martensite forms almost instantaneously, but bainite requires time for the transformation to take place.
So, a typical time in the salt is somewhere in the range of 18-20 minutes. I’ve seen parts held in salt for as short as 10 or 12 minutes and for as long as 30 minutes, but it depends on the thickness of the part, the material and, ultimately, the desired hardness we are going to reach. Now, interestingly enough, as opposed to a part that we harden to martensite and have to retemper or temper to balance the teeter totter, so to speak, with an austempering process, we do not need to temper afterwork because the parts are effectively tempered, so to speak, in the salt. So, we have a hardening operation that results in a banitic structure but we don’t need to temper. So, that’s one of the differences between hardening and austempering.
Again, the time in the salt will decrease as the transformation temperature increases and the time in the salt is similarly associated with the carbon content in the steel.
Let me give you a couple of examples: I mentioned in an earlier slide that SAE 1050, 1055, 1075 steel are typical steels that are austempered. Again, your austempering goes to put the hardness typically in the range of 40-45 Rockwell C, not nearly as hard as if we harden and quench them into oil or water, but certainly hard enough to give you a properly austempered part, giving you this part that is a combination of good hardness and yet a lot of ductility.
This, in a nutshell, is a brief summary of austempering. We’ve kind of said what it is -- it’s a process that’s going to get us a bainitic microstructure. We’ve looked at a little of the metallurgy of what we’re dealing with here and we’ve seen that it’s a different type of microstructure than is something like annealing or normalizing which gives you a primarily ferritic and pearlitic microstructure. And it’s different than hardening that gives you primarily a martensitic or tempered martensitic structure.
So, for those parts that require not only hardness but toughness, austempering is a process that should be considered by heat treaters.
Doug, that’s really the end of the presentation that I’ve prepared. We can certainly discuss it a little bit more if anyone has any questions.
DG: At the beginning, you were talking about pearlite and all that stuff, did we talk about austenite?
DH: Well, we talked about austenite because, again, that’s the temperature to which we heat the parts up to at the very beginning. In other words, to start the process, we heat the parts up to the austenite field, if you will. In other words, the parts are essentially red hot. They are above the proper transformation point that they turn into austenite.
DG: So, I assume that’s here, if you guys can still see the images: That’s austenite. The austenitic temperature is up above this dash line, right?
DH: That’s correct.
DG: And as you bring it down, you come through, perhaps, other, there’s a lot of different "ites" in heat treating, right? There’s austenite, pearlite, ferrite, bainite, martensite, you know, it sounds like a stalagmite and whatever those other things are in the caves, but all of those things basically are telling us about the orientation of the molecules inside the metal.
DH: Well, think of it this way, Doug: When we have a steel, its microstructure, if it isn’t hardened, its microstructure is typically body-centered cubic, which means the atoms are all lined up in a certain structure. Now, what we do when we heat it up is -- when it gets above the transformation temperature (that dotted line, for simplicity, in this example) the atoms will realign themselves from body-centered cubic to face-centered cubic and a face-centered cubic structure is called austenite. Then, when we quench it, until we move into the nose of the curve or past those red lines, we still maintain an austenitic crystal structure as we’re cooling. The ferrite, the pearlite and things occur when we cross over into those reddish lines in that area there.
I think you can do this- if we start off as austenite, and we slow, slow cool.
Slow, slow cool. We go all the way down like that. Keep going down, down, down, down, down. Okay, if we do something like that, (and I’ve got some pictures to show it better), but the idea being the fact that because we’ve fallen into the nose of the curve, we form a microstructure that is typically ferrite and pearlite. The first line you’ve drawn is indicative of an annealing process where we’re slow cooling inside the furnace. The second line you’ve drawn is more indicative of a normalizing process where we’ve cooled at a faster rate but still, in this case, we’ve fallen into the nose of the curve because it’s not that quick.
And to give everyone a perspective of the time element involved here, and I haven’t shown numbers, but the time element is for plain carbon steels, you may only have a few seconds to reach the nose of the curve. So, as a result of that, you have to move very rapidly where those turquoise lines are shown; you’re cooling at a very, very, very rapid rate to try to miss the nose of that transformation curve.
