4J32 Nickel-Iron Alloy Guide: Controlled Expansion Performance, Applications and Purchasing Checks
Date: 2026年9月14日 Categories: Nickel Views: 320
4J32 Nickel-Iron Alloy Guide: Controlled Expansion Performance, Applications and Purchasing Checks
4J32 is a controlled-expansion iron-nickel alloy from the same family as the better known 4J36, but it is not a substitute for it and it is not interchangeable with it. The addition of cobalt and a small amount of copper to a nominal 32 percent nickel balance shifts the thermal expansion behaviour to a still lower level over a narrower temperature window, which makes 4J32 the grade of choice for the most demanding dimensional-stability applications and an expensive mistake where a wider working range is required.
This guide is written from the stock side of the desk, where the questions are practical: what the alloy actually does over which temperature range, how the heat treatment changes the result, what forms are held, and what evidence should accompany a delivery. It covers chemistry, the standards framework, expansion and mechanical data, heat treatment for dimensional stability, applications, fabrication, and the purchasing and verification checks that prevent an unsuitable heat from being released into a precision assembly.
1. What 4J32 Is
Controlled-expansion alloys exploit the magnetostriction behaviour of iron-nickel alloys: near a specific nickel content the spontaneous magnetisation strain cancels the normal thermal expansion of the crystal lattice, and the macroscopic expansion coefficient drops almost to zero over a limited temperature interval. In the 36 percent nickel alloy, 4J36, this gives a coefficient of roughly one part per million per kelvin over the room-temperature range. Adding cobalt and copper and reducing nickel to about 32 percent pushes the coefficient lower still, and 4J32 is the result.
The trade-off is inherent to the mechanism. The effect only exists below the Curie temperature, and the cobalt-bearing composition places that transition lower than in plain 36 percent nickel alloy. Above the transition the alloy expands at the normal rate of a ferritic steel. That is why 4J32 is specified for narrow, well-defined temperature windows and why 4J36 remains the default grade for cryogenic and wide-range duty.
2. Chemistry
| Element | 4J32 range | 4J36 range | Why it matters |
|---|---|---|---|
| Ni | 31.5-33.0 | 35.0-37.0 | Nickel content sets the temperature of minimum expansion |
| Co | 3.2-4.2 | 0.50 max | Lowers the coefficient but also the useful temperature ceiling |
| Cu | 0.40-0.80 | 0.50 max (residual) | Fine-tunes the expansion curve and improves stability |
| C | 0.05 max | 0.05 max | Carbides pin the structure and disturb expansion uniformity |
| Mn | 0.40 max | 0.60 max | Residual from melting; high values degrade the expansion effect |
| Si | 0.25 max | 0.35 max | Keep low for strip surface quality and forming |
| P, S | 0.020 max each | 0.020 max each | Hot workability and grain boundary cleanliness |
| Fe | Balance | Balance | Matrix |
Because the expansion minimum is composition-sensitive, the usable nickel and cobalt windows are narrow and the melting practice must hold them tightly. A heat at the edge of the specified range will still be chemically compliant while missing the required expansion value, which is why a certificate that shows only chemistry is not sufficient evidence for a controlled-expansion delivery.
3. Standards and Designations
| Document or designation | Scope and comment |
|---|---|
| GB/T 15018 | Chinese designation standard for precision alloys, which establishes 4J32, 4J36 and the other 4J grades |
| GB/T 14985 series | Technical delivery conditions for expansion alloys in Chinese practice; always cite the current edition |
| ASTM F1684 | Iron-nickel and iron-nickel-cobalt alloys for low thermal expansion applications; the document most often cited for Invar 36 (UNS K93600) |
| DIN 1.3912 / FeNi36 | European designation for the 36 percent nickel grade; useful when a European drawing must be matched |
| MIL-I-23011 (historical) | US military specification for low-expansion iron-nickel alloys, still referenced on legacy drawings |
| UNS equivalents | 4J36 maps to UNS K93600. The cobalt-bearing 4J32 family has no single universally adopted UNS number, so the composition and the expansion requirement must be stated explicitly |
The practical consequence for a purchase order is that 4J32 should always be specified by composition plus expansion requirement plus product form, since the grade name alone does not pin the acceptance criteria the way a single ASTM specification would.
4. Thermal Expansion: The Property That Defines the Grade
| Temperature range | 4J32 | 4J36 |
|---|---|---|
| -60 to +20 C | Not the design window | Approx. 1.2-1.6 x 10-6 /K |
| 20 to +100 C | Approx. 0.5-1.0 x 10-6 /K, specification limits commonly at or below 1.0 | Approx. 1.2-1.5 x 10-6 /K, limits commonly at or below 1.5 |
| 20 to +200 C | Rises sharply as the Curie transition is approached | Approx. 3-5 x 10-6 /K |
| Above the Curie temperature | Normal steel-like expansion | Normal steel-like expansion above roughly 280 C |
Two cautions belong with this table. The first is that the low coefficient only appears after the correct heat treatment; a mill-rolled strip that has not been properly annealed can miss the requirement by a wide margin. The second is that the value quoted must be tied to a temperature range, because quoting a single coefficient without a range is meaningless in this family. Most specifications for 4J32 are written over 20 to 100 C, which is exactly why the grade suits laboratory instruments, laser hardware and precision metrology rather than wide-range cryogenic equipment.
