Rapid solidification of martensitic stainless steel atomized droplets
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INTRODUCTION
RAPID solidification is usually accompanied by undercooling of the molten phase, which enables an unusual microstructure to be achieved. The reduction of the particle size not only leads to an increase in the cooling rate, but it also decreases the probability for nucleation, which decreases with decreasing volume, i.e., powder particle diameter.[1,2,3] The result is that the degree of the undercooling increases with decreasing particle size and is not solely dependant on the cooling rate. Undercooling prior to the nucleation can result in one or more of the following changes:[4] (1) a refined structure; (2) extension of terminal solid solubility in the primary phase; (3) a morphology change of eutectic and/or primary phases; and (4) the formation of metastable phases. Extensive studies on the relative amounts of fcc and bcc phases in rapidly solidified types 304, 303, 316, and 308 austenitic stainless steel powders have been reported.[5–8] The results revealed that the fraction of the metastable bcc phase increases with decreasing powder particle size. The stable fcc phase is found usually in the larger particles. The structure difference is a result of different levels of undercooling. The same effect of the undercooling prior to nucleation is found in other alloys such as Fe-C, Ni-Cr, and Fe-Cr-Ni alloys.[9,10,11] One of the methods for producing rapidly solidified alloys is the atomization process, in which the molten metal is disintegrated by a jet of water or gas to form particles.[12] Particles produced from the liquid metal undergo a high cooling rate, in the range of 102 to 105 K s⫺1, as well as high undercooling, depending on the powder particle diameter and the process parameters used.[13] The present work is aimed at studying and characterizing the microstructure of 12Cr-Mo-V steel produced by gas atomization. Under equilibrium conditions, 12Cr-Mo-V steel solidifies as primary ferrite. Gas atomization gives a range of powder particle sizes, and this implies a range of cooling rates. In general, it is accepted that with decreasing particle N.H. PRYDS and A.S. PEDERSEN, Senior Scientist, are with the Materials Research Department, Riso National Laboratory, DK-4000 Roskilde, Denmark. Contact e-mail: [email protected] Manuscript submitted March 18, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
size, the cooling rate increases. Since the structure of the material is sensitive to undercooling and/or cooling rate, it is clear why a spectrum of microstructures is present in the powder particles. Therefore, an investigation of the microstructure was conducted in order to observe the effects of undercooling and cooling rate in different powder particle sizes. The following methods to characterize the powder particles have been used: scanning electron microscopy (SEM), optical microscopy (OM) equipped with software for image analysis, and X-ray diffraction (XRD).
II. EXPERIMENTAL PROCEDURE Commercial 12Cr-Mo-V martensitic stainless steel, with the composition given in Table I, was used as a bas
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