| The application of bimetal clad pipe in the highly corrosive oil and gas gathering and transportion system is a reliable and economical method to improve the anti-corrosion performance. Surfacing is widely used for the production of bimetal clad pipe as a key technology. But the influence of the welding procedure on microstructure and properties of surfacing layers has not been deeply researched.Therefore, in this article, on the bases of research about welding hot cracking,the inconel625alloy was surfaced on Q235B. The effect of welding current, forming coefficient(φ), the temperature of postwelding heat treatment(PW-HT) on the properties, microstructure and the composition distributions of surfacing layers have been discussed. The results are showed in the following:The welding current increased to140A accompanied with the emerging of the cracks which grew in length as the welding current enhanced to160A when SMAW was applied to restraint welding; On the other hand,the application of TIG resulted in no cracks even if the welding current was180A, under this condition,the cracks appeared only when trial panel was thickening to20mm,indicating the enhanced influence of thermal input and restraint intensity directed by the plate thickness on hot cracking susceptibility,and thus confirming that TIG is more conducive to thermal cracking control;the macroscopic hot cracking was intergranularly expanding and the fracture appearance consisting of equiaxed crystals was abundant in Nb. The microcracks which were solidification cracks mainly existed in the grain boundary of the columnar crystals or at the bottom along the growth direction, the precipitations of carbides rich in Nb at the fracture initiation location arised the augment of the hot cracking susceptibi-lity.The microstructure of surfacing layer deposited metal which was prepared by TIG is austenite with y-Ni single phase.The morphologgies of microstructure along the direction of temperature gradient are planecrystal,columnar cellular,columnar dendrite,equiaxed and transverse structure, respectively, and the grain size is smaller when getting closer to the top of layer.The alloy element precipitated phase at the grain boundary in the layer with high-φ shows segregation phenomenon existed in the grain,which increased the tendency to intergranular corrosion and was reflected by the etching effects of electrochemically in the oxalic acid solution.However,the segregation of alloy elements was inhibited by reheating effect produced by the multi-pass welding,which was demonstrated by the weaker etching effect of oxalic acid solution on bottom deposited metal, leading to more dense microstructures than the top deposited metal.The dilution of layer with high-cp increased with the enhancement of current and the dilution of180A with high-φ specimen can reach up to37%-47%.;for another aspect,the current had little influence on the morphologies of microstructure,but the etching effect of top micro structure firstly reduced and then increased following with the increasing of current, the lowest effect occurred at the current of165A.The width of liquid mixing zone significantly minished with the reducing of φ but slightly increased as the current augmented, the liquid mixing zone of110A with low-φ specimen is80μm in width which had a high dilution at39%,reflected by a severe etching layer generated in microstructure.The composition of the deposited metal out of the liquid mixing zone is close to substrate material,which had a totally low dillution rate at1%with the weaker etching effect.The microhardness within surfacing layers was ascending distribution from fusion line to the top and was increased in the addition of welding current but the decreasing of φ. The highest value reaching up to346.4HV appeared in150A with low-φ speciman which exeeds98.8HV more than150A with high-φ speciman, and the microhardness of base metal is slightly increased.The charge transportation resistance Rct and the dilution changes of deposited metal in3.5%NaCl solution are the same.The150A with high-φ sample deposited metal presents higher charge transportation resistance of210.5kΩ, and the150A with low-φ sample deposited in lower stratum presents the lower corrosion resistance of21.2kΩ. The165A with high-φ sample also showed the phenomena of damaging passivation film and repairing obviously during the anodic polarization.The pitting potential of165A with high-φ sample is low to about694mV, and the pitting potential of other samples are close to1000mV. Reduction in the cp and reheat effect after welding can effectively reduce the tendency of test specimen’s intergranular corrosion. The110A with low-φ and150A with low-φ sample’s Ra is2.1%and0%respectively. The Ra of180A with high-φ sample deposited metal in lower stratum is27.44%lower than the upper stratum, which is just67.38%.The segregation of alloy elements was inhibited by PWHT which reduce the tendency of intergranular corrosion.The microstructure of deposited metal grew denser and the mircohardness is decreased.Those effects is more effective with the temperature increased. The value of Ra declined to36.12%in the specimen at1020℃;The grain size in base metal increased with the enhancement of temperature and Widmanstaten in the HAZ is eliminated. However, the mechanical properties of specimen at1020℃decreased greatly reflected that Rm dropped47Mpa and the tensile elongation was decreased by50%because the Widmanstatten structure reappear in base metal by overheating. No cracking was found in the interface of the sample after the bending test.The heating temperature of postwelding heat treatment should not exceed1020℃and recommended820℃. |
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