The invention concerns a method for welding tubes, bolts and rods, or other profiles with an essentially circular or similar cross section consisting of one, two or more layers of material.
For many applications it is natural to use tubes, rods, bolts and other elements of relatively simple geometry consisting of one or several layered materials. In a number of cases the materials in such profiles may fulfill different functions. An internal core of copper may be surrounded by one or several layers of steel tubes with very different properties. The copper conducts electricity or heat, while the steel tube protects the copper and provides mechanical strength. A possible alternative to such a design will, in some cases, be an internal core of steel and an external lube of copper.
Another possible use of different materials in constructions is insulating tubes, where internal and external metallic tubes are separated by a material which insulates electrically and thermally. In some relations it can also be economically beneficial to use several metals in the same profile. As tubes for the oil industry, relatively inexpensive CMn-tubes are likely to be used. By coating such tubes with an internal layer of stainless metallic material, the external tube will be protected against corrosion. An external protective stainless steel tube is also a theoretical possibility. High pressure water pipes can also advantageously be prepared with an internal and external stainless coating.
In conventional welding it is very difficult to ensure good fusion of all layers in profiles consisting of several metals. Metals melt at different temperatures, and it is generally very difficult or time-consuming to join metals which are inside a bolt or between two layers of metals. It is often not desirable to mix materials, since this may give a weld with unwanted mechanical properties and defects. Conventional welding is also time-consuming compared with methods of both pressure welding and friction welding. In the case of automatic methods for welding, such as friction and flash welding, it is very difficult to ensure a uniform layer thickness and good mechanical properties for the product.
Forge welding is a relevant method for joining of tubes, rods and bolts. A forge welding process may consist of three distinct phases:
Traditionally, there has been insufficient focus on the establishment of contact and the subsequent contact mechanics and plastic deformation relating to pressure welding methods. However, it is of large significance for the quality of the weld that the bevel is closed and forged correctly. Particularly in relation to forge welding of multilayer tubes or bolts, it is important to ensure good contact in all parts of the profile in order to ensure satisfactory joining in all layers. As mentioned above, the layer that melts most easily can be smeared out and disturb the joining of the other layers. The challenges related to forge welding of multilayer materials are:
An object of the present invention is to provide both an optimal and robust method for enhanced diffusion or forge welding of tubes, rods and bolts. Furthermore, an object is also to provide a method by diffusion and forge welding of multilayer tubes, rods and bolts, where satisfactory joining is achieved in all layers.
By shaping the profile ends in a particular way it is possible to solve these challenges; this is achieved by the method which is described in the appended patent claim. More precisely, the invention comprises a method for joining tubes, rods, bolts and other axial symmetric profiles end-to-end, comprising shaping the profile ends by plastic deformation and/or machining processes such that they obtain a reduced cross section/thickness, local heating of the profile ends electromagnetically by induction and/or direct high frequency resistance heating, hot forging of the profile ends, one of the profiles' end surfaces being shaped such that its cross section consists of a double-arched curve, where the profile ends have varying distance in the radial direction, and where the tube profile ends initially meet with a blunt angle between the fitting surfaces.
The invention is illustrated in the appended drawings, in which
a depicts a cross section of a tube with a classical bevel shape for use in forge welding,
b depicts a so called double-arched bevel shape, a bevel shape consisting of both a convex and a concave part,
a shows an example of pre-forming the profile by plastic forming by upsetting and in subsequent turning, while
The invention will now be described in detail with reference to the drawings mentioned above.
The invention is a method for joining or welding tubes, bolts, rods and other profiles consisting of one, two or several materials, but where at least one of the layers is metallic. The profiles are preferably elongated and axially symmetric or similar, and the ends which are to be joined have similar shape. The materials of the profiles can be found in distinct layers which extend in the axial direction, and have the same distribution in each of the two parts. The material properties of the layers may differ significantly. A tube consisting of several layers of metals is referred to as multi metallic.
