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EnquireWith increasing frequency, metalworking shops are asked to machine parts and components from Invar alloy (UNS K), a 36% nickel-iron alloy known for its unique low expansion properties. Invar alloy has a rate of thermal expansion approximately one tenth that of carbon steel at temperatures up to 400 oF (204 oC). This characteristic makes the alloy a candidate for a growing number of applications &#;
(a) &#; where dimensional changes due to temperature variation must be minimized (radio and electronic devices, aircraft controls, optical and laser systems, etc.)
(b) &#; in conjunction with high expansion alloys in applications where motion is desired when the temperature changes (bimetallic thermostats, rod and tube assemblies for temperature regulators, etc.)
This alloy is available in two variations. One is the conventional Invar alloy, used generally for its optimum low expansion properties. The second is a variation of the basic alloy known as Free-Cut Invar &#;36&#;® alloy (UNS K and ASTM F). This alloy has shown improved machinability for applications where high productivity is important. It is also a 36% nickel-iron alloy, but with a small addition of selenium (Fig. 1) to enhance machinability.
Free-Cut Invar &#;36&#; alloy, the world&#;s first free-machining Invar alloy, has been used by machine shops that are producing high volumes of parts like controls for hot water heaters, filters for microwave instruments, precision parts for optical mounting in lenses, etc.
High-production shops have reported the free-machining alloy to be advantageous also when performing several different machining operations, particularly when parts have intricate shapes and/or require working to close tolerances.
Compared with the conventional Invar grade, the downside for Free-Cut Invar &#;36&#; alloy is negligible. Its coefficient of thermal expansion is only slightly higher than that of the basic alloy; not enough, generally, to make a difference in part performance.
With the free-machining alloy, there is a minimal loss in both transverse toughness and corrosion resistance. It also may be necessary to clean and passivate the free-cut alloy to remove selenides from the surface.
However, a good case can be made for the free-cut alloy because it machines without a hassle and permits parts productivity gains frequently reaching 250%. From the machinist&#;s point-of-view, it becomes difficult to justify not using the free-machining grade.
Both Invar 36 alloys are soft like Type 304 and Type 316 austenitic stainless steels; the free-cut variation, in particular, machines similar to those two stainless grades. They all have the same high work-hardening rate, which requires care in machining.
The standard Invar alloy produces stringy, gummy chips which &#;birdnest&#; around the tools and interfere with coolant flow. Chips have to be broken up using chip breakers. Chip breakers are also used with the free-cut alloy, but they do not have to be as deep as for the basic alloy because the free-cut chips are more brittle.
Large, sharp and rigidly supported tooling is recommended for both grades. A positive feed rate should be maintained for all machining operations to avoid glazed, work hardened surfaces. In some cases, increasing the feed and reducing the speed may be necessary. Dwelling, interrupted cuts or a succession of thin cuts should be avoided.
In general, the free-cut Invar alloy has produced a good surface finish as well as higher productivity. During all cutting operations, with both materials, care must be taken to ensure good lubrication and cooling.
The two grades are very ductile, thus readily cold headed and formed. Stamping from cold-rolled strip is easily accomplished. Parts may be deep drawn from properly annealed strip.
Fabrication does add stresses which, unrelieved, can change the thermal expansion behaviour. When that happens, parts placed in service as-fabricated may not meet design requirements. Thus, annealing and stress relieving thermal treatments may be needed to promote structural uniformity and dimensional stability.
After severe forming, bending and machining, relief of stresses induced by these operations can be accomplished by annealing at temperatures of 760 oC ( oF) to 980 oC ( oF) long enough to thoroughly heat through the section. However, these alloys will oxidize readily at such high temperatures.
When annealing cannot be done in a non-oxidizing atmosphere (vacuum, dry hydrogen, dissociated ammonia, argon, etc.) sufficient material must be present to allow cleaning by light grinding, pickling, etc., after annealing. For sections having light finishing cuts or grinding performed after annealing, stress relief is accomplished by heating to 315 oC (600 oF) to 425 oC (800 oF) long enough to uniformly heat through the work pieces.
There is no single set of rules or simple formula that is best for all machining situations. In addition to the materials used, job specifications and equipment must be considered in determining the most applicable machining parameters.
Furthermore, operations such as turning on automatic screw machines, turret lathes and CNC lathes involve so many variables that it is impossible to make specific recommendations which would apply to all conditions. That&#;s why the following parameters should serve only as a starting point for initial machine setup.
Properly ground tools are essential in turning Invar alloy. Fig. 2 illustrates suggested starting geometries for high-speed steel single-point turning tools. Tools with a 5 to 10o positive top rake angle will generate less heat and cut more freely with a cleaner surface.
As large a tool as possible should be used to provide a greater heat sink, as well as a more rigid setup. To ensure adequate support for the cutting edge, the front clearance angle should be kept to a minimum, i.e., 7 to 10o, as shown. The Invar alloys require tools ground with top rake angles on the high side of the 5 to 10o range to control the chips. They may also require increased side clearance angles to prevent rubbing and localized work hardening.
