PWHT 17-4PH 3

[tc7]

In reading the ASM Welding Hanbook Volume 6, pg 483-484, there seems to be a strong preference to weld 17-4PH material in the annealed condition, then after weld we should age harden to the desired condition (in my application, final condition will be ~H1050).

If this sequence is not possible, the ASM goes on to recommend a variation of the above (if we cannot weld in annealed condition) we may solution treat the finished weldment, then age harden it.

In either case, can anyone tell me the particulars of the solution treatment (i.e., temperature, holding time, heating rate and cool down rates) and also the age hardening particulars (i.e., temperature (1050 deg F in my case?), holding time, temperature rates)? A reference (I can’t find it in the ASM) for such information would also be appreciated.


Would also like to understand why in the postweld situation we need for a 2-step process, i.e., why solution treat (anneal) then age harden. Isn't the welding partially annealing the welded area?

Thanks in advance.

 

[EdStainless]

When you weld a PH grade you get all kinds of mixed structure in the weld and HAZ. You need to anneal in order to be starting from the same place in order to age harden.
You can weld in either the annealed condition or the overaged condition. You don't really want to weld in the higher strength conditions because of the risk of cracking.

Then you anneal, usually 1900F-1950F. I don't remember if AMS 5604 has any information on the annealing times.

Aging is usually +/-10F from the stated temp. For 1050F you would use a 4 hour age cycle.

Buy the applicable AMS spec. You need it.

For some kinds of general industrial use people weld annealed plate and then go straight into aging. The HAZ of the weld ends up at lower strength than the rest of the part because it is overaged (welding + aging).

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[TVP]

The appropriate SAE standard is SAE AMS 2759/3D. You can obtain it directly from SAE:

http://www.sae.org

 

[metengr]

There is also excellent information on welding and thermal treatments for 17-4PH, and it is free;

http://www.aksteel.com/markets_products/stainless.asp

Look up the 17-4 PH precipitation hardening
stainless steel data bulletin

 

[tc7]

I got the AMS 5604, also the SAE AMS 2759 and the AK Steel Bulletin. I think I'm just about set. Except for two questions;

1. If the designers decide that the annealed condition is satisfactory for the final weldment, then the PWHT should only consist of a solution heat treatment; but if they want a final condition of H1050 , then I still have to solution heat treat followed by precipitation hardening - IS THIS CORRECT?

2. For GTAW welding 1/2" plate and SMAW welding 3/4" plate what may be the recommended kJ heat input? (cannot find this in the AK Steel Web site or ASM handbook)

Thankyou again.
tc

 

[EdStainless]

You do not use these alloys in the annealed condition. The properties are rather unpredictable.
If you want low strength and high ductility then you use the overaged condition.
You two choices for fabrication are:
1. Start with annealed plate, weld with matching filler, anneal, age
2. Start with annealed plate, weld with matching filler, age
The second option will give you lower strength in the HAZs.

I am not sure on the heat input, all of my work is autogenous.

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Rust never sleeps
Neither should your protection

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[metengr]

tc7;
If you are going to weld 17-4PH, I would recommend using heat inputs for the SMAW and GTAW processes similar to welding on austenitic stainless steel. The 17-4PH grade does not require preheat for thickness less than 4".

Regarding PWHT, here is an excerpt from ASM Handbook Volume 6 (by the way, I would suggest you re-review this volume on welding of 17-4PH material, there is plenty of information)

Postweld Heat Treatment. In the as-welded condition, the weld metal and the high-temperature regions of the HAZs of welds in 17-4PH stainless steel have structures consisting primarily of untempered martensite plus a small amount of ferrite (Fig. 2a). Weldments exhibit an aging peak in the HAZ and a weld metal hardness that is only slightly less than that of the base metal in condition A (Fig. 3a). Weldments in 17-4PH stainless steel are not usually put into service in the as-welded condition except for repair welds where PWHT is impractical. In order to obtain weld properties approximating those of the base metal, PWHT is necessary. For single-pass welds made with the base metal in condition A, a simple aging treatment of 1 to 4 h at 480 to 620 °C (900 to 1150 °F) is usually sufficient. It simultaneously hardens the weld metal, HAZ, and base metal and lowers the residual stresses associated with the weld. Because only slight overaging occurs in the portion of the HAZ that is heated into the aging temperature range during welding (Fig. 3a), joint efficiencies of 97 to 100% are obtained (Ref 25).

In multipass welds, the repeated heating from the deposition of successive beads may leave a variation in structure from bead to bead that will result in nonuniform response to the aging treatment. Consequently, in the aged condition, weld yield and ultimate tensile strengths are only about 65% and 80 to 90%, respectively, of the base-metal values (Ref 25). Solution treating the weld before hardening reduces the weld metal and HAZ ferrite contents and improves weld metal uniformity and response to heat treatment. As a result, weld strength increases to 80 to 90% and 90 to 95%, respectively, of the base-metal yield and ultimate tensile strengths. For welds made with the base metal in the overaged condition, solution treatment is required if it is desired to heat treat the weldment to a higher strength level. In general, if the weld deposit is less than 13 mm (0.5 in.) in thickness, fairly good tensile properties can be obtained even if the solution treatment is omitted prior to aging. However, the toughness of the weld metal decreases with aging temperature above 540 °C (1000 °F), probably due to an unfavorable carbide morphology. Therefore, if weld deposits are 13 mm (0.5 in.) or greater in thickness and a postweld solution treatment is not feasible, an age-hardening temperature of 550 °C (1025 °F) or lower is suggested (Ref 25).

 

[tc7]

Good morning metengr-
Actually that excerpt from the ASM handbook was exactly what caused me to post my original questions. I found it to be somewhat confusing until yourself, Ed & TVP explained things more clearly to me.

As for the heat input question, I still haven't found a good source for that data, whether it is GTAW or SMAW on any material. Any suggestions?
tc

 

[metengr]

tc7;
In lieu of heat input, I would recommend you monitor the interpass temperature during welding with no preheat. I would recommend a maximum interpass temperature of 350 deg F.