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Heating elements are used in numerous consumer and industrial products. These elements are used to heat air in various applications such as clothes dryers, room heaters and heating ducts for performance testing of power-generation equipment. The heating elements are made from wire and strip products. They are typically wound into coils to maximize the surface area of the heating element to rapidly heat flowing air.
|Fig. 1. Example of an open-element heating coil arranged in an air heater frame|
Any material used in this application must be able to resist sagging at temperature and have an oxidation rate low enough to give the desired life at the heating-element design temperature of the heating element. The material must resist sagging (creep) because contact with adjacent wires or other metal in the system can cause a short circuit. Oxidation rates are generally parabolic as oxidation rates decrease with time due to the thickening of the oxide layer. However, it is important to also prevent spalling of this protective oxide because local exposure of fresh metal accelerates the oxidation rate. Local rapid oxidation of the wire consumes metal, which reduces the wire cross section causing hot spots. This can further accelerate local oxidation and promote local sagging at the higher temperature. Figure 1 shows a typical arrangement for an open heating coil.
Comparison of Heating-Element Alloys
The various demands placed on the heating-coil material require a chemistry that is balanced for both high-temperature oxidation resistance as well as sag resistance (creep strength). In addition, the processing of the wire is important for these same properties, especially the sag resistance.
Chemistry of Heating-Element Materials
The chemistry of common heating-element materials is shown in Table 1. The nickel in the wire has traditionally been added at higher levels to improve the creep or sagging resistance of the material. Note that the generic term “60Ni” is used to describe the Cronifer II and Nikrothal 60 as this material is produced by multiple suppliers. The 60Ni wire has been the most common heating-element material. The chemistry and processing of Cronifer 40 B wire has been developed to have creep resistance and oxidation resistance comparable to a 60% Ni, as shown in this article.
The reduced nickel content is important because the nickel content in the wire is the single-most costly element added to these materials. Reductions in the nickel content result in a lower-cost material.
|Fig. 2. Resistance of selected alloys as a function of temperature. Note that the hot resistance of the leaner-nickel alloys is higher than the traditional 60Ni material (30Ni = Cronifer 40 B).|
Physical Properties of Heating Elements
In order to reduce costs, the Cronifer 40 B was developed with lower nickel content than the traditional 60Ni Cronifer II material without sacrificing performance. Table 2 compares the physical properties of the Cronifer 40 B to other materials used for heating elements.
While the cold resistance of the leaner-nickel alloys is lower than the 60Ni Cronifer II material, the resistance curves as a function of temperature of the leaner-nickel alloys have a different shape than the 60Ni Cronifer II alloy so that hot resistance is higher for Cronifer 40 B than the traditional 60Ni material at temperatures above 500°C (932°F). The resistance curves as a function of temperature are shown in Figure 2.[2,3]
|Fig. 3. The sagging resistance of the 38Ni (Cronifer 40 B) compared to 60Ni|
High-Temperature Sag Resistance (Creep) of Heating Elements
High-temperature creep causes heating elements to sag. When the element sags enough to touch another element or the containment enclosure, a short circuit develops and the element fails. For this reason, the heating-element industry performs extensive testing to ensure that heating elements will maintain their position and not excessively sag during the life of the heating device. As a screening test, a single element can be used to evaluate the general sag resistance of a material at 1000°C (1832°F). Details of this test method are given elsewhere. In addition, proof testing in actual configurations is also mandated for safety reasons.
Results of the laboratory tests are given in Figure 3. In the laboratory testing of single coils, the average of four tests showed that Cronifer 40 B sagged 4 mm while the 60Ni wire sags an average of 5.5 mm. In these laboratory tests, the sagging resistance of Cronifer 40 B surpasses that of the higher-nickel alloy. This shows the excellent creep or sagging resistance of Cronifer 40 B. This also shows that nickel content alone does not dictate the creep or sagging resistance of a material.
|Fig. 4. Comparison of the oxidation life of various materials evaluated in the laboratory. The error bars are the 95% confidence interval (38Ni = Cronifer 40 B).|
High-Temperature Oxidation of Heating Elements
An accelerated lifetime test is used to compare the oxidation resistance of heating-element materials to achieve a result in less time. For this, the temperature is increased and the heating cycle is longer. The wire is heated electrically to a temperature of 1150°C (2102°F), with the current being interrupted for 15 seconds every two minutes.
