SEM investigation of minor constituents of carbide materials prepared from shungite rocks
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The report deals with a study on the behavior of carbides in low-W high-speed steel and low W-Co high speed steel.The total amounts of carbides in the several heat-treated specimens of the high speed steels were determined by electrolytic isolation method.Chemical compositions and crystalline structures of the isolated carbides were determined by chemical analysis and X-ray diffraction, and the shapes of the carbides were observed by electron microscope, and the following results were obtained.(1) In annealed states, the amounts of carbides in these steels are about 20-22% wt%, and about 85-95% of the W and V in the steels are concentrated in the carbides, while the most portions of the Si, Mn and Co are dissolved in the matrix. The carbides precipitated in these annealed steels are M6C, MC and M23C6.(2) In quenched states, the insoluble carbides in the high speed steels are only M6C, and it amounts to about 10 wt%, which is the same about 40% of the carbides in annealed state.(3) The more rises the tempering temperature, the more precipitate the carbides from the austenite. And, especially, the concentration percentage of the special alloy elements in the carbides increases suddenly at the tempering of 575°C, and MC carbide appeares in the steel by tempering at above 300°C.
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The research in the cryogenic treatment of tool steels and other alloy steels is important as it causes substantial increase in the wear characteristics of alloy and tool steels. The present day research of this technology indicates precipitation of submicroscopic carbides referred to as eta carbides. This work comprises of reviewing the carbide types that form during heat treatment of high speed steels and presenting the results of the experiment for finding eta carbide in the cryogenically treated AISI T42 high speed steels using advanced microscopy. The results of this work indicate the contribution from the other secondary carbides (other than eta carbides) in the enhancement of wear characteristics in high speed steels.
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The morphology and chemical composition of MC and M23C6 carbides in the cast nickel-based superalloy K52 aged in a high magnetic field was investigated.The results show there is no significant influence of a high magnetic field on the morphology and distribution of MC and M23C6 carbides.However,when aged in a high magnetic field,the content of strong carbide forming elements,such as W,Mo,Nb and Ti,in the MC and M23C6 carbides increases slightly,on the contrary,the content of weak or non-carbide-forming elements such as Cr,Co and Ni,in the MC and M23C6 carbides decreases obviously.These results indicate that under a high magnetic field,the affinity of carbon to the strong carbide forming elements is enhanced rather than to the weak or non-carbide-forming elements,therefore the content of strong carbide-forming elements in carbides increases and contrary that of the weak or non-carbide-forming elements decreases.
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Carbides in HSS roll were studied systematically. The results showed that there were two kinds of carbides in HSS roll, 1st and 2nd carbides. 1st eutectic carbides included the chrysanthemum-like MC mainly consisting of V and the plate-like M2C rich in Mo. 2nd carbides were complex carbides, concentrating Fe, Cr, Mo and V elements. The morphology, distribution and alloy concentration of the 1st eutectic carbides were studied using OM, SEM and EDS. During heat treatment, evolution of the 1st eutectic carbides was analyzed. It was found that the MC carbides had no change in morphology, components and distribution, while the M2C carbides decomposed to M6C and MC. In order to improve the properties of HSS, a method of increasing solidification rate was defined to refine the eutectic carbides. After refinement, the eutectic carbides distributed both along the grain boundaries and inside the grains.
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Effect of an Mg–Re complex treatment on eutectic carbides in bearing steel has been studied. Results show that the number, and the average maximum size of carbides tend to decrease and then increase with the increase in Mg content. When the Mg content is low, the addition of Re is not conducive to the refinement of carbides; When the Mg content is high, the number and size of carbides decreases with the increase of Re. The main reason for the refinement of carbides by Mg is that the diffusion of Mg hinders the diffusion of C and Cr, so that the formation and growth of carbides are inhibited. A large number of MgS-MnS can induce the precipitation of carbides, which also leads to the dispersion of carbides. While the Re‑Mg complex treatment, the inclusions containing Re and segregation of Mg is the main reason for the refinement of carbides.
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Carbides in new type Hf-Ta-containing nickel-based superalloy FGH98 Ⅰ powders prepared by plasma rotating electrode processing(PREP)before and after pre-heat treatment at different temperatures were studied.The results show that MC' type carbides in the initial powders can be divided into two groups.One is rich in Ti,Ta,Nb and the other contains the elements of Ta,Hf and Zr.The two types of carbides both contain a certain amount of non-carbide forming elements(Co,Ni)and medium carbide forming elements(Cr,Mo),and these carbides locate between interdendrites or cells and in block and granule shapes.Ti-Ta-Nb riched MC' type carbides in the powders can be transformed into MC type carbides and the total amount of Ti-Ta-Nb increases with increasing pre-heat treatment temperature.The Ta-Hf-Zr contained MC' type carbides can be decomposed and transformed into M23C6,M6C and MC type carbides.It is also shown that the precipitate and dissolution temperatures of M23C6 type carbides are 950℃ and 1150℃ respectively,and the co-existing temperature of M23C6 and M6C type carbide is within the range from 1000℃ to 1100℃.In addition,trance elements Hf and Ta in the powders take part in the reactions of carbides mainly in the form of carbides and γ' phases.
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Carbide particles in T91 steel after service for different time were obtained by electrolytic extraction method and their phase,amount,morphology and composition were analyzed by SEM,XRD and EDS.The results show that the carbides in T91 steel exist mainly in two forms: M23C6,which is the majority,and MX.With increasing service time,the M23C6 carbides coarsen gradually along grain-boundary,while the MX carbides are relatively stable.No significant increase of size and amount of MC carbides are observed in the T91 steel even after service for 105296 hours.Morphologically,the initial rod-like carbides are spheroidized gradually.Mo,Cr,V in solid solution diffuse into M23C6,which leads to the increase of Mo,Cr,V and the decrease of Fe in the carbides as service time increases.
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In order to investigate the several characteristics of high-speed steels, which were made in our works, we isolated carbides in steel by electrolysis, and studied the composition, the crystal structure and the shape of carbides by chemical and X-ray analysis, electron microscopy etc. This report, especially lays stress on the relation of cutting durability and carbides in high-C high-V high-speed steel. The results obtained were as follow: (1) The contents of carbides in annealed state was about 20% and existed in three types of carbides, M6C, MC and M23C6. The content of carbide MC in this steel was several times that in other high-speed steel. (2) In quenched state, the amount of insoluble carbides was about 8%. These carbides consisted nearly of MC alone. The remainder of carbides was only M6C, (3) With the progress of tempering, the carbide contents increased, especially suddenly at 575°C. But up to 650°C, the weight percentages between the two types of carbides, MC and M6C, remained unchanged. (4) This high-C high V (XVC1) steel, in spite of low W and no Co contents, indicated an as good cutting durability as W-Co (X00) steel. We consider, this is due to the presence of a number of small and hard carbides MC grains effectively serving for the purpose.
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