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.

Precipitation Behavior and Microstructural Evolution of Ferritic Ti–V–Mo Complex Microalloyed Steel

Precipitation behavior of （Ti, V, Mo）C and microstructural evolution of the ferritic Ti-V-Mo complex microalloyed steel were investigated through changing coiling temperature （CT）. It is demonstrated that the strength of the Ti-V-Mo microalloyed steel can be ascribed to the combination of grain refinement hardening and precipitation hardening. The variation of hardness （from 318 to 415 HV, then to 327 HV） with CT （from 500 to 600-625 ℃, then to 700 ℃） was attributed to the changes of volume fraction and particle size of （Ti, V, Mo）C precipitates. The optimum CT was considered as 600-625 ℃, at which the maximum hardness value （415 HV） can be obtained. It was found that the atomic ratios of Ti, V and Mo in （Ti, V, Mo）C carbides were changed as the CT increased. The precipitates with the size of 〈 10 nm were the V-rich particles at higher CT of 600 and 650 ℃, while the Ti-rich particles were observed at lower CT of 500 and 550 ℃. Theoretical calculations indicated that the maximum nucleation rate of （Ti, V, Mo）C in ferrite matrix occurred around 630 ℃, which was consistent with the 625 ℃ obtained from experiment results.

Tensile Deformation Behavior of Fe-Mn-Al-C Low Density Steels

Room temperature tensile tests of Fe-Mn-A1 C low density steels with four different chemical compositions were conducted to clarify the dominant deformation mechanisms. Parameters like product of strength and elongation, as well as specific strength and curves of stress-strain relations were calculated. The microstructures and tensile fracture morphologies were observed by optical microscope, scanning electron microscope and transmission electron mi-croscope. The tensile behavior of low density steel was correlated to the microstructural evolution during plastic de formation, and the effects of elements, cooling process and heat treatment temperature on the mechanical properties of the steels were analyzed. The results show that the tensile strength of steels with different cooling modes is more than 1000 MPa. The highest tensile strength of 28Mn-12Al alloy reached 1230 MPa, with corresponding specific strength of 189.16 MPa· cm^3·g^-1 , while the specific strength of 28Mn-10Al alloy was 178.98 MPa·cm^3·g^-1 , and the excellent product of strength and elongation of 28Mn-SAl alloy was over 69.2 GPa·%. A large number of ferrite reduced the ductility and strain hardening rate of the alloy, while the existence of κ carbides may improve the strength but weaken the plasticity. Some fine κ carbides appeared in the water-quenched specimen, while coarse carbides were observed in the air-cooled specimen. High temperature heat treatment improved the decomposition ki- netics of 7 phase and the diffusion rate of carbon, thus speeded up the precipitation of fine κ carbides. The dominant deformation mechanism of low density steel was planar glide, including shear-band-induced plasticity and microband- induced plasticity.

Rietveld refinement,microstructure,mechanical properties and oxidation characteristics of Fe-28Mn-xAl-1C(x=10 and 12 wt.%)low-density steels

The quantitative relationship between microstructure and properties of austenitic Fe-28Mn-xAl-1C（x=10 and 12 wt.%）low-density steels was evaluated using Rietveld method to refine X-ray diffraction（XRD）patterns.The results showed that a typical three-phase austenitic steel was obtained in the forged Mn28Al10（i.e.Fe-28Mn-10Al-1C）steel,which included about 92.85 wt.% γ-Fe（Mn,Al,C）（austenite）,5.28 wt.%（Fe,Mn）_3AlC_（0.5）（κ-carbide）,and 1.87 wt.% α-Fe（Al,Mn）（ferrite）.For the forged Mn28Al12（i.e.Fe-28Mn-12Al-1C）steel,nevertheless,only about 76.64 wt.% austenite,9.63 wt.%κ-carbide,9.14 wt.%ferrite and 4.59 wt.% Fe_3Al（DO_3）could be obtained.Nanometerκ-carbide and DO_3 were mainly distributed in austenite grains and at the interface between austenite and ferrite,respectively.The forged Mn28Al10 steel had a better combination of strength,ductility and specific strength as compared with the forged Mn28Al12 steel.The ductility of the forged Mn28Al12 steel was far lower than that of the forged Mn28Al10 steel.The oxidation kinetics of Mn28Al10 steel oxidized at 1323 Kfor 5-25 h had two-stage linear rate laws,and the oxidation rate of the second stage was faster than that of the first stage.Although the oxidation kinetics of Mn28Al12 steel under this condition also had two-stage linear rate laws,the oxidation rate of the second stage was slower than that of the first stage.When the oxidation temperature increased to 1373K,the oxidation kinetics of the two steels at 5-25 hhad only onestage linear rate law,and the oxidation rates of the two steels were far faster than those at 1323K for5-25 h.The oxidation resistance of Mn28Al12 steel was much better than that of Mn28Al10 steel.Ferrite layer formed between the austenite matrix and the oxidation layer of the two Fe-Mn-Al-C steels oxidized at high temperature.