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GRC Transactions, Vol. 34, 2010

991

KeywordsClad metal, cost savings, corrosion control, Detaclad®, dissimi-lar metal, explosion welded, pipe material, separator material, well casing material

AbstrAct

In many of today’s geothermal environments, high pressures, high temperatures and significant corrosion are unavoidable. These factors are driving engineering material selection for many components, including piping and pressure vessels, to higher al-loys of stainless steel, nickel based alloys and titanium. The costs of these materials can be significant in the scope of any geothermal project. Explosion welded clad metal offers a number of interest-ing and unique advantages over solid material. Economics is one of the key benefits. With a relatively thin layer of the higher cost corrosion resistant alloy being clad to a lower cost carbon or alloy steel, the total cost of construction using clad metals is frequently lower than using solid material. Explosion welding creates a metallurgical bond between similar and dissimilar metals. The strength of the weld allows the resulting material to act as one, with the benefits of both materials in the final product.

Explosion welding has been used for years in the oil and gas industry, chemical plants, and other indus-trial applications. This paper will focus on the process of making an explosion weld. It will also include the origins of the process and most recent developments. In addition, it will cover briefly how to fabricate equip-ment with clad materials. Finally, it will give examples of where explosion welded clad metals are already employed and where they may have application in the field of geothermal energy.

One primary focus of this paper is to continue to ex-pand the toolbox of engineers and designers that develop geothermal resources. It is important to have a broad understanding of all the technologies and products that exist in the world with application in the geothermal

community. Explosion welding is one technology with broad, but niche based, applications and is useful in many situations like geothermal energy recovery, where high temperature, high pressure, and corrosive environments are typical.

History of Explosion Welding

Explosion welding had its start as an observed phenomenon related to wartime explosions and the occasional resulting shrapnel and armor or other metals being ‘stuck’ together. It was understood that these metals would stick or weld together, but the mecha-nism and methodology to commercialize this knowledge was not well defined until DuPont developed the Detaclad® process in the 1960s. Subsequently, the Detaclad® process was licensed to a number of explosion welding companies around the world. Many of those licensed companies, as well as the Detaclad® trade name, have since been consolidated into Dynamic Materi-als Corporation.

Explosion Welded Materials for Geothermal Applications

Michael blakely1 and Jose Olivas2

1Director of Market Development2Director of sales

Dynamic Materials corporation, boulder cO UsA

Figure 1. Explosion Welding Process.

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One of the first significant uses of explosion welded material was for United States government coins. The process was used to clad slabs of material in three layers. Those slabs were then rolled to the appropriate gauge and coined.

Since its inception, explosion welding has found a number of different niche applications in a wide range of industrial markets. Highly dissimilar metal clad plates continue to be a key market for explosion welded material. Applications requiring combinations of steel, stainless steel, nickel, titanium, aluminum, zirconium, copper, brass, and many other materials are fabricated into large flat plates or smaller custom machined components.

Explosion Welding Process

Explosion welding, as with nearly all welding processes, has a number of pre- and post-processing steps that must be followed to create a high quality final product. Preparation and testing are key elements, along with the explosion itself. A general guide to the explosion welding process can be seen in Figure 1. A more detailed explanation follows.

Preparation

Explosion welding involves two or more plates of material. During welding, one plate remains stationary and is called the backer. The other plate, called the cladder, is typically thinner and driven into the backer. In preparation for explosion welding, the faces of the plates that will be welded are ground. This removes any dirt or oxides and achieves a uniform surface finish.

The plates are then positioned in parallel to each other, with a small gap between the two metals. The distance of this gap is one of the crucial elements in explosion welding, helping to determine the angle of impact, the velocity of impact, and the consistency of the jet. The gap is held constant by standoff devices in between the plates. These devices are designed to be consumed by the jet, and ejected from the resulting weld. Figure 2 shows the general arrangement of the plates during the welding process.

An explosive containment frame is then constructed around the perimeter of the cladder plate. The height of the frame is speci-fied to allow the proper amount of explosive to be loaded. The amount and type of explosive are also very important parameters. The amount of explosive determines the force generated during welding. The formulation can be adjusted to control the speed at which the explosive burns. The explosive layer is placed on top of the cladder plate. A detonator will be used to ignite this explosive.

WeldingUpon detonation, the explosion sweeps across the surface of

the cladder at approximately 7500 ft/sec, driving it into the backer material. Figure 3 shows the detonation front moving across the plate and the development of the jet.

The jet is very high energy. It consists essentially of spalled oxides and very small amounts of base material driven from the joint by the impact of the two plates.

