SA-387 Gr11 creep data 5

[misaj]

Does anybody have stress data for creep rate 0.01%/1000hr for SA-387 Gr11 at 500degC and 390degC? Is this kind of material data available anywhere on the web?

Regards
Misa

 

[ijzer]

Values can be found in STP 151 of 1953 published by ASTM.

@500 °C = 103 MPa
@390 °C = 189 MPa

 

[metengr]

Actually, the web site below is one that I have used to obtain elevated temperature tensile, creep and stress rupture data for various steels. All you need to do is to register (it is free).

http://mits.nims.go.jp/db_top_eng.htm

 

[misaj]

Thanks, ijzer and metengr

Misa

 

[misaj]

ijzer,

What is the exact name of the publication by ASTM from which you read the data for SA-387 Gr11? Perhaps I could order a copy from somewhere if it is still available?
I also need the same data for SA-335 P11?

metengr,

The site you have mentioned is full of useful information and it is free. Unfortunately there is no information available for Stress versus minimum creep rate (%/hr) for (1.25Cr-0.5Mo-Si steel plate) although there is information available for steel tube but only for temperatures 500, 550, 600 and 650degC.

Misa

 

[metengr]

Misa;
For the Cr-Mo family of alloy steels most users do not worry about creep deformation in applications where time independent properties are the controlling factor for design. In other words, below 500 deg C, ultimate tensile and yield strength values would control design in boiler and pressure vessel applications. In other words, below 500 deg C, the ASME B&PV Code considers tensile failure or permanent deformation concerns well before one would have 1% creep deformation in 100,000 hours or failure from stress rupture in x thousand hours. This is why I do not worry about creep rate or stress rupture of these materials below 500 deg C, as well as others.

 

[davefitz]

Don't forget- for longitudinal welds in P11 (or other creep affected pipes)that were not N+T'd, , there should be utilized a weld creep strength reduction factor- for P11 this is assocaited with so called type IV creep failures. For such highly stressed weld lines, the parent material creep curves are not the correct source for weld creep life.

 

[misaj]

davefitz,

Thanks for pointing this out. I was just about seek further advice because my interest in these materials stretches beyond what metengr is saying and which holds truth for majority of cases.
I am not a materials expert but after reading some reports about failures with P11 welded pipe it made me think that using a welded pipe made out of SA-387 Gr11 plate isn’t a great idea.
The problem is further emphasized with the fact that we are late in the construction and that seamless P11 in 28inch we are not going to find easily if at all. You have mentioned that there should be utilized a weld creep strength reduction factor – for P11 that is associated with so called type IV creep failures because for highly stressed weld lines the parent material creep curves are not the correct source for weld creep life.
Do you have any references where I can get more detailed information about this?

Regards
Misa

 

[davefitz]

misaj:

Yes and no. It has been a while since I was involved in these design measures. Perhaps Meteng can help.

The 2005 edition of B31.3 (process piping) has some text that provides suggestions for weld creep strength reduction factors. Likewise, the online engineering magazine <

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

has papers which describe results of tests of such P11, P22, and P91 welds, and as I recall the creep strength reduction factor was about 0.75 ( ie a 25% reduction in strength at a design temp of 1050 F). As I recall, there was a note in an older ( 1990) ASME sect I or B31.1 talbe that indcicated that the weld creep strength reduction factor of P22, P11 and P91 was about 0.75-0.80 for longitudinal welds , but I have not been able to find it again.

Historically, this problem became a crisis in 1988 in power plants following the failure of a P11 longitudinally welded hot reheat pipe at the SCE Mohave plant ( 11 mort). Since then, such piping has been replaced or placed on an annula NDT inspection schedule.

The root cause of the weld weakness ( overaged due to multiple weld reheat passes- and the thicker the weld , the worse the damage) can be corrected by normalizing and tempering but this can only be applied for pipe sections short enought to fit in an oven. As I recall,the largest dia pipe that mannesman can pilge is 48" OD.

 

[davefitz]

as an update on weld creep strength reduction factors:

as presented at the 2006 ASME PVP conference (Vancouver), papers by Takahashi and Tabuchi:

For P91, reduction factors as a function of temp;
below 550 C, F=1.0
575 C, F= 0.85 600C , F=0.75 625C, F=0.70
650C, F= 0.70

For P122 (HCM12A):
T below 550C, F=1.0 575 C, F=0.73 600C, F=0.68
625 C, F=0.50 650C, F=0.51
This is a major concern for P122, as the factor of 0.51 indicates that even butt welds are of questionable life for temps above 650C, unless a N+T is provided.

 

[metengr]

Just a point on clarification regarding long seam weld creep damage in both SA 387 Grade 11 and 22 plate materials;

For the Cr-Mo alloy systems, like 1.25%Cr- .5% Mo and 2.25% Cr - 1% Mo, long seam weld failures have occurred in both subcritically post weld heat treated and normalize and tempered post weld heat treated submerged arc welds because of weld metal composition and flux. I can recall one case, a hot reheat seam weld containing a 14" long crack from the Gallatin Unit 2 plant in the late 1980's. The crack was discovered by shear wave UT (after the Mohave incident) and formed mid-wall along the cusp region of the double-vee groove weld. The creep cavitation damage was not along the fine grained region of the base metal HAZ (as is typical with Type IV cracking), it was long the fusion zone of the weld metal because of a higher density of nonmetallic inclusions. These inclusions (which were formed because of oxygen and flux composition) acted as preferred sites for creep cavitation damage along the cusp region of the weld. Also, the creep deformation rate of the weld metal was not matched with the base metal, the creep deformation rate of the weld metal was much higher to that of the base metal, which is the reason nonmetallic inclusions became such a contributing factor.

I believe the long seam rupture of the Mohave reheat steam line was a similar occurrence, creep cavitation damage was confined to the weld fusion zone that started near the cusp region of the double-vee groove seam weld. Once the plane of weakness formed along the axial direction of the pipe, a preferred path for creep rupture occurred that was driven by hoop (pressure) stresses.

This brings us to the more advanced enhanced creep strength enhanced ferritic Cr-Mo-V alloy systems like Grade 91, and Grade 92. In these alloy systems, with more control on weld metal composition, the concern is Type IV creep damage in subcritically post weld heat treated weld regions or in SAW's that were N&T'd, and subsequently repaired using the SMAW process and given a local sub critical PWHT.

 

[davefitz]

not so sure about the mohave failure. According to the shift supervisor of the crew that perished ( he had left the job about 1 month prior to the accident) the region that failed had been weld repaired at least twice in the prior 2 yrs due to leakage at the seam weld. The boiler was a slurry coal fired unit which had excessive slag buildup in the furnace, which led to very high reheater steam temp ( more than 100 F above the design 1005 F). They had used all availabl reheat spray and yet could not hold final reheater stem temp below 1100 F- for over a year!

 

[metengr]

I heard about the prior weld repairs, as well. All this means is that the HRH seam weld was under distress at the time and they (the Mohave folks) did not recognize the symptoms of a much bigger problem (mid-wall and ID creep damage that started to progress to the OD surface).