Study of Carbide Dissolution and Austenite Formation during Ultra-Fast Heating in Medium Carbon Chromium Molybdenum Steel, Metals 2018, 8, 646; doi:10.3390/met8080646
Abstract
In this study, UltraFast Heat Treatment (UFHT) was applied to a soft annealed medium carbon
chromium molybdenum steel. The specimens were rapidly heated and subsequently
quenched in a dilatometer. The resulting microstructure consists of chromium-enriched
cementite and chromium carbides (in sizes between 5–500 nm) within fine (nano-sized)
martensitic and bainitic laths. The dissolution of carbides in austenite () during ferrite to
austenite phase transformation in conditions of rapid heating were simulated with DICTRA.
The results indicate that fine (5 nm) and coarse (200 nm) carbides dissolve only partially, even
at peak (austenitization) temperature. Alloying elements, especially chromium (Cr), segregate
at austenite/carbide interfaces, retarding the dissolution of carbides and subsequently
austenite formation. The sluggish movement of the austenite/carbide interface towards
austenite during carbide dissolution was attributed to the partitioning of Cr nearby the
interface. Moreover, the undissolved carbides prevent austenite grain growth at peak
temperature, resulting in a fine-grained microstructure. Finally, the simulation results suggest
that ultrafast heating creates conditions that lead to chemical heterogeneity in austenite and
may lead to an extremely refined microstructure consisting of martensite and bainite laths
and partially dissolved carbides during quenching.