Most clad metals are composites of a cladding metal such as stainless steel, nickel and nickel alloys, and copper and copper alloys welded to a backing material of either carbon or alloy steel. The two metals are welded together at a mill in a roll under heat and pressure. The steel clad plates are usually specified in a thickness of the cladding which ranges from 5% to 20% of the total composite thickness. The advantage of composite material is to provide at relatively low cost the benefits of an expensive material which can provide corrosion resistance, abrasion resistance, and other benefits with the strength of the backing metal.
Stainless Steel Clad Plate
Clad metals were developed in the early 1930s and one of the first to be used was nickel bonded to carbon steel. This composite was used in the construction of tank cars. Other products made of clad steels are heat exchangers, tanks, processing vessels, materials-handling equipment, storage equipment, etc.
Clad or composites can be made by several different welding manufacturing methods. The most widely used process is roll welding which employs heat and roll pressure to weld the clad to the backing steel. Explosive welding is also used and weld surfacing or overlay is another method of producing a composite material.
Clad steels can have as the cladding material chromium steel in the 12-15% range, stainless steels primarily of the 18/8 and 25/12 analysis, nickel base alloys such as Monel and Inconel, copper-nickel, and copper. The backing material is usually high-quality steel of the ASTM-A285, A212, or similar grade. The tensile strength of clad material depends on the tensile strength of its components and their ratio to its thickness. The clad thickness is uniform throughout the cross section, and the weld between the two metals is continuous throughout.
A slightly different procedure is used for oxygen cutting of clad steel. All of the clad metals mentioned above can be oxygen flame cut with the exception of the copper-clad composite material. The normal limit of clad plate cutting is when the clad material does not exceed 30% of the total thickness. However, higher percentage of cladding may be cut in thicknesses of 12 mm and over. The oxygen pressure is lower when cutting clad steel; however, larger cutting tips are used.
The quality of the cut is very similar to the quality of the cut of carbon steel. When flame cutting clad material the cladding material must be on the underside so that the flame will first cut the carbon steel. The addition of iron powder to the flame will assist the cutting operation.
Schedules of flame cutting are provided by clad steel producers as well as flame-cutting equipment producers. For oxygen flame cutting copper and copper-nickel clad steels the copper clad surface must be removed and the backing steel cut in the same fashion as bare carbon steel.
Copper and brass clad plate can be cut using iron powder cutting. Clad steels can be fabricated by bending and rolling, shearing, punching, and machining in the same manner as the equivalent carbon steels. Clad materials can be preheated and given stress relief heat treatment in the same manner as carbon steels. However, stress relieving temperatures should be verified by consulting with the manufacturer of the clad material.
Clad materials can be successfully welded by adopting special joint details and following specific welding procedures. Special joint details and welding procedures are established in order to maintain uniform characteristic of the clad material. Inasmuch as the clad material is utilized to provide special properties it is important that the weld joint retain these same properties. It is also important that the structural strength of the joint be obtained with the quality welds of the backing metal.
The normal procedure for making a butt joint in clad plate is to weld the backing or steel side first with a welding procedure suitable for the carbon steel base material being welded. Then the clad side is welded with the suitable procedure for the material being joined. This sequence is preferable in order to avoid the possibility of producing hard brittle deposits, which might occur if carbon steel weld metal is deposited on the clad material.
Different joint preparations can be used to avoid the possible pickup of carbon steel in the clad alloy weld. Any weld joint made on clad material should be a full-penetration joint. When designing the joint details it is wise to make the root of the weld the clad side of the composite plate. This may not always be possible; however, it is more economical since most of the weld metal can be of the less expensive carbon steel rather than the expensive alloy clad metal.
The selection of the welding process or processes to be used would be based on that normally used for welding the material in question in the thickness and position required. Shielded metal arc welding is probably used more often; however, submerged arc welding is used for fabricating large thick vessels and the gas metal arc welding process is used for medium thicknesses; the flux-cored arc welding process is used for the steel side, and gas tungsten arc welding is sometimes used for the thinner materials, particularly the clad side.
The selection of process should be based on all factors normally considered. It is important to select a process that will avoid penetrating from one material into the other. The welding procedure should be designed so that the clad side is joined using the appropriate process and filler metal to be used with the clad metal and the backing side should be welded with the appropriate process and filler metal recommended for the backing metal. For code work the welding procedure must be qualified in accordance with the specification requirements.
