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Chromium-molybdenum heat-resistant steel pipe welding process specification 1 Scope This standard is applicable to the welding construction of chromium-molybdenum steel steel for industrial pipelines and public pipelines. 2 Normative reference documents The provisions in the following documents become provisions of this standard through reference in this standard. For dated reference documents, their subsequent amendments (excluding errata content) or revised versions do not apply to the standard. However, parties that reach an agreement based on this section are encouraged to study whether the latest versions of these documents can be used. For undated referenced documents, the latest edition applies to this standard. GB 50235-97 "Construction and Acceptance Specifications for Industrial Metal Pipe Engineering" GB 50236-98 "Construction and Acceptance Specifications for Field Equipment and Industrial Pipe Welding Projects" DL/T 869-2004 "Technical Specifications for Welding in Thermal Power Plants" HYDBP018-2004 "Welding Material Management Procedures for Pressure Pipe Installation Projects" Ministry of Labor No. 1 "Boiler and Pressure Vessel Welder Examination Rules" 3 Prerequisites 3.1 Materials 3.1.1 Pipes, pipe fittings, etc. that enter the site as base material shall comply with the requirements specified in the corresponding standards and design documents, and shall have a material quality certificate or material re-inspection report. 3.1.2 Welding materials (hereinafter referred to as welding materials) 3.1.2.1 The welding consumables entering the site should comply with the requirements of the corresponding standards and technical documents, and have a welding consumable quality certificate. 3.1.2.2 The secondary library of welding materials at the construction site has been established and is operating normally. The management of welding materials shall be carried out in accordance with the requirements of the "Management Specifications for Welding Materials". 3.2 Main equipment and tools 3.2.1 Equipment Inverter welding machine or silicon rectifier welding machine, preheating and heat treatment equipment, high temperature oven, constant temperature box, dehumidifier, temperature and humidity measuring instrument, carbon arc gouging and other equipment are in good condition and have reliable performance. The measuring instruments are normal, certified and valid. 3.2.2 Tools Welding seam cleaning and grinding tools such as portable welding rod insulation barrels, angle grinders, wire brushes, chisels, and hammers are fully equipped. 3.3 The welding procedure qualification has been completed in accordance with the requirements stipulated in the corresponding regulations and standards. The welding process card has been compiled. 3.4 Welders shall comply with the requirements of the "Boiler and Pressure Vessel Welder Examination Rules" and have corresponding certified items after assessment. 3.5 Welding environment 3.5.1 The welding environment should meet the following requirements: 3.5.1.1 The minimum ambient temperature allowed for welding is 0℃ ； 3.5.1.2 wind speed: Manual arc welding is less than 8m/s, gas shielded welding is less than 2m/s ； 3.5.1.3 The relative humidity of the welding arc within 1m is less than 90%. 3.5.2 When the surface of the weldment is wet, covered with ice and snow, or during rain, snow, or wind, effective measures such as wind protection, rain protection, snow protection, cold protection, and preheating must be taken. Welding is not allowed without protective measures. 5 Process points 5.1 Bevel processing 5.1.1 The bevel form and bevel size of the pipe should be carried out in accordance with the requirements specified in the design document or welding process card. 5.1.2 The groove processing of butt weldments of unequal thickness shall comply with the requirements of the "Code for Construction and Acceptance of Industrial Metal Pipe Engineering". 5.1.3 Mechanical methods should be used for bevel processing, and thermal processing methods such as plasma cutting and oxyacetylene cutting can also be used. After the groove is processed by thermal processing, the oxide scale, slag and surface layer that affects the joint quality on the surface of the groove should be removed, and the uneven areas should be smoothed. 5.1.4 Appearance inspection should be carried out after bevel processing. There should be no cracks, delamination and other defects on the surface of the bevel. If required by the design documents, conduct magnetic particle or penetrant inspection. 5.2 Assembly 5.2.1 Before assembling the weldments, the oil, paint, dirt, rust, burrs and galvanized layer within a range of not less than 20mm on the groove and its inner and outer surfaces should be removed. 5.2.2 When assembling butt joints of pipes or pipe fittings, the inner walls should be flush, and the offset of the inner walls should not exceed 10% of the pipe wall thickness and should not be greater than 1mm. 5.2.3 When assembling butt weldments of unequal thickness, the end face of the thin part should be within the end face of the thick part. 5.2.4 Except for cold-stretched or cold-compressed pipes specified in the design documents, weldments shall not be forcibly assembled. It is also not allowed to use the thermal expansion method for assembly. 