FeCrAl & NiCr wires and strips
Category：
Metal electric heating element
Metal Electric Heating Elemens
- Description
-
Introduction to Iron-Chromium-Aluminum Electric Heating Alloy Materials I. Material Properties
(I) Basic Properties
Iron-chromium-aluminum electric heating alloys are ferritic alloys. They have high usable temperatures in air, such as 0Cr27AI, MO2 high-resistance electric heating alloys, with a maximum usable temperature of 1400℃, and a relatively long service life at higher operating temperatures. Due to the high allowable operating temperature and long life, a higher surface load can be selected for the components, resulting in faster heating and saving of alloy materials. This alloy has high resistivity; when designing components, selecting larger specifications of alloy materials can extend the service life of the components; selecting smaller specifications of alloy materials can reduce the space occupied by the components.
(II) Oxidation Resistance and Corrosion Resistance
1. Oxidation Resistance: The Al2O3 oxide film formed on the surface of the iron-chromium-aluminum alloy is dense in structure and adheres well to the substrate, and is not easily scattered to cause pollution. Al2O3 also has high resistivity and a high melting point, giving the oxide film excellent oxidation resistance and good carbon penetration resistance.
2. Corrosion Resistance: Iron-chromium-aluminum alloys exhibit good corrosion resistance in sulfur-containing atmospheres and when the surface is contaminated with sulfur-containing substances.
(III) Material Defects
1. Low high-temperature strength; as the temperature increases, the plasticity increases; when used above 1000℃, the components will slowly elongate and deform due to their own weight.
2. After long-term high-temperature use and subsequent furnace cooling, the alloy grains grow larger and become brittle, and cannot be bent in the cold state.
3. Iron-chromium-aluminum alloys containing molybdenum are prone to brittleness during processing into components, and special care must be taken during operation.
Advantages
Disadvantages
Iron-chromium-aluminum
High-temperature resistance
High surface load selectable, fast heating, material saving
Low high-temperature strength
Deformation at high temperatures.
High resistivity
Design advantages, long service life
Brittleness
Becomes brittle after cooling; molybdenum-containing materials are also prone to brittleness during processing
Excellent oxidation resistance
The alumina layer is not easily detached, and has good carbon penetration resistance
Corrosion resistance
Corrosion resistance to sulfur-containing atmospheres and sulfides
II. Usage Points
(I) Maximum Operating Temperature
The maximum operating temperature of a component refers to the surface temperature reached by the component in dry air and cannot be based on the furnace temperature or the temperature of the heated body.
The actual furnace temperature is lower than the heating element temperature, and the temperature difference varies depending on the structure of the resistance furnace, the heat transfer method, and the surface load.
In the case of open direct thermal radiation, the temperature of the electric heating element is about 100℃ higher than the furnace temperature; the temperature difference is even greater when embedded or sealed.
When the temperature of the electric heating element approaches the maximum operating temperature or the oxidation temperature reaches its limit, the amount of oxidation increases, the heat resistance and high-temperature strength deteriorate, the component is easily deformed, and even partial collapse or short circuit occurs, shortening the service life.
There are various grades of electric heating alloys, and the maximum operating temperature varies depending on the alloy composition and specifications.
Generally, for alloy components operating at the maximum operating temperature, the wire diameter is not less than 3.0 mm, and the strip thickness is not less than 2.0 mm.
The furnace atmosphere will also affect the maximum operating temperature of the component. Corrosive atmospheres interfere with the formation of the oxidation protective film, shortening the service life of the component.
(II) Temperature Coefficient of Resistance
The resistance of iron-chromium-aluminum electric heating alloy components changes with temperature, and the change is expressed by the temperature coefficient of resistance.
When the temperature coefficient of resistance is positive, the resistance increases with increasing temperature; when it is negative, the resistance decreases with increasing temperature.
Due to the different alloy compositions of iron-chromium-aluminum alloys, the higher the Cr and Al content, the smaller the temperature change coefficient. The 0Cr2AIMO2 high-temperature alloy material has a relatively small negative temperature coefficient of resistance.
