Continuous cooling transformation of undeformed and deformed low carbon pipeline steels
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Acicular ferrite
Continuous cooling transformation
Acicular
Beta ferrite
CCT diagrams and microstructure of 09CuPCrNi steels were investigated using dilatometric method and optical microscopy respectively.Compared with that of the CCT diagram of the conventional 09CuPCrNi steel,the transformation range of bainite is partly separated from that of ferrite when the adding amount of Mo is 0.33%wt,and they are completely separated when the amount of Mo is 0.41%wt,also the transformation ranges of pearlite and bainite are separated completely.
Continuous cooling transformation
Isothermal transformation diagram
Weathering steel
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Continuous cooling transformation (CCT) diagrams were determined for high strength low alloy (HSLA) steel by the dilatometric method as a function of different austenitising temperatures from 900 to 1300°C. The diagrams show significant suppression of polygonal ferrite at higher cooling rates and higher reheat temperatures (1100–1300°C), a prominent transformation region attributed to bainite or acicular ferrite at temperatures intermediate between those of polygonal ferrite and martensite formation. The CCT diagrams for the steel are characterised by the formation of acicular ferrite structure for a wide range of cooling rates. The initial austenite grain size did not affect the transformation temperatures of austenite to acicular ferrite at all cooling rates. The microstructure of steels cooled at very low cooling rates (<0·1 K s−1) exhibited a two-phase structure of acicular ferrite and polygonal ferrite. A model based on the rule of mixtures was used to predict the strengthening components in the steel in the tempered condition. The major contributions to strengthening in the alloy were found to be from solid solution strength, precipitation strength, and ferrite lath strengthening. The contribution to dislocation strengthening in the tempered condition was found to be negligible. The calculated strength is in excellent agreement with the experimental values.
Acicular ferrite
Continuous cooling transformation
Lath
High-strength low-alloy steel
Beta ferrite
Acicular
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The metallographic optical microscope and scanning electron microscope(SEM)were applied to analyze the morphology of acicular ferrite microstructures.Acicular ferrite microstructures with the same growth directions and nucleation of acicular ferrite in inclusions were identified.Distinct characteristics of nucleation of acicular ferrite at different grain boundaries were also discovered through the SEM. Nucleation and growth of acicular ferrite at straight grain boundaries were more regular than at large curvature grain boundaries.Vickers hardness of acicular ferrite microstructures measured was about 377HV3, but the value of Vickers hardness could not accurately reflect the intrinsic hardness in the theoretical analysis.
Acicular ferrite
Acicular
Vickers hardness test
Beta ferrite
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The curves of Continuous Cooling Transformation (CCT) of supercooled austenite for a new low-cost high-speed steel (GDL -4) were measured by expansion instrument and Metallographic-Hardness method. The microstructure and hardness characteristic of the steel’s transformation during the continuous cooling process were analysed. Results show that not only pearlite and martensite phase transition can be obtained at different temperature, but bainite phase zone can be obtained at the temperature from 303 ℃ to 383 ℃ as well. The Ac1of the steel is 890 ℃ and the critical cooling rate of the steel is 30 ℃/min. The bainite and pearlite can be obtained at range of cooling rate from 15 ℃/min to 11 ℃/min. The pearlite can be obtained when the cooling rate is less than 10 ℃/min.
Continuous cooling transformation
Cooling curve
Supercooling
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The dynamic CCT curves of 07MnNiMoVDR steel were determined by means of MMS-200 thermomechanical simulator.The phase transformation behavior and microstructure were investigated during continuous cooling.The results showed that with the cooling rate increase,the microstructure transformed gradually from ferrite and pearlite to bainite;and with different cooling rate,two phase transitions existed in the CCT diagram,that is,the pro-eutectoid ferrite and pearlite transformation area at low cooling rate and the bainite transformation area at middle cooling rate.