5.2.5 Weldments should be firmly padded when assembled, and measures should be taken to prevent additional stress and deformation during welding and heat treatment. 5.3 Positioning welding 5.3.1 The welding materials, welding process, welder and preheating temperature of positioning welding should be the same as the formal welding requirements. 5.3.2 The length, thickness and spacing of tack welds should be able to ensure that the welds will not crack during the formal welding process. 5.3.3 Check immediately after positioning welding. If there are any defects, they should be removed immediately and re-positioned. 5.3.4 The material of the assembly fixture that needs to be welded to the base metal during positioning welding should be the same as the base metal or the same category number. The base metal should not be damaged when removing the work clamps. After removal, the remaining weld scars should be polished and repaired until they are flush with the surface of the base metal, and penetrant inspection should be performed. 5.4 Preheating 5.4.1 Preheating before welding shall comply with the provisions of the design document or welding procedure card. When tungsten arc welding is used as the primer, the preheating temperature before welding can be reduced by 50°C according to the specified lower limit temperature. 5.4.2 For pipes requiring preheating before welding, preheating shall be carried out after the weldments are assembled and inspected to be qualified. When the outer diameter of the pipe is greater than 219mm or the wall thickness is ≥20mm, electric heating should be used to preheat it. For situations other than the above, in principle, electric heating should be used. When conditions are not met, flame heating can be used. 5.4.3 The preheating width is based on the center of the weld, and each side should not be less than 3 times the thickness of the weldment, and not less than 50mm. The temperature measurement method can be a contact thermometer or a temperature measuring pen. Measuring points should be evenly distributed over the entire circumference. 5.5 Welding 5.5.1 The welding process should be strictly implemented in accordance with the requirements of the welding process card. 5.5.2 Welding method 5.5.2.1 Pipe butt joints with pipe diameter ≤60mm or wall thickness ≤6mm shall be welded by tungsten arc welding. 5.5.2.2 For pipe butt joints with a pipe diameter >60mm or a wall thickness >6mm, tungsten arc welding is used for bottom welding and manual arc welding is used for cover welding. 5.5.2.3 The welding of corner joints, T-joints and casing joints generally uses manual arc welding. 5.5.3 Welding materials Principles for selecting welding materials should generally be welding materials that have the same chemical composition as the pipe to be welded, and the strength value of the weld metal should not be lower than the lower limit of the pipe standard value. Specific requirements shall be implemented in accordance with the provisions of the welding process card. 5.5.4 Welding sequence 5.5.4.1 The welding sequence and welding levels should be implemented in accordance with the requirements specified in the welding process card. 5.5.4.2 After welding, the welds of the base layer should pass the self-inspection before welding the sub-layer. 5.5.4.3 Welding of thick-walled and large-diameter pipes should use multi-layer and multi-pass welding. 5.5.4.4 Except for process or inspection requirements that require welding in stages, each weld should be welded continuously at one time. When welding is interrupted for any reason, measures to prevent cracks (such as post-heating, slow cooling, heat preservation, etc.) should be taken. When re-welding, you should carefully check to confirm that there are no cracks before continuing to weld according to the original process requirements. 5.5.4.5 Concealed welds that require inspection must pass the inspection before other processes can be carried out. 5.5.5 Welding process parameters 5.5.5.1 The welding current, welding voltage, welding speed, etc. during welding should be within the range specified in the welding process card. 5.5.5.2 The temperature between welding layers shall not be lower than the specified lower limit of preheating temperature and shall not be higher than 400°C. 5.5.6 Welding technology 5.5.6.1 Two-person symmetrical welding should be used for butt welding of pipes with a diameter greater than 194mm. 5.5.6.2 For pipes of medium and high alloy steel (chromium content ≥3% or total alloy content >5%), in order to prevent oxidation and overburning of the root welding joint, the inner wall should be filled with argon gas or mixed gas protection during welding. 5.5.6.3 Welding of thick-walled and large-diameter pipes should comply with the following requirements: 5.5.6.3 .1 The thickness of the underlying weld layer for tungsten arc welding shall not be less than 3mm ； 5.5.6.3 .2 The single layer thickness of other welding passes shall not be greater than the diameter of the welding rod used plus 2 mm. ； 5.5.6.3 .3 The swing width of a single weld pass shall not be greater than 5 times the diameter of the electrode used. 5.5.7 Operation precautions 5.5.7.1 It is strictly prohibited to strike an arc, test current, or weld temporary supports on the surface of the workpiece to be welded. 