Towards the understanding of rare earth microalloying on the improvement of thermal stability of intragranular austenite and mechanical property of TRIP steels
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Thermal Stability
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This study investigated the austenite stability and deformation behavior of cyclic quenching-austenite reverse transformation processed Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb medium Mn steel. A number of findings were obtained. Most importantly, the extent of the TRIP effect was mainly determined by an appropriately retained austenite stability rather than its content. Simultaneously, chemical elements were the key factors affecting austenite stability, of which Mn had the greatest impact, while the difference of retained austenite grain size and Mn content resulted in different degrees of retained austenite stability. Additionally, there were still large amounts of strip and granular-retained austenite shown in the microstructure of the CQ3-ART sample after tensile fracture, revealing that the excessively stable, retained austenite inhibited the generation of an extensive TRIP effect.
TRIP steel
Thermal Stability
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According to the actual heat treatment condition,the heat treatment process was simulated on a rapid phase transition device,and the influence of retained austenite on reversed austenite content was concluded by controlling the temperature after aging.The results showed that the content of alloying elements influenced the austenite content after the final aging process,and with the increase of retained austenite content after intermediate treatment,the content of austenite increased first and then decreased after final aging.Controlling the content of retained austenite strictly was one of the methods to adjust the content of reversed austenite in blade.
Content (measure theory)
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Metastability
Structural material
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Tempering
Diffusionless transformation
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Diffusionless transformation
Isothermal process
Isothermal transformation diagram
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As environmental temperature decreases, the amount of retained austenite is more likely to greatly reduce due to the thermal austenite–martensite transformation caused by the decreased thermal stability of retained austenite, probably making its amount lower than the required content. In the present study, the thermal stability of retained austenite in Cr–Ni weld metals was investigated to see whether sufficient retained austenite can be maintained at low temperatures. The specific experimental procedure is as follows: briefly, the samples were cooled in turn from room temperature to 0, −20, −40, −60, −80, −100 and −196°C; the amount of retained austenite at the above temperatures was measured using X-ray diffraction. Through investigating the dependence of the content of retained austenite on temperature, it was revealed that when the content of retained austenite is <20, retained austenite can be maintained until −196°C.
Thermal Stability
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Cryogenic Treatment
Diffusionless transformation
Volume fraction
Thermal Stability
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木硏究는 Ms點이 약 -28℃이며 炭素量을 0.33%까지 各組成으로 변화시킨 3種의 Fe-Ni-C 合金을 마련하여, austenite 組織에 직접 加工을 부여한 경우와 prior martensite 組織에 加工을 부여한 兩경우에 대하여, 加工度에 따른 austenite 安定化를 調査하여 相互比較하였으며, 또한 兩경우에 있어서 austenite 安定化에 미치는 炭素量의 影響도 아울러 調査하였다.
Fe-Ni-C 合金의 austenite 相에 各種 加工度를 부여해서 -196℃에 深冷處理한 結果, 加工度가 증가할수록 martensite 中의 殘留 austenite量은 증가하였으며, 同一 加工度에서는 炭素量이 많은 鋼일수록 加工에 依한 殘留 austenite의 증가량은 많았다.
한편 Fe-Ni-C 合金의 austenitet相을 -196℃에 深冷處理하여서 된 martensite 組織에 各種 加工度를 부여한 後, 이를 逆變態시켜 다시 -196℃에 再深冷處理를 行하였을때, 加工度에 의한 盞留 austenite 증가량은 加工度와 더불어 증가되지만, 同一 加工度에서는 炭素量에 관계없이 同一한 증가량을 나타냈다.
그러나 모든 加工度에 있어서 Fe-Ni-C 合金의 austenite 安定化는, normal austenite相에 직접 加工한 경우보다, martensite相을 加工하여 이를 逆變態시켜서 된 reverted austenite의 경우가 더 컸었다.
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The amount of residual austenite in martensitic roll steels is an important technological parameter of heat treatment, which affects the performance properties of the rolls. But determining its amount in roll steels is a complex and not fully solved scientific and technical problem. The aim of the work was to comparatively analyze the amount of residual austenite in the structure of alloy steel rolls by X-ray diffraction, ultrasonic methods and metallography analysis. However, the qualitative difference of microstructures in the content of the light phase - austenite, confirms the results of X-ray diffraction analysis. No correlation was found between the austenite content in the samples and their hardness. It was found that the X-ray method, based on the comparison of the intensities of the α- and γ-phase lines of iron, overestimates the value of the amount of residual austenite in some samples of roll steels. The results of the analysis of residual austenite by ultrasound rate showed better convergence. The amounts of residual austenite, calculated on the sample of stainless steel (100% γ-Fe), had reduced values (2.6-4.5%). The most accurate results on the amount of residual austenite gave the use of the established regression dependence with the selected standard (2.7-7.8%). This dependence is obtained at the speed of sound in austenite ~ 4000 m / s. It is determined that the application of the ultrasonic method allows to determine the content of residual austenite in the samples of roll steels quite quickly and accurately.
Metallography
Alloy steel
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