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Nickle tool steels match up with composites.

Nickel tool steels match up with composites

Since the 1940s, nickel-iron alloys have been associated with electrical, electronic, and electromechanical devices. They offered almost any degree of controlled thermal expansion, from very low (within certain temperature ranges) to higher than that of low-carbon steels. Alloys could virtually match the expansion characteristics of nonmetallics, but few ever thought these binary alloys were destined to become tooling for composites.

Early composites used low-temperature-curing matrix resins, and a variety of tooling materials served nicely for their molds. However, more recent sophisticated-matrix resins demand higher cure temperatures, and this has narrowed the range of acceptable tooling materials.

Also, as composite structures increase in size and complexity, tooling performance has become even more critical. The tooling material's coefficient of thermal expansion (CFE) is now a primary consideration. Because it is autoclaved with the composite assembly, the tool's CTE must closely match that of the composite. Nickel-iron alloys can do this, plus they are readily fabricated and provide good mechanical properties.


The most common Ni-Fe alloys contain from 30% to 80% nickel, with the balance essentially iron. Very tight control of residual elements and nickel content is necessary to maintain the alloy's expansion properties.

When approximately 28% or more nickel is present in iron, the crystal structure is austenite, but, because the Curie temperatures (where feromagnetics lose their magnetism) of these alloys are above room temperature, they are ferromagnetic and, as a result, exhibit abnormal thermal expansion.

The table compares thermal-conductivity, Curie-temperature, thermal-expansion, and specific-heat data for a number of Ni-Fe compositions.

Thermal expansion

Thermal expansion of Ni-Fe alloys changes significantly as nickel content is varied between 30% and 50%. Near room temperature, thermal expansion is less than 1 ppm/deg F for 36% nickel (by weight) in iron. However, thermal expansion increases tenfold as nickel increases from 36% to 50%. Also, Curie temperature increases from room temperature at 28% to 950 F for 50% Ni-Fe.

The figure illustrates thermal expansion with temperature for a number of Ni-Fe alloys. For those with nickel content greater than 36%, higher Curie temperatures extend their useful application range to higher temperatures.

Ni-Fe alloys of commercial interest for their low thermal expansion exhibit moderate strength levels and are not hardenable by heat treatment. For those with 30% to 50% nickel, tensile strengths for annealed alloys are typically 68 ksi to 84 ksi with tensile elongations greater than 25%.


With regard to fabrication:

Welding. Carpenter Invar 36 [R] alloy can be successfully welded using most standard arc-welding processes. In general, preparation should be similar to stainless steels. Joint designs should allow easy access to the weld because of relatively poor weld-pool fluidity. Also, weld volume should be limited to reduce shrinkage problems. Pre-heating and postheating are not required and should be avoided. A low interpass temperature (300 F max) should be maintained.

Welding is most commonly performed using gas tungsten arc or gas metal arc processes. Welding is best with a freshly ground thoriated-tungsten electrode. Shielding gases should be argon or argon/helium mixtures. When a filler metal is needed, use a matching-composition filler alloy (such as Carpenter Invarod [TM]) with the same thermal-expansion properties.

Machining. Ni-Fe alloys machine similar to, but not quite as good as, 316 austenitic stainless steel, or about 25% as well as AISI B1112.

Free-machining versions of some basic grades are available for improved machining productivity. Generally, these can be turned at 50 sfm to 95 sfm higher at the same or slightly higher infeed rates as their nonfree-machining counterparts.

Tool geometries normally used for austenitic steels are applicable. Cutting-fluid selection is important. For best machinability, use a 1-to-1 blend of sulfachorinated petroleum oil with 8% to 10% fatty oil and a paraffin blending oil, or a water-emulsifiable cutting fluid with polar and extreme-pressure additives.

Machining components should be degreased and cleaned as soon after machining as possible. Residual sulfur can cause grain-boundary embrittlement and other problems.

Thermal treatment. Although Ni-Fe alloys are not hardenable, annealing and stress-relief thermal treatments are necessary at times to promote structure uniformity and dimensional stability. This can be accomplished by annealing at temperatures of 1400 F to 1800 F.

Stress relief can be obtained for sections having light finishing cuts or grinding performed after annealing. This is accomplished by heating to 600 F to 800 F long enough to heat the workpiece uniformly throughout.

Earl L Frantz Product Application Manager Carpenter Technology Corp Reading, PA
COPYRIGHT 1992 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Author:Frantz, Earl L.
Publication:Tooling & Production
Date:Jan 1, 1992
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