The secret with austempering is that you have to put the brakes on before you form martensite, and that’s not as easy as you might think it is. But that’s one of the reasons why molten salt is an excellent medium to quench into.
Don’t mix up crystal structures with microstructures. The ferrite, the pearlite, the bainite, the martensite are microstructures whereas the crystallographic structures -- body-centered cubic, face-centered cubic, body-centered tetragonal- represent how the atoms realign themselves.
DG: Does anybody have questions for Dan?
Bethany Leone (BL): I was thinking about asking you, Dan, but you have already essentially answered it: How difficult is it to have that rapid cooling and then control it to remain quite stable for a long period of time? You hit on the first part of the question which is the salt quench does a good job in this instance. But how does a heat treater maintain that stability of temperature for such a long time?
DH: That’s a great question because one of the interesting properties of salt, molten salt, is the fact that it is a bath that’s extremely uniform in temperature. So, when, for example, the parts, the stampings, and other parts are conveyed through a furnace, they then drop off into a quench and there is a conveyer belt in the quench, under the salt, that the parts drop on to this conveyor belt and then move through the salt. So, if I want 20 minutes in the salt bath, I have to run the speed of the conveyor slow enough to allow that time to take place.
Now, not to confuse everyone, but there are other ways you can austemper: You can heat in molten salt and then quench in the molten salt. So, there is a molten salt you can actually preheat in molten salt, have a high heat in molten salt and then a quench in molten salt. A lot of people don’t use that for high volume production work, but they still use that.
But, yes, you need time in the salt for that transformation to fully take place.
DG: Any other questions?
Let me do a couple other things and, again, we can probably put this up on the screen, but we just recently, I believe, already released this -- the Heat Treat Radio interview we did with Bill Disler regarding salt quenching. That may be of interest to people who have an interest in what about salt quenching? You might want to reference that sometime so, feel free to look into that. You also can just search our website for "bainite" or "austempering" and you may come up with some additional articles.
So, that’s it. Dan, thank you very much. I appreciate it. Unless anybody else another question, I think we’ll sign off at this point.
A manufacturer in Poland turns to in-house heat treating capabilities rather than outsourcing its nitriding for stainless steel power generation parts. Steam turbine components will now be processed at the facility in Elbląg as a result of funding help from a government program part of the European Union.
Marcin Stokłosa Project Manager Nitrex Poland LinkedIn.com
P.W.P.T. POSTEOR Sp. z o. omade the decision to stop outsourcing to commercial heat treaters its steam turbine pieces to bring the nitriding in house. The NX series furnace, model NX-620 will streamline production of these parts with its automated capabilities. The pit type furnace from NITREXmeets requirement for nitriding, nitrocarburizing, and in-process oxidation. “The turnkey system also includes remote access software, an INS neutralizer for a clean and environment-friendly process, and a custom HMI for the end-user," describes Marcin Stokłosa, project manager at NITREX. "It is entirely automated, requiring little operator attendance or involvement."
POSTEOR hopes to reduce the challenges it was facing in outsourcing the components. With the nitriding furnace the company will have more positive control of the end results.
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Ever heard of binder jetting (BJT)? It's an evolving technology that is quickly catching up to metal injection molding (MIM). Compared to MIM, BJT has a lower cost per part rate, produces larger parts, and, because BJT is a cold process, it does not introduce residual stress inside the part.
Even though BJT is a cold process, sintering is a key step in BJT. Read this best of the web article to learn the ins-and-outs of sintering with binder jetting.
An excerpt:
"Vacuum sintering furnaces are usually the go-to choice for sintering of [binder jetting] parts, thanks to the ability to provide bright and shiny sintered parts, the tight process parameters control and the possibility to work with different debinding and sintering atmospheres."
We’ve assembled some of the top 101 Heat Treat Tips that heat treating professionals submitted over the last three years into todays original content. If you want more, search for “101 heat treat tips” on the website! Today’s tips are all things temperature: thermocouples, how to keep temperatures in check, TUS, and more.
