GENERAL INFORMATION
Cold-drawn wire from alloy Fechroma® is applied for resistance elements and heaters. For example, in alloy Fechroma®23 a high electric resistance and low temperature coefficient of electric resistance (TCER) are balanced.
Alloys Fechroma®23Н, Fechroma®23, Fechroma®15, Fechroma®13 possess high resistance to oxidation and corrosion in the most widespread industrial hostile environments, that is caused by the nature of chemically inert, dense superficial field oxide layer on the surface of AL2O3. Unlike feсhral, nickel-containing alloys, for example, can't be used not only in sulfur-containing environments, but also in protective CO-containing atmosphere at 800-950оС, cause at simultaneous oxidation and carburizing, on their surface there is a destruction of protective oxide with formation of so-called "green rot"
FECHRAL ADVANTAGES
The cost is 6-8 times lower than cost of Nichrome;
High operating temperature – up to 1350оС;
Higher melting point - 1500 оС, in comparison with 1400 оС for Cr20Ni80-N;
Low density – 7,28 g/cm3 it allows to save up to 30% of weight during production of heating elements;
The best corrosion resistance in air atmosphere, vacuum, argon, sulfur containing and carbon-bearing environments, water vapor.
FECHRAL DISADVANTAGES
Despite the available advantages, fechral has a number of disadvantages, which limit its technological application and possibility of replacement with nichrome alloys:
- has the increased fragility and the lowered durability that complicates production of heating elements;
- gain irreversible fragility as a result of formation of coarse-grained structure after heating above 1000оС;
- as fechral contain iron, this alloy is magnetic and can rust in the damp atmosphere at a normal temperature;
- has the low resistance of creep;
- interacts with fireclay lining and oxides of iron;
- during operation fechral heaters are significantly extended that can lead to their sagging.
FEATURES OF OPERATION OF HEATERS FROM FECHRAL
Alloys like fechral are inclined to chemical interaction with oxides and metals. For fechral, unlike nichrome, the ceramics from chamotte containing a significant amount of iron oxides isn't suitable. At operation temperatures of fechral above 1100оС the fire-resistant weight has to contain not less than 75% of alumina and the minimum quantity of iron oxides (less than 1%), up to 1100оС the fire-resistant weight can contain not less than 60% of alumina and less than 1% of iron oxides. In practice, ceramics coating in places of contact with heating elements made of fechral with high-alumina weight has proved itself well (mix of 30% of corundum fraction ≈25mkm, 45% of corundum fraction ≈ 5mkm, 25% of kaolin and water).
The destroying effect on oxide scale of fechral is caused by couples and splashes of some metals – copper, zinc. Aluminum. Lead. The contact of a surface of heater made from fechral with table salt, slags, enamels, asbestos and iron scale is inadmissible. Production of heaters from a rusty fechral wire or strip is inadmissible. For the purpose of prevention of premature failure of heaters from fechral alloys it is necessary to avoid sharp change of thermal power, especially in the course of a furnace warming up. For increase of service life of fechral heaters it is recommended to cool as seldom as possible heaters of high-temperature furnaces lower than 700-800 оС.
For increase of service life in a vacuum. also in carbon-bearing and nitrogen-containing environments it is recommended preliminary oxidation of fechral wire at 1100оС during 10-20h. The oxides of aluminum which are formed at the same time slow down metal distillation and interfere to penetration of carbon and nitrogen in it.
It is necessary to consider that heaters from fechral have long service life on condition of the high culture of their operation.
Classification
|
DIN |
1.4725 (CrAl 14 4) |
|
UNS |
К 91670 |
Chemical composition, %
|
Cr |
Fe |
Al |
C |
|
14.0-15.0 |
rest |
3.0-4.8 |
≤0.005 |
|
Mn |
Si |
S |
|
|
≤0.50 |
≤0.50 |
≤0.015 |
|
Mechanical properties at 20°С
|
Diameter, mm |
Elongation, %, not less |
Tensile strength, MPa |
|
0,12 – 0,5 |
18 |
588-735 |
|
0,5 – 1,00 |
18 |
|
|
1,0 – 12,0 |
25 |
PHYSICAL PROPERTIES
|
Temperature, оС |
Resistance μΩm |
Exten. 10-6/К |
|
20 |
1,25 |
- |
|
200 |
1,27 |
11,0 |
|
400 |
1,30 |
12,0 |
|
500 |
1,32 |
- |
|
600 |
1,34 |
13,0 |
|
800 |
1,39 |
14,0 |
|
1000 |
1,42 |
15,0 |
|
1200 |
- |
- |
| Fechroma®23H |
| Fechroma®23 |
| Fechroma®15 |