4J36 (Invar 36) Guide: Low Thermal Expansion, Standards, Applications and Quality Verification
Date: 2026年9月15日 Categories: Nickel Views: 320
4J36 (Invar 36) Guide: Low Thermal Expansion, Standards, Applications and Quality Verification
4J36, known internationally as Invar 36, is the reference low-expansion alloy: a nominally 36 percent nickel balance iron material whose thermal expansion is close to zero over the temperature range where most precision equipment operates. It was identified in the late nineteenth century, it is specified today in LNG carriers, composite tooling, cryogenic piping and optical metrology, and it is one of the few grades where the purchasing decision is influenced more by heat treatment and documentation than by the price per kilogram.
This guide is written from the stock side of the desk. We hold 4J36 strip, sheet, plate, bar, wire and tube, and the practical questions are always the same: what expansion value can be guaranteed and over which range, how the thermal history changes the result, which standard to cite, and what evidence should accompany the delivery. The sections below cover both grades of question, and they are written so that a buyer, a design engineer and an inspection engineer can each find the part that applies to them.
1. What Invar 36 Is and Why It Works
Iron-nickel alloys exhibit a magnetostrictive contraction as they are heated towards the Curie temperature. At a nickel content of roughly 36 percent, the spontaneous volume magnetostriction that accompanies progressive loss of magnetic order very nearly cancels the normal thermal expansion of the crystal lattice. The macroscopic result is a mean coefficient of thermal expansion of about 1.2 parts per million per kelvin between 20 and 100 C, against roughly 12 for stainless steel and about 11 for carbon steel.
Three consequences follow, and each of them shapes how the alloy is bought and used. The effect exists only below the Curie temperature, so above roughly 280 C the alloy expands like an ordinary steel. The effect depends on composition within narrow limits, so nickel content must be controlled tightly. And the effect depends on the metallurgical state, so heat treatment and cold work both change the measured value.
2. Chemistry of 4J36
| Element | Limit | Effect and reason for the limit |
|---|---|---|
| Ni | 35.0-37.0 | Sets the temperature of minimum expansion; deviation moves the curve and raises the measured coefficient |
| C | 0.05 max | Carbides disturb expansion uniformity and reduce ductility |
| Mn | 0.60 max | Residual from melting; contributes to oxidation and surface quality problems |
| Si | 0.40 max | Too high a level affects hot working and surface condition |
| Cr | 0.25 max | Chromium beyond residual levels alters the expansion curve |
| Co | 0.50 max | Cobalt is not part of this grade; the cobalt-bearing low-expansion grade is a separate specification |
| P, S | 0.025 max each | Hot workability and cleanliness; a free-machining variant deliberately adds sulphur and is a separate grade |
| Fe | Balance | Matrix |
Note how restricted the nickel window is compared with a corrosion-resistant alloy. A two percent band may look generous, but the minimum of the expansion curve is a sharp function of composition, so a heat at 35.1 percent nickel and a heat at 36.9 percent nickel will not deliver the same measured coefficient even though both meet the chemistry table. This is the technical reason why the certificate for a low-expansion alloy must include an expansion test result, not chemistry alone.
3. Standards, Designations and Test Methods
| Document | Role |
|---|---|
| GB/T 15018 | Chinese precision-alloy designation standard establishing 4J36 and the other 4J grades |
| GB/T 14985 series | Technical delivery conditions for expansion alloys in Chinese practice; cite the current edition on the purchase order |
| ASTM F1684 | Iron-nickel and iron-nickel-cobalt alloys for low thermal expansion applications; the standard most often cited for Invar 36 (UNS K93600) |
| UNS K93600 / DIN 1.3912 | International and European identifiers used to cross-reference drawings and certificates |
| ASTM E228 | Push-rod dilatometer method for linear thermal expansion, the usual acceptance test method for this family |
| ASTM E831 | Thermomechanical analysis method for linear thermal expansion, used where small specimens or thin strip must be tested |
| ASTM E112 / E18 | Grain size and hardness, used as supporting checks on delivered product |
Because the Chinese and international systems do not line up perfectly, the safest purchase order for 4J36 states the grade, the composition limits, the product form and condition, and the expansion requirement with its temperature range and test method, and then names the standard or standards that govern. That combination leaves nothing to interpretation at goods-in.
