Papers by Author: Can Dong Zhou

Abstract: Being examined by tensile tests at 820°C with initial strain rates of 2.5×10-4 s-1, 5.0×10-3 s-1and 1.0×10-1 s-1, the hot-rolled spay formed high speed steel (SF-HSS) had superplastic properties. With έ=2.5×10-4 s-1and 5.0×10-3 s-1, the σ-ε curves indicate that there has occurred dynamic recrystallization at the later stage of deformation.The tensile elongation decreases monotonously with strain rate increasing. The strain rate sensitivity m =dlogσ/dlog έ is about 0.23. In the sub-surface of fractured SF-HSS samples with έ=2.5×10-4 s-1,, most of holes on the subsurface are observed to distribute near the carbides and arranged along the tensile axis direction. The interface between coarse carbides and matrix is very weak sites where the holes are easy to nucleate and connect to cracks during deformation. In the necked region, fine carbide particles on the grain boundary have pinned the slip of dislocations and formed dislocation wall inside the grain. Dynamic recrystallization and some climb of dislocations has occurred. The superplastic deformation mechanism with έ=2.5×10-4 s-1 was dislocation creep mechanism controlled by dynamic recrystallization. During deformation, the role of some carbide in the materials was to retard the grain growth and keep SF-HSS having fine equiaxed grain size and remain stable.

Abstract: A structural steel, 35CrMoV steel, has been attempted firstly by explosive powder compaction followed by sintering (EPC-sintering). The nitrogen content of the steel was 0.15wt%, which was accordant with the definition of high nitrogen steel (HNS). The final density of the EPC-sintering steel was only about 6.9g/cm3, which indicated that the processing parameters must be modulated further. In the sample of this steel, some radial cracks were found around the center of the cross-section of the steel, resulting in no mechanical tests carrying out. Observing the majority of the rim region of the sample of this steel, the microstructures were very tight, suggesting that it was possible and successful to manufacture HNS through EPC-sintering. The characteristics of the EPC-sintering high nitrogen 35CrMoV steel were that the cementites in the pearlites were found to be extremely fine. There were many (Cr,Mo)23(C,N)6 carbonitrides precipitates in the matrix. Some precipitates were round and others were needle-like. Some were distributing orderly in matrix and crossing over the dislocations. The dislocation density in the EPC-sintering steel remained high.