We are currently in the process of issuing requisitions for clad pipe and would appreciate comments from anyone with experience in stress analysis of cladded pipe.

Based on the considered types of internal corrosion, API 5L X60 carbon steel pipe cladded with 3 mm thick high nickel corrosion resistant alloy Incoloy 825, is proposed for pressure containment. The cladding should be achieved either by metallurgical bonding or welding overlay. In addition, it is also proposed that Inconel 625 welding consumables are used for fillet and butt welding. Thus, on-site butt welds between cladded pipe and cladded fittings will unavoidably result in dissimilar metal welding (DMW). Since this type of welds involving stainless steel or nickel based fillers (e.g. Inconel, Incoloy - 625/825, etc) are not permitted for pressure containing welds in sour service per our client's standard, and back cladding on either side is mandatory for weld restoration of clad or weld overlayed joints, on-site butt welding of clad pipe and fittings of less than 30" diameter may be difficult if not impractical to achieve.

In addition, although both materials Inconel 625 and API 5L X60 have roughly the same SMYS of around 60,000 psi, it is somewhat a peculiar situation, as the composition of the weld deposit will be controlled not only by the electrode or filler metal but also by the amount of dilution from the two base metals. The created joint material properties could therefore, vary with the welding process, the operator technique, and the joint design and may result in producing a joint weaker than the base metal. There are indications by some suppliers that the strength of these joints could even yield Incoloy 825 mechanical properties with SMYS of 35,000 psi.

Hence, apart from being difficult to achieve, in our situation DMW welds could form week adhesive rings and may act as numerous stress concentration points, creating potentially situation where failure due to fatigue may occur much sooner than due to corrosion.

To ensure reliability in our design and without having control over the above-described processes, unless it is demonstrated that the yield strength of DMW joints is equal to or higher then the base pipe material, we believe that we have to rely upon the worst assumption and that is to presume SMYS for these joints of no more than 35,000 psi.

In order to assist us in our design we have asked potential suppliers to provide us with information on dissimilar metal butt and fillet welding procedures and with values for:

- SMYS for the created dissimilar metal butt and fillet welded joints
- Stress intensification factors to be used in pipe stress analysis calculations at dissimilar metal butt and fillet welded joints,
- Flexibility factors to be used for cladded fittings

In addition, it is our understanding that clad pipe doesn't have a continuous yield strength value across its full thickness and that the cladding layer will be less strong than the backing steel. Considering this, we have also requested the following information to be provided for the proposed cladded material:

- Modulus of Elasticity
- Coefficient of thermal expansion and
- Mechanical Properties:

Yield Strength and Tensile Strength

We believe that this is satisfactory for the purpose of our analysis, however any suggestion on matters that should also be considered in stress analysis of clad pipe would be highly appreciated.

MISA JOCIC
Lead Mechanical/Piping Engineer
Worley Arabia Limited
E-Mail: misa.jocic@worleyarabia.com

Dear Misa,

WOW let us know if your material suppliers adress your questions... if so we'll all start buying there! Your list is complete but I'm afraid your supplier will respond to your questions with a "HUH?" In which case you will be in the void where we all find ourselves from time to time.... Best of Luck!