Vacuum Investment Casting of Oxygen-Free Copper (OFHC): Why Controlling Oxygen Below 10 ppm Is Critical

Specifying OFHC copper for a precision electrical or vacuum component is one decision. Verifying that the casting delivered actually meets the OFHC specification is another — and the two are not the same thing. Investment casting — also known as lost wax casting — of OFHC copper requires a manufacturing environment that maintains oxygen control throughout the entire melt and pour cycle. The resulting component must be verified against the OFHC oxygen specification on the finished casting itself, not on the incoming raw material.

This guide covers where OFHC copper investment casting is genuinely mandated rather than merely preferred, the two OFHC grades and what each delivers for specific applications, the ASTM and EN specifications that govern correct OFHC designation, how to write an OFHC copper casting specification on a purchase order, and what certifications to request from a supplier. For the full grade selection comparison between OFHC and ETP copper — including applications where ETP is the correct and more economical specification.

For a complete overview of the investment casting and lost wax casting process across all copper, stainless steel, and aluminium alloy families, refer to our complete guide on Investment Casting: Process, Materials, and Industrial Applications

OFHC Copper Grades — ASTM and EN Specification Equivalents

Two grades of OFHC copper are available for investment casting, with different oxygen limits and purity minimums. The correct grade depends on the application:

Grade	ASTM / EN Designation	O₂ (Max)	Cu (Min)	Electrical Conductivity	Specify When
OFHC Standard (C10200)	ASTM B170 / B187 /CW008A (Cu-OF)	10 ppm	99.95%	≥101% IACS	Vacuum interrupters, H₂-atmosphere brazing, precision conductors, RF components — the standard OFHC specification for most industrial applications
OFHC Electronic (C10100)	ASTM B170 / CW009A (Cu-OFE)	5 ppm	99.99%	≥101% IACS	Ultra-high vacuum (UHV) systems below 10⁻⁷ mbar, scientific instrumentation requiring absolute minimum resistivity and maximum purity
ETP (reference) (C11000)	ASTM B152 / B187 / CW004A (Cu-ETP)	~400 ppm	99.90%	100% IACS	Standard electrical: bus bars, switchgear connectors, terminal blocks, general high-conductivity electrical hardware where H₂ brazing and vacuum outgassing are not requirements

The IS 191 Part 2 (Bureau of Indian Standards) corresponds to OFHC grades C10100 and C10200 and is the applicable Indian standard for OFHC copper in Indian procurement. IS 191 Part 1 corresponds to ETP copper C11000.

Where Vacuum Investment Casting of OFHC Copper Is Mandated

The following application categories represent the service conditions where OFHC vacuum investment casting is not an optional upgrade but the correct specification for component reliability and service life. In each case, ETP copper in the same application creates a specific and documented failure risk.

• Vacuum Interrupter Contacts and Electrodes

  Sector:  Medium Voltage Switchgear

  
  

  The risk with the wrong specification:  ETP copper inside a sealed vacuum interrupter releases trace oxygen into the vacuum envelope over service life, progressively degrading the vacuum quality and the dielectric performance of the device. As vacuum quality declines, the interrupter's ability to interrupt fault current is compromised — a failure that is progressive, not sudden, and may not be detected until the device fails in service.

  
  

  What OFHC vacuum investment casting delivers:  OFHC vacuum investment cast contact discs — in the complex disc and cup geometries that vacuum interrupters require — deliver the outgassing performance specified for sealed vacuum devices. The near-net-shape investment casting geometry minimises post-cast machining on the contact surface, preserving the dense conductor layer where the arc interruption occurs.

  
  

• Copper Components for Hydrogen-Atmosphere Furnace Brazing

  Sector:  Electrical Assembly and Power Equipment

  
  

  The risk with the wrong specification:  Bus bar connectors, terminal hardware, and electrical conductor assemblies that are furnace-brazed in a hydrogen atmosphere — standard practice for high-conductivity copper assemblies where a clean, oxide-free joint surface is required — will embrittle at ETP copper if the brazing temperature exceeds approximately 400°C. The resulting components appear visually intact but fail mechanically at the braze interface under subsequent mechanical or thermal cycling. This failure mode is irreversible and requires complete component replacement.

  
  

  What OFHC vacuum investment casting delivers:  OFHC copper investment castings eliminate hydrogen embrittlement risk at the base material level. The investment casting process produces the complex geometry of bus bar junctions and terminal connectors as near-net-shape components, reducing the number of joints in the assembly and the total brazing requirement. Fewer brazed joints in an OFHC copper assembly means fewer potential failure points.

  
  

• RF Waveguides, Cavity Resonators and Microwave Components

  Sector:  Telecommunications and Defence Electronics

  
  

  The risk with the wrong specification:  RF and microwave components operating at frequencies above approximately 1 GHz carry current in a very thin surface layer — the skin depth at microwave frequencies. Surface conductivity and surface quality govern the RF performance of the component. Oxygen-containing phases in the copper surface layer degrade conductivity at the skin depth and affect the Q-factor of cavity resonators. Outgassing inside evacuated microwave cavities creates contamination that degrades the dielectric properties of the cavity over time.

  
  

  What OFHC vacuum investment casting delivers:  OFHC C10200 vacuum investment castings deliver the surface conductivity and outgassing performance that RF and microwave components require. Investment casting produces complex multi-port waveguide cross-sections and compound cavity geometries as near-net-shape components — geometries that machining from billet produces at substantially higher material cost and longer lead time.

