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Heat treating becomes a science.

For the past 400 or 500 years, heat treating has had somewhat of a bad name. The heat-treating department was a private little hell hidden in the back of the shop, and the people who inhabited that murky inferno were considered alchemists. You gave them the parts you had sculpted out of metal, and they performed some mysterious magic on them, and lo and behold, the parts were indeed better, but nobody really understood why.

Remarkably, just in the past few years, heat treating has made some bold leaps into the 20th Century, and it is now ready to pounce into the 21st. Heat treating has become a science. It is finally being accepted as an equal in the metalworking fraternity, and it is now as ready for the factory of the future as the rest of the metalworking processes.

Heat treating is no longer magic, the process and its metallurgy are understood. To meet today's demands, it has become flexible, it has become productive, it has become competitive. And solid-state controls and sensor technology have given it the most desirable property of all: repeatability. A batch today will have identical characteristics with a batch run last week or next year. Heat treating has arrived!

Induction means production

Jack Cachat has been president of Tocco Inc, Cleveland, OH, for about 150 years or so. He knows just about everyone in this business. He knows induction heating's past and present, and is in a great position to predict its future.

As he sees it, the key trend today is using more robotics to automate heat-treating systems and make them flexible enough to process different types of parts. Although induction heating systems are usually in high-volume, dedicated lines, handling at best no more than three or four closely related parts, they now offer the flexibility for mid-volume tasks.

"The challenge today," Cachat reports, "is to design in more workhandling ability and greater flexibility than in the recent past because now we're seeing more variation in parts and smaller batch sizes. Also, with the effects of the new just-in-time philosophy coming to the auto industry from overseas, induction heating is a natural. Because you can shut it off and start it up again so easily, you can run smaller batches, and the problem of different sized parts is simply a matter of changing a certain amount of tooling--an inductor, locator, or transfer device that merely requires an adjustment.

"With induction heating, you can get the type of temperature you want, when you want it, and vary it readily. You can get good quality control with very precise timing of the heat-energy cycle in kilowatt seconds.

"But there are sill gaps remaining," Cachat admits. "One thing we don't have quite yet is a method to nondestructively test for hardness and depth of case as the part comes off the inductor, to prove that the part is satisfactory. Right now, most of that is done visually. We would like to be able to determine grain size and structure without having to cut and polish the part. We would love real-time metallurgical feedback, but there's nothing like that on the horizon yet."

Sensing temperature

"We are seeing the beginning of the use of radiation pyrometry to sense part temperature and produce a temperature-control signal that modulates either induction-heating time or power levels. This is something that will come. We're using kilowatt-second units to meter cycle time and power level, although we can add a radiometric temperature measurement to show that good control is being maintained. This confirms the operator's suspicion that temperature is being held within limits."

One of the problems is not being able to actually sight on some parts as they are being heated, although others are easy to see. "We will soon be able to integrate temperature readings to adjust operating parameters and gain finer control. We anticipate an increased demand for this more sophisticated level of control.

"Our kilowatt-second control is a digital control that precisely regulates heat input. It's been out for about three years and is in broad use. With the advent of high-power solid-state controls, we can get very rapid response controls for power level, time, and frequency--down to the fraction of a millisecond. It's both sharp and quick. In fact, in some cases, our switching time is actually too short and we have to be careful to apply the power gradually and not shock the part.

"The heating pattern is often custom programmed to the part. The basic programmable controller is ideal for our process, and it can be Allen-Bradley, GE, Modicon, Westinghouse, or whatever you want."

Energy densities are going slightly higher in capabilities. "Where we once used between 20 kW/sq in and 30 kW/sq in, we now see some cases where we go to 40 kW/sq in or even 50 kW/sq in. We use these higher power densities to produce shallower, finer cases by bringing the surface temperature up faster. In one example, we got the very shallow case we wanted with a heating cycle of only 0.4 sec."

Heat treaters are

accepted as equals

These newer capabilities affect part design and the relationship between heat treater and designer. "We try to work with design engineers to get them to stop specifying any more hardening than they really need," Jack explains, "but they are usually difficult to persuade. You have to go through a great many tests to prove to them that what you're telling them is true. They're the ones who take the rap if a part fails for insufficient hardening, so they'd rather be safe than sorry. And you can't blame them for that.

