Electric Furnaces Complement Heat Treat Requirements for Gears

Gear manufacturers find that furnaces heated by electric elements routinely harden, anneal and stress relieve gears, while reducing thermal processing costs.

0
1612

Furnaces heated by electric elements routinely harden, anneal and stress relieve gears. Today's heating elements operate at higher temperatures, last longer and offer gear manufacturers opportunities to reduce thermal processing costs. Energy and environmental concerns should further expand the use of electric furnaces.

Custom Electric Manufacturing Co., of Wixom, MI, designs original equipment and replacement heating elements for electric thermal processing equipment. For the past 40 years, it has worked with electric furnace builders and companies using electric furnaces to heat-treat gears. Gear Solutions asked Custom Electric to comment on the future rolE of electric furnaces in gear manufacturing.

Figure 1

Gas-Fired Furnaces vs. Electric Furnaces
It was not until the 1950s that the United States had a pipeline network and power grid sufficient to allow natural gas and electricity to be the primary energy sources for energy-intensive industrial applications like heat-treating. Today, heat-treating is a $20 to $25 billion industry serving more than 18,000 manufacturers. Gas-fired heat treat furnaces outnumber electric furnaces in the U.S. because natural gas is plentiful and, relatively speaking, affordable. In many parts of the world, like the Scandinavian countries, natural gas is scarce and electric furnaces are more prevalent.

“With respect to performance, both electric and gas-fired furnaces do an excellent job case hardening, through hardening, annealing, normalizing and stress relieving gears,” says Bob Edwards, President of Custom Electric. “However, there are characteristics of electric furnaces which are becoming better appreciated. For example, a uniform heat pattern is achievable with proper element placement. Energy efficiency exceeds 90 percent; compared to from 60 to 80 percent with gas furnaces. There is no need for expensive ventilation systems and gas recuperators. Electric furnaces also are quiet, safe and pollution free.”
Today’s electric furnace is a highly engineered piece of equipment. It is tightly constructed, better insulated and energy efficient. Electronics monitor and control every stage of processing.”

Figure 2

Not surprisingly, Edwards believes heating elements are the most important components of an electric furnace. “Heating elements may only account for about 10 percent of the costs to operate a large, modern heat treat furnace, but they dictate or influence the entire heat treat process, everything from furnace operating temperature and heat cycle times to maintenance costs and environmental compliance.”

Heating Elements
Heating elements are made from electrical resistance metals (molybdenum, platinum, tantalum and tungsten), electrical resistance alloys (Ni/Cr, Ni/Cr/Fe and Fe/Cr/Al) and nonmetallic materials (graphite, silicon carbide and molybdenum disilicide). There are many variations of these materials under various trade names. “The list is shorter for heat treating high performance parts on a production basis,” according to Vic Strauss, a Custom Electric Vice President with 25 years of heating element design experience. “In fact, one material, Ni/Cr, has been the dominant heating element alloy for more than half a century.”
The majority of heating elements manufactured by Custom Electric are made from 70/30 Ni/Cr and 80/20 Ni/Cr. These alloys perform well over a wide temperature range, tolerate different atmospheres and have a long service life in most applications. In recent years, Strauss says Fe/Cr/Al is also becoming a popular element alloy, particularly in higher temperature applications.

Figure 3

Better Heating Elements
Fe/Cr/Al elements have a long service life at temperatures up to 2300º to 2400ºF. This is attributable to the advent of the Sandvik/Kanthal APM® alloy, a powder metal alternative to standard Fe/Cr/Al. In many applications, Fe/Cr/Al elements have a service life from two to 4 times that of conventional alloys, often longer.

“Manufacturing heating elements from APM is a growing segment of our business,” says Strauss. We make bayonet elements, immersion heaters, plug/rack elements, rod overbend elements and helical coil elements from APM stock. Some customers switched from Ni/Cr elements to APM elements when nickel prices soar, but most are attracted by the material's long service life at higher temperatures.” Strauss described the experience of an automotive transmission plant to illustrate the benefits of APM elements.

APM Element Case History
The transmission plant project began with replacing corrugated 330 stainless steel heating elements in one of eight AFC-Holcroft austempering furnaces with APM bayonet elements and radiant tubes. The furnaces worked side-by-side during a two-month evaluation processing gears on the same schedule. Test results were so impressive the plant expanded the conversion program to include all eight austempering furnaces and 11 Lindberg gear-carburizing furnaces heated by the same style elements.

Figure 4

According to plant records, the average service life of stainless steel heating elements in the 19 furnaces was 30 weeks. The average service life of the replacement APM elements is four years. This dramatically reduced the frequency and time required to replace elements and contributed to a significant increase in furnace productivity.

Over a period of four years, the 300 lb. x 8-ft. long x 10-in. wide stainless steel elements in eight austempering furnaces needed replacement six times, each replacement resulting in six weeks of downtime. Based on a four-year service life, APM elements need to be replaced once, a task, which is completed in one day. Over four years, this reduced element replacement downtime from 1728 days to one day.

Stainless steel heating elements in the carburizing furnaces needed replacement five times every four years.  However, more time was required to do the job, 3960 days over four years. By reducing element replacement downtime in 19 furnaces, the plant gains more than 200 hours a year of additional gear heat-treat capacity.

Figure 5

Other benefits of the new bayonet element/radiant tube package included higher carburizing temperatures (1922º F compared to 1760º F), and a 50% reduction in heat cycle times.  When Custom Electric totaled the benefits of upgrading heating elements in 19 furnaces, the plant gained gear heat treat capacity equivalent to 22 additional furnaces and avoided spending $10 million on new equipment.

