Early stages of cementite precipitation during tempering of 1C–1Cr martensitic steel

Abstract

The precipitation of cementite (M 3 C) from as-quenched martensite during tempering at 500 and 700°C was investigated in a Fe-1C-1Cr (wt%) alloy. Tempering for a short duration at 700°C results in a Cr/Fe ratio in the core region of M 3 C precipitates which is equal to the bulk alloy composition, while a shell on the surface of the precipitates exhibits a higher Cr concentration. With a prolonged tempering up to 5 h, the shell concentration gradually increases toward the equilibrium value, but the core region has not yet reached the equilibrium value. After tempering for 5 s at 500°C, there is no Cr enrichment found at the M 3 C-matrix interface, while a transition to partitioning of Cr is found during the first 5 min of tempering at 500°C. These experimental results indicate that M 3 C grows without significant partitioning of substitutional elements at both temperatures initially, i.e., growth is carbon diffusion controlled. This stage is, however, very short, and soon after 5 s at 700°C and 5 min at 500°C, Cr diffusion becomes important. Calculations using the diffusion simulation software DICTRA and precipitation simulation software TC-PRISMA were performed. The diffusion simulations using the local equilibrium interface condition show excellent agreement with experiments concerning Cr enrichment of the particles, but the size evolution is overestimated. On the other hand, the precipitation simulations underestimate the size evolution. It is suggested that a major improvement in the precipitation model could be achieved by implementing a modified nucleation model that considers nucleation far from the equilibrium composition.

On the role of transmission electron microscopy for precipitation analysis in metallic materials

“…[218] This study indicates that the formation of cementite during tempering of the Fe-Cr-C alloy does not follow the full local equilibrium composition at the interface. [103] The actual composition of cementite is more close to para-equilibrium composition at the early stage of precipitation. Figure 5c demonstrates that ortho-equilibrium and para-equilibrium assumptions significantly influence the precipitation kinetics of cementite.…”

“…[214] With the developments of advanced sample preparation methods and high resolution of EDS and EELS in advanced TEM, these issues can be combatted and a reliable chemical composition analysis by high resolution EDS or/and EELS analysis on FIB lamellae with very thin and uniform thickness is now possible. [103,215,216] Figure 5a,b show TEM bright-field images and corresponding EDS mapping and line scans on cementite particles within a Fe-1Cr-1C alloy tempered for 5 s and 30 min at 500 C. The results show that the 5 s tempering sample has no obvious segregation of Cr atoms at the precipitate-matrix interface and the Cr/Fe atomic ratio within the precipitate is close to that of the alloy's matrix phase. In contrast, the 30 min tempering sample shows Cr enrichment at the interface zone.…”

“…However, the chemical resistance of precipitates may change during the precipitation process with a continuous evolution of the concentration of alloying elements within the precipitates, e.g., Cr concentration increases with the size of M 3 C in steel. [103] This evolution could increase or decrease the chemical resistance of the precipitates, which leads to the dissolution of precipitates in concentrated acid solutions, which are usually applied in the chemical extraction process. Hence, electrochemical dissolution could be a better choice in certain cases.…”

“…Moreover, the subsequent treatments of the residues after the particles have been separated from the bulk matrix are important, and precipitates with size down to 5-15 nm could be successfully collected due to electrostatic attraction effects. [98,99,103] The major challenges here are to find a proper electrolyte and the applied electrochemical potential that selectively dissolve the matrix rapidly without dissolving the precipitates. In addition, the possibility to dissolve large volumes of the bulk material makes electrolytic extraction advantageous in studying precipitate volume fraction and identification of multiple precipitate phases by combining with X-ray diffraction (XRD).…”

“…In addition, the possibility to dissolve large volumes of the bulk material makes electrolytic extraction advantageous in studying precipitate volume fraction and identification of multiple precipitate phases by combining with X-ray diffraction (XRD). [103,106]…”