Analysis of the Cause and Recommendations for Welding
1. The analysis of the failure
According to the above analysis results, it can be known that the main reasons for the failure of the slip-on flange are as follows:
(1) GB T 700-2006 standard stipulates that w(C) should be less than and equal to 0. 22% in Q235, and w (C) in this material is 0. 96%. In the classification of steel, w(C) less than 0.25 % is low-carbon steel, w(C) between 0.25% and 0.65% is medium-carbon steel, and w(C) greater than 0.65% is high-carbon steel.
(2) Metallographic analysis shows that the material structure is mainly cementite and pearlite, and the cementite is distributed in sheets, showing good hardness and high strength, while the metallographic structure of Q235 is mainly ferrite plus pearlite; ferrite has good plasticity, but low strength and hardness, while pearlite has good strength and hardness. Therefore, Q235 steel has relatively good strength and plasticity, and good weldability.
(3) The yield strength of Q235 steel should be 235MPa, the tensile strength between 370 and 500 MPa, the elongation after fracture greater than and equal to 26%, and the impact absorption energy greater than and equal to 27 J. The material has no yield stage, elastic deformation and plasticity deformation, and the tensile strength reaches 856.02MPa, twice that of Q235 steel. From the above results, the material of the flange is high carbon steel, and the welding operation instruction and processes adopted on the construction site are based on the base material of Q235 low carbon steel. Therefore, the root cause of the failure of the flange’s welding seam is the difference between the base material and the construction process documents.
2. Recommendations about welding
(1) The material of the flange is high carbon steel. This type of steel has a relatively high carbon content and poor weldability. Therefore, the flange and the pipe must be preheated before welding. The preheating temperature should be between 250 to 350°C. Keep the preheating temperature always higher than the interlayer temperature. To ensure the preheating effect, the preheating range should extend 150 mm to the outside of the flange.
(2) After the welding is completed, the flange should be placed in heat treatment equipment with a temperature above 650°C for stress relief treatment immediately, and the tempering temperature should be raised quickly. The temperature measuring device must be calibrated and have sufficient accuracy. The specific heat treatment time depends on the thickness of the workpiece.
(3) If there is no preheating equipment on the construction site, welding materials with good welding performance and good crack resistance can be selected. To ensure the welding quality, the welding speed must be reduced in the welding process.
(4) In terms of quality management, since the material of the flange is high-carbon steel and the material of the pipe is low-carbon steel, the welding of the two belongs to the welding of dissimilar steels. Therefore, welding procedure evaluation must be carried out before construction. Prepare welding work instructions. After passing the assessment, the welding operation instructions should be compiled again.
3. Conclusion
The flange in this failure case is high-carbon steel. According to design standards, technical requirements and construction costs, this type of steel is not used in thermal pipeline welding. The root cause of such failure of the welding seam is related to the management of welding materials. That is to say, the construction party did not re-inspect the material according to the regulations, and the supplier supplied the material subjectively or non-subjectively not based on the requirement. Therefore, in the future welding management process, the welding quality management work should be done well from the following aspects:
(1) Control the procurement process according to the ISO 9001-2000 standard. It is required to formulate criteria for selection, evaluation and re-evaluation based on purchased commodities, and to determine qualified suppliers accordingly.
(2) The procurement information of welding materials and raw materials must be clear and confirmed by the relevant technical department before procurement, and important materials need to be reviewed by the responsible welding engineer.
(3) The quality certificate inspection, packaging and appearance inspection and material re-inspection shall be carried out according to the specified requirements for the re-inspection of the factory, and composition sampling inspection of materials shall also be carried out; the important parts such as flanges and valves shall be fully inspected.
(4) A ledger is established for incoming materials, and unqualified materials, materials to be inspected, and qualified materials stacked in separate areas with obvious signs ensure the consistency of records, cards, and objects.
(5) According to the raw materials used in the welding process, the welding process, and the welding technical problems that may be encountered in the production and manufacturing should be solved. Appoint the responsible person for welding quality, and the person must pass the ability test according to the ISO standard to fundamentally ensure the reliability of welding.