The invention is based on a new development for all types of forge or pressure welding, including forge welding of only one material type, in that contact between the profiles is gradually established from one side of the profile to the other side, preferably in the direction opposite to the flow of reducing gas. For tubes, this usually corresponds to closing from the outside to the inside of the profile. While one of the end surfaces has a purely convex shape, the other may consist of both a convex and concave shape, here designated as double-arched shape. The end surface may be inclined at different angles relative to the direction of the profile axes, but it is always prepared in a way that ensures gradual closing from one side to the other side. The purpose of the described design is to ensure an optimal and robust mechanism for closing of the gap separating the pipe ends. For example, the design allows parts to be joined to be significantly displaced and angled relative to each other. During the closing the contact will be gradually established over the thickness, while a pressure wave and a zone with local plastic deformation is moving along the welding. This provides a kind of zipper mechanism, with good and well defined pressure and deformation conditions during welding. The double-arched shape of one of the end surfaces ensures that the ends do not meet in a sharp angle at the same time as the bevel surfaces are properly closed at the inner side of the profiles. The shape of the bevel can simply be adjusted in order to ensure best possible conditions during both welding and resistance or induction heating. It is pointed out that in the text, seam surface and the end surface are used as synonymous terms for the surface shown as 11 or 12 in
As mentioned, one of the end surfaces may have a purely convex shape. It can also have other shapes, such as conical or double-arched shapes. The double-arched shape may be symmetrical, corresponding to the double-arched shape of the other end surface. Such embodiments of the profiles ensure that the end surfaces to begin with meet (in a fitting surface) along one of the edges of the profiles, for example along the external edge, in a blunt angle which may approach 0°, and that there is a gradually larger distance between the end surfaces in the radial direction of the profiles as seen in a cross section. Also the shoulders/side surfaces of the profiles may have a double-arched shape, consisting of two circle segments and possibly a straight part.
A more robust solution is obtained by using bevel surfaces which are defined by pure convex lines in the cross section. The normals of the surface in the first touching point should then be almost parallel with the forge direction in heated condition. With a pure convex design of both bevel surfaces, there will, however, be a risk that the gap between the bevels is incompletely closed. The reason is that the surfaces near the end of the seam or fusion line meet at an angle and that, in many cases, and in particular in connection to welding of bi-metallics, it may be very difficult to enforce plastic deformation that ensures proper closing. An indent will in that case be formed. Another problem with purely concave bevel surfaces is that the distance between profiles must vary significantly across the gap between the bevels. To ensure even heating, the differences in the width of the gap over the thickness should be small. With small variations in the gap distance it will, however, be difficult to ensure gradual closing of the gap in the correct direction at the same time as the local plastic deformation at the surface becomes small.
The profiles shown in
The geometry may then be described by a total of nine independent parameters, for example A, B, D, E, F, ra=(R2/R1), rb=(R4/R3) and rc=(R6/R5). If E and F are known, then the sum of the radius Rc=R5+R6 may be determined by the expression:
R5 and R6 can then be determined if rc is known. If R6 and rc are close to 0, the curve to of the bevel surface will be purely concave. If R5 is close to 0 and rc is approaching infinity, the curve that defines the bevel surface will be purely convex. The Cartesian coordinates at any point on the bevel surface can be determined by trigonometric relations, if a suitable origo is selected. Hence, the curves can be described in simple manner, in both the 2- and 3-dimensional space.
A correction of the bevel shape must be made in order to compensate for thermal expansion of the material. The effect of the thermal expansion is that the bevel surface rotates slightly. The bevel surfaces must be shaped so that the normal to the planes in the first contact point, after heating and possibly skew clamping of the profiles, are parallel, or in total have a radial component in a direction which is parallel to the direction for closing of the welding.
The described shape is only an example of a double-arched shape. It is fully possible to describe double-arched shapes in alternative ways, either by using circle segments or polynomial functions. The advantage of the described double-arched shape is that there are very few parameters; only one parameter in addition to the two parameters for a straight line. All double-arched shapes that allow extensive adjustment and optimization of the shape of the bevel may advantageously be used for forge welding purposes, and are covered by the claims in the text.
