Hydrogen Embrittlement and Local Characterization at Crack Initiation Associated with Phase Transformation of High-strength Steel Containing Retained Austenite

Abstract

States of hydrogen present in high-strength steels for use as bearing steel SUJ2 and hydrogen embrittlement susceptibility were examined using thermal desorption analysis (TDA) and tensile tests. SUJ2 specimens containing the retained austenite phase (γ R ) in the martensitic phase exhibited three hydrogen desorption peaks in the TDA profile. Two of the peaks desorbed at higher temperatures decreased with a decreasing amount of γ R , indicating they corresponded to desorption associated with γ R . Fracture strength in the presence of hydrogen increased with a decreasing amount of γ R and with an increasing strain rate. For the specimens containing γ R and hydrogen, a flat facet at the crack initiation site and a quasi-cleavage (QC) fracture in the initial crack propagation area were observed on the fracture surface. Local characterization using electron back-scattered diffraction (EBSD) revealed that the flat facet on the fracture surface corresponded not to γ R but to strain-induced martensite. In addition, the facet was on the {112} plane of martensite, which is the slip plane or deformation twin plane of body-centered-cubic metals. The reason for high hydrogen embrittlement susceptibility of the specimens containing γ R was attributed to the straininduced phase transformation at the crack initiation site of the flat facet and in the initial crack propagation area of the QC fracture. Furthermore, the strain rate dependency of hydrogen embrittlement susceptibility is presumably ascribable to local plastic deformation, i.e., the interaction between dislocations and hydrogen.

Hydrogen Desorption Behavior Trapped in Various Microstructures of High-Strength Steels Using Thermal Desorption Analysis

“…pct retained austenite, hydrogen desorption behavior in a TDA measurement was affected by retained austenite present in a film shape with a maximum length of about 3 lm on the prior austenite grain boundary. [18] These previous reports on high-carbon martensitic steel with retained austenite suggest that film and/or plate-like retained austenite ranging from sub-lm to hundreds of lm in size has hydrogen trapping capability. In the present study, HC-M presumably contained similar retained austenite in terms of morphology, size, and remaining area within the martensitic microstructure.…”

“…As some researchers have reported, retained austenite and/or interfaces between retained austenite and the matrix can serve as hydrogen trapping sites in C-Mn-Fe alloys. [16,18,46] Tsubakino et al reported that the effective hydrogen diffusion coefficient (D H ) of two types of steels, namely, tempered martensite steel and steel with retained austenite of 26.3 vol. pct, having the same tensile strength of 1200 MPa and shaped as a plate from 20 to 100 lm in thickness, were D H % 1.5 9 10 À10 m 2 /s and D H % 1.3 9 10 À11 m 2 /s, respectively.…”

“…It has been reported that the rates of hydrogen diffusivity for steels containing retained austenite with the fcc crystal structure affected their hydrogen desorption profiles irrespective of the hydrogen trapping strength because the diffusion rate was lower than that of a single phase steel with the bcc crystal structure. [16][17][18] In view of these previous reports, when analyzing hydrogen desorption curves obtained by TDA, it is difficult to identify the state of hydrogen present (i.e., diffusible or non-diffusible) in different microstructures as well as to detect hydrogen trapping sites at the atomic scale. For judging whether the hydrogen trapping state in steels is detrimental or innocuous to the susceptibility to delayed fracture, it is necessary to comprehensively consider the results for not only the hydrogen desorption temperature but also the hydrogen release behavior at room temperature and activation energy of hydrogen desorption (E a ).…”

Effect of Rolling and Alloying Elements on the Impact Properties of Hypereutectoid Steels

“…Hypereutectoid steels possess high hardness without requiring complicated surface treatments such as carburization. For example, as a typical hypereutectoid steel, JIS-SUJ2 steel with high C and Cr contents (i.e., 0.95-1.10C and 1.30-1.60Cr), is practically used as a bearing steel with a martensitic microstructure as the matrix and globular carbide particles 1,2) . However, the metals demonstrate a trade-off relationship, i.e., hypereutectoid steels have high hardness but severe brittleness.…”

Effects of Cementite Particles on Impact Properties in High-hardness Hypereutectoid Steels

“…As a typical hypereutectoid steel, JIS-SUJ2 steel with high C and Cr contents is widely known as a bearing steel with a martensitic microstructure as the matrix and globular carbide particles. 3,4) However, as is generally known in the field of metallic materials, there is a trade-off between strength and ductility, meaning that as strength increases, ductility tends to decrease. Accordingly, hypereutectoid steels are characterized by high hardness and brittleness.…”