4. Thermal Expansion Performance
| Temperature interval | Mean coefficient, 10-6 /K | Comment |
|---|---|---|
| 20 to 100 C | approx. 1.2, limits commonly at or below 1.5 | The most frequently specified range for instruments and optical hardware |
| 20 to 200 C | approx. 3.0-4.0 | Expansion rises steeply as the Curie transition approaches |
| -100 to 20 C | approx. 1.0-1.8 | The low and stable behaviour that made the alloy attractive for cryogenic work |
| -196 to 20 C | Approx. 1.5-2.0 | Still an order of magnitude below steel, which is why it is used in LNG systems |
| Above approx. 280 C | Normal steel-like values | The magnetostrictive compensation is lost above the Curie temperature |
Three practical rules apply to this table. Always quote the temperature interval, because a coefficient without a range cannot be verified. Always confirm whether the requirement is a mean coefficient over the interval or a local value, because the two differ. And always relate the requirement to the delivered condition, because the same heat measured in the annealed state and in a cold-rolled temper will not give the same answer.
5. Physical and Mechanical Properties
| Property | Typical value |
|---|---|
| Density | approx. 8.1 g/cm3 |
| Curie temperature | approx. 280 C |
| Melting range | approx. 1425-1440 C |
| Elastic modulus | approx. 140 GPa |
| Poisson ratio | approx. 0.29 |
| Thermal conductivity | approx. 10-11 W/(m.K) |
| Electrical resistivity | approx. 0.78-0.85 micro-ohm.m |
| Magnetic behaviour | Ferromagnetic below the Curie transition; the low expansion depends on this |
| Condition | Tensile strength | 0.2 % yield strength | Elongation | Hardness |
|---|---|---|---|---|
| Annealed | approx. 450-550 MPa | approx. 240-310 MPa | 30-45 % | approx. 130-180 HV |
| Cold rolled, medium temper | approx. 600-750 MPa | Higher in proportion | 10-25 % | approx. 180-250 HV |
| Cold rolled, hard temper | Up to approx. 900-1000 MPa | High | Low, often below 5 % | Up to approx. 300 HV |
| Cryogenic service, approx. -196 C | Rises to roughly 700-800 MPa | Rises | Retains useful ductility | Not normally specified |
The strength figures look low, and they are: 4J36 is a soft alloy by comparison with stainless steel, and designs that treat it as a structural material usually run into stiffness and buckling problems rather than strength problems. Two other properties matter operationally: the alloy has effectively no chromium and therefore very poor corrosion resistance, so it must be protected with oil, VCI packaging or a coating; and its low thermal conductivity means heat stays where it is applied, which affects both welding and machining.
6. Heat Treatment: The Step That Determines the Result
The expansion measured on delivered material is a product of composition and thermal history. Annealing is normally carried out in the region of 830 to 900 C in a controlled atmosphere, followed by cooling at a rate that is part of the specification rather than the operator's choice. Where dimensional stability in service is critical, a stress relief at a lower temperature is applied after machining, and the most demanding metrology components are thermally cycled to confirm that they settle before release.
Two failure patterns appear repeatedly in incoming inspection. The first is a coil that meets the chemistry limits but misses the expansion limit because the final anneal was insufficient or the cooling was uncontrolled. The second is a part that meets the expansion limit as delivered but drifts in service because residual stress from machining was never relieved. Both are documentation problems as much as metallurgical ones: the fix is to require the heat treatment cycle and the expansion test in the certificate.
