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SA335 P11 + TP304L steel pipe welding and heat treatment

Attention:     Issuing time:2020-05-19 10:25
SA335 Gr.P11 + TP304L steel pipe welding and heat treatment
This paper introduces the application of nickel-based alloy welding consumables in the welding of dissimilar steel of SA335Gr.P11 + TP304L thick-wall pipe. First, a nickel-based alloy isolation layer with a certain thickness is pre-surfacing welded on the edge of SA335 Gr.P11 alloy steel groove. Post-weld heat treatment and non-destructive testing are performed after surfacing welding of the isolation layer to prevent sensitization of TP304L steel pipe.

1 Introduction
The principle of a nuclear power plant is that the nuclear energy released by an atomic nuclear reactor converts nuclear energy into steam kinetic energy through a set of power devices, and then into electrical energy. In order to withstand high temperature and high pressure, heat-resistant steel pipes such as SA335 Gr.P11 are widely used in the third-generation nuclear power plants. This paper analyzes the welding and heat treatment process of SA335 Gr.P11 heat-resistant steel pipe and TP304L austenitic stainless steel dissimilar steel.
2 Welding process analysis
2.1 Selection of welding methods
At present, during the installation process of nuclear power plant construction in China, the process selection is relatively mature and conservative, mainly to ensure the quality of nuclear power installation. The welding of A335 Gr.P11 + TP304L dissimilar steel is used for welding thin-walled tubes by manual tungsten arc welding For large thick-walled tubes, manual tungsten argon arc welding + manual electrode arc welding is used for welding.
2.2 Selection of welding groove
Bevel form adopts J type and V type
2.3 Process route selection
Because the design specification requires that the thick-walled tube SA335 Gr.P11 needs heat treatment after welding, the welding of the SA335 Gr.P11 + TP304L thick-walled tube must first be pre-surfacing welded with a certain thickness of nickel-based alloy isolation on the edge of the SA335 Gr.P11 alloy steel groove After the surfacing welding of the layer and the isolation layer, the post-weld heat treatment and non-destructive testing are performed. Finally, the processed groove is welded to the TP304L pipe, as shown in Figure 2. The thickness of the isolation layer after bevel processing is not less than 3mm.
2.4 Analysis of welding performance and selection of welding consumables
The welding material used according to the design requirements: the welding wire is ERNiCr-3 (φ2.0mm), and the electrode is ENiCrFe-3 (φ3.2mm).
3 Welding process
3.1 Using DC pulse
The advantages of GTAW welding DC pulse GTAW welding compared to DC GTAW welding: increase the depth ratio of the weld; prevent burn-through; reduce the heat affected zone range; increase the stirring effect of the molten pool, help reduce the phenomenon of porosity and unmelted, When welding at low current, the stiffness of the arc can be enhanced.
3.2 Shielding gas
Using argon-helium mixed gas GTAW welding, that is, the product welding seam uses 90% Ar + 10% He, or further increases the content of He gas in the mixed gas, the Ar gas arc is stable and soft, and the He gas arc has a large heat concentration and increases The feature of deep penetration eliminates the generation of unmelted defects in the welding of the dissimilar steel products.
3.3 Welding method
The welding seam of this dissimilar steel product should be GTAW bottom welding, manual arc welding (SMAW) filling and cover surface.
3.4 Notes
The use of nickel-based alloy consumables must pay attention to cleaning during the welding process. Oil and dirt on the surface of the weldment must be removed. During multi-layer multi-layer welding, it is necessary to carry out enhanced interlayer grinding and cleaning, because the surface of nickel-based alloy welding materials is prone to form oxide films such as NiO, etc. Because its melting point exceeds 2000 ° C, it is higher than the base material, and it is easy to form if it is not removed. Welding seams. In order to prevent shrinkage and deformation of the weld, the joint stress is concentrated. The butt pipe group is made of point fixing rods of the same material to be evenly fixed in the groove. Welding is performed by double symmetric welding. The welding should be even and alternating from both sides. The skill level of the two welders should be equal. . For heat treatment of pipelines, resistance heaters are used for heating, which is easy to operate, safe and reliable, and has strong traceability of heat treatment records.

4 Heat treatment of isolation layer
4.1 Heat treatment
After the welding of the isolation layer is completed, a stress-relieving heat treatment is performed immediately. If it cannot be performed immediately, post-heating should be performed before the pipe is cooled to room temperature. For the heat treatment parameters, see Table 1. Heat treatment adopts resistance heating local heat treatment, heat treatment equipment must be able to automatically control and adjust the temperature. During preheating and heating, the resistance wire is arranged at least 20mm away from the edge of the groove around the pipe, wrapped with inorganic thermal insulation cotton, and fixed with iron wire. The distance between the resistance wire and the edge of the groove should not be too large. When heating, the thermocouple should be evenly arranged up and down.

Number Project Heating temperature Holding time
1 Preheat 125-200℃ ≥30mins
2 Postherpetic fever 300-350℃ ≥1hour
3 Stress relief heat treatment 610±5℃ 2.4 minutes / mm, 30 minutes minimum

4.2 Matters needing attention
Attention should be paid to the heat treatment: (1) After preheating to the specified temperature and holding for half an hour before welding. (2) Post-weld heat treatment is required if post-weld heat treatment cannot be performed immediately. (3) For some reason, the welding is stopped halfway, and the preheating and postheating should be carried out again according to the original requirements. (4) When the heating temperature of post-weld heat treatment is below 300 ℃, the heating rate is not required. (5) After welding, the heat treatment is cooled to ≤300 ° C to stop energization and natural cooling; the temperature is reduced to ≤100 ° C and the heating device is removed. (6) All pre-heating, post-heating and post-weld heat treatment retention curve records. (7) The thermocouple temperature measurement end is close to the weld and avoid contact with the heating belt. Insulation cotton should at least cover the heating belt.
5 Conclusion
The method of pre-surfacing the isolation layer with nickel-based alloy consumables can solve the post-weld heat treatment problem of SA335Gr.P11 + TP304L thick-walled pipe, and avoid the sensitization of TP304L steel pipe. When multi-layer and multi-pass welding, the arc start and the arc-receiving joint are staggered. Pay attention to the cleaning of the inter-layer weld bead and smooth the transition. Especially the side wall of the base material should be very careful to prevent stress concentration and reduce slag inclusions. The occurrence of defects such as fusion improves the welding pass rate.

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