Selecting Controlled Expansion Alloys

Alloys with controlled expansion characteristics are employed in a wide range of applications where thermal size changes of the metal are a critical aspect of component design. Typically, these alloys expand when heated and contract when cooled.

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Composition and crystal structure collectively determine an alloy's thermal expansion behavior. The rate of expansion is usually expressed as an average coefficient of thermal expansion. The expansion rates for some alloys are non-linear. Therefore, specifying the upper and lower temperature limits is necessary when addressing the expansion coefficient needed in any material.

There are three general categories of controlled expansion alloys:

• Low thermal expansion - Primarily iron-nickel alloys with very low expansion coefficients within specific temperature ranges, used in electronic devices, instrumentation, and thermostats.
• Matching expansion - Alloys with expansion coefficients that match those of glasses and ceramics typically used in the electronics industry.
• High thermal expansion - Alloys with high expansion coefficients, used mainly in thermostat applications.

Low Expansion Alloys

Low expansion alloys boast exceptionally low coefficients of thermal expansion, ranging from less than 1.8 to approximately 9 x 10^-6 per °C (1.0 to approximately 5.0 x 10^-6 per °F). These alloys generally exhibit a unique expansion behavior related to ferromagnetism. Below its Curie temperature, each alloy shows very low thermal expansivity, a phenomenon known as the "Invar Effect", which involves lattice distortion countering normal thermal expansivity. Above the Curie temperature, the alloy's expansion rate significantly increases as it loses its ferromagnetic properties.

Despite several theories proposed to explain the "Invar Effect", its mechanism remains not fully understood. Some iron-nickel compositions offer very high magnetic permeability, making these alloys valuable in applications requiring high magnetic permeability such as transformers, cores, and relays. All alloys in this family are iron-nickel or iron-nickel-cobalt with a face-centered cubic crystal structure. As nickel content increases from 36%, both thermal expansivity and Curie temperature rise. Thus, selecting an appropriate alloy requires consideration of its useful temperature range, which may be limited by the Curie temperature.

Application Categories

Uses of low expansion alloys fall into two primary categories. The first one involves applications where minimal size changes due to temperature variations are crucial, such as structural components for measuring and control instruments, aircraft and missile control components, laser and optical systems, and wave guide tubes. The second category includes temperature controls utilizing a bimetallic strip, whereby a low expansion alloy is bonded to a high expansion alloy, causing the element to bend upon heating due to the difference in thermal expansion rates.

Among well-known low expansion alloys is Carpenter Technology Invar "36"® alloy (UNS K93601), a 36% nickel-balance iron alloy used in radio and electronic devices, precision optical laser measuring instruments, and bimetal thermostats. Two other notable alloys are Carpenter Technology Free-Cut Invar "36"® alloy (UNS K93602), and Carpenter Technology Super Invar "32-5" alloy, an iron-nickel-cobalt alloy with approximately half the thermal expansivity of Carpenter Invar "36" alloy at room temperature.

Any of four other low expansion alloys may be suitable for higher temperature ranges; for example, Low Expansion "39" alloy (ASTM B-753), Low Expansion "42" alloy (ASTM B-753), Low Expansion "45" alloy (ASTM B-753), and Low Expansion "49" alloy, which have been used in tunable capacitors, thermostats, thermoswitches, and glass sealing of fiber optics.

Fabrication

Low expansion alloys can be machined similarly to Type 316 austenitic stainless steel but produce gummy chips. Thus, using large, sharp, and rigidly supported tooling with slower speeds is recommended. These alloys are very ductile and can be readily cold headed, formed, or deep drawn from cold rolled strip. It's necessary to relieve stresses introduced by forming, bending, and machining through annealing at appropriate temperatures to maintain thermal expansion characteristics.

Matching Expansion Alloys

Matching expansion alloys are designed to have thermal expansion properties compatible with certain glass and ceramic materials. This characteristic enables a lasting metal-to-glass fusion in hermetically-sealed devices. Hermetic seals have been crucial in protecting vacuum tubes, transistors, semiconductors, and integrated circuits from environmental factors.

