Fe-Ni alloys containing nickel concentration minimum 35 percent exhibit an exclusive low thermal expansion coefficient and their size is almost constant at and around room temperature. Hence they are known as INVAR alloys. They describe anomalous magnetic properties, for instance, deviation of magnetization from the Slater Pauling curve by reducing the count of electrons in the valence shell and extensive reliance of curie temperature (TC – it a temperature at which ferromagnetic material becomes paramagnetic) on the mean distance of the constituting atoms of alloys. These irregularities are because of instability of 3d ferromagnetism in fcc lattice. Practically this irregularity has been noticed as a quick fall in curie temperature if large pressure is applied and a major variation in the magnetic properties by altering the nickel magnitude in the NiFe alloys.

Nickel is a non-carbide constituting agent that is miscible in iron in all characteristics. Nickel avoids an extensive grain development at the elevated temperatures and offers fine grain steels to be developed more conveniently. It offers to stabilize the austenite and hence reduces the critical temperature. It results in slightly vigorous heat processing. Nickel could be available in the steel grades up to 50 %. For 2 – 5 %, it adds strength and hardness with the large elastic limit, fine ductility and suitable resistance as well as reduced machinability. For concentration of range 30 – 40 % nickel reduces the thermal expansion coefficient and for content up to 50 % and higher, it improves magnetic permeability. High magnitudes of Nickel offer oxidation resistance at the elevated temperatures.

Invar alloys are a material that have extremely small coefficient of thermal expansion at room temperature below 2 x 10(-6) per K as compared to other standard metallic materials that have thermal expansion coefficient about 10 to 20 x 10(-6) per K and is commonly utilized in the industrial applications including telecommunication, aerospace and aviation, cryogenic applications such as liquefied natural gas tankers and others that need very high size stability with change in temperature or expansion properties similar to other materials like glass ceramics or complex. Besides of thermal expansion behavior, iron rich fcc Fe-Ni alloys describe several other exclusive properties including very negative pressure effects on the magnetization and on the curie point, a huge force volume magnetostriction (volume expansion resulted by applied magnetic field) and an irregular temperature reliance of elastic modulli.

So now we understood that:

What is Invar effect?

The face centered iron – nickel alloys containing nickel weight about 35 % attain very small thermal expansion over a variety of temperature limits. This effect is called as invar effect that has been observed in the several ordered and random alloys and also in the amorphous materials. Invar alloys also show anomalous behavior in their atomic volume, elastic modulus, heat capacity, magnetic properties and curie point. They are used in instrumentation such as in hair springs of watches.

Invar is used in applications that demand high extent of size stability in the variable temperatures. It is also utilized in the precision mechanical equipments in the diverse industries and not only in opto- mechanical engineering applications. Invar basically belongs to a family of low expansion iron-nickel alloys, the popular members are:

1. Invar consisting of 64 percent iron and 36 percent nickel called as Invar 36 or Nilo 36
2. SuperInvar consisting of 63 % iron, 32 % nickel, 5 % cobalt
3. Kovar consisting of 54 % Iron, 29 % Nickel and 17 % cobalt.

While using the word Invar refers to Invar 36 that is the most commonly used FeNi alloy in the opto –mechanical applications. Others are slightly mentioned in this article.

Magnetic Effects:

It is a known fact that magnetic field results in a variation in the dimensions of ferromagnetic materials, it is anticipated that the elastic moduli will also be affected. It is definitely true, however not always identified. When a magnetic field is applied to a ferromagnetic material, its E modulus alters by some amount known as dE hence the effect has come to be called as dE effect. The extent of this effect alters, definitely, with field strength. Although, even slight field may introduce a considerable dE in the precision measurements.

It has been described, for example, Invar pendulum’s period was changed by earth’s gravity which was eventually discarded by shielding the device in a special hox whenever it was moved. In many cases, the damping capacity of a vibrating body will decrease as a magnetic field is applied to the body. For an Armco Iron field, damping increased with the field strength and approached a highest at saturation magnetization. As an increase in damping is generally achieved by a reduction in dynamic modulus, it is similar to stating that modulus decreases with increase in field. The damping decreases with increase in magnetic field for longitudinal or torsional vibrations. The effect may be resulted from the stress induced rotation of the magnetic vectors.

