Cladding

Introduction

We are aware of PETROBRAS Technical Standard N-1707 Rev. C 11/2010 which contains in its items 3.1-NOTE and 4.2.2 a restriction on the use of conventional explosion clad plates with a base plate thickness of less than 19 mm. Item 4.3.5 also specifies conditions for Stress Relief Heat Treatment, stipulating the use of materials that are less susceptible to sensitization, i.e. low-carbon stainless steels. Therefore, we believe it is necessary and appropriate to make some technical considerations on the subject, with the aim of weighing up the limitation imposed by the Standard and trying to contribute with subsidies deemed useful for a better understanding of the explosion cladding process.

The explosion cladding process

The cladding of metals by explosives is an example of a solid-state welding process. It differs from fusion welding in that there is no phase change at the interface and the bond produced is due to the action of interatomic forces of attraction that develop between the atoms of the two joined surfaces. Most researchers agree that the explosion welding process occurs as a result of the formation of a high-velocity metal jet resulting from the oblique impact between the materials to be welded. The most important application is undoubtedly the production of bimetallic plates in carbon steel coated with stainless steel, titanium, copper and copper alloys such as naval brass, nickel and nickel alloys such as Inconel, Hastelloy and Monel and aluminum which are produced in large quantities by the explosive welding process mainly in the United States of North America, Germany, Great Britain, Japan, Sweden and Brazil. In practice, the materials to be welded are arranged horizontally, with the base plate on the ground and the cladding plate, or mobile plate, separated from the base plate by conveniently calculated spacers. A uniform layer of explosive is applied to the top surface of the mobile plate.The detonation of the explosive projects the mobile plate onto the base. The pressures generated in the impact region are such that, as a result of the high shear stresses produced, the shear strength of the materials becomes negligible, and the metal behaves like a low-viscosity fluid, which serves to remove all the oxide layers that would inhibit welding. When all the parameters are properly controlled, the weld interface has the appearance of a regular wave. The waviness, as well as being characteristic, gives rise to unique properties that distinguish materials obtained by explosion from others processed conventionally.

Predicting the parameters to obtain a certain type of interface involves some boundary conditions, such as:

1- The boundary condition for the formation of the metal jet is determined by the ratio between the collision velocity and the sound velocity of the base plate material or by the ratio between the velocity of the moving plate and the sound velocity of its material.

2- A critical impact pressure is essential, as welding will not take place below this pressure. This pressure can be associated with the minimum impact speed of the moving plate.

3- The existence of a critical flow velocity (collision point) or transition between the turbulent regime (wave formation at the interface) and the laminar regime (flat interface) has been observed by some researchers.

4- Above the minimum impact speed, the conditions for obtaining the best mechanical properties can be associated with the kinetic energy provided by the moving plate in the impact zone.

5- The separation distance between the moving plate and the base plate must be between limits that allow the moving plate to reach its maximum speed.

6- The detonation velocity of the explosive must be directly linked to the sonic properties of the sheet materials and their initial configuration (inclined or parallel).

Quality of Bimetallic Sheets Clad by Explosion

In the 1970s, during the phase in which metal processing technology was being implemented in Brazil, IPT carried out studies into the development and production of large bimetallic plates made of stainless steel with a thickness of 3 mm in carbon steel, as they were of interest to PETROBRAS. The conclusions are contained in at least two papers published at the time. The PETROBRAS Technical Bulletin, Rio de Janeiro, 20 (3): 193-203, July/September 1977, published the Report Experimental Production of explosion-welded clad plates, presenting the results of the technical and economic evaluation of the use of the explosion welding process developed by IPT in the experimental production of ASTM A 516, grade 70 carbon steel plates and ASTM A 240, type 304 stainless steel, based on the ASTM A 264 specification. The report was drawn up by engineers Sérgio Luiz Benini and José Paulo Silveira, both from DIOBI/SEGEN.

The conclusions were as follows:

4- The explosion welding process developed by IPT produces clad plates of satisfactory quality in relation to the requirements of the ASTM A 264 specification, when the thickness of the base plate is equal to or greater than 19 mm (3/4').

5- For base plates with a thickness of less than 19 mm, the welding process requires additional research. These plates may meet the ASTM specification mentioned above, provided that they undergo heat treatment after welding and that the consequences of this operation on the corrosion resistance of the stainless steel are evaluated.

6- The manufacturing process can provide appreciable savings in foreign currency and, under domestic market conditions, can compete with the rolling process for thicknesses of more than 20-25 mm (data relating to the carbon steel/stainless steel type 405 combination).

Additional studies were carried out by the IPT, in pursuit of the production of stainless steel explosion clad plates in carbon steel, with base plate thicknesses of less than 19 mm.