When designing electric heating elements, the resistance value at the actual operating temperature (hot resistance) is used as the basis. The resistance temperature correction coefficient Ct (Ct = Rt/R20, where Rt is the working temperature resistance and R20 is the room temperature resistance) is often used to indicate the temperature change of the alloy element. To calculate the working temperature resistance Rt, the room temperature resistance needs to be multiplied by the correction coefficient Ct.
In iron-chromium-aluminum alloys, aluminum is the main element that increases the resistivity. Although high-temperature and long-term use will cause aluminum loss, leading to a decrease in room temperature resistivity, the temperature correction coefficient Ct increases accordingly, and the resistivity remains almost unchanged in actual use.
(3) Brittleness
Iron-chromium-aluminum electric heating alloy products have sufficient ductility to meet the requirements of winding electric heating elements when they leave the factory.
However, after long-term use at high temperatures above 900℃, the alloy grains gradually increase, and the plasticity significantly decreases, causing high-temperature embrittlement.
The higher the temperature, the longer the time, and the slower the cooling speed, the more serious the embrittlement after cooling, and the more obvious the embrittlement of large-size components.
During use, collisions and violent vibrations should be avoided. After high-temperature use and cooling, do not stretch or bend the components; handle them gently during repair. If straightening or bending is required, it can be done after heating to 600℃-800℃. Provide adequate support during installation.
(4) High-temperature strength
The mechanical properties of alloy materials change with increasing temperature, and the tensile strength decreases with increasing temperature.
For example, the tensile strength of 0Cr2Al alloy at 900℃ is 34N/mm2, and the creep strength is 2.5N/mm2; at 1100℃, the creep strength is 0.3N/mm2, and it is easy to deform at high temperatures.
The strength and creep strength of nickel-chromium alloys are superior to those of iron-chromium-aluminum alloys.
When designing iron-chromium-aluminum elements, improper parameter selection or inadequate installation support may cause high-temperature deformation, leading to element collapse, short circuits, etc., affecting the service life. Reasonably determining the element shape and size and the layout of the support material can compensate for this defect.
3. Corrosion resistance
(1) Air
When iron-chromium-aluminum alloys are used at high temperatures in air, a surface oxide film is formed, protecting the alloy from further oxidation.
Below 800℃, the oxide film is composed of Fe2O3, Cr2O3, and Al2O3; above 1000℃, it is mainly composed of aluminum oxide. The thickness of the oxide film is related to the heat resistance of the alloy.
The iron-chromium-aluminum alloys produced by our company undergo heat treatment before leaving the factory, and the formed oxide film is impure and has poor corrosion resistance.
Pre-oxidation treatment of the components before use can generate a purer oxide film, extending the service life. The pre-oxidation method is to electrically heat the components in dry air to 100℃-200℃ below the maximum operating temperature, such as heating 0Cr2Al alloy to 1050℃ for 7-10 hours, and then slowly cooling with the furnace.
(2) Other atmospheres
1. Carbon monoxide, carbon-containing, carburizing reducing atmosphere: When the temperature is not very high, the surface oxide film of the component can prevent carbonization; above 1100℃, the stability of the component decreases, and it may be melted. Pre-oxidation or coating with a high-temperature inorganic glaze before use, and regular re-oxidation, can enhance the service life. In carburizing furnaces, the voltage can be reduced to prevent carbon deposition at the brick-laying points of the components, causing short circuits.
2. Halogen atmosphere: Iron-chromium-aluminum electric heating elements cannot be used in atmospheres containing halogens or other compounds. Trace amounts of halogen elements will severely corrode the components, and hand contact should be avoided during component manufacturing.
3. Sulfur-containing atmosphere: Iron-chromium-aluminum alloys are relatively stable in sulfur-containing atmospheres, but their service life will be reduced in sulfur-containing reducing atmospheres.
4. Hydrogen-containing and nitrogen-containing atmospheres: Pure hydrogen gas is harmless to iron-chromium-aluminum electric heating elements, and ammonia decomposition gas also has no effect, but the presence of ammonia will shorten the service life. The life of iron-chromium-aluminum alloys directly exposed to nitrogen is lower than that in air, and pre-oxidation treatment can keep it stable at high temperatures in pure nitrogen.