Carbide tools in single-point turning operations will allow higher speeds than high-speed tool steels. However, carbide tooling requires even greater attention to rigidity of tooling and the workpiece. Interrupted cuts should be avoided.
Either blade-type or circular cutoff tools are used for Invar alloy applications. Blade-type cutoff tools usually have enough bevel for side clearance, i.e., 3o minimum, but may need greater clearance for deep cuts. In addition, they should be ground to provide for top rake and front clearance.
The front clearance angle is 7 to 10o; a similar angle is used for top rake, or a radius or shallow concavity may be ground instead. The end cutting edge angle may range from 5o or less to 15o, with the angle decreasing for larger-diameter stock.
Angles for circular cutoff tools are similar to those for blade-type, including a top rake angle of 7 to 10o, as shown in Fig. 3. Since circular cutoff tools are more rigid than blade-type, they can withstand more shock. Therefore, they may be preferred for automatic screw machine operations where they are fed into drilled or threaded holes. Because of their size, they also dissipate heat better.
Carbide-tipped cutoff tools may be used. However, shock loading from interrupted cuts must be considered when selecting carbide.
Form tools are usually dovetail or circular. Speeds and feeds for form tools are influenced by the width of the tool in relation to the diameter of the bar, the amount of overhang and the contour or shape of the tool. Generally, the width of the form tool should not exceed 1½ times the diameter of the workpiece; otherwise, chatter may become a problem.
Dovetail form tools should be designed with a front clearance angle of 7 to 10o, and ground with a front rake angle of 5 to 10o. Angles for circular form tools are similar, as shown in Fig. 4. Higher rake angles within the 5 to 10o range may be used for roughing operations, and lower rake angles for finishing.
Design of the tool should incorporate enough side clearance or relief angles, typically 1 to 5 o depending on depth of cut, to prevent rubbing and localized heat buildup, particularly during rough forming. It may be necessary to round corners. A finish form or shave tool may be necessary to obtain the final shape, especially for deep or intricate cuts.
Carbide-tipped tooling may be used for forming operations so long as shock loading from interrupted cuts is duly considered.
Table 1 shows reasonable feeds and speeds for single-point and box tool turning of Carpenter Invar &#;36&#; alloy and Free-Cut Invar &#;36&#; alloys. Table 2 shows feeds and speeds for cutoff and forming operations.
Certain rules should be observed in drilling the Invar alloys &#; (a) work must be kept clean and chips removed frequently to avoid dulling the drill (b) drills must be carefully selected and correctly ground (c) drills must be properly aligned and the work firmly supported (d) a stream of cutting fluid must be properly directed at the hole and (e) drills should be chucked for shortest drilling length to avoid whipping or flexing, which could break drills or cause inaccurate work.
When working with the Invar &#;36&#; alloys, it is advisable to use a sharp three-cornered punch rather than prick punch to avoid work hardening the material at the mark. Drilling templates or guides may also be useful.
To relieve chip packing and congestion, drills occasionally must be backed out. The general rule is to drill to a depth of three to four times the diameter of the drill for the first bite, one or two diameters for the second bite, and around one diameter for each of the subsequent bites. A groove ground parallel to the cutting edge in the flute for chip clearance will allow drilling deeper holes per bite, particularly with larger-size drills. The groove breaks up the chip for easier removal.
Drills should not be allowed to dwell during cutting. Allowing the drill to dwell or ride glazes the bottom of the hole, making restarting difficult. Therefore, when relieving chip congestion, drills must be backed out quickly and reinserted at full speed to avoid glazing.
Drill feed is important in determining the rate of production. Carefully selected, proper feeds and speeds can increase both drill life and production between grinds. Feeds and speeds for various drill sizes are indicated in Table 3.
It is especially important to grind tools correctly. Fig. 5 shows suggested geometries for high-speed drills to be used with the Invar alloys.
Two types of holes are prepared for tapping &#; the open or through hole, and the blind hole. For open or through holes, taps of either the spiral-fluted or the straight-flute spiral-pointed type can be used, as shown in Fig. 6. They are especially desirable when tapping the relatively soft Invar alloys because they provide adequate chip relief.
The spiral-pointed tap cuts with a shearing motion. It has the least amount of resistance to the thrust, and the entering angle deflects the chips so that they curl out ahead of the tap. This prevents packing in the flutes, which frequently causes tap breakage. When backing out a spiral-pointed tap, there is less danger of roughing the threads in the tapped part.
Spiral-pointed taps should not be used in blind or closed holes unless there is sufficient untapped depth to accommodate the chips. To tap blind holes, special spiral-pointed bottoming taps are available. However, spiral-fluted taps with a spiral of the same hand as the thread are suggested, since they are designed to draw chips out of the hole.
Tapping speeds for both Invar alloys, using three standard tooling materials, are shown in Table 4.