7. Applications
| Application | Why 4J36 is used | Form usually supplied |
|---|---|---|
| LNG carrier membrane tanks and primary barriers | Very low contraction from ambient to liquefied gas temperature, keeping the barrier and insulation system stable | Thin strip, typically less than 1 mm |
| Cryogenic piping, valves and expansion joints | Low contraction limits thermal stress in restrained assemblies | Seamless tube, bar, forgings |
| Composite layup tooling for aerospace structures | Tool and part expand together, so the cured component holds its dimensions | Plate and block, up to large thicknesses |
| Optical benches, laser cavities, metrology frames | Dimensional stability against workshop temperature variation | Bar, plate, tube |
| Precision instruments and clock pendulums | Removes temperature as a source of error | Bar, wire, strip |
| Thermostatic bimetals | Acts as the low-expansion side of the element | Strip |
8. Grade Selection: 4J36 and Its Neighbours
| Grade | Character | Use it when |
|---|---|---|
| 4J36 (Invar 36) | 36 % nickel, coefficient about 1.2 over 20-100 C, stable over a wide span | General low-expansion duty, cryogenic and LNG work, tooling and instruments |
| 4J32 | 32 % nickel with cobalt and copper, lower coefficient over a narrower window | The most severe room-temperature dimensional requirement |
| 4J38 | Free-machining variant with deliberately added sulphur | Heavy machining with a slightly relaxed expansion requirement |
| 4J29 (Kovar) | Iron-nickel-cobalt with expansion matched to hard glass | Glass-to-metal and ceramic seals, not zero-expansion structures |
| 4J42 / 4J50 | 42 % and 50 % nickel grades with medium expansion | Ceramic and glass sealing where the thermal match is to the ceramic, not to zero |
The most common mistake is ordering 4J29 because it is also a controlled-expansion alloy used in electronic assemblies. 4J29 is a sealing alloy whose coefficient is deliberately matched to glass; it is not a low-expansion alloy and it will not behave like 4J36 in a thermally stable frame.
9. Fabrication Notes
9.1 Machining
4J36 is soft, gummy and prone to built-up edge. Sharp tooling, positive rake, generous feed and flood coolant give the best results, and clamping pressure should be kept moderate because the alloy deforms easily. Where dimensions are critical, plan the machining sequence so that the finishing cuts are taken after stress relief, and record the amount of material removed, because machining changes the residual stress state of the part.
9.2 Welding
Welding is possible but it changes the local expansion behaviour: the fusion zone and heat-affected zone do not reproduce the parent metal's coefficient unless a matching low-expansion filler is used and the joint is subsequently annealed, which is often impractical on a finished assembly. For dimensionally critical work, keep welds away from the functional dimensions, use the filler specified on the drawing, keep heat input low, and verify the expansion of a representative joint if the requirement is tight.
9.3 Corrosion protection and handling
With no significant chromium content, 4J36 rusts readily in humid conditions. Stock is normally supplied oiled or with VCI protection, and fabricated parts should be protected before storage or transport. Fingerprint and moisture marking on strip surfaces is a common cause of rejection in optical and instrument applications, so handling discipline is part of the specification.
10. Purchasing: Forms, Tolerances and Documentation
| Form | Common size range | Points to specify |
|---|---|---|
| Cold-rolled strip | 0.05-3.0 mm | Thickness tolerance, width, edge condition, flatness, temper, expansion limit |
| Sheet and plate | 0.5-50 mm, thicker by enquiry | Thickness tolerance, flatness, ultrasonically tested if for barrier or pressure use |
| Round bar | 3-200 mm diameter | Peeled or ground finish, straightness, condition |
| Wire | 0.1-8 mm diameter | Temper, spool size, surface cleanliness |
| Seamless tube | By enquiry | Wall thickness tolerance, hydrostatic test, expansion verified on the finished form |
| Cut-to-size pieces | From stock | Saw or shear quality, burr removal, protective packaging |
Four commercial factors drive the price and lead time of 4J36. Nickel content is the largest, because the alloy contains roughly a third nickel by weight. The required expansion limit matters, because tighter limits widen the number of heats that must be rejected. The product form matters, because thin strip with tight thickness tolerance requires more rolling and annealing passes than bar. And the documentation requirement matters, because an expansion test on every heat is real laboratory time. A buyer who relaxes an unnecessary expansion limit or accepts a sensible thickness tolerance can often improve both price and delivery without compromising the application.
11. Quality Verification at Goods-In
- Chemistry: confirm nickel, and check carbon, manganese and cobalt against the limits; a certificate that lists only nickel is not sufficient.
