Flow Stress Of Warm Compression Of Medium Carbon Martensite And Its Microstructure And Mechanical Property

In this work, the flow curves of martensite structure (M), ferrite plus pearlite structure (F+P) and spherical structure (S) in a medium carbon steel were determined by warm compression tests performed on a Gleeble3500 machine, and the corresponding microstructures and mechanical properties at room temperature were comparatively studied by TEM, SEM, hardness and tensile tests. The results show that the flow stress of M decreases with the increase in deformation temperature, and gradually close to the flow stress of F+P and S, or even below. These imply that the M warm working has the same feasibility as ferritic rolling, which is widely used in engineering. Further more, the mechanical properties of warm deformed steel with M as starting structure are much better than that of the same steel deformed with F+P or S, due to the warm deformation of M has a stronger effect of grain refinement than that of F+P or S.The flow stresses, such as peak stress, yield stress and stable stress of M are decreased with the strain rates at the temperatures range from 350 ℃ to 700 ℃. The flow stress of M is lower than that of F+P, but little higher than that of S, when the strain rate is lower than the critical strain rate, the temperature is higher than the critical temperature, or the strain is larger than the critical strain. It is found that the strain rate sensitive index and the work-softening effect of M are the largest among the three structures.The reason of above flow behaviors of M occurred during warm compression is the high dislocation density and interface density which make the structure energy state of M very high. Besides dynamic recovery and dynamic recrystallization, grain boundary sliding and dislocation diffusional creep are more important in work-softening of M than that of F+P.The quantitative observation by TEM shows that the mean grain sizes ofdynamic recrystallized ferrite are in the range of 0.56 urn to 1.73 urn, and the mean size of the carbide is about 32 ~ 60 nm for M warm compression at temperature of 550 -700 °C. But, for F+P warm compression , local dynamic recrystallization occurred at 600 °C, entirety dynamic recrystallization occurred at 650 -700 °C. The mean grain sizes of dynamic recrystallization for ferrite warm compression are 2.63 urn at 700 °C, and the mean size of the carbide is about 160 - 210 nm in that of F+P.Uniaxial tensile tests at room temperature show the yield strength of the M warm compression sample is increased by 3.92 -27.80 % compared to that of F+P, and increased by 22.68 -54.62 % to that of S. The elongation is almost the same. The mechanical property of M is higher than that of annealing structure after warm compression, probably because of the fine ferrite grain with uniformly distributed nano-sized carbide particles.