“The transmission plant project illustrates the benefits of updating electric heating elements in older equipment,” says Strauss. “Based on our experience, we estimate element upgrading is justified in about 20% of heat treat furnaces between 10 and 25 years old. After 25 years of service, element upgrading probably is justified 100 percent of the time.”
The availability of longer life heating elements is important to original equipment manufacturers because the frequent necessity to replace heating elements is a common criticism of electric furnaces.

“New alloys make element life and element replacement requirements less of an issue,” says Strauss. “If you compare the costs to replace combustion control hardware like burner tubes and nozzles, it is probably no more expensive to maintain an electric furnace then it is to maintain a gas-fired furnace.”

The Future of Electric Heating
“In speculating on future gear heat treat practices, I would never suggest electrically-heated furnaces will replace gas-fired furnaces,” says Edwards. “However, based on the many scenarios being discussed, I do believe the use of electric furnaces will increase.”

Environmental Compliance
There is not a heat treat facility in America not struggling to keep pace with rapidly changing Federal and State environmental regulations. Current rules are tough. Pending rules are tougher, particularly those pertaining to the discharge of carbon dioxide.  Depending on which proposals become law emission limits may be reduced 80 to 100, percent by 2050. This scenario favors electrically heated furnaces, says Custom Electric.

Energy Efficiency
Heat treat facilities consume huge amounts of electricity and natural gas. In today’s “Green” world, this raises a red flag. Some industry analysts suggest the heat treat industry should reduce energy consumption by as much as 80 percent by 2020. This scenario favors electric furnaces because they already are 90 percent energy efficient, says Custom Electric.

Renewable Energy
To protect the environment and lower oil imports, Washington politicians could mandate that 80 percent of America’s energy come from renewable sources by 2050. The plan is to replace or supplement electricity produced from fossil fuels with electricity generated by wind turbines, solar panels, hydroelectric turbines, biomass and heat trapped beneath the surface of the Earth. This scenario is ambitious and will be enormously expensive, but favors electric furnaces, says Custom Electric.
 

A Brief History of Electric Heating
Three men launched what today is the electric thermal processing industry, German physicist Georg Ohm, British physicist James Joule and American metallurgist Albert Marsh. In 1827, Ohm demonstrated current and voltage in an electric circuit is controllable by varying the size, shape and material of a resistor, i.e. a heating element.
In 1841, Joule discovered electricity converts to heat when passed through a resistor. In 1905, Marsh patented the first practical heating element material, an 80/20 Ni/Cr alloy. Ohm’s Law and Joule’s Law continue to dictate the design of electric heating elements. Ni/Cr is an excellent and widely used heating element alloy.
  High Performance Heating Element Alloys
70/30 Ni/Cr Widely used in air, vacuum and controlled atmosphere furnaces. A maximum operating temperature in air of 2250º F provides broad application flexibility. Heating elements made from 70/30 Ni/Cr have excellent oxidation and carburization resistance. Exposure to sulfur, chlorine and salt should be avoided. Other characteristics include good formability, good mechanical properties in the hot state and long service life.
80/20 Ni/Cr This Ni/Cr alloy provides good performance in clean, dry air at operating temperatures up to 2150º F. It also is suitable for use in inert and reducing atmospheres and vacuum furnaces. Exposure to sulfur, chlorine and salt should be avoided. Good oxidation and carburization resistance contributes to long service life.
Fe/Cr/Al Fe/Cr/Al elements provide greater flexibility at temperatures up to 2350º F. This is due to higher electrical resistivity and lower density than Ni/Cr alloys. A major attribute of Fe/Cr/Al is resistance to sulfur contamination. This material is not recommended for use in reducing atmospheres other than hydrogen. APM is a lightweight Sandvik/Kanthal Fe/Cr/Al powder metal alloy with superior hot strength and form stability at high temperatures. APM elements perform well in most furnace atmospheres and have excellent oxidation resistance. In many applications, element service life is from two to four times longer than other heating element alloys.

Supply and Demand
All fossil fuels used to generate electricity are under attack. Coal is dirty. Oil is imported. Oil shale is relatively new. Nuclear power plant construction is at a standstill because of safety concerns. Decades will pass before alternative energy sources dramatically change electricity-generating methods. Meanwhile, the demand for electricity continues to grow. All supply/demand scenarios indicate the cost of energy, gas, and electricity will increase.

Preparing for Change
“Manufacturers of heat treat furnaces and electric heating elements are acutely aware of the challenges facing energy-intense manufacturing processes,” says Edwards. “Our industry is ramping up efforts to provide solution. Energy consumption and energy costs, furnace design and construction and methods to reduce process cycle times and production costs top the agendas of organizations like the Industrial Heating Equipment Association, Metal Treating Institute, International Federation for Heat Treatment and Surface Engineering and the ASM Heat Treating Society.

Figure 6

Conclusions
“Short term, heat treat facilities with a history of using gas-fired furnaces will continue to use gas-fired furnaces and facilities with a history of using electric furnaces will continue to purchase electric furnaces,” predicts Edwards. “Long term, I expect increased use of electric furnaces because all nations are converting to energy produced from renewable resources. Also, the $20 to $25 billion gear industry is an important market to electric furnace and electric heating element manufacturers,” Edwards said. “I am confident furnaces and elements manufacturers will develop the technology necessary to support future gear manufacturing requirements.”