Invar 36 Round Bar | 4J36 (UNS K93600 / DIN 1.3912) Low‑Expansion Iron‑Nickel Precision Round Bar

Invar 36 round bar - equivalent to Chinese grade 4J36, UNS K93600 and DIN 1.3912‑is our premium nickel‑iron low‑expansion round bar engineered for machined parts that must hold tight dimensions despite temperature swings. Taking its name from the Latin "invariable", this alloy contains roughly 36 % nickel and delivers exceptionally low thermal‑expansion performance, remaining dimensionally stable from cryogenic conditions up to around +200 °C.

As an experienced manufacturer, we produce stress‑optimized Invar 36 round bar in hot‑rolled, peeled and centreless‑ground finishes, tightly controlling chemical composition, grain structure and residual internal stress to avoid post‑machining warping. It is the reliable material solution for precision‑instrument components, optical mounts, laser housings, cryogenic fixtures and composite‑tooling mandrels - scenarios where thermal drift would otherwise ruin measurement accuracy, optical alignment, CTE‑matched assembly or mould geometry. Every batch is supported with complete MTR documentation covering chemical analysis and measured thermal‑expansion test data, catering for prototype sample validation, small‑batch trials and large‑volume serial orders for global industrial customers.

Thermal Expansion: Average linear expansion coefficient:1.2‑1.5 × 10⁻⁶/°C (20‑100 °C). Stable low‑expansion performance from ‑250 °C up to +200 °C. Curie temperature ~230 °C; long‑term service temperature for precision components shall not exceed 230 °C.

Mechanical (Annealed State): Tensile strength 440‑680 MPa; Yield strength ≥240 MPa; Elongation 25‑35 %; HV ≤190.

Physical: Density 8.1 g/cm³; Melting point 1430 °C; Low thermal conductivity, ferromagnetic below Curie temperature.

Surface Quality: Peeled or ground bar free of cracks, laps, oxide scale and deep scratches; tight diameter tolerance and excellent straightness for downstream CNC machining.

Buyers often compare Invar 36 round bar with 4J32, 4J42, and 4J29. Here is the practical difference:

How to choose:

• Choose 4J36 (Invar 36) round bar when you need the widest low-expansion temperature range, proven dimensional stability, and the best cost-performance balance. This covers most precision instrument, optical, and cryogenic applications.
• Choose 4J32 (Super Invar) round bar when your operating window is narrow (−60 to +60°C) but your precision requirement is even higher. Expect higher cost and narrower availability.
• Choose 4J42 round bar when you need better ductility and machinability than 4J36, and your expansion requirement is less stringent.
• Choose 4J29 (Kovar) round bar when your application is glass-to-metal or ceramic-to-metal sealing in the 20–450°C range, not general low-expansion structural use.

One-line rule: general low-expansion precision work → 4J36 Invar 36 round bar. Extreme precision in a narrow window → 4J32 Super Invar. Sealing to glass/ceramic → 4J29 Kovar or 4J42.

Our Kovar vs Invar comparison guide clarifies key performance distinctions to help you avoid material mis‑specification.

• Near-zero thermal expansion. The average linear expansion coefficient is 1.2–1.5 × 10⁻⁶/°C in the 20–100°C range - roughly one-tenth that of ordinary steel. Across −60°C to +300°C, α remains ≤1.5 × 10⁻⁶/°C.
• Wide low-expansion range. Invar 36 maintains low expansion from −250°C to +200°C, making it suitable for both cryogenic and moderate-temperature applications.
• Good mechanical properties in annealed condition. Tensile strength 440–680 MPa, yield strength ≥240 MPa, elongation 25–35%. The annealed condition (HRA 65–75) provides a good balance of strength and machinability.
• Predictable magnetic behavior. Below the Curie temperature of approximately 230°C, Invar 36 is ferromagnetic with low expansion. Above 230°C, it becomes paramagnetic and expansion rises sharply.
• Proven across demanding industries. From satellite navigation resonant cavities to LNG membrane tanks, Invar 36 has demonstrated reliability where dimensional drift is unacceptable.

A professional Czech precision‑instrument manufacturer required Invar 36 ground round bar for optical‑alignment shaft blanks used in laboratory measuring equipment.

Client's original requirements & technical pain‑points

• Strict straightness and diameter tolerance for centreless‑ground bar; residual internal stress must be minimised, to avoid slow dimensional drift after customer's rough‑to‑finish CNC machining. Early trial material sourced from other suppliers caused minor warping after machining, leading to out‑of‑tolerance finished shafts.
• CTE value must stay within 1.2‑1.5 × 10⁻⁶/°C over 20‑100 °C; full thermal‑expansion test report required for local European incoming inspection.
• Mixed‑size batch including regular stock plus several non‑standard intermediate diameters; sample pre‑validation before mass delivery; documents need to meet EU industrial procurement requirements.
• Stable lead‑time for serial replenishment, considering long‑distance sea freight from China to Czech Republic.

