Effect Of Heat Treatment On Microstructure And Properties Of Nanoscale Precipitate-strengthened Steel

As a newly developed high-strength low-alloy steel,nanoscale precipitate-strengthened steel has good comprehensive mechanical properties by adopting the method of precipitated nanophase strengthening and traditional strengthening.Reasonable heat treatment process is of great significance to the microstructure and mechanical properties of nanoscale precipitate-strengthened steel.In this paper,the effects of different solution treatment and aging treatment on the microstructure and mechanical properties were studied.The low-temperature toughness mechanism and the changes of the yield ratio during the aging process were described emphatically.The nanoscale precipitate-strengthened steel was treated by solution treatment at 900? for 90 min and 913? for 60 min.After solution treatment,the microstructure was mainly low carbon lath martensite and a small number of polygonal ferrite.The effect of the two processes on the microstructure was not obvious.The hardness increased after solution treatment in both processes,but the slightly higher temperature 913? solution treatment could increase the microhardness more greatly in a relatively shorter time.The quenched steel had higher tensile strength and relatively lower yield strength.After solid solution treatment,due to a large number of lath structure,which had a hindrance to the crack propagation,the low temperature toughness was also excellent.The aging treatment of nanoscale precipitate-strengthened steel after solution treatment was carried out in the temperature range of 450° C ~ 690° C for 5min ~ 50 h to explore the effect of aging treatment on its microstructure and mechanical properties.Studies had shown that aging treatment does not change the matrix microstructure and phase composition.The precipitation strengthening of the nanoscale precipitated phase significantly improved the hardness and yield strength of the steel.Due to the precipitation strengthening and the temper softening of the matrix structure occurred during the aging process,the hardness decreased first and then increased and then decreased when the aging temperature was low.At high temperature aging,the hardness of the steel decreased with the increase of aging time.The plasticity could be significantly improved after the aging treatment of different processes.The steel had poor impact toughness after low temperature aging,and prolonging the aging treatment time could improve the toughness value.Aging at a higher aging temperature of 600° C ~ 660° C could maintain excellent low temperature toughness.After direct aging treatment,the microstructure of the steel was still a banded structure in which the grains were elongated.Although the plasticity was significantly improved,the improvement of hardness and strength was not significant.Due to the weak effect of the band structure on crack propagation,the low temperature toughness was poor.The direct aging treatment method was not suitable for improving the comprehensive mechanical properties.The secondary austenitizing treatment was carried out to compare the microstructure and mechanical properties with one time austenitizing treatment.It was found that the secondary austenitizing treatment does not change the microstructure and grain size of the steel,and had no obvious improvement on the mechanical properties.At the same time,the same aging treatment was applied to the steel under the two processes,and it was found that the secondary austenitizing treatment had no significant effect on the aging treatment affection.According to the analysis of the embrittlement phenomenon in the nanoscale precipitate-strengthened steel during aging treatment,it was considered that there was no traditional high-temperature temper brittle problem.The embrittlement phenomenon was mainly caused by the precipitation of nanoscale phase after aging.The nanophase should be kept in an appropriate overaging state by extending the aging time to improve the low temperature toughness.The yield strength and the low temperature toughness were not simply inversely related.The best match in the study range was the yield strength of 927 MPa,the-84? impact energy of 180 J,and the worst match was the yield strength of 859 MPa,the-84? impact energy of 44 J.During the aging process,the nanoscale precipitate-strengthened steel would undergo M2 C carbide transformation,which could prolong the yield strength retention platform.The yield ratio of the steel changed during the aging process by the combination of copper nanophase precipitation strengthening and temper softening.Under appropriate high temperature aging treatment,steel could easily achieve a good match between low yield ratio and high low temperature impact toughness.The best match in the study range was yield strength of 815 MPa,yield ratio of 0.9,-84? impact energy of 192 J.