NITROCARBURIZING TECHNICAL CONTENT

Heat Treat Radio #48: The World of Ferritic Nitrocarburizing with Thomas Wingens

Heat Treat Radio host, Doug Glenn, talks with Thomas Wingenspresident of WINGENS LLC – International Industry Consultancy, about the growing popularity of ferritic nitrocarburizing (FNC) and whom this process would benefit most. Listen and learn all about FNC and how it might be a help to your production process.

To find the previous episodes in this series, go to www.heattreattoday.com/radio.

Below, you can listen to the podcast by clicking on the audio play button or read the edited transcript.

 

 



The following transcript has been edited for your reading enjoyment.

Doug Glenn (DG):  We want to welcome Mr. Thomas Wingens who is from WINGENS LLC – International Industry Consultancy.  Thomas is no stranger to Heat Treat Radio.  Thomas, you’ve been here before and, in fact, you’ve got one of the more popular  Heat Treat Radio (as far as downloads).  It’s one of the ones we did several years ago, actually, on megatrends in the heat treat industry.  But, anyhow, Thomas, welcome back to Heat Treat Radio.

Thomas Wingens (TW):  Thankful to be back, Doug.

DG:  If you don’t mind, Thomas, let’s start off very briefly and give the listeners a brief idea of your history and your current activities in the heat treat industry.

TW:  My name is Thomas Wingens.  I am an independent consultant to the heat treat industry for 10 years now.  I have been in the heat treat industry for over 30 years.  As a matter of fact, my parents actually had a heat treat shop and I was born and raised above the shop.  We had various heat treat processes in our shop.  Vacuum heat treating we started in the early ’70s, but also atmosphere heat treating and nitriding.

Nitriding – I am also familiar with this, now for over 30 years.  I work with different companies and manufacturers on the one hand, but also other commercial heat treat shops (like Bodycote and Ipsen).  I am a metallurgist by trade.  I studied material science.

Today, I live in Pittsburgh, Pennsylvania with my family (not far away from you, Doug), and we really enjoy it here.

DG:  It’s very obvious you’ve got heat treat in your blood.  You were born and raised in Germany, but you’ve been here in the States for quite a few years now.  You’re well acquainted, and I think this is important, with not only the European technology that we’re going to talk about today – which is ferritic nitrocarburizing – but you’re also familiar with the U.S. market.  It gives you a good “in” in both of those markets and so a good perspective to share with our listeners.

This episode is basically going to just cover FNC, ferritic nitrocarburizing.  We want to start at the basic level and work down through a few questions for anyone interested in what it is, how to do it, and that type of thing.  If you don’t mind, FNC 101.

What is ferritic nitrocarburizing?

TW:  It is aligned with carburizing and nitriding into fusion treatment.  It is thermal process diffusion, not a coating.  As it is ferritic, it means it is not austenitic.  So, we’re not heating parts as high as we would do with carburizing or carbonitriding, which is more the range of 950 Celsius; nitriding in general is operated in a temperature range of 500 Celsius range and ferritic nitrocarburizing is in the 560 – 590 Celsius range.  We are not austenitic, and that makes a huge difference, especially when it comes to distortion.  We are treating with FNC parts which are ready to build in.  It is the final step, very often.  That is a huge difference.  We can do this because we do not experience any distortion.

FNC Image
Source: Paulo

DG:  So, you’re doing it at a lower temperature range, we don’t have to worry about distortion and things of that sort, and it is, more or less, the final step.

TW:  It is.  Like nitriding, the nitriding is taking place in the 500 – 540 Celsius, and usually the nitriding takes longer; it is up to 90 hours very often, so deep case nitriding is very popular for some applications.  The rise and the popularity of FNC is that we can achieve results very fast.  First of all, we are at elevated temperature versus nitriding as we are operating at 580 – 590 degrees Celsius.

But there is also the carbon content.  The additional carbon, in conjunction with the nitrogen, also accelerates the diffusion.  We are achieving faster diffusion layers with FNC than with nitriding.  So, shorter cycle times means lower costs and faster turnaround.  Instead of having 24 or 90 hours cycle times, we often have 4-6 hours.

