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Cladding of steels

• satyendra
• July 4, 2021
• 11 Comments
• Bonding, Brazing, Cladding of steel, corrosion resistance, Explosive bonding, Explosive welding, Hard facing, Laser cladding, Roll bonding process, Thermal surface treatment, Weld cladding, Weld overlaying, Work hardening,

Cladding of steels

There are several industries, such as, chemical and fertilizer industries, nuclear and steam power plants, food processing, and petrochemical industries etc., where corrosive environments are inevitable. Several process equipment / engineering components are exposed to the corrosive environment in these industries which reduces their service life. In such applications, materials (like duplex stainless steel) having high strength and corrosion resistant properties are desirable for long term reliability, and performance of the whole system.

Materials possessing the above properties are, however, costly. Hence, their use to manufacture process equipment / engineering components adds to the cost. To overcome this kind of situation, the service life of a component is to be increased by keeping the cost of the material reasonable. For this, the process of surface treatment is used. In this process, a filler metal or alloy is deposited on the base material which imparts the process equipment / engineering components the properties of resistance to wear, corrosion, heat, and cavitation, etc. Hence, the particular property requirement for the material surface is met without using large quantities of expensive steels or alloys. There are several surface treatment processes which are used for covering low grade cheap materials economically. These methods are coating, plating, buttering, metal spraying, and cladding etc. In these different methods, cladding is an important method which is used frequently.

Cladding is the bonding together of two dissimilar metals. It refers to the metallurgical process of coating a metal onto another metal under high temperature and pressure so as to protect the inner metal from corrosion. It is different from fusion welding or gluing as a method to fasten the metals together. There are different methods for cladding.

Cladding is a thermal surface treatment process in which a layer of hard or corrosion resistant material (such as high alloy steel) is placed on a cheaper substrate (mild steel), so as to increase the corrosion resistance, wear resistance, and / or hardness of the process equipment / engineering components with an objective of increasing the service life of the same. Unlike hardening in which the properties of the surface layer of substrate material (mild steel) to a certain depth are changed, cladding creates a new surface layer with different composition than the base material. In other words, cladding does not influence the microstructure of the base steel. It simply provides a protective layer of the filler material having different composition to that of the base steel. It is a process which finds its way in various industries for the improvement of wear, or corrosion, resistant properties of process equipment / engineering components.

The clad steel is a composite product consisting of a thin layer of high alloy steel in the form of a thin covering integrally bonded to one or both surfaces of the substrate backing material (mild steel). The principal object of such a product is to combine, at low cost, the desirable properties of the high alloy steel and the backing material for applications where full-gauge alloy steel construction is not needed. While the high alloy steel cladding furnishes the necessary resistance to corrosion, abrasion, or oxidation, the backing material contributes structural strength and improves the fabricability and thermal conductivity of the composite. Clad steels can be produced in plate, strip, tube, rod, and wire form.

Cladding on a surface can serve two fold functions. One is to improve surface dependent properties like resistance to wear under abrasion, erosion, and corrosion, and the other is to enhance the bulk dependent properties like hardness, strength, etc. which is known as hard facing. Clad materials are expected to have capabilities of serving its specific function in a hostile environment for a sufficiently long time economically.

Compared with solid corrosion-resistant alloy (CRA) materials, cost advantages can be achieved by using clad materials. Not only material costs, but also the costs of filler metals can be reduced. The thicker is the backing material; the lower is the overall costs while maintaining consistent corrosion resistance.

During cladding, unlike coating, material of several millimetres is deposited on a corrosion-prone material for protecting it from corrosion and / or imparting high strength on the surface to increase service life of the parent material. Cladding does not change the micro structure of the backing material as this process creates a new surface layer with a different composition than that of the backing material. Normally cladding layer is harder than the backing material. Compared with other techniques used for surface treatment by means of material deposition, cladding has some distinct advantages. It provides high hardness, corrosion and / or erosion resistance, good bonding, and favourable microstructure.

Cladding can be done by several processes like rolling, strip cladding, different types of welding such as electric resistance welding, shielded metal arc welding, submerge arc welding, overlay welding, electro-slag welding, gas tungsten arc welding, flux cored arc welding, gas metal arc welding, laser beam welding, oxy-acetylene welding, powder welding by laser, pulsed gas metal arc welding, tubular core covered electrodes, plasma process (hot-wire), submerged arc strip cladding and strip electro slag weld surfacing, electro-slag cladding, friction cladding, and laser engineering net shaping etc.

