Microstructure Formation And Evolution Of Austenitic Stainless Steel During Near-Rapid Solidification

Sub-high solidification, with typical cooling rate of 100-103 K/s, is a transition region between equilibrium solidification and rapid solidification. During this solidification pattern some solidification characteristics such as microstructure morphologies, element distribution, and interface stability change markedly. Therefore, investigation on theories of sub-high solidification plays an important role in improving solidification theory and solving the practical problems relevant to solidification. In this paper, prediction on the primary phase was conducted based on the phase selection theory. Directional solidification experiments and liquid quenching technology, water-cooling copper mold casting, and suction casting were performed to systematically investigate microstructure formation and evolution in austenitic stainless steels under sub-high solidification within the whole cooling rate range.Interface response functions for austenitic stainless steels were calculated. The effect of temperature gradient on the stability of the S/L interface and prediction of primary phase in austenitic stainless steels were done using the highest interface temperature criterion. It is found that plane front growth is destabilized more easily and the the velocity scope of plane front growth extends at low growth rate and the velocity scope of cellular/dendrite growth shrinks with increasing temperature gradient. For AISI 304 stainless steel, the interface temperature of ferrite is higher than that of austenite and ferrite grows steadily in the form of cellular/dendritic morphology when the growth rate is lower than 4.2 cm/s. When the growth rate exceeds 4.2 cm/s austenite will appear as primary phase. For 18Cr-8Ni alloy, ferrite solidifies as primary phase along and no transformation of solidification sequence occurs. Transition of solidification sequence occurs for 18Cr-8.76Ni, 18Cr-9Ni, 18Cr-10Ni, and 18Cr-11Ni alloys. With the increase of Cr/Ni, the critical growth rate for the transition from ferrite to austenite increases and it is more difficult for austenite to precipitate as primary phase consequently.Microstructure evolution in AISI 304 stainless steel within sub-high solidification was explored using systematic directional solidification experiments. It is found that ferrite morphologies develop gradually with the increase of the withdraw rate during directional solidification. Blocky ferrite, skeletal ferrite, lathy/lath like ferrite, cellular ferrite, and strip ferrite occurs in sequence with the increase of the withdraw rate.Blocky ferrite takes place during subsequent solid-state transformation on cooling under low cooling rate conditions. The two-phase coupled growth microstructure forms from the melt at the initial stage of solidification. Ferrite is arranged in the form of lath in the two-phase coupled growth microstructure. During the solid-state transformation on cooling, austenite becomes unstable due to local solution microsegregation when austenite grows. Some finer ferrite clusters precipitate in the unstable austenite region. These ferrite clusters transform into blocky ferrite at room temperature.The formation details of the lathy ferrite are revealed and the formation mechanism of the two-phase coupled growth microstructure is proposed. During the formation of the two-phase coupled growth microstructure, ferrite first nucleates from the melt and grows gradually into the melt. With the casting solidification, the melt temperature decreases. Here, if the melt undercooling is higher than the nucleation undercooling required for austenite before ferrite reaches a steady state growth, austenite will nucleate at the interface of ferrite. From the solidification path it is known that the melt temperature is undercooled for ferrite when austenite forms. Thus, after the formation of austenite the undercooling is higher than the required undercooling for ferrite. So, ferrite will form at the interface of austenite before austenite reaches a steady-state growth. The primary dendritic ferrite is suppressed and the coupled growth of ferrite and austenite takes place. Cellular and strip ferrite are observed at high cooling rate during directional solidification. Cellular ferrite is caused by the degeneration of second dendritic arm due to high cooling rate. Strip ferrite is a result of the instability and adjustment of the primary arm spacing.Some experiments by suction casting were performed to investigate the microstructures of austenitic stainless steels within high velocity scope of sub-high solidification. The macrostructure of as-cast strip by suction casting is composed of bilateral columnar zone and central equiaxed zone. Primary phase transforms from ferrite to austenite at the surface layer of the strip due to high cooling rate. Columnar ferrite changes to equiaxed morphology due to the different heat-extraction pattern in the centre of the strip. Mold material affects the proportion between columnar zone and equiaxed zone in the strip. The proportion of columnar zone in mold made of material with good heat conduction is much higher. In addition, remelting condition of the alloy has an important effect on the maicrostructure. Better heat insulation capacity of the crucible, higher proportion of the columnar region appears in the macrostructure of the strip.Effects of alloy composition and cooling rate on the solidified microstructures of austenitic stainless steels were investigated. For 18Cr-7Ni or 18Cr-8Ni alloy, Widmanst?tten structure is observed at low cooling rate and skeletal ferrite occurs at higher cooling rate. For 18Cr-9Ni, 17.93Cr-8.76Ni, 18.77Cr-10.59Ni, and AISI 304 stainless steel, no Widmanst?tten structure was observed and ferrite occurred in the form of skeleton at the low cooling rate. Ferrite morphology will change with the increase of the cooling rate. With further increase of the cooling rate austenite precipitates as primary phase instead of ferrite. At higher cooling rate, dendritic austenite transforms into cellular austenite. For 18Cr% alloy, increase of content of austenite stabilization elememt favors the occurrence of primary austenite. When Cr content and Ni content are fixed, austenite is easier to form as primary phase in the stainless steel with other elements than in the pure Fe-Cr-Ni alloy. Under the same (Cr/Ni)eq, the formation ability of primary austenite in austenitic stainless steels with other elements is lower than that of pure Fe-Cr-Ni alloy.