A comparison of the oxidation life of materials is shown in Figure 4. While the Cronifer III and Nikrothal NXT alloys are not as good as the 60Ni in resisting oxidation, Cronifer 40 B is the closest with 80% of the life of the 60Ni material. It reaches the lower part of the 95% confidence interval of the 60Ni alloy. All lean-nickel heating-element materials contain rare-earth additions that reduce the oxidation rates and prevent spalling of the protective oxide layer.[5-8] The Cronifer 40 B material has La added, and the others have Ce added. The addition of La is more effective at reducing oxidation rates in chromium oxide-forming materials and also better prevents spalling of the oxide than additions of Ce.
|Fig. 5. Comparison of 60Ni and 38Ni (Cronifer 40 B) materials in a proof test for an actual application. A. 60Ni (Cronifer II) Five years actual service; B. 60Ni (Cronifer II) 32,000-cycle proof test; C. 38Ni (Cronifer 40 B) 32,000-cycle proof test|
In order to assure performance and safe operation of devices with heating elements, it is required to perform proof testing. The heating-element configuration and design conditions that will be used in service are replicated in this evaluation process. During the proof test, the worst operating conditions (still air and maximum voltage) are used to assure safe operation in actual service.
Figure 5 compares the life of the 60Ni to that of Cronifer 40 B in a proof test. The normal sag after five years of service for a 60Ni wire is shown in the top photo for comparison, while the sag in the more severe proof tests is shown for 60Ni and Cronifer 40 B (38Ni) in the bottom two photos. Note that the sag of the 60Ni and 38Ni wires is similar for this design, and both materials have acceptable amounts of sag in this test. In this case the more expensive material is not required, and cost savings can be realized for the element design.
Cronifer 40 B is generally seen to perform similarly to the traditional 60Ni wire in most tests. In certain cases, however, the higher-nickel wire is still required. In actual design the material behavior is complicated due to radiation between coils. Therefore, the coil spacing, element spacing and other factors will dictate the final outcome as well as the material selection.
A new lean-nickel alloy has been developed with properties approaching that of the traditional 60Ni material. The following is a summary of the Cronifer 40 B (38Ni) material.
1. Cronifer 40 B has about 20% less nickel than the traditional 60Ni material and therefore offers the advantage of lower costs because nickel is more expensive than iron.
2. Further cost saving is possible due to the ability to use shorter-length heating elements when using Cronifer 40 B instead of 60Ni material due to the higher hot resistance.
3. In laboratory tests, Cronifer 40 B had creep strengths better than the traditional 60Ni material.
4. The oxidation resistance of Cronifer 40 B approaches that of the 60Ni material and has a life of 80% of the higher-nickel material.
5. The oxidation resistance of Cronifer 40 B is better than all other lean-nickel alloys by virtue of the higher chromium content and use of La to reduce oxidation rate and prevent spalling.
6. In full system-proof tests, Cronifer 40 B can meet the required life requirements. It is noted that system design can influence the life of heating-element materials, including Cronifer 40 B.
7. Cronifer 40 B can be considered to replace the more expensive 60Ni material, but proof testing is always required to assess the influence of design parameters on heating-element life. IH
For more information: Contact Larry Paul, ThyssenKrupp VDM USA, Inc., 122 E. Jefferson, Tipton, IN 46072, USA; tel: 765-675-9964; e-mail: Larry.Paul@ThyssenKrupp.com. Other author contact information: Dr. Heike Hattendorf, research & development, ThyssenKrupp VDM, Kleffstrasse 23, Altena, D-58762; tel: + 49-2392- 55-2945; e-mail: firstname.lastname@example.org and Craig Dykhuizen, executive VP, NOVA Industries, Inc., 5401 West Franklin Drive, Franklin, WI 53132; tel: 414-433-4335; e-mail: email@example.com