"I think the heat treating profession has gained a lot of respectability today. People listen to us a little more. There are several very good books out on the basics of this science. Sure, at one time they thought we were magicians. We're no longer alchemists to them; we're accepted as equals."

The integration of the heat-treating process into the machining process has been particularly successful in valve-seat hardening on cylinder heads. "Our equipment is fully integrated into a transfer line. They machine the valve seats; transfer the heads through burnishing operations to our equipment, which hardens the seats; and then they move on to other operations."

Yet, despite this example of success, most spindles for the new front-wheel drive cars are hardened today in separate operations. "There is a large potential market here for automation to combine these operations," Jack observes, "for both the hollow spindle used on front wheels and the solid spindles used on rear wheels."

Induction competitors

The competition faced by induction heating today includes the people promoting lasers, but Cachat doesn't think they will prove to be economically superior, except for very small parts. "And even there, I would recommend electron-beam methods, even over our own induction-heating methods, if the part's the right size.

"The laser has shown superiority over induction heating for rather small-bore cylinders (1" dia, for example, 6" to 8" deep), where it can produce a spiral-scan hardening and for a few larger bores where induction heating could not be done. This is obviously a design compromise; they don't really want a spiral hardening pattern, but they must take what they can get.

"Another potential competitor, but not very broadly used, is a welding system using an inert gas or heliarc type of welding head. This yields a very shallow pattern and has some inherent problems with starting and controlling the arc. Also, we've been experimenting with plasma approaches, but we have been unable to get the heat source into a small enough area to do any good. So far, it is a big torch and big flame that is not very useful or economically competitive with existing induction methods.

"There is very little work going on combining atmosphere with induction heating. The newer areas are ion nitriding and ion carburizing, but I don't see much practical development work there. The aerospace people are trying to squeeze extra performance out of critical parts, but the rest of the metalworking world doesn't really need to worry about these processes."

Induction's future

What's the future for induction heating look like? "The heat-treating industry, generally, will probably see some growth, but in a lot of specific instances it will be static. I anticipate a certain mortality, and we're beginning to see some of that in process equipment now.

"For us, we are very carefully watching the automotive industry--that's our big customer of the future, as well as the farm-equipment industry. In the past, we could help them set up a valve line that would run for a decade, but now they are changing their engine designs more frequently, and the way they manufacture parts.

"Now they are even serious about predicting plastic engine blocks and heads. I can't visualize this myself. I can remember the Crosley brazed engine. One of our people had one back in 1947 and we worked with them for awhile trying to find a way to induction braze it. It was a steel engine furnace-brazed together with copper, essentially for the same purpose people are considering plastics today--to cut costs. It worked, but I don't think that engine lasted for too many tens of thousands of miles."

Higher frequencies ahead

"Because of the demands today for designing special handling equipment, most of our engineering people are mechanical. But in R&D, we're almost 100 percent electrical," Cachat reports.

"The R&D challenge today is to go to higher frequencies. We need to get into the radio-frequency range. The generators of today are tube type and not efficient enough for the power levels we need. We need high power levels at much higher frequencies. The communications people get along fine with tube technology because they don't need high power levels. We need much higher efficiencies than the 50 percent offered by vacuum tubes. We need 75 percent, and we can't do that with tubes.

Our equipment is in the 1-kHz, 3-kHz, and 10-kHz nominal frequency range, all using solid-state convertors. The first is a current-source and the other two are voltage-source units. We have to properly tune these to get the maximum transfer of energy, and they normally run around 90 percent efficiency, which is much better than the old power motor/generator set could do. We will soon have a prototype ready in the 50-kHz range."

Vacuum comes out

of its vacuum

The big news in vacuum-furnace technology is that it has come out of the lab and beyond the specialty applications to compete right on the shop floor with the rest of the heat-treating processes. According to Daniel Herring, corporate metallurgist, C I Hayes Inc, Cranston, RI, "Vacuum technology is a very exciting field to be in today."

Vacuum processes in the past were quite expensive. Their high initial capital expenditure relegated vacuum to specialty jobs--custom, one-of-a-kind, high-quality, high-precision work where cost was not the critical factor; the quality of the end product was.

"As more people became aware of vacuum technology," Herring relates, "the technology began to catch up to their needs. With the development of fiber materials, all-graphite-insulated heating chambers, pressure quenches, etc, our industry began to look for the first time at something called 'productivity'--getting significant production rates by making the vacuum more versatile and flexible, and by getting its initial cost to the point where it could be offset by high production volumes."

