Yuanbing Li

Toughness improvement by Cu addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effect of microstructure and Cu addition in a simulated coarse-grained heat-affected zone (CGHAZ) of a high-strength low-alloy (HSLA) steel subjected to 100 kJ cm−1 heat input welding was studied. It has been observed that the primary microstructure in Cu-free HSLA steels is dominated by bainite, whereas, in Cu-bearing HSLA steels, the predominant microstructure is acicular ferrite. The acicular ferrite nucleated at intragranular complex inclusions consisting of Al and Ti oxides, covered with layer of MnS and CuS. The presence of high intensity of acicular ferrite and hard impingements between acicular ferrite laths or plates has contributed to the fine-grained and interlocked microstructure. The enhanced toughness in CGHAZ of Cu-bearing HSLA steel is attributed to the fine-grained interlocked microstructure of acicular ferrite.

In situ observation of acicular ferrite formation and grain refinement in simulated heat affected zone of high strength low alloy steel

Abstract The phase transformation from austenite to acicular ferrite in the simulated coarse grained heat affected zone of a high strength low alloy steel was investigated by means of analytical characterisation techniques such as in situ microscopy, transmission electron microscopy and electron backscattered diffraction analysis. The acicular ferrite grains nucleated on inclusions (Zr–Ti oxides) in coarse austenite grain grew in different directions and effectively partitioned coarse austenite grain into several finer and separate regions. The crystallographic grain size became small for coarse austenite grains due to the effective partitioning by acicular ferrite laths or plates.

Magnetic Field-Induced Precipitation Behaviors of Alloy Carbides M2C, M3C, and M6C in a Molybdenum-Containing Steel

The effect of a 12-T high magnetic field on alloy carbide precipitation in an Fe-C-Mo alloy during tempering at an intermediate temperature was investigated. Thin foils and carbon extraction replicas of the treated specimens were examined by transmission electron microscopy (TEM). The results show that the applied high field effectively promoted the precipitation of (Fe,Mo)6C alloy carbide. The concentration of Fe atom in Fe6−xMoxC carbide is increased whereas that of Mo atom decreased when the high magnetic field was applied. However, the high magnetic field almost had no detectable influence on the atom concentration in (Fe,Mo)2C and (Fe,Mo)3C carbides. First principle calculations have been performed to calculate the magnetic moment per iron atom of the carbides to explore the origin of the effect of the magnetic field. The influence of the high magnetic field on the precipitation behaviors of alloy carbides was closely related to the magnetic moment of (Fe,Mo)2C, (Fe,Mo)3C, and (Fe,Mo)6C. The magnetic field promotes the formation of the carbides with high total magnetic moment. The effect of the high magnetic field on the substitutional solute atom (Fe and Mo) concentration change in the three alloy carbides was attributed to their magnetization differences per Fe atom.

Fabrication and characterisation of porous mullite ceramics from high voltage insulator waste

Abstract High voltage insulator waste (HVIW) eliminated from high voltage transmission lines was used in the preparation of porous mullite ceramics. The fabrication and characterisation of porous mullite ceramics are described. The relationship between phase transitions, sintering temperature and physicochemical properties of the porous mullite ceramics are analysed and compared. The results show that the complex properties are the best when the mass ratio of HVIW/clay is 4∶1, the amount of pore former is 500 mL kg−1 and the sintering temperature is 1500°C, the cold crushing strength reaches 4·8 MPa. Increasing the sintering temperature could strengthen the specimens. The average pore diameter of the specimens enlarged with the increasing sintering temperature. Meanwhile, the crystalline and morphology of mullite grains advanced and the maximum grain diameter reached to 1·5 μm. This improvement was attributed to the fact that complex properties of specimens could be optimised by new formation of new mullite phase.

Optimal design on the high-temperature mechanical properties of porous alumina ceramics based on fractal dimension analysis

Fractal theory and regression analysis were employed for the first time to investigate the effect of pore size and pore distribution on high-temperature mechanical properties of porous alumina ceramics (PAC). In the present work, PAC with the comparable porosity, different pore sizes and pore distributions were prepared using carbon black as the pore-forming agent. Particular emphasis in this study was placed on the establishment of correlation between the thermal shock resistance and pore properties. The relationship between fractal dimension (Df) andthermal shock resistance parameter (Rst) in specimens presented the negative power function, indicating that low Df could benefit the improvement of thermal shock resistance in specimens. The results showed that the increase of pore size and pore sphericity leads to a reduced Df, the enhanced hot modulus of rupture (HMOR) and. The decrease of proportion of micro-pores below 2 μm, the increase of mean pore size and pore sphericity could result in the decrease of Df, and then improve Rst and HMOR of specimens. Based on the correlation between Rst and pore characteristics, PAC with improved thermal shock resistance could be achieved when their pore structure meets the above features.