By the way, Heat Treat Today introduced Heat Treat Resources this year; this is a feature you can use when you’re at the plant or on the road. Check out the digital edition of the September Tradeshow magazine to check it out yourself!
Temperature Monitoring When the Pressure is On!
Increasing in popularity in the carburizing market is the use of batch or semi-continuous batch low pressure carburizing furnaces. Following the diffusion, the product is transferred to a high-pressure gas quench chamber where the product is rapidly gas cooled using typically N2 or Helium at up to 20 bar pressure.
In such processes, the technical challenge for thru-process temperature monitoring is twofold. The thermal barrier must be capable of protecting against not only heat during the carburizing, but also very rapid pressure and temperature changes inflicted by the gas quench. From a data collection perspective, to efficiently perform temperature uniformity surveys at different temperature levels in the furnace it is important that temperature readings can be reviewed live from the process but without need for trailing thermocouples.
During the gas quench, the barrier needs to be protected from Nitrogen N2(g) or Helium He(g) gas pressures up to 20 bar. Such pressures on the flat top of the barrier would create excessive stress to the metal work and internal insulation / logger. To protect the barrier therefore a separate gas quench deflector is used. The tapered top plate deflects the gas away from the barrier. The unique Phoenix design means the plate is supported on either four or six support legs. As it is not in contact with the barrier no force is applied directly to the barrier and the force is shared between the support legs. The quench shield in addition to protecting against pressure, also acts as an additional reflective IR shield reducing the rate if IR absorption by the barrier in the vacuum heating chamber.
(PhoenixTM)
3 Tips to Meet Temperature Uniformity Surveys
Adjust the burners with some excess air to improve convection.
Make sure that the low fire adjustment is as small as possible. Since low fire will provide very little energy, it will make the furnace pulse more frequently and this will improve heat transfer by convection and radiation.
Increase internal pressure. This will “push” heat to dead zones allowing you to increase your coldest thermocouples (typically near the floor and in the corners of the furnace).
(Nutec Bickley)
Ways to Increase Temperature Uniformity in Heat Treat Furnaces
A (sometimes) simple way to increase uniformity in a furnace is to add a circulation fan. Circulation fans can be a quick way to add an additional 5°F tighter uniformity on a batch furnace application.
Be sure that the furnace is tuned optimally to reduce/eliminate any overshoot and oscillation around setpoint.
Eliminate any thermal lag by making sure that the control thermocouple and TUS thermocouples have similar sensitivity. If not, the control thermocouples can fall behind and cause the TUS thermocouples to overshoot and fail.
(L & L Special Furnace Co., Inc.)
Pack Your Thermocouples
When a thermocouple is used with an open-ended protection tube, pack rope or fiber between the thermocouple and the protection tube to prevent cold air infiltration from influencing the reading.
(Super Systems, Inc.)
A Good Fit
If a thermocouple fits loosely in a protection tube, avoid errors by ensuring that the tip maintains good contact with the tube.
(Super Systems, Inc.)
Introducing Your Common Thermocouple Types
What are the common thermocouple types?
Thermocouple material is available in types K, J, E, N, T, R, S, and B. These thermocouple types can be separated into two categories: Base and Noble Metals.
Types K, J, E, N, and T are Base Metals. They are made from common materials such as Nickel, Copper, Iron, Chromium, and Aluminum. Each base metal thermocouple has preferred usage conditions.
Types S, R, and B thermocouples are Noble Metals because they are made of one or more of the noble metals, such as Ruthenium, Rhodium, Palladium, Silver, Osmium, Iridium, Platinum, and Gold. Noble metals resist oxidation and corrosion in moist air. Noble metals are not easily attacked by acids. Some Noble metal thermocouples can be used as high as 3100°F.
(Pelican Wire)
Culprits of a Stable Thermocouple
Factors affecting the stability of a thermocouple:
The EMF output of any thermocouple will change slightly with time in service and at elevated temperatures. The rate and change are influenced by metallurgical and environmental factors. The four factors that can induce EMF drift are: Evaporation, Diffusion, Oxidation, and Contamination.
(Pelican Wire)
Does Length Matter?
Does the length of a thermocouple wire matter?
