3. Quality control
3.1 Inspection before welding
According to the metallographic analysis, impurities such as S and P in the base metal will be transferred to the surfacing layer in the surfacing process, and iron-based alloys are surfaced on the austenite matrix. The thermal conductivity and linear expansion coefficient of the two materials are different, causing the welding to be difficult. Therefore, the content of impurities such as S and P in the base metal should be strictly controlled to reduce the tendency of hot cracking. Before surfacing welding, check the base metal of the area to be welded to ensure that the welding material and base metal are not contaminated, and the cleanliness of the surface to be welded and the welding wire should be checked especially. After the liquid penetrant inspection, the penetrant and imaging agent should be removed well to avoid holes.
3.2 The checking of preheating
Preheating before welding can slow down the cooling rate after welding, which facilitates the overflow of diffused hydrogen in the weld metal, and avoids cracks caused by hydrogen. At the same time, it can also reduce the hardening degree of the surfacing zone and heat-affected zone, and improve the crack resistance of the welded joint. Preheating can also reduce welding stress, reduce the temperature gradient between the welded workpieces in the welding area, and help avoid welding cracks. Therefore, before welding, check whether the surface of the workpiece has sufficient preheating temperatures and the heating is uniform. When the workpiece is preheated and out of the furnace, a contact temperature measuring instrument should be used to measure the temperature of the surface to be surfaced to ensure that the welder performs welding within the temperature range and the quality of surfacing is guaranteed.
3.3 The inspection of the welding process
According to the analysis, the factors that affect the hardness of the iron-based alloy surfacing layer mainly include the surfacing method, dilution rate, thickness of the surfacing layer and material of the base metal, among which the hardness and dilution rate (mainly referring to the iron content) in the surfacing process have a close relationship. Therefore, small current, short arc welding, fast speeds, multi layers and multi-pass welding should be adopted as much as possible in the surfacing process, and the arc length should remain unchanged. It's important to control temperatures at the lowest layer and the welding joints between the layers to ensure that the base metal and deposited metal, the layers and channels of the deposited metal are fused and welded well and there are no defects such as holes and slag inclusions. Increase the frequency of temperature measurement, and pay attention to the change of ranges of temperatures; reduce the change range of the welding temperature field and welding stress, and prevent the tendency of welding cracks. Check the welding speed to avoid great tensile stress due to partial overheating. Reduce the burning loss of alloy elements and the dilution rate. Avoid the coarse austenite columnar crystals and segregation of impurities. In the surfacing process, the surfacing should be layer by layer along the circumferential direction of the sealing surface to ensure that the deposited metal’s thickness is uniform, and there should be no unevenness. The welding sequence should also be checked to reduce welding distortion.
3.4 The inspection of the heat treatment
After welding, the temperature of the workpiece should be ensured, and perform heat treatment in time, which can speed up the overflow of hydrogen in the welding seam and heat-affected zone, reduce the hydrogen residue in the structure, avoid the shrinkage rate of the surfacing layer being faster than that of the valve body, prevent internal tensile stress for the surfacing layer and base metal, and reduce the tendency of cold cracks. Post-welding heat treatment can improve the metallographic structure of the surfacing layer, increase the plasticity and toughness of the welded joint, thereby improving the comprehensive mechanical properties of the surfacing layer. For this reason, the stability inspection of the heat treatment furnace, measuring position of the thermocouple, and timeliness of the post-welding heat treatment should be paid much attention to.
3.5 Non-destructive inspection
Non-destructive inspection is the best means of testing the performance and quality of surfacing. After welding, the surfacing layer can be tested by liquid penetration according to the requirements of ASME IX. Non-destructive inspection should be carried out 48 hours after welding to prevent the surfacing stress from being fully released, and to ensure that there are no defects such as cracks on the surfacing sealing surface.
4. Conclusion
The API 000 third-generation nuclear power system puts forward higher quality requirements for valves and their components. The surfacing process of the sealing surface of nuclear valves has obvious welding and surfacing characteristics, so high requirements are put forward for the welding process and parameter selection. Through the research of surfacing iron-based alloys on austenitic stainless steel substrates, various process factors that generate cracks are controlled. Under the premise of ensuring the deposition efficiency, the quality of the surfacing is guaranteed and the conditions are created for the safe and stable operation of the system.
