Nitrogen Austenitic Stainless Steel Organizational Structure And Ductile-brittle Transition Mechanism

High nitrogen austenitic stainless steel (HNS) is a type of economy stainless steel with nitrogen and manganese partly or completely replacing nickel to obtain the austenite. Carbon and nitrogen alloying gives several beneficial effects on the steel which delivers high strength without significant reduction in fracture toughness and good corrosion resisting property. As a structural steel, the hot worked application, the welded application and the application at low temperatures depends on their microstructure variation. Because of nitrogen-bearing and carbon-bearing particularity, some problems, such as precipitation and growth of second-phases, the ductile to brittle transition which the microstructure variation of the steel leads to, etc., will probably occur in working and application process, which can reduce the properties of the steel. Therefore, the research on structural stability and the ductile to brittle transition of HNS is of great significance for alloy design, hot working, welding and low-temperature application in theoretical study and industrial application.In this paper, the structural stability at high temperature, the structural stability at intermediate temperature and the ductile to brittle transition at low temperature have been investigated by testing a high nitrogen austenitic stainless steel which contains 0.7 weight-percent nitrogen and carbon basically and testing other steels which contain different content of nitrogen and carbon relatively. The main achievements are summarized as the followings:(1) The structural stability of HNS at high temperature depends on carbon and nitrogen , especially nitrogen containing austenitic stainless steel. Within the range of alloy composition that we designs, HNS has a austenitic matrix with small quantity ferrite if the alloy contains 0.7 weight-percent nitrogen, HNS has a complete austenitic matrix under different solution treatment conditions, or if, the alloy exceeds 0.94 weight-percent nitrogen. Furthermore, shape, distribution and volume percent of the ferrite in the steels is influenced by different heat treating regime: it is the sensitizing temperature range that is higher than 1200℃as a solution treatment temperature, where the content ofδ-ferrite increases and its shapes are mostly sharp angles or polygon. When it is low at 1100℃for solution treatment temperature the state ofδ-ferrite in the steel almost has no change. The volume percent and block ofδ-ferrite increase gradually, and its shape is changed from round or oval to sharp angle or polygon between 1100℃and 1200℃.(2)The variation of mechanical property depends on the structural variation and the solid solubility of interstitial atoms(carbon and nitrogen) in HNS, the optimal solution temperature is at 1150℃for the steel: as it is higher than 1150℃, which causes the growth of grain and a large amount ofδ-ferrite precipitates in austenitic matrix. When it is low at 1150℃, several nitrides and carbides have not been dissolved sufficiently which delivers the reducing of strength. The high-temperature tensile test shows that the high-temperature brittle segment is caused by the growth or the melting of ferrite at higher temperature than 1200℃, the other one is caused by the appearances of precipitates carbide and nitride and brittle second phase (σ-phase), especiallyσ-phase in temperature range of 850℃to 1050℃, the steel has a good high-temperature ductility which delivers a excellent hot-working character in temperature range of 1050℃to 1200℃.(3) The kind, shape, distribution and volume percent of precipitates depend on the content of carbon and nitrogen in HNS, sensitization time and temperature of the steel during holding for intermediate temperature. With the increasing ageing time, the volume fraction of precipitates increases. Carbon and nitrogen of HNS can improve the mutual solubility each other. The content of carbon has no effect on kind of precipitates when the content of nitrogen is improved to certain extent, and nitrides exist in two kinds of forms: (a) Cr₂N, (b) (CrFe)₂N_1-x. The precipitating process of carbide and nitride (mainly M₂₃C₆ and Cr₂N ) precipitates in the steels during isothermal ageing shows that: precipitating as growing as cellular structure from meeting point of three intergranulars→chain-like along grain boundaries with a small amount precipitating in intracrystalline→growing from grain boundaries into intracrystalline gradually and joining to be layered tablet shaped, then distributing the whole crystal face. When containing a small amount ferrite in the steel, second-phases precipitate along the grain boundaries between ferritic matrix and austenitic matrix, then precipitating process gets the similar one which is described above.(4) There are two kinds of characteristics in the formation ofσ-phase: (a) The formation of nitrogen-depleted and carbon-depleted near the precipitates colonies will lead to its appearance. (b) The unstability of ferrite in matrix can cause the transformation from ferrite toσ-phase.(5) The isothermal precipitation kinetics curves (PTT) of precipitates were obtained. In the 1# steel, the carbide precipitate sensitive temperature of isothermal precipitation is 850℃with the corresponding incubation period about 90 second, precipitation of carbides hardly take place over 950℃, and the critical cooling velocity is 3.3℃/s. In the 3# steel, the nitride precipitate sensitive temperature of isothermal precipitation is 900℃with the corresponding incubation period about 12 second, precipitation of carbides hardly take place over 1000℃, and the critical cooling velocity is 20.8℃/s.(6) The mathematical models of precipitation of carbide and nitride, which are about relation between time and temperature, obtained in HNS at low-temperature(l#:, 3#: ). According to these models, the activation energy of carbide M₂₃C₆ and nitride Cr₂N were obtained.(7) The ductile to brittle transition temperature (DBTT) of HNS depends on carbon and nitrogen, especially nitrogen, and the DBTT is shifted from low temperature to high temperature with the content of nitrogen increasing, which accords with the equation DBTT=300C_N-303(℃) basically. The testing results show that: the DBTT of 1# steel is -133℃, the DBTT of 6# steel is -65℃, the DBTT of 3# steel is -18℃, the DBTT of 4# steel is -18℃, the DBTT of 2# steel is 17℃. The change of fracture patterns of HNS is dimple→dimple and cleavage-like fracture→transgranular fracture with cleavage-like fracture mainly, and fracture mode is mainly a transgranular fracture with few fracture along annealing twin boundary and grain boundary. In addition, the ductility and brittleness transition of HNS depends on cooling rate of solution treatment and ageing behaviour of the steel, the curve of ductile to brittle transition tends towards flat and the toughness of HNS is wors and worse with carbide and nitride precipitating.(8) With carefully analyzing, the brittle fracture of HNS has two factors at low temperature: (a) The formation of deformation twin and the interaction of dislocation, stacking fault and deformation twin can cause the brittle fracture between two slip planes, the interaction leads to stress concentration at the top of crack which will deliver the expansion of crack, so the steel is brittle. (b) the stacking fault energy is decreased with the content of nitrogen increasing which causes the increase of the slipping resistance among dislocations, so the stress concentration will centralize easily under the exogenous process which causes the decreasing of the toughness of HNS.