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Sintering of a hypoeutectic high chromium cast iron as well as its microstructure and properties
Accepted Manuscript Sintering of a hypoeutectic high chromium cast iron as well as its microstructure and properties Jinghong Gu, Pingan Xiao, Jianyong Song, Zhihua Li, Ruiqing Lu PII:
S0925-8388(17)33958-0
DOI:
10.1016/j.jallcom.2017.11.189
Reference:
JALCOM 43882
To appear in:
Journal of Alloys and Compounds
Received Date: 28 March 2017 Revised Date:
13 November 2017
Accepted Date: 14 November 2017
Please cite this article as: J. Gu, P. Xiao, J. Song, Z. Li, R. Lu, Sintering of a hypoeutectic high chromium cast iron as well as its microstructure and properties, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.189. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Sintering of a hypoeutectic high chromium cast iron as well as its microstructure and properties Jinghong Gua, Pingan Xiao a, *, Jianyong Song a, Zhihua Li a, Ruiqing Lu a College of Materials Science and Engineering, Hunan University, Changsha 410082, P. R. China
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Abstract A hypoeutectic high chromium cast iron (HCCI) was fabricated by super-solidus liquid phase sintering (SLPS) technology with gas atomized powders as raw materials. The effects of sintering parameters on densification, microstructure evolution and mechanical properties of the
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alloy were investigated systemically. The results showed that samples with full density could be fabricated and sintering temperature window suitable for SLPS is around 20
. The X-ray
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diffraction (XRD) revealed that the hypoeutectic HCCI mainly consists of martensite matrix and M7C3 carbide as well as small amount of austenite, and optical metallographic analysis showed that M7C3 carbides are of crystal square bars with uniform distribution in the matrix. As sintering temperature or holding time is raised, both grain and carbide are gradually coarsen, while the strength and toughness of the alloy increase at first and then decline. The mechanical properties of
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sintered hypoeutectic HCCI in an optimized sintering process were of hardness HRC63, bending strength 2241MPa, and impact toughness 8.0J/cm2. A model about microstructure evolution in sintered hypoeutectic HCCIs has been proposed.
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Key words: hypoeutectic HCCIs; SLPS; microstructure; mechanical properties; model
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*Corresponding author: Pingan Xiao E-mail address: [email protected](P. Xiao) Tel./fax:+86 073188821610
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ACCEPTED MANUSCRIPT 1. Introduction As an important abrasion-resistant materials, high chromium cast irons (HCCIs) have been widely used in mineral processing, cement manufacturing, slurry pumping and paper
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manufacturing industries[1,2]. The combination of high volume fraction of M7C3-type hard carbides and relatively ductile ferrous matrix (commonly austenite and martensite) is the main reason why HCCIs have excellent wear resistance[3-7]. However, practical applications show that
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the major problem faced by HCCIs is still a good balance of hardness and toughness. There are
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large amount of chrysanthemum-like carbides with sharp edge in normal HCCIs’ microstructure, which practically plays a role of both pores and stress concentration raisers and then deteriorates toughness. In order to improve carbides’ morphology much effort has been done, and some techniques & skills have been developed, such as multi-alloying[8-9], metamorphism and
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inoculation[10-15], semi-solid processing[16-17], rapid/directional solidification[18], and heat treatment[4,19-21]. Some of them, such as rapid/directional solidification and semi-solid processing, do notably change the morphology of carbides and effectively reduce their
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consecutiveness in space, and hence greatly increase HCCIs’ strength and toughness, but their
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high cost prohibits practical applications in industry. The others can in certain cases affect HCCIs’ microstructure evolution at low cost and improve their mechanical properties, but are not equally efficiency as the former ones. Typically, the bending strength of normal HCCIs is around 900MPa and impact toughness is around 4J/cm2 when a standard sample with 5×5×50mm is tested. Fabrication of HCCIs by P/M process with alloyed powder as starting materials can provide a unique thermodynamic condition for microstructure evolution in comparison with casting process. Firstly, atomized raw powders possess rapid solidification microstructure and offer excellent
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Therefore, a new type of HCCIs with unique microstructure and high performance could be developed.
In this paper, a HCCI was prepared by a pressing plus super-solidus liquid phase sintering
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(SLPS) process with gas atomized powders as raw materials. Its densification, microstructure
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evolution and mechanical properties were systematically evaluated respectively in order to make it have a good combination of strength and toughness. 2. Materials and experiment methods 2.1 Preparation of samples
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The raw powders of a hypoeutectic HCCI were prepared by nitrogen gas atomization and its chemical composition was listed in Table 1. The cast HCCI samples have the similar chemical compositions with the raw powders. First, initial charge materials were steel scrap, 1000 kg of raw
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material was melted in a medium frequency induction furnace at 1500 ±50
after removal of any dross and slag. The particles’ size was analyzed
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sand mold at 1500 ±20
,then poured into a
by a BT-9300H Laser particle size analyzer; it was revealed that their size distribution range was narrow with D50 around 20.11µm. As super-solidus liquid phase sintering (SLPS) technology was applied to obtain samples with full density, the powders were also detected by a Sta449C simultaneous differential thermal analyzer in order to determine their solidification temperature range which usually indicated the sintering temperature window. For a hypoeutectic HCCI as a Fe-C-Cr ternary alloy its solidification temperature range could be divided into two parts, the
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ACCEPTED MANUSCRIPT eutectic transformation temperature interval and the one above it. Obviously, the sintering window for SLPS in fabrication of PM HCCI with unique microstructure should be within eutectic transformation temperature interval where proeutectic austenite, carbide and liquid metal
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coexisted and the ratio of solid austenite plus carbide to molten metal could be regulated by changing sintering temperature. Otherwise, if sintering temperature was above eutectic transformation temperature carbides will disappear and their final morphology will be mainly
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depend on cooling speed after sintering. The result from differential thermal analysis exhibited
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that the temperature range of eutectic transformation is of 1232~1273°C.
