Nattaya Tosangthum

Effects of Cooling Rate and Carbon Content on Mechanical Property of Sintered Fe-Cr-Mo Alloys

Advanced high strength steel (AHSS) was prepared using the conventional ‘press and sinter’ process.The compacts of ultralow carbon Fe-Cr-Mo powder with carbon additions (base metal powder admixed with 0.1,0.2 and 0.3 wt.% graphite) and without carbon addition (plain base powder) were sintered in a vacuum furnace at pressure of 1.28 x 10-5MPa at 1280 °C for 45 min. After sintering, the sintered specimens were continuously cooled with different nitrogen gas pressures of 0, 2500 and 5000 mbars (or 0, 0.25, 0.5 MPa). Mechanical properties of the sintered alloys were strongly controlled by carbon contents and cooling rates after sintering. The sintered specimens, with 0.3 wt.% carbon and cooled by nitrogen of 5000 mbars, showing superior tensile strengths and good ductility, had microstructures dominated by carbide-free bainitic structures and some retained austenite. The sintered specimens with lower carbon contents and cooled under slower cooling rates, having lower tensile strengths but slightly higher ductility, had microstructures with lower bainite volume fractions and even without bainitic structures. The dominant phase in the sintered specimens with low strength but high ductility was ferrite.

Microstructure, Hardness and Wear Properties of Sintered Fe-Mo-Si-C Steels with Spheroidal Graphite Iron/Compacted Graphite Iron-Like

Sintered Fe-Mo-Si-C steels were prepared from pre-alloyed Fe-0.85Mo powder added with fixed 4wt.% silicon carbide powder and varied graphite powder contents. It was found that the graphite powder addition caused morphological change from black nodular to black vermicular particles and resulted in decrease of black nodular/vermicular particle fraction, increase of pearlite fraction and slight change of ferrite fraction. The black nodular particles were either graphite or Fe-Mo-Si-C/graphite core-shell particles whereas vermicular particles were totally composed of carbon. The microstructural features showed influence on mechanical property of the sintered Fe-Mo-Si-C alloys. Wear properties of the sintered steels were strongly affected by their microstructural components. The sintered Fe-0.85Mo+4wt.%SiC steels showed highest friction coefficient and volume loss. Addition of graphite to the sintered Fe-0.85Mo+4wt.%SiC steels, not only changed morphology and chemistry of black particles but also reduced friction coefficient and volume loss. The reduction of both determined wear properties were attributed to the presence of vermicular graphite particles.

Modification of Microstructure and Tensile Property of Sintered Fe-Cr-Mo-C Steel by Nickel Addition

Nickel is one of the alloying elements promoting the formation of acicular ferrite at the expense of proeutectoid ferrite. The Ni addition reduces the steady-state nucleation rates of grain boundary ferrite allotriomorph in Fe-C-Ni alloys. Due to such reasons, Ni was added to modify the microstructure of the sintered steel, investigated in this study, with the aim of improved mechanical properties. The sintered steel, produced from pre-alloyed Fe-Cr-Mo powder mixed with 0.3 wt.% C, was modified by Ni addition and cooling rate. The alloy compositional change was performed additions of varied Ni contents of 1.0, 1.5, 2.0, 2.5 and 3.0 wt.%. The material processing variables were investigated by using two different cooling rates of 0.1 and 5.4 °Cs-1. Under the cooling rate of 0.1 °Cs-1, the sintered Fe-Cr-Mo-3C steel without Ni addition showed dual-phase microstructure consisting of ferrite (soft phase) and bainite (hard structure). With Ni additions, the dual-phase microstructure was replaced by bainitic structure. Microstructural heterogeneity was observed due to the presence of Ni-rich areas, which increased with increasing added Ni contents. Under the cooling rate of 5.4 °Cs-1, the sintered Fe-Cr-Mo-3C steels with and without Ni additions showed bainitic structure. Microstructural heterogeneity was similar to that of slowly cooled steel. Ni additions not only modified the sintered steel microstructure but increased tensile strength and elongation. In general, Ni pushes the C-curve of pearlite transformation to the right hand side and lowers the martensite start temperature. The absence of ferrite from the sintered steel with only 1 wt.% Ni addition and slowly cooled at 0.1 °Cs-1 suggests that Ni strongly suppresses the austenite → ferrite transformation. In another word, Ni promotes bainite formation in the sintered Fe-Cr-Mo-Ni-C steels.

Effect of Aluminum Addition on AlxCoFeMnNiZn Multi-Component Production

Five multi-component alloy (MCA) formulations of CoFeMnNiZn (MCA01), Al0.5CoFeMnNiZn (MCA02), Al1.0CoFeMnNiZn (MCA03), Co5Fe5Mn30Ni20Zn40 (MCA04) and Al8.4Co4.6Fe4.6Mn27Ni18.4Zn37 (MCA05) were prepared by mechanical alloying and melting process (MAM). Five-component alloys of MCA01-MCA05 were designed using empirical formulae for high entropy alloys. Phase formation and microstructure were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that MCA01 was partially melted by MAM process. However, MCA02-MCA05 could be melted and cast by MAM process. The microstructures of as-cast MCA02 and MCA03 showed dendritic solidification. Nevertheless, the as-cast MCA04 showed microstructure similar to that of Ni-based superalloy, i.e., the as-cast MCA04 consisted of γ matrix and γ′ phase. Moreover, egg type core shell structure was found in the interdendritic regions of the MCA05 alloy. In addition, the Al-added MCA02 and MCA03 alloys showed crystal structures of FCC1, FCC2 and BCC. MCA04 alloy demonstrated crystal structure of FCC whereas MCA05 alloy had crystal structures of FCC and Primitive Cubic.