- Expansion test: review the dilatometer result, confirm the temperature interval, the test method and the calibration, and compare it with the ordered limit. This is the acceptance criterion that decides whether the material can be used.
- Heat treatment record: annealing temperature, atmosphere, cooling rate and any stress relief, recorded per load.
- Condition and temper: hardness or tensile results confirming the ordered temper, especially for strip where cold work is intentional.
- Dimensions: thickness or diameter with tolerance, width, edge condition, flatness and straightness.
- Surface and protection: freedom from rust, roll marks, scratches and contamination, and confirmation of the protective coating or VCI packaging.
- Traceability: heat number marked on each coil, plate or bar and linked to the mill certificate, with the piece-to-heat relationship documented on request.
- Grade separation: 4J36, 4J32, 4J38 and 4J29 look identical in the warehouse. Physical separation, labelling and a simple verification step prevent the substitution that would otherwise be discovered only when a finished assembly fails its thermal test.
12. Frequently Asked Questions
What is the expansion coefficient of Invar 36?
Typically about 1.2 parts per million per kelvin over 20 to 100 C, with specification limits commonly set at or below 1.5 for that interval. The value changes with the temperature range and with heat treatment, so any quotation or certificate must state the interval and the test method along with the number.
Why does Invar 36 stop being low-expansion at higher temperature?
The low expansion is produced by magnetostrictive effects that only exist while the alloy is magnetically ordered. Above the Curie temperature of roughly 280 C the alloy behaves like an ordinary ferritic steel and expands at a much higher rate. Applications above that temperature need a different material strategy entirely.
Is 4J36 the same as 4J32?
No. 4J36 contains about 36 percent nickel with no deliberate cobalt addition and gives a coefficient near 1.2 over a wide range. 4J32 uses about 32 percent nickel with cobalt and copper to reach a lower coefficient over a narrower window. Both are low-expansion alloys, but they suit different temperature windows and are not interchangeable.
Does 4J36 rust?
Yes. The alloy contains essentially no chromium, so it has very little corrosion resistance and will rust in humid conditions or from contact with moisture and fingerprints. Stock should be kept oiled or in VCI packaging, and fabricated parts should be protected before storage or shipping.
Can Invar 36 be welded without losing its properties?
The weld and heat-affected zone will not match the parent expansion behaviour unless a matching low-expansion filler is used and the joint is annealed afterwards. Where welding is unavoidable, keep joints away from critical dimensions, keep heat input low, and verify the expansion of a representative joint if the dimensional requirement is tight.
How is the expansion coefficient verified?
By dilatometry, most commonly the push-rod method of ASTM E228, or thermomechanical analysis per ASTM E831 where thin strip or small specimens must be tested. The report should state the temperature interval, the mean coefficient over that interval, the specimen condition, and the calibration status of the instrument.
What documentation should accompany a delivery of 4J36?
An EN 10204 Type 3.1 mill test report naming the heat and the standard, with chemistry including nickel, carbon and cobalt; the expansion test result with its temperature interval; the heat treatment cycle; the mechanical or hardness results confirming the temper; and dimensions. Heat numbers on the product must match the certificate.
Is Invar 36 available in small quantities from stock?
Yes. Controlled-expansion alloys are usually held as stock in strip, sheet and bar, and cut-to-length pieces, mixed sizes and prototype quantities are practical. Because the material is supplied from stock rather than rolled to order, delivery is measured in days, which matters when a development programme has a fixed test date.
13. Summary
4J36 is the reference low-expansion alloy and remains the default choice for cryogenic work, LNG systems, composite tooling and precision instruments. Its expansion behaviour is the reason it is bought, and that behaviour is a joint property of composition, heat treatment and cold work. A purchase is therefore only as good as its specification: state the grade, the expansion limit with its temperature interval, the test method, the condition, the form and the documentation, and verify the delivery against all of them. Protecting the material from corrosion and treating the heat treatment record as part of the product are what separate a successful project from an expensive recalibration.
Send us the grade, the form, the size, the required expansion coefficient and temperature interval, and the documentation you need. Our technical sales team will confirm the closest available stock, the condition in which it is held, and the certification that can be issued with the shipment.
