Alumel NiMn3Al Wire and Strip

Composition

Thermocouple alloy of alumel consists of nickel (94%), aluminium (max. 2,5%), mangan (to 2,2%), silicon (within 1,2%) and cobalt (1%), which is used for supporting a required thermoelectromotive force. To make special properties of an alloy, it has to be alloyed with boron (0,03%), zirconium (0,06%); as a consequence, a relatively new alloy is obtained.

Advantages

Specific electrical resistance constitutes 0,32 Ohm-mm²/m, melting point — 1440 °C, a coefficient of linear thermal expansion is 13,7•10−6/°C, density is — 8,48 g/cm³, annealing temperature — 950 °C. Due to a huge containing of nickel in a composition, the properties are defined by their physical characteristics — heat resistance and resistance to corrosion in an aggressive environment. An enhancement of these properties achieves by adding alloying components. It allows stabilizing an index of thermal electromotive force, increasing an index of plasticity, providing a long-term strength and use of this alloy at a temperature up to 1300 °C.

Demerits

An alloy is quite expensive due to a presence of pricey nickel in a composition. The main demerit is an exposure of devastating impact of sulfur when rising a temperature. It leads to embrittlement and corrosion. It can be fixed by increasing a percentage composition of silicon and at the same time by reducing a content of aluminium. Such changes enhance a running time of a thermocouple at a temperature more than 1000 °C.

Application

Alloy is widely used in a pyrometry in a form of wire and strip for producing thermocouples (diameter of a wire is 0,2 — 3,2 mm), heating controllers and compensating lead wires. A thermocouple consists of electric conductor fumes, that are soldered in one end and placed in oriented environment («hot junction»). Then, the exposed ends are going to be placed into a thermostatic regulator (so-called cold junction).

Thanks to alloyage, a thermocouple of this kind can stand a short-term effect of temperature up to 1300 °C working in an aggressive environment. Alumel compensating lead wires are intended for accessing transducers and for reducing an error in indication. By virtue of small sizes and high readability, alumel-chromel transducers are used in automobile construction, aerospace engineering, in physical and chemical laboratories.

Delivery of alloy grade NiMn3Al conducts in the following way:

A rod (with a help of cross-section 12,0 — 100,0 mm);
A wire (with a help of cross-section 0,10 — 12,00 mm);
A strip (0,1−5,0 mm of thickness, 3−600 mm of wideness);

Percentage composition:

Physical characteristics of NiMn3Al at a temperature 20 °C