Mingyue Sun

Characterizations of Dynamic Strain-induced Transformation in Low Carbon Steel

Dynamic strain-induced transformation of the low carbon steel Q235 at 770 ‐ C and 850 ‐ C leads to flne ferrite grains. The microstructure characterization and mechanism of the flne ferrite grain were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscattered difiraction (EBSD) technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries (HAGBs) are dominant in both of the deformation induced microstructures occurring below and above Ae3, with only a few low angle grain boundaries (LAGBs) existing in the grain interiors. It implies that the dynamic strain-induced transformation (DSIT) happens above and below Ae3 temperature and has the same phase transition mechanisms. The reflnement of ferrite is the cooperative efiect of DSIT and continuous dynamic recrystallization (CDRX) of ferrite. Besides, DSIT is deemed as an incomplete carbon difiusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q235 steel with reflned ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the reflned structure is partly responsible for the ductility loss.

The Effect of Quenching Media on the Microstructure and Mechanical Properties of SA508-3 Steel

Three different quenching media (water, oil and air) were used to compare the effect of cooling rate on the microstructure and mechanical properties of SA508-3 steel. The result has demonstrated that the microstructure for water-quenched specimen is the mixture of martensite and lower bainite, for oil-quenched specimen is the mixture of upper bainite，lower bainite and a little martensite, while for air-cooled specimen is mostly granular bainite. The product of water and oil Q&T was tempered martensite with qualified mechanical properties. The air-cooled granular bainite was translated to massive and allotriomorphic ferrite during tempering, which had poor mechanical properties. To avoid the formation of granular bainite, it is necessary to increase the cooling rate to above 5°C/s.

The Influence of Composition Segregation to the Microstructure and Mechanical Properties of SA508-3 Steel

A large scale ingot was dissected to study the segregation law of the elements. The influence of composition segregation to the microstructure and mechanical properties of SA508-3 steel was studied by the comparison of three positions on the ingot. Two precipitated phases were approved to be alloyed cementite and molybdenum carbide (Mo2C). It has demonstrated that the middle and upper parts of the ingot were almost the same in compositions except for a slightly difference in the carbon content. The upper part with more carbon included has relatively more carbide precipitation after the performance heat treatment. The bottom part of the ingot has the lowest carbon and molybdenum content, while the reduction in the amount of precipitated carbide was not observed. On the contrary, fine needle-like Mo2C are extensively distributed in the matrix of the bottom part besides a variety of coarse cementite rods. And the data of Energy Dispersion Spectrum (EDS) mapping has suggested that precipitation of Mo2C tends to bring the segregation of impurities. Many large inclusions were found in the bottom part of the ingot, which were considered to be the main reason for the strength loss of this area.

Optimization of Bending Process of Large Marine Crankthrow by Computer Simulation

The conventional method of bending the large crankthrow was investigated by computer simulation combined with manufacturing trial, and the typical forging defects, such as constricted waist, folded cracks and horn mouth on forged blank were analyzed. On the basis of these results, a novel forging shape of preformed blank was proposed using anti-transformation method by computer simulation. The FEM simulated results show that all the above defects can be avoided by carrying out the novel method, furthermore, the maximum resistance of the novel bending deformation was reduced to 72% of conventional process, and the weight of forging blank can be decreased by 15%. Finally, the optimum forging shape was applied to the actual process, the simulation results were confirmed by manufacturing trial, and qualified forged piece was gained. The FEM model established can be used for further optimization of other types of crankthrow.

Prediction of Mechanical Properties and Microstructure Distribution of Normalized Large Marine Crankthrow

Based on the measured CCT diagram of steel S34MnV, the parameters for the heat treatment of large marine crankthrow were designed, and the models for predicting microstructural evolution and mechanical properties were developed. By computer simulation, the temperature and phase volume evolutions in the controlled cooling process were predicted together with the final mechanical properties. Finally, the manufactural trial was carried out in heavy plant, the temperature evolution and final mechanical properties on the blank were obtained. The manufactural data agree well with the predicted results.

Effect of Microstructural Evolution on Mechanical Properties of 55NiCrMoV7 Steel

55NiCrMoV7 steel is extensively used as mould steel. However, how to obtain a product combined with both high strength and good toughness is still a problem in the heat treatment process. In current study, a series of 55NiCrMoV7 steel specimens were quenched with different cooling rates to obtain martensite, lower bainite and upper bainite, then tempered at 600°C, 620°C and 650°C, respectively. The micrographs show the evolution of microstructure with the variation of cooling rates and tempering temperature. Mechanical properties test has revealed that, 1) at the same tempering temperature, the tempered martensite has the highest strength and hardness, while the tempered lower bainite has the best impact toughness; 2) at the same ratio of yield strength to tensile strength (YS/TS), the ductility and toughness of martensite are better than that of bainite. Furthermore, fractographs of impact specimens show the ductile and brittle fracture behavior for martensite and upper bainite respectively.