In Situ Observation of Solidification Process of AISI 304 Austenitic Stainless Steel

Available online at www.sciencedirect.comScienceDirectJOURNAL. OF IRON AND STEEL RESEARCH, INTERNATIONAL. 2008, 15(6); 78-82In Situ Observation of Solidification Process of AISI 304Austenitic Stainless SteelHUANG Fu-xiang'，WANG Xin-hua'，ZHANG Jiong ming' ,JI Chen-xi'，FANG Yuan2，YU Yan'(1. Mtllurgical and Eoological Enginering School, University of Science and Technology Beiing, Bejing 10083, China;2. Advanced Technology Institute of Technology Center, Baoshan lron and Steel Co Ltd, Shanghai 201900, China)Abstract: The solidification process of AISI 304 stainless steel during cooling at a rate of 0. 05 K/s has been observedin situ using a confocal scanning laser microscope (CSLM). The results show that the δ phase appeared first in liquidsteel, as the temperature decreased, the r phase precipitated prior at 8-grain boundary at 1452.2 C, the liquid steeldisappeared at 1431.3 C, and then the r phase precipitated on the 8 frrie Based on the ScheilGulliver solidifiae-tion model, the solidification processes of AISI 304 stainless steel are simulated using the Scheil model in Thermo-Calc, and the simulation results agree well with the results observed in the experiment.Key words: stainless steel; confocal scanning laser microscope; strip casting; Thermo-CalcThe solidification mode of austenitic stainless been implemented in this study, and then based onsteels has a large effect on the surface quality ofthe Scheil-Gulliver solidification model, the solidifi-slabs manufactured by continuous casting and conse-cation processes of AISI 304 stainless steel are simu-quently on hot and cold rolled flat producs'1. Pre-lated using the Scheil model in Thermo Calc.vious studiesl2.3] have mainly investigated the solidi-lExperimentalfication mode of austenitic stainless steel using thequenching method. However, owing to the restric-A confocal scanning laser microscope (CSLM)tion of the testing method, it is difficult to visualizewas used to directly visualize the surface of the s0-the crystal growth and phase transformation duringlidifying samples. This equipment allows for the ob-the solidification of AISI 304 austenitic stainlessservation of liquid steel samples at high temperaturesteel. Recent advances in the confocal scanning laserand during controlled thermal fields.microscope (CSLM) have enabled the direct obser-The CSLM combined with an infrared imagevation of the high-temperature phase transformationfurnace is schematically ilustrated in Fig. 1. Theand microstructural evolution in steel(C. LIU ZZet He-Ne laser was rflected and scanned by an acous-al[5J observed the solid 8/γ phase transformation intic optic deflector (AOD) in the horizontal directionsitu on the surface of low carbon steels containing and by a galvano mirror in the vertical direction.different phosphorus concentrations using the CS-The scanning beam was confocally focused on theLM. YIN H et alL6J discussed the morphological in-surface of specimen via a polarizing plate, a long fo-stability of 8-ferrite/ Y-austenite interphase boundarycus objective lens, and through a view port coveredin low carbon steels. LIANG G F et al7] recently with a quartz disc. Reflection from the surface re-observed the 8→γ transformation in the AISI 304turned by the same path above, passed through a. stainless steel. To observe the in-situ solidification polar中国煤化工ens，and then fo-behavior of AISI 304 stainless steel, the CSLM has cusedpinhole[8].YHCNMHGFoundation ltem;ltem Sponsored by National Natural Science Foundation of China (50434040)Biograpby:HUANG Fu-xiang(1981-), Male, Doctor;E-mail; huang150429@ sohu. cm, Rerised Dnte: September 4, 2007Issue 6In Situ Observation of Solidification Process of AISI 304 Austenitic Stainless Steel●79●Crn/Nig<1.25VTRHe-Ne laserAF mode: L →L+r +L+8+r→y+δCCD image sensor1.25 Beam .1.481.95Modhulate beamHere, Nim and Crm can be estimated by the fol-splitterJ MirrorAODlowing equations:MirrorCr.