Protecting CDA 260 Brass from Dezincification while annealing

[xom1]

We currently anneal quite a bit of our raw CDA 260 brass and find that as we approach annealing temperatures of 900-1000 deg. F, our product dezincifies to such a great extent that no amount of mechanical or chemical finishing can make the part look correct again. Does anyone have any tips as to how to anneal 70%Cu/30%Zn brass? Thanks.

 

[btrueblood]

Are you doing this in an air furnace, or under a controlled (reducing) atmosphere?

 

[kenvlach]

Not enough detail. Atmosphere, t, T? Continuous or batch? Does a thick oxide scale form?

Probably, too much time at too high a temperature with too much oxygen present. Also, verify that the initial composition is correct ($ Cu = 1.44 x $ Zn, so cheating incentive), higher Zn might segregate as more oxidizable beta-phase.

Are the annealing parameters based on existing cold work? Cold-worked C26000 recovers about 75% of its elongation potential at 800 [sup]o[/sup]F (1 hour).

Combusted natural gas, air-to-gas adjusted to give 0.5 to 1% hydrogen after drying, is the most widely used protective atmosphere for the anneal of copper and copper alloys. Direct firing (w/o drying), even with slight excess of gas, may cause discoloration and staining of the metal by water vapor during cooling.
-- Metal Handbook Volume 4 Heat Treating, pages 719-728.

 

[xom1]

Our most recent attempts have been at 700 degrees to 950 degrees F for time durations of 1 hour, 2 hours, and 3 hours in standard atmospheric conditions.

 

[FinishingTalkdotcom]

The majority of the parts we anneal are 70% Cu / 30% Zn. We anneal at 500 degrees F for 8 hours, gas fired oven, normal atmosphere. I have completed exhaustive stress corrosion experiments with gases from ammonium chloride solutions (raised to pH of 10.0) and mercurous nitrate solutions. The result was that the above anneal time and temp was satifactory to acheive our desired resistance to stress corrosion cracking.
Obviously, this annealing process results in an oxide film on the brass, which can be removed with a warm solution of blended acid salts (sulfate/flouride) or a cold dilute solution of hydrogen peroxide/ammonium biflouride. The parts can then burnished to a bright finish.

 

[kenvlach]

The lower annealing temperature given by FinishingTalkdotcom minimizes the oxidation scaling problem experienced by the OP.

For more info on the acidic descaling solutions mentioned above, plus tumble cleaning, electrocleaning, electropolishing & bright dip solutions, see ASTM B281, 'Standard Practice for Preparation of Copper and Copper-Base Alloys for Electroplating and Conversion Coatings.'

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B281.htm?L+mystore+moba4412