Advice on Heat Treating 6061-T651 IAW ASTM B918 2

[tc7]

Attempting to restore T651 condition as a post weld heat treatment on 6061 T651 plate;
Table 2 of ASTM B918 for 6061 calls for Solution Heat Treatment of "plate" to bake at 960-1075 deg F and soak at this temperature for a minimum of 60 minutes – this based on Table 3 recommendations for plate thickness of .500 inches.

If the plate happened to run a little thicker, say .501 inches, then Table 3 requires a longer soak time of 90 minutes.

Question 1. This seems like a very wide spread in soak time for very little difference in thickness. My first question is: what actually occurs to 6061 during solution treatment and how critical are the soak time recommendations? If I were to soak the .500 thick plate for 90 minutes or even longer, am I degrading properties or structure?

Question 2. After the next step of quenching in water, what temper should I expect to have? The center column of Table 2 would indicate that I should expect T42 or T4 – am I reading this correctly? If not, please explain.

Question 3. Lastly, the Precipitation Treatment, or artificial aging, is how I should arrive at my final desired T651 condition: What is occurring during aging? Table 2 indicates I should hold the plate at 320 deg F for 18 hours. WOW! That is a long time. Are there other published allowable temps and times that will reliably get me to the desired T651 condition?

 

[CoryPad]

The time at temperature values are helpful suggestions based on how long it takes a given size part to reach the desired temperature. It actually is a continuous function, not a step function.

Answer 1: During solution heat treatment, solute elements are dissolved into the aluminium matrix. If you hold for longer, you don't degrade the structure or properties, you just expend extra energy/cost.

Answer 2: T4.

Answer 3: Precipitation of second phases occurs during artificial aging. This is a diffusion process, which is controlled by time and temperature. You can increase the temperature, which decreases the time requirement. However, you don't necessarily get identical properties. Yes, artificial aging times for Al alloys are long. However, the temperature is barely above boiling water, so the energy requirements aren't that high.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TVP]

tc7,

Keep in mind that your re-heat treat process is a T6 treatment, not T651. The difference between the two is stress relieving: T651 involves stress relieving by stretching the original sheet/plate.

MIL-H-6088 is now obsolete, but it has information on heat treating Al alloys:

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=5791

 

[tc7]

Thanks folks, very good points.

Not clear on why after Solution H/T & quench we arrive at T4 and not T0?

After the quench, is there any time limitations before we get into the precipitation treatment? Suppose we delayed a day or so before precipitation aging – any problems to consider?

 

[CoryPad]

There is no such designation as T0. Here is a listing of the basic tempers:

faq330-294

I am not aware of any time limitations. Since deformation after quenching is used to product T8 tempers, it is expected that there can be delays between quenching and artifical aging.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.