1% styrene-butadiene rubber (SBR) was added into the raw powders as lubricant to improve their compacting ability. Green compacts with dimensions of 80×17×13mm were firstly pressed with pressing pressure between 200~300MPa on a Y32-315A four-column hydraulic press, and
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then super-solidus liquid phase sintered to obtain samples with high density in a GSL1600X vacuum tube furnace. All samples were heated to sintering temperature at a heating rate of 2 °C min-1, and when holding time was finished, they were firstly cooled to 450°C with cooling rate of
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5°Cmin-1, and afterward went through gradual furnace cooling to room temperature. In order to
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understand the relationship among sintering, microstructure and mechanical properties, the influence of such sintering parameters as sintering temperature and holding time was systemically investigated.
2.2 Characterization The density of the sintered samples was measured by Archimedes method. Microstructure inspection was operated on a Leitz-MM6 optical microscope (OM) and/or a FEI QUANTA200
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ACCEPTED MANUSCRIPT environmental scanning electron microscope (SEM). To observe the three-dimensional morphology of carbides in sintered HCCI, a specimen was firstly etched in alcohol with 4% hydrochloric acid for more than 24 hours and then cleaned in an ultrasonic dispersing instrument
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with water before SEM inspection. Its phase constitution analysis was undertaken with a D8-advance X-ray diffraction instrument (XRD, Cu target, λ = 0.15405nm).
Mechanical property specification of the sintered HCCI covered hardness, impact toughness
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and bending strength. Hardness measurement was conducted by DUR-O-Test and each result was
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the average of more than 5 inspections. Specimens for both bending strength and impact toughness inspection were firstly wire cut and then ground to the size of 5 × 5 × 35mm and 5 × 5 × 50mm respectively. Bending strength and impact toughness were tested on an Instron3369 universal mechanical testing machine and a XJ-40A impact tester separately.
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3. Results and discussion
3.1 Microstructure of raw material powders
The gas atomized raw powders were spherical as shown in Fig. 1(a) and their microstructure
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was revealed in Fig.1(b) after etched by ferric chloride hydrochloric acid. There were many
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bouquet-like outgrowth structures with a distinct nucleation from which a lot of fine rods or flakes grew out as presented in Fig.1(b). This unique type of microstructure should have close relationship with rapid solidification of powders during gas atomization. According to literature [22] there were M7C3-type carbides in atomized HCCI powders, thus the fine rod or flake-like phases were M7C3-type carbides. 3.2 Effect of sintering process on microstructure evolution The influence of both sintering temperature and holding time on microstructure evolution of
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ACCEPTED MANUSCRIPT the hypoeutectic HCCI was systematically investigated. Its microstructures sintered at different temperatures were presented in Fig. 2 as holding time was kept the same as 120 min. White carbides were mainly distributed along grain boundaries discontinuously as shown in Fig. 2. The
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sizes of both grains and carbides in sintered HCCI were obviously much finer in comparison with normal cast one with almost the same composition as in Fig. 2(i), which was prepared by authors themselves, and the carbides were of uniform distribution and low continuity in space. With
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increase of sintering temperature, both grains and carbides was coarsen, and sectioning growth of
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the carbides also gradually took place.
SLPS belongs to liquid phase sintering. As sintering temperature was raised in the eutectic transformation temperature range of the alloy, the volume fraction of liquid phase became larger, which firstly appeared along grain boundaries and then sub-grain boundaries, and thus improved
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growth condition for carbide coarsing. Furthermore, the coarsening of carbides also had relationship with Ostwald ripening in liquid phase sintering [23]. As a result shown in Fig. 2, small carbides along boundaries and in grains gradually faded away and some large ones appeared
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along the boundaries. In addition, pores could be recognized as sintering temperature was lower
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than 1245°C due to too smaller fraction of liquid phase to get a sample fully densified. The influence of holding time on microstructure evolution of sintered samples was shown in
Fig.3 when sintering temperature was at 1257°C. It was revealed that samples were almost fully densified when holding time was as short as 1 hour. With extra holding time, coarsening and Ostwald ripening of both grains and carbides were easily observed and developed almost in the same evolution behavior as that shown in Fig. 2. However it could be noticed that the benefit of holding time on microstructure evolution was much weaker than that of sintering temperature by
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ACCEPTED MANUSCRIPT comparison Fig. 2 with 3. Raising sintering temperature not only boosted diffusion coefficient of atoms in the alloy, but also increased the liquid metal volume fraction involved, which provided fast track for atom diffusion, and hence effectively accelerated microstructure coarsening.