An additional factor should be identified. In few alloy structures, a solute element will occupy a specific lattice position relative to the magnetization vector. Hence, if a piece of iron containing carbon solution is magnetized, it will initially increase in length because of magnetostriciton then length will be reduced as a time function as the carbon atoms diffuse to energetically more favorable locations. It is a kind of reaction called as directional ordering and seems to offer several undiscovered ramifications. A specifically essential effect found is that damping effect of invar varies with the time at temperatures lower than curie point subsequent to thermal processing or after demagnetization. Again it refers to change in modulus as a time function. Hence care should be taken about the conditions in which invar alloy is used if its complete capacity is to be achieved.   

General knowledge in Mechanical applications

The significant feature of Invar is small coefficient of thermal expansion (CTE). Its value is 1 ppm per Kelvin at room temperature, although in most mechanical properties, CTE shows changes with change in temperatures. The CTE of Invar is the minimum among all metals as shown in the following table:

Metals	CTE
Aluminum	23.6 X 10(-6) per K
Copper	17 X 10(-6) per K
Gold	14.2 X 10(-6) per K
Iron	11.8 X 10(-6) per K
Nickel	13.3 X 10(-6) per K
Silver	19.7 X 10(-6) per K
Tungsten	4.5 X 10(-6) per K
Steel 1025	12 X 10(-6) per K
SS 316	16 X 10(-6) per K
Brass	20 X 10(-6) per K
Kovar	5.1 X 10(-6) per K
Invar	0.5 -2 X 10(-6) per K
Super Invar	0.3 -.1 X 10(-6) per K
Ceramics
Fused silica	0.4 X 10(-6) per K
Glass BK7	7.1 X 10(-6) per K
Borosilicate glass	3.3 X 10(-6) per K
Polymer plastics	100 to 200 X 10(-6) per K

It can be seen that CTE of Invar and Super Invar are much lower than the CTE of other metals.

Considering from the atomic level, thermal expansion is described by an increase in the average distance among atoms. Larger bonding energy between atoms in a material will result in smaller CTE hence the ceramics with comparatively stronger interatomic bonding offer smaller CTEs rather polymers and metals as shown in the above table. The small coefficient of expansion of fused silica can be described by a small atomic packing density that the interatomic expansion creates comparatively small macroscopic dimensional variations.

The CTE of Super Invar can be almost zero after specific heat processing however it is valid only for a controlled temperature limit. Slighter variation of CTE will result in temperature of alloy 36 to make it a Super Invar in few applications where temperature fluctuations occur considerably.

In real CTE of Invar depends on its temperature, machining processing and chemistry.

The low CTE of Invar is good for the opto – mechanical engineering where construction of systems that are stable at temperatures is required. As the light wavelength for the optical apparatus in the visible area of the spectrum is about 0.5 micro-m and system needs generally the optical elements that can tolerate to this level, a base metal with a small CTE is of great significance.

Discovery of Invar

Invar was discovered in 1896 by Charles Edouard Guillaume in Paris. It is popular as low expansion metal, Edouard found that CTE of Fe-NI alloy showed very small value when its chemistry is 36 percent nickel and 64 percent iron.

Properties

Invar appears and feels like steel. It makes sense because Invar is an iron based alloy in which iron is the base element. The iron based alloys are made in larger magnitudes as compare to other alloys. Keep in mind that steels and cast irons are iron-carbon alloys however Invar is an iron-nickel alloy as stated above. Invar hardly consists of 0.01 to 0.1 percent carbon. A highly pure Invar will comprise of below 0.01 percent of carbon. The magnitude of carbon in invar with other contaminants is a chief factor in its sequential stability. In addition of iron (Fe) and nickel (Ni), invar 36 may also contain cobalt (Co), chromium (Cr), carbon ©, manganese (Mn), phosphorous (P), silicon (Si), sulfur (S), aluminum (Al), magnesium (Mg), zirconium (Zr) and titanium (Ti).

The exact percentages of these elements in Invar will alter on the base of its dimension and manufacturer. A super invar consists of around 5 percent cobalt reducing nickel content by the same amount.

