Invar Alloy Machining – A Guide

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About Invar Alloys

With 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-Machining Variation

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.

Fabricating Characteristics

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.

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.

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.

Turning

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. 

Drilling

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.

Tapping

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.

Milling

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. 

Broaching

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. 

Reaming

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. 

Cutting Fluids

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.

Summary

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.

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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.

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After 100 Years, The Uses for Invar Continue to Multiply

The metals world in observed the centennial of the discovery of the low expansion alloy known as Invar (UNS K). This remarkable alloy has been so important to scientific advancement that it earned the Nobel Prize in for its inventor, Charles-Edouard Guillaume, the first and only scientist in history to be so honored for a metallurgical achievement.

Still exhibiting the "Invar Effect" that defies understanding, Invar has bred an entire family of low expansion, nickel-iron alloys that are used today in a wide variety of both commonplace and high technology applications. Commercial uses have proliferated in fields as diverse as semiconductors, television, information technology, aerospace and cryogenic transport.

The Discovery

Guillaume, employed by the International Bureau of Weights and Measures, was looking in for a metal for geodesic tapes and wires that would not change in length when exposed to temperature variations. In addition, he wanted a cost-effective material for reference bars of perfectly defined length that could be used as secondary standards throughout the world.

He had many heats of nickel-iron alloys melted, handicapped often by feed stock that was contaminated. In the process, experimenting with nickel contents of 30% to 60%, Guillaume discovered that the coefficient of expansion at room temperature was lowest at a nickel level of 36%.

With 36% nickel, in fact, the alloy exhibited the least amount of thermal expansion of any alloy known. Since Guillaume considered the expansion of his new alloy "invariable", it eventually became known as Invar.

Guillaume published a graph similar to Fig. 1 visualizing the unique thermal expansion behavior of an "Invar-Effect" alloy. The expansion characteristic of these nickel-iron alloys is broadly determined by ferromagnetism.

These alloys exhibit very low expansivity below their Curie temperature (the temperature below which they are ferromagnetic). This low thermal expansivity anomaly, often referred to as the "Invar Effect", is related to spontaneous volume magnetostriction where lattice distortion counteracts the normal lattice thermal expansivity.

Above the Curie temperature, 36% nickel and other nickel-iron alloys expand at a high rate because they are no longer ferromagnetic. A number of theories have been proposed to explain this phenomenon. Although these theories have provided some insight, the mechanism is not yet sufficiently understood.

 

Early Applications

Once the 36% nickel and its companion nickel-iron alloys were discovered, it didn't take long to find applications that could utilize their low thermal expansivity. Surveying tapes and wires, as well as pendulums for grandfather clocks became important early applications. Nickel-iron alloys were substituted for platinum for glass sealing wire at great cost savings in the 's. They also were used in light bulbs and electronic vacuum tubes for radios.

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Uses expanded further in the 's. Nickel-iron alloys were used as one of the bimetals in thermostats for temperature control. A copper-coated 42% nickel-balance iron alloy was being used in the lead-in seals of incandescent lights. A 56% nickel alloy was used to make measuring devices for testing gauges and machine parts.

World War II greatly increased the demand for "Invar Effect" nickel-iron alloys, particularly by the military. Demand escalated dramatically for alloys used in vacuum tubes and other products in the fast growing electronics industry.

Applications continued to spread in the prosperous 's and 's. The 36% nickel and other nickel-iron alloys were needed for low expansion components in bimetals for circuit breakers, motor controls, TV temperature compensating springs, appliance and heater thermostats, aerospace and automotive controls, heating and air conditioning, etc.

Glass-to-metal and ceramic-to-metal seals were in great demand. Since several of the "Invar-Effect" nickel-iron alloys had thermal characteristics similar to those of glass and ceramics, they were the natural choice for such applications. They were also used for the sealing needs of semi-conductors and microprocessors. These include pin feed-throughs, packaging and lid seals.

Newer Applications

The demand for thermostat metals continued to grow in the 's and 's. The 36% nickel alloy has been found quite useful for containers used to transport liquid natural gas on tankers. The alloy minimizes cryogenic shrinkage.

A 36% nickel-iron alloy has been used more recently for shadow masks in high-definition CRT (television) tubes. It has been used for this application in Japan and Europe as a solution to the "doming effect" of the shadow mask. In the U. S., the 36% nickel alloy with a high expansion alloy has been used in deflection springs which reposition the mask to the color phosphors.

Newer applications use nickel-iron alloys for structural components in precision laser and optical measuring systems, and wave guide tubes. These alloys have been used in microscopes, in support systems for giant mirrors in telescopes and in a variety of scientific instruments requiring mounted lenses.

36% nickel-balance iron alloys have been used for composite molds by the aerospace industry. New generation aircraft, in particular, need 36% nickel-balance iron alloys for molds that will hold tight dimensional tolerances while advanced composites are cured at moderately high temperatures. The "Invar-Effect" family of alloys, in fact, is helping to raise modern science to higher levels with applications in orbiting satellites, lasers, ring laser gyroscopes and a host of high-tech applications.

Nature of Expansivity As shown in Fig. 1, the thermal expansion curve for "Invar-Effect" alloys consists of a low expansivity portion and a high expansivity portion. Below their Curie Temperature, these alloys are magnetic and their expansivities are anomalously low. Above it, they expand at a normal, high rate. This high rate extends to the melting point of the alloy. It can be noticed that there is a transition range between the low expansion and high expansion components. This transition is evidently related to the deterioration of ferromagnetism as the alloy approaches the Curie Temperature

Below room temperature, these alloys have low expansivity. Below approximately liquid nitrogen temperature (-196°C), their expansivity declines to near zero. The low expansion portion of the curve is most important because its slope defines the coefficient of expansion, and its length describes the useful temperature range of low expansivity.

