In Situ Investigation of Grain Evolution of 300M Steel in Isothermal Holding Process

Abstract

The relationships between initial microstructures, process parameters, and grain evolutions in isothermal holdings have drawn wide attention in recent years, but the grain growth behaviors of 300M steel were not well understood, resulting in a failure in precise microstructure controlling in heat treatment. In this work, in situ observations were carried out to characterize the grain evolutions of 300M steel with varying holding time, holding temperatures, and initial microstructures. The intriguing finding was that the grain refinement by austenization of 300M steel was followed by a dramatic grain growth in the initial stage of holding (≤~600 s), and with increasing time (~600–7200 s), the average grain size appeared to have a limit value at specific temperatures. The austenization process accelerated the grain growth by generating large quantity of grain boundaries at the initial stage of holdings, and the growth rate gradually slowed down after holding for ~600 s because the driven force was weakened due to the reduction of grain boundary energy. The initial structure and the initial grain size of 300M steel had no obvious influences on the grain size evolutions. The mechanisms of grain growth were analyzed based on in situ observations and transmission electron microscope (TEM) characterizations. A grain evolution model considering the grain boundary migration of 300M steel was established for the isothermal holding process. Good agreement was obtained between the in situ observation results and the model calculation results. This investigation aimed to understand fundamentally the grain evolutions of 300M steel in the isothermal holding process.

Hot Workability of 300M Steel Investigated by In Situ and Ex Situ Compression Tests

“…A mixed microstructure of fine recrystallized grains and coarse initial grains was obtained at 900-1000 • C, as shown in Figure 4a-c. With increasing temperature, a full recrystallized microstructure was formed at 1050-1150 • C, as shown in Figure 4d-f, and with increasing temperature, the average grain size increased. The mean grain boundary migration rate was less than 0.2 µm/s at 900-1150 • C according to a previous study [12], and under the strain rate of 10 s −1 , the average grain size grew by 0.02 µm at most during the compression process, which was negligible compared with the average grain size (≥5 µm). Thus, the effect of grain coarsening due to grain boundary curvature could be neglected.…”

“…Up to date, it is generally accepted that the grain evolution is the intrinsic reason for flow stress evolution [9], and the grain morphology is able to interpret the deformation mechanisms of different domains in the processing maps. The microstructure evolution of 300M steel has been studied by Chen et al [10][11][12][13], Liu et al [14], and Zeng et al [15]. However, the influence of the dynamic recrystallization volume fraction on the processing map of 300M steel has not been systematically investigated.…”

A Flow Stress Model of 300M Steel for Isothermal Tension

“…But only a few small recrystallized grains could be seen in Figure 6 e, because under a low strain rate (0.01 s −1 ) and high temperature (1150 °C), small recrystallized grains gradually coarsened. The grains in Figure 6 g were relatively small because the deformation time was short, and the grains did not have enough time to grow [ 19 ].…”

Investigation of the Microstructural Evolution during Hot Stamping of a Carburized Complex Phase Steel by Laser-Ultrasonics

“…Even though grain growth appears during the several hours of carburization, the grain size is in the range of 10 ± 2 μm throughout the whole austenitization process. This can be linked to a grain refinement during the α→μ transformation upon heating [ 54 ]. The austenite grains primarily form at previous austenitic grain boundaries [ 55 ] and packet and block boundaries within martensite [ 56 ].…”