5. Water vapor-containing atmosphere: Water vapor will affect the formation of the surface oxide film protective layer of iron-chromium-aluminum elements, reducing the service life of the elements.
6. Salts and oxides: When iron-chromium-aluminum electric heating alloy elements come into contact with various salts and oxides, the surface oxidation protective layer will be destroyed, affecting the service life.
7. Enamel and Glaze: Harmful compounds in enamel and glaze can affect the service life of iron-chromium-aluminum components.
8. Molten Metal and Metal Oxides: Some molten metals and metal vapors react with and dissolve iron-chromium-aluminum heating alloys, iron oxide spots hinder the formation of oxide films, and lead oxide deposits on cooler parts of the furnace, corroding the components.
9. Refractory Materials: When using iron-chromium-aluminum heating alloy components, attention should be paid to the contact areas with refractory materials. Refractory materials containing alumina or magnesia, or clay refractory bricks with alumina content greater than
45%, should be selected. Reduce the SiO2 and basic oxide content, avoid using water glass as a binder, and avoid contact with rock wool and slag. At high temperatures, the supporting material must have sufficient insulation resistance to prevent damage to the components due to excessive leakage current.
IV. Usage Precautions and Lifespan
Usage Precautions: High-Temperature Deformation and Brittleness
. Low high-temperature strength, easy to deform above 1000℃, requires optimized support structure.
. Grain coarsening leads to brittleness after long-term use, bending is prohibited in the cold state, and repair requires heating to 600-800℃. Environmental Adaptability Guide
Environment Type
Applicable Suggestions
Carbon-containing/Carburizing Atmosphere
Pre-oxidation treatment + surface glazing, temperature ≤1100℃
Oxidizing/Vacuum
Not applicable (short-term use at low vacuum and low temperature is possible)
Sulfur/Hydrogen/Nitrogen Containing Gas
Applicable (nitrogen requires pre-oxidation)
Water Vapor/Salts
Avoid contact; easily corrodes components
Usage Temperature Specifications
. Component temperature ≠ furnace temperature; the actual temperature difference is affected by the furnace structure (e.g., approximately 100℃ temperature difference in an open furnace). . Recommended wire diameter ≥3.0mm, flat strip thickness ≥2.0mm (see the model and temperature correspondence table for details).
Refractory Material Selection
· Preferably AI2O₃ or MgO-based materials (AI2O₃ content > 45%), avoid materials containing SiO2/Fe2O₃.
Lifespan
The service life of iron-chromium-aluminum heating alloys is affected by many factors, including alloy composition, impurities, added elements, usage conditions, and usage methods, including furnace atmosphere, supporting material material, usage temperature, surface load, heat dissipation conditions, heating and cooling frequency, operation methods, and component design and installation conditions.
Selecting larger-specification alloy wires can increase the AI content per unit surface area, extending the service life of the components; reducing the surface load can lower the alloy wire temperature, extending the service life.
The iron-chromium-aluminum and nickel-chromium heating alloy materials produced by our company are tested for quality using accelerated life tests according to current national standards (GB1234-95). This test method is more stringent than actual usage conditions and only reflects the product's oxidation resistance, not the actual service life of the components. If the service life is significantly shortened during use, the alloy material and other influencing factors should be comprehensively analyzed.
Lifespan Optimization Strategies
1. Specification Selection: Prioritize the use of large-diameter wires/thick flat strips to improve the unit aluminum content.
2. Pre-oxidation Treatment: 1050℃ for 7-10 hours to strengthen the AI2O₃ film.
3. Load Control: Reasonably design the surface load to avoid accelerated oxidation due to overheating.
4. Installation and Maintenance: Handle with care, regularly check the support structure, and avoid vibration damage.
IV. Quality Assurance and Testing Standards
National Standard Certification: Strictly follows GB1234-95, passing the accelerated life test (0.8mm sample wire cycle test).
Actual Lifespan Indication: Affected by operating conditions, installation, and other factors, regular inspection and optimization of usage conditions are recommended (see Table 5 for high-temperature creep strength comparison).
V. Applicable Fields
Industrial furnaces, heat treatment equipment, ceramic kilns, carburizing furnaces, and other high-temperature scenarios. For customized solutions, please contact the technical team.