Comparison of properties of Invar 36 with Stainless steel 304 and other metals. See the following table:

Property	Invar	SS 304
Density	8.05 g/cm3	8 g/cm3
Young Modulus	141 GPa	193 Gpa
Poisson ratio	026	0.27
Micro yield strength	70 MPa	Above 300 Mpa
Coefficient of thermal expansion	1 x 10 (-6) per K	14.7 x 10 (-6) per K
Thermal conductivity	10.4 W/m-K	16.2 W/m-K
Specific heat	515 W s/kg K	500 W s/kg K
Specific stiffness	17.5	24.1
Thermal diffusivity	2.6 x 10(-6) m2 per sec	4.1 x 10(-6) m2 per sec
Thermal distortion (steady state)	0.10 micro –m /W	0.91 micro –m /W
Thermal distortion	0.38 s/m2 K	3.68 s/m2 K

Apparently, Invar offers equivalent mechanical properties to stainless steel. Still there are few variations between their properties. Young modulus, yield strength, thermal conductivity and specific stiffness are smaller for Invar.

Invar seems to be an exciting metal for applications that demand outstanding specification stability across various temperatures. It should be kept in mind although that specification states to how a component constructed of this material alters shape while variations in temperature, time and stress. To employ Invar suitably one should be aware of its temporal stability problems and thermal stability features.

CTE is just very small around room temperature and changes with change in temperature from its lowest level.  So CTE is temperature function represented as CTE (T). While studying about the thermal expansion for mechanical systems, variation in length of a component is calculated by:

dL(T) = CTE(T).L.dT  ————————————– equation (1)

However in several analyses, CTE (T) can be assumed invariable, it is recommended to use average coefficient for variable temperatures. Invar 36 offers CTE from -0.6 to 3 ppm  per K between -70oC to 100oC and can be bounded to 0.8 – 1.6 for temperatures 30oC to 100oC by a specific control of component while treatment. It is not usual that an operator needs to utilize the general formula:

dL(T) = L^T2∫_T1 dT. CTE(T) ——————————-equ (2)

Equation 1 is implemented when changes in CTE are small for slight temperature variations. For greater temperature changes, dCTE (T) /dT and equation 2 can be used.

In addition of changes in coefficient of thermal expansion, operator should note that its value for Invar is similar to fused silica and CTE of superalloy is equivalent to ULE.

Temporal stability of Invar

Invar’s size increases with ageing in fact at the stable temperature. Its enlargement with the time is based on several aspects such as time after final machining, magnitude of carbon, heat processing and ambient temperature.

Various analyses have been performed to evaluate the temporal stability of invar. Invar 36 has CTE 1 ppm per k and temporal stability 1 ppm per year. Increasing carbon percentage and other contaminants may cause increased temporal stability. Carbon is the chief element. It is found that growth in 0.02 percent carbon was smaller than invar containing 0.06 % carbon by 4ppm for 300 days.

The specification stability of Invar 36 is below 1 – 2 ppm per year. It needs very small carbon % about below 0.02 % and is increased by small magnesium and silicon content.

Moreover, heat processing is very crucial for Invar. It decreases the temporal growth of metal by aging it significantly. The temporal growth of invar is not persistent at the large rates, it diminishes with the passage of time and heat processing results in early ageing of invar hence lowering its temporal growth.

SuperInvar experiences phase transformation at low temperature limits that permanently damages its small coefficient properties. It happens at a temperature limit that is significantly based on chemistry. Moreover, Super Invar has majorly temperature based temporal stability and may be complicated to form. Normally SuperInvar is less commonly used in opto-mechanical applications than Invar 36.

Practical factors

Several practical factors important for an opto-mechanical engineer who needs to produce drawings and develop hardware using Invar.

Comparison of cost of Invar 36 with SS 304: Invar 36 is much costlier than stainless steel 304 almost five times more. Different opto-mechanical applications of invar include metering rods for telescopes, lens and mirror cells, interfacing spacers among optics and other shapes and laser cavity systems.

Machining: The ductility and hardness of invar made it tough to machine. Various machining engineers would admit that invar is tougher to machine than steel. In fact its machining order is not accepted by small scale machinists if the components are very composite, tightly toleranced or need high level of material removal. While machining of this metal, cutting devices wear significantly earlier and cutting speed will slow down. Hence machining of invar needs more patience on machinist side, larger lead period and more cost for the engineer to purchase the component.

CTE: The CTE of Invar 36 is very small as compare to austenitic stainless steel. Hence when steel LNG piping needs a process like pipe looping to take up the thermal shrinkage, using Invar alloy can offer straight piping, decreasing its production cost considerably.