DG:  Let’s do the comparisons again of the processes.  You’ve got nitriding which is probably the lowest temperature process, but it’s a much longer cycle.  If we’re moving up in temperature, probably ferritic nitrocarburizing would be next.  It’s going to be a much shorter cycle because you’ve got the addition of carbon as well, which is helping diffusion into the metals.  Then you’ve got nitrocarburizing or carburizing, both at much higher temperatures.  In fact, when you get to carburizing, you need to worry about distortion, I would assume, correct?

TW:  Exactly.  That makes a big difference because it is not the final step after carburizing or carbonitriding which is taking place at 950 degrees Celsius, or, if you go into a vacuum furnace with LPC, you can go even higher (up to 1000 Celsius).  Nevertheless, you’re in the austenitic field.  When your part is cooler when being quenched, you transform from austenitic to martensitic, and then you get distortion associated with quenching and the ensuing transformation.  That means you need to grind the parts to have finished parts.  That’s not the case with nitriding or nitrocarburizing or FNC.

DG:  As an example, can you list off some parts that typically go under FNC?  What are people typically ferritic nitrocarburizing? What types of parts?

TW:  Due to the fact that we have a couple of micron layer only, (that means you don’t have huge parts, for the most part), you are doing .3mm up to 3 or 6mm for deep case for windmill gears.  With the size of the part, usually the surface treatment layer is growing as well, so it really depends on the wear.

Nitriding certainly can be applied on large parts and it is done on very large parts, meaning 7 meter long extrusion screws and such; but it is because of the wear.  The work technique you have on a very unique surface layer with nitriding and nitrocarburizing is formed from friction.  When you have chemical wear, when you have fatigue wear, you get a couple of things.  One of them is you have compressive stresses that are holding up to some degree of fatigue, and then you have, of course, a high surface hardness of 1200 vickers.  You have a very high surface hardness and then if you have galling or pitting where metal on metal is wearing.  The nitriding layer is very supportive here.  But also, the chemical resistance is a very big factor.

A big part of the success of FNC is the combination with post oxidation.  That is a big part because the combination of ferritic nitrocarburizing with post oxidation leads not only to a mechanical strong surface with compressive stresses, it also has a very high corrosion resistance.  That combination is a wonder combination for several automotive parts.  A lot of components have been hard chrome plated in the past.  So you have several ball pivots, ball joints, in the car.  When you have an older car with chrome plated ball pivots, you maybe have heard an itchy noise, when the car makes a noise when you go over a curb or when you go up and down.  That is very often due to the fact that these ball joint pivots are corroded and were chrome plated.  That is a huge application.  That became the standard in the automotive industry.  Every ball joint is now FNC and post oxidized.

The other application that you see a lot is if you have a pneumatic trunk lift piston.  The piston, you remember, has been hard chrome plated so that you have the chrome finish.  You will see in a newer car, in the front hood, you have a gas piston that is FNC treated and post oxidized.  Everything that is exposed to corrosion, which are so many parts on the automobile, even the light building of the body.  This is something to mention.

[blockquote author=”Thomas Wingens, WINGENS LLC – International Industry Consultancy” style=”1″]A big part of the success of FNC is the combination with post oxidation. That is a big part because the combination of ferritic nitrocarburizing with post oxidation leads not only to a mechanical strong surface with compressive stresses, it also has a very high corrosion resistance. [/blockquote]

All of these components I’m mentioning here are body parts predominantly and have nothing to do with electrification or with internal combustion drive trains.  They are not impacted by that, so we will not see any change here in the future.  A lot of under body components, where there is stone chipping and all the corrosion, people are tending to use FNC and black oxide because they can make it on thinner sheet metal part with compressive stresses so they have higher strength built in and they have the corrosion protection on top of it.  It’s a good combination.  And, of course, it’s virtually distortion free.  You may see that on some parts, due to very high compressive stresses, there is a buildup on the corners, but other than that, it is virtually distortion free and that’s a big, big plus of FNC.