The main cladding techniques include hot roll bonding, cold roll bonding, explosive bonding, centrifugal casting, brazing, and weld overlaying, although adhesive bonding, extrusion, and hot isostatic pressing have also been used to produce clad steels. With casting, brazing, and welding, one of the metals to be joined is molten when a metal-to-metal bond is achieved. With hot / cold roll bonding and explosive bonding, the bond is achieved by forcing clean oxide-free metal surfaces into intimate contact, which causes a sharing of electrons between the metals. Gaseous impurities diffuse into the metals, and non-diffusible impurities consolidate by spheroidization. These non-melting techniques involve some form of deformation to break up surface oxides, to create metal-to-metal contact, and to heat in order to accelerate diffusion. They differ in the amount of deformation and heat used to form the bond and in the method of bringing the metals into intimate contact.

The hot roll bonding process, which is also called ‘roll welding’, is the most important commercial process since it is the major production method for manufacturing the high alloy clad steel plates. It is also known also as the ‘heat and pressure’ process because the principle involves preparing the carefully cleaned cladding components in the form of a pack or sandwich, heating to the plastic range, and bringing the alloy steel and backing material into intimate contact, either by pressing or by rolling. The product so formed is integrally bonded at the interface. The clad surface is has in all respects corrosion resistance, physical properties, and mechanical properties which are equal of the solid high alloy steel. It can be polished and worked in the same manner as solid high alloy steel.

The most common clad systems on a tonnage basis are carbon or low-alloy steels clad with 300-series austenitic stainless steel grades. The types of austenitic stainless steel cladding normally available in plate forms are (i) type 304 (18-8), (ii) type 304 L (18-8 low carbon), (iii) type 309 (25-12), (iv) type 310 (25-20), (v) type 316 (17-12Mo), (vi) type 316 Nb (17-12 Nb stabilized), (vii) type 316 L (17-12 Mo low carbon),(viii) type 317 (19-13 Mo), (ix) type 317 L (19-13 Mo low carbon), (x) type 321 (18-10Ti), and (xi) type 347(18-11Nb).

The carbon or low-alloy steel / high alloy steel plate rolling sequence is normally followed by heat treatment, which is generally needed to restore the cladding to the solution-annealed condition and to bring the backing material into the correct heat-treatment condition. The cladding thickness is normally specified as a percentage of the total thickness of the composite plate. It varies from 5 % to 50 %, depending on the end use. For most commercial applications involving carbon or low-alloy steel / stainless steel combinations, cladding thickness normally falls in the 10 % to 20 % range.

Hot roll bonding has also been used to clad high-strength low-alloy (HSLA) steel plate with duplex stainless steels. The micro alloyed backing metals contain small amounts of copper (0.15 % max), niobium (0.03 % max), and nitrogen (0.010 % max) and have mechanical properties comparable to those of duplex stainless steels. Typically these HSLA base steels have yield strengths of 500 MPa and impact values of 60 joules at minus 60 deg C. The shear strength of the cladding bond can be as high as 400 MPa.

The cold roll bonding process, which is shown schematically in Fig 1, involves three basic steps namely (i) the mating surfaces are cleaned by mechanical and / or chemical methods to remove dirt, lubricants, surface oxides, and any other contaminants, (ii) the materials are joined in a bonding mill by rolling them together with a thickness reduction which is in the range of 50 % to 80 % in a single pass which gives the materials an initial or green bond created by the huge cold reduction, and (iii) the materials then undergo sintering, a heat treatment during which the bond at the interface is completed. During the heat treatment diffusion occurs at the atomic level along the interface. This results in a metallurgical bond which is due to a sharing of atoms between the materials. The resulting bond can exceed the strength of either of the parent materials.

Fig 1 Cold roll bonding process

 Upon completion of this three-step process, the resultant clad material can be treated in the same way as any other conventional monolithic steel. The clad material can be worked by any of the traditional processing methods for strip metals. Rolling, annealing, pickling, and slitting are typically carried out to produce the finished strip to specific customer requirements, so that the material can be roll formed, stamped, or drawn into the needed part.

Clad steels prepared by this method show substantially the same microstructures as those which have been bonded by hot roll bonding process. Because of the high power requirement in the initial reduction, the cold bonding process is not practical for producing clad plates of any appreciable size. The single largest application for cold-roll bonded materials is stainless-steel-clad aluminium for automotive trim. The stainless steel exterior surface provides corrosion resistance, high lustre, and abrasion and dent resistance, and the aluminium on the inside provides sacrificial protection for the painted for the auto body steel and for the stainless steel.

Cladding by explosive welding uses an explosive detonation as the energy source to produce a bond between metal components. It can be used to join virtually any metal combination, which are metallurgically compatible or which are known to be not weldable by conventional processes. Also, this process can clad one or more layers onto one or both faces of a backing steel, with the potential for each to be a different metal type or alloy.

