Invar 36 – CNC Machining

Invar 36 is used for CNC machined components where dimensional change from temperature variation must be minimized.

Its machining behavior is dominated by low thermal conductivity and a tendency to work harden if the tool dwells, driving heat at the cutting edge and accelerating wear.

At ShvaveyMetal, Invar machining workflows are defined around rigid setups, positive feeds without dwelling, and heat management to protect geometry and surface integrity.

Invar 36 is a nickel–iron low-expansion alloy with nominal 36% nickel content, commonly supplied under standards such as ASTM F1684 and ASTM B753 depending on product form.

It is selected when thermal expansion control is a primary design constraint rather than strength-to-weight optimization.

Mechanical Properties (Typical – Annealed)

Property Typical Value Units
Density ~8.05 g/cm³
Yield Strength (0.2%) ~275 MPa
Ultimate Tensile Strength ~450 MPa
Elongation at Break ~35 %
Elastic Modulus ~141 GPa
Hardness ~70 HRB

Mechanical properties depend on product form and cold work level (annealed vs. cold worked).

Thermal & Physical Properties

Property Typical Value Units
Thermal Conductivity ~10.5 W/m·K
Coefficient of Thermal Expansion (CTE) ~1.3 µm/m·K
Curie Temperature ~279 °C
Melting Range ~1,427 °C

CTE is temperature-range dependent and rises above the Curie region, so expansion behavior must be tied to the operating range.

Chemical Composition (Representative)

Element Typical Content (%)
Iron (Fe) Balance
Nickel (Ni) 35.0 – 37.0
Manganese (Mn) ≤ 0.60
Silicon (Si) ≤ 0.35
Carbon (C) ≤ 0.10
Chromium (Cr) ≤ 0.50
Molybdenum (Mo) ≤ 0.50
Copper (Cu) ≤ 0.50
Phosphorus (P) ≤ 0.025
Sulfur (S) ≤ 0.025

Composition ranges depend on the governing specification and supplier certification.

Invar machining response is driven by heat concentration at the cutting zone and sensitivity to work hardening when the tool rubs or pauses.

Key machining characteristics include:

  • Low thermal conductivity that promotes cutting-edge heat and shortens tool life at aggressive parameters
  • Work hardening risk if feed drops or the tool dwells, increasing cutting forces on the next pass
  • Tendency toward “gummy” cutting and built-up edge without sharp tools and stable chip evacuation
  • Surface finish sensitivity to tool wear progression, especially in finishing passes
  • Dimensional stability in service can be excellent, but machining-induced stress and heat input can affect local stability if sequencing is not controlled

Machining is typically performed in an annealed condition with rigid workholding, sharp tooling, adequate coolant flow, and continuous positive feed to avoid dwelling.

Benefit Description
Very Low Thermal Expansion Enables tight dimensional control across temperature changes in service
Dimensional Repeatability in Use Supports metrology frames, optical structures, and stability-critical assemblies
Weld and Fabrication Compatibility Can be formed and welded with appropriate procedures and filler selection
Cryogenic Performance Maintains useful strength and toughness at very low temperatures
Predictable CTE-Based Design Supports assemblies where differential expansion must be managed

Invar 36 is commonly used for CNC machined components such as:

  • Precision fixtures and tooling for composite forming where thermal stability is required
  • Optical and laser system structures and mounts
  • Semiconductor and metrology components requiring low drift
  • Cryogenic hardware where dimensional change must be minimized

Application suitability depends on operating temperature range, required CTE behavior, and stability requirements after machining and any stress-relief steps.

Condition General Characteristics
Annealed Baseline condition for machining with stable microstructure
Cold Worked (Cold Drawn / Cold Rolled) Higher strength with reduced ductility; can increase residual stress and stability sensitivity
Stress Stabilized (Dimensional Stability Cycle) Applied when ultra-stable geometry is required over time and temperature

When long-term stability is critical, a defined stabilization cycle may be specified to reduce cold work stress effects.

  • Define the operating temperature range used to specify CTE performance and inspection acceptance criteria
  • Avoid thin asymmetric geometries that amplify machining stress effects on flatness and alignment
  • Use conservative finishing passes with sharp tooling to reduce surface damage and heat input
  • Prevent dwell marks and work hardened skins by maintaining positive feed and avoiding rubbing
  • Plan for stability steps (stress relief and stabilization) when ultra-low drift is required over time and temperature

Early alignment between stability requirements, machining route, and inspection method is required for dimensionally controlled Invar parts.

FAQ

What is Invar 36?

Invar 36 is a nickel iron alloy containing approximately 36% nickel, known for an exceptionally low coefficient of thermal expansion near room temperature. This dimensional stability across temperature changes is why it is specified for precision optical, metrology and scientific instrument components.

Why is Invar 36 difficult to machine?

Invar is gummy and work-hardens readily, producing built-up edge, poor chip breaking and rapid tool wear. Its low thermal conductivity concentrates heat at the cutting edge. Most critically, residual machining stress can undermine the dimensional stability the alloy is chosen for, so stress management is as important as cutting performance.

What is the recommended approach to machining Invar 36?

Sharp carbide tooling with positive rake, moderate surface speeds, heavy consistent feeds to cut beneath the work-hardened layer, and generous coolant. Interrupted or light cuts should be avoided as they promote rubbing and hardening. Roughing, intermediate stress relief and finishing in separate stages is standard for precision components.

What is Invar 36 tooling used for?

Invar is widely used for composite layup and curing tooling, because its coefficient of thermal expansion closely matches carbon fibre composites. This keeps tool and part dimensionally matched through autoclave cure cycles, avoiding the distortion and residual stress that occur with steel or aluminium tooling.

What tolerances can be achieved on machined Invar 36 parts?

Tight tolerances are achievable, but they depend on disciplined process control rather than machining alone: staged material removal, stress relief, controlled fixturing that does not induce distortion, and inspection under stable temperature conditions. For optical, metrology and precision instrument components, this process discipline is what delivers the required stability.

 

Why Machine Invar 36 at ShvaveyMetal

ShvaveyMetal machines Invar 36 with workflows focused on heat management, rigid process control, and sequencing that protects dimensional intent in low-expansion assemblies.

This approach supports repeatable geometry and inspection outcomes for temperature-stable components across prototype and production machining programs.