In a word, “Yes.” There are several factors when considering the maximum length of a thermocouple assembly. Total loop resistance and electrical noise. Total loop resistance should be kept under 100 ohms for any given thermocouple assembly. Remember, the total loop resistance would include any extension wire used to complete the circuit. Motors and power wires can create noise that could affect the EMF output.
(Pelican Wire)
Type N Thermocouple (Nicrosil/Nisil)
Type N Thermocouple (Nicrosil/Nisil): The Type N shares the same accuracy and temperature limits as the Type K. Type N is slightly more expensive and has better repeatability between 572°F to 932°F (300°C to 500°C) compared to type K.
(Pelican Wire)
Know Your Thermocouple Wire Insulations
Know your thermocouple wire insulations. When is Teflon® not Teflon®? Teflon® is a brand name for PTFE or Polytetrafluoroethylene owned by Chemours, a spin-off from Dupont. FEP is Fluorinated Ethylene Propylene. PFA is Perfluoroalkoxy Polymer. All three are part of the Fluoropolymer family but have different properties. Of the three compounds, PTFE has the highest heat resistance, PFA second highest and FEP third. The higher the heat resistance the more expensive the insulation. Keep that in mind when specifying the insulation and only pay for what you need.
One of the most important advances in batch integral quench (BIQ) furnace systems has been the development of innovative control systems. Many BIQ manufacturers have developed their own highly sophisticated and cutting-edge control systems.
This Technical Tuesday, we explore one such system written by Daniel Hill, PE, sales engineer at AFC-Holcroft. This original content article was originally published in the February 2021Air and Atmospheres, the IQ Edition.
Daniel Hill, PE Sales Engineer AFC-Holcroft Source: AFC-Holcroft
With continuing advancements of smart devices, integrated controls systems, and the Industrial Internet of Things (IIoT), we have at our fingertips an abundance of data, both the traditional and newly developed. How to convert that data into useful information, and more importantly how to leverage that information into day-to-day operations to reap the benefits, becomes the difference maker in a competitive landscape.
Recognizing these trends, AFC-Holcroft has built upon a suite of software modules that includes Remote Diagnostics™ to also offer Maintenance Module™ and Calibration Mode™. All three modules are in commercial service having been integrated with BatchMaster™ controls system features on Universal Batch Integral Quench (UBQ) furnaces. End-user response and adoption have been positive with new synergies and feedback leading to ongoing enhancements. In this article, we will discuss how these advancements are affecting the future of furnace diagnostics and some examples of their benefits in many day-to-day situations.
Dynamic Furnace Calibration & Diagnostics
Perhaps the most interesting of the three modules is the Calibration Mode, a patent-pending diagnostics software designed to dynamically test furnace operation for verification of proper functionality over a wide range of subsystems and devices. Notably, furnace and quenchant heating/cooling thermal loads are strategically cycled for response monitoring of typical production needs. Likewise, furnace atmosphere is cycled for response and stability. Additional systems and metrics such as agitation attributes, tray motion and positioning, time to quench, and elevator operation are activated and evaluated.
Once the calibration cycle is complete, the data and responses are compared to original commissioning data and design thresholds to generate a time-stamped diagnostic report with straightforward pass/fail results. With this module, the operator can ensure proper operation at a moment’s notice without additional external testing devices and have the data available to back it up.
It was designed to integrate optional device packages to elevate its diagnostics capabilities exponentially by targeting efficiency and optimization of operation including:
Combustion efficiency monitoring
Vibration monitoring
Power consumption monitoring
Process Troubleshooting
As its base premise, this module was created as a means for operators to proactively verify that the furnace is both fully operational and responding as designed. But what happens if it is not? Calibration Mode can be initialized at any time for deeper analyses by first reporting the current status of subsystems and devices and then for further comparison against initial commissioning data or cumulatively against any previous calibration iterations, making it a powerful tool for rapid diagnostics.
With the auto-generated reports and comparison tools in hand, operators or maintenance team members can pivot directly into troubleshooting any deficiencies identified for quick resolution. Once more, a follow-up can be run to ensure the deficiency is corrected after making appropriate adjustments or repairs. This saves valuable time and resources, improves availability, and likely increases profitability.