Fe-Sn Intermetallic Compound Synthesized via Mechanical Alloying and Liquid Phase Sintering

This investigation aimed to synthesize an iron (Fe)-tin (Sn) intermetallic compound (FeSn2) by using a two-step process of mechanical alloying (MA) and liquid phase sintering (LPS). We demonstrated experimentally that this process was able to produce the FeSn2 intermetallic compound. Two different mechanical alloying procedures for producing mechanically alloyed Fe-Sn powders were explored. In the first, mixtures of as-recieved Fe and Sn powders were mechanically alloyed. In the second, the as-received Fe powder was pre-milled first and the as-received Sn powder was then added and mechanically alloyed. Under the same liquid phase sintering conditions, the sintered materials produced via the first mechanical alloying procedure showed that the FeSn2 content increased with increasing sintering time and left small traces of unreacted Fe and Sn materials. In the sintered materials produced via the second mechanical alloying procedure, only the FeSn2 phase was observed for all sintering times. The two-step process using the second mechanical alloying procedure performed better; it synthesized more of the FeSn2 intermetallic compound.

Sintered Frictional SiC-Reinforced Cu-Base Composites

Lead-free frictional materials are important components in safety and power transmission parts of automobiles. In order to avoid using lead-containing friction modifiers, non-toxic ceramic particles are considered to be used as reinforcements. In this research work, copper-based friction materials have been developed by using press and sinter method. Pre-alloyed bronze (Cu-10Sn) powder was admixed with iron (Fe), graphite (C) and varied amounts of silicon carbide (SiC) powders. The admixed powders were compacted into disc-shape samples, which were then sintered at different temperatures in the range of 800-950 °C. It was found that sintered density and hardness of the sintered copper-based friction materials reduced with increasing SiC content. Microstructures of the sintered materials showed inhomogeneity due to uneven distribution of coarse Fe and SiC particles. The coarse SiC particles also prohibited bonding between metal powder particles. However, the sintered materials showed high room-temperature friction coefficients, which were in the range of 0.50-0.90, particularly the materials containing 4 wt. % of SiC particles.

Fe-Sn Intermetallics Synthesized via Mechanical Alloying-Sintering and Mechanical Alloying-Thermal Spraying

Iron (Fe)-tin (Sn) intermetallics were synthesized by using two different routes. These two routes had a common synthesis step, in which Fe powder (19 wt. %) was mechanically alloyed with Sn powder (81 wt. %) for 25 h under argon atmosphere. The mechanically alloyed powders were then treated with different heating routes. In the first route, the compacts of the mechanically alloyed powders were sintered at different temperatures for different times. It was found that the FeSn2 content increased with increasing temperature and time. There were small traces of Fe, Sn and FeSn found in the sintered materials. In the second route, the mechanically alloyed powders were plasma-sprayed using different currents of 300, 400 and 500 A. It was found that the porous coatings produced by plasma sprayng consisted of mixed Fe, Sn, FeSn2, SnO, FeO and Fe3O4 for all employed currents.

Intermetallic Electrode Structure Produced by Thermal Spraying of Water-Atomized Cu-Sn Powders

ntermetallic copper-tin compound (Cu6Sn5), acting as a negative electrode, is able to be intercalated and deintercalated by lithium ions during charging and discharging of a rechargeable lithium ion battery made of this intermetallic. Single Cu6Sn5 phase powders were successfully prepared by water-atomization of Cu-Sn alloys with subsequent heat treatment of the water-atomized powders. Although forming of the Cu6Sn5 intermetallic powders into applicable electrode structures can be produced by using some binders to hold the Cu6Sn5 powder particles to form a desired shape, the binder formulations are complicated and unrevealed. In our approach, we employed a thermal spraying technique to form a porous Cu6Sn5 intermetallic layer on a Cu plate. The porous Cu6Sn5 intermetallic layer was aimed to perform Li insertion and reversible extraction functions and the Cu plate to perform electron collector and supplier functions. It was found that the thermal spraying technique was able to form Cu6Sn5 intermetallic electrode structure. The coating layers produced under suitable spraying conditions contained Cu6Sn5 as a major phase.

Sintered Frictional Materials Based on Cu Powders

Sintered frictional materials (SFMs) were fabricated by using powder metallurgical process. Powder formulations were experimented to investigate effects of Sn, C and ZrO2 additions on mechanical and tribological properties of the sintered composites made for dry frictional materials applications. The developed SFMs consisted of non-lead friction material. Natural sand (SiO2) and ZrO2 were employed instead of lead oxide to provide frictional components. Interrelationships between chemical composition, sintering temperature, friction coefficient, wear behavior and mechanical property of the SFMs have been studied. It was found that sintering temperature affected hardness property of the SFMs. The hardness was also affected by SFM compositions. Friction coefficient increased with increasing ZrO2 content. Addition of natural sand resulted in decrease of the hardness of the SFMs. Graphite also affected hardness and friction coefficient of SFMs containing no sand. Employing prealloyed Cu-Sn powders provided SFMs with better mechanical properties compared to the SFMs made of admixed Cu and Sn powders.