It is no condition that the side surfaces of the profile ends are described by double-arched shapes. Simple line segments can be used, as well as complex polynomial functions. The advantage of the double-arched shape is that the surface of the weld has no sharp edges and uneven sections. Again, a double-arched shape described by two circle segments will represent the absolutely simplest description.
The figure on the left hand side shows a weld with reduced wall thickness. Such deviations will reduce the mechanical integrity of the weld and are not desirable. At the inside of the weld it may be desirable, for various reasons, that there be no cap. Also in this aspect the shape of the weld is not optimal.
At the right hand side, another weld is shown with a somewhat reduced thickness and with the cap at the inside. The deformation has taken place somewhat more in the inward direction than desirable. Furthermore, at the inside of the weld, the gap has been incompletely closed. The thermo-mechanical conditions have, during forging, not been adequate to close the inside of the welding. This may result in crack initiation and growth and stress corrosion during use. At the outer side of the weld, an undesirable folding has taken place. Both these effects can be observed when bevels not being double-arched are used.
It is emphasized that both the previous and the subsequent figures show the appearance of the profile ends in the hot condition. Because of the heating of the end surfaces and the thermal expansion, they will usually rotate somewhat relatively to each other. This must be taken into account during forming of the profile ends in the cold condition. The shape of the profile ends are here called convex, concave and double-arched. However, the double-arched embodiment also includes as border cases pure convex, concave and plane shapes. The precise shape should be tuned to the physical properties of the materials, the temperature picture and the desired final shape of the weld. A best shape can be found by solving a classical Optimization problem. The simplest shape of a double-arched bevel is one consisting of two circle segments. The circle segments may have different radii, and should preferably meet in a smooth transition. Where extra precision is required, the surfaces may be described by mathematical splines or similar.
During heating as well as upsetting, a reducing gas is used to remove oxides and prevent new corrosions of the profile ends. It has previously been shown that pure hydrogen or chlorine gas can be used, but it is now also shown that the gas can consist of a mixture of nitrogen and hydrogen; the composition depending on the material properties. The advantage of using a mixture of hydrogen (typically 5 to 20%) and nitrogen is that the gas is non-explosive. At high temperatures it is found that the nitrogen gas also contributes to removal of oxides on the surface of the steel at high temperatures.
The method makes use of numerical tools, such as finite element methods for rapid optimization of shape. In connection with the use of numerical modelling tools, it is of great significance that there is a large degree of certainty related to process conditions and material behaviour. For this reason, material testing is done in order to determine heat conduction properties and describing elastic and plastic behaviour of the material. The original distribution of temperature in the part can either be determined experimentally or by a satisfactory numerical model. It can also be determined by inverse analyses. In that case the temperature distribution should be described by a small number of parameters. Pressure, deformation and temperature conditions that secure a good weld are found through the planned experiments and, with the aim of contact mechanics, micromodels for adhesion are established.
Requirements for shape and properties of the weld are determined by the users. Requirements are given in standards. The object functions express the deviation between simulated results and requirements. The weighting of requirements is carried out in a rational manner, depending on how the weld is to be used and the requirements of the user. If, for a given bevel shape, one is not able to establish a weld with satisfactory quality, then the values of the bevel shape parameters are changed before new simulations are undertaken. The procedure is followed until a shape is found which is both optimal and robust. A number of different methods of optimization exist, which can be used in this connection. If, for a specific material, a certain temperature distribution and under certain process conditions, it is not possible to satisfy the user requirements, it may be possible to adjust process conditions and the original temperature distribution until a satisfactory result is achieved. It is of great significance that, during evaluation of the robustness of the method, one considers deviations which by nature are of a three dimensional character. This implies that an analysis of consequences misalignment, tilting and offset should be made.
