The surface to be overlaid shall be free of extrusion or scale like marks, and the roughness shall not be less than; The surface of seamless steel pipe or pipe fitting to be welded shall be polished with polishing wheel, and the surface shall be free of rust; Before surfacing welding of rolled pipe or barrel section, the weld on one side of the surface to be welded must be polished to be flush with the base metal; The surface of groove to be overlayed; The equipment for groove surfacing is generally used in high temperature and high pressure environment, and the surface to be welded is formed by machining. During the machining process of the surface to be welded, a machining plan drawing must be drawn according to different groove surfacing requirements, which should reflect the initial processing groove form and the groove form after surfacing. In the plan drawing, the backing for surfacing should also be fully considered;
Processing of various forms of groove:
1 — During processing, the metal at A shall be reserved and a 90 ° groove shall be made according to the drawing. The surfacing metal is B and the red part in the drawing. The subsequent processing to remove the metal at A and B is the required groove part (the red part); Tube sheet forgings shall be ordered according to the maximum outer diameter and thickness.
2 —- It is one of the groove of spherical head. In order to prevent the downward flow of deposited metal during surfacing, a gasket with the same material as the base material shall be added at the bottom of the groove metal flow direction. This gasket also processes a circle of grooves with bottom fillet transition depth of about 3mm and width slightly higher than the required surfacing layer height. If the head is pressed with steel plate, the base plate can be made into a cone, and its cone angle can be determined according to the upper and lower circle diameters of the head and the head height. When the stacking surface is to be processed, it can be processed and formed at one time according to the drawing.
3 —- It is another form of groove, which is mainly used on the cylinder section and the head. In order to ensure the quality of the surfacing layer and have a certain machining allowance, the surfacing layer must be welded 4-5mm more in the width and height direction. Therefore, some thinning treatment should be taken when adding the backing, but the original thickness must be flush with the surface to be welded when assembling. The grooves of the two are the same. The material of various gaskets used for surfacing must be the same as that of the base. When the bottom of the groove of the gasket transits to the height, there must be an R 3 or more fillets. During groove processing, the surface shall be free of extrusion or scale like traces, the roughness shall not be less than, and the angle tolerance shall be ± 1 °. The corner of the groove to be welded must be processed into a fillet transition with a radius of no less than 3mm. All surfaces to be overlayed must be strictly cleaned, oil, rust and other sundries on the surface shall be thoroughly removed, and 100% MT inspection shall be carried out.
Preparation of surfacing equipment for various parts and components. For surfacing of pipes and pipe fittings with diameters of DN150 ～ DN300, gas shielded welding is generally used for surfacing of small diameter pipes and pipe fittings. Its tooling is mainly horizontal lathe, as shown in the figure below. Clamp the workpiece on the head of the lathe, install the carbon dioxide gas shielded welding handle on the lathe tool holder through a fixture, and conduct surfacing according to different speeds and feed rates; When surfacing these workpieces, such as tube plate surface, head groove surface, nozzle and flange surface, the workpiece is clamped on the welding positioner. During installation, the center of the workpiece must coincide with the center of the positioner turntable. Generally, the circumference of the workpiece is drawn according to the concentric circles of different diameters on the positioner turntable, and then the workpiece is placed on the circumference and clamped with a clamp. The strip electrode surfacing is generally used for tube plate surface surfacing, For the surfacing of nozzle and flange surface, if it is not nickel based welding material, CO2 shielded welding is used, while nickel based welding material surfacing is mostly manual arc welding. When CO2 shielded welding is used, the welding handle should also be fixed on the rack with longitudinal movement. The rotary table of the positioner can be turned from the horizontal position to the vertical position, so that the welding position can always be kept at the best position according to different welding forms of the workpiece.
When CO2 shielded welding is used, if it is necessary to make the welding handle have a longitudinal movement function, it is only necessary to make a platform at an appropriate height and fix the welding handle on the semi-automatic gas cutting machine to achieve automatic surfacing. As the workpiece is heavy and long, the workpiece can be placed on the roller carrier at this time, and various methods can be used for welding.