DG:  That explains why it is growing in popularity.  I think that’s one thing you and I talked about earlier; there seems to be within the last, I don’t know, five years for sure, it seems like you’re hearing a lot more about FNC than you used to hear about.  Nitriding is still popular and carburizing is still popular, but you’re hearing a lot more about FNC, primarily because of the things you said.  Are there any other reasons, or is that primarily it?  Cost savings and good qualities.

TW:  If you look back, Doug, in the early days, in the beginning of the early nineties, I was running our nitriding department in our heat treat shop, and I had this little shaker bottle where it can determine the disassociation of ammonia and that determined the nitrogen potential.  The outcome was mediocre, to tell you the truth.  We did not clean the parts, we just put ammonia on it, and we had no way of controlling it other than the time and the temperature, so the outcome was a big variation. That’s why it was limited.  You could not find anything in the aerospace industry.  Nitriding was not accepted in aerospace at all.  Even in the automotive industry in the nineties, you did not find anything nitrided.  It was only used on tooling applications, and such.

But with the controls you have today, with the probes and sensors, you can determine everything, and you can see exactly what’s going on.  That has been a big factor.  There is the reproducibility of the layer you achieve and that is only possible with the good controls that you have and a better understanding of the process.

And, it is very important to mention, the cleaning of the surface.  There is no other heat treat process which benefits from good cleaning than nitriding and nitrocarburizing or FNC.  That makes a huge difference because you’re operating at a lower temperature and you don’t necessarily get rid of all the impurities and the ammonia gas, which, speaking of the process, really relies on the surface cracking of the material to dissociate in.  We have seen a huge impact if parts are not cleaned well on the different surface layers of FNC where we have missed the wide layer in total and such, so that is a big difference.

DG:  And the cleaning, I assume, besides just particles, I assume we’re talking about removal of grease and chemicals and things of that sort so that there can be good diffusion.

TW:  Exactly.  The surface has to be active.  The chips and the dirt to remove, that’s the easy part, but you have, sometimes, salts and residue from cutting and forming, especially the forming agents, sulfur phosphate, which are very hard to remove, especially for parts that are often FNC treated, like deep drawn parts or cheap metal components that are cut and there we see a big difference if they’re not cleaned well.

DG:  Run our listeners through a typical FNC process.  How does it happen?

TW:  I think it’s important to mention, as we haven’t done it yet, that we have three different processes.  We have salt bath nitriding or nitrocarburizing, gas, and plasma.  Each process has pros and cons.

FNC Image
Source: Bluewater Thermal Solutions website

The salt, there is a [cleaning] process or…QPQ, there are a lot of names out there for salt bath nitrocarburizing.  It is wonderful in that you just dunk it in, it’s quick.  The problem is the cyanide salts.  You have to carry it over, you have to clean it, you have to appropriately handle it, store it, and not everyone likes to do that.  Other than that, you have wonderful mechanical results with salt bath nitrocarburizing.

And then there is the plasma process.  The plasma is excellent for certain geometries, not so much for bulk.  You can place the parts in the furnace; it’s wonderfully clean and environmentally friendly.  Everything is good.  The problem is twofold: it is hard with bulk loads, it’s not as flexible on various parts and the other is with the post-oxidation, you cannot do it with plasma because it technically doesn’t work, so you need the… of gas nitriding in the plasma furnace to have the oxidizing part of the process, if you wish to go that route.

Having that said, the most widely accepted process is gas nitriding and gas nitrocarburizing.  Everyone knows that in pit furnaces this is one of the arrangements.  You put the parts in the furnace either vertically pit or modern now love horizontal arrangements, so if there is a loader you just have a batch.  Then you either purge with nitrogen gas or with a newer equipment that have a vacuum pump, so they have a vacuum purge system and instead of flushing with a lot of gas that draw a vacuum, they heat up the load and the convection to 580–590 degrees Celsius.  That can be done with so called “pre-oxidation process.”