The IPT paper Quality of Stainless Steel-Carbon Steel Bimetallic Sheets Obtained by Explosion published in METALURGIA-ABM Magazine, Vol. 34, No. 250, Sept. 1978, contains the experiments carried out, the results obtained, the evaluation tests and the conclusions. The qualification tests were carried out in the Materials and Metallography Laboratories, supervised by Metallurgical Engineer Tibério Cescon from the Metallurgy Division of the Institute. The tests carried out led to the conclusion that the results obtained were within the limits of the requirements of the ASTM A 264 standard, with one exception: some specimens with a total thickness of 14.3 mm did not pass the bending test. Furthermore, all the specimens that failed were cut with the largest dimension parallel to the wave front at the interface. All those cut with the length transverse to the wave front passed the test. From this observation it can be deduced that the waves at the interface have the characteristics of 'hardening fringes'. To explain this effect, an analogy can be made with corrugated cardboard: specimens cut to length parallel to the corrugation resist bending and break easily. However, when the specimens are cut transversely to the corrugation, they offer little resistance to bending and do not break when bent up to 180°. Using this analogy, which is all too simple, and assuming that the waves at the interface act like 'ferrules', let's consider what might happen in a bending test when the line is transverse to the direction of wave propagation and the carbon steel is under tension: as the load increases the tension at the points furthest from the stainless steel face, the 'ferrules' (bends) reach the yield strength before the stainless steel does.

Deformation begins in the friezes along the line under load. The stiffness of the adjacent fibers forces the flow to continue along the line under load. As the steel is relatively thin compared to other bimetallic plates, and given that the stainless steel has been hardened by cold working, the stainless steel dominates the bending test, causing the steel to break. The test results and problems described seem to be the only ones of which, to our knowledge, nothing has yet been reported in international literature, up to the time of our studies. This is possibly because explosive welding of relatively thin sheets is not commercially competitive in countries where stainless steel is produced on a large scale, or because it is used for the production of bimetallic sheets. The conclusion of this study was that Brazil was in a position to produce bimetallic plates within the required specifications, but with a technical limitation in the total thickness, probably 15.8 mm, below which the bending test requirements could not be met without stress relief by heat treatment. Later, in the 1990s, studies conducted by the

Eng. João Bosco da Silva Bastos (private/internal information) made it possible to conclude and recommend the following procedures:

1- In explosion clad plates with carbon steel thicknesses such as 15.8 mm base plate and lower, use austenitic stainless steels type 304L/316L instead of 304/316, which is a design condition for pressure vessels.

2- Carry out TTAT according to ASME Section VIII, Div I with controlled and rapid cooling after passing the temperature of 450 °C to avoid possible sensitization. The heat treatment aims to reduce the stresses introduced by the explosion welding, restoring the ductility of the carbon steel, enabling the material to withstand the bending test as prescribed in ASTM A264.

3- These two associated solutions drastically reduce the formation of chromium carbides with a consequent reduction in the loss of chromium in the structure of the 304L/316L stainless steel, preventing a reduction in corrosion resistance or corrosion at grain boundaries due to the formation of these chromium carbides as a result of the TTAT.

Final considerations

In the study of the scientific basis of Explosive Metal Processing, a fundamental point is the study of the interaction between explosive and metallic material, based on the three fundamental principles of physics applied to the interaction of materials of different impedances:

1st- Law of conservation of mass (mass intercepted by a wave front = mass behind the wave front);

2nd- Law of conservation of momentum (impulse = quantity of movement);

3°- Law of conservation of energy (work = change in kinetic energy + change in internal energy).

No resulting equation used to determine the parameters for explosion welding takes into account the influence of conditions external to the materials. What governs are the intrinsic conditions of the materials. As shown above, the welding of different materials using the energy released from an explosion occurs if there is sufficient impact, and this occurs if the moving plate has a minimum speed for this. In vacuum cladding, the properties of the materials do not change. Because it is in a vacuum, it can be imagined that less impulse (less explosive) is needed to give the moving plate the minimum speed required for a good welding impact, i.e. impact energy equal to or slightly greater than the plastic deformation energy of the materials and the formation of a metal jet, which is a mandatory condition for welding to take place. In other words, the speed of the moving plate and, consequently, the impact, are the same as in the open-cut process. It should also be pointed out that atmospheric resistance can be considered negligible given the magnitude of the explosive's detonation pressure.We therefore agree that the vacuum process has a significant advantage when it comes to environmental problems, with the suppression of explosion noise allowing the process to be carried out on flat, inhabited sites. Barring a more accurate assessment, we have our doubts about any other advantages this process may have over materials processed in the open air. Finally, with the arguments presented here, we would like to consider the limit recommended in the well-crafted PETROBRAS Standard N-1707: 19 mm as the minimum thickness for the base plate, in the so-called conventional explosion clad plates, since from a technical point of view, bimetallic plates are routinely produced by explosion, with smaller thicknesses, meeting the requirements of the specifications. In the hope that this limit, which is reflected in the national product, will be reviewed, MULTICLAD places itself entirely at the disposal of PETROBRAS for any further studies that may contribute to enriching the foundations of the attractive technology of Metal Processing by Explosion, which has been implemented and used industrially in our country, with guaranteed quality.