1.25Cr1Mo-V - code allowable stresses 1

[davefitz]

We have been supplied with a steam turbine inlet stop valve of cast 1.25Cr-1Mo-V. I have not been able to find an ASME tabulated list of allowable stresses vs temperature. Is there an equivalent DIN or EU or BS or Japanese material spec that I can use to arrive at an equivalent ASME allowable stress?

 

[stanweld]

The design of cast valves and turbine casings made from this alloy has been proprietary to GE, Toshiba, Seimens, etc., even though the alloy has been in use for decades. Toshiba provided some design info that stated 6870 psi at 1077 F, which is somewhat > than that (5100 psi at 1075 F) for P22 alloy steel. They normally design the welded ends to match that of the connective P22 pipe design stress at temperature.

 

[davefitz]

stanweld;
Thanks for the info. Can you advise where I can get the Toshiba data.

In our case, The design Temp is 1063 F, and Toshiba provided a weld end prep of 21.26" x 3.02" thick. The EPC vendor goofed big time and simply butted a 18" OD x sched 140 ( 1.56" aw) P91 pipe against the valve, buttered it with P22 filler, and the weld failed after 1 yr operation.

 

[stanweld]

I have required the P91 weld to 1 1/4 CR-1Mo-V valve to be made with ER90S-B9 & E9015-B9. Because the design mismatch is so great between the two alloys, a forged or machined P91 transition piece is initially welded to the valve in the shop. This permits a simple P91 to P91 butt weld to be made in the field.

 

[davefitz]

I agree the forged P91 transition piece is required; the main error of the EPC vendor is failure to supply this transition piece. Suplied design violates code in regards to supplying required minimum wall thickness as well as exceeding the 30 degree max thickness taper at butt weld thickness transition.

Failure occurred in the carburized zone of P91 pipe.

 

[stanweld]

It's easy to see why the joint failed; Low strength weld metal and P91 HAZ coupled with poor weld geometry. I suspect that failure was more thermal fatigue related, assuming that the plant is operating in a merchant capacity.

With respect to obtaining design allowables from GE, Toshiba, etc., what little info I have was obtained through months of requests from the Owner/Purchaser. I have never been able to obtain same (through the Purchaser or any other)from GE or Seimens.

 

[GRoberts]

I think the alloy you are referring to is ASTM A356 Grade 9?, or a proprietary grade of it.

I don't have the information with me so I can't provide a lot of details, but I have heard that ASME is incorperating some of the ASTM A356 cast alloys in Section IX, which means it would have to be added to Section II or the B31 codes I think. You might check with an ASME committee member if you have a connection.

 

[davefitz]

GRoberts;

Thanks for the tip. It will help if ASME recognizes the commonly used cast material, and it would help the general public even more if B16.25 and B31 explicitly inlcuded a paragraph dealing with butt welding of dissimilar strength alloys.

Although it seems elementary to me that the weld thickness must be equal to or greater than the minimum wall thickness required of the minimum strength material ( considering tube, valve, and filler), the EPC vendor still does not admit the failed weld doesn't meet code.

 

[stanweld]

You indicated that the weld geometry didn't meet FIG. 127.4.4 of B31.1. If so, the weld did not meet Code. From an engineering standpoint, it makes little sense to weld two higher strength (high temperature) materials with weld metal that is weaker than either, especially when thermal cycling is enevitable.

If failure occurred out of the warranty period, retrieving costs for repair from the EPC contractor could prove difficult. Legal costs would far exceed the repair cost.

 

[davefitz]

It is under warranty, but the peculiar warranty is one where we pay all warranty repair costs. Some warranty. Must have borrowed the warranty provisions from Chevy.

I would like to think is is negligence; they repeated this error on 3 duplicate units, and all 3 will be out of service for over a month until we obtain the required transition joints. The lost power costs are staggering, considering otehr plants are out of service for scheduled maintenance.

 

[stanweld]

I have old Toshiba welding procedures that specify the use of "B3" filler metals for making that joint, so negligence is extremely difficult to prove since the EPC contractor may have simply been following Toshiba's guidelines. Poor or nonexistant visual inspection of the joints to verify joint geometry in accordance with Code can certainly be cited.

Appropriate PWHT is required for both the P91 and the 1 1/4 Cr-1Mo-V alloy. Inappropriate PWHT will adversely affect high tempearture properties of the weld joint.

 

[davefitz]

Stanweld:
I think you are probably 100% right, except that the EPC contractor did not follow the Toshiba procedure: Toshiba had provided an explicit weld detail for a 3.02" thick weld. But B31.1 does require the Owner to inspect the nonboiler external piping weld.