Laser Powder Bed Fusion and Heat Treatment of the Martensitic Age-Hardenable Steel (1.2709)

The primary objective of this study is to clarify the fundamental question of whether, in principle, it is possible to dispense with a prior solution annealing process in favor of a direct aging heat treatment for specimens of maraging stainless steel grade X3NiCoMoTi18-9-5 (1.2709) produced by laser powder bed fusion (LPBF). The waiver of a solution annealing process would significantly increase the process efficiency and thus support a sustainable and resource-friendly production of such components. Therefore, the hardness, microstructure, and the present phases of specimens in as-built + aged condition (AB + A) and solution-annealed + aged (SOL + A) are examined during this study. Initially, an extended parameter study is performed using a Renishaw AM 250 LPBF system equipped with a pulsed mode laser system to achieve the highest possible apparent density. As test specimens, small cubes are produced for parameter study and are analyzed for porosity by means of optical microscopy. To investigate the relationship between microstructure and hardness in different material states, one series of specimens is aged directly after LPBF processing in the as-built state (AB + A). For comparison, the other series was solution annealed at 820 degrees C for 60 min, quenched in water and then aged (SOL + A). A maximum hardness value of 614 HV1.0 is achieved for specimen aged at 490 degrees C for 120 min in as built condition (AB + A), while 624 HV1.0 was measured for specimen aged at 490 degrees C for 180 min in conventionally solution annealed + aged (SOL + A) condition. Significant austenite reversion is not observed at aging temperature of 490 degrees C in both cases. Aging of specimens at temperatures of 540 and 600 degrees C resulted in reduction of specimen hardness due to higher percentage of austenite reversion. No significant difference between the hardness values of AB + A and SOL + A specimens is observed. It can therefore be concluded that, in principle, conventional solution annealing and ageing can be dispensed with in favor of direct aging. However, as the results are based on small sized specimens, further investigations into the scalability are needed. The rapid solidification and intrinsic cyclic reheating during laser powder bed fusion (LPBF) result in precipitation during the LPBF process which, in the cast steels, is expected after solution annealing followed by quenching and aging. The waiver of solution annealing for maraging steel specimens produced via LPBF manufacturing can increase the process efficiency and thus support a sustainable and resource-friendly production of such components.image (c) 2024 WILEY-VCH GmbH