Overlay welding sequence The rotary table of the positioner is horizontal when the tube plate of the heat exchanger is overlayed. The strip electrode surfacing or CO2 surfacing is used. When the strip electrode surfacing is used, it starts from about 100mm from the center to the outer circle circle by circle. The center and the outermost circle are generally filled with electric arc welding or CO2. The first step of surfacing is to use the welding materials for the transition layer. When the transition layer is completely overlayed and tested to be qualified, the corrosion resistant layer can be overlayed. When overlaying the flange and nozzle, the flange neck and the inner surface of the nozzle shall be overlayed first, and then the flange sealing surface shall be overlayed. Groove surfacing – all workpieces requiring groove surfacing are generally used to avoid repeated heat treatment after welding. Therefore, groove surfacing is made of nickel based welding materials. The specific surfacing sequence is shown in the following figure: tube plate groove surfacing: install the tube plate at the center of the positioner rotary table to make the rotary table turn 90 °. During surfacing, weld according to the number sequence in the figure. During surfacing, rotate the rotary table at a certain angle to facilitate surfacing. Sufficient machining allowance shall be ensured during surfacing, and the groove shall be processed according to the drawing after welding. When surfacing on the head or double V-groove head of small workpieces or the upslope of workpieces whose weight is less than 2500Kg, since it can be conducted on the positioner, the welding position can be adjusted to the best, and the surfacing shall be conducted according to the sequence shown in the right figure. The V-groove surfacing only needs two positions: 1 and 2. The surfacing sequence of double V groove on the cylinder. When surfacing the first part, the cylinder shall be placed on the roller carrier, and the surfacing shall be as high as possible. When surfacing the second part, the cylinder section shall be placed vertically, and when surfacing the third part, the cylinder section shall be placed on the roller carrier. Manual arc welding shall be adopted during surfacing, and the vertical lathe shall be used to turn the groove to the drawing after welding. Cylinder surfacing The inner part of the cylinder is only required to place the cylinder section on the roller carrier for surfacing according to the process.
Anti deformation measures for overlaying The tube sheet must be overlayed along the circumference, not along the radial direction. Longitudinal surfacing shall not be used for barrel section surfacing. When strip electrode surfacing is used, concentric circle surfacing in circumferential direction shall be used. When manual arc welding is used for surfacing, the symmetrical backward welding method along the cylinder circumference is adopted. For all kinds of groove surfacing welding, circumferential symmetric welding must be used when welding the first part. If conditions permit, circumferential symmetric backstepping method can also be used when welding the second part. If radial welding is used, after each electrode is welded, another electrode must be welded in the symmetrical direction. The difference between the next symmetrical point and the previous one must be about 90 °. Local repair welding is not subject to the above measures. The quality of surfacing layer requires that the overlap amount of weld bead shall be controlled within 6-8mm when strip electrode surfacing is used. When electrode arc welding is used for surfacing, the overlap overlap between weld beads shall not be less than the width of weld bead. After surfacing, the height difference between weld beads shall not be greater than 0.5mm. Undercut defects between weld beads and between weld and base material are not allowed on the surface of surfacing layer.
After surfacing of the transition layer and corrosion resistant layer, UT inspection shall be carried out respectively to detect the fusion between the weld and the substrate and the height of each welding layer. After surfacing, the spatter and impurities on both sides of the weld bead shall be cleaned timely, and the surface of the surfacing layer shall be free of defects such as pores and slag inclusions. After surfacing, the geometric dimensions of the workpiece must meet the requirements of the drawing, and the welding deformation must be strictly controlled. If there is a requirement to take deformation prevention measures in the process, the measures must be in place. The interpass temperature must be strictly controlled during the surfacing process. Generally, the interpass temperature during the transition layer welding is higher than the interpass temperature of the corrosion resistant layer surfacing. When welding nickel based welding materials, it is necessary to keep the environment clean and reduce the generation of dust. If it is impossible to designate a special welding site, a simple room with smoke exhaust facilities can be used for welding in the room. During surfacing, self inspection must be carried out carefully to ensure that the welding is qualified at one time.