Continuous cooling transformation
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Study on continuous cooling transformation (CCT) behavior is an essential issue before thermo-mechanical processing for a new steel. In this study, dynamic CCT characteristics and microstructural evolution of a novel Cu-bearing pipeline steel with different Cu content (1.06%, 1.46% and 2.00%) were investigated by means of a combined method of dilatometry and metallography. The microstructure developed at a cooling rate range of 0.05 to 30°C/s consisted of pearlite, polygonal ferrite, quasi-polygonal ferrite and acicular ferrite. More Cu addition could lower the transformation temperature for austenite to ferrite and lead to an increase in the driving force for the acicular ferrite transformation, resulting in a full acicular ferrite for 2.0 Cu steel at cooling rate above 2°C/s. The precipitation behavior of Cu-rich phase during continuous cooling showed that Cu precipitation could occur in the acicular ferrite, which made a hardness peak on the hardness vs cooling rate curve of 2.0Cu steel at the cooling rate of 2°C/s. However, no Cu precipitate was detected in the acicular ferrite at higher cooling rate for 2.0 Cu steel. Higher supersaturation of Cu in austenite and a short incubation period of Cu-rich phase precipitation were assumed to allow the Cu precipitation to occur in the auto-aging after acicular ferrite transformation.
Acicular ferrite
Continuous cooling transformation
Acicular
Beta ferrite
Metallography
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Continuous cooling transformation (CCT) diagrams were determined for high strength low alloy (HSLA) steel by the dilatometric method as a function of different austenitising temperatures from 900 to 1300°C. The diagrams show significant suppression of polygonal ferrite at higher cooling rates and higher reheat temperatures (1100–1300°C), a prominent transformation region attributed to bainite or acicular ferrite at temperatures intermediate between those of polygonal ferrite and martensite formation. The CCT diagrams for the steel are characterised by the formation of acicular ferrite structure for a wide range of cooling rates. The initial austenite grain size did not affect the transformation temperatures of austenite to acicular ferrite at all cooling rates. The microstructure of steels cooled at very low cooling rates (<0·1 K s−1) exhibited a two-phase structure of acicular ferrite and polygonal ferrite. A model based on the rule of mixtures was used to predict the strengthening components in the steel in the tempered condition. The major contributions to strengthening in the alloy were found to be from solid solution strength, precipitation strength, and ferrite lath strengthening. The contribution to dislocation strengthening in the tempered condition was found to be negligible. The calculated strength is in excellent agreement with the experimental values.
Acicular ferrite
Continuous cooling transformation
Lath
High-strength low-alloy steel
Beta ferrite
Acicular
Microalloyed steel
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The transformation of super-cooling austenite in a commercial pipeline steel was investigated by measuring the continuous cooling transformation (CCT) diagram and the hot simulation test. Based on the obtained results, a thermo-mechanical control process (TMCP) has been proposed, which can obtain a mixed microstructure mainly consisted of acicular ferrite, and the detailed features of acicular ferrite are also analyzed. Results indicate that the increase of cooling rate can increase the content of acicular ferrite in the final microstructure of the pipeline steel under the present experimental conditions.
Acicular ferrite
Continuous cooling transformation
Acicular
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The transformation behavior and microstructural characteristics of API X65 pipeline steel were investigated by dilatometry and microstructural observation. Microhardness measurements were used to verify the observed microstructures. The test steel is imported from abroad and is used extensively in Iran natural gas transmission projects. The continuous cooling transformation curves of the test steel were constructed. The results showed that with increasing the cooling rate from 0.5 to 40°C/s, the microstructure changed from polygonal ferrite, quasi-polygonal ferrite-pearlite to acicular ferrite. The microstructure was dominated by acicular ferrite in cooling rates higher than 5°C/s. The results can be used to design the optimum thermo-mechanical control process (through the selection of proper cooling rate) in domestic manufacturing of the test steel.
Acicular ferrite
Continuous cooling transformation
Acicular
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As an optimal microstructure of pipeline steels, acicular ferrite is widely found in steels used in oil and gas pipeline transportation because it possesses both high strength and good toughness. In this paper, the microstructure of acicular ferrite and its continuous cooling transformation (CCT) diagrams of six steels with different carbon and alloy additions have been studied by using dilatometry, optical metallography. And the effects of different hot deformation processes on the CCT diagrams and microstructures have also been studied. Furthermore, the effects of microalloyed elements and hot deformation on continuous cooling transformation have been discussed. The results show that lower carbon content and alloy additions such as Mn, Nb, Ti, Mo, Ni and/or Cu in steels will promote the formation of acicular ferrite. The hot deformation promotes the acicular ferrite transformation and refines the microstructures of final products.
Acicular ferrite
Continuous cooling transformation
Acicular
Metallography
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