Calibration Mode™ Screen Source: AFC-Holcroft
Compliance with Industry Specifications
Whether following automotive (CQI-9), aerospace (AMS2750), military, energy, or other specifications, universal themes requiring the user to implement regular assessments, surveys, and the monitoring of the process equipment are paramount. Today, the Calibration Mode is being used to supplement these efforts in a number of ways:
Creating reports with Calibration Mode for job audit in annual Heat Treat System Assessments (HTSAs)
Producing objective evidence for process equipment validation before and after a major rebuild or modification
Collecting and analyzing data over time and reacting to it
Running with TUS as another layer of equipment verification
Running with Quench System Monitoring as another layer of verification
PPAP, Control Plan, & PSW Inclusion
The production part approval process (PPAP) must be a collaboration between the customer and heat treater to ensure a clear understanding of all elements before, during, and after the processing. Calibration Mode can be utilized as a verification tool initially when processing parts for a PPAP to document the furnace capability and that it is meeting original OEM specifications. If repair or rebuilds are required while that PPAP is still valid, Calibration Mode can be run to demonstrate the equipment in operational condition on a Part Submission Warrant (PSW). Moreover, Calibration Mode can be incorporated into control plans both as a control method for ongoing production and also as part of a reaction plan to diagnose nonconformance.
Dedicated Equipment Vs. Changeover of Equipment Running Multiple Processes
A major benefit of batch processing equipment is the inherent flexibility it offers–especially to commercial heat treaters who are serving multiple customers with many different processes. Often customer specifications for heat treating include clauses preferring dedicated equipment with strict allowances on changeover of equipment. Typically, changeover of equipment for multiple processes requires customer personnel to review and approve the heat treater’s changeover procedures and must include verification prior to start of production (including atmosphere). This changeover process must be documented in the heat treater’s process control plan. Consider running Calibration Mode at changeovers as a means to consistently verify conditions and provide documentation to the benefit of all involved.
Intelligent Preventative Maintenance
Maintenance Module™ also takes advantage of the latest advancements. This module is designed and pre-programmed to include the OEM recommended preventative maintenance tasks based upon pre-defined intervals, sometimes utilizing conditional statements, or where appropriate, predictive algorithms based upon operating time, temperatures, and number of cycles. This database of tasks and report queries provides an intelligent roadmap for the preventative maintenance of the furnace. As such, maintenance task statuses are elevated and flagged for action complementary to accrued usage and actual conditions so that all-important resources can be prioritized and best served.
Maintenance Module™ Screen Source: AFC-Holcroft
Leveraging Diagnostics through the Cloud
Finally, Remote Diagnostics™ was conceived to increase furnace uptime and availability through the analysis of equipment performance data. Real, data-driven reliability and maintainability (R&M) information supports continuous improvement efforts. As a first step, the abundance of data delivered through the IIoT is aggregated to effectively parse, diagnose, and highlight operational inefficiencies.
Next, virtual conference sessions are led by AFC-Holcroft personnel to collaborate with users on best practices, identify training needs, aid in knowledge capture, and provide optimization plans moving forward in a classic continuous improvement cycle.
Interestingly enough, it has become a source of synergy for continuous improvement efforts by allowing us to better understand users’ needs and incorporating them into designs and equipment of the future.
Addressing important day-to-day situations, Calibration Mode, Maintenance Module, and Remote Diagnostics are helpful tools for the forward-thinking BIQ furnace operator.
About the Author: Daniel Hill, PE, is a sales engineer with AFC-Holcroft, based in Wixom, Michigan. AFC-Holcroft is a leading North American manufacturer of industrial furnace systems used in the heat treatment of ferrous and non-ferrous metals.
Vacuum Sintering Furnace from Signature Vacuum Systems, Inc. Source: Signature Vacuum Systems, Inc.
A United States defense contractor will expand their heat treat capabilities with a custom ceramic sintering vacuum furnace. The furnace will be provided with a 36” diameter x 48” high work zone in a graphite hot zone rated for 3362°F (1850°C).
