Clad Metals

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Understanding Clad Metal Composition and Manufacturing

Clad metals represent sophisticated composite materials that combine the economic advantages of carbon steel with the specialized properties of premium alloys. These composites consist of a cladding metal—typically stainless steel, nickel and nickel alloys, or copper and copper alloys—permanently bonded to a backing material of carbon or alloy steel. The manufacturing process involves welding these two distinct metals together at specialized mills using roll welding techniques under carefully controlled heat and pressure conditions.

The engineering specifications for clad composite plates typically require cladding thickness ranging from 5% to 20% of the total composite thickness. This design philosophy maximizes the economic benefit by providing the specialized properties of expensive materials—such as corrosion resistance and abrasion resistance—while leveraging the structural strength and cost-effectiveness of the backing metal.

Historical Development and Industrial Applications

The development of clad metals traces back to the early 1930s, with nickel-bonded carbon steel representing one of the first commercially successful applications in tank car construction. This innovation addressed the critical need for materials that could withstand corrosive environments while maintaining structural integrity and economic viability.

Modern industrial applications for clad steels have expanded significantly to include heat exchangers, processing tanks, materials-handling equipment, and storage vessels. These applications capitalize on the unique combination of properties that clad metals provide, making them indispensable in chemical processing, petroleum refining, and other demanding industrial environments.

Manufacturing Processes and Material Specifications

Clad metals can be produced through several distinct manufacturing methods, each offering specific advantages for different applications. Roll welding remains the most widely utilized process, employing controlled heat and roll pressure to create a metallurgical bond between the cladding and backing materials. Alternative methods include explosive welding and weld surfacing or overlay techniques, each selected based on specific application requirements and material combinations.

The cladding materials encompass a broad range of specialized alloys, including chromium steels in the 12-15% range, stainless steels primarily of 18/8 and 25/12 compositions, nickel-base alloys such as Monel and Inconel, copper-nickel alloys, and pure copper. The backing material typically consists of high-quality steel conforming to ASTM-A285, A212, or equivalent specifications.

The mechanical properties of clad materials depend on the tensile strength of individual components and their proportional thickness ratios. A critical characteristic of quality clad materials is uniform cladding thickness throughout the cross-section and continuous welding between the two metals, ensuring consistent performance across the entire composite structure.

Fabrication Techniques and Processing Methods

Oxygen Cutting Procedures

Oxygen cutting of clad steel requires modified procedures compared to conventional carbon steel cutting. Most clad metals can be oxygen flame cut, with the notable exception of copper-clad composite materials. The standard limitation for clad plate cutting occurs when the cladding material exceeds 30% of total thickness, although higher cladding percentages may be successfully cut in thicknesses of 12 mm and greater.

The cutting process requires reduced oxygen pressure and larger cutting tips compared to standard carbon steel cutting. Critical to achieving quality cuts is positioning the cladding material on the underside, allowing the flame to initially cut through the carbon steel backing. The addition of iron powder to the cutting flame significantly assists the cutting operation and improves cut quality.

For copper and copper-nickel clad steels, the copper cladding surface must be removed before cutting, with the backing steel then cut using standard carbon steel procedures. Copper and brass clad plates can be effectively cut using iron powder cutting techniques.

Mechanical Fabrication Methods

Clad steels demonstrate excellent fabricability through conventional metalworking processes including bending, rolling, shearing, punching, and machining. These operations can be performed using the same techniques and equipment employed for equivalent carbon steels. The composite materials also respond well to preheating and stress relief heat treatment procedures similar to those used for carbon steels, though stress relieving temperatures should be verified with the clad material manufacturer to ensure optimal results.

Advanced Welding Procedures for Clad Materials

Joint Design and Welding Sequence

Successful welding of clad materials requires specialized joint details and carefully developed welding procedures to maintain the uniform characteristics of the composite material. Since clad materials are utilized specifically for their special properties, weld joints must retain these same properties while achieving the required structural strength through quality welds in the backing metal.

The standard procedure for creating butt joints in clad plate involves welding the backing or steel side first using procedures appropriate for the carbon steel base material. Subsequently, the cladding side is welded using procedures suitable for the specific cladding material. This sequence prevents the formation of hard, brittle deposits that might occur if carbon steel weld metal were deposited directly on the cladding material.

Process Selection and Filler Metal Considerations

The selection of welding processes depends on factors normally considered for the specific materials, thicknesses, and positions involved. Shielded metal arc welding finds the most frequent application, while submerged arc welding is preferred for large, thick vessels. Gas metal arc welding serves medium thickness applications effectively, flux-cored arc welding is utilized for the steel side, and gas tungsten arc welding is sometimes employed for thinner materials, particularly on the cladding side.

Process selection must prioritize avoiding penetration from one material into the other. The welding procedure should ensure that the cladding side is joined using appropriate processes and filler metals compatible with the cladding metal, while the backing side receives welding with processes and filler metals recommended for the backing metal.

Specialized Welding Techniques

Root Pass Control and Penetration Management

The backing side or steel side welding begins the process, with root pass penetration depth requiring precise control through proper procedure and filler metal selection. The objective is producing a root pass that penetrates through the backing metal weld joint into the root face area without contacting the cladding metal. Low-hydrogen deposits are recommended to minimize the risk of brittle weld formation.

Excessive penetration causing root bead melting into the cladding material results in brittle deposits requiring weld removal and remaking. Conversely, insufficient penetration necessitates excessive back gouging and increased quantities of expensive cladding material weld metal. The steel side joint should be welded at least halfway before beginning any cladding side welding.

Cladding Side Preparation and Welding

The cladding side joint preparation involves gouging to sound metal or into the root pass from the backing steel side. This can be accomplished through air carbon arc gouging or chipping, with sufficient depth to penetrate into the root pass ensuring full joint penetration. Grinding is not recommended as it tends to wander from the joint root and may mask unfused areas by metal smearing.

For thin materials, gas tungsten arc welding may be employed, while thicker materials utilize shielded metal arc welding or gas metal arc processes. Filler metal selection must ensure compatibility with the cladding metal composition, and special techniques should minimize penetration into the steel backing material by directing the arc on the molten puddle rather than the base metal.

Material-Specific Welding Procedures

When welding copper or copper-nickel clad steels, high nickel electrodes (ECuNi or ENi-1) are recommended for the first pass, with remaining passes using electrodes matching the cladding metal composition. For stainless steel cladding, the initial pass should utilize electrodes with richer alloying element analysis than required to match the stainless cladding, with subsequent passes using electrodes compatible with the cladding metal composition.

Quality Control and Inspection Requirements

Special quality control measures must be established when welding clad metals to prevent undercut, incomplete penetration, and lack of fusion. Additionally, specialized inspection techniques must be implemented to detect cracks or other defects in weld joints, particularly on the cladding side that may face corrosive environments. These enhanced quality control measures ensure that the finished welded assemblies maintain both the structural integrity and corrosion resistance properties that justify the use of clad materials in demanding applications.

The successful application of clad metals in modern industry depends on understanding their unique properties, appropriate fabrication techniques, and specialized welding procedures. When properly manufactured and fabricated, these composite materials provide an optimal balance of performance and economy for applications requiring both structural strength and specialized surface properties.