| Quenching-partitioning-temepring(Q-P-T) steel exhibits excellent comprehensive mechanical property. The ultimate tensile strength and elongation of Fe-0.38C-1.44Mn-1.52Si-0.61Cr-0.048 Nb steel are1628 MPa and 22.98% respectively after Q-P-T treatment, therefore the product of strength and elongation(PSE) reaches 37411MPa%. However,the sensitivity of the hydrogen embrittlement(HE) shows the positive dependence of strength, which results in the deterioration of mechanical property, especially the plasticity, significantly, lowering service life with unpredictable failure. Therefore, it is of great importance to understand the mechanism of HE in Q-P-T steel. In this work, this steel was selected to understand the relationship between HE feature and microstructural difference after Q-P-T and traditional quenching-tempering(Q-T)treatment, then the possible improvement process was proposed. The mechanical property after electrolytic hydrogen charging was measured by slow strain rate tensile(SSRT) test, and the microstructure and fracture surface were observed by optical microscopy(OP), scanning electron microscopy(SEM) and transmission electron microscopy(TEM). X-ray diffraction(XRD) was also applied to the possible phases in the steel. The main achievements are concluded as follows:1. The ultimate tensile strength of Q-P-T and Q-T specimens are 1628 and 1963MPa respectively because of different retained austenite concent.Meanwhile, the corresponded elongation are 22.98% and 8.03%. Therefore,Q-P-T steel exhibits good combination of strength and plasticity.2. SSRT curve shows plasticity of both Q-P-T and Q-T specimens decreases continuously with the increase of charging time. Elongation Loss(Iδ) of Q-P-T and Q-T steel reach 20%, 62.5% respectively after charging for 1 minute; The values become as 34.3%, 82.5% respectivelyafter charging for 2 minutes. 10 minutes later, it was 84.8% as well for Q-P-T specimen. Microstructural observation shows the fracture feature changes from ductile to brittle as the charging time extended. Due to the existence of considerable retained austenite, Q-P-T steel shows better HE resistance for its low strength level.3. After refinement of microstructre by cold rolling and subsequent heat treatment, the HE resistance of both Q-P-T and Q-T specimens becomes worse in the present experiments, as the Iδ of refined Q-P-T and Q-T specimens reach 89.7% and 89.3% respectively even only after charging for 1 minute. The corresponded fracture mode is typical cleavage.This incredible results may be attributed to the higher dislocation density in microstructure and more diffusion path of hydrogen by refinement. Of course, the detailed investigation is further required to verify this results.4. The stacking fault probability of retained austenite in Q-P-T steel has no relationship with hydrogen content by XRD measurement.Therefore, the stability of retained austenite was not changed so much during charging, and induced hcp( ?) was not formed at all. Retained austenite may not play the key role for HE effect.5. When there are some amount of proeutectoid ferrite in the specimen by critical annealing, the strength of the Q-P-T steel decreases a little. However, the HE resistance was improved significantly. Even after charging for 100 minutes, Iδ is only 47.1%. The possible reason is that ferrite provides additional plasticity deformation capability and blunts the tip of cracks. The low HE susceptibility of the specimen is expectable by retarding crack propagation. |
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