Heat Treatment and Conductivity in CuCr Castings: What to Specify on the Drawing and How to Verify It

The Bottom Line

• Choosing the right CuCr alloy for the application is critical - but it is only half the job. Conductivity in CuCr castings is not fixed by alloy composition alone - it is set after casting, by the solution treatment and aging cycle that follows, and that step matters just as much as the alloy choice itself.

  
  

• A casting that skips or shortchanges aging will still pass every dimensional check on arrival, look identical to a correctly aged part, and only reveal its shortfall once it is in service carrying current or holding clamping force under load.

  
  

• Solution treatment alone leaves conductivity low: dissolving chromium into solid solution is a necessary first step, but it leaves conductivity around 40-50 percent IACS - aging is what recovers it.

  
  

• Aging is what restores conductivity and adds strength together: precipitating the dissolved chromium back out as discrete particles typically brings conductivity up to 80-85 percent IACS while simultaneously raising strength through precipitation hardening.

  
  

• Hardness testing is a fast proxy, but conductivity testing is the real verification: specify both on the drawing, and request a heat treatment certificate alongside the material certificate for every heat.

Why Heat Treatment Determines Conductivity in CuCr Castings

Heat treatment determines conductivity in CuCr castings because chromium behaves very differently depending on whether it is dissolved in the copper matrix or precipitated out of it. Dissolved chromium atoms sit inside the copper lattice and scatter conduction electrons, which is exactly what keeps conductivity low straight after solution treatment.

Aging gives those atoms time and temperature to migrate out of solution and cluster into nanoscale precipitates - once they are out of the lattice, electron scattering drops sharply and conductivity recovers, while the precipitates themselves pin dislocations and raise strength. This is the mechanism that makes CuCr castings usable at all: a strengthening route that does not have to trade away conductivity, provided the aging step is actually carried out to completion.

Solution Treatment and Aging Are Two Different Jobs

Solution treatment and aging are frequently discussed as a single step, but they do opposite things to the microstructure and it matters that both happen in the right order and for long enough. Solution treatment heats the casting to dissolve chromium fully into the copper matrix - skipping this step means there is nothing left to precipitate later, no matter how the aging cycle is run.

Aging then holds the casting at a controlled elevated temperature for a set duration, to let the dissolved chromium precipitate back out as the nanoscale particles that do the actual strengthening and conductivity recovery. Research on cast and wrought CuCr consistently shows conductivity climbing from roughly 45 percent IACS after solution treatment alone to the 80-85 percent IACS range once aging is complete, with hardness following a similar trajectory into the 120-180 HV range.

The table below summarises the typical property shift between solution treatment alone and a complete solution-treat-and-age cycle for CuCr castings.

Property	Solution Treated Only	Solution Treated + Aged
Electrical conductivity	~40-50% IACS	~80-85% IACS
Vickers hardness	Low, unaged condition	~120-180 HV
Typical tensile strength (cast)	~230-290 MPa (unaged)	~400-550 MPa

What Under-Aged CuCr Castings Look Like on Arrival

The uncomfortable part of this mechanism for buyers is that under-aged CuCr castings give away almost nothing on a routine incoming inspection. Dimensions, surface finish, and visual appearance are all governed by the casting and machining steps, not by heat treatment - a casting that received a shortened aging cycle, or none at all, will measure correctly and look correct. The shortfall only shows up in service: lower-than-specified conductivity means more resistive heating at rated current, and lower-than-specified hardness means the part loses clamping force or wears faster than the datasheet promised. By the time a contact is running hot or a bolted joint has lost torque in the field, the root cause is rarely traced back to an incomplete aging cycle that happened months earlier at the foundry.

Hardness Testing vs Conductivity Testing: Which One to Trust

Vickers (HV) or Rockwell B (HRB) hardness testing is fast, cheap, and a reasonable proxy for whether aging happened at all - a casting that reads far outside the 120-180 HV range (roughly 70-100 HRB for correctly aged CuCr) for its condition has clearly not been aged correctly. But hardness is a proxy, not the specification itself, and it does not directly confirm conductivity. Eddy-current conductivity testing, reported in percent IACS and typically performed per ASTM E1004, is the direct measurement of the property that actually matters for current-carrying CuCr castings, and it should be run on a sample from every heat rather than assumed from a hardness reading alone. The two tests together - hardness as a quick screen, conductivity as the real acceptance criterion - give a buyer a defensible basis for accepting or rejecting a heat lot.

