Fundamental Aspects of Functional Grading via Powder Hot Isostatic Pressing - Development of microstructure and diffusional processes

Functional grading of a ferritic steel (SA508 grade 3) and austenitic stainless steel (316L)
powder via hot isostatic pressing (HIP) consolidation is investigated as an alternative to
dissimilar welding. Fundamental aspects of this method such as the development of
microstructure and diffusional processes during the HIP process of such mixed powder are
studied in this work. A change of the post-HIP α to γ ratio with HIP temperature was observed
in mixed powder specimens, combined with the formation of a ferritic lath zone which affects
the properties of the specimens (throughout this work the term ferritic laths refers to a ferritic
phase of unknown structure which is likely to consist of martensite, bainite or a mixture of
both). These effects were attributed to extensive diffusion of alloying elements from 316L into
SA508 grade 3. Simulation and experimental quantification of diffusion in diffusion couple
experiments were applied to derive an empirical model for estimating α-Fe boundary migration and the formation of the ferritic lath zone and phase fractions for various temperatures and dwell times of the HIP process.

Keywords
Hot isostatic pressing, Functional grading, SA508, 316L