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What Is Expansion Alloy

Expansion Alloy is a precision alloy with abnormal thermal expansion characteristics, also known as thermal expansion alloy, which is widely used in the fields of electronics industry, precision measuring tools, precision instruments and low temperature engineering.

Expansion alloy is a kind of precision alloy with abnormal thermal expansion characteristics, also known as thermal expansion alloy, which is widely used in the fields of electronics industry, precision measuring tools, precision instruments and low temperature engineering.

General metals and alloys expand when heated, and the amount of expansion increases linearly with the increase of temperature, but the thermal expansion curve of some alloys has a bending point at a certain temperature (the intersection of the tangents of two line segments with different slopes, as shown by Tk in Figure 1 ), the thermal expansion coefficient below the inflection point is much lower than the normal thermal expansion coefficient above the inflection point, a phenomenon called anomalous thermal expansion characteristics.

Expansion alloys are divided into low expansion alloys and constant expansion alloys, the latter also known as sealing alloys. The average expansion coefficient of the low-expansion alloy below the inflection point is lower than 3×10-6℃-1; the average expansion coefficient of the constant expansion alloy below the inflection point is about (4~10)×10-6℃-1.

The expansion alloys mainly include Fe-Ni series, Fe-Ni-Co series and Fe-Ni-Cr series alloys, etc. High chromium steel and Co-Fe-Cr series alloys are also used as expansion alloys, but the amount is not large.

Alias Thermal Expansion Alloy
Features Anomalous Thermal Expansion
Property Precision Alloy
Application Direction Electronic industry, precision measuring tools, precision instruments, etc.
Classification Fe-Ni series, Fe-Ni-Co series, etc.

In addition to a specific thermal expansion coefficient, the expansion alloy also requires good sealing, weldability, corrosion resistance, machinability and ease of machinability according to different uses, and it is not allowed to cause expansion within the operating temperature range. A distinct phase transition. The chemical composition of the expanded alloy must be accurately controlled during the manufacturing process, and its products are generally rods, plates, strips, wires and pipes.

Interrelated Expansion Alloy

1.FeNi29Co17 FeNi29Co17 glass sealing alloy

Grade: 4J29 (Kovar) Also called Kovar alloy (Fe54%, Ni29% and Co17%), its expansion coefficient is 4.7×10-6/℃.

Executive standard: YB/T 5231-93

Uses: used to make matching seals with hard glass. It is suitable for electric vacuum devices such as launch tubes, oscillator tubes, ignition tubes, transistors, and tube-sealed plugs and relay housings.

It has a certain linear expansion coefficient in the temperature range of -60℃~+40O℃, and can be matched with hard glass and sealed firmly.

Availability: strip, wire, round bar, forging

2.Porcelain sealing alloy Grade: Vacodil42, 4J42, 4J33, 4J34

Executive standard: YB/T 5234-93

Uses: used for making and sealing with ceramics. It is suitable for matching sealing between electric vacuum devices and 95% Al2O3 ceramics. It has a linear expansion coefficient similar to that of 95%Al2O3 ceramics in the temperature range of -60℃~+600℃.

Availability: strip, wire, round bar, forging.

3.Low expansion alloy FeNi36, FeNi32CO4Cu, Invar, Invar alloy

Brand: 4J32, 4J36 4J38, 4J40, executive standard: YB/T 5241-93 The performance characteristics of the product are mainly used for: making 4J32, 4J36 alloy and high temperature and low temperature instrument parts with high precision requirements in the range of ambient temperature.Expanded 4J40 alloy and free-cutting 4J38 alloy. 4J36 has a very low coefficient of thermal expansion and can maintain a fixed length over a wide temperature range.

What Is Low Expansion Alloy

In 1896, the French physicist CEGuialme discovered a wonderful alloy. The thermal expansion coefficient of this alloy is significantly reduced near the magnetic temperature, that is, the Curie point, and the so-called abnormal thermal expansion phenomenon (negative anomaly) occurs, so that it can be very wide around room temperature. In the temperature range, a small or even close to zero expansion coefficient is obtained, and this alloy is called a low expansion alloy. The composition of this alloy is 64% Fe and 36% Ni. It has a face-to-center structure. Its grade is 4J36. Its Chinese name is invar, and its English name is invar, which means that the volume remains unchanged. Also known as non-expanding steel.

The Performance Of Expansion Alloy

1.Small expansion coefficient

Invar alloy is also called invar, its average expansion coefficient is generally 1.5 × 10-6 °C, nickel content reaches 1.8 × 10-8 °C at 36%, and does not change at room temperature -80 °C ~ 100 °C .

2.Strength and hardness are not high

The carbon content of Invar alloy is less than 0.05%, the hardness and strength are not high, the tensile strength is about 517Mpa, the yield strength is about 276Mpa, and the Vickers hardness is about 160. Generally, the strength can be improved by cold deformation. still has good plasticity.

3.Low thermal conductivity

The thermal conductivity of Invar alloy is 0.026~0.032cal/cm·sec·℃, which is only 1/3-1/4 of the thermal conductivity of 45 steel.

4.High plasticity and toughness

Invar alloys have high elongation, area reduction and impact toughness, elongation δ= 25-35%, impact toughness αK=18-33 kg m/cm2.

5.Other Properties

Low expansion alloys are mostly used in components that require approximately constant dimensions at a certain ambient temperature.There are:

  • Precision instruments, components in optical instruments, such as arms of precision balances, pendulums of standard parts, balance wheels, external compensation of clocks, etc.;
  • Length rulers. Geodetic baseline ruler;
  • Various resonant cavities, waveguides for microwave communication, standard frequency generators, etc.;
  • Blades and support rods of standard capacitors;
  • Liquid natural gas, liquid hydrogen, liquid oxygen, etc. Storage tanks and transport pipes;
  • passive layers of thermal bimetals;
  • shadow masks in high-resolution cathode ray tubes (picture tubes);
  • molds for composite parts in the aerospace industry;
  • For satellites, lasers, ring laser gyroscopes and other advanced high-tech products

The Processability Of Expansion Alloy

Due to the high nickel content of Invar alloy, the hardenability and hardenability of steel are improved, and the gas resistance, corrosion resistance and wear resistance of steel are improved. According to the analysis of chemical composition, metallographic structure, mechanical and physical properties of Invar alloy, the machinability of Invar alloy is similar to that of austenitic stainless steel, but it is more difficult to process than austenitic stainless steel. It mainly has the characteristics of large cutting force, high cutting temperature, and fast tool wear. Therefore, invar alloy appears soft, sticky and very plastic during the processing process, the chips are not easy to break, and the friction between the chips and the front to the surface is increased. The wear of the tool is aggravated, which not only reduces the durability of the tool, but also reduces the machining accuracy of the workpiece. Therefore, when processing Invar alloys, high-performance carbide-coated tools and new processing methods must be used to achieve To make the cutting process go smoothly, as long as the method is correct, the difficult-to-machine Invar alloy can be easily machined.