The Model VBS-12 ordered, will increase production capacity and is the fifth furnace from the supplier, Signature Vacuum Systems, Inc., to be manufactured and installed in this contractor’s facility.
“Our strength[…] is solving problems and delivering solutions,” said Greg Kimble, president of Signature Vacuum Systems, Inc. “We have enjoyed the progression of this relationship over the years and we are committed to providing quality products and dependable services.”
(photo source: Defence-Imagery at Pixabay.com)
Unless identified otherwise, all other images from unsplash.com.
“Successful heat treating begins by understanding the make-up of the steel that is to be treated.”
Heat Treat Today’sTechnical Tuesday feature provides an overview of the heat treatment process and the benefits wrought from heat treating in salt baths. The article also illuminates details to understand part composition and the austempering and quenching process as a whole.
The author of this Original Content article, Jerry Dwyer, market manager at Hubbard-Hall, has previously written for Heat Treat Today on the topic of polymer quenchants as an alternative to water and oil quenching. Read more here.
Heat treating is a process in which metal is heated to a predetermined temperature and then cooled in a particular manner to alter its internal structure for obtaining a desired degree of physical, mechanical and metallurgical properties. The purpose is to obtain maximum strength (i.e., increase the metal’s hardness) and durability in the material.
Numerous industries utilize heat treated parts, including those in the automotive, aerospace, information technology, and heavy equipment sectors. Specifically, manufacturers of items such as saws, axes, cutting tools, bearings, gears, axles, fasteners, camshafts, and crankshafts all rely on heat treating to make their products more durable and to last longer.1
The heat treating processes require three basic steps:
Heating to a specified temperature.
Holding at that temperature for the appropriate amount of time.
Cooling according to prescribed methods.
Understanding the Part Material
According to the ASM International’s Heat Treating Society, about 80 percent of heat treated parts are made of steel, such as bars and tubes, as well as parts that have been cast, forged, welded, machined, rolled, stamped, drawn, or extruded.1
SAE Designation. (Image source: Jerry Dwyer. Reference source #3.)
Successful heat treating begins by understanding the make-up of the steel that is to be treated. The American Iron and Steel Institute (A.I.S.I.) and the Society of Automotive Engineers (S.A.E.) utilize a four-digit system to code various types of steel used in manufacturing. The alloying element in the AISI specification is indicated by the first two digits, and the amount of carbon in the material is indicated by the last two digits. The first digit represents a general category of the steel groupings, meaning that 1xxx groups within the SAE-AISI system represent carbon steel. The second digit represents the presence of major elements which may affect the properties of steel; for example, in 1018 steel the zero in the 10xx series depicts no major secondary element. The last two digits indicate the percentage of carbon concentration. SAE 1018 indicates non-modified carbon steel containing 0.18% of carbon, while SAE 5130 indicates a chromium alloy steel containing 1% chromium and 0.30% carbon.
Carbon steel has a main alloying constituent of carbon in the range of 0.12% to 2.0%. Plain carbon steel is usually iron with less than 1% carbon, plus small amounts of manganese, phosphorous, sulfur and silicon. Carbon steel is broken down into four classes based on carbon content:
Austempering is one of several heat treatments that is applied to ferrous metals and is defined by both the process and the resultant microstructure of the work. In steel, it produces a bainite (or a plate-like) microstructure.
Typical Austempering Heat Treatment Cycle in Ductile Iron
When heated to temperatures below 730°C (1346°F), the pure metal iron has a body-centered cubic structure; if heated above this temperature, the structure will change to a face-centered cubic. On cooling, the change is reversed, and a body-centered cubic structure is once more formed. The importance of this reversible transformation lies in the fact that up to 2.0% carbon can dissolve in a face-centered cubic, forming what is known as a “solid solution.” While in a body-centered cubic iron state, no more than 0.02% carbon can be dissolved this way. The solid solution formed when the carbon atoms are absorbed into the face-centered cubic structure of iron is called austenite.
Austempering Process Steel Structuring
When quenched, carbon is precipitated from austenite not in the form of elemental carbon (graphite), but as the compound iron carbide Fe3C, or cementite. Like most other metallic carbides, this substance is usually very hard; as the amount of carbon increases, the hardness of the cooled steel will also increase.