Continuous flow

"To make this happen, we needed continuous-flow vacuum processes. Our continuous-vacuum oil-quench furnace, loaded at one end and unloaded at the other, is typical of the push today toward productivity.

"We are looking at vacuum-carburizing furnaces capable of producing 200 lb/hr to 800 lb/hr. This covers the same territory as batch equipment rated at 400-lb to 2500-lb batches, when you translate batch ratings into lb/hr of cycle time. So our equipment can compete directly with what the batch equipment people are doing. Our own batch equipment runs as small as 20 lb/hr and up to 2500 lb/hr."

Vacuum has gone beyond the limited high-temperature alloy applications and use on only very special parts. With the new emphasis on productivity and cost, vacuum furnaces are becoming competitive with conventional atmosphere-type equipment. "The type of part to consider for vacuum heat treating has become nearly universal. We do a lot of tool steels and very-high-speed steels in vacuum, taking advantage of the 2000 F to 2300 F temperatures. We do a lot of vacuum sintering, especially stainless-steel powder-metal parts. But far-more-ordinary parts can be considered practical.

"As corporate metallurgist, I'm interested in the quality of the end product, its metallurgy, being able to control case depths to a thousandth of an inch uniformly throughout a workload regardless of load size. Hardening saw blades is a good example--getting the M2 edges to an Rc 65 range and taking the D6A backing into the mid Rc 50s, i.e., getting that blade up to the best possible metallurgical condition to produce the most cuts for the money, the right combination of application properties of hardness, toughness, strength, impact resistance, etc.

"I see a trend toward maintaining the fine metallurgy that vacuum has given us for years and producing big increases in productivity of the final heat-treated product. It has always offered superior metallurgy, and now we're combining it with heat-treating productivity."

Pressure quench

The addition of pressure quenching enables vacuum processes to replace a lot of salt-bath equipment. The obvious benefit is eliminating the need to work with cyanide or caustic salts.

Original vacuum-furnace quenching pressures were negative or still within the vacuum range, -10" Hg to -16" Hg. Now the part is heat processed and then transferred to a quench chamber where the quenching gas is highly pressurized.

"You can envision it as using 1000 gas molecules to quench a hot part in the past," Herring explains. "Those molecules absorb a certain amount of heat and exit through a heat exchanger and come back as cold gas. Today, we send 1,000,000 molecules. With positive-pressure (up to 100 psig) quenching, we have high-density gas that far more effectively transfers the heat. The process becomes infinitely faster, and you can simulate certain types of oil quenching and salt-bath quenching. Pressure quenching has been a very significant development."

Vacuum carburizing

A third major development, Herring feels, is a refinement on a well-established heat-treating technique--vacuum carburizing. It has been doing well because vacuum is faster than conventional atmosphere carburizing. You get a metallurgically clean surface--a very important issue--because any rust or oxide on the part prior to carburizing will be cleaned up. And because the part surface is so clean, you can carburize extremely fast. With certain steels, you can carburize at 1900 F, as opposed to conventional methods at 1700 F, which adds to processing speed and productivity.

"The big differences in our latest equipment," Herring explains, "is that the process is now self-cleaning and can be either a continuous-flow or batch-type operation. Vacuum carburizing has been held back by the need to remove the buildup of soot from both the workload and the furnace. This affects your ability to carburize, and is a high-maintenance operation. Now our new units self-clean after each carburizing run."

The in-house movement

Commercial heat treaters are seeing the need to specialize in vacuum or provide a custom service, recognizing that they can get a premium for the product they produce. But a bigger trend is the movement of the vacuum processes in-house. More people are doing their own vacuum processing than going outside for it, reports Herring.

"When the capital costs for vacuum equipment were much higher, the trend was to send out for that service to a specialty heat treater. Now, with greater productivity, the process can be justified in-house in most cases. It has become more of an in-house appliance--something you can turn on and off. It doesn't have to have a man in attendance at all times.

"Both continuous-vacuum and vacuum-carburizing processes now lend themselves to automation. We have even put robotic workhandling equipment on our machines, and taken some of the loading responsibility. This makes the heat treater now more of an operator simply loading and unloading baskets."