Our technical solutions to overcome challenges

• Metallurgy & heat‑treatment optimisation: We adopted vacuum induction melting to control S, P impurities at low level. After hot rolling and peeling, we added dedicated high‑temperature full annealing plus low‑temperature stabilisation treatment before centreless grinding. This step largely eliminated mill‑origin residual stress, the root cause of post‑machining warping.
• Precision grinding & straightness control: Multi‑pass centreless grinding with intermediate straightening process. Each bar piece was inspected for diameter, roundness and straightness before packing.
• Full‑set testing documentation: Every lot performed independent CTE lab testing. MTR 3.1 documents contained measured CTE curve, mechanical and chemical data, fully satisfying Czech‑side incoming quality audit.
• Flexible batch arrangement: We first delivered small validation samples. After customer passed lab test and machining verification, we organised mixed‑size mass production, reserved proper production buffer to match sea‑transit cycle for Czech destination.

Project Outcome

The customer completed CNC turning without unexpected warping or deformation. Final optical‑alignment shafts maintained excellent dimensional stability under laboratory temperature fluctuation. We have kept repeat serial‑order cooperation with this Czech client ever since.

Our take‑away: For Invar 36 round‑bar applications, surface dimension alone is not enough. Controlled heat‑treatment and residual‑stress management determine whether your finished precision parts can hold tolerance long‑term.

Supplying Invar 36 Round Bar to a Czech Precision Instrument Manufacturer

Multi‑pass Centreless Grinding With Intermediate Straightening Process For Invar 36 Round Bar

Invar 36 Round Bar Ready To Ship For Czech Precision‑Instrument Client

We support diversified custom requirements for Invar 36 round bar:

• Custom diameter, tolerance, length, cutting‑to‑length service.
• Multiple surface options: hot‑rolled black surface, peeled, centreless ground finish.
• Custom metallurgical state: annealed, stress‑stabilised for ultra‑precision downstream machining.
• Pre‑sales technical consultation: we analyse your working temperature, machining workflow and risk of residual‑stress deformation; suggest proper material condition for your project.
• Sample validation, small‑batch trial and large‑volume serial orders are all welcome.

• Precision instruments & metrology: gauge blocks, calipers, micrometers, CMM guide rails, optical platform supports
• Astronomy & optics: telescope tubes, mirror bases, laser interferometer guide rails, optical benches, laser housings
• Aerospace & navigation: inertial navigation components, gyroscope structures, accelerometers, satellite resonant cavities
• Cryogenic & energy: LNG membrane tank components, cryostat fixtures, low-temperature test equipment
• Composite tooling: carbon fiber and fiberglass hot-press mold mandrels
• Nuclear & precision structural: reactor structural components, bimetallic parts, length standards, precision gears

Not sure whether Invar 4J36 or Kovar 4J29 is right for your application? Read the full Kovar vs Invar selection guide first, then confirm your specification below.

Kovar vs Invar selection guide

We are a specialized Chinese manufacturer and exporter of high‑performance special alloys, focusing on low‑expansion iron‑nickel alloys, glass‑to‑metal sealing alloys and nickel‑based alloys. Our product range covers Invar 36 (4J36, UNS K93600), Super‑Invar 4J32, Kovar 4J29 and other custom Fe‑Ni grades, available as round bar, sheet, strip, wire, forging and semi‑finished blanks. With full‑in‑house production lines including vacuum induction melting, hot forging, rolling, wire‑drawing, heat‑treatment and precision finishing, we strictly manage raw‑material screening, in‑process parameter monitoring and finished‑product lab testing to stabilize chemical composition, grain structure and residual stress for consistent batch quality.

More than a material supplier, our metallurgical engineering team provides free technical support including grade selection, processing advice and heat‑treatment guidance to solve practical technical challenges such as post‑machining deformation and thermal‑mismatch issues. Each shipment comes with complete MTR test reports, and we flexibly accommodate sample evaluation, small‑batch trial production and mass‑volume orders. Our alloys are widely supplied to global clients across aerospace, optical precision instruments, electronic packaging, cryogenic energy and composite tooling industries. Feel free to share your specifications or drawings, our technical‑sales team will deliver reliable and cost‑effective alloy solutions for your project.

Our Factory

 

Vacuum Melting Furnace

Automatic Cold Drawing Line For Expansion Alloy Bars

Straight Pipe Cutting Machine

4-Tensile Testing Machine

Flaw Detector

Hardness Tester

Q1: Why do finished Invar 36 parts warp even if bar dimension is qualified?

A: The main root cause is residual internal stress inside raw bar stock. For high‑precision parts, stress‑relieved & stabilised Invar 36 round bar is strongly recommended, rather than only focusing on diameter tolerance.

Q2: What long‑term operating temperature is allowed for Invar 36 round bar?

