| Super ferritic stainless steels(FSS)are those steels whose corrosion resistance is far prior to general FSSs,similar with super austenitic stainless steels(ASSs)and some nickel base alloys.Super FSSs have been widely used in the chemical,petroleum and food processing industries,and also used for handling corrosive gases in water treatment and pollution control equipment.However,there are still some issues in both theoretical study and practical experience such as insufficient understandings of precipitation mechanism,low impact toughness,shortage of production technology and use technology.Hence,it is quite necessary to study the effects of alloy composition and processing technology on microstructure,property and precipitation behavior of Super FSSs.Effects of alloy composition,production processed and use technology on microstructure and properties of designed FSS were systematically studied in this paper.Precipitation behavior at different temperatures and impact toughness enhancement of FSS were emphatically studied.Addition of Cr and Mo can raise Super FSSs' recrystallization temperature and improve their pitting corrosion resistance and sulphuric acid corrosion resistance.With the increase of Cr and Mo,? phase is more prone to precipitate through widen of precipitation temperature region and transition of precipitation pattern from grain precipitate to mixture of grain and grain boundary precipitate.Furthermore,Cr and Mo increase the strength and hardness of material by solid solution strengthening and precipitation strengthening.Addition of Ni improves the Charpy impact toughness and sulphuric acid corrosion resistance remarkably but has almost no effect on precipitation of ? phase and pitting corrosion resistance.Hot rolling processes have a significant impact on microstructure and properties of hot rolled plates and hot annealed plates.With the decrease of finishing delivery temperature,the strength and hardness of hot rolled plate are increased and recrystallized temperature is decreased as well as reduction of grain size.Compared with water cooling,furnace cooling promotes precipitation of ? phase which lead to rise of material strength.Microstructure and precipitation temperature,size and amount of ? phase are decided by annealing temperature.Annealed at temperature under 1020°C,there will be a mount of ? phase precipitated.And with the increase of annealing temperature,the size of ? phase increases and the quantity of ? phase reduces.Annealed at temperature above 1020°C,there won't be ? phase precipitated.Meanwhile,annealing temperature also influences uniformity and size of recrystallized grain,then influences elongation,strength and hardness of material.Furthermore,improvement of microstructure and precipitation promotes the improvement of material impact toughness.In super ferritic stainless steel there are five typical precipitations including TiN?NbC??? phase?? phase and ? phase.TiN and NbC particles are dispersed uniformly in the metal matrix.The nose temperature of ?? phase is 475°C,and its size and shape change with prolongation of aging time.? phase is a kind of metastable phase which is easily to precipitate from 600°C to 1000°C,and with the increase of aging time,? phase will transformed to ? phase aged from 700°C to 950°C.? phase is mainly distributed on the grain boundary while ? phase precipitates both in the interior of grain and on the grain boundary.The nose temperature of ? phase is 800°C and for ? phase it is 850°C.When aged at 1000°C,? phase disappears and ? phase can be found only in the interior of grain.The welded joints of laser welding and pulsed TIG welding are both composed of single ferritic phase,and they are mainly composed of central equiaxed grains and columnar grains at the edge of the weld.For laser welding in the welded joint precipitated phase is mainly TiN,but for pulse TIG welding a small number of ? phase is found besides carbides and nitrides of Ti and Nb.Precipitation of ? phase should attribute to higher heat input and slower cooling rate of pulse TIG. |
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