Alloys with a specified temperature coefficient of linear expansion

The temperature coefficient of linear expansion (TCLE) is a characteristic that indicates by how many times the linear dimensions or volume of a product will change at constant pressure when temperature rises by 1 K. For steels with specified TCLE this value is regulated, making it possible to precisely calculate linear dimensions of parts from precision alloys when using them under certain temperature regimes.

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Classification

Precision alloys with specified TCLE are divided into groups according to their magnetic properties: ferromagnetic metals and non-magnetic steels. Within groups they are divided into subgroups by TCLE value.

Ferromagnetic materials:

• with minimal temperature coefficient — iron-nickel alloys with nickel content from 30 to 40%; at working temperature up to 100°C their temperature coefficient does not exceed 3.5·10⁻⁶ K⁻¹;
• with low and medium TCLE — iron-nickel materials with addition of cobalt, copper, or chromium; the former have a coefficient from 3.0–7.0·10⁻⁶ K⁻¹, the latter — 5.5–12·10⁻⁶ K⁻¹.

Non-magnetic alloys:

• with high TCLE — manufactured with addition of manganese, have a temperature coefficient above 15·10⁻⁶ K⁻¹ in the temperature range 20–200°C, distinguished by good ductility and elevated strength;
• with low — have a nickel base, alloyed with copper, tungsten, molybdenum, titanium, chromium; have high corrosion and heat resistance; temperature coefficient is from 6 to 14·10⁻⁶ K⁻¹ in the range −80…+900°C.

Regardless of TCLE, all precision non-magnetic materials are distinguished by high elasticity and resistance to mechanical effects.

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Features of alloys with specified TCLE

The main factors that affect characteristics of alloys with specified TCLE are their chemical composition and production technology. To achieve the required temperature coefficient value, tungsten, beryllium, cobalt, copper, titanium, manganese, chromium, aluminum, vanadium, or molybdenum are added to base metals.

Concentration of alloying elements directly affects the temperature coefficient of linear expansion. For example, copper and cobalt increase the temperature range of TCLE stability, while chromium, conversely, decreases it.

The required coefficient value in ferromagnetic steels is achieved by decreasing or increasing nickel quantity in the alloy. For example, invar — an iron-nickel alloy with 35–36% nickel (steel grade 36N) — has a working temperature range of −100…+100°C. When nickel quantity is increased to 42%, platinite (alloy 42N) is obtained, which retains its properties at temperatures already up to 300°C.

When manufacturing alloys with low TCLE, one of the following subsequent processing methods is used to stabilize parameters over time:

• holding for 48 hours at 95°C;
• annealing for 1 hour at 315°C;
• quenching at temperatures from 830 to 870°C.

When producing metals with higher coefficients, quenching at 1,100…1,150°C with subsequent cooling in water is used. Annealing at temperatures up to 1,050°C may also be used.

Alloys with a specified temperature coefficient are produced as bar, wire, cold-rolled strip, band, and tube.

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Applications of precision alloys with specified TCLE

Developing new precision alloys with specified thermal properties had a high influence on technology development in radio and cryogenic engineering as well as quantum electronics. Where even minimal influence of magnetic fields of ferromagnetic materials proved unacceptable, non-magnetic metals with minimal TCLE found wide use: steel grades 72TF, 75TM, 75NM-VI, 76NKhVG93TsT, 80NMV, 80NMBKh3, 95KhK, and 96Kh.

Non-magnetic precision alloys with high TCLE (56DGNh, 70GNDh) are used in manufacturing thermobimetals.

Ferromagnetic materials with minimal temperature coefficient are used in meteorology, geodesy, and when equipping potentially hazardous pipelines.

Alloys with low and medium TCLE are used for soldering with ceramics and glass. For example, a material such as Kovar (29NK) is manufactured at Petersburg Plant of Precision Alloys as cold-rolled strip. It is used for vacuum-tight joints in metal-glass and metal-ceramic structures when manufacturing aircraft and space satellite parts.

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