New skills

The use of microprocessors, programmable-controllers, and computer-interface technology on nearly everyone's vacuum equipment today has made the heat-treating process programmable for automatic operation. "One of my first jobs in the heat-treating business was loading and unloading furnaces, and being taught from the ground up how to operate this equipment," Herring recalls. "Back then, you were always in attendance at the machine while it was running. It was very labor intensive. Heat treating was much more of an art than a science. You would look in and make sure the workload was up to temperature, and with a little black magic you somehow made good parts.

"Today, the trend is to remove all that labor. Those skills, to a degree, are being lost, but they have been replaced by a definite science.

"There is no question that vacuum technology will be merged into the factory of the future. That's certainly one of the reasons for the development of continuous processing. It fits nicely with the idea of manufacturing flow through a plant.

"When manufacturing processes report directly to management--on-line, real time--they lend themselves to fully automating and integrating manufacturing and heat treating. When every manufacturing step is reported via a computer hierarchy to management, they can make active decisions based on real-time analysis. The data is real, not historical.

"Physically, vacuum can go right out on the manufacturing floor. You don't have any open flames; it's a controlled environment; and it can fit right in with the other machine tools. So does induction heating, which is certainly more like a machine tool than a heat-treating furnace, but this is not true for the atmospheric heat-treating processes. They are too dangerous."

What kinds of users need vacuum? "We see two types of people being converted to vacuum technology," Herring replies. "The conventional atmosphere heat-treating people, when their equipment reaches the end of its useful life, seem to be upgrading with vacuum processes. The other group is the people looking at in-house heat treating for the first time who are choosing vacuum technology for its simplicity."

Plasma needs time

Herring does not see much of an immediate future for plasma processes. "We see much more emphasis on vacuum-process productivity than on the new ion processes being used in the high-tech laboratories. We've scanned all the different ion processes: ion nitriding, ion implantation, plasma carburizing, etc. The trend there is more toward a laboratory operation--a 'onesy' process that will sit off in the corner, and people will say, 'Yeah, we've got one of those but we don't use it much.' It's a long way from breaking into the conventional-atmosphere realm of productivity.

"The ion processes will someday handle big batches of parts. Consider plasma carburizing. This is a vacuum furnace to which you add a power supply to generate the plasma. The gain in speed that the plasma process offers, we think, is directly offset by the efficiency you can get with our latest vacuum carburizing furnace. So we don't see it making inroads into production vacuum applications for at least another decade or so.

"Personally, I feel plasma processes are going to have to suffer through the same learning curve that we in vacuum carburizing have gone through. Our first commercial testing was in 1968, almost 20 years ago. So, if the plasma processes officially started in the early 1980s, they've got 20 years of learning to do to reach the point where they are as competitive as vacuum carburizing is today. You have to work at it to make a process like that practical and ready for head-to-head competition."

Vacuum leadership

So how does our technology compare these days? "I feel that in vacuum processes, the US is by far the technology leader. A lot of our foreign competition are phenomenal methods engineers--they can take a product and make it better. A camera, hand-held calculator, or a furnace. But in terms of continuous vacuum, pressure quenching, or vacuum carburizing, the US applications are far and away showing technology leadership.

"In machine tools, I've heard that the average age of a tool in Japan is eight years versus 22 here in the US, and I would suspect that the numbers are similar in heat treating. Just as the US is strong worldwide in computers because most of that technology was developed here, we're as strong worldwide in the furnace industry because we have the leading technology for many of these processes here. Yet, by and large, the Japanese and German industries have more in-place current equipment than we do."

Controls get fine tuning

One of the best reasons for the leadership of US technology is the development of fine-tuned controls--controls that yield exact repeatability, batch to batch, year to year. For an inside view of controls, we talked to Tiny Thorson, application engineering manager, Industrial Process Control Section, Industrial Instruments Div, Barber-Colman, Loves Park, IL.

"In all secondary thermal-processing industries, automation to get repeatability and accuracy for in-product quality is the driving force behind everything right now," Thorson explains. "The dollar value of the load is up, you're affecting the physical properties of an end product, and there are quite a few steps to assure an end result. Low reject rate, improved product quality, and decreased energy use without sacrificing process time are the key issues."

Real-time control

As Tiny Thorson sees it, "Controlling a thermal process is no longer just heating a material, holding at temperature, and then quenching it. We must become involved with other measurements of reactive gases and materials that are introduced.