Explosive bonding uses the very-short-duration, high-energy impulse of an explosion to drive two surfaces of steels together, at the same time cleaning away surface oxide films and creating a metallic bond. The two surfaces do not collide instantaneously but rather progressively over the interface area. The pressure generated at the resulting collision front is intense and causes plastic deformation of the surface layers. In this way, the surface layers and any contaminating oxides present are removed in the form of a jet projected ahead of the collision front. This leaves perfectly clean surfaces under pressure to form the bond. Fig 2 also gives the wavy interface which characterizes normally the explosive bonds.

Fig 2 Explosive bond cladding process

Two basic geometric configurations of the explosive bonding process which are normally used are (i) angle bonding, and (ii) parallel-plate bonding. Angle bonding is normally used for bonding sheet components and tubes, where the needed bond width does not exceed 20 times the flyer plate thickness. The more normally used parallel-plate geometry (Fig 2) is applicable for welding larger flat areas, plate, and concentric cylinders. The energy of bonding typically creates sufficient deformation and hence flattening or straightening is needed before further processing. Flattening is carried out with equipment of the same design used in plate and sheet production.

Explosive bonding is an effective joining method for virtually any combination of steels. The only metallurgical limitation is sufficient ductility and fracture toughness to undergo the rapid deformation of the process without fracture. Normally accepted limits for these properties are 10 % and 30 joules minimum, respectively.

A completely different approach is used for the cladding of the seamless pipes. It consists of the utilization of the horizontal centrifugal casting technology. First, well-refined liquid steel is poured into a rotating metal mould with flux. After casting, the temperature of the outer shell is monitored. At a suitable temperature after solidification, the liquid high alloy steel is introduced. The selection of the flux, the temperature of the outer shell when the liquid high alloy steel is introduced, and the pouring temperature of the high alloy steel are the most important factors for the achievement of a sound metallurgical bond. By controlling these different parameters it is possible to achieve minimum mixing at the interface and maintain homogeneous cladding thickness and wall thickness.

Centrifugal casting is followed by heat treatment to solution anneal the cladding and quench and temper the outer pipe for achieving the needed mechanical properties. Finally, the pipe is machined externally and internally to remove the shallow inter-dendritic porosity in the bore and achieve the needed dimensions and surface finish.

In case of furnace brazing, the high alloy steel cladding and the backing material, in their respective final gauges, are assembled as a multi-layer sandwich, with a brazing alloy placed between each pair of surfaces to be bonded. The sandwich is heated under continuous vacuum to a temperature at which the brazing alloy melts and forms an inter-metallic alloying zone at the interface of the high alloy steel and the backing metal (normally mild steel). A broad range of brazing filler metals can be used to join high alloy steels to carbon or low-alloy steels. The normally used brazing filler metals are silver-base alloys.

In recent years, weld cladding processes are being applied widely in several industries such as chemical, and fertilizer plants, aviation, mining, agriculture, power generation, and food processing etc. as a cost effective engineering solution. These are used to deposit a surface protective layer on the corrosion-prone low carbon or low alloy steel against corrosive environment. Weld overlay cladding techniques were originally developed at Strachan & Henshaw, Bristol for applying on the defence (Navy) components subjected to extreme pressure and shock loading. These clad components come in contact with sea water, but needed less maintenance. Cladding techniques are employed in sub-sea components for making outer layer to be corrosion resistant against corrosive saltwater solution. A number of parts of the submarines are clad with Inconel 625. Pressure vessels used in power plants are being clad to provide anti-corrosive as well as strong surface layer to survive severe working conditions at high temperatures. Residual stresses developed in clad pressure vessels are evaluated, and these data are used in their designing process. Weld cladding is also being used to provide for enhanced performance of components in service, or to repair worn or corroded components. Large worn out gears can be preliminary repaired by depositing metallic materials using a cladding technique.

Weld overlaying refers to the deposition of a filler metal on a backing steel (substrate) to impart some desired property to the surface which is not intrinsic to the underlying backing steel. There are several types of weld overlays such as weld claddings, hardfacing materials, build-up alloys, and buttering alloys.

A weld clad is a relatively thick layer of filler metal applied to a carbon or low-alloy steel backing material for the purpose of providing a corrosion-resistant surface. Hardfacing is a form of weld surfacing which is applied for the purpose of reducing wear, abrasion, impact, erosion, galling, or cavitation. The term build-up refers to the addition of weld metal to a backing steel surface for the restoration of the component to the required dimensions. Build-up alloys are normally not designed to resist wear, but to return the worn out component to, or near, its original dimensions, or to provide adequate support for subsequent layers of the hardfacing materials. Buttering also involves the addition of one or more layers of weld metal to the face of the joint or surface to be welded. It is different from build-up since the main purpose of buttering is to satisfy some metallurgical consideration. It is used primarily for the joining of dissimilar metal with the backing steels.