At impact, the two plates are brought into intimate contact by the immense pressures of the process. The result is a metallurgi-cal weld between the metals. The joint happens almost instantly and the plates do not see any changes in chemistry, significant bulk heating nor significant changes in mechanical properties from the process.

TestingExplosion welding is a reliable and repeatable process. The

environments in which the clad will be used are typically critical to a system or facility, and therefore it is important to subject clad metal to rigorous testing. There are ASME and ASTM specifica-tions that govern most cladder materials and clad systems. For example is ASME SA-265 governs the production and testing of nickel and nickel base alloy clad steel plate while ASTM B898 governs titanium clad plate.

Mechanical PropertiesSteel is the most common base metal for clad plates. The steel

alloy selection and specification are established by the purchaser based primarily upon strength and operating temperature require-ments. The cladding metal is primarily selected for corrosion or wear properties. Although ASME Code will allow the strength of the cladding metal to be considered in design strength calcu-lations for many metals, that is rarely the case. The base metal must comply with all of the technical requirements of the Base Metal Specification including tensile strength, impact properties, grainsize, and others as applicable. When heat treatments are required as part of the cladding or fabrication process, these heat treatments are typically simulated on test coupons at the time of metal manufacture to minimize subsequent retesting.

The most common bond strength test is the shear test. The specific geometry for the shear test specimen is defined in the ASME/ASTM specifications. With the exception of the specifi-cation for copper clad, all ASME and ASTM clad specifications require a minimum bond shear strength of 20,000 psi (137.9 Mpa). Figure 2. General Arrangement.

Figure 3. Welding Mechanism.

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Detaclad® proprietary specifications require a higher minimum bond strength of 35,000 psi (241 MPa) for stainless steel and nickel alloys. Production test results are typically in the range of 6o to 80 ksi (413-551 MPa) for stainless steel and nickel alloys explosion clad to steel.

It is difficult to measure the tensile strength of a clad bond since the cladding metal layer is typically thin, commonly in the 0.125” (3mm) range. An extensive study of the bond tensile strength of Detaclad® production plates demonstrates that the bond tensile strength is consistently higher than the shear strength, which is an easily measurable property.

Non Destructive TestingAll explosion welded clad plates are ultrasonically tested for

bond and base metal integrity. The procedures of ASTM A578 Specification for Straight Beam Ultrasonic Examination of Plain and Clad Steel are commonly used. The Clad Metal Specifica-tions present a range of defect acceptance/rejection criteria from which the purchaser can select more, or less, severe requirements dependent upon the severity of his application. Detaclad® plates are ultrasonically inspected over 100% of the clad surface. Typi-cally the sound bond area of the plate is well in excess of 99%.

Supplemental RequirementsThere are a number of application specific supplemental tests

which can be performed on clad plates when agreed upon between the purchaser and manufacturer. These include bend tests, bond tensile strength tests, angle beam UT, penetrant tests, and others. A detailed list of testing options is presented in Detaclad Speci-fication DMC 100.

Advantages of Explosion Welded Materials

Explosion welding is a versatile process that can join nearly any metal to any other metal. It is very common for steel, alloy steel or stainless steel to be requested as the ‘backer’ plate to give the resulting product strength and structure in the operating environment, but at the lowest possible cost. The cladding metal is usually thin, and typically not taken into account during the pressure based thickness calculations. Therefore, almost any corrosion resistant alloy can be specified and welded. Figure 4 gives some general economic comparison of clad metal versus solid corrosion resistant alloy materials. As you can see from the figure, two things influence the economics. One, the thicker the system, the more advantageous it may be to consider clad and two, as the value of the cladding material goes up, the relative savings also goes up.

Another advantage of explosion welded material is the ability to use the relative material savings to upgrade the alloy used. For example, at certain thicknesses, the economics may be similar between solid 2205 and 825 clad materials. In this case, clad may be employed and offer a longer equipment life than the alterna-tive solid choice.

To be clear, there are some added costs in fabrication that must be taken into account. It typically takes more hours to construct a pipe or pressure vessel from clad material as compared to solid material. Another consideration may be the restoration of the clad metal in the area of a welded joint. However, the filler metal

used to join the backer plate is considerably less expensive than the filler metal used to weld the corrosion resistant alloy or restore the cladding layer offering overall savings. As less of the high cost alloy filler metal is required, it becomes more likely fabrication costs can be kept down.

Another advantage of using a clad metal product instead of a solid alloy product is the ability to attach external structures and supports directly to the piping or pressure vessels. A completed nickel alloy vessel, with steel on the outside (from the clad), can have steel attachments (such as ladders, insulation clips, stiffening rings, and saddles) welded directly to the steel shell. An additional benefit is that attachments can be made of lower cost carbon steel and not alloy.