Some people, especially if you have higher alloy chrome 4140 – “chrome alloyed” steels – they’re better to nitride if they are pre-oxidized on the way up.  Other than that, you would nitrocarburize ammonia gas, when you do gas nitriding, in conjunction with either endogas or CO2 gas.  Both, in combination, over a cycle time of 1 hour to 4 hours, soaking time and process time, and then you cool down with gas.  Not with the ammonia.  A lot of people make that mistake.  They heat up with ammonia or maybe even cool down with ammonia, but that is not correct.  Depending on, of course, what you’re trying to achieve, the best way is to flush it out because you have different disassociation processes going onto the surface and you have whatsoever surface combination nitrides if you don’t do it properly.

DG:  Are we gas cooling with nitrogen then?

TW:  You’re better off cooling with nitrogen.  Or you go interrupted cooling and then you oxidize on your way down, then you have this so called post oxidation.  You cool down to 300 – 350 C, and you have an FEO to layer which is dense, which is important.  You don’t want to have a flaky one or rough one, you want to have a dense oxidized layer as a surface and then you continue to cool.  That is basically the recipe of FNC.

DG:  I didn’t ask you before, and I should ask you: metals with which you can nitride or FNC, are they basically all steels?  Are there some steels you can’t do it with?  How about aluminum?  Titanium?  What can you do FNC with and what can’t you do it with?

TW:  I would say that nitriding is applied to a much broader spectrum of steels and even other alloys, let’s say.  People even do titanium and nickel alloys and try to put in nitrogen surface, calling it nitriding.  That is much broader.  FNC with nitrocarburizing is typically done with low carbon steels or carbon steels rather than high alloy steels.  That is why we have sheet metal parts very often.  So, low carbon or plain carbon steels.

DG:  And that’s maybe another reason, Thomas, why it’s become a more favorable process, right?  You can get some of the mechanical properties out with less expensive materials.  Is that safe to say?

TW:  Yes, that can be part of it.  But you should have a pre-hardened material, that’s important.  You need some carbon content in to have some hardness which sustains the high hardness of the surface.  It’s all prehard metals, for the most part.  Not necessary, but it certainly helps if you have some strength in the sub-strength which is supporting the hard layer.  It truly depends on your application.  But, you’re right: you can save on the materials to some degree and still get the mechanical properties that you’re looking for, especially in combination with the carbon.

DG:  Two final questions that maybe will help some of our listeners who are thinking about moving into the FNC direction.  The first question is, Who are the companies, and I know we can’t be exhaustive here, but who are some of the companies that actually manufacture this type of equipment that they could speak to?  And secondly, What are some of the things that companies ought to be asking themselves before they decide to go down the FNC rabbit trail, if you will?

So, first a list of companies if you have them.  We’ll try to be more exhaustive in our transcript of this.  If we miss any here, we’ll list them in the transcript.  But if you could rattle off a few that you’re comfortable with.

TW: There are the plasma people, that is RÜBIG GmbH & Co KG and Eltropuls and PlaTeG.  On the salt side, you have HEF Group, Degussa, and Kolene.  On the gas nitriding, you have Lindberg/MPH and Surface Combustion.  On the horizontal, very recently over the last 20 years, a very popular design is a horizontal vacuum perch retort nitriding and nitrocarburizing furnaces.  There you seen Ipsen, a German company called KGO, but also you have SECO/WARWICK with some proprietary designs (zero flow is also a good concept), and lately Gasbarre came into this business and Solar as well; they have the vacuum purge nitriding firms.

DG:  I want to back up a little.  On the salt bath companies you mentioned several, I also know Ajax Electric, also Upton Industries.  I don’t know if they do FNC units, but I’m assuming that they do.  There are a lot of other companies.

TW:  Salt bath is unique to salt.  There are only two, or three maybe, companies left in the world who supply these salts.  It’s more popular in Japan, by the way.  Anyway, it’s not as big as the gas process.

DG:  So, I’m a company thinking about maybe converting from some other surface hardening process over to FNC.  What kind of questions should I be asking myself?

TW:  It all starts, of course, with the product and the application.  Then you need to understand the wear and the corrosion methods.  That has to be well understood.   If that leads to FNC being the most suitable solution for this application, you need to understand the details of how you want to build up the surface layer, the thickness of your diffusion layer, the compound layer, the wide layer on the top and if you want to do post oxidation, so you will also need to do the oxide layer, which by the way, very often needs to be polished at the end, as well, to increase corrosion resistance.  These kinetics need to be well understood and the wear and what you want to achieve with this.

