How Vanadium Affects P91 Material

Very good answer sb and very informative sites. I have learned from all of your posts. I would like to thank you for your correct information. I have welded allot of SA 335 P11 and p22 but only ran into the p9 a few times. I would assume that the vanadium is at least part of the reason for a lower inter pass temp. while welding and for the PWHT requirements? From this welders point of view welding on anything with more chrome-molly than p22 is a pain because of the need for backing gas and purging. Either this or I am just lazy when it comes to backing gas. I was overseeing a job a few years back where we were replacing several super-heater valves on a header from a 1200 psi boiler when I ran into a 5% chrome molly globe valve that was welded into a P22 pipe. We had a heck of a time trying to get a root in that weld till we figured it out. I never did figure out why someone put it in there and in the end bought a new valve from the correct alloy and installed it.

P91 is chromium-molybdenum steel pipe, no? Construction material, I think.

It is a Cr, Mo V Steel - Usually used for high temperature pipes. It is creep resistant steel. ASTM A335 (if I remember well) and is covered under ASME code.

It is not construction material - used for carrying boiler steam to turbines (The use here I don't know)

Hi sorry I was not so clear in my original post.

The material is P91 steam pipe on a thermal power plant.

We have found the Vanadium to be higher than in the chemical requirements table in SA-335 the maximum V % is 0.25 and we have at one point 0.45%

The independant inspectors who carried out the PMI accepted the material as P91 Then when we were revieving the documentation we noticed the discrepency.

ASME SA 335 Grade 91 have a vanadium content percent range (min. 0.18% & max. 0.25%), so the pipe material with V 0.45% is not in accordance with ASME code requirements, and inturn must be rejected. May be you can proceed a chemical analysis to check exactly how much the vanadium content.

Note. ASME SA 335 is a specification for seamless ferritic alloy steel pipe for high-temperature services. We used to use Grade P5 (Cr 4-6%, Mo 0.45-0.65% & without any limit requirements for V) in piping of water-tube boilers, and all weldment require preheating, inter pass in addition to PWHT as per code.

what about the method of purging of this grade ? the minimum preheat temperature is 200°C;

Thanks SB,

That was a perfect answer our third parties PMI machine was reading 0.2 high and they didn't bother to explain this point! Just accepted and printed their report without an explanation.

I learnt this while looking at all possible outcomes

Excess Vanadium Increased vanadium is likely to reduce grain size and improve creep resistance through VN precipitation, However, it may cause problems with stress relief cracking (reheat cracking) due to grain interior hardening, ductility and possibly toughness. Whether or not a higher level is acceptable comes down to the performance. If it meets the performance requirements and those requirements are sufficient to pick up on potential problems then there shouldn't be much to worry about. If no cracking of the welds has occurred during PWHT and no other problems have been spotted in qualification tests (toughness and strength)

It sounds to me that you got a different alloy than you ordered or that there was a malfunction in the manufacture. At any rate I would be required to reject the material as not matching the ASME requirements as per my quality control manual. The vendor should have spotted this problem before he ever sent it out to you as well as the manufacturer before he sold it to the vendor. I sometimes wonder if the vendors really go over the MTR's for materials before they buy/sell it and this seems to me to be evidence of them not doing this. This is a very good reason to never pay for material before being inspected and/or accepted. As a quality control manager if I sign off on materials as being in accordance to ASME or other codes you can bet It will right before I accept. The bad part about this is that the material may not be readily available and could have a long lead time thus hurting the whole project to re order if you have a guaranteed finish date with liquidated damage clause in your contract.

If you used Niton XRF, this is very common to have over or under values. I would check with spark or wet chemistry and from my experience I can bet 2 cents that your material is ok, means V is between .18 to .25