The use of ceramic-to-metal seals has increased substantially, with “moly-manganese” being a standard process to metallize ceramic surfaces. Well-brazed joints are consistently achieved by adjusting temperature, time, atmosphere, and fixturing during the process.

Sealing Alloy Choices

Specifying the right alloy from ten glass and ceramic sealing options helps match the thermal expansion characteristics of the part. For instance, Carpenter Technology Kovar® alloy (ASTM F-15) offers strong hermetic seals with hard borosilicate glasses and ceramic materials, while Ceramvar® alloy (ASTM F-1466) is suited for applications requiring a better match with alumina ceramics.

Other popular alloys include Carpenter Technology Glass Sealing "42" alloy (ASTM F-30; Alloy No. 42), Carpenter Technology Glass Sealing "42-6" alloy (ASTM F-31), and Carpenter Technology Glass Sealing "49" alloy, each offering benefits relevant to specific applications, such as bubble-free seals or soft glass compatibility. .

Fabrication

Iron-chromium glass sealing alloys are ductile, enabling ease of stamping and deep drawing for mass production. With linear expansion rates, they don't exhibit the "Invar Effect". Additionally, these alloys are mechanically harder, making them easier to machine than iron-nickel alloys. Manufacturers can control variables to meet fabrication requirements such as composition, mechanical properties, and quality.

High Expansion Alloys

Often iron-based with nickel and chromium, high expansion alloys can also extend to certain stainless steels. Examples include Carpenter Technology High Expansion "22-3" alloy and Carpenter Technology High Expansion "19-2" alloy, used in bimetal thermostat applications.

Carpenter's Carpenter Technology High Expansion "72" alloy (ASTM B-753), a unique non-ferrous alloy with higher thermal expansivity, is employed for greater flexing in bimetal strips due to its high expansion rate. Combining low and high expansion alloys allows for controlled mechanical bending in temperature-responsive devices, such as thermostats.

These thermostats are crucial in preventing overheating of equipment, safeguarding circuit breakers, and ensuring the safe operation of home appliances and industrial systems. Their design depends on matching the high and low expansion alloys to achieve desired deflection under temperature changes.

High expansion iron-nickel-chromium alloys exhibit good fabricability and compatibility with low expansion iron-nickel alloys, making them suitable in various applications.

Summary

Controlled expansion alloys are manufactured using premium melting practices and specialized processing to ensure optimal properties. Such alloys offer predictable behavior during fabrication processes due to their well-controlled physical, mechanical, and expansion characteristics.  These alloys are available in various standard forms and can be customized for specific properties such as chemistry, cleanliness, thermal expansivity, and electrical resistivity.

By Leslie L. Harner
Carpenter Technology Corporation
Reading, PA
USA

When designing your tooling or forming die, it's crucial to choose the right alloy. There isn't a universal alloy that fits all applications, so understanding your specific needs is crucial. This article provides tips on selecting the best alloy based on application, requirements, and challenges.

If you need further assistance, MetalTek is available to help.

Factors in Selecting the Best Alloy for Your Tooling or Forming Dies

Consider both physical and mechanical properties when selecting an alloy.

• Mechanical properties entail how the metal performs under different forces, such as strength, ductility, and wear resistance.
• Physical properties are measurable aspects like density, melting point, and the coefficient of expansion.

Typically, mechanical properties guide alloy selection, but keeping physical properties in mind is essential. Let’s dive deeper into the specific properties to consider for your tooling application.

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Yield Strength

Yield Strength refers to the point at which the material no longer returns to its original shape after the stress is removed. It's measured in Newtons per square millimeter (MPa) or pounds per square inch (PSI). Selecting an alloy with a higher yield strength than the forces applied to the tooling helps prevent deformation, ensuring consistent parts and longevity.

