| Ferritic stainless steels have been extensively applied in manufacturing processing such as chemical and petrochemical industries and so on, due to their superior corrosion resistance in certain environments and the superb low-cost especially. However,Ferritic stainless steels are liable to some embrittlement phenomena which is related to sigma phase formations, grain growth without phase transition, sigma phase formation and a precipitations.00Cr14Sn and00Cr16Sn produced by Tai Yuan Iron and Steel Corporation are two ferritic types with different Sn contents, which are named ultra-pure ferritic stainless steels with ultra-low C and N (C+N≤150ppm) contained and Ti and Nb as double-stabilizing elements.Due to the addition elements of Cr and Sn, the steels present higher corrosion resistance than other low-Cr FSS. Nb and Ti stabilizing elements form fine carbides and nitrides, which have prevented sensitization and improved the weldability and mouldability.The aim of the present work was to investigate the microstructural changes under the welding thermal cycle (above1300℃). The effects of the precipitations reactions and the elements addition of Sn on the mechanical and corrosion properties were determined. Automatic TIG (direct current electrode negative) welding process (without filler) was used to generate the welding samples00Cr14Sn and00Cr16Sn of0.8mm thick plates. Two welding processing techniques were employed: continuous TIG and pulse TIG. Simultaneously, these welding joints including base metal and welding joints with the two welding processes of O0Cr14Sn and00Cr16Sn were evaluated respectively with mechanical and corrosion resistance properties. The mechanical properties including Vickers hardness and tensile properties and the ductility by cupping experiment; the corrosion resistance tests containing electrochemical corrosion resistance test and intergranular corrosion resistance test and salt spray test. Conventional optical microscopy was employed for microstructural studies. Scanning electron microscope was used for fractographic examination.With the results of welding tests about the design of welding parameters, the optimum welding parameters were determined. The microstructure of welding joints are consist of grown HAZ(heat affected-zone) and bulky columnar zone and equiaxed crystal zone,which was similar to the classic cast structure underwent the welding heat cycle and constitutional rapid cooling. Meanwhile, those areas were all susceptible to form Nb(C,N) and Ti(C,N) precipitations. From the thermodynamics respects, the interstitial Nb(C,N) and Ti(C,N) were a type of thermodynamic balanced faults. These rigid interstitial atoms dispersed on the soft and flexible matrix, which both improved the ductility and toughness and hardness and strength. Consequently, we could obtain such alloys with superb comprehensive properties. On average, the properties of samples used pulse TIG were closest approached the base metal. Given to double-stabilizing, theses samples all exhibited good resistance to IGC. The increase of Sn contents is responsible for the improvement of resistance to corrosion, especially to resistance to chloridion. This phenomenon can be attributed to the formation of double electrode layer.Tungsten inert gas arc welds exhibited the great mechanical and corrosion properties approached to the base metal. Sn addition resulted in substantial improvement in resistance to corrosion without adversely affecting the ductility. The Nb(C,N) and Ti(C,N) were the typical fractured second phases. In each welding specimens, the growth of grain size is not so significant which can be attributed to the exist of (Nb,Ti)C and (Nb,Ti)N precipitations. Thus, the materials studied exhibited the excellent comprehensive properties. |
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