利用热膨胀仪、同步辐射高能X射线衍射、XRD和TEM等对经淬火+两相区处理+不同温度回火处理的9Ni钢中逆变奥氏体含量、逆变奥氏体在室温单轴拉伸过程的形变诱导相变及其对强度的影响规律进行了研究. 结果表明, 经780 ℃淬火和680 ℃两相区处理后, 实验钢中不含逆变奥氏体, 而在随后的回火过程中产生一定量的逆变奥氏体. 逆变奥氏体含量随回火温度的上升先升高而后降低, 600 ℃回火时逆变奥氏体含量最高. 在室温拉伸过程中, 逆变奥氏体的形变诱导相变开始于宏观屈服之后, 在颈缩前基本全部转变成马氏体, 从而屈服强度随回火温度的升高先减小后增大, 600 ℃回火时屈服强度最小; 抗拉强度随回火温度的升高而增大, 640 ℃回火时抗拉强度最大.
9Ni steel has been widely used in liquid natural gas tanks and pipelines because of its excellent toughness at low temperature after quenching, larmellarizing and tempering heat treatment. Recently, in the cryogenic field it is used in some forgings, which have a strict demanding on the strength of this material. In order to clarify the relationship between the strength and the reversed austenite in the 9Ni steel after different temperature tempering, a systematic investigation on the amount of reversed austenite, deformation induced phase transformation (DIPT) of reversed austenite and its influence on the mechanical properties of 9Ni steel has been carried out by dilatometer, in situ synchrotron high-energy X-ray diffraction, XRD and TEM. The experimental results indicated that the amount of reversed austenite showed a parabolic trend with increase of tempering temperature and obtained the highest value after 600 ℃ tempering. And the DIPT of reversed austenite occurred after yielding during uniaxial tension test. This phenomenon induced that the yield strength of the experimental steel decreased to a minimum value after 600 ℃ tempering, and then, the value increased with further the increase of tempering temperature. However, the tensile strength of experimental steel increased with the increase of tempering temperature and reached the maximum after 640 ℃ tempering, because almost all of the reversed austenite transforms to martensite before necking.
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