Nimonic 80A

By Niekie3 (***)

Date 05-06-2002 17:48

I have not had experience with this particular alloy, but precipitation hardening alloys pretty much all work the same way, so I can certainly give you answers from first principles.

I am just wondering what you are trying to achieve? Do you want to soften the alloy to perform some kind of repair and then later re-heat treat it to achieve the properties again?

We must remember that the solution treatment actually puts all the precipitates back into solution. This is required if you already have an over-aged structure. You would then subsequently perform the age hardening treatment to grow the precipitates to the optimal size. It is these precipitates that result in the strengthening of the material.

What exactly do you want achieve with your heat treatment?

Regards
Niekie Jooste

By Diego Date 05-07-2002 12:09

Thanks Niekie! The exhaust valves must retain their high-temperature properties (creep resistance, corrosion resistance, strength) after the reconditioning process (building up a few layers of matching Nimonic 80A). I understand the principles behind age hardening, but when it comes to welding age-hardenable superalloys I have found little information. For instance, I've read that that this type of alloy should only be welded in the "soft" condition, which means solution treating to above 1000 C. However, I have also read that "no prior heat treatment is necessary".

Main concern is to avoid strain age cracking during welding and during PWHT. Any thoughts?

By Niekie3 (***)

Date 05-07-2002 20:59

Generally you will need to re-heat treat your component following welding because you will have areas of over-ageing, under-ageing and no-ageing depending on where you look in your component. (Caused by temperature gradients.)

The heat treatment condition before welding will therefore have no bearing on your final properties, because you will be re-establishing these properties with your heat treatment. This heat treatment (following welding) will therefore have to include the solution heat treatment, fast cooling and subsequent ageing treatment.

It is true that cracking can be minimised by getting the material in a soft condition prior to welding. This can be either by solution heat treatment only, or by overageing with or without a solution treatment. Exactly how to achieve the overaging condition you would have to get from the suppliers of the material, or by performing some hardness tests.

Hope this helps
Niekie Jooste

By Diego Date 05-16-2002 08:17

Niekie,
Thanks for the info! I agree now that solution treatment is necessary as first stage in PWHT. THe problem now is ramping up to solution temp. I'm thinking it might be a good idea to soak at 600C for long enough to overcome mass effects and then ramp up as fast as possible to get through the aging range ASAP. Do you know what a good rate is to traverse the aging temperature range?
Thanks again!
Diego

By Niekie3 (***)

Date 05-16-2002 17:45

Hi Diego

The rate at which you ramp up the temperature to the solution heat treatment temperature is not a big issue, because any aging or over aging that may occur will be eliminated by the solution heat treatment. What is important here is the solution temperature and keeping it at this temperature long enough to ensure complete dissolution of the pricipitates.

Another critical step is the cooling rate from the solution temperature. With some alloys, cooling in still air is adequate. With some other alloys, water quenches are required. You will need to get this info. from the suppliers. The next critical step would be the aging temperature and time. Again, this info must come from the suppliers.

Regards
Niekie Jooste

By Diego Date 05-17-2002 14:25

Thank you again, Niekie. It's good to get recent information on the topic... I'm getting most of my information from Nimonic books from 1974, 1987, etc! But I'm sure that strain-age cracking (which does occur on ramp-up to solution temperature) is still a problem for 80A and still requires some solution other than pre-weld heat treatment to soften the base metal. Do you have experience with strain-age cracking? If ramping up quickly through the aging range is not a viable solution then something else must work... perhaps heating in vacuum?

Yes, cooling rate from solution temperature is my next big issue! I am looking to buy a furnace right now to accommodate for 80A. Is an air cool really all that is necessary? No stepping-down slowly?