"In the carburizing process, for example, in the past there were very crude approaches to measuring carbon potential in the furnace: manual sampling of the gas with a dew-point bottle, and then going back through cross-referenced tables to find your operating temperature to deduce a crude indication of carbon potential. Then came automatic dew-point methods, followed by infrared analysis of the cell relative to the CO content (directly related to gas-equilibrium content) to yield free-carbon levels.

"All of these were awkward. There were many such sampling systems and variations on the market. All depended on taking out a gas sample and then inferring what process correction was required.

"Several years ago, the insitu-type probe came along, a zirconium-oxide-based sensor that reads directly in the atmosphere and responds in real time to changes in gas-equilibrium constant to yield real-time control, after processing by a microprocessor through a very complex series of nonlinear equations over the temperature range.

"Now, you can read directly in real time the true free-carbon potential. If, for example, you are heat treating fuel-injector nozzles that require a very light case hardening, a too-heavy carburizing would ruin the load, and too light would not accomplish the desired results. With the carbon-potential probe and microprocessor control, this process can be fully automated. By measuring the temperature near the probe and furnace temperature, you can control the enriching gas and exposure time to yield a very precise depth for a specific material. You have eliminated the human element, the need for the operator to interpret measurements and sometimes make changes on a whim or guess based on his prior experiences.

"The result is case hardening controlled to the nearest thousandth of an inch. Now the sum-total experiences of the heat treater, metallurgist, and process engineer are used to determine the proper recipe. They run tests to confirm it, program the process, and run the process exactly that way each and every time. All motions and times for quench, carburizing, diffusion, etc are fully controlled. There's no guesswork.

"Now you can get reproducibility, load to load. A recipe from a batch that was run last year will be the same when run today, there's no need for a designer to add extra carbon depth to cover for process error. The process is more efficient because the time and fuel use is cut to the minimum. There's no wasted effort.

"Another driving factor today is the desire to use lesser-skilled operators or eliminate the need for extensive training. A good operator doesn't need the skills he used to require. He doesn't have to know the recipe or even enter the setpoint numbers. They are set up initially and retained in computer memory, cassette tape, or whatever. No human error can enter in. If it was right before, it will be right this time. The equipment will be set exactly the same. The system tells him when to load and unload."

Sensor technology

What's new in process metrology? "Temperature is still being measured by thermocouples; the only change is the elimination of the need for nonlinear calibration charts. This is now part of the preprocessing of analog measurements, usually part of the temperature transmitter to transform these measurements into true digital readout. With our experience with low-level analog signal measurement with noise rejection, we can bring those thermocouple measurements directly into our control system's preprocessor, linearizing and digitizing them at the same time before transferring them to the central processor. We can linearize to withion 0.2 degree on almost all thermocouples.

"As the physics of the process begin to be better understood, the desire for total control increases. You add more process variables under automatic control. In chemical vapor deposition, the desire is to lay down multilayers of micron-thick titanium nitrate or titanium chloride/aluminum oxide to increase tool life, for instance, five to ten times. There, you're operating under partial pressure and controlling mass flows of specific gases. This calls for introducing accurate measurements of time, temperature, partial pressure, etc, and a whole new recipe that becomes quite complex. We can do this to a certain degree today, but we use external sensors for mass flow."

Is the programmable controller adequate for thermal processes? "In most of the process-control jobs that we do, 95 percent of the logic written has to do with the interrelationships of measurements and times and only 5 percent with opening or closing valves or moving work. The programmable logic controller, on the other hand, is 99 percent concerned with controlling the movement of materials or devices such as machine tools.

"Thermal processes are not much concerned with motions, but rather with a high degree of analog measurements and manipulation of those measurements such as ramp changes or step changes in set points and maintaining accurate partial-pressure relationships for the proper process chemical and thermal reactions. Control loops must be added for speed, motion, position, etc.

"The typical programmable controller is not designed to fit the conditions seen by the process engineer and the heat-treat operator. That's why we developed our own special control system we call EDAC."

Ion trends

How do you feel about the newer ion and plasma processes? "Such esoteric processes as ion nitriding and plasma carburizing, CVD, and PVD are coming on strong, and they too will be coming in-house," Thorson feels. "People will want to do these processes themselves because there are physical properties they need to have to become competitive and they need to develop them and control them, not farm them out.

