In-situ formation of borides and enhancement of powder metallurgy properties (original) (raw)
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Structure formation in sintering iron-boron carbide powder composite
Soviet Powder Metallurgy and Metal Ceramics, 1991
The use of boron carbide for producing wear-resisting powder composites based on iron requires examination of the processes of structure formation during sintering of these materials. It has been shown that gas-transport processes in the Fe--C--B system at T _< 1000~ lead to breakdown of B4C particles and formation of a layer of iron boricles on the internal surface of the pores [1]. However, this temperature does not ensure the required intensity of sintering the iron base of the powder material and this reduces its strength characteristics. An increase of the sintering temperature in the Fe--C--B system leads to a change in the structure formation mechanism. As a result of formation of the liquid phase (temperature at the start of melting is in the range 1000-1100~ [2]) the sintering process is accelerated. In this work, we examine the mechanism of phase and structure formation in sintering a Fe--C--B composite with the liquid phase (LP) taking part, in order to examine the effect of the process conditions on the properties of the material. Investigations were carried out on iron powders of WPL-2 grade and commercial boron carbide (GOST 5744--74) with particles 50-100 ~tm in size. Specimens of the Fe-3 wt.% B4C composition were compacted to a density of 85 and 87% and sintered for 2 h at 1050~ To ensure that the breakdown of B4C does not manage to end by the gas-transport mechanism at T < 1000~ the compacts were heated to 1050~ at a rate of 10-12 deg/sec. The pattern of structure formation in sintering changes qualitatively as a result of the appearance of a new phase around B4C particles . According to the results of x-ray microanalysis, the composition of this phase is close to the composition of borocementite (around 75 iron, 15 carbon, and 10 at.% of boron) . As a result of similar angular positions of the diffraction lines of this phase at Fe3C , it was not possible to identify this phase by the method of phase x-ray diffraction analysis. It is characteristic that this structural component has a sharp boundary with the B4C particle and changes to boride needles Belorussian Republican Scientific Production Association for Powder Metallurgy.
Gas transport processes in sintering of an iron-boron carbide powder composite
Soviet Powder Metallurgy and Metal Ceramics, 1989
As a rule the interaction of the components in an iron-boron carbide powder composite in solid-phase sintering has been investigated from the point of view of the phase composition and morphology of the compounds formed. It is known [i, 2] that iron starts to actively interact with boron at 873-973 K with the formation of iron boride Fe2B. With a sintering temperature not exceeding 1373 K the boride phases Fe2B and FeB located in the vicinity of a B4C particle are present in an Fe-B4C powder composite . However, the rules of formation of iron borides under conditions of sintering of powder composites have been studied insufficiently fully. This makes difficult the development of powder metallurgy materials with a specified structure and properties. In connection with this it was desirable to investigate the processes of formation of iron borides occurring under conditions of solidphase sintering.
Aluminothermic powder boriding of steel
Applied surface science, 2004
Subject of thermodynamical analysis are the probable boron transport reactions, boron precipitation and boride forming in the reaction space with the use of aluminothermic boriding mixtures and activators such as NaF, NH 4 F, Na 2 OÁ4BF 3 and (NH 4 ) 2 Á4BF 3 . Aluminothermic boriding of carbon and alloyed steels have been carried out in a powder boriding mixture containing B 2 O 3 -20%, Al-7%, Al 2 O 3 -72.5%, and (NH 4 ) 2 OÁ4BF 3 -0.5% at a temperature of 1160 K and within a 6 h period. The thickness and the microhardness of the boride layers have been measured and a layer-by-layer chemical spectrum analysis has been made. The proposed structure of an aluminothermic mixture is determined to be suitable for boriding of lowcarbon and low-alloyed steels. The boride layers obtained are distinguished for considerable thickness and high microhardness. #
Liquid Phase Sintering of Boron-Containing Powder Metallurgy Steel with Chromium and Carbon
Liquid phase sintering is an effective method to improve the densification of powder metallurgy materials. Boron is an excellent alloying element for liquid phase sintering of Fe-based materials. However, the roles of chromium and carbon, and particularly that of the former, on liquid phase sintering are still undetermined. This study demonstrated the effects of chromium and carbon on the microstructure, elemental distribution, boride structure, liquid formation, and densification of Fe-B-Cr and Fe-B-Cr-C steels during liquid phase sintering. The results showed that steels with 0.5 wt pct C densify faster than those without 0.5 wt pct C. Moreover, although only one liquid phase forms in Fe-B-Cr steel, adding 0.5 wt pct C reduces the formation temperature of the liquid phase by about 50 K (°C) and facilitates the formation of an additional liquid, resulting in better densification at 1473 K (1200°C). In both Fe-B-Cr and Fe-B-Cr-C steels, increasing the chromium content from 1.5 to 3 wt pct raises the temperature of liquid formation by about 10 K (°C). Thermodynamic simulations and experimental results demonstrated that carbon atoms dissolved in austenite facilitate the eutectic reaction and reduce the formation temperature of the liquid phase. In contrast, both chromium and molybdenum atoms dissolved in austenite delay the eutectic reaction. Furthermore, the 3Cr-0.5Mo additive in the Fe-0.4B steel does not change the typical boride structure of M 2 B. With the addition of 0.5 wt pct C, the crystal structure is completely transformed from M 2 B boride to M 3 (B,C) boro-carbide.
