Influence of solidification induced composition gradients on carbide precipitation in FeNiCr heat resistant steels

Abstract

Secondary precipitation of Cr-rich carbides in heat resistant austenitic stainless steels has been investigated both experimentally and using finite element simulations. The microstructural evolutions in two commercial grades were characterized using electron microscopy. A special emphasis was given on the peculiar spatial distribution of M23C6 secondary carbides exhibiting precipitate free zones surrounding primary carbides, and high density precipitate zones extending with longer aging times. Solidification induced chemical composition gradients were clearly exhibited in the as-cast alloys with significant chromium depletion in the vicinity of primary M7C3 carbides. It is then proposed that these gradients play a major role in the secondary precipitation mechanism. Classical nucleation and growth theories have been adapted to account for (i) the flux of solutes with large difference in diffusion coefficients, (ii) the initial composition gradients in the matrix and, (iii) the chemical driving force for nucleation and growth of M23C6 carbides. Within this framework, the whole kinetics has been reproduced. It clearly shows that the spatial distribution of secondary carbides that play a key role in the creep resistance of these alloys is the result of a complex interaction between initial composition gradients in as-cast alloys and solute flux resulting from phase transformation during aging.

Microstructure influence on creep properties of heat-resistant austenitic alloys with high aluminum content

“…Appearance and dissolution of this phase during ageing or creep can be explained as follows. As shown by Roussel et al [28], depletion in Cr and enrichment in Ni at the primary Cr-carbide/matrix interface after solidification (Figure 12). The presence of these chemical gradients at the matrix/carbide interface after solidification can act as an additional driving force for the heterogeneous nucleation of the NiAl phase.…”

“…In good agreement with the phase diagrams of Figure 5, only MC (bright contrast) and M23C6 carbides (darkest phase) are observed in alloys 1, 2 and 5 (Figure 6a, Figure 6b and Figure 6e respectively). Two types of M23C6 carbides are present: coarse M23C6 carbides originating from the transformation of the primary M7C3 carbides and finely dispersed secondary carbides M23C6 resulting from the release of C in the matrix during the transformation of M7C3 into M23C6 [26,28]. In alloys 3 and 4 (Figure 6c and Figure 6d respectively), an additional phase in light grey contrast is observed.…”

“…At cracking service temperature (950-1100°C), the creep resistance of these austenitic alloys is mainly guaranteed by the control of secondary precipitation of MC (M stands for metal which is mainly Nb and Ti), and M 23C6 (M is mainly Cr) carbides. M23C6 carbides are precipitated owing to transformation of the primary carbides M7C3 (M is mainly Cr) formed during solidification [23][24][25][26][27][28]. However, the influence of high Al level (>3.5 wt.…”

Modeling of interstitials diffusion during debinding/sintering of 3D printed metallic filaments: Application to titanium alloy and its embrittlement

“…The coupling with an evolving diffusion profile from the outside to dynamically modify the driving force of precipitation is, on the other hand, rarely encountered. Such coupling has already been proposed in steels [23,24] and aluminium [25], but never in Ti64 to model the precipitation during debinding/sintering processes. In this paper, we evaluate the effect of the debinding conditions (temperature, holding time, heating rate, atmosphere) on both microstructural and mechanical properties of Ti64 scaffolds obtained by DIW.…”

Influence Mechanism of Silicon on Carbide Phase Precipitation of a Corrosion Resistance Nickel Based Superalloy

“…Large amounts of research papers have reported that the distribution, size, amount, morphology, and type of carbide phase all have a great influence on the properties of the alloy [15][16][17][18]. Some researchers have found that the element concentration, element type, element ratio, heat treatment temperature, time, and other factors all have influence on the precipitation of the carbide phase [19][20][21][22][23], however, in essence, carbide is formed by element diffusion, while carbide formation needs the required metal element and carbon element reach the necessary element concentration for carbide formation, which rely on element diffusion [24][25][26][27][28][29]. Some researchers have shown that Si has an effect on the precipitation rate of alloy and can delay the transformation from ε/ε'-carbides to cementite [30,31].…”