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Since liquid phase was firstly generated along grain boundaries in raw powders and interfaces between them, which would be also converted into grain boundaries in sintering, and then along subgrain boundaries in SLPS process, carbides at these places (or boundaries) grew fast
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and could play a role in preventing grains from merging with each other. Moreover, grains or
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sub-grains in raw powders could be split apart during SLPS and these smaller solid particles would be rearranged into close-packing in the presence of capillary force as they were soaked in a liquid metal sea. As results, PM HCCI was of equiaxed grains with small size and no proeutectic austenite dendrites could be recognized in its matrix, shown in Fig. 3.
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3.3 The influence of sintering temperature on densification and hardness The influence of sintering temperature on density and hardness of sintered HCCI was shown in Fig.4 with the holding time of 2 hours. It revealed that the change of both density and hardness
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performed almost in the same way as a function of sintering temperature. It rapidly rose at first
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and then reaching a steady peak, and finally gradual declining if temperature was further increased. When sintering temperature was in 1260 ~ 1265°C density and hardness reached their maximum values of 7.63g/cm3 (the theoretical density is 7.70 g/cm3) and HRC63 respectively. Elevating temperature would increase the volume fraction of liquid metal during SLPS, which greatly enhanced densification of a sample. Thus, both density and hardness were improved simultaneously. With carbon equivalent higher than 2.7%, the alloy’s molten liquid phase was of low viscosity and good flowability. Thus, a smaller volume fraction of liquid phase could be
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ACCEPTED MANUSCRIPT demanded to ensure obtain sintered samples with high density and hardness, which in turn made the sintering temperature window of SLPS available for PM HCCIs wider, as shown in Fig.4. However, if sintering temperature was raised too close to the upper limit of the alloy’s eutectic
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transformation temperature range, excess volume fraction of liquid phase would not only lead to specimen deformation, but also cause serious microstructure coarsening and non-uniform distribution of carbides, which resulted in the decline of both density and hardness. As it can be
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seen from Fig.4, suitable temperature range of SLPS for the HCCI was from 1245 ~ 1265°C.
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3.4 Phase constitution of sintered HCCI
The X-ray diffraction patterns of HCCIs sintered at different temperatures as well as their raw powders were shown in Fig.5. It was revealed that they were composed of M7C3-type carbides, martensite and austenite, and their matrix was mainly of martensite instead of austenite as in
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normal cast HCCIs. The austenite content in raw powders was distinctly higher in comparison with PM HCCIs, which lead to the XRD strongest peaks of M7C3-type carbides and martensite were overlapped as shown in Fig.5. However, as sintering temperature was close to 1270 , the
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amount of austenite apparently increased.
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HCCIs were of excellent hardenability and their martensite transformation could be carried out in air cooling. Hence their amount of martensite at ambient temperature had close relationship with its martensite start temperature Ms. It was found that martensite precipitated on the matrix’s side along the interface between carbides and the matrix as shown in Fig. 2(i), where there was a chromium-poor zone in the matrix nearby the carbides due to precipitation of eutectic carbides. However this belt-like region was quite narrow in cast HCCI because of high diffusion coefficient of elements during eutectic transformation. High chromium content in HCCI’s matrix would
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ACCEPTED MANUSCRIPT improve its hardenability but lower its Ms, and as a result, martensite could be detected in chromium-poor zone (in a cast HCCI) and the other part of the matrix usually remained as supercooled austenite. On the contrary, eutectic transformation lasted for a quite long time in the
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fabrication process of a PM HCCI as SLPS was applied, which means that the chromium-poor zone could extend all over the matrix because of uniform and full precipitation of vast carbide particles. Hence, the average chromium content in its matrix should be normally lower than that of
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a cast HCCI, and the former’s Ms was higher than the latter’s. The results of chromium content
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inspection by energy dispersive spectroscopy (EDS) at the center of the matrices of both PM and cast HCCIs in Fig.2 (c) and (i) were of 7.31% and 13.55% respectively, which confirmed our point of view on Ms difference between them. Therefore, there was much more amount of martensite in PM HCCIs than that in cast ones. In addition, both the volume fraction and Cr content of liquid
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phase would become larger and higher separately as temperature of SLPS was raised, resulting in less carbide precipitation and a matrix with higher Cr content. Accordingly, austenite’s Ms of the alloy was reduced and there were more supercooled austenite in its matrix as shown in Fig. 5. By
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the way, shift of the strongest XRD peak between raw powders and sintered parts has no
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relationship with change of diffraction angle of the phases but their constitution fraction because their diffraction peak is very close and overlaps with each other. Therefore, change of sintering temperature not only put strong influence on densification and
microstructure evolution of sintered HCCIs, but also on their matrix’s phase constitutions. 3.5 The influence of sintering temperature on the mechanical properties The effect of sintering temperature on bending strength and impact toughness of sintered HCCIs was shown in Fig.6. The results indicated that both of them increased firstly as temperature
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ACCEPTED MANUSCRIPT was elevated, and then reached their maximum while sintering temperature was at 1245°C, 2241MPa and 8.0J/cm2 respectively, and finally declined as it was further raised. It was worth to mention that the bending strength and impact toughness of heat treated cast HCCI mentioned
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above were of 1008MPa and 2.44J/cm2 correspondingly. Hence, HCCIs’ mechanical properties could be improved effectively when they were manufactured by the PM process designed by us. As it was presented above, the morphology of carbides was deeply modified in PM HCCIs, and
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their distribution uniformity was greatly improved as well, which could vastly reduce stress
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concentration caused by carbides.