Reaction Sintered Materials Based on Boron Carbide and Silicon Carbide (Review)
The particulars of phase and structure formation during reaction sintering of materials based on boron carbide and silicon carbide are examined, and the actual technological aspects of reaction sintering are analyzed. The main physical and mechanical characteristics of reaction sintered material based on silicon carbide and boron carbide and their relation to the structure of the material are discussed.
Journal of the European Ceramic Society, 2022
The feasibility of fabricating novel boron carbide-silicon carbide composites by spark-plasma sintering (SPS) of B 4 C+Si powder mixtures at only 1400 • C was investigated. First, it is shown that B 4 C can be fully densified at 1400 • C if ~20 vol% Si aids are used, leading to bi-particulate composites constituted by boron carbide (major phase) and SiC (minor phase). The formation of these composites is due to the fact that Si acts as a reactive sintering additive during SPS. Lower and higher proportions of Si aids are not optimal, the former leading to porous bi-particulate composites and the latter to dense triplex-particulate composites with some residual free Si. Importantly, it is also shown that these novel boron carbide-SiC composites are fine-grained, nearly-ultrahard, moderately tough, and more affordable to fabricate, a combination that makes them very appealing for many engineering applications. Second, it is demonstrated that during the heating ramp of the SPS cycles a eutectic melt is formed that promotes full low-temperature densification by transient liquid-phase sintering if sufficient Si aids are used. Otherwise, a subsequent stage of solid-state sintering is required at higher temperatures once the eutectic liquid has been consumed in the in-situ formation of SiC. And third, it is demonstrated that during SPS the original B 4 C undergoes a gradual isostructural crystallographic transition towards a Si-doped carbon-deficient boron carbide that is more relevant with increasing proportion of Si aids, and it is identified that the carbon source for the formation of SiC is almost exclusively the carbon exsoluted from the B 4 C crystals themselves during their isostructural transition. Finally, implications of interest for the ceramic and hard-material communities are discussed.
Materials Research Bulletin, 2000
In this study, the effect of particle size of powder used in the boronizing process with solid boron-yielding substances on the boride layer was investigated. Hot-shaped AISI 1020, AISI 1030, AISI 1040, and AISI 1050 structural steels were used as the base materials. EKabor HM powder was used as the boronizing agent, and was classified into four groups according to particle size. The boronizing process was carried out with each group at 900°C for 2, 3, 4, and 5 h. The microstructure, microhardness, and layer thickness of the boronized materials were investigated.
2013
Powder of AISI 316L austenitic stainless steel modified with 0,3 wt.% elemental boron was sintered in dry hydrogen atmosphere at temperature of 1240°C. Microstructures were analyzed by means of optical and scanning electron microscope. Chemical composition of phases was determined by EDS. Obtained results were used as a basis for thermodynamic simulation of crystallization process using Scheil-Gulliver Modified solidification model Thermo-Calc software program. On the basis of experimental work it was concluded that two-step crystallization of borides during cooling from sintering temperature take place.
A method of making boride and vitreous compound by powder metallurgy
Journal of Materials Processing Technology, 2003
A procedure of making borides and vitreous compounds has been developed through the reaction between boron and a chemical reductor. Several chemical reducing elements were used, like aluminum, magnesium, coke, etc. As raw material for boron, different compounds were used: amorphous boron, boric acid and boron oxide. The incorporation of the reducing elements into the boron was achieved by using powder technology.
Combined Elemental Synthesis of Boron and Silicon Carbides
Combined synthesis from powders of elements B, C, and Si at 1400, 1500, and 1650°C is used to prepare heterophase powders in the system SiC–B 4 C containing 80, 57, and 30 (mol.%) boron carbide. Powders containing only SiC and B 4 C phases are prepared at 1550°C from a mixture with 5% excess silicon given vibration grinding for 60 h. The powder has a unimodal particle size distribution and d 50 = 3.5 mm with a volume concentration of 12% submicron particles.