When a satisfactory result has been obtained, it must be validated through systematic experiments. By conducting a large number of measurements it is possible to find out whether possible deviations between experiment and model are due to measurement errors or modelling errors. In the case that the deviation is due to modelling errors, the model has to be further examined and it may be necessary to carry out specific experiments that reveal the cause of possible errors. If deviations are due to measurement errors, measurement equipment must be calibrated. When a satisfactory agreement between the model and the experiment exists, the weld can be certified for relevant combinations of bevel, material and process conditions. All results are stored in a database which is gradually expanded as new experimental data are established.
The basis of the method is a clear definition of the customer requirements regarding the weld shape and properties, 509. Requirements are usually expressed in standards but, if desirable, particular requirements can be put forward by the customer.
The target shape of the weld shall normally be described by two functions f(z) and g(z). The variable z is here the distance from the fusion/seam line in the direction of the axes. The function f(z) is the difference between the radial coordinate for a point at a distance z at the outer surface of the part and the outer diameter of the part, OD. The function g(z) similarly is the difference between the inner diameter of the part, ID, and the radial coordinate for a point at a distance z from the fusion line at the internal surface of the part. Hence, the following situations may arise:
f(z)>0, g(z)>0: the thickness of the weld in position z shall be larger than the thickness of the part
f(z)<0, g(z)<0: the thickness of the weld in position z shall be less than the thickness of the part.
It is quite possible to demand that f(z)=g(z)=0 for all z, which means that the geometry of the weld shall be equal to the geometry of the part. Functions of the following type are normally used:
f(z)=A exp(−Bz2)
Here, A is the maximum deviation from the OD, of the part, while B states how rapid the shape deviation tends towards 0 in the axial direction. A similar function may be applied for g(z). Normally a requirement is that the value of A is less than 10% of the wall thickness. It is of course fully possible to put A=0.
The customer may also prescribe requirements for the mechanical and metallurgical properties of the weld. These requirements can not be used directly in an analysis. The properties of the weld depend on the thermo-mechanical treatment of the base material and of die contact conditions during welding. In order to relate the properties to the parameters from the analyses experience data 508 are used, as well as models for contact and adhesion, 508, 509. The models are established by dedicated small scale experiments and inverse modelling. This means that the shape and the parameters of the models are determined by a routine which minimizes the deviation between the model and the measurement. In any case the models link temperature, pressure, deformation and time to the quality of the weld. The simplest type of such a model is a special value which states whether sufficient pressure, sufficient temperature or sufficient degree of deformation that must be achieved to ensure satisfactory welding. It is also possible to demand that combinations of the given parameters shall satisfy specific requirements. Model data are material dependent, and must be established for each individual case.
Central to the method for analysis is the use of numerical tools for evaluation of the weld shape and properties, 510. The finite element method (FEM) permits analysis of complex forming operations for complex material behaviour and geometry. The part which is formed and welded is subdivided into a number of small elements. For the simplest formulations, in each corner of the element there will be a node which is exposed to forces causing deformations in agreement with a described behaviour of material. The relationship between forces and displacement for a group of nodes belonging to one or several elements may be expressed by a set of algebraic equations.
Usually the forming problems are non-linear. This requires use of an iterative routine for determination of offset changes as a result of a change in the load. In the case boundary conditions, such as contact between tools and a part, are known, the non-linear equations can for example be solved with a Newton-Rapson technique. The result is a description of displacements and internal forces in the part over time during forging.
Forge welding occurs at a high temperature and temperature gradient, and during a gradual change of the temperature. The finite element model includes calculations of temperature changes during forging, and there is a two-way connection between the mechanical and the thermal calculations. Plastic deformation generates heat and contributes to heating, while the behaviour of the materials is affected by the temperature. The basic equation for the mechanical calculations is Newton's 2nd law, while the basic equation for the thermal problem is the equation for conservation of energy. Additionally constitutive relations describing the behavior of the material are required.