 

• Stator Bar Connectors and Conductors for Hydrogen-Cooled Generators

  Sector:  Power Generation

  
  

  The risk with the wrong specification:  Large-scale generators — typically above 100 MW — use hydrogen as a cooling medium for the stator winding. Copper conductor components in contact with or adjacent to the hydrogen cooling circuit at elevated temperatures require OFHC copper to prevent progressive embrittlement that would develop in ETP copper over the generator's operating life. Generator refurbishment cycles are measured in decades; component failures in a hydrogen-cooled generator involve significant safety and production risk.

  
  

  What OFHC vacuum investment casting delivers:  OFHC vacuum investment cast stator bar connectors and terminal components — produced in the complex geometries that stator end-winding configurations require — provide the long-term reliability that generator design life demands. Investment casting produces multi-port connector geometry in a single operation, eliminating the fabricated joints that represent additional potential failure points in a hydrogen environment.

  
  

• Precision Conductors for Scientific Instruments and Measurement Equipment

  Sector:  Scientific and Research Applications

  
  

  The risk with the wrong specification:  Precision measurement instruments — resistance standards, current transformers, and measurement system conductors — require conductor material whose electrical properties are both at specification and consistent across production batches. Batch-to-batch conductivity variation in standard copper grades introduces systematic measurement error that is difficult to separate from the signal being measured.

  
  

  What OFHC vacuum investment casting delivers:  OFHC C10200 vacuum investment castings deliver conductivity at ≥101% IACS with batch-to-batch consistency verified and documented on the heat certificate. The dense, homogeneous conductor cross-section produced by vacuum investment casting eliminates the conductivity variation that oxygen-containing phases introduce in standard atmospheric-cast copper. For the most demanding applications, C10100 provides the highest achievable purity in a commercially produced copper casting.

For electrical switchgear and bus bar applications, see “Investment Casting of High Conductivity Copper & Brass Parts for Electrical Switchgear” and “Casting vs Fabrication of High Conductivity Bus Bars.”

For the complete range of copper alloy casting capabilities, materials, and applications, refer to the ultimate guide “Copper, Brass, and Bronze Investment Casting: Metallurgy, Process Control, and Industrial Applications.”

The OFHC Specification — What to Verify and How to Request It

OFHC copper investment castings must be verified against the oxygen specification on the finished casting, documented in a heat-specific material certificate. The incoming raw material certificate alone is not sufficient verification — it confirms the grade of the copper entering the foundry, not the oxygen content of the casting that leaves it.

The oxygen verification relevant to the buyer is the chemical analysis of the finished casting from the production heat in question, performed to the applicable standard, showing a result below 10 ppm (C10200) or below 5 ppm (C10100). This analysis should appear on the material certificate as a specific measured value, not as a conformance declaration or a reference to the incoming stock grade.

What to Request	Document	Standard Reference	What It Confirms
Chemical composition of casting	Material test certificate	EN 10204 Type 3.1  (or 3.2 for critical applications)	Oxygen content of the FINISHED casting below 10 ppm (C10200) or 5 ppm (C10100)
Conductivity of casting	Conductivity test report	ASTM E1004 (eddy current) or four-point probe	Finished casting achieves ≥101% IACS — confirming the OFHC condition in the cast component
Dimensional and visual inspection	Inspection report or CMM report	Per drawing specification	Near-net-shape accuracy and surface condition of delivered castings
Heat traceability	Heat number on certificate	ISO 9001:2015 / EN 10204	Each casting can be traced to its specific melt heat and the analysis certificate for that heat

When writing the specification on a purchase order, state the UNS designation (C10200 or C10100) or the EN designation (CW008A or CW009A), the maximum oxygen content as a numerical value, the required material certificate type (EN 10204 Type 3.1 as a minimum), and conductivity verification requirement.

Specifying 'OFHC copper' without the grade designation and oxygen limit leaves ambiguity that a supplier may not resolve in the buyer's favour.

When OFHC Is Mandated — and When ETP Copper Is the Correct Specification

OFHC vacuum investment casting is the correct specification when the component will be hydrogen-atmosphere brazed above 400°C, when it will operate inside a vacuum enclosure, or when it will be continuously exposed to hydrogen at elevated temperatures in service. In all three conditions, ETP copper creates a specific failure mode that OFHC eliminates.

For the large majority of copper investment casting applications — bus bar junctions, switchgear connectors, terminal blocks, distribution board connectors, and general high-conductivity electrical hardware operating at standard service temperatures in air — ETP copper (C11000) is the correct and more economical specification. 100% IACS conductivity is adequate for these applications, and the service conditions do not create the interactions that make OFHC necessary.

The full application-by-application decision between OFHC and ETP copper, with a decision matrix by service condition and a comparison of all relevant properties, is in the OFHC vs ETP grade selection guide.

Source OFHC Copper Vacuum Investment Castings from Pahwa MetalTech

Pahwa MetalTech produces vacuum investment castings of OFHC copper grades C10200 with oxygen below 10 ppm documented on every heat certificate. Full chemical analysis and conductivity verification on samples from each production heat.

To discuss an OFHC copper casting requirement or request a quotation — contact us or write to us at info@pahwametaltech.co.in