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 (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.

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.

The current work elucidates the phase transformation and carbide dissolution during rapid heating followed by short austenization and quenching to room temperature. For this work, dilatometry experiments were carried out in medium carbon low alloy steel containing chromium and molybdenum (42CrMo4).The thermal cycle of the sample was analyzed with the use of Thermocalc  and Dictra. The simulation results indicate that the high heating rate and short austenization time limits diffusion and the alloying elements segregate at the interfaces between carbides and ferrite. In addition, the segregation of alloying elements at the interfaces in conjunction with the rapid heating rate and the short dwelling time leads to incomplete dissolution of carbides and the formation of austenite with varying sizes and different carbon content, which in some cases reaches 1% wt; this of course can be of significance as it affects phase transformations and lead in retaining the austenite during quenching. The carbide morphology, size and type could be revealed only after Transmission Electron Microscopy (TEM) analysis. The microstructure consists of bainite, martensite and undissolved carbides confirming the results of simulation. Further investigation in the effect of chemical composition and size of austenite during bainitic and martensitic transformation is necessary and thorough study on the effect of the alloying elements on the transformations mentioned above.

No abstract

No abstract