Invar also normally called Nilo 36 or 64FeNi in the United States, is a nickel based steel alloy popular for its exceptionally small coefficient of thermal expansion. It is a solid solution single phase alloy. It is an essential material for use in scientific devices, It is also produced by Heanjia Super-Metals in China. Popular invar grades possess coefficient of thermal expansion up to 1.2 x 10(-6) per K or 1.2 ppm per oC. Although additionally pure grades can show smaller CTE up to 0.62 – 0.65 ppm per oC. Some grades may even exhibit negative thermal expansion properties. It is utilized in applications that need high specification stability for example in precision apparatus, clocks, seismic creep gauges, YV shadow mask frames, motor valves and antimagnetic watches. But Invar is susceptible to creeping.

Invar a controlled expansion alloy attaining very small expansion at ambient temperatures and is commonly used in applications that need lowest expansion. It is an Iron-Nickel alloy containing 36 percent nickel and remaining iron. Its lowest expansion at the ambient temperature limits makes it significant in several operations that demand high dimension constancy such as:

Locating equipments, bimetal thermostats, latest composite molds for aerospace engineering, size stability instruments and optical equipments, containers for LNG tankers, transmission lines for LNG, echo boxes and filters for telecommunication, magnetic shielding, small electrical transformers, meterology devices, scientific instruments, temperature maintainers such as regulators, clock balance wheels, pendulum clocks, precision condenser blades, radar & microwave cavity resonators, specific electronic enclosing, seals, spacers, special frames, large voltage transmission lines, CRT applications such as shadow masks, deflection clips and electron gun parts.

In addition of controlled thermal expansion, Iron-Nickel alloy 36 offers medium strength and fine toughness at temperatures below the liquefaction of helium about -452oF or -269oC. These characteristics combined with good weldability and required physical properties make alloy 36 a suitable option for several cryogenic applications. A refined form of Invar 36 is used in the LNG membrane containers. It is formed at HSM in wire, sheet, plate, strip, ribbon, tape and foil forms.

Specifications

 ASTM standard A 658 describes Invar or Nilo 36 plates made for welded pressure vessels. It is delivered in annealed form to meet the ASME boiler and pressure vessel code needs. The highest permissible stress in tension for ASME equivalent, SA 658 provided in section VIII, division 1 of ASME code is 16,200 psi or 112 N/mm2.

Heat processing

Annealing – ASTM A 658

Heating to 1450oF above or below 50oF (790 oC above or below 28oC), sustaining at this temperature for 30 minutes per inch of thickness, cooling in air. The hardness offered subsequent several annealing processing are described in the following table:

Temperature		Cooling	Received Hardness , Rockwell B
1200 of	650 oC	Air	87 to 88
1500 of	815 oC	Air	77 to 78
1800 of	980 oC	Air	70 to 71
1900 of	1040 oC	Air	66 to 68

 Stability annealing

Three level heat processing is done to obtain the desired combination of low expansion coefficient and great size stability:

1. Heating to 1525oF or 830oC, keeping for 30 minutes per inch of thickness, water cooling.
2. Reheating to 600of or 315oC, keeping for 1 hour per inch of thickness, air quenching.
3. Reheating to 205oF or 96oC, keeping for two days, air quenching.

Mechanical characteristics

Tensile properties & hardness

The standard room temperature (RT) mechanical characteristics of annealed and cold processed Invar 36 are described in the following table:

Property	Annealed	Cold processed, 15 %	Cold processed 25 %	Cold processed 30 %
Tensile strength,	71400 psi, 492 N/mm2	93,000 psi, 641 N/mm2	100,000 psi, 690 N/mm2	106,000 psi, 731 N/mm2
Yield strength 0.2 % offset	40000 psi, 276 N/mm2	65,000 psi, 448 N/mm2	89,500 psi, 617 N/mm2	95,000 psi, 655 N/mm2
Elongation in 2 inch	41 %	14	9	8
Reduction in area	72 %	64 %	62 %	59 %
Brinell hardness	131	187	207	217

Invar 36 is not susceptible to notching, the ratio of notched tensile strength to unnotched tensile strength is 1.10 at RT and at -320oF or -196oC.

Stability of mechanical characteristics after exposure to low temperatures for prolong periods

The mechanical characteristics of 36 % NiFe alloys are not influenced by the exposure to low temperatures for prolong time length. Subjecting to time length of many thousand hours at -320of or -196oC in presence or absence of applied stress has not changed its mechanical characteristics even in the case of notch sensitivity.