The three-dimensional morphology of carbides in both the PM HCCI and the cast one was studied by deep etching and submitted in Fig. 7. It could be easily recognized that carbides in cast HCCI were contiguous in space with each other, and their petals were so fully developed that they
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grew in clusters, between which narrow gaps or hollow tubes could be observed. They would certainly result in serious stress concentration, especially around the sharp edges of a petal, and deteriorated its strength and toughness. On the contrary, the development of carbides in PM HCCI
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was habited to some extent for the low sintering temperature and limited volume fraction of liquid
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phase during SLPS. In practice, they grew in the form of square bars with little petals. Their continuity in space was also much lower than those in cast HCCI. No structures like pores, gaps and hollow tubes could be found in or between them. Therefore, their strength and toughness were surely much higher than those of cast ones for their unique microstructures. Basing on the results on PM HCCIs and analysis on their microstructure evolution and mechanical behaviors mentioned above as well as the nature of SLPS, we proposed a model of microstructure evolution of PM HCCIs during SLPS as it was shown in Fig.8. The total process
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ACCEPTED MANUSCRIPT could be divided into three phases according to their microstructure characteristics. At the initial stage liquid phase is firstly generated at grain boundaries near interfaces between the raw powders in a green compact as shown in Fig. 8 (a), where it reaches sintering temperature earliest. Powders
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will be rearranged and gradually closely packed as shown in Fig. 8 (b), because of capillary force and lubrication resulted from liquid phase which increases as holding time passes. At the middle stage as shown in Fig. 8 (c) more and more liquid phase are formed along boundaries and finally
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causes the powders break into many single grains or sub-grains. At the moment grains practically
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soak in a liquid phase sea and will be closely packed again by their moving or turning because of capillary forces, as shown in Fig. 8 (d), which effectively enhances densification of a compact being sintered. Meanwhile, some carbides born at or near grain boundaries of raw powders will float into liquid metal, and then their coarsening preferentially carries out because of the high atom
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diffusion coefficient in molten metal as holding time passes. Ostwald ripening also takes place among grains and carbides respectively. As a result, carbides in the liquid are further coarsened, but those in solid grains gradually become smaller or even disappear. Nevertheless, there were still
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some carbides in solid grains, and the coarsening of grains is still acceptable because carbides
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along grain boundaries do play a role against their ripening. 4. Conclusions
A hypoeutectic HCCI (20wt.%Cr) with full density has been manufactured by SLPS, and the
suitable sintering temperature window was around 20°C. There were only M7C3-type carbides in the form of square bars, which uniformly distributed along grain boundaries, and its matrix consisted of martensite and small amount of austenite. The sintered HCCI was of high performance, and their optimized mechanical property combination could be as following:
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Acknowledgements This research work is supported by the National Science Foundation of China No.
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51574119.
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Captions of figures Table 1 Chemical composition of aerosolized HCCI powders (wt%).
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Fig.1 Microstructure of aerosolized HCCI powders: (a) SEM morphology of gas atomized HCCI, (b) OM microstructure of etched HCCI powders.
Fig.2 The microstructure of PM HCCI sintered at different temperature: (a) 1235°C,(b) 1240°C, (c)
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1245°C, (d) 1250°C, (e) 1255°C, (f) 1260°C, (g) 1265°C, (h) 1270°C, (i) The conventional cast
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HCCI
Fig.3 The Effect of different holding time on microstructure evolution: (a) 60min, (b) 90min, (c) 120min, (d) 150min.
Fig.4 The effect of sintering temperature on density and hardness.
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Fig.5 X-ray diffraction patterns of sintered HCCI as well as raw powders. Fig.6 The relationship between bending strength and impact toughness with sintering temperature. Fig.7 SEM deep etched microstructure: (a) as-cast HCCIs, (b) as-sintered.
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Fig.8 Schematic diagram of powder metallurgy liquid phase sintering microstructure evolution: (a)
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Initial stage, (b) Middle stage, (c) Final stage, (d) The final microstructure. (Red for carbide, light green for liquid metal)
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Table1 Chemical composition of aerosolized HCCI powders (wt.%). Mo
Cu
Si
Mn
V
2.7
19.6
1.5
1.0
0.8
0.9
1.0
S/P
Fe
≤0.05
Bal
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Cr
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1.PM HCCIs (hardness HRC 63, bending strength 2241MPa and impact toughness 8.0J/cm2). 2.Suitable temperature range was from 1245 to 1265°C (practical applications in industry).
