The current work focuses on complex multiphase microstructures gained in CrMo medium carbon steel after ultra-fast heat treatment, consisting of heating with heating rate of 300 C/s, 2 s soaking at peak temperature and subsequent quenching. In order to better understand the microstructure evolution and the phenomena that take place during rapid heating, an ultra-fast heated sample was analyzed and compared with a conventionally treated sample with a heating rate of 10 C/s and 360 s soaking. The initial microstructure of both samples consisted of ferrite and spheroidized cementite. The conventional heat treatment results in a fully martensitic microstructure as expected. On the other hand, the ultra-fast heated sample shows significant heterogeneity in the final microstructure. This is a result of insufficient time for cementite dissolution, carbon diffusion and chemical composition homogenization at the austenitization temperature. Its final microstructure consists of undissolved spheroidized cementite, nano-carbides and martensite laths in a ferritic matrix. Based on EBSD and TEM analysis, traces of bainitic ferrite are indicated. The grains and laths sizes observed offer proof that a diffusionless, massive transformation takes place for the austenite formation and growth instead of a diffusion-controlled transformation that occurs on a conventional heat treatment.@en

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 (γ) 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.@en

The current study focuses on the composition of the constituents that are obtained from ultra-fast heating and the composition of the parent austenite that was achieved at the peak temperature. In order to analyse the content and distribution of important elements such as carbon, manganese, chromium and molybdenum in the microstructure, simulation software was used. It is also found that the content of these elements shows a gradient in the microstructure in relation to the distance from the undissolved cementite. This leads to carbon-rich areas in the vicinity of cementite and carbon-depleted areas as the distance from the cementite grows. The former creates ideal conditions for the retainment of austenite due to the higher carbon content while the latter creates conditions that enable the formation of bainite in the microstructure.@en

The current study focuses on the composition of the constituents that are obtained from ultra-fast heating and the composition of the parent austenite that was achieved at the peak temperature. In order to analyse the content and distribution of important elements such as carbon, manganese, chromium and molybdenum in the microstructure, simulation software was used. It is also found that the content of these elements shows a gradient in the microstructure in relation to the distance from the undissolved cementite. This leads to carbon-rich areas in the vicinity of cementite and carbon-depleted areas as the distance from the cementite grows. The former creates ideal conditions for the retainment of austenite due to the higher carbon content while the latter creates conditions that enable the formation of bainite in the microstructure.@en