Application of Molecular Dynamics Calculations to Elucidation of the Mechanism of Hydrogen-Induced Crack Initiation in Fracture Toughness Tests Using Tempered Martensitic Steels

• Published in: ISIJ International Vol. 62; no. 10; pp. 2107 - 2117
• Main Author: Matsubara, Kazuki
• Format: Journal Article
• Language: English
• Published: The Iron and Steel Institute of Japan 15.10.2022
• Subjects: fracture toughness; grain boundary; hydrogen embrittlement; martensite steel; molecular dynamics calculations
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.ISIJINT-2022-153

It is well known that the presence of hydrogen causes deterioration of the mechanical properties of steel, which appears in the forms of reduced fracture toughness, shorter fatigue life, etc., and these phenomena are recognized as hydrogen embrittlement. Here, the effect of hydrogen on crack initiation in fracture toughness tests was investigated using a combination of experimental and computational approaches. Tempered lath martensitic steel was subjected to fracture toughness tests with a monotonically rising load in air and high-pressure hydrogen gas environments. While cracking propagated continuously within grains in the air environment, cracking in the hydrogen environment grew by linking of isolated interfacial failures ahead of the main crack tip. To understand the nucleation mechanism of isolated failure in the presence of hydrogen, tensile simulations of twist grain boundaries (TGBs) rotated around the axis at various misorientation angles were conducted using molecular dynamics (MD) simulations. While dislocation emission from TGB rotated 70° is the dominant deformation mode in the absence of hydrogen, rupture along TGB rotated 110° and 170° without stress relaxation due to dislocation emission is the dominant deformation mode in the presence of hydrogen. As a consequence, it is indicated that the origin of hydrogen-induced isolated crack initiation in the vicinity of a fatigue precrack is rupture along the block boundaries within the martensitic structure due to hydrogen-induced inhibition of dislocation emission from grain boundaries (GBs).