Advanced Materials Research Vol. 51

Abstract: High strength and high hardness can be readily attained after sintering when sinter-hardening grade powder metallurgy alloys are used. However, fast cooling rates greater than 60°C/min are usually required. This increases the cost of the sintering equipment and maintenance. To lower the required minimum cooling rate, the homogeneity of the alloying elements in the matrix and the hardenability of the material must be improved. Among the various popular alloying elements, nickel and carbon are the two most non-uniformly distributed elements due to their repelling effect. It is found that to improve their homogenization, the addition of Cr and Mo can alleviate the repelling effect between Ni and C. As a result, weak Ni-rich/C-lean ferrite and austenite are eliminated and replaced by hard bainite and martensite. A tensile strength of 1323 MPa and a hardness of 39 HRC are attained in sinter-hardened Fe-3Cr-0.5Mo-4Ni-0.5C compacts without any quenching treatment.

Abstract: The effect of morphology and distribution of sulfides on tensile, impact and bending fatigue properties of non-quenched and tempered steel 49MnVS3 has been investigated in this paper. Microscopic structure and morphology of sulfides are observed, and impact fracture and fatigue fracture have been analyzed by SEM. The results show that the morphology of sulfides is mostly strip and distributes in ferrite, which affects mechanical properties and fatigue life. The length direction of sulfide strip is parallel to the rolling direction of steel. When the length of sulfide is short relatively and is approximate to the shape of particles. The impact properties and bending fatigue performance of 49MnVS3 are higher. Under those conditions, there are more ductile characteristics in their impact fracture and the fatigue fracture. The reasons for the effect of sulfide morphology on the mechanical and fatigue properties are explained.

Abstract: The development of oil&gas steel products in Baosteel in the past two decades is reviewed. After years of R&D works of steel products used in energy industry such as drilling, exploitation, collection, transportation, and storage of oil and gas, the alloy system and manufacturing processes of oil&gas steel products have been established in Baosteel. The oil&gas steel products of Baosteel consist of two major categories, i.e. the Oil Country Tubular Goods used in underground service, and the pipes used for pipeline construction on the ground. Currently, the product quality has been maintained stable, and the involved products have been extensively used in worldwide oil&gas exploration and transportation industry.

Abstract: The influences of the different Si (0.723%~4.5%) and Cr (2.0%~8.0%) contents on the microstructures and properties of Cr-Si-Mn white cast iron were investigated. It is shown that with increasing of amounts of Si and Cr elements, carbide undergoes an evident change in the morphology from the continuous net to isolated stripe and becomes clearly finer, even forms chrysanthemum-like microstructure which is usually found in high Cr white cast iron. Additionally, the amounts of the carbides increase too. The XRD analysis shows that the carbides are a mixture of Fe3C and Fe7C3 phases. Furthermore, the hardness of carbide and matrix is also found to progressively increase with increasing of amounts of Si and Cr elements. The hardness of the matrix in as-cast white cast iron is over HV400, suggesting that the matrix consists of martensite and bainite phases. The impact toughness of the samples declines evidently when Si content excesses 3.0wt%. It is also revealed that the bainite matrix in the Si-Cr white cast iron has a higher impact abrasive wear resistance than others, which is almost not dependent upon heating temperature and cooling rate. When Cr content approaches 5wt%, the impact wear resistance of the new cast iron is comparable to that of the traditional high Cr cast iron.

Abstract: In this paper, using high tensile strength and yield strength of hot-rolling acid washing thin steel sheet instead of cold-rolling steel sheet as the vehicle frame is investigated. The high pure and finer grain steel can be obtained by deep deoxidation technology, deep desulfurization technology, low nitrogen and microalloy-technology of Boron/Niobium/Titanium. High yield strength (σs>340 MPa) and high specific elongation hot-rolling thin steel sheet have been produced in the EAF-CSP short flow product line. The mechanical property, shape, surface quality and tolerance of steel sheet after hot-rolling can be controlled by optimizing joining-casting-rolling and controlling-rolling-cooling technology. Based on the forming test, cold-bending test, welding test and fatigue test results, it can be seen that cold-bending property, forming ability and fatigue strength of hot-rolling acid washing steel sheet is better and meet the product requirement of vehicle frame. This investigation can resolve the fracture problems of QUANSHUN vehicle, improve the working life, provide reliable automobile and reduce the production cost.

Abstract: The precipitate behavior of copper in the high purity structural steel was investigated by means of transmission electron microscope (TEM), and aging hardening mechanism was investigated based on the corresponding phase transformation mechanism. The results show that lots of Cu rich clusters exist in supersaturated ferrite matrix in solid solution, which evolve to B2-like structure during aging. It is found that the hardening in the initial stage is controlled by the coherency relationship of the B2-like structure with matrix that forms the obstacle of the dislocation motion, while the decrease in hardness after the peak is attributed to the loss of coherency, which should highly likely be the dominant reason of aging hardening in Cu bearing high purity steels.

Abstract: Mg bulk metallic glass rod, ribbon and composite plate were synthesized via injection casting, melt spinning, and spray forming, respectively. The BMG composite plate has various microstructure combinations, from fully amorphous phase to mixture of nanocrystals, crystals and amorphous phase. XRD analyses were done on the rod and ribbon and throughout the vertical locations of the plates at the maximum thickness. Glass transition temperature (Tg) and onset crystallization temperature (Tx) were measured with DSC, and GFA (Glass Forming Ability) was estimated with various GFA models. The microhardness of the spray-formed Mg-Cu-Gd layered composite plate at various positions and Mg-Cu-Gd rod and ribbon were measured.

Abstract: In this study, the crystal structure and mechanical property of AZ80-2%Li Mg alloy are investigated. Experimental results show that the density of AZ80 Mg alloy can be reduced to be 1.71 g/cm3 by addition of 2wt% Li. The as-extruded AZ80-2%Li Mg alloy exhibits a two-phase microstructure of α and β phases, with fine strips along the extrusion direction. The fracture strain increases significantly with increasing deformation temperature, and the tensile strengths hardly change at below 100oC. The tensile strength increases, but fracture strain decreases, with increasing strain rate. The 400°C solid-solution treated AZ80-2%Li Mg alloy will produce the AlLi precipitate after aging at 170°C, and produce the Mg17Al12 precipitate after aging at 250°C. The AZ80-2%Li Mg alloy with aging at both 170°C and 250°C will exhibit the phenomenon of precipitation hardening.