The term weld cladding normally denotes the application of a relatively thick layer (higher than 3 mm) of weld metal for the purpose of providing a corrosion-resistant surface. Hard facing produces a thinner surface coating than a weld cladding and is normally applied for dimensional restoration or wear resistance. Typical backing steel components for which weld-cladding is carried out include the internal surface of carbon and low-alloy steel pressure vessels, paper digesters, urea reactors, tube sheets, nuclear reactor containment vessels, and hydro-crackers. The cladding material is normally a high alloy steel or a nickel-base alloy.

Weld cladding is normally done by using submerged arc welding process. However, flux-cored arc welding (either self-shielded or gas-shielded), plasma arc welding, and electro-slag welding processes can also produce weld claddings. Filler metals are available as covered electrodes, coiled electrode wire, and strip electrodes. For very large areas, strip welding with either submerged arc or electro slag techniques is the most economical weld cladding

Weld cladding is an outstanding method to impart properties to the surface of substrate steel which are not available from that of a backing steel, or to conserve expensive or difficult-to-obtain materials by using only a relatively thin surface layer on a less expensive or abundant baacking steel. Several inherent limitations or possible problems are to be considered when planning for the weld cladding. The thickness of the required surface is to be less than the maximum thickness of the overlay which can be obtained with the particular process and filler metal selected.

Welding position is also to be considered when selecting an overlay material and process. Some processes are limited in their available welding positions (e.g., submerged arc welding can be used only in the flat position). In addition, when using a high deposition-rate process which shows a large liquid pool, welding vertically or overhead can be difficult or impossible. Some alloys shows eutectic solidification, which leads to large liquid pools which solidify instantly, with no ‘mushy’ (liquid plus solid) transition. Such materials are also difficult to weld except in the flat position.

The economics of high alloy steel weld cladding are dependent on achieving the specific chemistry at the highest practical deposition rate in a minimum number of layers. The fabricator is to select the filler wire and welding process based on the specified surface chemistry and thickness, along with the backing steel. The most outstanding difference between welding a joint and depositing an overlay is the percentage of dilution represented by % dilution = a / (a + b) x 100 where ‘a’ is the amount of backing steel melted and ‘b’ is the amount of filler metal added.

For the high alloy steel cladding, a fabricator is to understand how the dilution of the filler metal with the backing steel affects the composition and metallurgical balance, such as the proper ferrite level to minimize hot cracking, absence of martensite at the interface for bond integrity, and carbon at a low level to ensure corrosion resistance. The prediction of the micro-structures and properties (such as hot cracking and corrosion resistance) for the high alloy steels has been the topic of several studies. In these studies, four micro-structure prediction diagrams have found the widest application. These include the Schaeffler diagram, the DeLong diagram, and the two Welding Research Council (WRC) diagrams (WRC-1988 and WRC-1992).

Although each weld cladding process has an expected dilution factor, experimenting with the welding parameters can minimize dilution. A value between 10 % and 15 % is normally considered optimum. Less than 10 % raises the question of bond integrity, and higher than 15 % increases the cost of the filler metal. Unfortunately, most of the welding processes have considerably greater dilution.

Because of the importance of dilution in weld cladding as well as hardfacing applications, some of the welding parameters as given below are to be carefully evaluated and controlled. Several of the parameters which affect dilution in weld cladding applications are not so closely controlled when arc welding is performed.

• Amperage – High amperage (current density) increases dilution. The arc becomes hotter and it penetrates more deeply, and more backing steel melting occurs.
• Polarity – Direct current electrode negative (DCEN) gives less penetration and results in lower dilution than direct current electrode positive (DCEP). Alternating current results in a dilution which lies between that provided by DCEN and DCEP.
• Electrode size – The smaller is the electrode, the lower is the amperage which results in less dilution.
• Electrode extension – A long electrode extension for consumable electrode processes decreases dilution. A short electrode extension increases dilution.
• Travel speed – A decrease in travel speed decreases the amount of backing steel melted and increases proportionally the amount of filler metal melted, thus decreasing dilution.
• Oscillation – Larger width of electrode oscillation reduces dilution. The frequency of oscillation also affects dilution. The higher is the frequency of oscillation, the lower is the dilution.
• Welding position – Depending on the welding position or work inclination, gravity causes the weld pool to run ahead of, remain under, or run behind the arc. If the weld pool stays ahead of or under the arc, there is lesser backing steel penetration which results dilution to occur. If the pool is too far ahead of the arc then there is insufficient melting of the surface of the backing steel and coalescence does not occur.
• Arc shielding – The shielding medium, gas or flux, also affects dilution. Different shielding mediums in order of decreasing dilution are (i) granular flux without alloy addition (highest), (ii) helium, (iii) carbon di-oxide, (iv) argon, (v) self-shielded flux-cored arc welding, and (vi) granular flux with alloy addition (lowest).
• Additional filler metal – Extra metal (not including the electrode),added to the weld pool as powder, wire, strip, or with flux, reduces dilution by increasing the total amount of filler metal and reducing the amount of backing steel which is melted. For weld cladding the inside surfaces of large pressure vessels, wide beads produced by oscillated multiple-wire systems or strip electrodes have become the means to improve productivity and minimize dilution while offering a uniformly smooth surface