Then, of course, you have to see the design.  If you have sheet metal components which are cut, the cutting corner usually receives a higher layer and then the corners themselves that built up due to stresses, so there are a couple of minor things that need to have attention.  Then, of course, you need to have an expert who really refines the process, and that has to be done in conjunction with good controls.  There are two or three companies in the market.  UPC is one of them.  Oh, I forgot to mention Nitrex, a big brand.

DG:  UPC is part of Nitrex, but they also do the process.

TW:  Right.  Very important.  Somebody who really understands the nitriding and the control part of it.  UPC Marathon, they have very good controls.  SSI also has the probes.  There is STANGE in Germany as well.  You have two or three companies which have good knowledge in the controls and the probes and how to control this nitriding process.  Then you can build up your desired layer system.  In the layers, you have a diffusion, then you have a compound, a white layer, and then maybe you have an oxide layer on top and that needs to be well understood.  And, of course, as mentioned before, it is essential to have  parts cleaned thoroughly and if you maybe need a polishing afterwards.  Then, of course, how you put them in the furnace (placement) so that the gas can uniformly penetrate the parts.  These are the essential things.

DG:  There you have it, folks.  That’s FNC 101.  Those are the basics in ferritic nitrocarburizing from Thomas Wingens.  Thank you, Thomas.  I appreciate it very much.  I know that if people have questions, you, specifically, would be more than happy to help them out.  The company again is WINGENS LLC – International Industry Consultancy.  Thomas, www.wingens.com.

TW: That’s it!

 

 

 

 

Doug Glenn, Publisher, Heat Treat Today

Doug Glenn, Heat Treat Today publisher and Heat Treat Radio host.


To find other Heat Treat Radio episodes, go to www.heattreattoday.com/radio.

Heat Treat Radio #48: The World of Ferritic Nitrocarburizing with Thomas Wingens Read More »

Nitrocarburizing for Automotive and Large-Volume Production

Mark Hemsath

Conventional wisdom says that batch processing is for smaller volumes. Anytime large volumes of 1 million or more parts per year are envisioned, for instance with ferritic nitrocarburizing, the go-to technology is a roller hearth or other continuous systems like rotary retort or mesh belt furnace. In this article, which originally appeared in Heat Treat Today’s June 2019 Automotive print edition, Mark Hemsath urges end-users and engineers who use, or specify, continuous systems to not undervalue automated batch processing for large volume production.


There are a number of trends in the automotive arena:

  • More parts are being light-weighted. This means they need more precise and repeatable heat treating.
  • Parts need to be cheaper and lighter. The trend we see are increased and more sophisticated stampings.
  • The trend is away from carbonitriding and toward ferritic nitrocarburizing due to less distortion on lighter parts.
  • Gears and such are smaller and require exact carburizing, minimized quench distortions, and less hard machining.

A deep discussion of all of these is beyond this article, but we will touch on each as we focus on nitrocarburizing for large-volume production.

Batch v. Continuous

What is the difference between a classic “batch” furnace and a classic “continuous” furnace? The answer is material handling. By definition, heat treating is a “batch” operation. In virtually all instances, the product must be brought to temperature and held—or “soaked”—for a specific time. Ferritic nitrocarburizing is no different. This ramp heat, hold, and cool is a “batch”. Thus, virtually all heat treating is batch and only material handling is the difference. The basic difference is that in batch we move the product in its cold state and heat it in one place (batch). In continuous furnaces, we move it while it is heating.

Advances in Material Handling

Figure 1: Roller hearth conveyor furnace with heating section, cooling tunnel and after cooling. Note the right angle turn via automatic conveyors to meet space requirements.

Advanced, fully automated, and reliable material handling has made great advances over the last two decades from more recent industries like Amazon, where millions of packages need to be moved through the shipping process, to older industries like heat treating which moves steel parts through furnaces and other equipment. Automation, such as conveyors with self-driven rollers and photo sensors or proximity switches, or robots and automated self-guided vehicles—all coordinated by a PLC—have made material handling more reliable. Manufacturers have a lot of options.