I wish I had suppliers to call about this. I will be receiving these exhaust valves from service, they will have been previously reconditioned by other repair shops who used their own weld wires and heat treatments, etc. Most I can do is chemically analyze to determine the base metal! Any thoughts on where else I can receive this sort of information?
Regards
Diego

By Niekie3 (***)

Date 05-20-2002 19:05

Hi Diego

For strain-ageing to occur, you need to have some kind of plastic deformation to have taken place in your component. It is possible that when the valve was originally made, it was subject to significant plastic deformation. But, the reason that it was made from a precipetation hardening material would suggest that the original manufacturer would have performed a solution heat treatment followed by an ageing treatment to obtain the best properties from the material. This heat treatment would have eliminated any cold deformation effects and you should not have to worry about significant strain-ageing effects.

Minor strain ageing effects may occur in the HAZ, (Due to plastic deformation of the material during cooling, following welding.) but this would be so localized that it should not lead to cracking.

Heating the component in a vacuum would have no influence on this aspect. It may reduce surface oxidation though.

Hope this helps

Regards
Niekie Jooste

By Niekie3 (***)

Date 05-23-2002 17:27

Looked up in my old stahlschlussel what the working parameters are for this material. This is what I found:

Solution heat treatment: 1065°C - 1095°C for 8 hrs followed by cooling in air.
Temper (Overage): 835°C - 865°C for 24 hrs
Age: 685°C - 715°C for 16 hrs followed by air cooling.

For the repair, you can use the following electrodes:

ENiCr19Nb or ENiCrFe-3

Here is my suggestion to you regarding the repair procedure:

1)Assuming that your valve was properly heat treated following its last repair, you need not worry too much about cracking. If however you want to be conservative, then subject the part to the solution annealing treatment before you do anything. (This will minimize any posibility of cracking.)
2)Perform the relevant weld repairs. (I assume it is weld build-up repairs.)
3)Perform the solution anneal treatment again. (This will once again leave the material relatively soft.)
4)Do any machining that will be required, leaving some material for final grinding. (Do this before aging so that machining will be easier.)
5)Perform the aging treatment.
6)Perform final grinding procedure to obtain final sizes.

It is my opinion that you need not worry too much about strain aging effects because, as I mentioned in a previous post, you should have no significant straining of the material during the repair procedure.

Hope this helps

Regards
Niekie Jooste

By Diego Date 05-24-2002 14:08

Thanks for the extra info, Niekie!

Just making sure about the strain-age cracking problem. Most literature I've gleaned on gamma-prime age-hardenable alloys (including the Welding Handbook) recommends welding in the solution annealed condition for precisely that reason.

Machining before aging is a great idea! Simplifies the furnace requirement as well, since an elaborate step-down isn't needed if the part is just removed and allowed to cool in air after both steps.

I've seen that 3-stage treatment as well. The intermediate stage is to maximize carbide precipitation (Cr23C6, Cr7C3), so components operating at high temperatures don't continue to harden in service. I'm opting for the simpler 2-stage treatment (without overaging) for this reason; the valves will not see such high temperatures. (OK, plus 24 hours is not very practical! If anything we may reduce this step somewhat).

Speaking of shortening the steps, I've been revisiting my old Nusselt numbers and heat transfer equations trying to calculate exactly which soak times are required for valves of specific shape and size. "The Nimonic Alloys" (Betteridge and Heslop) documents 8hrs/24hrs/16hrs as "standard" for Nimonic 80A "bars"... but what thickness bar? Doesn't say! Ah well, wouldn't be engineering if there weren't problems...

Thanks again!
Diego

By Niekie3 (***)

Date 05-24-2002 18:37

Hi Diego

I believe that the times given for the treatments are not dependent on the section thickness. This is so because even relatively large sections will heat up pretty much at the same rate throughout the section. Larger sections may take longer to heat up than smaller sections, but but once at soaking temperature, the precipetation or dissolution mechanisms should take the same time.

It is therefore important to place thermocouples on the components themselves, to monitor the temperature, rather than only monitoring the furnace atmosphere temperature.

Regards
Niekie Jooste