4J28 Expansion Alloys Strip Glass Seal Expansion Matching Corrosion Resistance

Expansion Alloy 4J28 Strips: Glass-Seal Expansion Matching and Corrosion Resistance

Product Overview

Expansion Alloy 4J28 Strips (Alloy 27) are precision-rolled iron-chromium strips formulated to closely match the coefficient of thermal expansion of typical soft sealing glasses, ensuring hermetic, stress-free bonds between metal and ceramic or glass. Between – 50 °C and 400 °C, 4J28 exhibits a mean linear expansion coefficient of approximately 4–8 × 10⁻⁶ /°C, minimizing thermal stresses in glass-to-metal seals. The alloy’s robust chromium-rich matrix (27–29 % Cr) and low carbon, phosphorus, and sulfur (≤ 0.12 % C; ≤ 0.02 % P, S) confer excellent corrosion resistance in mild atmospheres and compatibility with sealing glasses. Standard strip dimensions cover 0.2 – 3.5 mm thickness (cold-rolled) and up to 300 mm width; larger widths and thicknesses via hot-rolling/forging can accommodate broader applications. 4J28 Strips are engineered for reliable sealing in vacuum tubes, hermetic feedthroughs, glass-encapsulated sensors, and precision electronic packages, where dimensional stability and corrosion resistance are paramount. Their formability allows tight-tolerance bending and stamping without springback, while weldability supports complex assembly. For full mechanical property data—tensile, yield, hardness—and thermal-conductivity curves, please consult primary alloy producers like Special Metals (Invar/Glass alloys), Rolled Alloys (precision strip), and Haynes Intl.

Chemical Composition

Mechanical Properties

Physical Properties

Conformance Standards

• AMS 4951 – Iron-chromium low-expansion alloy

• DIN 17 327 – Nickel-iron sealing alloys

• ISO 464 – Glass-to-metal sealing alloys

Applications

• Vacuum & Electronic Seals – Glass-to-metal feedthroughs, vacuum tubes, hermetic connectors

• Precision Instruments – Pressure sensors, gas discharge tubes, photomultiplier bases

• Ceramic Package Encapsulation – Sealing of ceramic circuit substrates to metal headers

• Thermomechanical Assemblies – Bimetallic thermostats, expansion joints

• Medical Devices – Hermetic seals in implantable electronics and sensors

Frequently Asked Questions

Q1: What makes 4J28 Strips ideal for glass-to-metal seals?
A1: Their thermal expansion closely matches soft glass (4–8 × 10⁻⁶ /°C), eliminating stress and preventing seal cracks during thermal cycling.

Q2: Can 4J28 Strips be welded or brazed to other alloys?
A2: Yes—standard electron-beam and laser welding, plus active brazing alloys, bond 4J28 to Kovar, stainless, and nickel alloys without compromising expansion matching.

Q3: Where can I get certified mechanical data?
A3: For certified tensile, yield, hardness, and thermal conductivity data, consult the technical bulletins from Special Metals (Alloy 27), Rolled Alloys, and Haynes International.