Various high-speed steel cutters are shown in Fig. 7. Tooling with carbide inserts also may be used for the two Invar grades. As a general rule, the finest finishes are obtained with helical or spiral cutters running at high speed, particularly for cuts over 19 mm (0.76 in.) wide.
Helical cutters cut with a shearing action and, as a result, cut more freely and with less chatter than straight-tooth cutters. Coarse-tooth or heavy-duty cutters work under less stress and permit higher speeds than fine-tooth or light-duty cutters. They also have more space between the teeth to aid in chip disposal.
For heavy, plain milling work, a heavy-duty cutter with a faster, 45o left-hand spiral is preferred. The higher angle allows more teeth to contact the work at the same time, thereby reducing chatter. Table 5 shows reasonable feeds in inches per tooth for both alloys based on depth of cut, milling speed, cutter diameter and type of tooling used.
High-speed steel broaches should be used for the Invar materials. A broach is a simple tool to handle because the broach manufacturer builds into it the necessary feed and depth of cut by steps from one tooth to another. Basically, a broach can incorporate the roughing cut, the semi-finished cut and the final precision cut &#; as shown in Fig. 8 &#; or any combination of these operations.
Table 6 shows normal broaching parameters for both the Invar alloy and the free-cut alloy. Of course, proper lubrication and cooling are also important. Sulfochlorinated oils diluted with paraffin, rather than water-soluble oils, are suggested.
Several typical high-speed reamers are shown in Fig. 9. Carbide-tipped reamers also may be used with these alloys. Spiral-fluted reamers with a helix angle of approximately 7o are suggested. There is less tendency for this type of reamer to chatter, and better chip clearance is secured. This is particularly true for interrupted custs, such as in a keyway.
Left-hand (reverse) spiral reamers with right-hand cutting or rotation are suggested. Right-hand spiraling of the flutes with right-hand rotation helps the tool to cut more freely, but makes it feed into the work too fast.
When tapered holes must be reamed, any one of the standard taper reamers, ground for Invar alloy, will provide a satisfactory finish. However, the hole first must be carefully drilled or bored.
Feeds and speeds for both roughing and finishing operations are listed in Table 7 for both high-speed steel and carbide tooling. When reaming, cutting fluid be must considered to avoid overheating. Besides providing good lubrication, the cutting fluid must be a coolant to carry away the heat that would otherwise burn the cutting edges of the reamer.
The cutting fluid also must be kept clean. Reaming produces slivers and very fine chips which can float in the cutting fluid and get into the work very easily, damaging the finish, especially if the machine is equipped with a recirculating system.
Two types of cutting fluids can be used in machining the Invar alloys &#; sulfochlorinated oils recognized for their ability to prevent seizing, and emulsifiable fluids which have greater cooling capacity. Most machining operations require a sulfochlorinated oil.
When machine shops working the Invar &#;36&#; alloy experience problems, they might re-examine their procedures and correct some of the most common causes. For example:
A &#; Parts productivity is not satisfactory, finishes are not acceptable, difficult shapes cannot be machined properly. Solution: try the free-cut variation of Invar alloy.
B &#; Machined surfaces are glazed and work hardened. Solution: Be sure to maintain a positive feed rate.
C &#; Tools are chattering, not cutting cleanly, producing chips that interfere with coolant flow. Solution: Could be caused by using tools with improper geometry. Follow guidelines given in tool diagrams.
D &#; Tool heats excessively. Solution: Make sure the tool is heavy enough to carry off generated heat. Also check the cutting fluid. It might be too rich in sulphur-base oil; thus should be cut back with a coolant such as paraffin-base oil.
The information provided above is freely available in the public domain, and while we endeavour to keep the information up to date and correct, we make no representations or warranties of any kind.
In no event will we be liable for any loss or damage including without limitation, indirect or consequential loss or damage, or any loss or damage whatsoever.
Should you choose to use any of the information below it is strictly at your own risk.
Please contact City Special Metals if you have any further questions or would like to place an order for Invar.
03
Jul
by alloy
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:
Invar consisting of 64 percent iron and 36 percent nickel called as Invar 36 or Nilo 36
SuperInvar consisting of 63 % iron, 32 % nickel, 5 % cobalt
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
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 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) = LT2&#;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 oF 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
of
650 oC
Air
87 to 88
of
815 oC
Air
77 to 78
of
980 oC
Air
70 to 71
of
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:
Heating to oF or 830oC, keeping for 30 minutes per inch of thickness, water cooling.
Reheating to 600of or 315oC, keeping for 1 hour per inch of thickness, air quenching.
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,
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
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 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
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
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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
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.
0.
0.
0.
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
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 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 of
6.31
6.67
5.33
5.50
6
77 of to of
7.06
7.17
6.11
6.12
6.51
77 of to of
7.48
7.56
6.64
6.66
7.06
77 of to of
7.70
8.12
7.10
7.10
7.38
77 of to 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 . They were utilized in light bulbs and electronic vacuum tubes for radios.
The application area extended in . 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 to . 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 and s. 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.
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