Nichromes: varieties, composition, properties and characteristics
Nichrome — the main material for the production of heating elements for electric ovens. Nichrome is specially designed for this purpose and, therefore, satisfies everyone to the maximum extent requirements for such materials.
Nichrome is the general name of a group of alloys consisting, depending on the degree of the alloy, of 55-78% nickel, 15-23% chromium, with additions of manganese, silicon, iron, aluminum. The first group includes alloys consisting mainly of nickel and chromium, the iron content in them is low (0.5-3.0%), which explains their name. The second group includes alloys containing, in addition to nickel and chromium, also iron.
Nichrome, which is a further development of chromium-nickel refractory steel, is a very heat-resistant material because it has an extremely strong protective film of chromium oxide Сr2О3 with a melting point higher than that of the alloy and withstands periodic heating and cooling.In addition, it has good mechanical properties at both normal and high temperatures, creep resistance and sufficient plasticity, so it is easy to process and especially well welded.
The electrical properties of nichrome are also quite satisfactory, it has high resistance, low temperature coefficient of resistance, the phenomena of aging and growth are absent. Binary alloys have the best electrical and at the same time good mechanical properties. At the same time, these alloys have excellent heat resistance, which is why they can work up to 1100 ° C.
The higher the content of chromium in the alloy, the higher the content in its protective film Сr2О3, the more refractory it is and the better the material is resistant to oxidation. But as the chromium content increases, the machinability of the material deteriorates at the same time, and when the chromium content reaches 30%, drawing and cold rolling are no longer possible. Therefore, as a rule, the chromium content in them does not exceed 20%.
The addition of iron to the alloy somewhat improves machinability and increases its resistance, but worsens the temperature coefficient of resistance and significantly reduces its heat resistance. Nevertheless, in cases where the working temperature does not exceed 1000 ° C, it is permissible to use a triple alloy, since it is cheaper and contains less deficient nickel.
Iron-rich nichrome (the term adopted abroad, where it is widely used, we have the Kh25N20 alloy to which it refers) is even cheaper, requires even less nickel and has excellent mechanical properties, although its heat resistance is even lower.It can be used in ovens with a temperature not higher than 900 ° C. All nichromes are non-magnetic alloys. Nichrome is available in wire and ribbon form.
Nichrome was first proposed in 1906 by Marsh. It is currently produced abroad by many companies under different names. Double and triple alloys are produced, molybdenum is added to some brands. In our country, a double alloy with a chromium content of 20 — 23 and a nickel content of 75 — 78% is produced (Kh20N80), in addition, a similar alloy with titanium (Kh20N80T) is produced, but it is slightly less heat resistant and received only limited application. Triple alloys are produced with a content of chromium 15 — 18 and nickel 55 — 61% (Х15Н60). The high price and shortage of nichrome led to an intense search for other alloys, cheaper and more accessible and capable of simultaneously replacing it in certain conditions.
The main technical characteristics of nichrome:
Triple nichrome Х15Н60 — (ЕХН60): Сr — 13 — 18, Ni — 55 — 61. Density at 0ОС — 8200 kg / m3… Specific electrical resistance ρ, 10-6 ohm xm — 1.11 (20 ОВ), 1, 18 (400 OB), 1.2 (600 OB), 1.21 (800 OB), 1.23 (1000 OB). Specific heat capacity — 0.461 x 103 J / (kg x OS). Coefficient of thermal conductivity — 16 W / (mx OS). Maximum operating temperature ОFrom nichrome, depending on the diameter of the wire in mm — 900 (0.2), 950 (0.4), 1000 (1.0), 1075 (3.0), 1125 (6.0 and more ).
Double nichrome Х20Н80 — (ЕХН80): Сr — 20 — 23, Ni — 75 — 78. Density at 0ОС — 8400 kg / m3… Specific electrical resistance ρ, 10-6 ohm x m — 1.09 (20 ОВ), 1 .13 (600 OB), 1.11 (800 OB), 1.12 (1000 OB). Specific heat — 0.44 x 103 J / (kg x OS). Coefficient of thermal conductivity — 14.2 W / (mx OS).Maximum working temperature ОFrom nichrome, depending on the wire diameter in mm — 950 (0.2), 1000 (0.4), 1100 (1.0), 1150 (3.0), 1200 (6.0 and More ▼ ).