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Samples of Invar The coefficient of thermal expansion of nickel/iron alloys is plotted here against the nickel percentage (on a mass basis) in the alloy. The sharp minimum occurs at the Invar ratio of 36% Ni.
Invar, also known generically as FeNi36 (64FeNi in the US), is a nickel&#;iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α). The name Invar comes from the word invariable, referring to its relative lack of expansion or contraction with temperature changes,[1] and is a registered trademark of ArcelorMittal.[2]
The discovery of the alloy was made in by Swiss physicist Charles Édouard Guillaume for which he received the Nobel Prize in Physics in . It enabled improvements in scientific instruments.[3]
Properties
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Like other nickel/iron compositions, Invar is a solid solution; that is, it is a single-phase alloy. In one commercial version it consists of approximately 36% nickel and 64% iron.[4] The invar range was described by Westinghouse scientists in as "30&#;45 atom per cent nickel".[5]
Common grades of Invar have a coefficient of thermal expansion (denoted α, and measured between 20 °C and 100 °C) of about 1.2 × 10&#;6 K&#;1 (1.2 ppm/°C), while ordinary steels have values of around 11&#;15 ppm/°C.[citation needed] Extra-pure grades (<0.1% Co) can readily produce values as low as 0.62&#;0.65 ppm/°C.[citation needed] Some formulations display negative thermal expansion (NTE) characteristics.[citation needed] Though it displays high dimensional stability over a range of temperatures, it does have a propensity to creep.[6][7]
Applications
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Invar is used where high dimensional stability is required, such as precision instruments, clocks, seismic creep gauges, color-television tubes' shadow-mask frames,[8] valves in engines and large aerostructure molds.[9]
One of its first applications was in watch balance wheels and pendulum rods for precision regulator clocks. At the time it was invented, the pendulum clock was the world's most precise timekeeper, and the limit to timekeeping accuracy was due to thermal variations in length of clock pendulums. The Riefler regulator clock developed in by Clemens Riefler, the first clock to use an Invar pendulum, had an accuracy of 10 milliseconds per day, and served as the primary time standard in naval observatories and for national time services until the s.
In land surveying, when first-order (high-precision) elevation leveling is to be performed, the level staff (leveling rod) used is made of Invar, instead of wood, fiberglass, or other metals.[10][11] Invar struts were used in some pistons to limit their thermal expansion inside their cylinders.[12] In the manufacture of large composite material structures for aerospace carbon fibre layup molds, Invar is used to facilitate the manufacture of parts to extremely tight tolerances.[13]
In the astronomical field, Invar is used as the structural components that support dimension-sensitive optics of astronomical telescopes.[14] Superior dimensional stability of Invar allows the astronomical telescopes to significantly improve the observation precision and accuracy.
Variations
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There are variations of the original Invar material that have slightly different coefficient of thermal expansion such as:
citation needed
][example needed
]α &#; 5.3 ppm/°C
, matching that of silicon, is widely used as lead frame material for integrated circuits, etc.[citation needed
]5 ppm/°C
) and form strong bonds with molten borosilicate glass, and because of that are used for glass-to-metal seals, and to support optical parts in a wide range of temperatures and applications, such as satellites.[citation needed
]Explanation of anomalous properties
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A detailed explanation of Invar's anomalously low CTE has proven elusive for physicists.
All the iron-rich face-centered cubic Fe&#;Ni alloys show Invar anomalies in their measured thermal and magnetic properties that evolve continuously in intensity with varying alloy composition. Scientists had once proposed that Invar's behavior was a direct consequence of a high-magnetic-moment to low-magnetic-moment transition occurring in the face centered cubic Fe&#;Ni series (and that gives rise to the mineral antitaenite); however, this theory was proven incorrect.[15] Instead, it appears that the low-moment/high-moment transition is preceded by a high-magnetic-moment frustrated ferromagnetic state in which the Fe&#;Fe magnetic exchange bonds have a large magneto-volume effect of the right sign and magnitude to create the observed thermal expansion anomaly.[16]
Wang et al. considered the statistical mixture between the fully ferromagnetic (FM) configuration and the spin-flipping configurations (SFCs) in Fe
3Pt with the free energies of FM and SFCs predicted from first-principles calculations and were able to predict the temperature ranges of negative thermal expansion under various pressures.[17] It was shown that all individual FM and SFCs have positive thermal expansion, and the negative thermal expansion originates from the increasing populations of SFCs with smaller volumes than that of FM.[18]
See also
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References
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For more information, please visit Soft Magnetic Alloy.
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Above are typical milling cutters, with suggested geometries.
Today's metalworking shops frequently machine parts and components from Invar alloy (UNS K), a 36% nickel-iron alloy known for its unique low-expansion properties. Although the material has been fairly popular for years, shops can still benefit from a better understanding of its machining parameters.
Produced by Carpenter Technology Corp., Reading, Pa., the alloy has a rate of thermal expansion approximately one tenth that of carbon steel at temperatures up to 400 8F (204 8C).
The 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 to enhance machinability.