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3.Increasing the cost (combination of heat treatment and sintering).
S0925-8388(17)33958-0
DOI:
10.1016/j.jallcom.2017.11.189
Reference:
JALCOM 43882
To appear in:
Journal of Alloys and Compounds
Received Date: 28 March 2017 Revised Date:
13 November 2017
Accepted Date: 14 November 2017
Please cite this article as: J. Gu, P. Xiao, J. Song, Z. Li, R. Lu, Sintering of a hypoeutectic high chromium cast iron as well as its microstructure and properties, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.189. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Sintering of a hypoeutectic high chromium cast iron as well as its microstructure and properties Jinghong Gua, Pingan Xiao a, *, Jianyong Song a, Zhihua Li a, Ruiqing Lu a College of Materials Science and Engineering, Hunan University, Changsha 410082, P. R. China
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Abstract A hypoeutectic high chromium cast iron (HCCI) was fabricated by super-solidus liquid phase sintering (SLPS) technology with gas atomized powders as raw materials. The effects of sintering parameters on densification, microstructure evolution and mechanical properties of the
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alloy were investigated systemically. The results showed that samples with full density could be fabricated and sintering temperature window suitable for SLPS is around 20
. The X-ray
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diffraction (XRD) revealed that the hypoeutectic HCCI mainly consists of martensite matrix and M7C3 carbide as well as small amount of austenite, and optical metallographic analysis showed that M7C3 carbides are of crystal square bars with uniform distribution in the matrix. As sintering temperature or holding time is raised, both grain and carbide are gradually coarsen, while the strength and toughness of the alloy increase at first and then decline. The mechanical properties of
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sintered hypoeutectic HCCI in an optimized sintering process were of hardness HRC63, bending strength 2241MPa, and impact toughness 8.0J/cm2. A model about microstructure evolution in sintered hypoeutectic HCCIs has been proposed.
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Key words: hypoeutectic HCCIs; SLPS; microstructure; mechanical properties; model
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*Corresponding author: Pingan Xiao E-mail address: [email protected](P. Xiao) Tel./fax:+86 073188821610
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ACCEPTED MANUSCRIPT 1. Introduction As an important abrasion-resistant materials, high chromium cast irons (HCCIs) have been widely used in mineral processing, cement manufacturing, slurry pumping and paper
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manufacturing industries[1,2]. The combination of high volume fraction of M7C3-type hard carbides and relatively ductile ferrous matrix (commonly austenite and martensite) is the main reason why HCCIs have excellent wear resistance[3-7]. However, practical applications show that
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the major problem faced by HCCIs is still a good balance of hardness and toughness. There are
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large amount of chrysanthemum-like carbides with sharp edge in normal HCCIs’ microstructure, which practically plays a role of both pores and stress concentration raisers and then deteriorates toughness. In order to improve carbides’ morphology much effort has been done, and some techniques & skills have been developed, such as multi-alloying[8-9], metamorphism and
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inoculation[10-15], semi-solid processing[16-17], rapid/directional solidification[18], and heat treatment[4,19-21]. Some of them, such as rapid/directional solidification and semi-solid processing, do notably change the morphology of carbides and effectively reduce their
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consecutiveness in space, and hence greatly increase HCCIs’ strength and toughness, but their
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high cost prohibits practical applications in industry. The others can in certain cases affect HCCIs’ microstructure evolution at low cost and improve their mechanical properties, but are not equally efficiency as the former ones. Typically, the bending strength of normal HCCIs is around 900MPa and impact toughness is around 4J/cm2 when a standard sample with 5×5×50mm is tested. Fabrication of HCCIs by P/M process with alloyed powder as starting materials can provide a unique thermodynamic condition for microstructure evolution in comparison with casting process. Firstly, atomized raw powders possess rapid solidification microstructure and offer excellent
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Therefore, a new type of HCCIs with unique microstructure and high performance could be developed.
In this paper, a HCCI was prepared by a pressing plus super-solidus liquid phase sintering
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(SLPS) process with gas atomized powders as raw materials. Its densification, microstructure
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evolution and mechanical properties were systematically evaluated respectively in order to make it have a good combination of strength and toughness. 2. Materials and experiment methods 2.1 Preparation of samples
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The raw powders of a hypoeutectic HCCI were prepared by nitrogen gas atomization and its chemical composition was listed in Table 1. The cast HCCI samples have the similar chemical compositions with the raw powders. First, initial charge materials were steel scrap, 1000 kg of raw
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material was melted in a medium frequency induction furnace at 1500 ±50
after removal of any dross and slag. The particles’ size was analyzed
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sand mold at 1500 ±20
,then poured into a
by a BT-9300H Laser particle size analyzer; it was revealed that their size distribution range was narrow with D50 around 20.11µm. As super-solidus liquid phase sintering (SLPS) technology was applied to obtain samples with full density, the powders were also detected by a Sta449C simultaneous differential thermal analyzer in order to determine their solidification temperature range which usually indicated the sintering temperature window. For a hypoeutectic HCCI as a Fe-C-Cr ternary alloy its solidification temperature range could be divided into two parts, the
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ACCEPTED MANUSCRIPT eutectic transformation temperature interval and the one above it. Obviously, the sintering window for SLPS in fabrication of PM HCCI with unique microstructure should be within eutectic transformation temperature interval where proeutectic austenite, carbide and liquid metal
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coexisted and the ratio of solid austenite plus carbide to molten metal could be regulated by changing sintering temperature. Otherwise, if sintering temperature was above eutectic transformation temperature carbides will disappear and their final morphology will be mainly
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depend on cooling speed after sintering. The result from differential thermal analysis exhibited
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that the temperature range of eutectic transformation is of 1232~1273°C.
