Diffusion processes during cementite precipitation and their impact on electrical and thermal conductivity of a heat-treatable steel

Abstract

The thermal conductivity of heat-treatable steels is highly dependent on their thermo-mechanical history and the alloying degree. Besides phase transformations like the martensitic c ! a 0 or the degree of deformation, the precipitation of carbides exerts a strong influence on the thermal conductivity of these steels. In the current work, thermal and electrical conductivity of a 0.45 mass% C steel is investigated during an isothermal heat treatment at 700 C and correlated with the precipitation kinetics of cementite. To include processes in the shortterm as well as in the long-term range, annealing times from 1 s to 200 h are applied. This investigation includes microstructural characterization, diffusion simulations, and electrical and thermal conductivity measurements. The precipitation of carbides is connected with various microstructural processes which separately influence the thermophysical properties of the steel from the solution state to the short-term and long-term annealing states. In the early stages of cementite growth, an interstitial-dominated diffusion reaction takes place (carbon diffusion in the metastable condition of local equilibrium non-partitioning). Afterwards, substitutional-dominated diffusion controls the kinetics of the reaction. The electrical and thermal conductivity increase differently during the two stages of the carbide precipitation. The increment is associated to the binding of alloying elements into the carbides and to the reduction of the distortion of the martensitic matrix. Both factors increase the electron density and reduce the electron and phonon scattering. The correlation of the precipitation kinetics and the thermophysical properties are of general interest for the design of heat-treatable steels.

The Effect of Hot-Mounting on the Microstructure of an As-Quenched Auto-Tempered Low-Carbon Martensitic Steel

“…To avoid convergence errors, a binary Fe-0.126% C system was selected. This approximation should not amount to significant errors as the early growth of cementite is expected to occur without appreciable diffusion of substitutional alloys [20,21]. The validity of using the binary approximation was also checked by making some calculations for the ternary Fe-0.126% C-1.66 Mn system as shown below.…”

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

“…The above-mentioned reports on low-temperature tempering of M 3 C suggest that nucleation and growth depends on the temperature and alloying, leading to different compositions of the initial nucleus and the early growth is controlled by PE, while later stages of growth are influenced by the diffusion of substitutional elements to and within the M 3 C. With increasing temperature, the controlling mechanism of growth has been proposed to change from PE to NPLE [10,22,30], and in the case of pearlitic and upper-bainitic phase transformations this transition from PE to NPLE has been reported [10,22,31]. In the case of M 3 C precipitation, a systematic experimental and modeling work is lacking to clarify the interface condition during early stages of precipitation of M 3 C at high tempering temperatures.…”

Evaluation of Effectiveness of Heat Treatments in Boron Steel by Laser Thermography

“…As described in other work [10] the thermo-physical properties of materials are related to their microstructure, and then the thermal properties can be used as an index for monitoring microstructure changes. Thus, the main idea is to consider thermal diffusivity (α) of material as a suitable index to indicate that heat treatment has taken place.…”