Gr.91 Chromemoly Hydrogen Bakeout

[FluidicTech]

We have a situation where we have butt welded two pieces of 12" Sch 140 (1.125" nom wall) SA 335 P91 CMS pipe. Preheat temperature was held at 400 degrees F until completing the joint. However, after completing the weld it was aloud to cool to ambient temperature. Generally, we would have allowed this to cool to not less than 200F and then raised the temperature back up to 700 F and held for 3 hours and then aloud to cool to ambient. Then we would PWHT for 2 hrs at 1400 F.

Our customer's specifications are written so that the part should not cool below 200 F before the intermediate heat treat. Is there any harm in allowing the part to cool below 200 F before the intermediate heat treat? ASME B31.1, the governing code, states that the preheat temperature shall be maintained until the PWHT unless an adequate intermediate heat treatmet is performed. Now that the part has been sitting at ambient temperature for days, can we perform the intermediate heat treat, allow it to cool in still air, and then PWHT later? Or would it be better to perform the intermediate heat treat and then immediatly follow with the 1400 F PWHT.

What is our best recovery?

 

[metengr]

FluidicTech;
Perform an RT (x-ray) and wet fluorescent MT of the weld and if acceptable, perform PWHT. The purpose of the intermediate thermal treatment prior to delayed PWHT is to assure removal of hydrogen. Since you missed this step before cool down, I would perform the NDT as I mentioned.

 

[DVWE]

I agree with metengr, it's pointless to do a hydrogen bake out at this point.

 

[Kwan]

If you missed the bakeout I would be concerned about the embrittlement. NDI may not pick up the microscopic cracking. You should consult with a material lab to SEM, charpy, etc. Just my opinion.

 

[metengr]

Kwan;
Embrittlement? Why? These are air hardenable steels and are prone to delayed cracking using conventional welding processes. The reason for cooling to 200 deg F is to allow for complete transformation of austenite to untempered martensite. In this untempered condition, martensite is prone to delayed cracking from exposure to moisture or hydrogen. Embrittlement is not the issue.

 

[FluidicTech]

This part will be radiographically examined after PWHT per code anyhow. I don't have any problem doing it an additional time before the PWHT.

However, I am curious what happens below 200 F that makes this irreversible so to speak. In other words, why can the hydrogen be released by holding the temperature at 700 F if the weld is not allowed to cool below 200 F, but once the weld is allowed to cool below 200F that the intermediate heat treat no longer works.

My guess is that any entrapped water is in the vapor state and will eventually be released from the weld if held at a substantial temp for a period of time. But if it is aloud to cool below 200 F some sort of reaction occurs. Anyone care to set me straight?

Any other suggestions?

 

[metengr]

It has to do with the diffusivity of hydrogen that is dissolved in the weld deposit during welding. Hydrogen diffusivity is a function of temperature - as the temperature increases hydrogen diffusivity increases. This also holds true for solubility of hydrogen in steel.

At around 400 deg F or higher, the rate of diffusion of hydrogen atoms significantly increases meaning hydrogen atoms can easily diffuse out of the weld and surrounding base metal prior to cooling to room temperature. If the hydrogen is trapped in the weld during cooling to room temperature without the benefit of a post weld bake (intermediate thermal treatment to allow for the diffusion of hydrogen atoms), there is significant risk of delayed cracking from hydrogen that is dissolved.

If you need to delay PWHT, the hardened weld and base metal are at risk of cracking from dissolved hydrogen especially as one cools to room temperature and remains at room temperature versus cooling to 200 deg F and immediately going to PWHT.

 

[stanweld]

In addition to the above by metengr, the dehydrogenation, intermediate PWHT at 600 F will provide some stress relaxation and will temper untempered martensite (to a degree) making the weld less susceptible to stress corrosion cracking if exposed to moisture/water prior to Code PWHT.

 

[FluidicTech]

To be on the safe side we are going to grind out the welds, bevel again, weld it up again and perform the bakeout before PWHT.

Thanks for all of the input.

 

[metengr]

FluidicTech;
If you go this route, you will be delaying PWHT correct? If not, you won't need the bakeout. Also most important, use H4 designation low hydrogen electrodes, not standard, low hydrogen!

 

[FluidicTech]

Metengr,

Yes, the code PWHT will still be delayed so the bake out will be necessary. Thanks again for the help.