Fatigue properties

The RT fatigue strength at 10(8) cycles of polished rotating beam samples of annealed Nilo 36 is about 40,000 psi. The axial fatigue properties of cold rolled 0.040 inch or 1 mm thick sheet at RT and at -100oF or -73oC are noticed. The standard fatigue strength at 10(7) cycles for sheet components analyzed in alternating plane bending at stable strain amplitude are 27 x 10(3) to 31 x 10(3) psi or 186 to 214 N/mm2 at room temperature and 39 x 10(3) to 41 (3) psi or 269 to 283 N/mm2 at -320of or -196oC.

 Welding

For pressure vessel making, Invar 36 falls into the class of P-10g. The ASME tensile need for the weld metal is 65,000 psi or 448 N/mm2 at least. The welds of equivalent strength to the base metal are obtained using suitable filler metals like enhanced Nilo 36.

Welding of Invar 36 is straightforward. Although while equivalent thermal and mechanical properties are needed, either TIG or short circuiting enhancement of MIG (metal inert gas) welding procedures should be utilized. Argon is used as shielding gas however helium –argon combination may also be utilized. Generally, welding processes and safety measures are not much more severe as compare to welding of AISI 300 series of stainless steel grades.

The mechanical properties of TIG welded invar 36 are shown in the following table:

Condition	Temperature		Tensile strength		Yield strength, 0.2 % offset		Elongation, 1 inch	Charpy V notch impact strength
	oF	oC	Psi	N/mm2	Psi	N/mm2	%	Ft-lb	J
As welded	70	21	70,200	484	44,600	308	26 – 30	46	62
	-320	-196	120,800	833	88,800	612	23	–	–
	-423	-253	126,100	869	110,100	759	17 – 20	19	26
	70	21	72,000	496	43,600	301	25 – 30	56	76
	-320	-196	122,400	844	89,300	616	22	–	–
	-423	-253	129,800	895	105,500	727	20 – 20.5	24	32

Nilo 36 is also readily resistance welded using the same welding variable used for annealed austenitic stainless steels. The spot welds meeting the entire needs of MIL W 6858 have been produced in similar and dissimilar sheet thickness pairs from 1/16 inch to ¼ inch or 1.6 to 6.4 mm. Where thermal expansion factors allow, Nilo 36 alloy can be welded readily to itself and a different iron and Nilo alloys using general purpose filler metals such as Inconel Filler Metal 92 and Hastelloy W. The characteristics are described in the following table:

 Butt Weld Mechanical Properties of TIG welded different metals- Mechanical properties at the specified temperatures:

Nilo 36 welded to	Filler wire		Tensile strength		0.2 % yield strength		Elongation in 2 inch %	CVN
			Ksi	N/mm2	Ksi	N/mm2		Ft-lb	J
Nilo 36	92		79 Ksi	545	61 Ksi	421	27 %	77	104
SS 304	92		75 Ksi	517.1	42 Ksi	290	32 %	55	75
	W		76 Ksi	524.1	43 Ksi	297	28 %	–	–
SS 304L	92		76 Ksi	524.1	41 Ksi	283	33 %	54	73
	W		75 Ksi	517	38 Ksi	262	45 %	–	–
SS 316	92		75 Ksi	517.1	43 Ksi	297	26 %	40	54
	W		76 Ksi	524.1	43 Ksi	297	30 %	–	–
SS 321	92		75 Ksi	517.1	44 Ksi	303	27 %	45	61
	W		76 Ksi	524.1	45 Ksi	310	25 %	–	–
SS 347	92		76 Ksi	524.1	44 Ksi	303	24 %	38	52
	W		76 Ksi	524.1	45 Ksi	310	25 %	–	–
SS 1020	92		68 Ksi	469	46 Ksi	317	18 %	35	48
	W		68 Ksi	469	46 Ksi	317	20 %	–	–
-320of or -196oC
Nilo 36	92	137 Ksi		945	97 Ksi	669	35 %	46	62
SS 304	92	133 Ksi		917 W	53 Ksi	365	21 %	39	53
	W	129 Ksi		889 W	50 Ksi	348	20 %	–	–
SS 304L	92	135 Ksi		931 W	43 Ksi	297	28 %	30	41
	W	132 Ksi		910 W	44 Ksi	303	26 %	–	–
SS 316	92	133 Ksi		917 W	89 Ksi	614	20 %	28	39
	W	132 Ksi		910 W	79 Ksi	545	–	–	–
SS 321	92	138 Ksi		952 1	53 Ksi	365	29 %	29	39
	W	126 Ksi		869 W	54 Ksi	372	20 %	–	–
SS 347	92	141 Ksi		972 1	72 Ksi	496	30 %	20	27
	W	136 Ksi		938 W	71 Ksi	490	25 %	–	–
SS 1020	92	131 Ksi		903 0	95 Ksi	655	14 %	19	26
	W	123 Ksi		848 W	79 Ksi	545	12 %	–	–