Forge welding of rotational symmetrical parts, such as tubes, may ideally be modelled as a problem in two dimensions. With this is meant that only radial and axial displacements are allowed. Forces may act in the tangential direction, but this is of less significance in solving the system of the equations. Simplification to only two dimensions makes it possible to carry out a large number of calculations and experimenting on a number of combinations of geometry parameters during a short time period. Thus, such calculations are perfectly suited ter optimization studies. Three dimensional analyses are necessary in order to evaluate possible deviations from axi-symmetric conditions, for example due to shape or process deviations. Such deviations may be due to relative tilting or offset of the parts.
The finite element method is foremost a mathematical tool. All information about material behaviour and process conditions must be described prior to the calculations. Establishment of the material data and data about boundary conditions occur through experience and analyses. Plastic flow data at different temperatures are established by ring upsetting in isotherm conditions, 506. Adhesion experiments are conducted under controlled conditions with small samples and almost isothermal conditions. Data from both types of experiments are compared with results from models describing different phenomena.
In connection with the solution of the heat conduction problem, it is important to determine the heat convection coefficient as well as the emissivity. At the surface of the part both natural and forced convection may take place. The heat transfer coefficient is determined through representative experiments with very good control of temperature and circulation, 502. The radiation is normally determined by optical means, and in order to determine heat transfer coefficient and emissivity, analytical and numerical models for the experimental set up are used. The models are then implemented in the analyses of forge welding, 503. Also for other boundary conditions such as, for example, friction, submodels are established prior to the analyses of the welding.
The temperature distribution prior to forging greatly affects the outcome of the process. The temperature has a first order effect on both the final geometry of the weld and on the pressure and deformation during forging. The temperature also influences the metallurgy. The distribution of temperature for forge welding is determined by the heating method, normally high frequency resistance heating or induction heating. The temperature profile may to a large extent be adjusted and optimized. Usually the temperature distribution can be approximately described by the function T(z):
T(z)=(TMAX−T0)exp(−KZ)+T0
where TMAX is the maximum temperature during forging, T0 is preheating temperature and the K is a parameter which determines the extension of the temperature field. The temperature distribution and the shape of the pipe end should be adapted to each other by optimization, but there are some limits to such adaptation. The determination of the original temperature distribution is done by heating experiments or by numerical simulation tools, 504. By solving Maxwell's equations for high frequency current, as well as the equation for conservation of energy, it is possible to estimate temperature distribution. Such a calculation will of course demand precise determination of material parameters such as the permeability, resistivity, the heat transfer coefficient and the specific heat capacity. The analysis makes possible optimal adaptation of the temperature distribution to the subsequent deformation conditions. At the first iteration of an optimization study for forge welding, however, a temperature distribution based on data from previous welding experiments with similar materials and process conditions, 505 is used.
It is of great significance for the optimization study that the geometry of the bevel is described precisely with as few parameters as possible, 500.
For the different shape parameters a set of reasonable values for the shape parameters in 501 should initially be selected. This selection is based on experience. Also, a type of analysis is developed which enables very rapid determination of natural selection of the ratios A/B and C/D. In this analysis, first a part whose shape is initially described by the functions Fi(z)==0 and Gi(z)==0 is studied. The part is subject to tensile forces simultaneously with application of a temperature distribution as described above. When subject to tensile forces, the reduction of the part's cross-section begins immediately with plastic deformation in the warm zone. The ratios A/B and C/D are continuously monitored. In order to give best possible imitation of the conditions during forging, the development of heat transfer and ductility are inverted. It is worth noting that the method is not intended to be an exact inverse analysis, but rather a starting point for the real inverse analysis. To ensure that the start analysis provides reasonable results, validation calculations with a traditional forward analysis are undertaken.
Prior to a numerical analysis, the customer requirements must be converted to objective criteria for use in evaluation of the results from the analyses 512. A basic requirement is that the final shape of the weld shall be in agreement with the target shape. The functions Fc(z) and Gc(z) describe the external and the internal shape of the weld after the forging. The functions f(z) and g(z) described above describe the target form after forging. The deviation between target and actual shape can for example be described by the difference:
It is also possible to more strongly emphasize on the negative deviations, if thickness reductions are not desirable. Other shape deviations, such as systematic displacement of the bevel against the inside or the outside, can also be quantified.