Microstructural characteristics and impact toughness in YS690MPa steel weld metal for offshore structures

The fine-grained mixed microstructure of acicular ferrite (AF) and bainite in YS690MPa steel weld metal contributes to attain high-impact toughness. The morphology and evolutionary mechanism of fine-grained mixed microstructure in this weld metal were investigated. Single or multiple AF grains were nucleated on complex inclusions by forming Mn-depleted zones, where Mn spontaneously diffused into Ti oxide inclusions due to the cation vacancies. It is in good agreement with the theoretical calculation by first principle. The bainite nucleated on austenite grain boundary and then assisted the pre-formed AF to partition the austenite grain into small and separate regions. Furthermore, the later formed ferrite nucleated on the broad surface of pre-formed ferrite plates and grew in those small regions with limited grain size. All of them resulted in the formation of fine-grained mixed microstructure, which provided excellent impact toughness in this weld metal with dimples and quasi-cleavage fracture surface combination.

Processing and properties of Cfibre/SiCfillers/SiOC composites using solvent free liquid polysiloxane as matrix source

Abstract Carbon fibre reinforced SiOC composites (denoted as Cfibre/SiCfillers/SiOC) were prepared by slurry coating and polymer infiltration pyrolysis (PIP) process. Low viscosity liquid polysiloxane (PSO) and SiC powder were combined at a 1∶1 weight ratio to produce a blend (S-PSO), which was employed as matrix source. Heat treated carbon fibre fabric was adopted as the reinforcement. The lamination process was determined on the basis of cure and rheology investigations on S-PSO. The effects of PIP cycles and temperature of heat treatment of the carbon fibre on the mechanical properties of Cfibre/SiCfillers/SiOC were examined. The results indicate that composites using carbon fibres annealed at 1700°C as reinforcement reached a maximum flexural strength of 300 MPa after six PIP cycles. The resistance of the Cfibre/SiCfillers/SiOC composite to oxidation was also evaluated. Without any protective coatings, the composite retained 60% of its strength after oxidation at 800°C for 3 h in a static air environment.

In-situ microscopy study of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by TiN particle

ABSTRACT High-strength low-alloy steels subjected to high heat input welding are susceptible to failure due to low toughness caused by grain coarsening. The effect of TiN on grain refinement in the simulated heat-affected zone (HAZ) was investigated. Because of small amount of Ti addition, abundant dispersed nanoscale TiN precipitates were formed. The TiN precipitates tended to be stable at high temperature and effectively retarded the austenite grain growth by refining the grain size during thermal cycle. Furthermore, the TiN also covered on the surface of Al–Ti complex oxide with MnS and caused low interface energy with ferrite. The acicular ferrite grains nucleated on complex inclusion in austenite grains at intermediate temperature and induced the austenite grain transform to the fine-grained mixed microstructure of acicular ferrite and bainite. The crystallographic grain size became small in the simulated HAZ due to the effective pinning effect and acicular ferrite formation.

Effects of Sintering Temperature on Properties of Green Porous Mullite Ceramics Fabricated by Insulators Waste

ABSTRACT In this research, green porous mullite ceramics with high porosity, high strength and low thermal conductivity were fabricated by using insulator waste as the main raw material. The effects of sintering temperature on linear shrinkage rate (LSR), bulk density, apparent porosity, cold crushing strength (CCS), mean pore diameter and thermal conductivity of the porous mullite ceramics were investigated. As the sintering temperature increased from 1300o to 1500oC at intervals of 50oC, the LSR initially decreased from 1300o to 1400oC, and then increased from 1400o to 1500oC, while CCS and thermal conductivity increased constantly from 3.2 to 8.5 MPa and from 0.187 to W.m–1.K–1 respectively. Meanwhile, the crystallization and morphology of the mullite grains were optimiz ed wit h t he incr easing sintering temperature. Comprehensive properties of specimens can be adjusted by changing the sintering temperature. GRAPHICAL ABSTRACT

Effects of Added Al Powder on the Properties of Sialon-ZrN Composites Fabricated by Aluminothermic Reduction and Nitridation

Sialon-ZrN composites were synthesized from zircon and aluminium via an aluminothermic reduction and nitridation process. The effects of extra Al powder on the product phase compositions and morphologies were studied. Products with different phases and morphologies result from different surplus amounts of additional Al. With increases of Al, after firing at 1500°C, the amount of ZrSiO4 decreased and the amount of sialon increased. The phase evolution of the specimens indicated that 8 mol-% extra Al powder contributed to the best decomposition of zircon and led to improved formation of sialon-ZrN composites.