According to the above analysis results, it can be known that the main reasons for the failure of the slip-on flange are as follows:
(1) GB T 700-2006 standard stipulates that w(C) should be less than and equal to 0. 22% in Q235, and w (C) in this material is 0. 96%. In the classification of steel, w(C) less than 0.25 % is low-carbon steel, w(C) between 0.25% and 0.65% is medium-carbon steel, and w(C) greater than 0.65% is high-carbon steel.
(2) Metallographic analysis shows that the material structure is mainly cementite and pearlite, and the cementite is distributed in sheets, showing good hardness and high strength, while the metallographic structure of Q235 is mainly ferrite plus pearlite; ferrite has good plasticity, but low strength and hardness, while pearlite has good strength and hardness. Therefore, Q235 steel has relatively good strength and plasticity, and good weldability.
(3) The yield strength of Q235 steel should be 235MPa, the tensile strength between 370 and 500 MPa, the elongation after fracture greater than and equal to 26%, and the impact absorption energy greater than and equal to 27 J. The material has no yield stage, elastic deformation and plasticity deformation, and the tensile strength reaches 856.02MPa, twice that of Q235 steel. From the above results, the material of the flange is high carbon steel, and the welding operation instruction and processes adopted on the construction site are based on the base material of Q235 low carbon steel. Therefore, the root cause of the failure of the flange’s welding seam is the difference between the base material and the construction process documents.
2. Recommendations about welding
(1) The material of the flange is high carbon steel. This type of steel has a relatively high carbon content and poor weldability. Therefore, the flange and the pipe must be preheated before welding. The preheating temperature should be between 250 to 350°C. Keep the preheating temperature always higher than the interlayer temperature. To ensure the preheating effect, the preheating range should extend 150 mm to the outside of the flange.
(2) After the welding is completed, the flange should be placed in heat treatment equipment with a temperature above 650°C for stress relief treatment immediately, and the tempering temperature should be raised quickly. The temperature measuring device must be calibrated and have sufficient accuracy. The specific heat treatment time depends on the thickness of the workpiece.
(3) If there is no preheating equipment on the construction site, welding materials with good welding performance and good crack resistance can be selected. To ensure the welding quality, the welding speed must be reduced in the welding process.
(4) In terms of quality management, since the material of the flange is high-carbon steel and the material of the pipe is low-carbon steel, the welding of the two belongs to the welding of dissimilar steels. Therefore, welding procedure evaluation must be carried out before construction. Prepare welding work instructions. After passing the assessment, the welding operation instructions should be compiled again.
3. Conclusion
The flange in this failure case is high-carbon steel. According to design standards, technical requirements and construction costs, this type of steel is not used in thermal pipeline welding. The root cause of such failure of the welding seam is related to the management of welding materials. That is to say, the construction party did not re-inspect the material according to the regulations, and the supplier supplied the material subjectively or non-subjectively not based on the requirement. Therefore, in the future welding management process, the welding quality management work should be done well from the following aspects:
(1) Control the procurement process according to the ISO 9001-2000 standard. It is required to formulate criteria for selection, evaluation and re-evaluation based on purchased commodities, and to determine qualified suppliers accordingly.
(2) The procurement information of welding materials and raw materials must be clear and confirmed by the relevant technical department before procurement, and important materials need to be reviewed by the responsible welding engineer.
(3) The quality certificate inspection, packaging and appearance inspection and material re-inspection shall be carried out according to the specified requirements for the re-inspection of the factory, and composition sampling inspection of materials shall also be carried out; the important parts such as flanges and valves shall be fully inspected.
(4) A ledger is established for incoming materials, and unqualified materials, materials to be inspected, and qualified materials stacked in separate areas with obvious signs ensure the consistency of records, cards, and objects.
(5) According to the raw materials used in the welding process, the welding process, and the welding technical problems that may be encountered in the production and manufacturing should be solved. Appoint the responsible person for welding quality, and the person must pass the ability test according to the ISO standard to fundamentally ensure the reliability of welding.
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