Beijing Wenton technology

Invar 36 Alloy (4J36)

1. Product Overview

Invar 36 (domestic grade 4J36, UNS K93600) is an iron-nickel ultra-low-expansion precision alloy containing about 36% nickel. Near room temperature it shows an extremely low coefficient of linear thermal expansion: about 1.2–1.8×10⁻⁶/°C (typically 1.2–1.5×10⁻⁶/°C) over 20–100 °C, only about one tenth that of ordinary carbon steel, which makes it the most widely used low-expansion metallic material today. Its value lies not in strength or corrosion resistance but in an almost unique property: near room temperature its dimensions change very little with temperature, approaching "no dimensional change with temperature". This makes it an irreplaceable structural material in precision instruments, cryogenic engineering and composite curing tooling.

Note that the Invar effect depends on the ferromagnetic state of the material: once the service temperature approaches or exceeds the Curie temperature (about 230 °C, some sources give about 279 °C), the low-expansion characteristic disappears quickly and the expansion coefficient rises back to the level of ordinary austenitic steel. Therefore 4J36 is a material for "low-temperature to medium-temperature range" dimensional stability, not a high-temperature material.

The word "Invar" comes from the English word invariable, because near room temperature it has an almost "unchanging dimension", hence the name. The physical origin of low expansion is the mutual cancellation of magnetostriction and lattice thermal vibration: as temperature rises, lattice vibration tends to expand the material, while the volume magnetostrictive contraction caused by the decrease in spontaneous magnetization of the ferromagnetic alloy tends to offset that expansion. At a nickel content of about 36%, the two effects are almost exactly balanced near room temperature, which macroscopically appears as an extremely low coefficient of linear thermal expansion.

In practice, 4J36 is often used to solve two kinds of problems: the first is the need for "size stability across temperature ranges", such as satellite structures, laser bases and precision gauges; the second is the need for "thermal matching with other materials", most typically carbon-fiber composite curing tooling — the composite has a very low coefficient of thermal expansion, and if the tooling is made of steel or aluminum, the dimensional mismatch between tooling and part during heating and cooling leaves residual stress and warpage in the part.

2. Chemical Composition

Invar 36 (4J36) — Chemical Composition (wt%, balance Fe)

3. Designations, Standards and Cross-Reference

Invar 36 / 4J36 appears in various forms across countries and standard systems.

1. Common designations and international cross-reference

2. Common applicable standards and their scope

We can supply with dual GB/T and UNS/ASTM certification, meaning that the same batch of material is delivered with chemical composition and property reports to both the domestic and the foreign standard, which simplifies incoming inspection for export projects and foreign customers. If a project drawing states only "Invar 36" without a standard revision, it is advisable to confirm this at the inquiry stage, so that the two parties do not differ in their understanding of the expansion-coefficient temperature range and the mechanical property values.

3. Key Performance Parameters

This section presents four groups of core data for 4J36 — physical properties, thermal expansion, mechanical properties and chemical composition — which respectively support thermal and structural design, dimensional accuracy checks, strength checks and purchasing acceptance. Unless otherwise stated, values are reference values for the annealed or standard-specified condition.

(1) Physical Properties

Invar 36 / 4J36 typical physical properties (room temperature, reference values)

(2) Thermal Expansion Properties

Invar 36 / 4J36 mean coefficient of linear thermal expansion (reference values; measured reports take precedence)

The values in the table above show visible scatter, for three reasons: differences in standard systems (different definitions of the start and end of the temperature range), deviations in composition within the 35.0%–37.0% nickel range, and differences in heat treatment and cold-work condition.

(3) Mechanical Properties

Invar 36 / 4J36 room-temperature mechanical properties (reference values and the minimum values required by common standards)

Annealed 4J36 has strength comparable to ordinary low-carbon steel, elongation up to 30%–45% and ample plastic reserve, and can be cold rolled, stamped, bent and deep drawn. Where higher strength is required it can be strengthened by cold working, but cold working comes at the expense of dimensional stability and must be followed by stress-relief annealing.

4. Available Forms and Size Range

4J36 is available as plate, strip, foil, bar, wire, tube, forgings and rings, and can be customized to size and tolerance. The table below gives typical size ranges; actual availability depends on the production schedule and stock at the time the order is confirmed, and special sizes outside the normal range require separate review.

Table 5-1 Typical available forms and size ranges (actual availability confirmed at order)

Table 5-2 Common delivery conditions, tolerances and documentation options