VI. Available Specification Range, Surface State, and Dimensional Deviation
(1) Resistance value: bright fine wire wound around the axis
Φ> 0.05--0.12mm ±8%
Φ> 0.12-0.17mm ±7%
Φ>0.17-0.32mm±6% Φ>0.32-1.0mm ±5% Cold-rolled alloy strip material ±5%
(2) Specifications and surface condition:
Variety
Specification range (mm)
Surface condition
Cold-drawn alloy wire
1.0-10.0
Oxidized annealing
Hot-rolled alloy coil
8.0~§12.0
Cold-rolled alloy strip
Thickness 0.1~3.5 Width 5.0~200.0
Oxidized annealing
Hot-rolled alloy strip
Thickness 3.0~6.0 Width 12.0~60.0
Bright wire on coil
§0.07~§1.0
Bright annealing
Bright flat wire on coil
Thickness 0.05~1.5 Width 0.5~6.0
Bright annealing
. Bright annealing: Alloy material annealed under ammonia decomposition gas protection, supplied in soft state.
. Oxidized annealing: Alloy material annealed in air, supplied in white or oxidized color. Strip materials are generally supplied in oxidized color after annealing, unless otherwise specified.
(3) Dimensional deviation: Cold-drawn alloy wire
Diameter (mm)
Allowable deviation (mm
Diameter (mm)
Allowable deviation (mm)
>0.05~0.10
±0.006
>1.00~3.00
±0.03
>0.10~0.20
±0.008
>3.00~5.50
±0.04
>0.20~0.30
±0.010
>5.50~7.50
±0.05
>0.30~0.50
±0.015
7.50~10.00
±0.08
>0.50~1.00
+0.02’-0.03
Cold-rolled alloy strip material table
Thickness (mm)
Allowable deviation (mm)
Allowable deviation (mm)
Width (mm)
Slit
Unslit
0.1~0.18
±0.010
5.0~10.0
-0.6
>0.18~0.30
±0.015
>10.0~15.0
-0.4
-0.7
>0.30~0.50
±0.020
>15.0~20.0
-1
>0.50~0.70
-0.1
>20.0~30.0
-1.2
>0.70~1.00
±0.030
>30.0~40.0
-1.3
>1.00~1.20
-0.01
>40.0~60.0
-0.7
±1.0
>1.20~1.60
±0.040
>60.0~100
>1.60~2.20
±0.050
>100~150
1
±5.0
>2.20~3.50
±0.07
>150~200
Hot-rolled alloy strip
Thickness (mm)
Allowable deviation (mm)
Thickness (mm)
Allowable deviation (mm)
3.5~5.0
±0.25
12~-60
+2.0’-1.0
>5.0~6.0
±0.30
Bright flat wire on coil
Thickness (mm)
Allowable deviation (mm)
Thickness (mm)
Allowable deviation (mm)
0.05~0.10
±0.010
≤1.0
±0.08
>0.10~0.20
±0.015
>1.0~1.50
±0.10
>0.20~0.30
±0.020
>1.50~2.0
±0.12
>0.30~0.50
+0.020’-0.030
>2.0~3.0
±0.18
0.50~0.70
±0.030
>3.0~4.0
±0.20
>0.70~1.50
±0.040
>4.0~6.0
±0.28
Special Note: Dimensional deviation and resistance value cannot be simultaneously satisfied.