=wcr+ wmo+1.5ws +0.5wNb(1)DOarao mirorNig = wWw + 30wc +0.5wMn + 30wN(2)Lens中With Eqn. (1) and Eqn. (2), the equivalent con-Sampletentsof Cr and Ni (Cra and Nig) for the AISI 304Gas outlet， Gas inletstainless steel under study are obtained as 18. 28 and10. 11, and therefore, the Crg/Nig is equal to 1. 81. .Quartz windowHalogen laumpThus, the solidification mode of the steel falls intoInfrared gold image fumnaceFA mode. In other words, the δ ferrite firstly pre-cipitated from liquid, then the three phase reactionFig, 1 Schematic of CSLM with infrared image furnace(ferrite, austenite, and liquid) at the terminal solid-ification stage, and δ > r continuing below the soli-The steel sample was machined into a disc, mir-dus linelo].ror polished, and set into a high purity alumina cru-Fig. 2 presents a representative micrograph ofcible. The sample and crucible were installed intonucleation and growth of δ phase in liquid and Ythe furnace at one focal point of the gold plated ellip-phase precipitated at 8-grain boundary during the so-soidal chamber as shown in Fig.1. During the ex-lidification of AISI 304 stainless steel when the steelperiment, ultra pure argon gas was itroduced into is cooled at 0. 05 K/s. The 8 phase first appears inthe chamber to prevent the sample surface fromliquid steel when critical supercooling occurs, andreoxidation. The sample was subsequently heated togrows with the temperature decreasing, as shown inits melting point by a halogen lamp located at theFig.2 (a) and (b). As known, Cr, as the ferriteother focal point of the chamber at a program-con-stabilizer, will partition to the δ ferrite, and Ni, astrolled heating rate. The temperature control andthe austenitic stabilizer, will partition to the remai-measurement was crried out with a thermocouple in ning liquid during sldifiction; therefore, the rcontact with the alumina cruciblel9).phase precipitates (L+δ→y) prior at the 8-grainThe progress of the melting during heating andboundary at 1 452.2 C，as shown in Fig.2 (c).the crystal growth during cooling were monitored onA previous study{4] has indicated that the cruci-a CRT as the confocal laser images on the CCD. The ble acts as a set for heterogeneous nucleation; theimages were simultaneously recorded on a video tapesolidification of δ ferrite proceeds from the bottom ofat a rate of 30 frames per second.the container and the segregated liquid steel will re-2 Results and Discussionmain on top until the tips of the growing solid den-drites emerge on the surface. The solid-liquid inter-2.1 Solidification process of AISI 304 stainless steelface is finger-like but has no faceted shape. The sol-For austenitic stainless steels, the solidificationo id fraction at 1 452.2 C when the r phase precipi-mode is related to Ni equivalent (Nig) and Cr equiv-tates at δ-grain boundary can be estimated by com-alent (Cr_), which are used to simplify a multi-puting the volume of solid δ ferrite in Fig.2 (c) bycomposition system into the Fe-Cr-Ni ternary sys-Photoshop software ( according to the difference oftem. Depending on the ratio of Ni equivalent and Cr brigh中国煤化工。and the liquid).equivalent (Cra/ Nig), the solidification modes ofThe; about 0.6, that isaustenitic stainless steels can be divided into the fol- to sayTMYH.CNMHGat8-grainboundary.lowing four types[10] :when the solid fraction is about 0.6 at 1 452.2 C dur-A mode: L→L+y→Ying the solidification of AISI 304 stainless steel.80●Journal of Iron and Steel Research, InternationalVol 15Table 1 Composition of AISI 304 stainless steel(mass percent, %)_CompositionCMInCrNiContent0.040. 560. 990. 0290. 005.17. 