How Austenitic Is a Martensitic Steel Produced by Laser Powder Bed Fusion? A Cautionary Tale

Abstract

Accurate phase fraction analysis is an essential element of the microstructural characterization of alloys and often serves as a basis to quantify effects such as heat treatment or mechanical deformation. Additive manufacturing (AM) of metals, due to the intrinsic nonequilibrium solidification and spatial variability, creates additional challenges for the proper quantification of phase fraction. Such challenges are exacerbated when the alloy itself is prone to deformation-induced phase transformation. Using commonly available in-house X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) and less commonly used synchrotron-based high-energy X-ray diffraction, we characterized nitrogen-atomized 17-4 precipitation-hardening martensitic stainless steel, a class of AM alloy that has received broad attention within the AM research community. On the same build, our measurements recovered the entire range of reported values on the austenite phase fractions of as-built AM 17-4 in literature, from ≈100% martensite to ≈100% austenite. Aided by Calphad simulation, our experimental findings established that our as-built AM 17-4 is almost fully austenitic and that in-house XRD and EBSD measurements are subject to significant uncertainties created by the specimen’s surface finish. Hence, measurements made using these techniques must be understood in their correct context. Our results carry significant implications, not only to AM 17-4 but also to AM alloys that are susceptible to deformation-induced structure transformation and suggest that characterizations with less accessible but bulk sensitive techniques such as synchrotron-based high energy X-ray diffraction or neutron diffraction may be required for proper understanding of these materials.

Microstructural Control of a Multi‐Phase PH Steel Printed with Laser Powder Bed Fusion

“…[ 40–43 ] However, various studies on 17‐4PH steel printed by LBPF present very different and more complex microstructures, encompassing various fractions of martensite, austenite, and ferrite, depending on laser speed, power, printing strategy, and shielding gas. [ 44–58 ] Most recently, Haines et al. observed austenite, ferrite, and martensite all present in 17‐4PH, [ 59 ] whereas An et al found mostly ferrite with little martensite.…”

The Effect of Varying Parameters of Laser Surface Alloying Post-Treatment on the Microstructure and Hardness of Additively Manufactured 17-4PH Stainless Steel

“…The 17-4PH grade currently seems to be the most studied precipitation-hardening stainless steel used in additive manufacturing [25], in which the microstructure and properties in the additively manufactured state have been thoroughly investigated in the works published in the last few years in relation to different technologies, primarily selective laser melting [26][27][28][29] and material extrusion [30]. In addition to this obviously increased interest in post-treatments, focusing primarily on surface quality improvement should be also mentioned.…”