5. Physical and Mechanical Properties
| Property | 4J32 (typical) | 4J36 (typical) |
|---|---|---|
| Density | approx. 8.2 g/cm3 | approx. 8.1 g/cm3 |
| Curie temperature | Below that of plain 36 % nickel alloy | approx. 280 C |
| Elastic modulus | approx. 140-150 GPa | approx. 141 GPa |
| Thermal conductivity | approx. 10-11 W/(m.K) | approx. 10-11 W/(m.K) |
| Electrical resistivity | approx. 0.75-0.85 micro-ohm.m | approx. 0.82 micro-ohm.m |
| Magnetic behaviour | Ferromagnetic below the Curie transition | Ferromagnetic below the Curie transition |
| Property | Annealed bar or strip | Cold worked strip |
|---|---|---|
| Tensile strength | approx. 450-550 MPa | Rises substantially with reduction, can exceed 900 MPa |
| 0.2 % yield strength | approx. 240-320 MPa | Rises with reduction |
| Elongation | 30-45 % | Falls sharply, often to a few percent at full temper |
| Hardness | approx. 130-180 HV | Up to approx. 300 HV at high reduction |
The strength values are modest by alloy standards, and engineers used to stainless steel must design accordingly. In exchange, the alloy is ductile, machinable and formable, and it can be supplied as very thin strip with tight thickness tolerance, which is what precision instrument work actually requires.
6. Heat Treatment and Dimensional Stability
The expansion coefficient of 4J32 is not a fixed property of the composition; it is a property of the composition plus the thermal history. A typical production route is solution annealing in the upper part of the working range followed by controlled cooling, and in many cases a stabilisation treatment at lower temperature. The specific cycle must be agreed against the required expansion value and the finished part, because the same heat can meet or miss a 1.0 x 10-6 /K limit depending on how it is treated.
- Annealing: carried out in a controlled atmosphere to avoid oxidation and contamination, with the cycle recorded for each load.
- Controlled cooling: cooling rate through the critical interval is part of the specification, not a workshop preference.
- Stress relief and stabilisation: used to remove the residual stress that would otherwise cause dimensional drift in service, and to stabilise the expansion after machining.
- Thermal cycling: for the most critical metrology parts, repeated cycling between service temperature limits is used to confirm that the part is dimensionally stable before release.
Because heat treatment is decisive, a controlled-expansion delivery should include the heat treatment cycle in the documentation, not just chemistry and mechanical results. Where the end user has qualified a supplier's cycle, any change to that cycle is a change to the product.
7. Applications and Grade Selection
| Application | Typical grade | Reason |
|---|---|---|
| Laser cavities, optical benches, metrology frames | 4J32 | Lowest expansion over a room-temperature window |
| Precision instruments and gauges | 4J32 or 4J36 | Depends on the width of the temperature window |
| LNG and cryogenic piping, tanks and membrane systems | 4J36 | Stable behaviour over a wide and mostly low temperature range |
| Composite tooling for aerospace layup | 4J36 | Large plates and blocks with predictable expansion |
| Glass-to-metal and ceramic sealing | 4J29 (Kovar) | Expansion matched to hard glass, not to zero expansion |
| Thermostatic and bimetallic elements | 4J36 with a high-expansion partner | The expansion difference drives the element |
The most common selection error we see is 4J32 being ordered where 4J36 is adequate. The second most common is 4J29 being confused with the low-expansion grades because both are used in electronic and instrument assemblies. The two alloys solve opposite problems: 4J29 is designed to match the expansion of hard glass so that a seal survives, while 4J32 and 4J36 are designed to expand as little as possible.
8. Fabrication Notes
8.1 Machining and forming
These alloys machine much like a soft austenitic material: stringy chips, a tendency to work harden, and a need for sharp tooling, positive rake and generous coolant. Cold forming of strip is common and the grades are ductile in the annealed condition, but the amount of cold work must be recorded because it changes both strength and the expansion response. Where a part will be annealed after forming, the forming strain and the annealing cycle together determine the final expansion.
8.2 Welding and joining
The low-expansion grades can be welded, but the weld and heat-affected zone do not reproduce the parent metal's expansion behaviour unless the joint is re-annealed, which is rarely practical on a finished assembly. For that reason, critical low-expansion structures are usually designed with mechanical joints, brazed joints with a compatible filler, or adhesive bonding rather than welded joints. Where welding is unavoidable, the joint should be located away from the dimensionally critical portion of the part, and the procedure should be qualified with expansion measurement if the dimensional requirement is tight.
