Improved Nickel Chromium Wire Heating

Improvement of Resistance heating wire and resistance heating strip in iron-chromium-aluminium (FeCrAl) alloys and nickel-chromium (NiCr) alloys for the manufacturing of electric heating elements. Using FeCrAl alloys instead of NiCr alloys result in both weight-saving and longer element life.

Resistance Heating Wire and Strip

FeCrAl Resistance Heating Alloys

Resistance heating alloys based on iron-chromium-aluminium for maximum element temperature of 1425°C (2600 °F). Fecral resistance heating alloys are characterized by high resistivity and capabiltity to withstand high surface load.

NiCr resistance heating alloys

Nickel-chromium-based resistance heating alloys suitable for element temperatures up to 1250°C (2282°F). Nickel Chromium resistance heating alloys are characterized by very good mechanical properties in the hot state as well as good oxdiation and corrosion properties.

Product forms and size ranges

Resistance heating alloys are available in the following product forms and sizes:

• Round wire: 0.10-12 mm (0.00394-0.472 inch)
• Ribbon (flat wire): thickness: 0.023-0.8 mm (0.0009-0.031 inch)
  Width: 0.038-4 mm (0.0015-0.157 inch)
  Width/thickess ratio max 40, depending on alloy and tolerance
• Strip: thickness 0.10-5 mm (0.00394-0.1968 inch), width 5-200 mm (0.1968-7.874 inch)

Improvement of Heating Properties of NiCr

In the nickel chromium alloys the chromium is easily soluble in the nickel. The solubility is highest at 47% concentration at the eutectic temperature and lowers at 30% at the normal room temperature. The group of industrial nickel chromium alloys is based on firm solution of nickel chromium metals. This nickel chromium alloyoffers large resistance to oxidation conditions at the high temperature as well as it as appreciable wear resistance.

With the inclusion of little concentration of chromium in the Nickel the alloy's potency to oxidation is improved. The reason is that the dispersion rate of oxygen has been improved.This process alters after the addition grades increased by 7% chromium and improves to increased level by 30%. Beyond this grade, minor changes occur.

The oxidation resistance of nickel chromium heating wire to the production of extensively adherent secured level. The adherent and coherent level can further be enhanced with the inclusion of little concentrations of other materials like zirconium, silicon, cerium, calcium and more. The level produced is the combination of nickel and chrome oxides. These add up to produce nickel chromite that possesses spinal shape.

Significant improvement in the resistance of nickel chromium heating wire is noticed with the chromium addition such that 20% chromium is said to be the suitable material amount for resistance in the electrical equipments. Such combination provides excellent electrical features with fine potency and ductility that the material suitable for drawing.

The little upgradation for this composition can be made to improve the wire for certain operations. With the inclusion of suitable reactive alloy metals an alteration in the properties is certain. The performance conditions of nickel chromium alloy wire are extremely influenced by its composition.

However the concentration alternations have minor impact on mechanical features, large concentration of reactive metals causes to avoid the flaking of scale while periodic heating and cooling.

The nickel chromium wire with binary expression of 90/10 is used for heating operations and it has highest performance temperature of 1100 degree Celsius. Moreover nickel chromium heating wire is also used in thermocouples.The combination of nickel and chromium in 90:10 ratio is more preferred for thermocoupling as compare to 95:5 nickel chromium addition.

Alloy Electrical Resistance Heating Elements

The article relates to a special nickel-chromium heat resistant alloy particularly to electrical resistance heating elements made of such alloys and having improved service life when subjected in use to elevated temperatures, especially under conditions involving repeated heating and cooling.

Nickel-chromium alloys used for electrical resistance heating elements contain small amounts of both calcium and other rare earth metals for the purpose of increasing the service life and these alloys contain small amounts of other elements like silicon. In making the alloys, the cerium is commonly used as Mischmetall, and the term cerium is used herein to mean not only cerium itself but also any other rare earth metals present. In practice the silicon contents of nickel-chromium alloys containing both calcium and cerium have hitherto been very low of the order of 0.5%. While such nickel-chromium alloys have been found beneficial when employed as electrical resistance heating elements, demands by industry for heating elements with greatly improved service lives have placed further burdens on such alloys with the result that the problem of providing improved alloys to meet the needs of industry has been greatly accentuated. Although many attempts were made to meet the needs and demands of industry, none were entirely successful when carried into practice commercially on an industrial scale.