Hard facing materials include a broad variety of alloys, carbides, and combinations of these alloys. Conventional hardfacing alloys are normally classified as carbides, nickel-base alloys, cobalt-base alloys, and ferrous alloys (high-chromium white irons, low-alloy steels, austenitic manganese steels, and stainless steels). Stainless steel hardfacing alloys include martensitic and austenitic grades, the latter having high manganese (5 % to 10 %) and / or silicon (3 % to 5 %) contents. Both cobalt-containing and cobalt-free austenitic stainless steel hardfacing alloys have been developed.

Hard facing alloy selection is guided mainly by wear and cost considerations. However, other manufacturing and environmental factors are also to be considered, such as backing steel, deposition process, as well as impact, corrosion, oxidation, and thermal requirements. Normally, the hardfacing process dictates the hardfacing or filler metal product form. Hard facing alloys normally are available as bare rod, flux-coated rod, long-length solid wires, long-length tube wires (with and without flux), or powders. The most popular processes with the consumable forms normally associated with each process are (i) oxy-fuel / oxy-acetylene (bare cast or tubular rod), (ii) shielded metal arc (coated solid or tubular rod, stick electrode),  (iii) gas-tungsten arc (bare cast or tubular rod), (iv) gas-metal arc (tubular or solid wire), (v) flux -cored open arc (flux cored tubular wire), (vi) submerged arc (tubular solid wire), (vi) plasma transferred arc (powder), and (vii) laser beam (powder).

The build-up alloys include low-alloy pearlitic steels, austenitic manganese (Hadfield) steels, and high-manganese austenitic stainless steels. For the most part, these alloys are not designed to resist wear but to return a worn part back to, or near, its original dimensions and to provide adequate support for subsequent layers of true hardfacing materials. However, austenitic manganese steels are used as wear-resistant materials under mild wear conditions. Typical examples of applications where build-up alloys are used for wearing surfaces include tractor rails, railroad rail ends, rolling mill table rolls, and large slow-speed gear teeth.

Martensitic air-hardening steels are metal-to-metal wear alloys which, with care, can be applied (without cracking) to the wearing areas of machine components. Hence, these materials are normally referred to as machine hardfacing alloys. Typical applications of this alloy family include undercarriage components of tractors and power shovels, rolling mill work rolls, and crane wheels.

Cobalt-base hardfacing alloys have been traditionally used for hardfacing nuclear plant valves (check valves, seat valves, and control valves), since they normally show high corrosion resistance and superior tribological behaviour under sliding conditions. However, even the (normally low) corrosion and sliding-wear rates of these hardfacings lead to a release of particles with a high cobalt content. The particles are entrained in the coolant flow through the core, and Co 60, which is a strong emitter of gamma radiation, is produced. The activated particles are incorporated into the oxide layers of primary system components and contribute considerably to the occupational radiation exposure of maintenance personnel during the inspection, repair, or replacement of components. Additionally, material loss has been found for cobalt-base hardfacings used for control or throttle valves which are exposed to high flow velocities, indicating that this type of alloy has a limited resistance to erosion-corrosion and cavitation attack.

Detailed investigations of contender replacement cobalt-free, iron-base alloys have been carried out since the late 1960s. Cobalt-free alloys have been developed. These alloys can be deposited successfully on stainless and carbon steel substrates with gas-tungsten arc welding, in any position and with no preheat, using controlled heat input techniques. Cobalt-free alloys are characterized by high wear resistance and anti galling properties, and they have a micro-structure consisting of an austenitic matrix containing eutectic alloy carbides. The cobalt-free alloys meet or surpass the performance of cobalt alloys with respect to corrosion, material loss due to wear, and maintenance of the sealing function of the valve. Cobalt-containing austenitic stainless steels have been developed for the repair of the cavitation erosion damage of the hydraulic turbines.

