4J29 glass-sealed stainless steel tube

Basic Introduction

4J29 alloy, also known as Kovar alloy, exhibits a linear expansion coefficient similar to that of borosilicate hard glass within the temperature range of 20–450°C. It has a high Curie point and excellent low-temperature microstructure stability. The oxide film of the alloy is dense and can be well wetted by glass. It does not react with mercury, making it suitable for use in instruments containing mercury discharge. It is a primary sealing structural material in electrovacuum devices.

Heat Treatment System

For standard testing of expansion coefficient and low-temperature microstructure stability, samples are heated in a hydrogen atmosphere to 900°C ± 20°C, held for 1 hour, then further heated to 1100°C ± 20°C, held for 15 minutes, and cooled to below 200°C at a rate not exceeding 5°C/min before removal.

4J29 Application Overview

This alloy is an internationally recognized typical Fe-Ni-Co hard glass sealing alloy. It has been used extensively in aviation industries with stable performance. It is mainly used for glass sealing in electrovacuum components such as transmitting tubes, oscillator tubes, ignition tubes, magnetrons, transistors, sealed connectors, relays, lead wires of integrated circuits, chassis, casings, and brackets. In application, the selected glass must match the expansion coefficient of the alloy. Low-temperature microstructure stability must be strictly verified based on the operating temperature. Appropriate heat treatment during processing is necessary to ensure good deep-drawing properties. When using forged materials, airtightness must be rigorously inspected.

4J29 Microstructure

After treatment according to the specified heat treatment system in section 1.5, the alloy should not exhibit martensitic structure after being frozen at -78.5°C for ≥4 hours. However, if the alloy composition is improper, austenite (γ) may transform to acicular martensite (α) to varying degrees at room or low temperatures, accompanied by a volume expansion effect. This increases the alloy’s expansion coefficient, leading to a sharp rise in internal stress in sealed components and potentially causing partial damage. The main factor affecting low-temperature microstructure stability is the chemical composition of the alloy. From the Fe-Ni-Co ternary phase diagram, nickel is the primary element stabilizing the γ phase, and higher nickel content favors γ phase stability. Increased total deformation rate enhances structural stability. Composition segregation may also cause local γ→α phase transformation. Additionally, coarse grains can promote γ→α transformation.