Patent Publication Number: US-6214420-B1

Title: Process and plant for metallization of cast-iron pipes

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for metallization of metal substrates and relates more particularly to the hot metallization of pipes, in particular cast-iron pipes which are obtained by means of vertical continuous casting. 
     Among the known processes for the hot metallization of pipes, the following are known: 
     customized hot-galvanization; 
     the continuous coating of steel plates with Zn or ZnAl; 
     methods for thermal spraying with Zn or ZnAl alloys; 
     zinc-coating of pipes. 
     The existing solutions based on galvanization have the following drawbacks. 
     Customized hot-galvanization coats the pipe internally and externally and the pipe undergoes an increase in temperature such that the results of other prior operations carried out on the pipe at lower temperatures risk being affected. 
     Hot galvanization by movement through a liquid poses the problem, currently unsolved, of tightness with respect to the liquid bath between the vertical pipe and the liquid metal, as well as the problem of maintaining a bath which is clean (without matte) and has a stable composition over time. 
     Cold metallization associated with annealing would ensure good distribution if rigorous conditions for preparation of the pipe surface are satisfied and would probably require heat treatment in an inert atmosphere. 
     The cost of such a treatment cycle would be considerable and as a result would offer limited advantages from an economic point of view. 
     The pipe to be coated, emerging after continuous casting, is not rough, which makes fixing of a cold coating impossible without carrying out surface preparation. 
     In order to obtain a well-distributed coating, a pipe without an oxide skin is necessary and the working temperature must be high or else the pipe handling time must be long. 
     SUMMARY OF THE INVENTION 
     The invention aims to provide a uniformly distributed coating which has a good impact resistance. 
     Moreover an object of the invention is to provide a coating which adheres very well to the outside of a pipe of varying position and diameter, continuously moving in a vertical direction and without undergoing rotation, while preventing the pipe from being subject to heat annealing. 
     Furthermore an object of the invention is to provide a coating which is “self-healing” in respect of any damage suffered. 
     Finally, an object of the invention is to provide a coating which contributes to corrosion resistance in underground conditions. 
     The invention therefore relates to a process for metallization of a metal pipe obtained by vertical casting and displaced vertically in a continuous manner with an upward movement, characterized in that it consists in: 
     cooling the pipe in an atmosphere which is inert with respect to oxidation up to a temperature of 700 to 900° C.; and 
     spraying onto the pipe, cooled beforehand to the aforementioned temperature, a zinc-based metallization alloy with the aid of a set of spray guns arranged so as to surround the path of the pipe to be metallized. 
     According to a particular characteristic feature of the invention, the spray guns are made to oscillate about the path of the pipe with a frequency and an angular amplitude adapted to the speed of displacement of the pipe with a view to obtaining a coating of uniform thickness. 
     The invention also relates to a plant for metallization of a metal pipe obtained by means of upward vertical continuous casting and displaced vertically in a continuous manner, characterized in that it comprises, along the path of the pipe, a metallization station comprising a passage for the pipe, a metallization enclosure, spray guns mounted inside the said enclosure on an oscillating plate surrounding the said path, means for supplying each gun with metallization material and means for causing oscillation of the support plate and for displacing the guns inside the said enclosure with an oscillating movement, the frequency and angular amplitude of which ensure the formation of a coating of uniform thickness. 
     The invention relates moreover to a metal pipe obtained by means of vertical casting, characterized in that it has a metallization coating obtained by the process defined above. 
     According to another particular characteristic feature, the spray guns are three in number and are arranged on the plate at 120° from one another. 
     According to another particular characteristic feature, the spray guns are flame guns or arc guns. 
     According to another characteristic feature of the invention, the metallization material is a wire of Zn x Al 1-x  alloy and the means for supplying metallization material comprise for each gun a wire store and an unwinding device operated by the oscillation movement of the plate supporting the guns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood more fully upon reading of the description which follows, provided solely by way of example and with reference to the accompanying drawings, in which: 
     FIG. 1 is a diagrammatic, vertically sectioned view of a metallization plant according to the invention; 
     FIG. 2 is a diagrammatic view sectioned along the line  2 — 2  of FIG. 1; 
     FIG. 3 is a partial diagrammatic side view of the metallization plant according to FIG. 1; 
     FIG. 4 is a partially sectioned view of a flame gun of the plant; and 
     FIG. 5 is a micrograph of a coating obtained by the process according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The metallization plant shown in FIG. 1 has, along the path of a pipe T obtained in an upward vertical continuous casting plant (not shown) and above this plant, a shaft  1  for passage of the pipe T being manufactured, into which there is injected, via a duct  2 , nitrogen for allowing the pipe T to be displaced between he casting plant outlet and the metallization plant in an atmosphere protecting the surface of the pipe T against oxidation. 
     The shaft  1  has a lower part  3  which the pipe gradually cools during its upward movement from 1100° C. to 1000° C. and a water-cooled upper part  4  situated immediately below the metallization zone and inside which the temperature of the pipe cools from 1000° C. to a temperature of between 700 and 900° C. 
