Thoughts on Precipitation Hardening

Materion discusses copper beryllium and precipitation hardening.

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While copper beryllium will increase in hardness and lose ductility as it ages from its solution annealed condition, it will begin to lose hardness and become more ductile once it achieves its maximum hardness of 36-42 HRC.  Copper beryllium sold into the molding industry is aged slightly past its peak hardness to provide more ductility and conductivity than a peak-aged product.  As most moldmakers who regularly use copper beryllium know, there is no way to age copper beryllium mold plate to make it harder.  Any further precipitation hardening of copper beryllium mold plate will not increase its hardness by 2 to 5 HRC nor will it cause the material to start cracking.

It should also be noted that it is impossible to age copper beryllium from the heat of the molding process. The temperature of the molten resin is not high enough and the copper beryllium has too much conductivity and heat capacity for the mold surface temperature to increase much above that of the cooling water outlet.  If the mold surface did get hot enough to age copper beryllium, it would not cool plastic and no one would have a reason to use it.   This holds for any other high conductivity alloy.

However, chrome plating incorrectly applied can lead to cracking because of the thermal expansion mismatch between copper and chromium.  This mismatch results in high interfacial stresses that crack the chrome plating once the mold surface starts to heat up from room temperature.  These cracks can cause the plating to chip off and act as stress risers that can cause the cracking to propagate into the copper.  A layer of nickel between the copper and chromium would have prevented this as the nickel's thermal expansion is midway between the copper and chromium, which would reduce the interfacial stresses, and the nickel's ductility would prevent cracks in the chrome plating from propagating into the mold.

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