Study of Carbide Dissolution and Austenite Formation during Ultra-Fast Heating in Medium Carbon Chromium Molybdenum Steel

Abstract

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.

Effect of Ultra-Fast Heat Treatment on the Subsequent Formation of Mixed Martensitic/Bainitic Microstructure with Carbides in a CrMo Medium Carbon Steel

“…The carbides were assumed with spherical shape and initial size was set to 5 nm as derived by TEM observations. According to Papaefthymiou [40] and De Knif et al [27], with increasing heating rates normal to the ultra-fast, a shift in the Ac1 and Ac3 temperatures to higher values was observed. Specifically, as shown in Figure 2b, for 300 °C/s heating rate, the austenite transformation starts at 810 °C (Ac1) and ends at 900 °C (Ac3) which are higher than the temperatures calculated on equilibrium phase diagram.…”

“…For austenite grain size larger than 7 µm the B s temperature is shifted to the right hand side of the diagram of Figure 6a, which explains why bainite cannot form with this cooling rate and only martensite forms rapidly at temperatures below the M s temperature (292 • C). Nevertheless, during UFH, large heterogeneity is expected in the distribution of carbon in the PAGs based on references [40,41]. Therefore, another CCT diagram was plotted in Figure 6b.…”

Grain-Boundary Interaction between Inconel 625 and WC during Laser Metal Deposition

“…Uniform distribution of carbide particles in the whole volume of the coating is expected to improve coating wear resistance. The appearance of TCP phases can further improve wear resistance and hardness, but weaken elastic properties when compared to nano-reinforcement [35,36,37]. …”

“…The particle size of 5 µm was chosen in order to achieve uniform carbide distribution in the whole volume of the material. Additionally, this grain size allowed to avoid dissolution of a significant amount of WC in the metal matrix [35,36,37,38]. The powder mixture of Inconel 625 and WC was used to enhance homogenous distribution of carbide in the material.…”

Ferrite-to-Austenite and Austenite-to-Martensite Phase Transformations in the Vicinity of a Cementite Particle: A Molecular Dynamics Approach

“…The microstructure development of medium-carbon low-alloy steels containing Cr and Mo under ultrafast heat treatment conditions (rapid heating, peak austenitization followed by quenching) considers the role of undissolved carbides, such as cementite, which influence the austenite formation. Thus, the role of the cementite interface on the microstructure formation in quench-partitioning (QP) and also in ultrafast heating (UFH) steel are under intense discussion [16][17][18][19][20][21][22][23][24][25].…”