Welding 17/7 PH with 630 (17/4 PH) appropriate heat treatment 1

[Kim Taylor]

We have a project that requires welding together .050" wall 17/7 PH 2" OD tubing. The tubing is structural only and welds together at an angle (TYK configurations) with fillet welds. The filler metal selected is 630 (17/4 PH chemistry).
I had .050" strips of 17/7 PH welded with the 630 filler metal and had them heat treated to the normal 17/7 PH specification of TH1050 and some of them heat treated to the normal 17/4 PH specification of H900.
I then had the samples pull tested along with some 17/7 PH non heat treated.
I was trying to determine the appropriate heat treatment as the small welds would be partially 17/7 chemistry and partially 17/4.
The results are:
17/7 PH with the TH1050 HT - 92,550 psi
17/7 PH with the H900 HT - 110,400 psi
17/7 PH with no HT - 120,050 psi
Can anyone explain why the non heat treat samples was the strongest?

Kim Taylor PE CWI
Mechanical Engineer

 

[metengr]

What was the original heat treatment condition of the 17-7 PH stainless steel before welding??? Did you re-solution anneal and age the weld coupon or other heat treatment after welding? How did you conduct your pull test with fillet welds?? Lots of missing information.

 

[EdStainless]

I have seen this done a couple of ways.
Of course using 17-7PH filler is easiest.
The required 17-7PH HT is solution anneal at 1950F, condition at 1400F, cool to room temp and then age at 1050F.

The best compromise is to weld the 17-7 in the conditioned state (1400F treatment).
Then after welding age the entire thing at 1050F.
This will develop full strength in the 17-7, and T1050 strength (which is similar) in the 17-4.

If you expose the 17-4 filler to 1400F you will end up with it very over aged and soft (but with great ductility).
The as welded is strong because the cooling from welding left it in the aging range long enough to gain some strength.

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P.E. Metallurgy, Plymouth Tube

 

[HakiNoir]

17-7 PH (semi-austenitic) has Al precipitate while 17-4 (martensitic) has Cu; thus the aging behavior would be different. As mentioned by metengr, the initial state of 17-7 is also important. You have to see the cross section and see how the welding affect the 17-7 and 17-4 microstructure; I guess this would be a good research project. The PH SS are fun and challenging stuff! I am dealing with the one right now:

http://www.eng-tips.com/viewthread.cfm?qid=426698

 

[EdStainless]

We even did some trial welding 17-7PH to 4130.
The 4130 was Q&T with 1050F anneal.
The 17-7PH was in the conditioned state (1400F only).
We welded with 17-4PH filler and then aged/stress relieved the whole thing at 1050F.
Tensiles and impacts were all great. The 4130 was weakest at about 150ksi.

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P.E. Metallurgy, Plymouth Tube

 

[Kim Taylor]

Thank you for your responses. Thanks to your input, we decided to purchase the 17/7 filler rods and sample weld, heat treat and tension test another round of coupons. The base metal is 0.050" 17/7 PH Cond A.
We welded 8 coupons (two pieces 0.050" x 1.00" x 4.00" with a 1" overlap between with a fillet weld (.050" x 0.050" legs x 1" length)
We had four of the coupons heat treated to TH1050.
We had the four heat treated and the four non-heat treated coupons "pull-tested" with the following results:
The Heat Treated coupons averaged an Ultimate Tensile Load of 4,495 Lbs.
The Non Heat Treated coupons averaged and Ultimate Tensile Load of 5,313 Lbs.
All coupons failed at the weld.
The Heat Treated coupons did not register a decline at the yield point whereas the Non Heat Treated yield averaged 3,271 Lbs.

If you apply math to the area of a .050" x 1.00" fillet weld A=.050"x.707x1"=.035 in2 this would give us average stress' and yields' of:
UTS Heat Treated - 128,429 psi
UTS Non Heat Treated - 151,800 psi
YS Non Heat Treated - 93,457 psi

I expected the heat treated stress to be higher than the non heat treated coupons. Can anyone explain these results?

Kim R. Taylor PE CWI
Mechanical Engineer

 

[metengr]

First, your testing the shear strength of the fillet lap welds because if I understand your coupon geometry, the stress applied is under shear load and not tension loading across the fillet weld.

My suggestion is to use a groove weld joint design on a coupon of this size, and pull apart, which will be under tension and provide more meaningful results.

 

[EdStainless]

Kim, when you say heat treated to TH1050 I presume that you mean conditioned at 1400F and then aged at 1050F.
Though without going back to anneal I am not sure what properties you should expect.


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P.E. Metallurgy, Plymouth Tube