     A sliding mask  5  made of heat-insulating material is arranged inside the shaft  1  at the level of the joint between the lower part  3  and the upper part  4  of the latter and its movement inside the upper part  4  allows cooling of the pipe T to be controlled by means of positioning between the pipe and the wall of the said cooled part. 
     Above the shaft  1  there is arranged a metallization enclosure  6  comprising a bottom  7  inclined downwards, from the center towards the periphery, and provided with suction ducts  8 . 
     In its central part, the bottom  7  is connected to a drop prevention cover  9  through which the pipe T to be coated passes with a minimum amount of play so as to prevent the sprayed metallization product which does not adhere to the pipe wall from falling towards the bottom of the plant. 
     The enclosure has moreover an oscillating plate  10  on which spray guns such as flame metallization guns  11  are mounted. 
     Each gun is supplied with metallization wire  12  in a manner which will be described with reference to FIGS. 2 to  4 . 
     The guns  11  are for example three in number and are arranged on the plate  10  at 120° from one another. 
     Each gun  10  sprays a jet  13  of metallization material onto the pipe T during its upward movement. 
     The spray guns may also be arc guns. 
     They may also consist of devices for atomization of liquid metal. 
     Flame guns, however, have the advantage that they constitute a heat technology which does not disturb the cooling kinetics of the product. 
     They ensure an excellent performance with regard to the spraying of the atomized droplets of alloy because these droplets are not driven away from the substrate as, for example, in the case where arc guns are used. 
     The flow of flame guns is finer than that of arc guns. 
     They require only one wire of metallization material, while two wires are required for arc guns. 
     The upper wall  14  of the enclosure  6  also has suction ducts  15 . 
     The enclosure  6  has mounted on it a water-cooled casing  16  allowing the metallized pipe to emerge form the metallization plant at a temperature of 750° C. and forming a thermal screen which prevents the plant from becoming overheated. 
     The plant shown in cross-section in FIG. 2 has a platform (not shown) through which the pipe being formed passes and on which the metallization chamber provided with the annular plate  10  is mounted. 
     The plate  10  is driven in rotation with an oscillating movement by a suitable mechanism, not shown. 
     As mentioned further above, in the present example, said plate carries three flame metallization guns  11 . 
     This number of three ensures the best compromise between the cost of the plant and the uniformity of the thickness of the coating. However, the number of guns may be different from three. 
     The amplitude of angular displacement of the plate  10  as well as its frequency are a function of the angle of dispersion of the jets  13  of molten metal which are sprayed by the guns  11  as well as the speed of displacement of the pipe T so as to allow application of a metallization layer of uniform thickness. 
     Each metallization gun  11  which has means for supplying oxygen and gas by means of suitable hoses (not shown) has associated with it a device for supplying metallization product in the form of wire  12 . 
     Each device comprises a wire store  20  and an unwinding device denoted by the general reference  21 , intended to convey the metallization wire  12  from the wire store  20  to the associated gun  11 , taking account of the oscillating movements to which the gun  11  mounted on the oscillating plate  10  is subjected. 
     The wire  12  advantageously consists of an alloy containing 5 to 15% of Al. 
     It is also possible to use a lined wire consisting of a Zn core surrounded by an Al sheathing, in which case, after melting, a Zn 45 Al 55  alloy may be obtained. The lined wire may also consist of an aluminum core and a Zn sheathing. 
     The best lined wire or the best alloy is chosen in order to arrive at a desired metallization composition of the Zn Z Al 1-Z  type. 
     FIG. 3 shows one of the spray guns  11  mounted on the oscillating plate  10  of the plant as well as the wire store  20  and the associated unwinding device or unwinder  21 . 
     The metallization wire  12  is positioned in coil form inside a drum  22 , for example a drum of the standard type for oil products, provided with a central core  23  on which the wire coil (not shown) is treated inside the drum. 
     Alongside the drum there is arranged a frame  25  comprising a bracket  26  which supports a wire straightening device  27  with four rollers with which there is associated an entry cone  28  for the wire  12 , directed towards the drum  22 . 
     At the exit of the wire straightening device  27  there is arranged a set of wheels, a first wheel  30  of which is mounted on a slanting support  31  fixed at a point  32  of the vertical upright of the bracket  26 . A second wheel  34  is mounted oscillating about a point close to the point  32  for fixing of the first wheel  30  by means of a movable support or bracket  35  from which a weight  36  is suspended. 
     The weight  36  is fixed to a rod  37  hinged on the bracket  35 . 
     A third wheel  38  for redirecting the wire towards the plate, arranged above the oscillating plate  10 , is fixed to the structure of the plant, not shown. 
     At the exit of the third wheel  38  there is provided a second wire straigthened device with four rollers  40 . 
     The gun  11  has mounted at its rear end, opposite to its spray nozzle  42 , a fourth wheel  44  or wheel for admitting wire into the gun, mounted on a pivoting support and allowing modification of the inclination of the metallization wire  12  according to the oscillations of the plate  10 . 