Application in Geothermal systems

There are three main classes of materials regularly investi-gated in the most aggressive geothermal environments. Stainless steel and nickel alloys have already seen application in these environments, and titanium is gaining acceptance. As noted before, increases in thicknesses of these expensive materials (usually driven by high pressures and temperatures) can lead to a strong economic advantage for clad metals. Some applica-tions for clad materials in geothermal applications include well casings, surface piping, crystallizers and separators. Figure 5 provides an example of a typical explosion welded clad appli-cation, employing nickel alloy clad in a horizontal geothermal separator vessel.

More specifically, southern California has been an area of early adoption of clad metal in geothermal environments. For twenty years, 625 nickel clad has been employed by CalEnergy in geothermal projects in this area. In addition, new projects in Salton Sea, California continue to be prime examples of em-ploying explosion welded clad metals in designs. Much of the equipment for a new geothermal project in this area, including the high pressure separator (625 Nickel based alloy clad), standard pressure crystallizer (2205 duplex stainless steel clad / Alloy 825 Nickel based alloy clad), and low pressure crystallizer (2205

Figure 4. Relative Cost of Clad Versus Solid Alloy.

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duplex stainless steel clad / 825 Nickel based alloy clad), as well as much of the connecting pipe and related equipment, are be-ing constructed of explosion welded clad metal. The material is being supplied to the fabrication source as large, flat plates and formed heads. The clad is subsequently formed and welded to produce the pressure vessels and piping. While explosion weld-ing is perfect for large formed pieces of equipment, it is not ideal for smaller forgings or very thin equipment. The thin sections will be made from solid alloy material and the forgings will have deposits of corrosion resistant metal applied to their surface. This process is known as weld overlay and as discussed previously, it will also be employed at the seams between the plates during fabrication to restore the cladding at the joints. Weld overlay is a widely used and well understood process of cladding these areas and components. Figure 6 gives an example of explosion welded duplex clad steel plates fabricated into a manifold for a Salton Sea based geothermal facility.

Geothermal well casings made from clad metal have been an area of interest for nearly twenty years. The casings that have been de-signed to date are nickel alloy clad to carbon steel. One method of construction is lined seamless pipe, with mechanically expanded liners and explosion welded ends. The other method of construction is clad plates rolled and welded, where the cladding material is explo-sion welded over the entire length. Current status of both clad casing types include fully qualified designs and processes, with threaded corrosion resistant alloy ends on each of the sections of pipe. Forty foot long casing spools with twelve inch nominal diameters are typical for geothermal applications, other sizes may also be available and clad metal suppliers or well casing manufacturers should be consulted as to specific availability. Economics of nickel clad casing material is usually compared to sol-id ruthenium enhanced titanium casing pricing. As titanium prices rise, the economics shift in the direction of nickel clad. Conversely, as titanium prices drop, nickel clad solutions

become less interesting. While there has been significant interest in clad casings in the recent past, there are no active examples of implementation in a geothermal power system.

conclusions

Explosion welding is a well understood, industrialized process of cladding high value corrosion resistant alloys to other construc-tion materials. The cladding can be similar or dissimilar to the base metal. The process of explosion welding relies a great deal on the preparation for welding to ensure weld quality. Similarly, explosion welded material undergoes extensive testing based on the importance of many of the end market applications. Explo-sion welded materials have a broad but niche application in a wide range of industries. Geothermal energy recovery is one of those applications. When systems are implemented in areas of the world with hot, high pressure, corrosive geothermal fluid, the ability to clad high cost corrosion resistant alloys to lower cost steel or stainless steel can yield an economic advantage in the construction of a wide range of equipment.

referencesJ.G. Banker, E.G. Reineke, “Explosion Welding”, ASM Handbook, Vol. 6,

Welding, Brazing, and Soldering, 1993, pp 303-305.

Nobili, “Explosion Bonding Process”, Nobelclad Technical Bulletin, Nobel-clad, Rivesaltes, France, March 1999.

“Specification for Nickel and Nickel Base Alloy-Clad Steel Plate”, Specifica-tion ASME SA-265, American Society of Mechanical Engineers, 2007.

“Standard Specification for Reactive and Refractory Metal Clad Plate.” Speci-fication ASTM B898-99, ASTM International, Conshocken, PA 1999.

Figure 5. Nickel Clad Geothermal Separator Vessel.

Figure 6. Duplex Clad Geothermal Manifold.