The Effect of Process Parameters and Post-fabrication Heat Treatment on the Microstructure and Mechanical Properties of Martensitic Steel SS420 Developed through Laser Powder Bed Fusion Technique

In this study, stainless steel grade 420 (SS420) martensitic steel coupons were developed through a laser powder bed fusion (L-BBF) process with the same laser heat input achieved through varying the two primary process parameters, i.e. laser power and scanning speed, to correlate the effects of these two primary parameters on the microstructures vis-à-vis mechanical properties under the condition of the same laser heat input. To understand the subsequent effect of the formed microstructures during heat treatment, the coupons were further subjected to a two-step heat treatment process, different from the heat treatment process applied for the conventionally developed SS420 material. The microstructural evolution in building direction (z) and x-y plane was compared. The designed heat treatment schedule showed a better combination of strength and ductility than that of their as-built components. Moreover, the sample produced through the control of laser heat input showed the best combination of strength and toughness response, which is attributed to a combination microstructure consisting of a few retained austenite islands and a finer cell structure decorated with fine Cr-rich carbides on the tempered martensitic matrix. These results demonstrate the way of developing a superior grade of additively manufactured SS420 material through proper control of the primary processing parameters.