Attachment: Quick Reference Chart
Table 1: Performance parameters of iron chromium aluminum electric heating alloy
Grade
1Cr13AI4
1Cr21AI4
0Cr21AI6
0Cr23AI5
0Cr25AI5
0Cr21AI6Nb
Performance
Main chemical composition (%)
Fe
Balance
Balance
Balance
Balance
Balance
Balance
Cr
12.0~15.0
17.0~21.0
19.0~22.0
20.5~23.5
23.0~26.0
21.0~26.0
Al
4.0~6.0
2.0~4.0
5.0~7.0
4.2~5.3
4.5~6.5
5.0~7.0
Re
Appropriate amount
Appropriate amount
Appropriate amount
Appropriate amount
Appropriate amount
Appropriate amount
Mo
-
-
-
-
-
-
Nb
-
-
-
-
-
Added amount 0.5
Maximum operating temperature (°C)
950
1100
1250
1250
1250
1350
Resistivity ρ
1.25±0.05
1.23±0.05
1.42±0.05
1.35±0.05
1.42±0.05
1.45±0.05
Melting point (°C)
1450
1500
1500
1500
1500
1510
Resistance temperature
800℃
1.132
1.154
1.046
1.07
1.04
0.99
Correction factor
1000℃
1.15
1.172
1.052
1.087
1.04
0.99
(Ct)
1200℃
-
-
1.058
1.084
1.047
0.99
Rapid life value
Temperature (°C)
900
1050
1200
1250
1250
1350
Hours (h)
≥80
≥80
≥80
≥80
≥80
≥80
Density
7.4
7.35
7.16
7.25
7.15
7.1
Elongation (%)
≥16
≥10
≥12
≥12
≥12
≥12
Thermal conductivity
52.7
46.9
63.2
60.2
46.1
46.1
Specific heat (20℃)
0.49
0.49
0.52
0.46
0.494
0.494
Coefficient of linear expansion
15.4
13.5
14.7
15
16
16
Tensile strength (Mpa)
588~735
637~784
637~784
637~784
637~784
637~784
Repeated bending number (F/R)
≥5
≥5
≥5
≥5
≥5
≥5
Microstructure
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Ferrite
Magnetic
Magnetic
Magnetic
Magnetic
Magnetic
Magnetic
Magnetic
Table 2: Relationship between wire diameter/thickness and maximum temperature
Maximum operating temperature
Grade
Diameter (mm)
0Cr27AI7MO2
0Cr21AI6Nb
0Cr25AI5
0Cr23AI5
0Cr21AI6
1Cr21AI4
1Cr13AI4
0.15~0.40
-
-
900~1025
900~1025
900-1025
800~875
625~700
0.41-0.95
-
-
1025-1125
1025~1125
1025~1125
875~975
700~825
1.0~3.0
1225~1350
1175~1300
1125-1200
1125-1200
1125-1200
975-1050
825-900
>3.0
1400
1350
1250
1250
1250
1100
950
Table 3: Furnace atmosphere adaptability guide
Maximum operating temperature
Grade
Furnace atmosphere
0Cr27AI7MO2
0Cr21AI6Nb
0Cr25AI5
0Cr23AI5
0Cr21AI6
Dry air
1400
1350
1350
1250
1300
Tidal air group
1200
1150
1100
1050
1100
Hydrogen
1400
1350
1300
1250
1300
Nitrogen
980
950
900
900
900
Ammonia decomposition gas
1200
1150
1100
1050
1100
Exothermic gas 10CO, 15H2, 5CO2, 60N2,
1150
1150
1100
1050
1100
Endothermic gas CO, 40H2, 40N2
1050
1050
1000
950
1000
Temperature coefficient of resistance
Alloy grade
CU p1
Temperature ℃
20
100
200
300
400
500
OCr27AI7MO2
Ct
1
0.992
0.986
0.981
0.987
0.976
P1
1.53
1.518
1.509
1.501
1.496
1.493
0Cr21AI6Nb
Ct
1
0.997
0.996
0.994
0.991
0.99
P1
1.45
1.446
1.444
1.441
1.437
1.436
0Cr25AI5
Ct
1
1.002
1.005
1.008
1.013
1.021
P1
1.42
1.423
1.427
1.431
1.438
1.45
0Cr23AI5
Ct
1
1.002
1.007
1.014
1.024
1.036
P1
1.35
1.353
1.359
1.369
1.382
1.399
0Cr21AI6
Ct
1
1
1.002
1.006
1.011
1.02
P1
1.42
1.42
1.423
1.429
1.436
0.352777778
1Cr21AI4
Ct
1
1:011
1.025
1.042
1.061
1.058
P1
1.23
1.244
1.261
1.282
1.305
1.301
1Cr13AI4
Ct
1
1.005
1.014
1.028
1.044
1.064
P1
1.25
1.256
1.268
1.285
1.305
1.33
Figure 1-4: Oxide film structure, corrosion cases and component installation examples
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