Austenite grain coarsening behavior during pseudo‐carburizing is investigated in a 18CrNiMo7‐6 steel, Nb microalloyed, and Nb–Ti microalloyed 18CrNiMo7‐6 steels, which are designed to evaluate the effects of Nb–Ti microalloying on suppressing austenite grain growth at elevated temperatures appropriate to high temperature carburizing operations. Pseudo‐carburizing heat treatments, without carburizing atmosphere, are performed in the temperature range of 850–1150 °C for 0.5–8 h. Transmission electron microscopy (TEM) incorporated with EDS spectra is used to investigate the morphology and size distributions of precipitates in pseudo‐carburized specimens. The results show that Nb–Ti microalloying has an obvious advantage in restraining austenite grain growth and increasing the grain coarsening temperature. Even in the temperature range of 1000–1150 °C, Nb–Ti microalloyed steel produces fine and uniform grain structures. The fine grain sizes observed in Nb–Ti microalloyed steel are attributed to the pinning effect of Nb–Ti precipitates that hinder austenite grain coarsening. The key reason for the significant difference of grain growth mechanisms between the base steel and Nb–Ti microalloyed steel is because Nb–Ti microalloying changes grain growth kinetic of the base steel. The results indicate that Nb–Ti microalloying can be successfully used to suppress grain growth in gear steels for high temperature carburizing.
X70 (steel A) and X80 (steel B) pipeline steels were fabricated by ultra fast cooling (UFC). UFC processing improves not only ultimate tensile strength (UTS), yield strength (YS), yield ratio (YS/UTS), and total elongation of both steels, but also their Charpy absorbed energy (A K ) as well. The microstructures of both steels were all composed of quasi polygonal, acicular ferrite (AF), and granular bainite. MA islands (the mixtures of brittle martensite and residual austenite) are more finely dispersed in steel B, and the amount of AF in steel B is much more than that in steel A. The strength of steel B is higher than that of steel A. This is mainly attributed to the effect of the ferrite grain refinement which is resulted from UFC processing. The finely dispersed MA islands not only provide dispersion strengthening, but also reduce loss of impact properties to pipeline steels. UFC produces low-temperature transformation microstructures containing larger amounts of AFs. The presence of AF is a crucial factor in achieving desired mechanical properties for both steels. It is suggested that the toughness of the experimental steel increases with increasing the amount of AF.
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