     The metallization wire  12  is removed from the storage coil contained inside the drum  22 , passes through the entry neck  28 , is straightened a first time inside the straightening device  27 , passes over the first wheel  30 , then underneath the second wheel  34 , and then again over the third wheel  38 . It is kept tensioned by the weight  36  acting on the oscillating bracket  35  supporting the second wheel  34 . 
     It is again straightened by the second wire straightening device with four rollers  40  and enters into the flame gun  11  after passing over the fourth wheel  44 . 
     The gun  11  has a device for driving the wire so as to bring it into the melting zone. 
     This device known per se and not shown comprises a servomotor with an incorporated tacho generator, a transmission assembly with reducing gears and pulleys for driving the wire and a device for pneumatic clamping of the pulleys, which, by means of a piston, ensures clamping of the wire between them. 
     As can be seen more clearly in FIG. 4, the flame gun  11  which has a base  46  fixed to the plate  10  is provided at its end opposite to the metallization nozzle  42  with a cradle  48  comprising bearings  49 ,  50  on which a fork member  51  supporting the fourth admission wheel  44  is mounted in an oscillating manner. 
     The bearings  49  and  50  are oriented radially with respect to the oscillating plate  10 . 
     A guide  52  for the metallization wire  12  at the entry of the wheel  44  is moreover mounted on the fork member  51 . 
     The bearing  50  of the cradle  48  which is located closest to the melting zone of the gun  42  has an axial passage  56  through which the wire  12  passes. 
     The gun has, moreover, connection sockets  58 ,  60  intended to receive hoses, not shown, supplying the gun with gas and oxygen. 
     The gas used may advantageously be propane, acetylene or natural gas. 
     The ZnAl alloy of the wire is therefore conveyed towards the melting zone of the spray gun  11  so as to be melted there into fine droplets and sprayed. 
     The pipe T to be metallized being at the right working temperature, i.e. at about 800° C. and free from surface oxides owing to the nitrogen atmosphere which surrounds it in the metallization zone, receives jets  13  emitted by the guns  11 , consisting of a mist of ZnAl droplets which adhere thereto. 
     As a result of the translational movement of the pipe together with the alternating rotational movement of the guns  11 , it is possible to ensure a good uniformity in the thickness of the coating  62  obtained. 
     The alloy remains liquid on the pipe, but does not run owing to a capillary effect and also because of the very rapid surface oxidation and surface solidification of the external coating thus obtained. 
     During the entire cooling phase which follows the metallization phase and which lasts about 15 minutes, the alloy reacts with the cast iron of the material which forms the pipe, so as to form intermetallic compositions of the type Fe x , Al( 1-x ) charged with a small proportion of interstitial Zn. 
     The result obtained is a pipe coated with an external coating which is formed such that it is continuous and perfectly adherent. 
     The suction system associated with the enclosure and connected in particular to the suction ducts  8 , provided in the bottom  7 , and suction ducts  15 , provided in the upper wall  14  of the said enclosure, allow recovery of the alloy droplets which have not reached the pipe. 
     By way of example a certain amount of numerical data relating to the operating parameters and to the nature of the metallization alloy used is given below. 
     The feed speed of the wire  12  is 3 m/min., its diameter is 4 mm and its linear mass is 70 g/m. 
     The production yield is of the order of 50%. 
     As mentioned above, the wire is made either in the form of a Zn 85 Al 15  alloy or in the form of a lined wire with a Zn core, making it possible to obtain a Zn 45 Al 55  alloy. 
     The angle of rotation of the spray guns  11  is 95°. 
     The pitch of the spraying cycle is 70 mm. 
     In the tests carried out it has been possible to obtain a coating of 100 g/m 2  to 500 g/m 2  with a Zn 85  and Al 15  alloy. 
     As shown in FIG. 5, the micrograph of a section through the coating obtained clearly shows the formation of a well-distributed interface. 
     In this figure, in fact, it can be seen that a well-distributed intermetallic interface  67  has formed between the cast-iron wall  65  and the ZnAl coating layer  66 . 
     During the impact tests carried out on the coated pipe it was possible to reach values of up to 150 J, i.e. up to deformation of the pipe without deterioration of the coating. 
     The corrosion characteristics of the coating obtained are on a par with those which could be obtained by means of customized hot galvanization on a same substrate and with the same alloy. 
     During coating, the pipe is not subject to any annealing effect. 
     The metallization process according to the invention has the following advantages with respect to the state of the art. 
     It allows the thickness of the metallization coating to be controlled by adjustment of the feed speed of the wires of metallization alloy. 
     It enables a continuous linear coating to be obtained. 
     It enables a well-distributed anti-corrosion coating to be obtained. 
     It allows easy application of a sealant owing to the roughness conferred to the initially smooth pipe by the metallization coating. 
     Finally it allows lined wire as well as other alloys to be used.