The normal procedure, assuming that the material is properly prepared and fitted is as follows. The backing side or steel side would be welded first. The depth of penetration of the root pass must be closely controlled by selecting the proper procedure and filler metal. It is desirable to produce a root pass which will penetrate through the root of the backing metal weld joint into the root face area yet not come in contact with the clad metal.
A low-hydrogen deposit is recommended. If penetration is excessive and the root bead melts into the clad material because of poor fitup or any other reason the deposit will be brittle. If this occurs the weld will have to be removed and remade. However, if the penetration of the backing steel root bead is insufficient the amount of back gouging will be excessive and larger amounts of the clad material weld metal will be required. The steel side of the joint should be welded at least half way prior to making any of the weld on the clad side. If warpage is not a factor, the steel side weld can be completed before welding is started on the clad side.
The clad side of the joint is prepared by gouging to sound metal or into the root pass made from the backing steel side. This can be done by air carbon arc gouging or by chipping. The gouging should be sufficient to penetrate into the root pass so that a full penetration of the joint will result. This will determine the depth of the gouging operation. It is also a measure of the depth of penetration of the root pass. Grinding is not recommended since it tends to wander from the root of the joint and may also cover up an unfused root by smearing the metal. If the depth of gouging is excessive, weld passes made with the steel electrode may be required to avoid using an excessive amount of clad metal electrode.
On thin materials the gas tungsten arc welding process may be used, on thicker materials the shielded metal arc welding process or the gas metal arc process may be used. The filler metal must be selected to be compatible with the clad metal analysis.
There is always the likelihood of diluting the clad metal deposit by too much penetration into the steel backing metal. Special technique should be used to minimize penetration into the steel backing material. This is done by directing the arc on the molten puddle instead of on the base metal.
When welding copper or copper nickel clad steels a high nickel electrode is recommended for the first pass (ECuNi or ENi-1). The remaining passes of the joint in the clad metal should be welded so that the copper or copper nickel electrode matches the composition of the clad metal.
When the clad metal is stainless steel the initial pass which might fuse into the carbon steel backing should be of a richer analysis of alloying elements than necessary to match the stainless cladding. This same principle is used when the clad material is Inconel or Monel. The remaining portion of the clad side weld should be made with the electrode compatible with or having the same analysis as the clad metal. The procedure should be designed so that the final weld layer will have the same composition as the clad metal.
On heavier thicknesses where the weld of the backing steel is made from both sides it is important to avoid allowing the steel weld metal to come in contact or to fuse with the clad metal. This will cause a contamination of the deposit which may result in a brittle weld.
When welding thinner gauge clad plate and inside clad pipe it may be more economical to make the complete weld using the alloy weld metal compatible with the clad metal instead of using two types of filler metal. The alloy filler metal must be compatible with the steel backing metal. The expense of the welding filler metal may be higher, but the total weld joint may be less expensive because of the more straightforward procedure. Joint preparation may also be less extensive using this procedure.
For medium thickness, the joint preparation is a single vee or bevel without a large root face. The root face is obtained by grinding the feather edge to provide a small root face. If possible the face of the weld will be the steel or backing side of the joint. The backing side or steel side is welded first using the small diameter electrode for the root pass to insure complete penetration.
If the composite is a pipe or if it must be welded from one side, the buttering technique should be used. In this case the filler metal must provide an analysis equal to the clad metal and be compatible with the backing steel. Weld passes are made on the edge of the composite to butter the clad and backing metal. The buttering pass must be smoothed to the design dimensions prior to fitup. The same electrode can be used to make the joint.
When welding heavy, thick composite plate the U groove weld joint design is recommended instead of the vee groove in order to minimize the amount of weld metal. The same principles mentioned previously are used.
When the submerged arc welding process is used for the steel side of the clad plate caution must be exercised to avoid penetrating into the clad metal. This same caution applies to automatic flux-cored arc welding or gas metal arc welding. A larger root face is required and fitup must be very accurate in order to control root bead penetration.
The submerged arc process can also be used on the clad side when welding stainless alloys. However, caution must be exercised to minimize dilution of a high-alloy material with the carbon steel backing metal. The proper filler metal and flux must be utilized. To minimize admixture of the final pass it is recommended that the clad side be welded with at least two passes so that dilution would be minimized in the final pass.
Special quality control precautions must be established when welding clad metals so that undercut, incomplete penetration, lack of fusion, etc., are not allowed. In addition, special inspection techniques must be incorporated to detect cracks or other defects in the weld joints. This is particularly important with respect to the clad side which may be exposed to a corrosive environment.
