K94100, ASTM F30, 1.3917, NF A54-301, FeNi42
Note that the designation Invar is a registered trademark of Aperam Alloys Imphy.
Invar 42 is a 42% nickel-iron alloy with a consistently low rate of thermal expansion at temperatures up to approximately 650°F (343°C). It is often used as core rod material uniformly coated with an 18 to 28 volume % copper sheath by manufacturers of copper coated wire. The thermal expansion of the composite wire provides a suitable match for glass sealing to soft glass.
Alloy 42 has been used for the glass-to-metal seals in electronic tubes, automotive and industrial lamps, transformer and capacitor bushings and other glass-to-metal and ceramic-to-metal applications.
The chemical composition of Invar 42 is indicated in the table below:
|
% |
C |
Si |
Mn |
S |
P |
Ni |
Fe |
Al |
Co |
|
Alloy 42 |
≤0.05 |
≤0.3 |
≤0.8 |
≤0.02 |
0.02 |
41.5~ 42.5 |
Bal |
＜0.10 |
＜1.0 |
Invar 42 density, melting point, inflection point, thermal conductivity, and coefficient of expansion is indicated in the table below:
|
Density |
Melting Point |
Inflection Point |
Thermal Conductivity |
Coefficient of Expansion |
|
8.11 g/cm³ |
1435 °C |
370 °C |
10.5 W/m• °C |
5.3 µm/m °C (20 – 100 °C) |
|
0.293 lb/in³ |
2615 °F |
700 °F |
72.8 btu•in/ft2•h °F |
2.9 x 10-6 in/in °F (70 – 212 °F) |
Properties
The typical mechanical properties of Invar 42:
|
Annealed |
|
|
|
Approx. tensile strength |
<600 N/mm² |
<87 ksi |
|
Approx. operating temperature |
up to +300 °C |
up to +570 °F |
|
Hard Drawn |
|
|
|
Approx. tensile strength |
700 – 900 N/mm² |
102 – 131 ksi |
|
Approx. operating temperature |
up to +300 °C |
up to +570 °F |
Refer to NACE (National Associate of Corrosion Engineers) for recommendations.
The Invar alloys are usually supplied and used in the annealed condition (residual cold work distorts the coefficients of thermal expansion). Annealing times may vary due to section thickness.
The typical heat treatment of Invar 42:
|
Type |
Temperature |
Time |
Cooling |
|
Anneal |
850 – 1000 °C (1560 – 1830 °F) |
0.5 Hr |
Air or water |
The forging temperature should be 2150/2200°F (1177/1204°C). Avoid prolonged soaking to prevent sulfur absorption from the furnace atmosphere.
For clean blanking of Glass Sealing 42, a Rockwell hardness of about B90 is suggested. Where any sharp bends are involved in forming finished parts from strip or rods, a hardness of not over Rockwell B93 is suitable.
A silicon carbide wheel is desirable, preferably a soft wheel which will wear without loading. For finish grinding, a satisfactory grade to start with is No. 80 grit.
Any of the conventional welding methods can be used. When filler rod is required, Glass Sealing 42 is suggested.
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