Inflection temperature can be determined by a slope intercept method. It is the temperature at which the slopes taken from the low expansivity and the high expansivity curve portions of the expansion curve intersect. Measuring the Curie Temperature is laborious. Whereas, the inflection temperature can be determined simply from expansion curves.

In his early investigations, Guillaume learned how variations in nickel content affect the coefficient of expansion of the nickel-iron alloys. He found that expansion reached a minimum with 36% nickel (Fig. 2), and that on either side of this value the expansion coefficient increased dramatically.

 

Alloys with nominally less than 36% nickel are seldom used for controlled expansion applications primarily for two reasons: (a) these alloys can transform to martensite, which drastically increases the expansivity, and (b) they have low Curie Temperatures, which reduce the temperature range over which they may be used. Therefore, the alloys with 36% or more nickel are usually considered for controlled expansion applications.

The Invar Family

All of the alloys in the Invar family are nickel-iron or nickel-iron-cobalt alloys, and all exhibit face-centered cubic crystal structure. As nickel content increases from 36%, thermal expansivity and Curie Temperature also increase. Curie Temperature increases from 280°C (536°F) for 36% nickel to greater than 565°C (°F) for 50% nickel.

The 36% nickel alloy's low coefficient of expansion, along with its off-the-shelf availability, make it one of the most commonly used materials for applications requiring low expansivity. But, depending on the temperature range of interest, it may not be entirely suitable for some applications. Although it exhibits the lowest thermal expansivity, it also has the lowest Curie Temperature. That limits its useful temperature range.

For applications around ambient temperature requiring the lowest expansivity, the 36% nickel is the obvious choice. It has been the most widely used nickel-iron alloy in applications such as electronic devices where dimensional changes due to temperature must be minimal, for structural members in precision optical measuring devices, and as the low expansion side in bimetal thermostats.

For certain applications, however, other alloys in the Invar family may be more suitable. In each case, alloy selection should take into account the temperature range for the intended application, as well as the coefficient of expansion desired over that range. The relative expansion rates for the "Invar-Effect" alloys are shown in Fig. 3.

 

Two alloys in this family (Fig. 4) have been found suitable for unique low expansion requirements. FCarpenter Technology Free-Cut Invar "36"® alloy (UNS K), with a slight increase in expansion properties, has been shown to offer improved machinability for applications where high parts productivity is important. This alloy has been used for aircraft controls and a variety of electronic devices

The second alloy - Carpenter Technology Super Invar "32-5" - is an iron-nickel-cobalt alloy which exhibits approximately one half the thermal of Carpenter Technology Invar "36"® alloy at or near room temperature. It has been used for structural components and supports for optical and laser instruments.

 

Fig. 4 - Typical properties and chemical composition of the Invar family of alloys.

Any one of four other nickel-iron alloys may be particularly suitable for service in higher temperature ranges. For example, Low Expansion "39" alloy (ASTM B-753) has a useful low thermal expansivity extending to approximately 250°C (482°F). It has been used as the low expansion element in thermostat bimetal products.

Carpenter Technology Glass Sealing 42 alloy, sometimes known as 42 alloy (ASTM F-30), has been commonly used for hermetic sealing to certain glasses. It is also has been used for high reliability ceramic- and plastic-sealed semiconductor packages.

Carpenter Technology Low Expansion "42"® thermostat alloy (ASTM B-753) has a virtually constant low rate of thermal expansion at temperatures up to about 350°C (662°F), while Low Expansion "45" alloy (ASTM B-753) has a relatively constant rate of thermal expansion to about 450°C (842°F). Both metals have been used in thermostats and thermoswitches. The thermal expansivity of the higher-nickel alloy approximates the thermal expansivity of alumina ceramics over certain temperature ranges. The alloy in this family with the highest nickel content, Carpenter Technology Glass Sealing "52" alloy (ASTM F-30) has been used for glass sealing of certain "soft" glasses.

Fabricating Parts

The entire group of "Invar-Effect" nickel-iron alloys machine similar to, but not as well as a Carpenter Technology 316 austenitic stainless steel. They are readily machinable, although they are soft and do produce gummy chips. Therefore, large, sharp and rigidly supported tooling is recommended, with slower speeds. Where high productivity and good surface finish are important, a big edge goes to the Carpenter Technology Free-Cut Invar "36" alloy, the free-machining alloy variation.

All of the alloys in the family 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 behavior. When that happens, parts placed in service as fabricated may not meet design requirements. To prevent such incident, 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°C (°F) to 980°C (°F) long enough to thoroughly heat through the section. However, nickel-iron alloys will oxidize readily at these 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°C (600°F) to 425°C (800°F) long enough to uniformly heat through the work piece.

Summary

Invar is a critical alloy that has endured for more than a century. It is in a league of its own, having grown in importance over the years and given life to a stream of new technologies.

The group of "Invar-Effect" alloys represents a significant, growing volume in today's specialty metals universe. Below their Curie Temperatures, these alloys exhibit anomalously low thermal expansivities. A minimum in low thermal expansivity is reached at approximately 36% nickel. Increasing the nickel content increases the thermal expansivity and also raises the Curie temperature.

Collectively, this family of nickel-iron alloys has been found suitable for a host of applications that require different expansivities over a broad temperature range. Because of the widespread and growing demand for their special properties, these alloys seem likely to make an even greater contribution to modern science as they begin their second century on Earth.

***

By Leslie L. Harner, Product Application Manager, Electronic & Magnetic Alloys

Carpenter Technology Corporation
Reading, PA
USA

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