Stress relieving copper at 200°C for 2 hours

Question

I'm not a material scientist, but I have some deep drawn electrolytic copper parts 99.9% that generally fold or don't bend very well or consistently when post processing in their drawn form. The machinists who draw the parts suggest stress relieving at 200°C for 2 hours. We have tried it and it works excellently for post stamping bending and processing, but I'm just trying to confirm it wont affect the material or make it weaker or reduce it's tensile or shear strength? I have looked at various charts and graphs and searched the Internet high and low and can't find an answer. Is anyone out there able to advise on this? Thanks, team.

Answer 1

By holding the copper at elevated temperature for a while, you are doing what is called annealing.

The inner structure of metals consists of individual crystalline areas called grains. As you work crystalline materials like copper, the grains break into smaller pieces and the material gets harder to work. The good news is if the temperature is above the re-crystallization temperature, the grains grow into each other and the metal becomes softer. I looked it up on the web and found various numbers for copper. One place says "Commercially pure copper: Recrystallizes at 180°C." and another says more like 400°C. I will speculate it is not a hard threshold but more of a rough boundary, the hotter it gets, the faster the crystals can recombine. It also depends upon the number and types of impurities. An interesting aside, unlike iron, if you anneal copper you can cool it quickly and it stays soft.

Something I remember from Material Science class in school is pure gold has a re-crystallization temperature below room temperature so you can hammer gold out forever. That's why they can make gold leaf thin enough to see through.