Dear Sirs.

can we weld CS shoe on SS Pipe with SS pad at 425C temp ?

E(alphass-alphacs)*dt comes very high - will CS side be highly stressed and crack can initiate!

Some FEA findings indicate the same. Please advise there is any reason for worry at this temp 425C for such CS-SS weld?

Can better weld procedure reduce such crack possibility?
reg,
sam

Differential coefficients of thermal expansion will result in fatigue failure at some point, but when is a function of shoe design and system cycling.

Of course, this doesn't really matter as much as the fact you'll suffer galvanic corrosion of both SS and CS, anyways, but at least this way you'll first eat up your pad before you eat up your pipe.

You have missed one point 425 deg C design temperature. Carbon steel shoe will expand less than ss pad attached with pipe by welding. E×(ass-acs)*DT is very high!
Even for a small no of thermal cycles, will it not initiate fatigue crack due to dissimilar metals held together!

I suppose I wasn't particularly clear on that point. Yes, if you use a traditional shoe design, you'll have problems.

If you must use CS shoes, then I can suggest the following:

An engineered welded-on shoe will last longer have fewer problems than a traditional welded-on shoe, and such an engineered apparatus might look more like vessel saddles and less like a pipe shoe, and might not really be a feasible option, depending on pipe size.

However, before you get to that point, you might consider a clamp on design or pipe saddle.

I saw once this tipe of shoes. They have slots cut into the CS shoe.

Welding is an issue for such different material groups at this high temperature. However, with modern day techniques, welding has become better over the years.

Following are the points you need to worry for your case:

- WPS has to be qualified for all requirements required to weld such joints.
- Need to take care of differential expansion for the portion of shoe outside insulation for the welding seam
- perform a FE check for your shoe in case it sees high horizontal loads (for eg: if you also plan to use it as axial stop)

I suggest to use trunnion support of same material (SS) in lieu of shoe for all vertical and guide functions. Even the same can be used for axial stop function provided loads are not high(perform local stress qualification).

In my opinion, if possible use SS Pad and SS shoe(Ribs), to save costing of shoe materials,Bottom plate of shoe may use CS, there will be no issue.
But for SS Piping, SS PAD + SS Rib of SHOE + CS Plate(Bottom) will be advisable.
Higher temp like 425 Deg C for CS, it is risk.

Sam,

You have already answered your own question for the reasons with the crack initiation under DT and FEA studies.

Reducing the welding length (keeping in the middle only) to the required might reduce the stress (leaving the contact length the same as the shoe length). It may work for the longitudinal shear but I would not recommend to use them under this kind of temperature since the pipe has loading in different directions, and you may find the weld length is not sufficient for the load. It is not good especially for the transverse loading under the shoe.

I would recommend the clamped pipe shoes for high temperature application. You still need to know loading and select the shoe type accordingly. In case large shear forces, and torsion is available at the shoe pipe connection you may add small attachment onto the piping only to take them adequately. This type shoes will not be effected by cycling thermal load that much either. I suggest you to see the catalogue from LISEGA or equivalent.