can we N+T using induction heating?

[davefitz]

There is a possiblity that we may need to remove incorrectly heat treated HP main steam line sections ( SA 335 P91) form a commissioned power plant and send them out for N+T, or else replace them.

Is it feasible to N+T at the site using either induction heating or a portable furnace? N+T at 1850 F, typical size is 16 " NPS with bent els, each piece may be 40 ft long?

It is beginning to look like there are a lot of plants with this issue.

 

[davefitz]

also, to achieve the required min cooldown rate of -9 F/min to avoid ferrite formation, we would force cool the pipe with fanbed air thru the pipe ID following the heat up to 1850 F.

 

[metengr]

davefitz;
Fro Grade 91 pipe material, I would not recommend induction heating. You should use a furnace for the N&T to assure a uniform austenitizing treatment prior to rapid cooling and tempering.

I would seriously look at sending the Grade 91 pipe spools to a heat treatment shop versus building a portable furnace in the field. For pipe spools up to 40 feet in length you need a rail car type furnace. There are several forging shops around the US that can handle this size, and have quenching and tempering furnaces (BendTec, Scott Forge,,,,).

 

[Yallapragada1022]

I agree with metengr. I also don't think induction heating is a good option. Again as metengr said, I would recommend sending them out to a heat treat (car bottom furnace) shop.

Rao Yallapragada

 

[JohnSterling]

Thanks for the info davefitz. What's the history at this plant? i.e. how did you determine that these spoolpieces were inadequately/incorrectly fabricated?

Can you share any more info? for example did this affect common main steam piping, or boiler piping on one unit (or both?

 

[davefitz]

The information remains confidential at this time.

I can only say,in a general sense, there are several recent gas fired combined cycle plants , for which the hot bent els were fab'd by a small forge in Mississippi/Louisianna area, have shown low hardness ( < 180 Hb), microstructure shows ferrite/pearlite with nil martensite, P9 instead of P91 creep properties, expected life implies "rejuvenation" via N+T to be scheduled. This mom and pop forge has been supplying hot bent P91 els to these kind of plants for at least 6 yrs, which means some are approaching the life of P9 , instead of a 50+ yr creep life with P91 metallurgical properties.

Unfortunately, it appears that the EPC vendors that were flush with work a few years ago did not know that , according to the ASME , the "designer" is responsible for specifying the hot bending procedure- it is not correct to assume the forge will have a metallurgist that will catch these errors. The correct specification of N+T, correct cooldown rate, and post fab NDT apparently was not uniformly applied in those boom years.

 

[pjdobson]

Interesting problem. I agree with Mssrs metengr and Yallapragada 1022, however I have also heard of stories told at the last ASME Section II Creep Strength Enhanced Ferritics meeting in Louisville, KY in Nov. 2006 that there were instances of P91 material having soft spots in straight lengths of pipe. One suspected culprit is the furnace that the material was heat treated in, particularly the car-bottom type furnace. The recommended practice when using this type of furnace is to ensure that the pieces that are being heat treated are N&T and the proper temperatures and that proper heat-up and cool-down is observed by the heat treating shop.

I would be interested in finding out how many other plants that davefitz was referring to has this problem.

 

[JohnSterling]

There seem to be many "stories" circulating on these issues, but few hard facts. Does anyone know of any conferences in the US where this will be discussed publicly?

Any idea why there is so little info out there on this important topic?

Cheers.

 

[pjdobson]

JohnSterling: I believe there was an HRSG User's Group meeting in the 3rd quarter or 4th quarter of 2006. That would be one good place to start. I don't think that the proceeds from that meeting are available yet. If anyone else knows more about this meeting and when/how the information from that meeting would be available, that would be greatly appreciated.

 

[stanweld]

We have found a number of SA-234 WP91 fittings that were improperly heat treated (some annealed as per WP22). Based on the large amount of testing that we have performed to date, I would suspect about 2% to 5% of the fittings installed throughout the industry have been improperly heat treated. We have also seen improperly normalized (max cooling rate at 300F/hr in the furnace) on induction bends from major suppliers. Pending litigation precludes any further discussion as to who/when/where.

I would advise hardness testing of all purchased fittings and induction bends and that they meet a 190 BHN minimum hardness ( I would actually like to have a minimum hardness of 200 BHN to mitigate effects of PWHT at the spool fabricator and additional PWHT at the site). Our studies have shown that fittings with hardness < 170BHN do not possess the required tempered martensitic microstructure.

We will be performing 1000 hr creep testing on removed P91 induction bends having ferrite microstructures and highly tempered (hardness approximately 175 BHN) martensitic microstructures. I expect to have data this summer to share.

 

[pjdobson]

stanweld: Along with the hardness testing, I would also recommend that metallographic replication be performed as there is a chance that, even if an elbow passes hardness requirements, the microstructure is not Grade 91 but something lower like Grade 9.

 

[metengr]

pjdobson;

I would also recommend that metallographic replication be performed as there is a chance that, even if an elbow passes hardness requirements, the microstructure is not Grade 91 but something lower like Grade 9.

Not possible if the material is Grade 91. Grade 9 material is not supplied in a quenched and tempered condition. Replication would only be useful if the hardness of the Grade 91 material is out of range.

 

[stanweld]

Replications should be done when the hardness is less than 185-190 BHN. We have seen appropriate microstructures in P91 having hardness in the 170's and low 180's but we have also seen abnormal microstructures that may have resulted from overtempering slightly above the lower critical.

We have also found P9 and P5 fittings (mixed steels) stamped as WP91; PMI should always be done.

When performing replications, it is important to remove the decarburized zone which we have verified as being as great as 0.040-inch; in the majority of cases seen to date, the decarb layers are only 0.005 to .010-inch.