The temperature of the quench tank is set so that the material is rapidly cooled down at a rate fast enough to avoid transformation to intermediate phases such as ferrite or pearlite and then held at a temperature that falls within the bainite region but staying above the martensitic phase. The bainitic microstructure that is formed as a result of austempering imparts high ductility, impact strength, and wear resistance for a given hardness; a rifle bolt was one of the first applications for this process.
The salt quench also provides low distortion of work with repeatable dimensional response. The materials have increased fatigue strength and is, in general, more resistant to hydrogen and environmental embrittlement.
Heat Treat with Salt Baths
Salt bath heat treatment is a heat treatment process comprising an immersion of the treated part into a molten salt, or salts mixture.2 There are numerous benefits of heat treatment in salt baths, the most prevalent is that they provide faster heating. A work part immersed into a molten salt is heated by heat transferred by conduction (combined with convection) through the liquid media (salt bath).2 The heat transfer rate in a liquid media is much greater than that in other heating mechanisms, such as radiation or convection through a gas.2
Using salt baths also helps with a controlled cooling conditions during quenching. In conventional quenching operation, typically either water or oil are used as the quenching media and the high cooling rate provided by water/oil may cause cracks and distortion. Cooling in molten salt is slower and stops at lower temperature and avoids may of the pitfalls associated with a faster quench.2
Salt baths also provide low surface oxidation and decarburization, as the contact of the hot work part with the atmosphere is minimized when the part is treated in the salt bath.2 There are additional advantages to salt heat treat:
Wide operating temperatures: 300°F -2350°F
Most of the heat is extracted during quenching by convection at a uniform rate.
Salt gives buoyancy to the work being processed to hold work distortion to a minimum.
Quench severity can be controlled or manipulated by a greater degree by varying temperature, agitation and water content of the salt.
Excellent thermal and chemical stability of the salt means that the only replenishment required is due to drag-out losses.
Nonflammable salt poses no fire hazard.
Salt is easily removed with water after quenching.
About the Author: Jerry Dwyer is Hubbard-Hall’s market manager for product groups pertaining to heat treating, phosphates and black oxide. To learn more or get in touch, please visit Hubbard-Hall’s website.
Time for a solid case study. In this Heat Treat TodayTechnical Tuesday feature, José P. Sanchez, part of Ceramics Business Unit in Nutec Bickley, describes the installation of several high-temperature and low-temperature ovens and kilns at one of the largest plants of a worldwide refractory product manufacturer.
Read this Original Content piece to get the details about how this project was executed and what measures were implemented for successful follow-up service.
About the Client
The client is recognized worldwide for the manufacture of refractory products used in the steel industry and on this project we worked with one of its largest plants.
This project comprised the design, manufacture and installation of 13 ovens and two kilns with temperatures ranging from 390°F to 3,000°F (200°C to 1,650°C) and retrofits to two existing kilns, as part of a major redesign at the client’s plant.
The Challenge
Nutec Bickley had to design, manufacture/modify and install 17 ovens / kilns in the space of 12 months, so we had to be especially careful organizing ourselves in the following aspects:
Ensuring we had the required space in our plant for the pre-assembly of all these units
Coordinating the execution of the project in which more than 80 people participated directly
Coordinating the parallel installation of multiple pieces of equipment in the customer’s plant, where at times there were more than 30 of our personnel on site at the same time
Our Solution
One of the decisive factors in winning this contract was the fact that other suppliers only had the capacity to manufacture either low temperature ovens or high temperature kilns, with Nutec Bickley being the only one with the capacity to manufacture both.
Additionally, we demonstrated our experience and ability to design ovens/kilns in temperature ranges from 210°F right up to 3,270°F (100°C to 1,800°C).
(photo source: Nutec Bickley)
We offered an ideal option, enabling the client to take on a single supplier to supply all of this equipment and to have it delivered it as a turnkey project.
