Improvement of the Corrosion Resistance for Martensitic Stainless Steel By Laser Thermal Processing
Authors: Manaka Tomoyo , Tsutsumi Yusuke , Goto Mitsuhiro , Kadowaki Mariko , Murase Yoshiharu , Katayama Hideki , Ishimoto Takuya , Hanawa Takao 
Published: December 2024

Introduction

Stainless steels are widely used as medical devices. Martensitic stainless steels can be hardened by quenching. Therefore, in the medical field, martensitic stainless steels are mainly used for knives and surgical instruments with cutting edges. However, carbon, a necessary additive for the hardenability of martensitic stainless steels, reduces their corrosion resistance. Therefore, it is theoretically difficult to achieve both high hardness and high corrosion resistance in stainless steels. To ensure the safety and longevity of devices made of martensitic stainless steels, improved corrosion resistance is required. In this study, an improved corrosion resistance of martensitic stainless steels was tried by laser thermal processing. Rapid heating and rapid cooling process by laser irradiation is expected to suppress the formation of corrosion-inducing (MnS) inclusions. The effects of the laser thermal processing on the corrosion resistance, microstructure, and hardness of the specimen surface and substrate layers were investigated.

Materials and methods

The specimens were commercially available type 420J2 stainless steel plates. Each plate was irradiated with a laser of 10 mm diameter at a rate of 100 cm min-1. The irradiation powers were 950 W, 1100 W, 1650 W, 2000 W, 2500 W, 3000 W, 3500 W, and 4000 W. The surface and cross-sectional metallography and inclusions of the laser-irradiated specimens were observed and analyzed using OM, SEM/EDS, and EBSD. Corrosion resistance was also evaluated by anodic polarization tests. A mixed aqueous solution of 0.1 M Na2SO4 and 0.585 M NaCl at 297 ± 3 K was used as the test solution. A Pt plate and a Ag/AgCl (saturated KCl aqueous solution) electrode were used as the counter and reference electrodes, respectively. The potential sweep rate was 20 mV min-1.

Results and discussion

The specimens surface after the laser thermal process were observed. The surfaces of the specimens processed at 950 W or 1100 W were smooth, while those processed at 1650 W or higher had a wavy shape. The specimens with exposed cross sections were chemically etched. A light-colored area on the surface layer, and a dark-colored area around and inside the surface layer were formed. The light-colored area is the layer whose microstructure has been changed by laser irradiation. In addition, the light-colored area of the specimens processed at 1650W or higher were divided into two parts. Defined as phase A (upper part of light-colored area), phase B (lower part of light- colored area), and phase C (dark-colored area), in order from the surface. The main phase of the unprocessed specimen is ferrite with a small amount of residual austenite. EBSD and XRD results indicated that the main phase of phase A is martensite with some austenite phase. Additionally, Vickers hardness of phase A significantly increased compared to the unprocessed specimen. This result confirms that phase A is a martensitic phase, indicating that rapid heating and rapid cooling by laser irradiation caused the martensitic phase to form. Corrosion resistance of phase A layer was better than unprocessed specimen. The pitting potential of the phase A layer increased with irradiation power, with the highest value at 3500 W. Some cracks appeared on the surface of the specimen processed at 4000W. These cracks are the reason for the lower corrosion resistance. Inclusions consisting of spherical Si oxides smaller than 1 µm were observed in the microstructure of the phase A layer; no MnS inclusions were observed. The presence of fine spherical Si inclusions and the wavy surface suggest that phase A was formed by melting and then rapidly solidifying due to laser irradiation. MnS was solidly soluble in the matrix phase by melting, and its formation was inhibited by rapid solidification. As a result, MnS inclusions in phase A were reduced and the corrosion resistance of phase A was improved. On the other hand, phase C showed similar hardness and corrosion resistance to the unprocessed specimen, and there was no difference in corrosion resistance depending on irradiation power. Thus, although phase C was heated by the laser, no melting or microstructural transformation occurred. In other words, laser thermal processing does not degrade the properties of the inner layer. The corrosion resistance, hardness, and microstructure of phase B were intermediate between those of phases A and C.

Conclusion

420J2 stainless steel was subjected to laser thermal process at various power to evaluate microstructure and corrosion resistance. Laser thermal process suppressed the formation of corrosion-induced inclusions in the surface layer, and the surface layer transformed to martensitic structure. Laser thermal processing succeeded in modifying the surface to have both high corrosion resistance and hardness.

Figure 1