low hardness for p91 elbows

[huguette]

we receive P91 elbows Brinnel Hardness 134-137. Straight pipe has HB 184-232.
Is this HB acceptable for 24" sch 40 elbow designed to 20 years , temperature 532 oC and pressure 36 bar ?
On what others parameters this impact?
What to do to correct this?

 

[metengr]

I would assume that the elbows were supplied under an ASME SA 182 or ASME SA 234 designation? If so, the specification only calls for a maximum hardness of 248 BHN for material supplied in the normalized and tempered condition. Are the elbows clam shell welded (double seam) or made from seam welded pipe or seamless pipe?

The hardness of the elbows does seem lower than I would expect for P91 material, but this could be related to lower carbon content (0.08% minimum) or local decarburization of the OD surface affecting your hardness readings. Where are you taking your hardness readings? When we have purchased P91 elbows, we take hardness readings on the weld prep surface after it has been machined on the end of the elbow.

I would double check the heat treatment condition of the supplied elbows to determine if the proper heat treatment was performed in accordance with an approved specification like ASME SA 182 -
Normalize at 1900-2000 deg F
Air cool
temper @ 1350 deg F minimum

If you temper at too high a temperature, you could exceed the lower critical transformation temperature. However, this would increase the hardness upon air cooling, which apparently you do not show with your hardness data.

The low hardness could indicate the material did not go thru a proper normalize and temper heat treatment after fabrication. If so, I would follow the requirements of ASME SA 182 and re-heat treat the elbows.

 

[davefitz]

You have a big problem.

It looks like the foundry had overtempered the elbows after bending. One can cold bend P91 up to 15% work hardening, but beyond that it must be hot bent then normalized and tempered; ie, heated to over 1800F then cooled to 1350 F then cooled to 200 F etc. If the post bending heat treatment did not achieve the full normalizing temperature of 1800 F but did exceed the austenizing temp of 1450 F, then the material was likely overtempered. That means creep life is but a fraction of ASME assumed.

A similar issue occurred in England at Plant West Burton, but on forged end caps ( flat header ends) . Several of these end caps had a parent metal hardness of below 190 Hb, and failed after 3 years operation ( failure was at weld soft zone, but tracked back to overtetmpered parent metal).

Another issue related to hot bent piping is if the elbows are integral to the straight runs prior to bending: if so, then the entire pipe run needs to be N+T since if one were to only N+T of the bent section it would require there to be a section of straignt pipe that did not recieve full N+T but which would be overtempered and unsuitable for high temp service.

See papers on this subject on the online tech journal at <

http://www.ommi.co.uk>

by A. Shibli et al.

 

[unclesyd]

metengr has given some good advice. Here is a little more advice to protect everyone both money wise and most importantly safety.

Before you proceed get with the supplier/manufacturer and verify if what you have is what was ordered. It is up to the supplier/manufacturer to satisfy you that they have met your requirements as specified. These elbows should have a paper trail and a certifiable MTR.
Then you need to do a Positive Material Identification (PMI) to cover yourself.

 

[davefitz]

By the way, to correct this , you might need to cut and isolate the entire section that has hardness below 195 Hb, then N+T this entire pipe section, then reweld it back to the rest of the piping system. Sounds like a busy weekend.

 

[stanweld]

Just a few more thoughts regarding your situation.

First, as unclesyd stated, perform PMI to verify the material as P91. More than once, we have found moxed steels, namely 5 Chrome alloy steel marked as WP91. In one case, not our job, a low out occured in superheater crossover piping.

Second, verify the hardness. I have had to recheck low hardness readings a number of times and found them acceptable. In somme cases untrained personnel performed the test; in others faulty equipment was to blame.

Finally, if the material meets the chemistry of P91 and has the low hardness recorded, you will have to renormalize and temper. You may have to scrap some or all of the material if you cannot satisfy the A-234 or A-182 testing requirements.

 

[huguette]

Thank you all.
Grateful for your information.
...We have a big problem... It is P91 checked

 

[davefitz]

all is not lost yet.

a) as stanweld says, most hardness readings are not correct; these readings should be confirmed by an experienced outfit.

b) can confirm the crystal structure is adequate or not by taking a boat sample and having it analyzed

b) if all tests show inadequate properties ( but still meets ASME spec ) , can possibly rationalize delaying repair by etching a set or grid of creep detectors onto the surface of the highest stressed/ lowest hardness areas and monitoring every 3 months, and if the rate of creep growth exceeds alarm rate then must repair ASAP. But for liability reasons you will need a more informed opinion than mine.

 

[stanweld]

davefitz,
A cautionary note. On some failed T91 tubes recently reported by EPRI and subsequently discovered to have been tempered at 1500 F by the manufacturer, microstructural examination did not reveal an inapprpriate micro-structure. The hardness of those tubes was not nearly as low as reported in this case, however.
huguette,
Were these ells, hot bends from pipe? I have seen some truly scary practices by some manufacturers. In some cases the hot bending procedures indicated forming at approximately 1600 F without subsequent normalizing and tempering. Also, many Engineering specifications that I have read have not addressed the proper requirements for hot or cold bends in P91 pipe and I (as Constructor)have been forced to mandate appropriate requirements.