Micro Structural Evolution And Plastic Deformation Of Quenched And Partitioned Commercial Steels

Quenching and partitioning (Q-P) heat treatments can be used to generate good combination of high strength and good plasticity in steels, and encouraging progress has been made in theoretical and applied studies of the process in the past few years. However, most of the studies are focused on the development of a new generation of advanced high strength steels (AHSS), while less attention are paid on the application of Q-P in traditional steels to enhance their mechanical properties. Additionally, theoretical perspectives such as the mechanism of carbon-enrichment of the austenite during partitioning treatment, the mechanism of the enhancement of plasticity and toughness works during deformation, etc. remain to be further developed. In this work, the experiments of the Q-P heat treatments are carried out on several traditional steels, e.g.27SiMn,35CrMnSi,35CrMo,35CrMoSi,38CrMoAl,60Si2Mn,9SiCr (in Chinese grade), and the results are compared with traditional quenching and tempering (Q-T) and austempering (AT) heat treatments after the optimization of the Q-P processing parameters. Moreover, the stability of retained austenite (RA) in the Q-P treated steels is studied systematically by cryogenic and tempering experiments. Meanwhile, acoustic emission (AE) technology and molecular dynamic (MD) simulations are applied to investigate the mechanism of the enhancement of plasticity and toughness by RA.Various Q-P heat treatments with different quenching temperature (QT), partitioning temperature (PT) and partitioning time (tp), are carried out on27SiMn,35CrMnSi,38CrMoAl,35CrMoSi steels. Firstly, the RA fraction follows the relationship with QT predicted by the CCE model qualitatively in35CrMoSi steel while different laws are found in other steels; Secondly, the variation of RA fraction versus the PT are similar in all of the experimental steels, i.e. the highest fraction of RA are attained in the range of350～420℃while a decreased RA fraction attained at lower or higher temperatures; Thirdly, the RA fraction decreases as the tp increases in the range of2～30min for35CrMnSi and35CrMoSi steels when partitioned at420℃maintaining at the level of10vol.%. Based on these results and combined with the consideration of engineering applications, it is suggested that the proper Q-P parameters acquiring desired microstructures should be quenched to attain about75%martensite and partitioned in the range of350～420℃for less than30min. Optical microstructural evidences that bainite transformation of austenite contributes to the carbon-enrichment of untransformed austenite are found in35CrMnSi and35CrMoSi, which accounts for the influence of PT and tp on the RA fraction along with the CCE model. When the proper partitioning process (partitioned at350～420℃for less than30min) was applied, both of carbon partitioning between martensite and austenite predicted by the CCE model and bainite transformation would take place. The carbide-free bainite is formed due to the effect of alloying elements in the steel, and the most austenite is stabilized by the two mechanisms. Only the carbon partitioning takes effect at lower temperatures and bainite with Fe3C formed at higher temperatures or at350～420℃for increased time, which induces the decrease of the RA fraction.A novel process termed "multi-cyclic quenching and partitioning"(M-Q-P), aiming at tailoring the RA fraction in an enlarged range, is developed based on the Q-P principle and applied in35CrMnSi steel successfully. For35CrMnSi steel,5times of Q-P heat treatment can increase the content of RA from8vol.%to17vol.%. As a result, the ultimate elongation of the steel is improved from17.4%after the typical Q-P heat treatment to27.1%after5times of Q-P treatment. Meanwhile, the improved combination of strength and ductility for steels by typical Q-P heat treatment is retained by the M-Q-P heat treatment.The Q-P heat treatments enhanced the combined mechanical properties of high strength and effective ductility for35CrMnSi steel, as compared with traditional heat treatments such as Q-T and AT. The mechanical properties would degenerate to a lower level as similar to Q-T heat treated steels once the RA degenerated by tempering for the Q-P treated steel. Additional AE signals with high amplitude and high energy were produced during the tensile deformation of the35CrMnSi steel with RA in microstructures (obtained via Q-P and AT heat treatments), and the additional AE signals would not appear again once the Q-P steel is tempered at high temperature. Combined the AE features with the Optical microstructural and fractography analysis, it is found that the additional AE signals are produced by the strain induced martensitic transformation of RA.Strain induced FCC→BCC phase transformation in a bi-crystal model of pure Fe containing interphase boundaries with a Bain orientation is investigated by MD simulation using the Meyer-Entel interaction potential. Under quasi-static tension and compresstion, homogeneous nucleation and heterogeneous growth of the BCC phase in the FCC crystal is observed. The phase transformation behavior induced the yielding and the yielding strength of the compression is higher than that of the tension. Moreover, the new-formed BCC phase has a K-S orientation with the FCC phase and no motion of the original interface is observed.The MD simulations of strain induced FCC→BCC phase transformation in a tri-crystal model of pure Fe containing interphase boundaries with a K-S orientation is also carried out using the Ackland potential. The heterogeneous nucleation of dislocation from the interphase boundaries induced the yielding and the BCC phase nucleates at the fault band as the strain increase. On the contrary, the FCC→BCC phase transformation has accomplished before the yielding via the motion of initial interphase boundaries under the quasi-static compression. When the tension is completed at a constant engineering strain rate, the yielding strength increases as the strain rate increases and the FCC phase is more stable at a higher strain rate.The results of cryogenic and tempering experiments show that the RA in Q-P treated steels are stable in the temperature range of-80～400℃. However, the strength and ductility are both decreased after tempering at low and medium temperatures due to the reduction of mechanical stability of RA in the Q-P treated steels. At the same time, the ductility is picking up while the strength is consistently decreasing as the tempering temperature is elevated continuously and the mechanical properties are at the same level as the Q-P treated steels.