 

Corrosion

The ordinary corrosion rates of Invar 36 are less than 1 mills per year or 0.025 mm per year in the industrial and marine conditions as shown in the following table:

Media	Duration		Corrosion rate
Industrial	10 years		0.7 mpy	0.02 mm/yr
Seawater	5 years		0.1 to 0.3	0.003 to 0.008
Localized corrosion rate
Stress corrosion
Marine	4 years, U Bend test		3 to 11	0.08 to 0.28
Kure Beach, N.C.
Pitting
Immersion in running marine water at speed of 2 fps or 0.6 m/sec at ambient temperature		Sheet material perforates quickly, .019 inch or 0.5 mm – within 16 days,0.060 inch or 1.5 m  within 49 days

 

Invar 36 is slightly resistant to stress corrosion cracking in tests when subjected to marine water or conditions. Quick fracture in acid chloride conditions of pH 2 is noticed at the high temperatures. It undergoes intense pitting in the widely exposed surfaces in the sea water running at 2 feet/sec or 0.6 m /sec.

 Physical Properties

Invar 36’s thermal expansion characteristics in the annealed condition are described in the following table. Cold processing often decreases the thermal expansion rate whilst chemistry changes usually increase the expansion rates.

Temperature		Mean coefficient of thermal expansion
oF	oC	Per oF	Per oC
-400 to 0	-240 to -18	1.20 x 10(-6)	2.16 x 10(-6)
-200 to 0	-129 to -18	1.10 x 10(-6)	1.98 x 10(-6)
0 to 200	-18 to 93	0.70 x 10(-6)	1.26 x 10(-6)
200 to 400	93 to 204	1.50 x 10(-6)	2.70 x 10(-6)
400 to 600	204 to 316	6.40 x 10(-6)	11.52 x 10(-6)

 

Property Data of Invar and other Low expansion alloys

Property	Invar 36	Free cue Invar 36	Low expansion 39	Low expansion 42	Low expansion 49
Chemistry
Carbon	0.12	0.12	0.08	0.10	0.10
Manganese	0.35	0.90	0.40	0.50	0.50
Silicon	0.30	0.35	0.25	0.25	0.40
Chromium	–	–	–	–	–
Nickel	36	36	39	42	49
Fe	rem	Rem	Rem	Rem	Rem
Physical properties
Specific gravity	8.05	8.05	8.08	8.12	8.25
Density, lb/cu-inch	0.291	0.291	0.292	0.293	0.298
Thermal conductivity (20 to 100oC)
Cal/cm3/sec/oC	0.0250	0.0250	0.0253	0.0257	0.030
Btu/hr/sq ft/F/inch	72.6	72.6	73.5	74.5	90
Electrical resistivity
Micro-ohm-cm	82	82	–	72	48
Ohms/cir mil-ft	495	495	–	430	290
Curie temp, oC	280	280	340	380	500
Melting point, oC	1425	1425	1425	1425	1425
Specific heat	0.123	0.123	0.121	0.120	0.120
Coefficient of thermal expansion
25oC to 100oC	1.18	1.60	2.20	4.63	8.67
25 oC to 200 oC	1.72	2.91	2.66	4.76	9.38
25 oC to 300 oC	4.92	3.99	3.39	4.88	9.30
25 oC to 350 oC	6.60	7.56	4.68	5.02	9.25
25 oC to 400 oC	7.82	8.88	6	5.63	9.14
25 oC to 450 oC	8.82	9.30	7.22	6.90	9.65
25 oC to 500 oC	9.72	10.66	8.17	7.78	9.72
25 oC to 600 oC	11.35	12	9.60	9.90	10.80
25 oC to 700 oC	12.70	12.90	11	11	11.71
25 oC to 800 oC	13.45	13.60	11.95	11.99	12.57
25 oC to 900 oC	13.85	14.60	12.78	12.78	13.29
25 oC to 1000 oC	–	–	13.42	–	–
77 of to 212 of	0.655	0.89	1.22	2.57	4.80
77 of to 392 of	0.956	1.62	1.48	2.54	5.20
77 of to 572 of	2.73	3.32	1.88	2.71	5.17
77 of to 662 of	3.67	4.20	2.40	2.78	5.14
77 of to 752 of	4.34	4.93	3.34	3.14	5.07
77 of to 842 of	4.90	5.45	4.01	3.83	5.36
77 of to 932 of	5.40	5.92	4.54	4.32	5.40
77 of to 1112 of	6.31	6.67	5.33	5.50	6
77 of to 1292 of	7.06	7.17	6.11	6.12	6.51
77 of to 1472 of	7.48	7.56	6.64	6.66	7.06
77 of to 1652 of	7.70	8.12	7.10	7.10	7.38
77 of to 1832 of	–	–	7.45	–	–
Mechanical properties (annealed)
Tensile strength, psi	65,000	65,000	75,000	82,000	85,000
Yield strength, psi	40,000	40,000	38,000	40,000	40,000
Elongation in 2 inch, %	35	35	30	30	35
Hardness, Rockwell	B-70	B-70	B-76	B-76	B -70
Elastic modulus, x 10(6) psi	20.5	20.5	21	21	24