By employing higher and critical silicon content in conjunction with a very low and critical cerium content, it is possible to considerably increase the service life of an electrical resistance element made of such an alloy. To be forgeable, the alloy must also contain a critical amount of calcium. The objective is to provide a special heat resistant nickel-chromium alloy having improved service life at elevated temperatures. And also provide special electrical resistance heating elements characterized by improved performance at elevated service temperatures, especially under conditions involving repeated heating and cooling.

A critical amount of silicon proves improvement on the average life in hours of a nickel-chromium alloy containing a critical amount of cerium and tested in accordance with designation B 76-39 of the American Society for Testing Materials (ASTM). The critical effect of cerium on the average life in hours of a nickel-chromium alloy containing a critical amount of silicon is likewise tested in accordance with the aforementioned ASTM designation B 7 6-39.

Nickel-chromium alloys of for electrical resistance heating elements contain from 10 to 2.5%, chromiumr from 0.0.05 to 0.051% calcium. 0.01 to 0.1% cerium and 1.15 to 2% sililcon and the balance (except for impurities) being of nickel. Preferably these elements are present in newer ranges, namely from 15 to. 2.5% Chromium, to. 0.03% calcium, from 0.025 to 0.06% cerium and from 1.4 to 1.6% silicon. Although these heating elements are concerned with nickel-chromium alloys, as distinguished from nickel-chromium-iron alloys, iron is present as an impurity in the raw materials and as a consequence the alloys of the heating element may contain upto 2% iron. Moreover some of the nickel in an amount up to 15% of the total alloy may be replaced by cobalt. Nickel Chromium alloys contain various other elements without detriment, namely up to 1% aluminum, upto 0.3% carbon, upto 0.16% copper and upto .3% manganese. The impurities present may includey traces of various other elements like titanium.

In evaluating heat resistant alloys of the type containing nickel and 20% chromium for use as electrical resistance heating elements, an accelerated life test is employed in accordance. with the American Society for Testing Materials designation B I6-39. In this test, the alloy specimen in the form of a wire measuring about 12 inches long and having a diameter corresponding to not larger than No. 20 American Wire Gauge (AWG) nor smaller than No. 22 AWG i. e., within the range of about 0.025 inch to 0.032 inch, is subjected to intermittent heating and cooling under prescribed conditions at a temperature of about 1177°C., the heating being accomplished by passing electric current through the wire. The service lives of electrical resistance wires of a number of alloys have been measured in accordance with the aforementioned ASTM test. The percentage compositions of some of the alloys tested and the results obtained. The first alloy was a typical nickel chromium alloy. The second alloy had a silicon content which, though much higher than usual, was still below the preferred range. The third alloy has silicon and cerium contents both within the preferred ranges. The results of tests conducted on the alloy provided indicated markedly improved service lives are obtained when the alloy contains about 0.03% to 0.05% cerium, particularly when the alloy contains 1.4% to 1.6% silicon.

The way in which the service life varies with the silicon content shows the average lives obtained with electrical resistance wires of alloys containing about 0.010% calcium, 0.04% cerium, 0,2% aluminum, 20% chromium and 0.4% iron and of varying silicon contents. It is seen that as the silicon content rises above the normal low figure, there is no appreciable increase in the service life until it is about 1.0%. At this figure the service life begins to increase at a rate which itself rapidly increases. Between 1.15 and 1.4%, the increase is most striking, the service life rising to at least three times the value at 0.6% silicon. When the silicon content exceeds 1.4%, the rate remains approximately constant up to 2%.

The critical nature of the cerium content shows the average lives obtained with electrical resistance wires of alloys containing about 0.010% calcium, 0.2% aluminum, 1.5% silicon, 20% chromium and 0.4% iron, and of varying cerium contents. It is seen that in the narrow range of 0.03 to 0.05% cerium, the best lives are obtained. Generally, alloys provided as above exhibit service lives at 1177°C., as determined by the ASTM designation B 76-39 of the order of about 400 hours and higher, while the best commercially available alloys of the 80-20 nickelchromium type of alloy which do not show good lives of this order when tested in this manner. The term service life in this specification refers in all cases to the total life to burn-out of the wire, since this occurs before the resistance of the wire has increased by 10 percent.

So, an electric resistance heating element made of a nickel-chromium alloy consisting essentially of 15% t0 25% chromium, 0.01% to 0.03% calcium, 1.4% to 1.6% silicon, 0.03% to 0.05% cerium, upto 15% cobalt, upto 2% iron, upto 3% manganese, upto 1% aluminum, upto 0.16% copper, upto 0.3% carbon, and the balance consisting essentially of nickel, the electric resistance heating element being characterized by markedly improved service life of at least 400 hours when subjected to intermittent heating and cooling at an elevated temperature of 2150°F in accordance with ASTM designation B 76-39.

