Welding & Heat Treatment of 17-4 3

[abassett]

If a specification allows for welding after solution and before age, can anyone tell me what the microstructure of the weld-metal looks like (more or less)?

Also, one spec allows welding in the aged condition, and re-aging the part. But it limits this practice to 2 times. Does solution reset the part to zero (metallurgically speaking)? Thanks.

 

[abassett]

One more thing....

For 1st question, I'm sure more information is needed to give a precise answer. Please assume cooling-rate resulting from TIG weld-repair of .250 section casting, with copper-backing plate, and try to get me in the ball-park. Thanks.

 

[metengr]

abassett;
What was the filler metal specification used for weld repair? If it was a matching chemical composition like AWS SFA 5.9 ER630, the weld metal microstructure will appear and respond just like the 17-4 base metal.

Yes, solution treatment after welding will allow the casting to be aged to the desired heat treatment condition.

 

[abassett]

Thanks metengr. Yes, it is a matching filler metal, but if the part doesn't get solution heat-treated after weld, what happens to the Cu in the weld metal? Is most of it dissolved anyway? Otherwise, when it goes to age, no precipitation hardening takes place in the weld metal.. right?

2nd question - are you saying there is no cumulative deleterious effect of aging a part multiple times, so long as it is solution treated first every time?

 

[MikeMet]

The answer to your second question is yes, fully solution annealing the welded part does put it back to square one with regard to how many time you can welding and only age after welding.

As to the first, the weld nugget in 17-4PH sst looks like most weld nuggets in any base metal with dentrites and directional solidification. It does not look like the tempered martensitic base metal. You will have untempered martensite in the weld and in the base metal HAZ, which is why you need to perform PWHT. Post weld aging tempers the HAZ and weld marteniste. The weld metal will not look like the base metal after a post weld aging since you haven't caused recrystallation, only some homogenization and precipitation of phases in the weld. To get the weld to look like the base metal you have to solution anneal, and even then you can still find the weld with etching.

The reason for limiting the number of times a part can be given post weld aging treatment is you begin to overage the base metal and degrade the properties. Solution annealing restores the base metal.

The ASME Code requires the part be fully solution annealed after welding. However, you can successfully perform only aging, typically for 4 hrs at temmperature, after welding and get predictable properties.

 

[SJones]

If you are trying to work out what will be the optimum heat treatment procedure, you need to have some idea of the strength, toughness and corrosion resistance effects of the heat treatment options and their bearing on the service performance of the casting. Try reviewing the following papers:

Science and Technology of Welding and Joining, Vol 4, (5), 1999, pp 295-301

Ditto, Vol 11, (5), 2006, pp 502-508

They have some nice optical and electron micrographs

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04