WP91 alert 3

[stanweld]

We recently received WP91 fittings which exhibited stress corrosion cracking. Hardness ranged from 270 to over 400 BHN (at SCC initiation site). SCC resulted in failure on preliminary hydro. I do not have identity of fitting manufacturer. Just to let all users know - the problems with P91 fittings remain. Let the Buyer Beware.

 

[metengr]

Thanks, stanweld. Also, while on the subject of creep strength enhanced ferritic steels, Grade T23 boiler tubing is now a concern if this tubing was supplied by tube mills other than V&M or Sumitomo. The material should be fully bainitic (tempered) after quench and temper heat treatment. However, I have heard through various reputable sources that the Grade T23 supplied by other tube mills is lower in hardness after quenching because this material contains a fair amount of ferrite in addition to bainite. The tube mills were following the ASME approved Code Case from 1995; however, the tube material still contained ferrite. Cause for concern....

 

[Mr168]

Thank you both for the heads up. I'm going to bring the fittings issue up on our next QC conference call.

 

[davefitz]

Related to the T23 excess ferrite, the lack of adequately fast cooldown seems to be a problem similar to that found at some fabrication shops which were to N+T P91 parts. There seems to be a general lack of understanding of the TTT curve behavior of these alloys and their need for a fast cooldown for proper metalurgical structure.

On visits to shops that were to fabricate P91 components, I had witnessed operation of the N+T ovens and found that the plant did not have established procedures to ensure the parts would be cooled faster than the required rate to ensure minimal ferrite formation. The plant personnel were not even familiar with the need to cool the parts quickly to avoid ferrite formation, and instead deliberately slowed the rate of cooling based on past practice with P22 parts.

bainitic T23 also has a ( seemingly ) generous allowable rate of cooling to ensure bainite forms, but a thick walled part that is not deliberately quenched ( ie air cooled instead of oil bath)might not achieve the required cooldown rate. See valourec T23 booklet for TTT curve and required cooldown rate.

ASTM spec allows for more precise QC testing requirements than the standard /default value of hardness test of only one piece per batch. More users should demand more pieces be tested and also require the temp vs time historgrams of monitored pieces be saved.

 

[metengr]

Related to the T23 excess ferrite, the lack of adequately fast cooldown seems to be a problem similar to that found at some fabrication shops which were to N+T P91 parts. There seems to be a general lack of understanding of the TTT curve behavior of these alloys and their need for a fast cooldown for proper metalurgical structure.

No, this is not the case for T23. It is a chemical composition issue that was developed for the original code case. Cooling rate has nothing to so with this particular problem.

 

[Welder4956]

After seeing Dr. Masuyama's slides on the effects of chemistry changes and cooling rate changes, I think cooling rate is just as important as chemistry. There seemed to be some unspoken key to the manufacturing process that V&M overcame by having a 4:1 Ti/N ratio to ensure sufficient free boron. I'm not certain what that "key" is that lets Sumitomo manufacture with a ratio as low as 0.33:1, but cooling rate does have an important effect.

 

[0707]

Is it acceptable to weld A335 P9 with P91 filler metals?

 

[davefitz]

I can't comment on T23- but there are to be 2 conferences on this subject this year ( April 09 near Zurich, and June 09 in Louisville)

regarding P91 cooling rates, it is suggested one goes back in history and re-reads the final report by ORNL ( V Sikkha) on P91 optimization. There, one finds several TTT curves for the purpose of optimizing normalization times and temperatures and cooling rates. Some highlights include:

a) normalization time and temp affect crystal size, which in turn affects creep strength and also the propensity to form ferrite upon cooling; a high normalization temp yields large crystals, low creep strength, and lower propensity to form ferrite upon cooling. I am not sure if modern solid state thermodynamics programs reflect this trend. But I have seen foundries use a relatively high normalization temp ( higher than ORNL recommends) plus slow cooling rates- I would expect lower than optimum creep strength with that combo.

b) a slow cooling rate can yield high ferrite content if the cooling rate is constrained in the temp range 1800-1400 F- as seen in the Mannesman valouurec P91 booklet for a specific assumed normalization time and temp batch

b)It is known ( from similar measurenment error associated with boiler fluegas measurements) that an error on the order of 100F can be expected at the monitoring thermocouple attached to the workpiece being normalized at 1900 F, due to radiation of heat to the colder enclosure- so the means of attaching and shielding the T/C to the workpiece will affect the accuracy of the monitoried and controlled normalizing temp, and the resulting crystal size and metallurgy.

 

[metengr]

Welder4956;

The key with Grade T23, unlike Grade 91 or 92 steels, is having enough free boron to promote hardenability to achieve 100% bainite, which is why chemistry is most important and cooling rate is less important. The recent revision to the ASME Code Case had nothing to do with cooling rate it was chemistry that needed to be re-evaluated. If the boron is sufficiently present to enable complete bainite transformation upon quenching.

 

[stanweld]

0707
It is highly recommended that you not weld Grade 9 with "B9" (P91 matching)weld filler metals. They are much slower to respond to PWHT and your ability to meet B31.3 hardness requirements will be affected. Longer hold times will be required.

 

[metengr]

Agree with stanweld. I would use B8 for welding standard 9Cr-Mo steels.

 

[deadrange]

welder4956,

I have seen the literature for grade T23 that was presented at the last meeting. In my brief review of the material change, I have a feeling that the Sumitomo way of achieving enough free boron is twofold. 1. The nitrogen levels are toward the low side of steelmaking limits for a grade of this nature. While not completely atypical, they are lower than what I would suspect is the norm. By having a lower Nitrogen level, it allows mechanism 2 to occur. 2. Titanium is not the only nitrogen scavenger that steelmakers are able to use. We all know that Boron is a nitrogen scavenger, but since we want free Boron, not the best solution. But you will notice that the Boron levels on the sumitomo material was fairly high. Conventional Boron treated grades typically run in the 0.0015 Boron range. The material Sumitomo presented was significantly higher. Also notice the Niobium (Columbium) is higher, also another Nitrogen Scavenger.
I think it would might be possible to achieve the structure and hardness requirements using lower Ti/N ratios. but to be on the safe side 3.5:1 is a good number.
Steelmaking addition practice may be what allows Sumitomo to achieve the properties using a lower Ti:N ratio. But that's just my opinion.

 

[metengr]

deadrange;
I was also present at the recent ASME B&PV code meeting in Montreal and was very concerned about the proposed change in B, N and other elements. My main concerns were that no heats that were produced to verify this recommended chemical composition change. I did support the Code Case change after discussion with V&M and several key committee members that the proposed change is better then what we currently have and this change would steer us in the direction of promoting a fully bainite microstructure. So, I supported the change.

 

[deadrange]

metengr,

The change in Nitrogen should only further enhance free Boron, as does increasing the Boron content. The change in the minimum Boron content is not huge. In my opinion there is no change. However, based on the information gathered I think steel mills will aim to a higher Boron content to help have free Boron. The addition of titanium to the specification makes complete sense to me.
My experience with Boron treated steels is that Titanium is the most economical and consistent alloying addition to allow Boron to remain free.
The more intriguing element to me is Nickel. I do not quite understand why the nickel is so high to start. This is not a typical residual level for any steelmaking operation.

 

[stanweld]

To update all; the high hardness, stress corrosion cracked fittings were manufactured in the USA and heat treated in a gas fired furnace, shipped to Korea for fabrication into HRSG componenets, hydrotested in Korea, shipped overseas to the USA with final Section I Hydrotest in the USA. The most recent metallurgical examination tends to favor over tempering, well above the lower critical temperature most likely caused by direct flame impingement, rather than undertempering.