Equipment Supplied
The turnkey solution comprised the following equipment and technologies:
A high temperature convection oven (1,110°F/600°C) with nitrogen injection
Equipped with stainless steel baffles to direct gases into the chamber and achieve the required temperature uniformity
Equipped with a thermal incinerator to reduce VOCs
Two Carbell Kilns
Operating temperature: 1,600°C (2,910°F)
Maximum operating temperature of 1,700°C (3,090°F)
Nine temperature control zones
High alumina bubble brick insulation
High velocity burners
Ceramic fiber lined exhaust vent
Incinerators in the exhaust for volatile gases
Two Retrofits of Existing Equipment
A) Modification of a Carbell kiln from another plant
We renewed the MCC and control panels.
We updated the PLC and display.
The combustion system was modified to comply with the NFPA-86 standard.
B) Transfer of a kiln from another plant
Insulation was supplied using our Jointless system.
Updated to meet NFPA-86 standard
12 Low-Temperature Convection Ovens (390°F to 660°F/200°C to 350°C)
(photo source: Nutec Bickley)
Two of them with forced cooling using extractors
Two of them with an incinerator to burn the volatile gases being vented to the atmosphere, greatly reducing pollution emissions
The other equipment was adapted to install more incinerators in the future.
Steel paneled ovens and mineral wool insulation
Design of baffles to direct gases into the chamber and achieve the required temperature uniformity
Project Benefits
Provided a comprehensive solution in the agreed timescale
Optimization of the use of floor space and improvements in operational logistics
SCADA system to monitor the uniformity of temperature in each cycle
Alarms in the event of sub-optimal temperature or where there is some discrepancy in the heating zones
Higher process quality and traceability in the case of rejects
Using some equipment from other plants reduced investment costs
Quickly service the 17 new, as well as the other, equipment
HSE supervisors in the field to monitor the operations of our mechanics and electricians
Knowledge to follow any type of safety standard in the design of combustion equipment. In this case it was NFPA 86-2019 (Class A oven and furnace design).
Opened our facilities at Nutec Bickley for visits to review progress of the oven manufacture
About the Author: José P. Sanchez is part of the Ceramics Business Unit in Nutec Bickley, in charge of sales in LATAM for kilns and major retrofits in the ceramic industry. He has been an active participant of multiple projects involving kilns and ovens in numerous industrial sectors, mostly refractories for the steel & aluminum industry.
A Pennsylvania producer and provider of steel wire and rod products, Liberty Wire in Johnstown, PA, underwent an intensive nitrogen-methanol controls upgrade for a continuous furnace line. The process included a new control system and panel for a continuous annealing furnace line to process coiled wire products.
SSI Controls Matrix (photo source: Super Systems Inc.)
Mike Cassidy, the Liberty Wire Controls Analyst, led the installation with Super System Inc. (SSI) project engineers, “I’m very pleased with SSI and the new system... I’m looking forward to working with them in the future.”
The SSI Matrix control system was incorporated to control the automated flow and mixing of the process gases. SSI HMI and eFlo 2.0 meters were also integrated to provide Liberty with the latest in hardware, software and communications technology.
Today's episode delves into the term "austempering". What is it? Why do heat treaters need to use it? For what applications is it necessary? Join Doug Glenn, publisher of 
But before we do, I’ve put on the screen a brief definition of austempering. It’s certainly a heat treat process. It’s used in medium to high carbon, both plain and alloy steels, as well as cast irons (an example being ductile iron) and we’re trying to produce a microstructure called bainite which is probably a foreign word to most of you and I’ll endeavor to explain it in a moment.
DH: Well, think of it this way, Doug: When we have a steel, its microstructure, if it isn’t hardened, its microstructure is typically body-centered cubic, which means the atoms are all lined up in a certain structure. Now, what we do when we heat it up is -- when it gets above the transformation temperature (that dotted line, for simplicity, in this example) the atoms will realign themselves from body-centered cubic to face-centered cubic and a face-centered cubic structure is called austenite. Then, when we quench it, until we move into the nose of the curve or past those red lines, we still maintain an austenitic crystal structure as we’re cooling. The ferrite, the pearlite and things occur when we cross over into those reddish lines in that area there.