1% styrene-butadiene rubber (SBR) was added into the raw powders as lubricant to improve their compacting ability. Green compacts with dimensions of 80×17×13mm were firstly pressed with pressing pressure between 200~300MPa on a Y32-315A four-column hydraulic press, and
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then super-solidus liquid phase sintered to obtain samples with high density in a GSL1600X vacuum tube furnace. All samples were heated to sintering temperature at a heating rate of 2 °C min-1, and when holding time was finished, they were firstly cooled to 450°C with cooling rate of
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5°Cmin-1, and afterward went through gradual furnace cooling to room temperature. In order to
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understand the relationship among sintering, microstructure and mechanical properties, the influence of such sintering parameters as sintering temperature and holding time was systemically investigated.
2.2 Characterization The density of the sintered samples was measured by Archimedes method. Microstructure inspection was operated on a Leitz-MM6 optical microscope (OM) and/or a FEI QUANTA200
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ACCEPTED MANUSCRIPT environmental scanning electron microscope (SEM). To observe the three-dimensional morphology of carbides in sintered HCCI, a specimen was firstly etched in alcohol with 4% hydrochloric acid for more than 24 hours and then cleaned in an ultrasonic dispersing instrument
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with water before SEM inspection. Its phase constitution analysis was undertaken with a D8-advance X-ray diffraction instrument (XRD, Cu target, λ = 0.15405nm).
Mechanical property specification of the sintered HCCI covered hardness, impact toughness
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and bending strength. Hardness measurement was conducted by DUR-O-Test and each result was
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the average of more than 5 inspections. Specimens for both bending strength and impact toughness inspection were firstly wire cut and then ground to the size of 5 × 5 × 35mm and 5 × 5 × 50mm respectively. Bending strength and impact toughness were tested on an Instron3369 universal mechanical testing machine and a XJ-40A impact tester separately.
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3. Results and discussion
3.1 Microstructure of raw material powders
The gas atomized raw powders were spherical as shown in Fig. 1(a) and their microstructure
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was revealed in Fig.1(b) after etched by ferric chloride hydrochloric acid. There were many
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bouquet-like outgrowth structures with a distinct nucleation from which a lot of fine rods or flakes grew out as presented in Fig.1(b). This unique type of microstructure should have close relationship with rapid solidification of powders during gas atomization. According to literature [22] there were M7C3-type carbides in atomized HCCI powders, thus the fine rod or flake-like phases were M7C3-type carbides. 3.2 Effect of sintering process on microstructure evolution The influence of both sintering temperature and holding time on microstructure evolution of
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ACCEPTED MANUSCRIPT the hypoeutectic HCCI was systematically investigated. Its microstructures sintered at different temperatures were presented in Fig. 2 as holding time was kept the same as 120 min. White carbides were mainly distributed along grain boundaries discontinuously as shown in Fig. 2. The
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sizes of both grains and carbides in sintered HCCI were obviously much finer in comparison with normal cast one with almost the same composition as in Fig. 2(i), which was prepared by authors themselves, and the carbides were of uniform distribution and low continuity in space. With
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increase of sintering temperature, both grains and carbides was coarsen, and sectioning growth of
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the carbides also gradually took place.
SLPS belongs to liquid phase sintering. As sintering temperature was raised in the eutectic transformation temperature range of the alloy, the volume fraction of liquid phase became larger, which firstly appeared along grain boundaries and then sub-grain boundaries, and thus improved
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growth condition for carbide coarsing. Furthermore, the coarsening of carbides also had relationship with Ostwald ripening in liquid phase sintering [23]. As a result shown in Fig. 2, small carbides along boundaries and in grains gradually faded away and some large ones appeared
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along the boundaries. In addition, pores could be recognized as sintering temperature was lower
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than 1245°C due to too smaller fraction of liquid phase to get a sample fully densified. The influence of holding time on microstructure evolution of sintered samples was shown in
Fig.3 when sintering temperature was at 1257°C. It was revealed that samples were almost fully densified when holding time was as short as 1 hour. With extra holding time, coarsening and Ostwald ripening of both grains and carbides were easily observed and developed almost in the same evolution behavior as that shown in Fig. 2. However it could be noticed that the benefit of holding time on microstructure evolution was much weaker than that of sintering temperature by
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ACCEPTED MANUSCRIPT comparison Fig. 2 with 3. Raising sintering temperature not only boosted diffusion coefficient of atoms in the alloy, but also increased the liquid metal volume fraction involved, which provided fast track for atom diffusion, and hence effectively accelerated microstructure coarsening.
