The Effect Of Shot-peened And Annealing On Intergranular Corrosion Resistant And Grain Boundary Character Distribution Of 304 Stainless Steel

The 304 austenitic stainless steel has been extensively applied in the fields such as petroleum processing, chemical engineering and power stations, etc. Because of its good mechanical property and corrosion resistance nature. However, it is vulnerable to intergranular corrosion in specific environment and thus causes destruction of the components. In order to increase the intergranular corrosion resistance of 304 austenitic stainless steel, in this thesis, shot peening approach is adopted to control the microstructure of the surface layer; studies are done on the influence of the shot intensity, annealing temperature and annealing time on the intergranular corrosion and grain boundaries characteristic distribution; furthermore, analyses are made on the formation of low-energy CSL grain boundaries as well as its mechanism with the corrosion cracks. The following conclusions are drawn:1. Shot-peening causes plastic distortion on the surface of the 304 austenitic stainless steel, where the original single-phase austenite has turned into dual-phase austenite and martensite. With the increase of the shot-peening intensity, the corrosion rate of the specimens increases gradually, and the nature of the intergranular corrosion resistance becomes worse and worse.2. Shot-peening combined with annealing treatment can change the surface microstructure of austenitic stainless steel. Under the same annealing process, it is easier for the lower intensity shot specimens to produce annealing twin which can block the grain boundary network. The corrosion resistance nature gradually increases with the decrease of the shot intensity. Under the same shot-peening process, the longer the annealing time spends, the larger the number of the annealing twin on the surface layer increases, the better the nature of corrosion resistance becomes. When annealing is made at 927℃, the corrosion rate is the lowest and the corrosion resistance nature is the best.3. Compared with the based material, the shot-peening specimen has larger corrosivity, larger corrosion rate and smaller resulting impedance, which become more and more obvious with the increase of the shot intensity. The specimens with a power of 0.2A annealed at 927℃for 24 hours which have been0.2, have lager corrosion potential, smaller corrosion current density and obviously larger Nyquist capacity reactance arc radius. Therefore, the specimen has smaller corrosivity, lower corrosion rate and the best corrosion resistance nature. Since the intergranular chromium depletion of the stainless steel after sensitization makes the passive film in this area unstable, negative resistance capacity reactance appears in the low-frequency phase in the specimens which have been annealed for 2 hours at 927℃. The passive film began to dissolve and the corrosion rate increases. At last, the corrosion rate is larger than that of the based material.4. The austenitic stainless steel, which has been handled by shot-peening and high temperature annealing, can produce a large number of low-energy CSL grain boundaries. These special grain boundaries can disperse the continual and network structured free grain boundaries and block the intergranular corrosion crack propagation channels so as to increase the intergranular corrosion resistance of the material.