Carburized gear steel has a high-hardness case layer with excellent wear and fatigue resistance and a low-hardness core with high toughness. Such different microstructures imply different susceptibilities to hydrogen embrittlement (HE). However, a few or no studies have explored the HE behavior of carburized gear steel. Herein, the HE behavior of a vacuum-carburized gear steel 20Cr2Ni4A was investigated via an electrochemical hydrogen-charging and slow strain rate tensile test. For comparison, another group of specimens was prepared by a conventional quenched and tempered (QT) treatment. The volume fraction of retained austenite was significantly higher in the case layer of the carburized specimen (13.8%) than in the core and the QT specimen (4.6%). The retained austenite in the case layer showed a mainly irregular block-type morphology with wide size distribution. The room-temperature diffusible hydrogen content in the hydrogen-charged carburized specimen were almost identical to the QT specimen but the nondiffusible hydrogen content was significantly higher in the former than in the latter. Meanwhile, the hydrogen diffusion coefficient was notably lower in the hydrogen-charged carburized specimen than that in the QT sepcimen because the former retained higher fractions of austenite and cementite. The QT specimen exhibited superior strength and ductility. After hydrogen charging, the strength of the QT specimen remained almost unchanged but the total elongation notably decreased, causing the HE index (HEI), as evidenced using the relative total elongation loss, being 54.3%. Relative to the QT specimen, the carburized specimen achieved a higher tensile strength (increase by 34.6%) but a much lower ductility (total elongation and reduction of area reductions by 66.5% and 92.4%, respectively). The carburized specimen underwent premature brittle fracture before yielding, indicating susceptibility to HE. In fact, the HEI was as high as 90.9%. Mixed intergranular and quasi-cleavage fractures were observed in the surface embrittled region of the hydrogen-charged QT specimen. This region roughly corresponded to the maximum hydrogen diffusion distance. Meanwhile, the hydrogen-charged carburized specimen exhibited an embrittled internal-surface region with a certain width of intergranular fracture, and a long crack had propagated along the circumferential direction near the effective case depth. The microstructure, strength level, and residual stress are thought to mainly explain the abovementioned differences between the carburized and QT specimens.
Fig.1 Schematics of the heat treatment cycles for non-carburized specimens (a) and carburized specimens (b) (OQ—oil quenching, AC—air cooling, Cp—carbon potential)
Fig.2 OM images of the quenched and tempered (QT) specimen (a), the case (b) and the core (c) of the carburized specimen, and the XRD spectra of the carburized and QT specimens (d)
Fig.3 EBSD maps of the case (a) and the core (b) of the carburized specimen, and the QT specimen (c), and the retained austenite size distributions of the carburized and QT specimens (d)
Fig.4 Vickers hardness distributions along the transverse cross-section of the carburized and QT specimens (a), and distribution of residual stress along the cross-section of the carburized specimen (b)
Fig.5 Variations of hydrogen content (CH) with different air exposing time (t) of the QT (a) and the carburized (b) specimens after hydrogen-charging
Fig.6 Slow strain rate tensile (SSRT) curves of QT (a) and carburized (b) specimens before and after hydrogen-charging
Specimen
Condition
σ / MPa
A / %
Z / %
HEI / %
QT
Uncharged
1462
16.4
62.2
54.3
Hydrogen-charged
1466
7.5
33.0
Carburized
Uncharged
1968
5.5
4.7
90.9
Hydrogen-charged
812
0.5
0.0
Table 1 Summaries of the SSRT results of the QT and carburized specimens
Fig.7 Low (a, c) and high (b, d) magnified SEM images showing representative facture surfaces of the QT specimen before (a, b) and after (c, d) hydrogen-charging
Fig.8 SEM images showing the fracture surfaces of the carburized specimen without hydrogen-charging
Fig.9 SEM images showing the fracture surface of the carburized specimen after hydrogen-charging
Fig.10 EBSD map (a) and the retained austenite (RA) size distribution (b) of the case of the carburized specimen after SSRT
Fig.11 Simulation of stress distribution of the carburized specimen under tensile testing
Fig.12 Schematics of the hydrogen embrittlement (HE) mechanism of the carburized specimen (A—maximum diffusion distance, B—crack, C—interface between the case layer and the core matrix)
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