8.3 Surface and contamination control
Avoid sulphur and chlorine-bearing marking materials, and control shop contamination, because these alloys are used in assemblies where surface chemistry and cleanliness affect both corrosion resistance and, in instrument applications, magnetic behaviour.
9. Purchasing Checks and Stock Forms
| Form | Common size range | Notes for the buyer |
|---|---|---|
| Cold-rolled strip | 0.05-3.0 mm thickness | Specify thickness tolerance, width, edge condition and flatness |
| Sheet and plate | 0.5-50 mm | For tooling and structural frames; confirm annealed condition |
| Round bar | 3-150 mm diameter | Peeled or ground finish for instrument parts |
| Wire | 0.1-8 mm diameter | Used for bimetals and precision elements |
| Tube | By enquiry | Seamless or welded, with expansion verified on the finished form |
The information that lets a stock supplier confirm availability the same day is: grade and composition requirement, product form, exact size with tolerance, condition (annealed, or cold worked to a specified temper), required expansion coefficient with the temperature range, quantity with cutting requirements, and the documentation level. Because these alloys are held as stock rather than rolled to order, cut-to-length strip, mixed sizes and prototype quantities are practical, which is often what determines whether a development programme stays on schedule.
10. Quality Verification
- Chemistry: verify nickel, cobalt and copper against the requirement, since these three elements control the expansion behaviour. A certifying report that lists only iron and nickel is incomplete.
- Expansion measurement: a dilatometer test reported with the temperature range and the method, calibrated to a traceable standard. This is the decisive acceptance test for a controlled-expansion alloy.
- Heat treatment record: annealing and cooling cycles recorded per load, since the expansion result depends on them.
- Mechanical check: hardness and tensile results, which confirm the ordered temper and detect accidental over-cold-working.
- Dimensions: thickness tolerance, edge quality and flatness for strip; diameter and straightness for bar.
- Traceability: heat number on each coil, sheet or bar, linked to the mill certificate.
- Identity check at goods-in: the grades in this family look identical, so a simple instrument check plus heat-number verification prevents the most common mix-up, which is 4J36 supplied against a 4J32 enquiry.
11. Frequently Asked Questions
What is the difference between 4J32 and 4J36?
4J36 is the 36 percent nickel alloy with an expansion coefficient of roughly 1.2 to 1.5 parts per million per kelvin over the room-temperature range and stable behaviour across a wide temperature span. 4J32 reduces the nickel content to about 32 percent and adds cobalt and copper, which lowers the coefficient over a 20 to 100 C window at the cost of a narrower useful range. Both are controlled-expansion alloys, but they are selected for different temperature windows rather than substituted for one another.
Can the expansion coefficient be guaranteed by chemistry alone?
No. Chemistry sets the potential, and heat treatment determines the achieved result. For that reason, a controlled-expansion purchase order should require an expansion test reported with its temperature range, plus the heat treatment cycle, rather than relying on a chemistry certificate only.
Does cold working change the expansion behaviour?
Cold work raises strength and reduces ductility, and it also affects the expansion response and the dimensional stability of the finished part. Where the expansion requirement is tight, the amount of cold work must be controlled and the part should be stress relieved or annealed according to the qualified cycle.
Why is my low-expansion component not dimensionally stable over time?
Dimensional drift in these alloys usually comes from residual stress introduced by machining or forming, or from an incomplete stabilisation treatment. A stress relief or stabilisation cycle, and thermal cycling where the application justifies it, normally resolves the problem. Chemical composition variation can also contribute, particularly if the nickel or cobalt content sits at the edge of the range.
Is 4J32 magnetic?
Yes. Below its Curie transition these alloys are ferromagnetic, which is exactly the mechanism that produces the low expansion. Above the transition they become paramagnetic and the low-expansion effect disappears. Applications that require both low expansion and low magnetic permeability are therefore not well served by this family.
Can 4J32 or 4J36 be welded?
They can be welded, but the weld metal and heat-affected zone will not reproduce the parent expansion behaviour unless the joint is re-annealed, which is often impractical. For dimensionally critical assemblies, mechanical fastening, brazing with a compatible filler or adhesive bonding is usually preferred, with welding placed away from the critical dimensions.
How should the expansion requirement be written on a purchase order?
State the grade, the required mean coefficient with its temperature range, the product form and condition, the size and tolerance, the required test method and the documentation level. A specification that reads only 4J32 leaves the acceptance criterion undefined, and the resulting argument at goods-in is both avoidable and expensive.
12. Summary
4J32 is the grade to choose when the dimensional requirement is severe and the operating window is narrow: laser and optical hardware, precision metrology and instrument frames. 4J36 remains the general-purpose controlled-expansion alloy for cryogenic and wide-range duty. Success with either grade depends on three things: specifying composition and expansion together, controlling heat treatment and cold work, and verifying the delivery with an expansion test and a heat treatment record rather than a chemistry certificate alone.
Send us the grade, the form, the size, the required expansion coefficient and the temperature range, and our technical sales team will confirm the closest available stock and the documentation that can be issued with the shipment.