And a heat resistant nickel-chromium alloy adapted for the manufacture of electrical resistance heating elements and consisting essentially of 15% to 25% chromium, 0.01% to 0.03% calcium, 1.4% to 1.6% silicon, 0.03% to 0.05% cerium, upto 15% cobalt, upto 2% iron, upto 3% manganese, upto 1% aluminum, upto 0.16% copper, upto 0.3% carbon, and the balance consisting essentially of nickel, characterized by improved service life of at least 400 hours when subjected as an electrical resistance wire to intermittent heating and cooling at an elevated temperature of 2150°F in accordance with ASTM designation B 76-39.

Nickel-chromium heating element alloy having improved operating life

The "80/20" nickel-chromium alloy in wire or strip form is used extensively as the heating element in resistance heating applications. An accepted means for evaluating the performance of a heating element is by ASTM life test B76-65. In this test, a constant temperature of 2175°F on a 0.0253 inch diameter wire, maintained by resistance heating, is applied at "2 minute on - 2 minute off" intervals until failure by burnout occurs. This life test may be significantly accelerated by raising the wire being tested to a temperature of 2200°F, while keeping all other test conditions the same. In addition, carrying out the test as a constant temperature test, by changing the power supplied to the sample during the test, is a more severe test than a constant voltage test or constant current test which have been used in the past. In a constant voltage test, the input voltage is maintained constant throughout the test. Because of high temperature oxidation, the effective diameter of the wire decreases, causing an increase in resistance. This in turn cause a decrease in electrical current flowing through the wire, because of the constant voltage. The net result is a decrease in power supplied to the wire, and a significant decrease in test temperature. Therefore, the test temperature toward the end of a constant voltage life test could be 100°F lower than the initial temperature. On this basis, the constant temperature test is much more severe than the constant voltage test and results from these tests should not be directly compared without an understanding of the boundary condition of these two tests. The average life to failure at 2200°F of a commercial 80/20 nickel-chromium alloy produced is 197 hours.

The beneficial effect of zirconium upon operating life of the 80/20 nickel-chromium alloy heating elements is known. The addition of calcium and zirconium to such an alloy increases its operating life. The addition of aluminum with calcium and zirconium to nickel-chromium-iron alloys also does the same. Subsequently the addition of calcium, aluminum and rare earths to improve life of nickel-chromium-iron alloys over lives obtainable for such alloys containing calcium, aluminum and zirconium. Zirconium has also been added to nickel-chromium-iron alloys of the superalloy type, high temperature resistant and corrosion resistant

However, the addition of zirconium to nickel-chromium alloys for the purpose of extending life of heating elements of these alloys has several attendant disadvantages, including a detrimental effect upon workability of the alloys at addition levels approaching 0.2 weight percent, loss of zirconium during charging into the alloy melt, and variations of such charge losses from melt to melt. All of these factors have led to difficulty and expense in producing heating element alloys of predictably long operating lives by the addition of zirconium.

It is felt that significant increases in the operating life of an 80/20 nickel-chromium alloy without attendant processing difficulties would enable longer life of heating elements incorporating these alloys, or alternatively enable smaller size heating elememts without a corresponding reduction in operating life, and that accordingly such increases in operating life would be an advancement in the art.

The addition of from about 0.1 to 0.75 weight percent of hafnium to a nickel-chromium heating element alloy having a nominal base composition of 20 weight percent chromium, 1.4 weight percent silicon, trace amounts of Ca, Al and B, up to 0.5 weight percent total, balance essentially nickel, significantly increases operating life of the alloy as a heating element over alloys which do not contain hafnium. For example, the average operating life at 2200°F of the nominal base composition alloy plus about 0.17 to 0.58 weight percent hafnium is about 250 hours, more than 100 hours greater than the average life of the nominal base composition alloy containing neither hafnium nor zirconium. The alloys thus made would find use in resistance heating applications where longer operating lives or smaller sizes of heating elements are desired. It appears that an increase in operating life of from about 20-25 percent over that of the common alloy could be achieved by the use of hafnium additions in the amounts specified to the 80/20 nickel-chromium based alloy.

Thus, a nickel-chromium heating element alloy consisting essentially of a base composition in weight percent within the range of: 18 to 22 percent chromium, 1.0 to 1.6 weight percent silicon, balance essentially nickel, characterized in that the alloy contains from about 0.1 to 0.75 weight percent hafnium, whereby the operating life of the alloy as a resistance heating element is improved.