The solubility of carbides in steel in relation to the original condition

Study of Structural Inheritance of Austenite in Nb‐Microalloyed 18CrNiMo7–6 Steel

“…Fine austenite grains form predominantly at and along the PAGB, so also on the sample surface and to a small extent within the grain, see Figure , for example. This austenite is varyingly called globular or granular or spherical austenite. In the grain interior mainly only acicular austenite forms.…”

“…Thus, acicular austenite formation precedes the RV‐RX of the martensite laths. Lipchin and Kokovyakina found in 40S steel that the new austenite nucleates on the subgrain boundaries of prior tempered martensite and is elongated due to rapid C diffusion along the lath boundaries, so also due to lower surface or interface energy by nucleation on these low angle boundaries. Although lath boundaries do have a certain inherent stability, this could be further increased by the presence of fine precipitates pinning them, TiC for instance, or also solute or impurity atoms…”

“…In essence the heating rate affects the misfit dislocations at interfaces and thus controls the formation and relative amounts of the two types of austenite, globular, and acicular. Globular austenite formation is influenced mainly by changes in the misfit dislocations at martensite or bainite/carbide interface, whereas acicular austenite formation is governed by changes in those at martensite or bainite/austenite interface…”

“…Effect of heating rate on globular austenite : With increase in heating rate from 1 °C min −1 to 150 °C min −1 to 100 °C s −1 , the number of globular austenite nulcei forming at the ferrite/carbide interface increases . The reason for this is the weak ferrite/carbide bond (ferrite here implies martensite or bainite) and the difference in the coefficient of thermal expansion (CTE) between them . Changes in heating rate affect the interfacial stresses and misfit dislocation generation differently.…”