Microstructures of Laser Deposited 304L Austenitic Stainless Steel
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ABSTRACT Laser deposits fabricated from two different compositions of 304L stainless steel powder were characterized to determine the nature of the solidification and solid state transformations. One of the goals of this work was to determine to what extent novel microstructures consisting of single-phase austenite could be achieved with the thermal conditions of the LENS process. Although ferrite-free deposits were not obtained, structures with very low ferrite content were achieved. It appeared that, with slight changes in alloy composition, this goal could be met via two different solidification and transformation mechanisms. INTRODUCTION Near net shape fabrication using the LENS (Laser Engineered Net Shape) process has matured to the point that complex three-dimensional shapes can be reliably processed from a CAD file. However, relatively little work has been conducted to determine to what extent the processing conditions, especially the relatively rapid cooling rates, can be utilized to produce unique or novel microstructures. The austenitic stainless steels are often used for their elevated temperature and corrosion properties, and sometimes for their non-ferromagnetic characteristics. However, in cast or welded materials the properties can be degraded for a number of reasons. These materials often exhibit a two-phase structure of austenite with several percent ferrite (0-10%) and in the absence of ferrite, exhibit a considerable amount of microsegregation. At elevated service temperature the ferrite can transform to sigma phase greatly reducing toughness. Ferrite can also affect corrosion behavior and its magnetic characteristics can limit the use of cast material in critical applications. In the absence of ferrite, the high degree of segregation can also degrade corrosion resistance. However, it must be noted that cast, welded and wrought materials containing several percent ferrite are acceptable for many engineering applications. A large amount of work has been conducted on high-energy-density (HED) welding of the 300 series stainless steels and this work has shown that alloy composition can have a large effect on weld structure and the development of unique microstructures [1-9]. In this study, the ability to develop unique structures, i.e. fully austenitic, in LENS built material of 304L alloy was investigated by varying: (1) the Creq/Nieq ratio of the starting powders within the commercial composition range, (2) the solidification velocity via the travel speed of deposition, and (3) the cooling rate via block build (faster) and shell build (slower) deposition geometries. MATERIALS Thermodynamic programs can be used both to predict the alloy phase diagram and to provide input into solidification models for predicting solidification behavior and microstructures [5,10,11,12]. Using these concepts, two alloys were designed with different Creq/Nieq ratios [13] within the commercial 304L composition range. The heat compositions and Cr~q/iNiq ratios are given in Table I. The specific Creq/Nieq ratios were cho
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