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.
Highlights
The 300M steel is extensively used in manufacturing of large structural parts of aircraft landing gear and nuclear power plants seal head due to its excellent mechanical properties [1].These parts are usually heat-treated to meet specific mechanical performance requirements
The microstructure evolutions could be observed on the upper surfaces of specimens by a confocal laser scanning evolutions could be observed on the upper surfaces of specimens by a confocal laser scanning microscope (CLSM) due to the alloy elements’ volatilization at grain boundaries
4a many small austenite grains emerged in initial coarse grains, resulting in a significant grain refinement compared with initial grain size (38 μm) at 25 C by metallography
Summary
The 300M steel is extensively used in manufacturing of large structural parts of aircraft landing gear and nuclear power plants seal head due to its excellent mechanical properties (σb ≥ 1800 MPa) [1]. Grains could either be refined or coarsened depending on the holding time and temperatures [2] It was reported by Karthikeyan et al [3] that the grain size of 9Cr-1Mo steel was refined from 26 to 12 μm via high temperature holdings. In situ methods allowed direct observations of grain size variations without altering the test conditions, and they were successfully used in the characterization of microstructure evolutions of steels for the past few years. The influences of holding temperatures, holding time, initial structures, structures, and initial grain sizes on grain evolutions of steel will be investigated by in situ and initial grain sizes on grain evolutions of 300M steel will be investigated by in situ observations. The grain size evolution of 300M model of steel in the isothermal holding process will be established
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