Cavitation refers to the formation of vapour bubbles, or cavities, in a fluid which is moving across the surface of a solid component. These vapour bubbles are caused by localized reductions in the dynamic pressures of the fluid. The collapse of these vapour cavities produces extremely high compressive shocks, which leads to the local elastic and / or plastic deformation of the metallic surfaces. These repeated collapses (compressive shocks) in a localized area cause surface tearing or fatigue cracking, which leads to the removal of small metallic particles from the exposed surface. This eventually results in serious erosion damage to the metallic surfaces and is a major problem in the efficient operation of hydraulic equipment, such as hydro-turbines, runners, valves, pumps, ship propellers, and so on. The damage caused by cavitation erosion frequently contributes to higher maintenance and repair costs, excessive downtime and lost revenue, use of replacement power (which is very expensive), reduced operating efficiencies, and shortened equipment service life.

The outstanding cavitation erosion resistance of cobalt-containing austenitic steels comes from a patented chemistry formulated to yield the highest work-hardening rate, with a high interstitial carbon and nitrogen content. For the same reason, and in order to stabilize a fully austenitic structure, nickel has been replaced by manganese and cobalt, which are balanced with silicon and chromium to give good corrosion resistance.

Studies at the ‘Institut de Recherche d’Hydro-Quebec’ have determined that the elements most favourable to cavitation resistance, in decreasing order, are carbon, nitrogen, cobalt, and silicon. The combination of carbon and nitrogen has an equivalent effect, whereas chromium and manganese show a neutral effect within the 8 % to 12 % Co range. Nickel is detrimental. These studies have allowed the formulation of alloys with the appropriate amount of austenitizer (carbon, nitrogen, cobalt, and manganese) and ferritizer elements (chromium, silicon, and molybdenum) to stabilize the austenite phase at room temperature. Cobalt alone is not sufficient as an austenitizer, since it only lowers very slightly the martensitic transformation temperature. Hence, it is to be supplemented with manganese, carbon, or nitrogen. For increasing the ductility and the corrosion resistance, carbon can be replaced by nitrogen.

The composition of cobalt-containing austenitic steels provides a balance of elements in such a way that an essentially austenitic gamma phase with low stacking fault energy is obtained in an as-welded and solidified weld overlay. This meta-stable face-centered cubic (fcc) gamma-phase transforms under stress to a body-centered cubic (bcc) alpha-martensitic phase showing fine deformation twins. The phase transformation and twinning absorb the energy of the shock waves generated by the collapsing of the vapour bubbles. Such behaviour is similar to that of cavitation-resistant high cobalt alloys, which show a transformation from a fcc gamma-phase to a hexagonal close-packed (hcp) epsilon-phase in addition to twinning.

In the ‘incubation’ period of the alloy surface under a cavitation condition, the hardness increases as deformation twins form on the surface. The metal loss during this period is normally minimal, and the surface is smooth and hardened. Unlike the case for other alloys, such as 300-series stainless steels, this incubation period is long and high hardness levels (450 HV) are reached in the steady state. After the surface is fully hardened, further cavitation causes damage by initiating fatigue cracks and subsequent detachment of particulates at the intersections of the deformation twins. Since the twins are relatively small and the metal particles also small, the result is a uniform and slow degradation of the metal surface.

The work- hardening or strain-hardening coefficient increases markedly for the cobalt-containing steel. Decreasing the nickel and replacing it with cobalt results in a decrease in yield strength and in an important increase in ultimate tensile strength. Although the initial strain-hardening coefficient for these steels is quite similar, it increases to a very high value at larger strains (upto 1.26) for cobalt-containing steels. This larger strain hardening is associated with a faster initial martensitic transformation, gamma to alpha, of the less stable austenite phase. The higher the cavitation resistance, the less the plastic deformation needed to transform the fcc gamma austenitic phase to the bcc alpha martensitic phase. For the cobalt-containing steel, only 5 % elongation is needed to produce some 25 % transformation.

Cobalt-containing austenitic steels are about ten times more resistant to cavitation erosion than the standard 300-series stainless steels.  Although cobalt-containing steels can become less ductile because of their high work-hardening coefficient, their ductility is good enough to be welded or cast without cracking. The as-welded hardness is around 25 HRC, with work-hardened materials reaching 50 HRC. With a tensile elongation between 10 % and 55 %, the annealed yield strength is around 350 MPa, and the ultimate strength can exceed 1000 MPa. The corrosion resistance is fair, comparable to that of type 301 stainless steel, being somewhat limited by the higher carbon content. However, The materials are adequate for most of the applications.

