Masing Behavior and Microstructural Change of Quenched and Tempered High-Strength Steel Under Low Cycle Fatigue

Low cycle fatigue behavior of a quenched and tempered high-strength steel(Q960 E) was studied in the strain amplitude ranging from ± 0.5% to ± 1.2% at room temperature. As a result of fatigue loading, the dislocation structural evolution and fracture mechanism were examined and studied by transmission electron microscopy and scanning electron microscopy(SEM). The results showed that this Q960 E steel showed cyclic softening at different strain amplitudes, and the softening tendency was more apparent at strain amplitude of ±(0.6–1.2)% than that at ± 0.5%. The reduction in dislocation density with increasing strain amplitude is responsible for the softening tendency of cyclic stress with the strain amplitude. The material illustrates near-Masing behavior at strain amplitude ranging from ± 0.6% to ± 1.2%. The near-Masing behavior of Q960 E high-strength steel can be the result of stability of martensite lath at different strain amplitudes. Partial transformation from martensite laths to dislocation cells is responsible for the derivation from ideal Masing behavior. In the SEM examination of fracture surfaces, transgranular cracks initiate on the sample surface. Striations can be found during the crack propagation stage.

Influence of Finishing Cooling Temperature and Holding Time on Nanometer-size Carbide of Nb-Ti Microalloyed Steel

The nanometer-size carbides formed in ferrite matrix of Nb-Ti microalloyed steel at different finishing cooling temperatures and holding time have been investigated. The characteristics of nanometer-size carbides in ferrite were observed by transmission electron microscopy, and mechanical properties of ferrite were detected by a nano-hardness tester. The results showed that interphase precipitation and diffusion precipitation were observed at different finishing cooling temperatures, and the interphase precipitation was planar and curved. Sheet spacing of inter-phase precipitation increased with the increase of finishing cooling temperature and changed a little when holding for 50--1000 s. Interphase precipitation shows higher nano-hardness at 640℃ compared with diffusion precipitation at 600℃, and the contribution of interphase precipitation to the mechanical properties of ferrite was larger than that of diffusion precipitation.

Microstructure-mechanical property relationship in Ti-Mo microalloyed steel with random and interphase precipitates

The relationship between microstructure and mechanical properties of Ti-Mo microalloyed steels composed of ferrite and bainite with nanometer-sized carbides and isothermally transformed at different temperatures and time was systematically investigated by tensile test, hardness test, and transmission electron microscopy. Ferrite formed at high temperatures exhibited both planar/curved sheet-like dispersions of interphase precipitates and random dispersion precipitates, with the interphase precipitates being the dominant morphology. In contrast, bainite formed at low temperatures exhibited only random dispersion precipitates. Furthermore, random dispersion precipitates and interphase precipitates were observed within the same ferrite grains. The mechanical properties of the ferrite specimen were superior to those of the bainite specimen. The stress-strain curves of both specimens indicated continuous yielding, high strength, and sufficient tensile elongation. The strengthening of the ferrite specimen was attributed to grain size strengthening, dislocation strengthening, and precipitation hardening, and the degree of precipitation strengthening was approximately 300 MPa.