The deviation D can be calculated for continuous functions from z=0 to infinity. In numerical calculations, discrete values for the shape deviation are used. The deviations are calculated in each node on the surface of the part in the element model. Node position deviations are summed up and weighted.
In connection with the accurate analysis of the results from numerical calculations and deviations between the calculated and the target shape 511, it is important to know that, in connection with plastic deformation, it may be assumed that the mass is conserved and the material is incompressible. If thermal expansion and elastic compression are neglected, it can then be assumed that the part volume in the first time step will be just as large as the volume in the last. If the forge length is not determined a priori by the user, the forging length will be adapted to the analyses, such that the final shape of the welding is in best possible agreement with the target shape. This must be the case after unloading and cooling. If a material between two analyses is heated further, the forge length will be adjusted according to the thermal expansion and change in forge pressure. The method takes account of thermal and mechanical conditions in the early simulation steps. The effect of pressure and temperature is estimated with use of the thermal elastic equations.
Shape constitutes the primary optimization criterion. It is also possible to include, in the object function itself, deviations between target pressure and calculated pressure, and target and calculated deformation at the contact surfaces. Other relevant parameters can also be included. A better solution is however to include requirements for pressure, strain and temperature as implicit and explicit constraints in connection with the optimization of shape. Solutions which do not satisfy the minimum requirements for so pressure, damnation and temperature cannot be considered as optimum.
Another optimization requirement is that the solution is robust. By this one means that the probability of experiencing a welds which do not satisfy requirements for shape or properties due to result of natural process variations, shall be very small and satisfy the customer requirements. Variability in the process shall be much smaller than the tolerances which are set (ref Six Sigma approach). Different methods are implemented for robust optimization. For robust optimization it must be assumed that a stochastic distribution is associated with the object function. There is both an expected value μD and standard deviation σD for the deviation D. During robust optimization, a so-called metamodel is established for μD and σD, with basis in a larger set of simulations. This is a surface in several dimensions in the parameter space, a response surface (ref. R. H. Myers and D. C. Montgomery: Response Surface Methodology, Wiley, 2002). A minimum is sought for the response surface for μD. It is also possible to search for minimum standard deviation for different parameter combination, or a minimum of a weighted sum of the expected value and the standard deviation. However, it is more common to demand that the sum of the expected value for D, and three times the standard deviation for D, is not larger than a given threshold value. This ensures that the in selected bevel produces results better than required usually at sufficient level of safety. If there are any explicit or implicit restrictions on parameters, one should also consider natural deviations which may occur for the parameters in the model, for example associated with the original bevel shape or temperature distribution. The result of the robust optimization may be a movement of the optimum away from the limits defined by constraints, and to flat parts of the response surface. A similar method is described in M. H. A. Bonte, A. H. van den Bogaard and R. van Ravenswaaij, Robust Optimization of metal forming processes, Proceedings of the 10th ESAFORM Conference, Zaragoza, Spain, pp. 9-14. When using response surfaces or similar, the interpolation must be controlled afterwards.
Different optimization techniques are used for determination of optima, in the purely deterministic as well as the stochastic case. A number of methods can be used to search for local optima on smooth surfaces (distributions of D). Reference is made to general literature related to optimization theory.
The innermost feedback arrow, 501 between 510 and 511 indicates that searches are undertaken until the optimum has been found. This may take place whereby evaluations of the object functions can be made between each simulation. As stated, above it is often more appropriate and efficient to establish a meta-model, a response surface, through simulation, and then search for minimum, validate the result and thereafter carry out calculations iteratively in order to obtain a better estimate. Both methods can be used in the algorithm.