The choice of a material for a particular application depends on such factors as cost, availability, appearance, strength, fabricability, electrical or thermal properties, mechanical properties, and corrosion resistance. Clad metals provide a means of designing into composite material specific properties which cannot be obtained in a single material. Self-brazing materials, such as copper-clad stainless steel (Cu/SS or Cu/SS/Cu), provide an example of the unique properties designed into a clad material.

Clad brazing materials are produced as strips, using the cold roll bonding technique. The strips comprise a backing steel which is clad with a brazing filler metal on either one or both sides. These products are used primarily in high volume manufacturing operations, such as the production of heat exchangers, brazed bellows, and honeycomb structures. The use of a self-brazing sheet reduces the total part count, simplifies the assembly operation (since the brazing filler metal is always present on the core material), and reduces assembly time and hence the cost. In addition, there is no need for the application of flux or for its subsequent removal. This not only saves the initial purchase cost of the flux, but also the waste-management cost associated with the disposal of the spent material.

Clad steels designed for corrosion control can be categorized as (i) noble metal clad systems, (ii) corrosion barrier systems, (iii) sacrificial metal systems, (iv) transition metal systems and (v) complex multilayer systems. Proper design is necessary for providing maximum corrosion resistance with clad metals.

Noble metal clad systems are materials having a relatively inexpensive backing steel covered with a corrosion-resistant metal. Selection of the substrate steel is based on the properties needed for a particular application. The cladding metal is chosen for its corrosion resistance in a particular environment, such as seawater, sour gas, high temperature, and automobiles. A wide range of corrosion-resistant alloys clad to steel substrates have been used in the industrial applications. One example is type 304 stainless steel on steel. Clad metals of this type are typically used in the form of strip, plate, and tubing. The noble metal cladding ranges from commonly used stainless steels, such as type 304, to high-nickel alloys, such as Inconel 625. These clad metals find different applications in the marine, chemical process-tube elements for boilers, scrubbers, and other systems involved in the production of chemicals.

Another group of normally used noble metal clad metals uses aluminum as a substrate. For example, in stainless-steel-clad aluminum truck bumpers, the type 302 stainless steel cladding provides a bright corrosion-resistant surface which also resists the mechanical damage (stone impingement) encountered in service. The aluminum provides a substrate with a high strength-to-weight ratio.

The combination of two or more metals to form a corrosion barrier system is most widely used where perforation caused by corrosion is to be avoided. Low-carbon steel and stainless steel are susceptible to localized corrosion in chloride-containing environments and can perforate rapidly. When steel is clad over the stainless steel layer, the corrosion barrier mechanism prevents perforation. Localized corrosion of the stainless steel is prevented. The stainless steel is protected galvanically by the sacrificial corrosion of the steel in the metal laminate. Hence, a thin pore-free layer is only needed. The carbon steel clad to type 304 stainless steel can be used for tubing and for wire in applications needing strength and corrosion resistance.

Carbon steel cannot be used when increased general corrosion resistance of the outer cladding is needed. A low-grade stainless steel with good resistance to uniform corrosion but poor resistance to localized corrosion can be selected. For seawater service, type 304 stainless steel which is clad to a thin layer of Hastelloy C-276 provides a substitute for solid Hastelloy C-276. In this corrosion barrier system, localized corrosion of the type 304 stainless steel is arrested at the C-276 alloy interface.

The most widely used clad metal corrosion barrier material is copper-clad stainless steel (Cu/430 SS/Cu) for telephone and fibre optic cable shielding. In environments in which the corrosion rate of copper is high, such as acidic or sulphide-containing soils, the stainless steel acts as a corrosion barrier and thus prevents perforation, while the inner copper layer maintains high electrical conductivity of the shield.

Sacrificial metals, such as magnesium, zinc, and aluminum, are in the active region of the galvanic series and are extensively used for corrosion protection. The location of the sacrificial metal in the galvanic couple is an important consideration in the design of a system. By cladding, the sacrificial metal can be located precisely for efficient cathode protection. A clad transitional metal system provides an interface between two incompatible metals. It not only reduces galvanic corrosion where dissimilar metals are joined, but also allows welding techniques to be used when direct joining is not possible.

In several cases, materials are exposed to twin environments such as one side is exposed to one corrosive medium, and the other side is exposed to a different one. A single material is not able to meet this requirement and a critical material is needed in large quantity. In small battery cans and caps, copper-clad, stainless-steel-clad nickel (Cu/SS/Ni) is used where the external nickel layer provides atmospheric- corrosion resistance and low contact resistance. The copper layer on the inside provides the electrode contact surface as well as compatible cell chemistry. The stainless steel layer provides strength and resistance to perforation corrosion.

