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 steel with an as-deposited specimen (AD) and after three types of heat-treated specimens (BM, TBMM, and TM) was investigated. At least three types of different hydrogen traps were identified in each microstructure of LAM AerMet100 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 ~21.4 kJ/mol for main reversible hydrogen traps in wrought AerMet100 steel, the less Ed value in LAM AerMet100 steel is closely related to the composition change of M2C carbides. In all of the H pre-charged samples, the diffusible and total H concentration of the TM specimen and the TBMM specimen are about 3-4 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 LAM AerMet100 steel is on the order of 10-9 cm2/s, and decreases with increasing density of dominantly reversible H traps brought about by heat treatment. Furthermore, compared with wrought AerMet100 steel of a similar yield strength (~1750 MPa), the LAM AerMet100 steel has a comparable Deff of about 2.8×10-9 cm2/s.