Corrosion, Oxidation Resistance of High-Temperature Heating Alloys
Since the development of commercial electric heating elements at the beginning of the 20th century, oxidation and corrosion resistance have been primary goals in the creation of new high-temperature resistance and construction alloys and ceramic materials.
The first accepted production-quality resistance-alloy family was the nickel-chrome (NiCr) and nickel-chrome-iron (NiCrFe) alloys, patented as Nichrome in 1905. The NiCr alloys – especially the 80% nickel, 20% chrome variant – became the standard for industrial electric heating elements. NiCr was an excellent choice and remains so to this day due to its high-temperature oxidation resistance and ability to operate in most common heating atmospheres, including oxidizing, nitrogen and hydrogen.
The 1930s introduced a new family of alloys to the available choices for heating elements. Iron-chrome-aluminum (FeCrAl) under the tradename of Kanthal was developed. Due to the nature of FeCrAl’s unique alumina coating, which forms during operation, a corrosion-resistant alloy with even higher operating temperatures was now available. A resistance alloy protected by a thin coating of alumina allowed for atmospheres that were considered to be too caustic for conventional alloys to be exposed to directly. This alumina surface coating resists reactions with many reactive materials, including sulfur and carbon, which can pose issues for nickel-based alloys.
Both of these families of alloys were further expanded on, with additional variants to increase their useful ranges. The AA, 70/30 and C grades allow for their use in common carburizing environments that might otherwise result in green rot of NiCr alloys. The AF and APM grades increased the oxide stability and strength of the base material to the FeCrAl family to allow for high-temperature, corrosion-resistant electric heating elements and structural materials.
The expansions of both families of alloys allow for the creation of alloy protection tubes. Along with the alloy tubes, several different ceramics – including Sialon and silicon-carbide-based tubes – lend their corrosion-resistant properties to the systems that are contained within. Silicon-carbide elements that would otherwise be greatly limited within a hydrogen environment are capable of providing their high power output from within alloy tubes. Bayonet-style elements of both ceramic and metallic construction can be immersed into aluminum holding furnaces with the use of Sialon tubes. Atmospheres with airborne contaminates, such as those found in incineration furnaces and those with recirculated atmospheres, can be separated from their heat source by one of the assorted alloy protection tubes.
Heating-Element Design Applications
The availability of such a wide range of options both complicates the design of electric heating systems and greatly expands the applications where energy-efficient electric systems can improve upon existing heating systems. When engineers design the heating system within a furnace, the configuration of the heating elements and the power required are often dictated by the chamber size and application. The operating temperature, chamber atmosphere and potential corrosive materials determine the alloy choice, and it is this decision where engineers must weigh the benefits of the alloys and ceramic materials available. These small issues make up the bulk of the decision-making process when starting a new application.
New developments in heating-element design and use of the materials available can often help to improve existing systems either by extending service life to decrease maintenance costs or increasing throughput. Examples of this are fairly commonplace to the heating-element design engineers at Custom Electric. Many materials have excellent oxidation and corrosion resistance, but it is the job of heating-element design engineers to also take into account strength.
Carburizing furnaces are fairly common among automotive parts suppliers. One such customer was using graphite heating elements within ceramic tubes to protect the elements from the carburizing environment in their two continuous pusher and two batch Ipsen furnaces. Both the elements and tubes suffered reduced life due to thermal shock and breakage. Obviously, metallic materials would solve the issue with thermal shock, but addressing the carburizing atmosphere is equally as important.
To this, APM extruded tubes were used for protection because they would operate without issue in this environment. The graphite elements within were replaced with APM bayonet elements to provide the same power levels, eliminate the breakage issue and remove the need for the water cooling and nitrogen-gas shield inside the tube.
One of the secondary benefits of this change was the ability to change elements when hot, which saves a significant amount of downtime during element replacements. The service life increased from an average 12 months for graphite elements to 36 months for the new APM bayonets. The upgrade allowed the customer to reduce its annual replacement element purchases from 120 to fewer than 40 pieces per year.
The average service life of the ceramic tubes was 18 months as compared to the APM tubes, which have a service life of over five years. Financially, the cost savings were significant. Lost production due to downtime was reduced by $200,000 per year. Overall maintenance costs have been reduced by $68,000 per year. In addition to these savings, overall productivity of the furnaces has increased by over 30%.
Furnace Atmosphere Challenges
Many companies operate furnaces with difficult atmospheres without even realizing it. Unless someone monitors their heating-element usage, they can be unaware of how long their elements are lasting until the problem reaches a climax. A metal stamping company experienced this with their five furnaces utilizing coiled NiCr heating elements. Each of these furnaces was operating under ambient-air conditions and well within the capabilities of the 80/20 NiCr elements being used.
Once the average service life was determined to be as little as six weeks, a call was made to determine if anything could be done. After a quick inspection of the heating elements and several questions about the furnace, it was determined that the parts were covered in oil before stamping and the atmosphere of the furnace had a variable carbon potential. It was sufficient enough to cause the elements to green rot, resulting in the embrittlement of the coils. Due to vibrations in the furnace, the elements would then randomly break and fail. Once this was identified as the cause, it was an easy fix to convert the heating alloy to a different NiCr (70/30) alloy, which is far more resistant to green rotting. Choosing the correct alloy extended the service life of the coils from as little as six weeks up to two years.
Corrugated Stainless Elements
Corrugated stainless steel elements are a style of element that was somewhat popular for some time but is not used very often anymore. Stainless steel 330 has decent high-temperature oxidation resistance, but it is not ideal for use as a heating element.
A customer with eight such austempering furnaces sought assistance in extending the service life of the heating system. The existing stainless steel elements had an average service life of 30 weeks because they were not well suited to the temperature and atmosphere of the application. In addition to their short service life, the maintenance time of replacing the failed elements was up to six weeks. Over the eight furnaces, this equates to just less than 14,000 hours of lost production per year.
To remedy the situation, bayonet heating elements were designed to be operated within APM tubes. The APM material offers superior oxidation resistance, extending the service life from seven to 48 months, and the replacement time was reduced from six weeks to one day. The end result was increasing the productivity of each furnace by 50% and saving the company $2 million in new equipment purchases.
Today, more facilities are monitoring energy consumption, in particular those related to thermal processing. Losses can occur from poor furnace or heating-element design, resulting in lost power and poor material choices, which increase downtime, maintenance and material costs. Choosing the most appropriate alloy for electric heating elements and/or protection tubes can make the difference between a furnace that merely operates well enough to keep a process operational and one where maximum profit can be obtained with a minimum investment. It is often possible with existing systems, especially those that have been in operation for years, to have a furnace or heating-element specialist offer options to greatly improve their performance to provide maximum production for your investment.
For more information: Contact Bob Fouquette, chief engineer, Custom Electric Manufacturing Co., 48941 West Rd., Wixom, MI 48393; tel: 248-305-7700; e-mail: firstname.lastname@example.org; web: www.custom-electric.com. Kanthal is a brand of Sandvik AB. Products include Kanthal®, Kanthal APM™, Kanthal APMT™ and Nikrothal®.