A: Curie temperature is approx 230 °C. For precision‑critical components, long‑term continuous working temperature shall not go above 230 °C, otherwise ultra‑low‑expansion performance will disappear. Short‑time peak temperature up to 450 °C is acceptable only for non‑precision structural usage.

Q3: Do you support sample testing for European customers such as Czech buyers?

A: Yes. We supply 2‑5 KG sample lots with complete MTR documents. Samples can be shipped by air express for quick material validation before mass order.

Q4: What surface condition should I select for CNC precision turning?

A: For high‑precision downstream machining, centreless‑ground + stress‑stabilised Invar 36 round bar is preferred, to minimise post‑machining deformation risk. Please share your machining workflow and tolerance requirement and our engineers will give targeted advice.

Q5: Is Invar 36 round bar corrosion‑resistant?

A: Moderate corrosion resistance. It works fine under normal atmospheric environment. For high‑humidity or corrosive‑medium conditions, follow‑up surface treatment such as passivation or plating shall be arranged on finished components.

Q6: Can you supply documents satisfying European incoming‑inspection standards?

A: Yes. Each shipment provides MTR 3.1 including actual measured CTE test data, which fits incoming‑audit requirements for industrial clients in Czech Republic and other EU countries.

Q7: What is Invar 36 round bar used for?

A: Invar 36 round bar (4J36, UNS K93600) is a low expansion precision bar used for precision instrument components, optical mounts, laser housings, cryogenic fixtures, composite tooling mandrels, and aerospace structural parts. It is chosen when temperature changes must not change the dimensions of a machined part - anywhere dimensional stability is critical.

Q8: What is the thermal expansion coefficient of Invar 36 round bar?

A: In the 20–100°C range, the average linear expansion coefficient is 1.2–1.5 × 10⁻⁶/°C - roughly one-tenth that of ordinary steel. Across −60°C to +300°C, α remains ≤1.5 × 10⁻⁶/°C. The low-expansion range extends from −250°C to +200°C.

Q9: What is the Curie temperature of Invar 36?

A: The Curie temperature is approximately 230°C. Below this temperature, Invar 36 is ferromagnetic and maintains its low expansion. Above 230°C, it becomes paramagnetic and the expansion coefficient rises sharply. This is why Invar 36 should not be used for long-term service above 230°C if low expansion is required.

Q10: What diameters of Invar 36 round bar are available?

A: We supply Invar 36 round bar from 3 mm to 300 mm in diameter. Standard lengths are 1 m to 6 m; custom cut lengths are available. Diameter tolerance can be h9, h11, or custom. Surface condition: peeled, ground, or polished.

Q11: Why does Invar 36 round bar need to be stress-relieved?

A: Invar 36 work-hardens rapidly during machining. Bar stock with high residual stress can distort after machining, especially when removing large amounts of material. Stress-relieved (annealed) bar ensures dimensional stability in the finished part. This is why we supply Invar 36 round bar in the annealed condition (HRA 65–75) with a controlled cooling cycle.

Q12: What is the difference between Invar 36 and 4J42 round bar?

A: 4J42 has a higher nickel content (~42%) and a slightly higher expansion coefficient than 4J36, but better ductility and machinability. It is commonly used for electronic packaging and glass/ceramic sealing where expansion matching is less stringent than Invar 36 applications. Choose 4J42 when machinability matters more than the absolute lowest expansion.

Q13: What is the difference between Invar 36 and 4J29 (Kovar) round bar?

A: 4J29 (Kovar) contains 29% Ni and 17% Co, and its expansion matches hard glass in the 20–450°C range. It is used for vacuum tubes, glass-to-metal seals, and ceramic-to-metal seals. Invar 36 is a general-purpose low-expansion alloy for structural and precision applications. They serve different purposes - choose based on whether you need glass/ceramic sealing or general dimensional stability.

Q14: What is the maximum service temperature of Invar 36 round bar?

A: Long-term service should not exceed 450°C due to oxidation concerns. However, for low-expansion performance, the practical limit is 230°C (Curie temperature) - above this, the low-expansion property is lost. If your application requires low expansion above 230°C, Invar 36 is not suitable.

Q15: Is Invar 36 round bar corrosion resistant?

A: Invar 36 has moderate corrosion resistance. It is stable in general atmosphere, fresh water, and mild corrosive media. However, in humid or strongly corrosive environments (seawater, acids, alkalis), surface protection is required - such as chrome plating, passivation, or electroless nickel plating. Electroless nickel (5–8 μm) can reduce oxidation rate at 400°C by approximately 40%.

Q16: Can Invar 36 round bar be welded?

A: Yes. Invar 36 has good weldability and is suitable for TIG, plasma, and laser-TIG hybrid welding. Recommended parameters for laser-TIG hybrid welding: laser power 4–6 kW, welding speed 0.5–1.0 m/min, current 150–170 A. Post-weld annealing at 600°C × 1 h is required to relieve heat-affected zone stress and preserve dimensional stability.

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