What to Specify on the Drawing

Most drawings for CuCr castings name only the alloy - for example, "C18200" - and stop there. That is not enough: alloy designation confirms composition, not the electrical conductivity a correctly heat-treated casting must actually deliver, and IACS is precisely the property most drawings leave unspecified despite it usually being the reason CuCr was chosen in the first place. Getting correctly aged CuCr castings starts with what the purchase order and drawing actually say, not with trusting a supplier's default process. The drawing should call out the heat treatment condition explicitly - solution treated and aged per the relevant alloy standard (for example, UNS C18200, EN/DIN equivalent CW105C) - rather than leaving temper unspecified.

The purchase order should require a minimum conductivity value in percent IACS and a hardness range, tested per heat and reported on a certificate separate from the standard material certificate. Requesting the heat treatment certificate alongside the material certificate closes the gap that a purely dimensional incoming inspection leaves open.

Where Correctly Aged CuCr Castings Matter Most

Verifying CuCr castings against a conductivity and hardness spec matters most where the part is doing real mechanical or electrical work, not just holding a shape. Creep deformation in high-amperage terminals and thermal creep in power switch contact carriers both depend on the strength that only correctly completed aging delivers - an under-aged casting loses clamping force faster under sustained load, which is exactly the failure mode these two articles cover in detail. Correctly aged CuCr also resists thermal softening far better than pure copper, typically retaining its mechanical properties up to roughly 475-525 degrees C, which is exactly the margin these two failure modes rely on when a contact runs hot under sustained load.

The same dependency runs through contact and terminal geometry: tulip contact architecture, VCB support electrodes, and GIS disconnector links and jaws all rely on the alloy hitting its full strength and conductivity spec, since these are precisely the components where a relaxed or under-conductive part raises resistance across an entire current path. Where design complexity pushes a component toward casting in the first place, the geometry and waste inflection point covers when that switch from forging or machining makes sense - heat treatment verification is part of the total cost picture on either side of that decision.

Which CuCr Variant Are You Actually Specifying

Not every application calls for exactly the same heat treatment condition, and the drawing should specify the standard rather than leave it generic. C18150 vs C18200 covers the temper and condition differences called out across this alloy family's standards - for example, the TF00 solution-treated-and-aged designation - which is exactly the kind of detail that belongs on a drawing rather than left implicit. For alloy selection questions beyond straight CuCr, CuCrZr vs CuCr and what is CuCrZr copper are useful starting points.

Sourcing Larger CuCr Castings

Heat treatment verification becomes more, not less, important as casting size increases, since larger sections age less uniformly and a foundry's furnace capacity and process control matter more. If your requirement runs above 20 kg per casting, sourcing CuCrZr investment castings above 20 kg covers the additional process and furnace considerations that come with scaling this alloy family up in size.

Complex CuCr Castings Need a Foundry That Understands the Metallurgy

As CuCr casting geometry gets more complex - thin sections next to thick bosses, cored internal passages, variable wall thickness - heat treatment response stops being uniform across the part almost automatically.

A furnace cycle that ages a simple bar shape correctly does not necessarily age every section of a complex casting to the same conductivity and hardness, since thermal mass and section thickness both affect how evenly a casting heats and cools during solution treatment and aging. This is exactly where the difference between a foundry that treats heat treatment as a controlled, verified metallurgical process and one that treats it as a generic outsourced step shows up in the finished part.

Buyers sourcing complex CuCr components are better served by a supplier like Pahwa MetalTech who can demonstrate process control and testing data across the section thickness range in their own castings, not just a certificate of conformance copied from a generic heat treatment vendor.

Three Signs Your CuCr Castings Were Not Properly Aged

• Conductivity below roughly 75-80 percent IACS on an eddy-current test, when the specification calls for the standard 80-85 percent IACS range.

  
  

• Hardness reading outside the 120-180 HV range typical for a correctly aged casting in this alloy family.

  
  

• No heat treatment certificate accompanying the material certificate - only a certificate confirming chemistry and dimensions, with no record of the solution treatment and aging cycle actually applied.

Conclusion: Specify the Heat Treatment, Not Just the Alloy

Naming the right alloy on a drawing is only half the specification - verifying CuCr castings for conductivity and hardness after heat treatment is what actually confirms the part will perform as expected in service. For the broader property set of this alloy family, the CuCrZr investment casting materials and process guide covers composition and mechanical properties in full, the CuCrZr investment casting vs hot forging comparison covers why casting preserves the precipitation-hardening response that forging can compromise, and why pure copper fails for high temperature switchgear applications covers the elevated-temperature case for choosing this alloy family over pure copper in the first place.

Request a Heat Treatment Verification Review for Your CuCr Castings

If you specify or receive CuCr castings for switchgear or connector applications, Pahwa MetalTech can review your current drawing callouts and incoming inspection process against the conductivity and hardness verification steps that actually confirm correct aging. Contact our engineering team to discuss your specification and certification requirements.