 

Invar’s Applications

Traditional applications

After invention of Invar 36, soon its applications were also found where low thermal expansion was required. Surveying tapes and wires and pendulums for grandfather clocks were crucial traditional uses. Invar alloys replaced platinum for glass sealing wire and saved a large cost in 1920. They were utilized in light bulbs and electronic vacuum tubes for radios.

The application area extended in 1930. Nilo alloys were utilized as bimetals in thermostats for temperature control. A copper coated Nilo 42 alloy was being utilized in lead in seals of incandescent lights. A 56% Nickel-iron alloy was utilized to construct measuring equipments for testing gauges and machine components.

During second world war, invar’s significance was widely increased, specifically in military. The applications continued to extend from 1950 to 1960. Nilo 36 and other Nilo alloys were required for controlled expansion parts in bimetals for circuit breakers, motor controls, TV temperature control spring, equipment and heater thermostats, aerospace and automotive controls, heating and air conditioning.

Glass to metal and ceramic to metal sealing were in huge demand. Various Invar effect inducing Nilo alloys have thermal properties identical to glass and ceramics, these became a recommended option for such applications. They were also utilized for sealing applications of semiconductors and microprocessors such as in pin feed throughs, packing and lid sealing.

Latest applications

The need for thermostat metals increased in 1980 and 1990s. Invar 36 has been discovered very significant for containers that are utilized to transport liquid natural gas on tankers. It reduces the cryogenic contraction.

Nilo 36 has been more commonly used in shadow masks in high definition CRT TV tubes. In America, it has been used in deflection springs for reposition the mask to the color phosphors. In Japan and Europe a doming effect of shadow mask used iron-nickel 36 alloy.

Latest applications include structural parts in precision laser and optical measuring equipments and wave guide tubes. Invar alloys are utilized in microscopes, large mirrors in telescopes and different scientific instruments that include mounted lenses.

Nilo 36 is used for composite molds in aerospace industry. Modern generation of aircraft, specifically include invar 36 for molds that keep tight specification tolerances while advanced complex components are used at medium high temperatures. Invar helps in increasing modern science to larger levels with applications in orbit satellites, lasers, ring laser gryoscopes and high tech applications.

At lower than room temperature, invar alloys have small expansion. Below liquefaction temperature of nitrogen about -196oC, their expansion stops to zero.

Fe-Ni alloys containing nickel below 36 % content are hardly used in controlled expansion applications basically for two causes: they can attain martensite configuration that abruptly increases the expansion and these alloys offer small curie temperature that decreases their application temperature limits. Hence Invar 36 containing 36 % nickel and other alloys with higher nickel percentage are accepted as low expansion alloys.

Heanjia Super-Metals produce various forms of Invar in our production unit located in Hebei China.  Contact us for ordering Invar alloys today.