Fabrication of Clad Plate (Continued) Welding

Welds in clad materials are made with the same fit-for-service requirements as solid material welds, with one notable exception. Due to the presence of a backing steel substrate, care must be taken to avoid welding procedures that produce chemically diluted welds on the cladding side. The normal procedure for butt welding clad plates is to weld the backing steel side with the appropriate carbon or alloy steel consumable first, then weld the cladding side with the high alloy consumable as shown in Figure 4. This process avoids depositing carbon steel on high alloy, a situation that can result in sufficient dilution of nickel, chromium and molybdenum to produce an air hardenable intermetallic which may be susceptible to weld cracking.

Figure 4

Step 1 Edge preparation. Step 2 Plate fit-up before welding. The lip of steel above cladding protects steel weld from high alloy pick-up. Step 3 Steel side welded using steel electrode. Note that steel weld has not penetrated into the cladding. Step 4 Cladding side prepared for welding by Arcair gouging, chipping or grinding. Step 5 welding the cladding side, first pass complete. Step 6 Completing the joint. Finish pass complete, cladding side.

Design considerations may render welding from both sides impossible. In this case, welding can be accomplished from the cladding side only as shown in Figure 5. While more demanding and requiring more control of dilution, usually with higher alloy consumables (overmatching), functional, corrosion resistant welds can be produced. It is important to note that during this process, care should be exercised to avoid contaminating the initial passes into the backing with the cladding, particularly with the high alloys, so as to avoid producing brittle intermetallics. This is easily accomplished by proper joint design or sufficiently stripping back the cladding.

Figure 5

Step 1 Beveled joint design to minimize grinding Step 2 Deposit root pass - GTAW or SMAW Step 3 Deposit 2nd pass SMAW to fully penetrate 1st bead Step 4 Deposit 3rd and 4th bead to fill joint

Light thickness clad plates are usually most economically welded using full alloy welds. This has been particularly true with the high performance clads used in flue gas desulfurization applications where the advent of the new generation of synergic feedback "GMAW" equipment has proven to be a major advancement in the production of low heat input welds necessary to reduce dilution and produce sound, corrosion resistant welds. GTAW has also been used for these purposes, though it is usually less efficient. In these cases, the sequence of welding may involve the cladding side first.

Welding of straight chromium (400-series stainless) clad, nickel clad, copper and copper-nickel clad steels involves slightly special considerations, but are readily weldable like the 300-series stainless clads. Consult BLP to discuss your special needs.

Stress relieving may be required by the design specifications or as a result of the ASME Pressure Vessel Code and would be based on the thickness of the backing steel. The 400-series stainless clads should be stress relieved in the range of 1100�F--1350�F with consideration given to the backing properties. The 300-series stainless steels can be subject to carbide precipitation during stress relief, but this can be minimized by the use of extra low carbon or stabilized grades.

Preheating before welding is a requirement determined by the nature of the backing steel. A302, A204 and A387 are some of the common alloy steels requiring preheating. Preheating may also be considered for heavy thickness clad plate or for highly restrained joints.