Welding is used for the cladding of high alloy steel to carbon or low-alloy steels. For preserving the desirable properties, high alloy clad steel can be welded by either of the two following methods, depending on plate thickness and service conditions.

• The unclad sides of the plate sections are bevelled and welded with carbon or low-alloy steel filler metal. A portion of the high alloy steel cladding is removed from the back of the joint, and high alloy steel filler metal is deposited.
• The entire thickness of the high alloy steel clad plate is welded with high alloy steel filler metal.

When the non high alloy steel portion of the plate is comparatively thick, as in most pressure vessel applications, it is more economical to use the first method. When the non high alloy steel portion of the plate is thin, the second method is frequently preferred. When welding components for applications involving high or cyclic temperatures, the difference in the coefficients of thermal expansion of the backing steel plate and the weld is to be taken into consideration. All high alloy steel deposits on carbon steel is to be made with filler metal of sufficiently high alloy content to ensure that normal amounts of dilution by carbon steel does not result in a brittle weld. The filler metals are to be carefully selected. High alloy steel filler metal is deposited only in that portion of the weld where the high alloy steel cladding has been removed, and carbon or low-alloy steel filler metal is used for the remainder. The back gouged portion of the high alloy steel cladding is to be filled with a minimum of two layers of high alloy steel filler metal. An additional layer is desired if high weld reinforcement at the cladding surface can be tolerated. The welding procedure is to be carefully controlled to achieve the desired weld metal composition in the outer layers of the weld. Chemical analysis of sample welds is to be made before joining clad plates intended for use under severely corrosive conditions.

In some applications, a narrow protective plate of the wrought high alloy steel of the same composition as the cladding is welded over the completed weld for ensuring uniformity of corrosive resistance. The fillet welds joining the protective plate to the cladding is to be carefully inspected after deposition. These welds, of course, are made with high alloy steel filler metal.

In depositing the high alloy steel weld metal, the first layer is to be sufficiently high in alloy content to avoid cracking as a result of normal dilution by the carbon steel base metal. A stringer bead technique is needed to be employed and the penetration is to be held to a minimum. If the proper weld metal composition is not achieved after the second layer has been deposited, a portion of the second layer is required to be ground off and additional filler metal is to be deposited to achieve the desired composition.

The most frequent method of joining high alloy steel-clad carbon or low-alloy steel plate with a weld which consists entirely of high alloy steel. This method is frequently being used for joining thin sections of high alloy steel clad plate. The basic welding procedure which is followed for both the butt and comer joints consists of depositing a high steel weld from the carbon steel side, using a filler metal sufficiently high in alloy content to minimize difficulties (such as cracking) resulting from weld dilution and joint restraint. The welding is done after bevelling and fitting up the plate up for welding. After the high alloy steel weld has been deposited from the carbon steel side, the root of the weld is cleaned by brushing, chipping, or grinding, as required, and one or more layers of high alloy steel filler metal are deposited. The filler metal composition is to correspond to that normally employed to weld the type of high alloy steel used for cladding.

Laser cladding is a method of depositing material by which a powdered or wire feed stock material is melted and consolidated by use of laser in order to coat part of a substrate. Laser cladding uses the high energy density generated by a laser beam to form a molten pool in a base material for metallurgically bonding with a filler material using a diffusion type of weld. Laser cladding is a weld build-up process and a complementing coating technology to the thermal spray. It is increasingly being used instead of plasma transferred arc welding process and easily outperforms conventional welding methods like tungsten inert gas welding for advanced weld repair applications.

In laser cladding process, the laser beam is focused on the work piece with a selected spot size. The powder coating material is carried by an inert gas through a powder nozzle into the melt pool. The laser optics and powder nozzle are moved across the work piece surface to deposit single tracks, complete layers or even high-volume build-ups. Fig 3 shows the laser cladding process and schematic of the equipment.

Fig 3 Laser cladding process and equipment

The laser coating / cladding can be done on shafts, rods and seals, valve parts, sliding valves and discs, exhaust valves in engines, cylinders and rolls, pump components, turbine components, wear plates, sealing joints and joint surfaces, tools, blades, moulds, etc. These clad rods and rolls are used in different industries. Clad tools can be used as cutting, punching, or die tools. At present, one step laser deposition (1SLD) process is becoming popular among laser cladding processes. Another new popular technique, namely, laser direct metal deposition (LDMD) is an additive technique based upon the mechanism of fusing metallic powders delivered by either a lateral or coaxial nozzle to a backing steel or substrate to build three-dimensional objects directly from a computer aided design model. Applications of the laser direct metal deposition process have been found in the aerospace and automotive industries for repair and tooling.