The outermost feedback arrow, 502′ between 510 and 511, indicates that a search for a set of initial and boundary conditions are terminated if, after a certain number of searches, it is not possible to obtain a satisfactory result, i.e. a weld which has the target shape and properties. In this case, the initial and if possible, the boundary conditions must be changed. In that case, the cap of the weld is not sufficiently extended in the longitudinal direction; the routine will modify the distribution of the temperature field such that more plastic deformation takes place at a distance from the weld. At the same time a message is given, regarding the old temperature distribution, that there is no bevel which could give a satisfactory shape on the weld. The user of the routine is also given the opportunity to change the target shape of the weld, or to modify the requirements for shape and properties.
When a weld with satisfactory shape has been established, a comparison is made, 514 of results from numeric modelling with results of experiments, 513, where parts with the suggested optimal shape are joined. During the welding, sensors continuously record temperatures, displacements, forces and shape. Then, the properties of the welds are checked (hardness, yield limit, fracture resistivity, ductility and fatigue resistivity) by destructive testing and metallurgical analyses. If there are significant deviations between numerical and experimental results, it will be evaluated whether these are due to errors in modelling or measurement. Inconsistent measurements indicate that there are one or several measurement errors. If the measurement results are consistent, but there is a deviation between model and measurement, the initial and boundary condition of the model are checked. In particular it can be necessary to change the temperature distribution, so that it is in better agreement with the experimental data.
When model and experiment are in good agreement, the method can be certified for a given combination of material and bevel shape. For this purpose there are standards for conventional welding methods. To the extent that the requirements in these standards are relevant, they are also used for forge welding. However, the systematic method described above ensures a weld with satisfactory properties, which can be used in spite of very significant variations in the input parameters. All experiences which are gained through simulation and forging are stored in a database for later use in connection with qualification of the method for other materials and welding parameters. The relationship between result and parameters is stored in a regression formula or in an artificial neural network (ANN).
For profiles consisting of several material layers, as illustrated in
When welding tubes with several layers, the innermost layer is often very thin. In this case, the inside of the tube cannot be machined without the inner layer being completely removed by machining or significantly reduced in thickness. It has previously been suggested that the thickness of the inner layer should be maintained while material is removed only from the outside of the profile. This is an unsatisfactory solution, and in particular for the case where the internal layer comes in contact first. First of all, it will in be difficult to maintain contact pressure and, for that reason, no satisfactory weld is established in the outer layer. Instead, a large internal cap is formed with a large kerf in the internal layer. Hence it is of great significance that the bevel is more or less centrally situated in the tube wall and that closure occurs as prescribed from the outside to the inside and generally in the direction against the flow of the reducing gas in a gradual is manner.
The following two methods are suggested in welding of tubes consisting of several materials:
Prior to the turning of the tube end 70, the tube may be expanded plastically with a conical tool 73. The degree of the expansion depends on tube dimensions, but the tube should be expanded more than the thickness of the inner layer 72,
The welding progress itself, in the above method, is illustrated in
The other method consists of shaping both the internal layer and the rest of the tube, such that they almost behave independently of each other during plastic deformation. In in this case, a groove is made between the internal coating and the rest of the tube,
In the case of forge welding of bolt and rod consisting of an internal core and an external layer,
In the case where the steel constitutes the internal core 132, and is surrounded by copper 131, the ideal geometry will depend on the heating process. However, it will in all cases be advantageous to shape bevels for steel and copper separately. If the copper melts or gets a significantly higher diffusivity, the copper may pollute the steel bevel and prevent satisfactory bonding between the steel parts. By using part profile ends, as described above, it is possible to avoid this type of treatment,
For the described methods for joining multilayer tube bolts, far better results are usually obtained when each part layer in the ends of the tubes/bolts/profiles is given convex and double-arched shapes, respectively, as explained previously. However, it is also possible to join such profiles when every part layer is given a classical plane forming.
Number | Date | Country | Kind |
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20075787 | Nov 2007 | NO | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/NO2008/000399 | 11/10/2008 | WO | 00 | 5/13/2011 |