Papers by Author: Hui Guo

Abstract: The influence of tempering temperature on the microstructure and mechanical properties of low carbon low alloy steel was investigated. The results show that tempering temperature has considerable influence on both yield strength and tensile strength. With the increase in tempering temperature, the yield strength increases first and then decreases after it reaches the highest point at 600°C with a strength of 843MPa, while the tensile strength decreases fastly from 550°C to 650°C and keeps stable after increasing drastically at 720°C. The yield ratio is about 0.60 except at 600°C and 650°C with a high yield ratio of 0.90, while the total elongation has little change. It is concluded that the major change of mechanical properties after tempering has a connection with the decomposition of M/A(martensite/austenite) islands, the recovery of dislocations and the precipitation of alloy elements.

Abstract: The microstructure evolution and precipitation behavior of two low carbon steels are studied, with 0.05C-0.77%Nb added in one steel and (0.03C-)1.63Cu-0.74%Nb added in the other as a comparison. In the Cu-Nb steel tempered at 600°C for 18 hrs, there are two peaks in the particle size distribution figure, one between 2-3nm formed by NbCN precipitates, and the other, 10-12nm for Cu precipitates. The TEM observation on carbon replica shows that the average particle diameter of NbCN precipitate is 2.81±0.78nm in C-Nb steel, while 4.23±0.95 nm in Cu-Nb steel with lower carbon. The analysis shows that this size increase of NbCN not only decreases the precipitation strengthening, but also weakens significantly the pinning effect on the dislocations, which results in a more serious microstructure softening in Cu-Nb steel.

Abstract: A Fe-0.05C-2.94Mn-1.87Si steel is heat treated using a two-stage isothermal holding process to obtain allotriomorphic ferrite and bainite. Two kinds of allotriomorphic ferrite are obtained, one with only carbon partitioning and the other, alloying element partitioning. It is observed that the allotriomorphic ferrite stimulates the adjacent bainite to select the similar variant on the side where near K-S relationship is maintained between ferrite and prior austenite. The longer the border length of the allotriomorphic ferrite, the larger the stimulated bainite area. The statistical measurement shows that the alltriomorhpic ferrite with alloying element partitioning stimulates such bainite variant selection as well as that with only carbon partitioning.

Abstract: . A low carbon steel is austenitized and isothermally held at 680°C to form allotriomorphic ferrite and followed by a holding at lower temperature to form bainite. The morphology of allotriomorphic ferrite/bainite interfaces is studied using optical microscope. Three kinds of combination are observed: Type I: interface on one side is clear while on the other side, unclear; Type II: unclear on both sides; Type III: clear on both sides. Clear interface indicates a large difference in the orientation between the bainite and the ferrite, and unclear interface, a very small difference. The statistical counting shows that the ratio of Type I is about 80-82%, and that of Type II, 7-8%, and Type III, 9-11%. It is observed that this ratio does not change with the austenite grain size and bainite forming temperature. And the clear and unclear side of allotriomorphic ferrite may have different influence on the nucleation rate of bainite at allotriomorhic ferrite/prior austenite interface.

Abstract: To improve the toughness and weldability, the carbon content of the steels has to be deduced, and more and more attention has been attracted to the low carbon and ultra-low carbon steels. To strengthen the microstructure Cu and Nb-bearing steels are developed. However, the knowledge on influence of combined addition of Cu and Nb is still in lack. The microstructure and mechanical properties are studied in the 6-mm thick as-rolled and tempered ultra-low carbon steel plates with varied copper and niobium content. The microstructure and mechanical properties are studied in the 6-mm thick as-rolled and tempered ultra-low carbon steel plates with varied copper and niobium content. The experimental results show that if niobium is added without copper, the increase of niobium addition does not have a significant influence on the phase transformation and mechanical properties before tempering. The strength and toughness of those copper-free niobium steels do not vary significantly after tempered at different temperatures, while the strength of niobium steels with 1.8% copper added increases after tempered in the range of 450-650°C and reaches a peak at 500-550°C. If combined with 1.8% copper, the increase of niobium addition from 0.08% to 0.16% improves the hardenabililty and strength significantly, and the strength peak after tempering moves to a lower temperature. The strength of air-cooled niobium steels with 1.8% copper added is usually higher than those water-cooled, while after tempered at a proper temperature, the strength of the latter becomes higher than the former.