5.5.7.2 During the welding process, the quality of arc starting and arc closing should be ensured, and the arc crater should be filled during arc closing. 5.5.7.3 Joints of multi-layer and multi-pass welds should be staggered. And only after passing the self-inspection layer by layer can welding the subsequent layers. 5.5.7.4 When welding pipes, drafts should be prevented inside the pipes. 5.5.7.5 After welding, the weld should be cleaned and the welder code should be marked after passing the self-inspection. 5.6 Post-weld heat treatment 5.6.1 Post-weld heat treatment shall be carried out in accordance with the requirements specified in the heat treatment process card. 5.6.2 Post-weld heat treatment should be carried out immediately after passing the visual inspection after welding. Otherwise, post-heat treatment should be carried out, the temperature should be 300~350℃, the constant temperature time should not be less than 2 hours, and the heating range should be the same as the post-weld heat treatment requirements. 5.6.3 The heating width of post-weld heat treatment, calculated from the center of the weld, shall not be less than 3 times the wall thickness of the pipe on each side, and shall not be less than 60 mm. 5.6.4 The insulation width of post-weld heat treatment, starting from the center of the weld, shall not be less than 5 times the wall thickness of the pipe on each side to reduce the temperature gradient. 5.6.5 The post-weld heat treatment temperature, constant temperature time and heating and cooling speed of the weld should be strictly implemented in accordance with the provisions of the heat treatment process card. 5.6.6 During heat treatment and heating, strive to achieve uniform temperature on the inner and outer walls and both sides of the weld. At constant temperature, the temperature difference between any two measuring points within the heating range should be less than 50°C. 5.6.7 During heat treatment, the temperature measuring points should be symmetrically arranged on both sides of the weld center, and should not be less than two points. The measuring points on horizontal pipes should be arranged symmetrically up and down. 5.6.8 After heat treatment of welded joints, records and labels should be kept. 5.6.9 The heat treatment quality of welded joints is generally checked by hardness measurement. 5.6.10 The heat treatment temperature of welded joints of different chromium-molybdenum steels should be selected according to the lower limit of the one with the higher alloy content. 5.6.11 The post-weld heat treatment of welded joints composed of weldments of different thicknesses shall be carried out according to the thickness. 6 Quality inspection 6.1 The inspection of welds shall be carried out in accordance with the requirements specified in the design documents or corresponding standards. 6.2 Appearance inspection of welds 6.2.1 For welds with unqualified appearance, inspection of other items or welding heat treatment is not allowed. 6.2.2 The surface of the weld should be well formed. The edge of the weld should smoothly transition to the base metal. The surface of the weld should not be allowed to have defects such as cracks, pores, and lack of fusion. The outer dimensions and surface defects of the weld shall comply with the requirements specified in the design documents or corresponding standards. 6.3 Non-destructive inspection of welds Non-destructive inspection of welds shall be carried out in accordance with the requirements specified in the design documents or corresponding standards. 7 Rework When welded joints have excessive defects, they must be reworked and the following regulations must be followed.: 7.1 The weld repair process should be subject to qualified welding process assessment. 7.2 Weld repairs should be performed by welders with corresponding qualifications. 7.3 For unqualified welded joints, the reasons should be identified, countermeasures should be taken, and repairs should be carried out. After repair, it should be re-inspected. 7.4 Rework generally uses mechanical digging to remove defects. For thick-walled pipes, carbon arc gouging can also be used to remove defects. After confirming that the defects have been removed, the repaired parts must be beveled and ground before repair welding can be carried out. 7.5 The number of repairs on the same part of the weld shall generally not exceed two times. 8 Safety Precautions 8.1 Safety inspection of the equipment must be done before starting the welding machine. 8.2 Workers must use labor protection supplies correctly. 8.3 There must be no flammable or explosive items in the work site and nearby areas. 8.4 When there is water accumulation and humidity at the welding site, necessary moisture isolation measures should be taken. 8.5 The connection between the workpieces of the electric welding machine should be reliably fixed on the welding parts using clips. 8.6 It is not allowed to place the powered welding clamp on the weldment. Reliable measures should be taken to insulate the joints and damaged parts of the welding clamp connection, and be updated in a timely manner in serious cases. 8.7 When placing oxygen bottles and acetylene bottles, a sun protection shed must be set up and located 5m away from the welding site. The distance between the above two bottles should also be more than 5m. 8.8 Protective glasses should be worn when using an angle grinder to grind welds.