Impact of retained austenite on the aging response of additively manufactured 17-4 PH grade stainless steel

Abstract

Additively manufactured precipitation hardened (PH) martensitic grade stainless steels display wide variations in their heat treatment response and mechanical properties. Much of this variability can be attributed to the selection of the atomizing gas and the resulting variations in powder composition. Differences in nitrogen compositions present in argon and nitrogen atomized powders led to the formation of a wide range of retained austenite levels between 0 and 81 vol% and significant differences in mechanical properties in as-deposited and aged 17-4 PH grade stainless steels. Material fabricated using argon atomized powder contained little to no retained austenite and displayed mechanical properties in the peak-aged condition similar to those in wrought alloys (1359 ± 59 MPa ultimate tensile strength and 10.1 ± 0.8% elongation to failure). On the other hand, materials fabricated with nitrogen atomized powder displayed between 20% and 81% retained austenite, leading to discontinuous yielding driven by the strain-induced transformation of the retained austenite to martensite. Even with the presence of this discontinuous yielding, the materials fabricated using the nitrogen atomized powders consistently displayed strengths 300 MPa higher in the as-deposited, 200 MPa higher in the peak-aged, and 500 MPa higher in the over-aged conditions than materials fabricated using the argon atomized powders. By directly comparing the local strains and microstructures in nitrogen atomized materials, a [011]γ//[001]α relationship in line with the Bain transformation process was present between the retained austenite and transformed martensite across the as-deposited and aged conditions.