Free-machining variation
Free-Cut Invar "36" alloy, the first free-machining Invar alloy, is used by machine shops producing high volumes of parts like controls for hot water heaters, filters for microwave instruments, and precision parts for optical mounting in lenses.
Compared with the conventional Invar grade, the downside for Free-Cut Invar "36" alloy is negligible. Its coefficient of thermal expansion is slightly higher than that of the basic alloy, which is not enough 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 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 often boosts production by as much as 250%.
Both Invar 36 alloys are soft like Type 304 and Type 316 austenitic stainless steels and have the same high workhardening rate, requiring care in machining. The free-cut variation, in particular, machines similar to the two stainless grades.
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 chipbreakers. Chipbreakers are also used with the free-cut alloy, but they do not have to be as deep as those used for the basic alloy because the free-cut chips are more brittle.
Large, sharp, and rigid tooling is recommended for both grades. A positive feedrate should be maintained for all machining operations to avoid glazed, workhardened surfaces. In some cases, increasing the feed and reducing the speed may be necessary. In addition, shops should avoid dwelling, interrupted cuts, or a succession of thin cuts.
In general, the free-cut Invar alloy produces a good surface finish and increases productivity. However, during all cutting operations, both materials require proper lubrication and cooling.
Machining parameters
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.
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. So, the following parameters should serve only as a starting point for initial machine setup.
Turning
Properly ground tools are essential in turning Invar alloy. In addition, as large a tool as possible should be used to provide a greater heat sink, as well as a more rigid setup. The front clearance angle should also be minimized to ensure adequate support for the cutting edge.
Invar alloys require tools ground with positive top rake angles on the high side of the 5 8 to 10 8 range to control the chips. They may also require increased side-clearance angles to prevent rub-bing and localized workhardening.
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 (3 8 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 8 to 10 8. A similar angle should be used for the top rake, or a radius or shallow concavity may be ground instead. The end-cutting edge angle may range from less than 5 8 to 15 8, 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 8 to 10 8.
Since circular cutoff tools are more rigid than blade-type and can withstand more shock, they are preferred for automatic screw machine operations where they are fed into drilled or threaded holes. Because of their size, they also dissipate heat better.
While shops can use carbide-tipped cutoff tools, they should also consider the shock loading from interrupted cuts.
When turning Invar alloys with form-tools, shops need to keep in mind that speeds and feeds 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, if the width of the form-tool exceeds 1.5 3 the diameter of the workpiece, chatter becomes a problem.
Dovetail form tools should be designed with a front clearance angle of 7 8 to 10 8 and ground with a front rake angle of 5 8 to 10 8.
Angles for circular form tools are similar. Higher rake angles within the 5 8 to 10 8 range may be used for roughing operations, while lower rake angles are more applicable for finishing.
Tooling side clearance or relief angles need to be 1 8 to 5 8 depending on depth of cut. This prevents rub-bing and localized heat buildup, particularly during rough forming. It may also 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 also be used for forming operations as long as consideration is given to shock-loading from interrupted cuts.
Drilling
Certain rules need be to observed in drilling Invar alloys. For example, the workpiece must be kept clean and chips removed frequently to avoid drill dulling. In addition, drills must be carefully selected and correctly ground. Also, drills must be properly aligned and the work firmly supported.
Other requirements include directing a stream of cutting fluid at the hole. And, finally, drills should be chucked for the shortest drilling length to avoid whipping or flexing.
When working with Invar "36" alloy, it is advisable to use a sharp three-cornered punch rather than a prick-punch to avoid workhardening the material at the mark. Drilling templates or guides would also be useful.
To relieve chip packing and congestion, drills must occasionally 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 approximately one diameter for each of the sub-sequent bites.
Grinding a groove parallel to the cutting edge in the flute for chip clearance allows the drilling of deeper holes per bite, particularly with larger-size drills. The groove also breaks up the chip for easier removal.
Allowing the drill to dwell or ride during cutting 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.
While drill feed is important in determining the rate of production, carefully selecting proper feeds and speeds can increase both drill life and production between re-grinds.
Tapping
For open or through holes, shops can use either spiral-flute or straight-flute, spiral-pointed taps. Both provide adequate chip relief in tapping the relatively soft Invar alloys.
The spiral-pointed tap cuts with a shearing motion and has less resistance to the thrust. And, the entering angle deflects the chips so that they curl out ahead of the tap, preventing packing in the flutes. Also, when backing out a spiral-pointed tap, there is less danger of roughing the threads in the tapped hole.
Spiral-pointed taps should not be used in blind or closed holes unless there is sufficient untapped depth to accommodate the chips. In order to properly tap blind holes, special spiral-pointed bottoming taps are necessary. However, spiral-fluted taps with a spiral of the same hand as the thread can do the job, since they are designed to draw chips out of the hole.
Milling
Carbide inserts may be used for both 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 more freely and with less chatter than straight-tooth cutters. Coarse-tooth or heavy-duty cutters work under less stress, permitting 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, 45 8 left-hand spiral is preferred. The higher angle lets more teeth contact the work at the same time, thereby reducing chatter.
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