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Since liquid phase was firstly generated along grain boundaries in raw powders and interfaces between them, which would be also converted into grain boundaries in sintering, and then along subgrain boundaries in SLPS process, carbides at these places (or boundaries) grew fast
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and could play a role in preventing grains from merging with each other. Moreover, grains or
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sub-grains in raw powders could be split apart during SLPS and these smaller solid particles would be rearranged into close-packing in the presence of capillary force as they were soaked in a liquid metal sea. As results, PM HCCI was of equiaxed grains with small size and no proeutectic austenite dendrites could be recognized in its matrix, shown in Fig. 3.
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3.3 The influence of sintering temperature on densification and hardness The influence of sintering temperature on density and hardness of sintered HCCI was shown in Fig.4 with the holding time of 2 hours. It revealed that the change of both density and hardness
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performed almost in the same way as a function of sintering temperature. It rapidly rose at first
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and then reaching a steady peak, and finally gradual declining if temperature was further increased. When sintering temperature was in 1260 ~ 1265°C density and hardness reached their maximum values of 7.63g/cm3 (the theoretical density is 7.70 g/cm3) and HRC63 respectively. Elevating temperature would increase the volume fraction of liquid metal during SLPS, which greatly enhanced densification of a sample. Thus, both density and hardness were improved simultaneously. With carbon equivalent higher than 2.7%, the alloy’s molten liquid phase was of low viscosity and good flowability. Thus, a smaller volume fraction of liquid phase could be
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ACCEPTED MANUSCRIPT demanded to ensure obtain sintered samples with high density and hardness, which in turn made the sintering temperature window of SLPS available for PM HCCIs wider, as shown in Fig.4. However, if sintering temperature was raised too close to the upper limit of the alloy’s eutectic
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transformation temperature range, excess volume fraction of liquid phase would not only lead to specimen deformation, but also cause serious microstructure coarsening and non-uniform distribution of carbides, which resulted in the decline of both density and hardness. As it can be
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seen from Fig.4, suitable temperature range of SLPS for the HCCI was from 1245 ~ 1265°C.
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3.4 Phase constitution of sintered HCCI
The X-ray diffraction patterns of HCCIs sintered at different temperatures as well as their raw powders were shown in Fig.5. It was revealed that they were composed of M7C3-type carbides, martensite and austenite, and their matrix was mainly of martensite instead of austenite as in
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normal cast HCCIs. The austenite content in raw powders was distinctly higher in comparison with PM HCCIs, which lead to the XRD strongest peaks of M7C3-type carbides and martensite were overlapped as shown in Fig.5. However, as sintering temperature was close to 1270 , the
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amount of austenite apparently increased.
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HCCIs were of excellent hardenability and their martensite transformation could be carried out in air cooling. Hence their amount of martensite at ambient temperature had close relationship with its martensite start temperature Ms. It was found that martensite precipitated on the matrix’s side along the interface between carbides and the matrix as shown in Fig. 2(i), where there was a chromium-poor zone in the matrix nearby the carbides due to precipitation of eutectic carbides. However this belt-like region was quite narrow in cast HCCI because of high diffusion coefficient of elements during eutectic transformation. High chromium content in HCCI’s matrix would
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ACCEPTED MANUSCRIPT improve its hardenability but lower its Ms, and as a result, martensite could be detected in chromium-poor zone (in a cast HCCI) and the other part of the matrix usually remained as supercooled austenite. On the contrary, eutectic transformation lasted for a quite long time in the
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fabrication process of a PM HCCI as SLPS was applied, which means that the chromium-poor zone could extend all over the matrix because of uniform and full precipitation of vast carbide particles. Hence, the average chromium content in its matrix should be normally lower than that of
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a cast HCCI, and the former’s Ms was higher than the latter’s. The results of chromium content
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inspection by energy dispersive spectroscopy (EDS) at the center of the matrices of both PM and cast HCCIs in Fig.2 (c) and (i) were of 7.31% and 13.55% respectively, which confirmed our point of view on Ms difference between them. Therefore, there was much more amount of martensite in PM HCCIs than that in cast ones. In addition, both the volume fraction and Cr content of liquid
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phase would become larger and higher separately as temperature of SLPS was raised, resulting in less carbide precipitation and a matrix with higher Cr content. Accordingly, austenite’s Ms of the alloy was reduced and there were more supercooled austenite in its matrix as shown in Fig. 5. By
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the way, shift of the strongest XRD peak between raw powders and sintered parts has no
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relationship with change of diffraction angle of the phases but their constitution fraction because their diffraction peak is very close and overlaps with each other. Therefore, change of sintering temperature not only put strong influence on densification and
microstructure evolution of sintered HCCIs, but also on their matrix’s phase constitutions. 3.5 The influence of sintering temperature on the mechanical properties The effect of sintering temperature on bending strength and impact toughness of sintered HCCIs was shown in Fig.6. The results indicated that both of them increased firstly as temperature
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ACCEPTED MANUSCRIPT was elevated, and then reached their maximum while sintering temperature was at 1245°C, 2241MPa and 8.0J/cm2 respectively, and finally declined as it was further raised. It was worth to mention that the bending strength and impact toughness of heat treated cast HCCI mentioned
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above were of 1008MPa and 2.44J/cm2 correspondingly. Hence, HCCIs’ mechanical properties could be improved effectively when they were manufactured by the PM process designed by us. As it was presented above, the morphology of carbides was deeply modified in PM HCCIs, and
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their distribution uniformity was greatly improved as well, which could vastly reduce stress
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concentration caused by carbides.
