| In recent years,with the elimination of old coal fired boilers,the austenitic stainless steel heat exchanger prepared by vacuum brazing process is being widely used.The structure of the brazing joint quality related to welding material and welding process,due to the difference of the material thermal physical properties of brazing cooling process,the special finned tube brazing structure has uneven cooling shrinkage,local plastic deformation and residual stress are generated,and the deformation and residual stress will influence the overall performance of heat exchanger.At present,the brazing technology of finned tube heat exchanger has not been reported on the reliability of heat exchanger.The influence of brazing technology on reliability of copper brazing stainless steel finned tube heat exchanger was studied.Based on the finite element software ANSYS,with the method of sequential coupling simulation vacuum brazing process of cooling finned tube structure.The temperature field of finned tube structure with time is obtained,and the results of corrugated deformation of S43000 ferrite stainless steel fin,AISI 316 L austenitic stainless steel tube and copper brazing combined welded structure are obtained.And the local stress concentration and local plastic deformation of austenitic stainless steel tube at the corresponding finned gap were simulated.Fill the size of solder and the microstructure of brazing is analyzed,it is found that there is a tendency of intercrystal penetration along austenitic stainless steel in copper filled metal.Tensile test of standard brazed joint shows that the fracture of welds occurs in stainless steel,and the welding strength of welds is higher than that of stainless steel,welding process has little effect on the mechanical properties of stainless steel tube.However,after adopting finned tube brazing structure,the elongation of the tube decreases significantly.Corrosion potential of copper in the weld area is lower than AISI 316 L stainless steel,and therefore,galvanic corrosion occurs near the weld.The accelerated simulation experiment of simulated high-pressure water circulation equipment was used to evaluate the corrosion resistance of weld tissue under extreme conditions.The results showed that: after 60 days of immersion and the test of 3.5% Na Cl hot brine circulation,a small amount of spot corrosion occurred in the weld,but no leakage was found.For brazing stainless steel tube deformation and stress,after studied the different annealing temperature on the tube cross-section microhardness,tensile strength,plasticity,the effects of intergranular corrosion resistance,the results show that 800 ?,holding time of 2 hours annealing state tube local strain zone is eliminated,and plastic to improve,resistance to intergranular corrosion is improved.From the practical application,the main failure form of unannealed heat exchanger is thermal fatigue cracking,some heat exchangers fail due to copper embrittlement crack and thermal corrosion perforation caused by overheating. |
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