Fatigue Strength

Fatigue can lead to fractures under repeated or fluctuating stresses below the material's tensile strength. The fatigue limit is the maximum stress the material can withstand in a given number of cycles. Fatigue strength is essential when designing components subjected to repetitive loads. Depending on how many parts your tooling needs to produce, this factor is critical in choosing the appropriate alloy.

Hardness

Hardness is a material's ability to resist permanent indentation and deformation. The harder the material, the better it resists wear. Hardness is measured through various methods, which assess a material’s resistance to scratching, abrasion, or cutting by a harder material. This property is crucial for ensuring durability and resistance to wear in tooling.

Coefficient Of Thermal Expansion

Coefficient of thermal expansion measures how much a material expands when heated. In forming processes that involve elevated temperatures and heat/cool cycles, selecting alloys with low coefficients of thermal expansion ensures better dimensional accuracy.

Shear Strength

Shear strength is considered in applications where directional forces cause the internal structure to slide against itself. This property is crucial when the direction and magnitude of the stress create extensive shear or torsion loads.

Common Metal Alloys for Tooling Applications

While non-ferrous or non-metallic materials are sometimes used, ferrous alloys are most common. Popular groups include specialty alloys, stainless steels, tool steels, and carbon steels.

Specialty & Nickel-Based Alloys

Heat Resistant Stainless Steel

Heat-resistant alloys withstand temperatures above 1200°F (670°C) while maintaining stress rupture and creep strength. Key groups include high chrome nickel austenitic alloys, nickel-based alloys, and cobalt chrome nickel-based alloys, such as MTEK 20-25, MTEK 20-25MA, and MTEK 25-12.

MTEK 625 (Inconel 625®)

MTEK 625, also known as Inconel 625®, excels in high heat applications, maintaining tensile strength and resisting deformation. It also offers superior resistance to corrosion, oxidation, and scaling, making it ideal for extreme pressure and heat environments.

MTEK Invar (K93600, FeNi36)

MTEK Invar, containing 36% nickel, offers great strength and dimensional stability across temperatures from cryogenic to 500°F due to its nearly zero coefficient of thermal expansion. It's suitable for precision instrumentation and tooling for aerospace and electronics.

MTEK Alloy X

MTEK X, a nickel-based alloy, provides high-temperature strength and oxidation resistance. It resists stress-corrosion cracking and maintains good ductility in high-temperature environments. The alloy includes about 22% chromium and 18% iron.

Stainless Steels

Stainless steels are popular for their temperature and corrosion resistance, primarily due to their passive surface films, created by chromium. Common stainless steels used in tooling include MTEK 304, MTEK 304L, MTEK 309, and MTEK 310.

Tool Steels

Tool steels, rich in elements like molybdenum, cobalt, and vanadium, vary in handling repeated loads and impacts at different temperatures. They are primarily used in an annealed condition before heat treatment. Classes include Cold-work, Hot-work, and High-speed tool steels, with common grades being MTEK A2, MTEK A6, MTEK D2, MTEK D5, and MTEK H13.

Carbon Steels

Carbon steels are primarily composed of iron and carbon, categorized by their carbon content: low, medium, and high. They offer a balance between machinability, strength, and wear resistance, with low-carbon steels being easily machinable but less strong, and high-carbon steels providing greater wear resistance but being more difficult to machine and weld.

MetalTek’s Top Three Picks To Consider

There isn't a universal alloy for every tooling application, but MetalTek’s top three picks include:

• #1) MTEK Invar: Great strength and toughness, with superior coefficient of thermal expansion.
• #2) MTEK 625: Ideal for high heat applications, maintaining tensile strength at elevated temperatures.
• #3) MTEK 20-25: Balanced strength and toughness across a range of temperatures.

To learn more about the best alloy for your application, contact us.

Disclaimer: The information in this publication is for informational purposes and does not replace good engineering practices. MetalTek International assumes no legal liability for its accuracy or completeness.

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