The three-dimensional morphology of carbides in both the PM HCCI and the cast one was studied by deep etching and submitted in Fig. 7. It could be easily recognized that carbides in cast HCCI were contiguous in space with each other, and their petals were so fully developed that they
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grew in clusters, between which narrow gaps or hollow tubes could be observed. They would certainly result in serious stress concentration, especially around the sharp edges of a petal, and deteriorated its strength and toughness. On the contrary, the development of carbides in PM HCCI
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was habited to some extent for the low sintering temperature and limited volume fraction of liquid
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phase during SLPS. In practice, they grew in the form of square bars with little petals. Their continuity in space was also much lower than those in cast HCCI. No structures like pores, gaps and hollow tubes could be found in or between them. Therefore, their strength and toughness were surely much higher than those of cast ones for their unique microstructures. Basing on the results on PM HCCIs and analysis on their microstructure evolution and mechanical behaviors mentioned above as well as the nature of SLPS, we proposed a model of microstructure evolution of PM HCCIs during SLPS as it was shown in Fig.8. The total process
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ACCEPTED MANUSCRIPT could be divided into three phases according to their microstructure characteristics. At the initial stage liquid phase is firstly generated at grain boundaries near interfaces between the raw powders in a green compact as shown in Fig. 8 (a), where it reaches sintering temperature earliest. Powders
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will be rearranged and gradually closely packed as shown in Fig. 8 (b), because of capillary force and lubrication resulted from liquid phase which increases as holding time passes. At the middle stage as shown in Fig. 8 (c) more and more liquid phase are formed along boundaries and finally
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causes the powders break into many single grains or sub-grains. At the moment grains practically
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soak in a liquid phase sea and will be closely packed again by their moving or turning because of capillary forces, as shown in Fig. 8 (d), which effectively enhances densification of a compact being sintered. Meanwhile, some carbides born at or near grain boundaries of raw powders will float into liquid metal, and then their coarsening preferentially carries out because of the high atom
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diffusion coefficient in molten metal as holding time passes. Ostwald ripening also takes place among grains and carbides respectively. As a result, carbides in the liquid are further coarsened, but those in solid grains gradually become smaller or even disappear. Nevertheless, there were still
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some carbides in solid grains, and the coarsening of grains is still acceptable because carbides
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along grain boundaries do play a role against their ripening. 4. Conclusions
A hypoeutectic HCCI (20wt.%Cr) with full density has been manufactured by SLPS, and the
suitable sintering temperature window was around 20°C. There were only M7C3-type carbides in the form of square bars, which uniformly distributed along grain boundaries, and its matrix consisted of martensite and small amount of austenite. The sintered HCCI was of high performance, and their optimized mechanical property combination could be as following:
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Acknowledgements This research work is supported by the National Science Foundation of China No.
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51574119.
References
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Captions of figures Table 1 Chemical composition of aerosolized HCCI powders (wt%).
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Fig.1 Microstructure of aerosolized HCCI powders: (a) SEM morphology of gas atomized HCCI, (b) OM microstructure of etched HCCI powders.
Fig.2 The microstructure of PM HCCI sintered at different temperature: (a) 1235°C,(b) 1240°C, (c)
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1245°C, (d) 1250°C, (e) 1255°C, (f) 1260°C, (g) 1265°C, (h) 1270°C, (i) The conventional cast
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HCCI
Fig.3 The Effect of different holding time on microstructure evolution: (a) 60min, (b) 90min, (c) 120min, (d) 150min.
Fig.4 The effect of sintering temperature on density and hardness.
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Fig.5 X-ray diffraction patterns of sintered HCCI as well as raw powders. Fig.6 The relationship between bending strength and impact toughness with sintering temperature. Fig.7 SEM deep etched microstructure: (a) as-cast HCCIs, (b) as-sintered.
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Fig.8 Schematic diagram of powder metallurgy liquid phase sintering microstructure evolution: (a)
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Initial stage, (b) Middle stage, (c) Final stage, (d) The final microstructure. (Red for carbide, light green for liquid metal)
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Table1 Chemical composition of aerosolized HCCI powders (wt.%). Mo
Cu
Si
Mn
V
2.7
19.6
1.5
1.0
0.8
0.9
1.0
S/P
Fe
≤0.05
Bal
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1.PM HCCIs (hardness HRC 63, bending strength 2241MPa and impact toughness 8.0J/cm2). 2.Suitable temperature range was from 1245 to 1265°C (practical applications in industry).
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3.Increasing the cost (combination of heat treatment and sintering).