44a)1 456.3C 1534s(b)1453.7C 1577SL' I 452.2C 1598(1448.9 C 1 658 s8,8δ40 μum，(a)，(b) The 8 phase appeared in liquid steel;(e), (d) The r phase preipitated prior at -grain boundaryFig, 2 Insitu observation of growth behavior of 8 phase during solidifcation of AISI 304 stainless steelAs the solidification continues, the δ ferrite alsophasis on the geometric aspects and the other em-continues to grow, and the amount of r phase pre-phasizing on the thermodynamics and kinetics. Bothcipitated (L+δ→γ) at δ-grain boundary graduallytechniques have been applied successfully to someincreases, as shown in Fig.3 (a). The liquid disap-practical cases.pears at 1 431.3 C, and the solidification ends, asIn a thermodynamic simulation, for example,shown in Fig.3 (b). Then, the r phase precipitatesThermo-Calc，precise thermodynamic descriptionsonly on the δ ferrite, as shown in Fig.3 (c) and (d).ona system are utilized; while it is very importantto know the final state that the system is trying to2.2 Simulation result for solidification of AISI 304reach, this can greatly simplify the kinetics and oth-stainless steeler factors. The Scheil module within Thermo-CalcThe solidification of multicomponent steels andapplies the Scheil-Gulliver model to deal with the so-alloys often involves various types of non-equilibri-lidification problems of alloys, by assuming that theum interactions, linear or non-linear dynamicdiffusion ceffcients of components in the liquidprocesses, and random fluctuation in the system.phase are infinitive fast whereas in solidified phasesAn ambitious process simulation shall take into ac- are zerol1.count the mass transfers, energy transports, andFig. 4 depicts the simulation result for the solid-momentum transmissions contributed by various ap-ification process of AISI 304 stainless steel under aproximationB. However, to consider all these in onecooling temnerp sten (0. 1 C) using the Scheilsimulation software is extremely complicated, andmodel中国煤化工: showed that the δhence, various approximations and simplificationsfirst fC N M H Gthe Y phase precip-are always required. , There are two groups of ap- itated' at 8-grain boundary wnen the solid fractionproaches using different methods, one with the em- was about 0.66 at 1 451 C, and the liquid disappeIssue 6In Situ Observation of Solidification Process of AISI 304 Austenitic Stainless Steel●81.(a1446.2 C 1 705 s1431.3C 1 765s IY81419 C 2148s1373.6C 2863s740 um(a) L+8+r;(b) Liquid steel disappeared at 1 431.3 C: (c)， (d) r phase precipitated on the 8 frriteFig3 In situ observation of precipiating behavior of Y phase at 8-grain boundary for AISI 304 stainless seelmicroscope. The δ phase first appeared in liquid dur-1 470ing the solidification of AISI 304 stainless steel.(2) The γ phase precipitated at 8-grain bound-14602ary when the solid fraction was about 0.6at 1452.2 Cduring the solidification of AISI 304 stainless steel,145114503and the liquid steel disappeared when the tempera-ture reached 1 431.3 C.(3) The simulation results by Thermo-Calcshow that during the solidification of AISI 304 stain-less steel, the L+δ→γ occurred when the solid frac-1430 t， 1 LQUID=Ltion was about 0.66 at 1 451 C, and the liquid dis-2 LIQUID) BCC. _A23 LIQUID BCC A2 FOC A1appeared at 1 425 C. The simulation results agree0.66very well with the results obtained in the experi-00.40.60.81.0ment,Mass fraction of solidReferences:LIQUID=L, BCC A2= Ferrite (8), FCC. A1= Austenite (》Figp 4 Scheil-Culliver module simulation result for[1] Spccarotella A, Ridolfi M R, Picht G. Control of δ→r Tran5soldification process of AISI 304 stainless steelformation During Solidification of Stainless Steel Slabs in theMould []. Steel Research, 2003, 74(11); 693.ared at 1 425 C. The agreement between the predic [2] Hunter A, Ferry M. Phase Formation During Sldificetion ofted and experiment results was extremely good.AISI 304 Austenitic Stainless Steel [J]. Scripta Materialia,2002, 46(4); 253.3 Conclusions3]中国煤化工Ateraive Phase Forma-Fuless Seels [D]. Materials(1) The solidification process of AISI 304MHCNM H G:181.stainless steel during cooling at a rate of 0.05 K/s [4] LiangGF, WangCQ, WuJC, et al. In Situ Observation ofwas observed in situ using a confocal laser scanningGrowth Behavior and Morphology of Delta-Ferrite as FunctionJournal of Iron and Steel Research, InternationalVol 15of Solidification Rate in an AISI 304 Stainless Steel [J]. 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