Effects of Cooling Rate after Hot Forging on Precipitation of Fine Particles during Subsequent Normalizing and Austenite Grain Growth during Carburization of Al- and Nb-microalloyed Case-hardening Steel

Abstract

This study deals with austenite grain growth during high-temperature carburization of an Al-and Nb-microalloyed case-hardening steel. The grain size after carburization-simulated heating for 5 h at 1 050°C decreased with the increase in the cooling rate from hot forging-simulated heating for 1 h at 1 250°C. The increase in cooling rate led to the decreases in the volume fractions and sizes of AlN and Nb(C,N) particles precipitated during cooling, and AlN disappeared when the cooling rate increased to 16°C/min, while Nb(C,N) still slightly exited at 16°C/min. Because of oversaturation caused by cooling within a finite time, further precipitation occurred during the subsequent normalization for 3 h at 1 070°C, resulting in the formation of AlN-Nb(C,N) combined particles. When the cooling rate increased, the volume fraction and number density of these combined particles increased while their size decreased. Therefore, a higher cooling rate causes a larger pinning effect on grain growth during carburization; thus grain size after carburization decreased with the increase in cooling rate. Transmission electron microscopy confirmed the formation of a coherent AlN-Nb(C,N) interface due to good lattice matching between the crystal planes of AlN (1120) and Nb(C,N) (220). This led to the preferential nucleation of AlN on the Nb(C,N) particles, thereby forming stable AlN-Nb(C,N) particles.

Effects of Normalizing Temperature on the Precipitation of Fine Particles and Austenite Grain Growth during Carburization of Al- and Nb-Microalloyed Case-Hardening Steel

“…4, some of AlN particles were combined with Nb(C, N) particles. AlN (1120) plane is well matched with the Nb(C, N) (220) plane, and the AlN-Nb(C, N) interface has high coherence, according to our recent study 18) . Therefore, it was considered that AlN precipitated on Nb(C, N) with a low interfacial energy.…”

“…In our previous study, we focused on an Al-and Nb-microalloyed case-hardening steel and found that forging-simulated heating at 1250 °C for 1 h completely dissolved AlN and Nb(C, N) 17) , and we also found the effects of the cooling rate after the forging-simulated heating on the precipitation behaviors of particles, as follows 18) . When cooled at 16 °C/min, a small number of Nb(C, N) particles precipitated during cooling, whereas the AlN particles did not precipitate.…”

“…The EDS analysis performed at room temperature after cooling confirmed that the dark-contrast particles were AlN. It was considered that the reason why AlN precipitated on Nb(C, N) was due to the good agreement between the lattice spacings of the AlN (1120) and Nb(C, N) (220) planes 18) . Thus, in the bainite region, AlN precipitated on the plate-like cementite particle, while in the proeutectoid ferrite region, it precipitated on the Nb(C, N).…”

“…The stable crystal structure of AlN depends on the heat treatment temperature. When steel is annealed at high temperatures, hexagonal wurtzite-type AlN is formed with plate-like or rod-like morphologies, and it does not contain other heavy elements such as Cr and Mn 18,[24][25][26][27] . In contrast, at relatively low temperatures, NaCl-type AlN becomes stable in steels 24,28) .…”

AlN and Nb(C,N) Composite Precipitation Behaviors and Their Effects on Austenite Grain Growth in SCr420H High‐Temperature Carburized Gear Steel

“…Furthermore, the precipitation mechanism and sequence for AlN-Nb(C,N) remain unclear. Saito [32] reported that the Nb(C,N) precipitates served as the role of nucleation sites for AlN due to a coherent AlN-Nb(C,N) interface, but Leap [33] claimed Nb(C,N) precipitation occurred heterogeneously on preexisting AlN precipitates. It is worthwhile to further investigate the synergistic effect of Al-Nb microalloying on precipitation behaviors and austenite grain growth.…”