A continuous furnace like a roller convey-or—or “roller hearth”—furnace conveys the product while it is heating (Figures 1, 5 & 8). A mesh belt furnace conveys parts while heating, and a rotary retort furnace (Figure 4) moves parts via a heated rotating barrel to the next process step which is typically cooling or quenching. Moving parts while hot is a challenge, but reliable high volume heat treating is why these furnaces have seen such success over the years. Roller furnaces and rotary retort furnaces are still built and used in a wide variety of industries, and they make sense for a number of reasons. Lower energy use is one main factor.

With robots placing the load, both batch and continuous processes can be fully automated. With such options, batch processing has increased in use.

Automated Batch

Figure 2: The doors have actuators for automatic opening.

A leading manufacturer of heat treating furnaces has implemented the high volume automation approach many times using batch technologies. In 2013, a fully automated batch FNC installation for gears was installed for processing 1 million gears annually.[1] As a result of this success, the customer added more batch furnaces to the line.

The furnaces in Figures 2 and 3 are retort-based nitriding and ferritic nitrocarburizing furnaces. With automatically opening doors, complete PLC control, and automated batch load movement, no humans are needed. A load car operates in both directions for a heavy load of two metric tons or more, allowing furnaces to be placed facing each other.

Automated, High-Volume System Design

Figure 3: This line consisted of pre-oxidizing ovens on one side to save time in the more expensive FNC furnaces. Cooling stations after heating are also added to reduce time in the batch furnace and make the parts safe for handling.

As mentioned, the company supplied nitrocarburizing technology using its ZeroFlow™ method (Figures 2 and 3) for an automated thermal treatment line for the production of a variety of gears. The line consisted of six large, front-loaded retort-style batch furnaces, a four-chamber vacuum washer, two ovens for pre-activation in air, additional post-cooling of the furnace charges, and an automatic robotic loader/unloader, which ensured charge transport within the system (seen in Figure 3). The automated line also included safety monitoring. System workload dimensions were 32″ wide x 32″ high x 60″ long with a gross workload capacity of 4,400 pounds. Production totaled 2,000 pounds of gears per hour. Good equipment design, retort technology, and use of ZeroFlow control technology resulted in a very successful project.

Cooling the Load and Vacuum Purging

Figure 4: Whirl-Away Quench on a Rotary Retort line for small part efficient quenching/cooling.

There are advantages to continuous furnaces like a conventional roller hearth furnace; however, special options like fast cooling and vacuum purging present challenges to these conventional furnace designs. In batch, this is usually not a problem. Vacuum (and even cooling) is more difficult to attempt in continuous variations due to sealing challenges in the chamber designs. An example of a good solution is the rotary retort furnace shown in Figure 4, which offers single piece quenching where each piece falls into a water or oil quench and is “whirled-away,” a continuous furnace design which works well for small parts with a relatively small footprint. In batch, the whole load needs to be quenched together; this can present challenges that understanding the part needs and configurations can lead the process engineer to different solutions.

In a roller furnace, slow cooling means the furnace gets longer (Figure 1).

Variations in Continuous Batch – Semi-Continuous Processing

Figure 5: Hardening roller conveyor furnace with integral pre-heat and oil quench system

In Figure 6, an automated batch hardening line is shown. In Figure 7, the same process is shown, but with an added pre-heat chamber to allow faster processing via the pre-heat and use the single quench in a more productive manner. An oil quench is an expensive piece of equipment. The cycles are also always much shorter for quenching than heating, so we want to maximize the use of the quench. In a pure batch system, you need one quench per furnace. In the semi-continuous approach, the quench is used more frequently and there is higher productivity per capital dollar invested. In a roller hearth or rotary retort installation, the quench can be properly sized to handle all of the heating production. In an installation using pure batch systems, there might be 3 to 6 quench tanks. In a fully continuous roller furnace, there would be one quench (see Figure 5).

Figure 6: This automated batch line is for low pressure carburizing and vacuum hardening, with oil quench, automated washer, and batch temper furnace. The smart loader makes the cell fully automated.