"Plasma carburizing in a vacuum furnace has been around for 30 years and not really taken hold. Now, the economics and accuracy requirements have changed greatly. That furnace sits there when there's no load in it, eating up too much energy just to maintain temperature above 1400 F because you cannot allow the endothermal gas to enter the furnace below 1400 F or you would risk an explosive atmosphere. Look at all that wasted energy! Even redesigned for higher efficiency, they still have significant energy consumption when they're not producing carburized parts.

"Now, plasma carburizing is coming on strong because it is a batch process where the heat evolves from the plasma itself, from the ion flow discharge. You get the same end result and you use only the exact energy needed. The plasma does both: part heating and carburizing. There's no waste!"

Case history

Thorson cites the following testimony of one of his satisfied customers to show what the latest control technology can do for the heat-treating process. According to Lou Binkus of Stanadyne, "I've been here in the heat-treating department for over 30 years, working with all types of equipment and processes. When we purchased our first new atmosphere furnace from Surface Div, we specified an SPC-1000 Barber-Colman EDAC control system. Just during the first year of operation, our EDAC has more than paid for itself in rework savings alone. We are now doing big-volume light-case work that we couldn't do before. The complete, accurate control provided by the EDAC allows us to hold case tolerances of 0.003" to 0.005". Once we have the parameters established, we can run the job over and over again exactly as before. We have increased our production rates and improved our quality.

"Before EDAC, furnaces required much more operator attention. Our operators can now do other things while the EDAC controls the furance automatically. The Insta-Set automatic setup feature is the greatest thing we've ever had in heating treating. It provides total process control. All manual control and chance of error are eliminated."

"Unfortunately, very few people at this time are at this level of automation," Thorson admits. "But everyone is looking at this from the standpoint of the economics they have to maintain to stay in business. All the way from the aerospace manufacturer, the people processing aluminum, to the heat-treating shops. Some of our customers in the bearing industry have 25 to 50 systems already--a mixture of new equipment and retrofits. This is an area of rapid change, and becoming very competitive, now that the ball is starting to roll.

"But to call this 'heat-treat sophistication' is to use scare tactics," he warns. "It's not adding process complexity, but reducing it. You can human interface now so well with displays on CRTs and LED matrixes that you can get much more information than you ever had before, yet in terms that the operator can easily understand. So, for the user, it is an actual decrease in sophistication."

Automation leadership

How well has the US adapted this technology? "The integration of heat treating into the totally automated factory--the factory of the future we hear so much about--is coming, but you won't see it here in the US as quickly as everybody would like to see it. The Japanese already have some fully automated plants.

"I don't think this integration will ever go so far as to bring heat treating out onto the machining floor. Induction heating, yes, lends itself for going right in-line. You will see totally automated, separate heat-treating facilities--parts in, treated parts out without human intervention. The Japanese have done this; the identity of the part produces the proper program, with automatic handling throughout, and the only operator involvement is an overseer in a control room. They can automatically track parts all the way through the process without operators, downloading recipe programming as the parts roll in and certifying each step in the process. This is clearly the next generation in heat treating."

Atmospheric progress

In this country, about 60 percent of all industrial gases are exothermic, used primarily for protective atmospheres in basic steel processes. Most of the remaining 40 percent are reactive endothermic gases used primarily in hardening and carburizing, plus some powder-metal sintering and brazing.

Manufacturers either generate their own endothermic gas from natural gas or buy nitrogen-based systems from industrial-gas suppliers. As Brian Sheehy, manager, heat-treat applications, Airco Industrial Gases Div, Murray Hill, NJ, explains, "Endothermic generators have been around since WWII, a very complicated piece of machinery, running at high temperature, with lots of moving parts such as pumps and valves with limited life. Since the mid-70s, the industrial-gas people, such as ourselves, have offered nitrogen-based substitutes for endothermic generators. Even the furnace people will admit that this was a revolutionary innovation--combining nitrogen and methanol to produce any gas composition you might want."

Most typically, the user stores liquid nitrogen outside his plant, vaporizes it to ambient temperature outside as it is needed, and pipes it into the furnace in a spray with liquid methanol. "Fortunately for us, methanol vaporizes and disassociates very well at low temperatures (300 C)," Sheehy explains. "You can use all the existing methods of endothermic-gas control such as oxygen probe, infrared CO.sub.2 analysis, and even good old dew point.

"All the gas companies will supply you with a flow-control panel, but we have gone further and are offering a carbon controller, a microprocessor that takes the millivoltage from an insitu furnace-oxygen probe and provides a digital readout in percent carbon. The O2 measurement provides almost instantaneous feedback."

