Enhancement of Corrosion Resistance and Hardness for Type 420J2 Martensitic Stainless Steel Via Laser Powder Bed Fusion Process
Authors: Tsutsumi Yusuke , Ishimoto Takuya , Manaka Tomoyo , Ueda Kyosuke , Kadowaki Mariko , Katayama Hideki , Matsugaki Aira , Nakano Takayoshi , Hanawa Takao 
Published: December 2024

In the field of materials engineering, physical properties such as mechanical properties are considered as the most important properties, however, chemical properties such as corrosion resistance are also critical properties that cannot be ignored. In particular, Japan is island country where metallic materials are frequently used in coastal areas. Therefore, the problems of failures and accidents caused by corrosion reaction of structural materials are more serious than in other countries.
Recently, laser powder bed fusion (LPBF) process which is a type of additive manufacturing (AM), has been attracting attention as an advanced processing tool for fabrication of metallic materials in various industrial fields. The direct-forming process by AM with the applicability of intricate-structured materials is strong advantage for manufacturing value-added and relatively small products such as medical implants or aerospace assemblies.
Our previous study demonstrated the excellent corrosion resistance of LPBF-processed type 316L austenitic stainless steel, focusing on its crystallographic planes and grain boundaries [1]. Therefore, in this study, we investigated the corrosion resistance of LPBF-processed type 420J2 martensitic stainless steel. Martensitic stainless steels have the lowest corrosion resistance among all kinds of stainless-steel types such as precipitation hardening, ferritic, austenitic, and duplex, because of minimum chromium content without other corrosion-resistant elements, and high content of carbon. The primary advantage of martensitic stainless steel is superior hardness, which can be achieved quenching heat treatment. In other words, corrosion resistance and hardness are conflicting properties for stainless steels, and it is difficult to realize coexistence.
In this study, the electrochemical and the non-electrochemical corrosion tests were performed to evaluate the corrosion behavior of LPBF-processed and commercial 420J2 stainless steels. The microstructural characterization and hardness tests were also performed.
Commercial 420J2 stainless steel powder was used as the primary material for specimen fabrication by LPBF in this study. The nominal composition of 420J2 stainless steel was Fe-12Cr-0.3C. The LPBF process was performed in an argon atmosphere using a 3D printer. To examine the evolved texture and planes of interest of the specimens, the z-axis was defined as the build direction, and the x- and y-axes were defined as the laser scanning directions. In this study, we fabricated cubic specimens with dimensions of 11 mm × 11 mm × 11 mm. The specimens were cut mechanically to expose their yz-, xz-, and xy-planes. No post-processing was performed on the LPBF specimens.
Anodic polarization measurement (linear sweep voltammetry) was performed using a potentiostat (HABF-501G, Hokuto Denko, Japan) connected to a function generator (HB-111, Hokuto Denko, Japan) with an analog cable. A saturated calomel electrode (SCE) and platinum electrode were used as the reference and counter electrodes, respectively. The specimens were fixed in a polytetrafluoroethylene holder with an O-ring. The exposed area contacting the electrolyte was 0.35 cm2 (6.7 mm in diameter). After immersing the specimens in a simulated body fluid (physiological saline: 0.9 mass% NaCl aqueous solution, aerated) at 310K, their open circuit potentials (OCPs) were recorded for 10 min. Then, a gradient anodic potential was applied at a constant sweep rate of 1 mVs-1 from the initial potential of −50 mV from the OCP. The measurement was stopped when the current density limit of 1 mAcm-2 was recorded.
Figure shows the polarization curves of LPBF-processed and commercial 420J2 stainless steel specimens in physiological saline. The pitting potentials of LPBF specimens were significantly higher than those of commercial specimens. These experimental results indicate that localized corrosion resistance was effectively improved as like 316L austenitic stainless steel in the previous study [1]. The results of the hardness test showed that LPBF specimens was 53.50 ± 0.25 HRC, regardless of the measuring position. The hardness of the commercial specimens varied from 54 to 27 HRC depending on the tempering treatment conditions after the primal quenching.
Thus, LPBF is found to be an ideal process that can simultaneously enhance corrosion resistance and hardness of martensitic stainless steels. The experimental results of another corrosion resistance evaluation and inclusion extraction will be presented in the session.
Reference:
[1] Tsutsumi Y et al. Additive Manufacturing 45 (2021) 102066.

Figure 1