Case History and Take-Aways

The automated batch system referred to in Figures 2 and 3 went online in 2014 and is currently operating at full capacity, while meeting the stringent requirements of the automotive industry. It achieved the planned production goal of 1 million gears per year with 99% process reliability and 98% equipment availability. The customer previously had a continuous conventional pusher furnace. The new line achieved an 80% reduction in the consumption of ammonia from that consumed using in the pusher furnace to nitrocarburize. Endothermic gas was also eliminated by the supply of a new methanol CO generator as the carbon source in the process.[1]

Figure 7: Triple chamber vacuum hardening line with oil quench and pre-heat chamber. Tray flow is right to left.

The take-away from this successful project is that in order to increase production even more, automated batch systems need to exhibit two factors to compete with a continuous system like a roller hearth furnace. First, the loads need to be optimized and very densely packed. Second, the batch loads need to be larger than the continuous loads. A standard size of 40″ x 40″ x 60″ has since been created which has 50% more volume than the unit in the example above. Making the furnace a bit larger is not that difficult. Additionally, in a recent application, CFC tooling has been utilized to assure more dense loading geometry with much lighter parts, giving reliable rack geometry for a load of 1,000 pieces.

Gas Usage – Benefit Batch

Figure 8: Cooling tunnel and exit of continuous roller hearth furnace for instrument transformer electrical steels annealing.

The biggest advantage of batch furnaces is the lower process gas usage. In continuous furnaces, in order to keep the process safe and clean, pressure must be maintained by flowing a significant volume of gases. With the constant opening of doors during the process and the need to keep operating pressures high enough to prevent air infiltration, atmosphere gas usage is always high. To keep the costs down, gases are typically generated with the use of an endothermic generator (40% Nitrogen, 40% Hydrogen, and 20% CO) or a lean exothermic generator with a low dewpoint. In all instances, the generator is another piece of thermal equipment to maintain and purchase.

Energy Costs – Benefit Continuous

In most instances, batch processing uses more energy—or more expensive energy—such as electricity. Electricity costs can vary tremendously from location to location whereas natural gas prices are more consistent and lower. Batch nitriding furnaces are available in gas-fired heating options at an added capital cost. However, the batch process still uses more energy per pound. If electricity is available at a reasonable rate, then the difference is not as great on a per pound basis. In a recent analysis, it was estimated that an electrically heated batch system came to cost the equivalent of about $0.06 per pound of FNC operating costs, versus $0.03 per pound of FNC operating costs in a continuous gas-fired variation (energy and consumables only).

Summary

Batch or continuous in large volume scenarios is no longer a clear-cut answer. Your heat treating professional and your furnace suppliers should understand this. There are literally dozens of variables that need to be assessed, and only after a careful analysis tailored for each customer can an optimized solution be designed with either batch or continuous furnace solutions.

Notes

1. Hemsath et al, “Nitrocarburizing Gears using the ZeroFlow Method in Large-Volume Production”, Thermal Processing, 10/2015

About the Author: Mark Hemsath is Director of Nitriding and Special Vacuum Furnaces at SECO/VACUUM Technologies, LLC and acting Thermal General Manager at SECO/WARWICK Corp. in Meadville, Pennsylvania. With 30 years of experience in the industrial furnace and heat treat equipment market, he is in charge of all North American atmosphere furnace sales, gas nitriding, and gas carburizing. This article originally appeared in Heat Treat Today’s June 2019 Automotive print edition and is published here with the author’s permission.

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What Gun Aficionados Say About FNC Barrels

 

Source: Shooting Illustrated

 

Applying the slow quench-polish-quench (QPQ) to a rifle barrel

Steve Adelmann at Shooting Illustrated gets the conversation going among gun aficionados about the pros and cons of using a surface treatment on rifle barrels called “liquid salt bath ferritic nitrocarburizing non-cyanide bath”, more commonly known as FNC. Experts weigh in on nitrided barrels and agree that accuracy in the heat treating process known as slow quench-polish-quench (QPQ) produces long-term results that substantially outweigh other types of barrel treatments.

Read more: “The Pros and Cons of Nitride Barrel Finishes”

 

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