Hydrogen Diffusion and Trapping in Laser Additively Manufactured Ultra-High Strength AerMet100 Steel

Abstract

Hydrogen trapping and the permeation behavior of laser additively manufactured (LAM) AerMet100 (UNS K92580) steel with an as-deposited specimen (AD) and after three types of heat-treated specimens (bainite microstructure [BM], tempered bainite and martensite microstructure [TBMM], and tempered martensite [TM]) was investigated. At least three types of different hydrogen traps were identified in each microstructure of the LAM steel, including both reversible and irreversible H traps. For as-deposited microstructure, the main reversible H trap states are related to the precipitation of M3C carbides associated with a detrapping activation energy (Ed) of 17.3±0.2 kJ/mol. After heat treatment, the dominant reversible hydrogen trap states in the tempered martensite microstructure have a different Ed value of 19.3±0.5 kJ/mol, which is attributed to the precipitation of highly coherent M2C carbides. In comparison with the reported Ed value of approximately 21.4 kJ/mol for main reversible hydrogen traps in wrought UNS K92580 steel, the lower Ed value in the LAM steel is closely related to the composition change of M2C carbides. In all of the H precharged samples, the diffusible and total H concentration of the TM specimen and the TBMM specimen are about three to four times higher than that of the AD specimen and the BM specimen. The TM specimen with tempered martensite microstructure has the highest diffusible and total H concentration due to its high density of dominantly reversible H traps. The effective hydrogen diffusion coefficient (Deff) of the LAM steel is on the order of 10−9 cm2/s, and decreases with increasing density of the dominant reversible H traps brought about by heat treatment. The LAM steel has a comparable Deff of about 2.8 × 10−9 cm2/s compared to the wrought steel of a similar yield strength (∼1,750 MPa),