The case for nitrogen

Today, about 30 percent of all endothermic and exothermic gas users have converted to nitrogen-based systems. The remainder still use endothermic generators or disassociated ammonia. "It's not really economical to run a heat-treating operation on bottled nitrogen," Sheehy admits. "Or convenient. Sooner or later you'll run out of gas at a critical time. The breakoff point between liquid nitrogen and bottled gas use is about 100,000 cu ft/month usage rates or something like 80,000 lb of parts/month. Thus, even the small captive operation should be considering liquid-nitrogen systems.

"And cost of the gas is not the only consideration. Safety, flexibility, and reliability of the natural-gas source are important too. When you consider that the cost of the atmosphere consumed is at most 10 percent of part cost, you might well feel that it's worth 11 or 12 percent to get better quality, fewer rejects, and less maintenance costs. And in initial capital costs, a large generator system is probably more expensive than a nitrogen-methanol system.

"Nitrogen-atmosphere use really took off after the 1976-77 energy crisis and severe weather caused natural-gas shut-offs. Utilities are now trying to woo these industrial users back, promising to never do that again, but who knows what will really happen?"

Obviously, the term "generator" is anathema to the nitrogen-sellers such as Airco. But they too have a crisis; the rising cost of electricity is hurting them because their nitrogen-producing cryogenic processes use lots of electricity. "All the industrial-gas producers are looking very seriously at pressure-swing adsorption (PSA) and vacuum-swing adsorption (VSA) techniques. These separate nitrogen from air without going the cryogenic route, much like a huge molecular sieve. For our medium-sized regional centers (2-million to 10-million cu ft/month), this will enable us to lower our cost of generating nitrogen."

Still cooking with gas?

Will the atmospheric processes be yielding to other heat-treating methods soon? Sheely doesn't think so. "With induction heating, for example, you get different properties than with carburizing where you can actually change the chemistry of the metal. Induction just heats the surface of the metal, and with a quench you get a hardened surface with no change in core-metal properties. It is usually more brittle, less ductile than carburized steel, which clearly has superior fatigue, wear, and strength properties.

"In vacuum hardening, the trouble has been the massive capital outlay it requires. I don't think they've made a vacuum furnace yet that can handle a 2000-lb load. They can heat up and cool down rapidly, and use higher

temperatures. But if you just raise temperature from 1700 F to 1900 F willy nilly to speed up the process--without checking first with your metallurgist--you could ruin the grain structure. Grain coarsening is a common problem. Quenching afterwards can crack the steel, cause poor ductility, poor strength, etc. You must also consider basket life at these higher temperatures.

"The vacuum people have also admitted that they need fans to stir up the atmosphere, and they must operate with some pressure. How else can you get molecular distribution in a vacuum? And when you are essentially spraying parts with natural gas, how do you handle high-density loadings? You've got to spread parts out neatly so that gas can get to them. So I see the vacuum people with a lot of problems to address.

"As for the plasma people, I cannot speak authoritatively, but I don't see them competing in the general marketplace. They're like the microwave oven, good at certain tasks, but not replacing every thermal appliance in your kitchen."

Future trends

Sheehy sees the inevitable rise in the cost of steel affecting all of heat treating. "People will be looking at the cost of waste from scale, pickling, shot blasting, etc, and they will also be evaluating heat-treat processes, asking themselves if they are really worth it. Forgers, for example, can sweep up big piles of scale from around a forging press, so they are looking at new processing methods. Rockwell International is looking at feeding nitrogen atmospheres into induction-heating tubes for forging billets to prevent oxidation. If they could get away from those big material allowances for the decarbing process, they could forge to closer tolerances and save material.

"An Ipsen spokesman has said that by the year 2000, he didn't think there would be designed to use as-forged, without carburizing or other treatment. He's banking on the use of more exotic steels and assumes everyone will be willing to pay extra for material. I doubt this will happen."

Generators wear out. With the tax benefits from new purchases, when the economy is healthy and people can spare the capital, they will replace them. When its not, they hold back. We feel the nitrogen system costs are now very competitive with endothermic generators, so when they are ready to make a change, they will definitely want to talk to us."
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

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Author:Sprow, Eugene E.
Publication:Tooling & Production
Date:Nov 1, 1984
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