Influence of Laves Phase Precipitation and Coarsening on High‐Temperature Strength of Ferritic Stainless Steels

Abstract

Downsizing trends in the design of internal combustion engines require ferritic steels with greater strength at elevated temperatures. One method of improving the hightemperature strength is precipitation hardening with intermetallic phases such as the Laves phase. Thermodynamic calculations show, that the elements Nb and Si contribute to the Laves phase formation strongly. In this work, the influence of intermetallic precipitates on the mechanical properties of three different ferritic Fe-Cr stainless steels was investigated and compared to a reference material. The three main hardening mechanisms -precipitation-hardening, grain refinement, and solid-solution strengthening -were studied with appropriate alloy compositions and thermo mechanical treatment. Investigations were performed with uniaxial compression tests of samples aged isothermally at 9008C for up to 1440 h. It is shown that, the solid solution effect of Mo and W increases the high-temperature strength about 40%, also after long-term annealing. The contribution of the Laves phase precipitates on the high-temperature strength is rather small due to their rapid coarsening.[ Ã ] N. Nabiran, W. Theisen

Ferritic Stainless Steels for High-Temperature Applications: Stabilization of the Microstructure by Solid State Precipitation of MX Carbonitrides

“…Sim et al [11] also reported a rapid coarsening rate of Laves phase precipitates at high temperatures (700 °C). These precipitates coarsen rapidly and their contribution to precipitation hardening is comparatively small, although they inhibit grain growth and thus stabilize the microstructure [9]. However, coarse Laves phase precipitates on the grain boundaries can have a detrimental effect on the toughness of these steels [12].…”

“…Due to the low stability of M 23 C 6 in low-carbon ferritic steels at higher temperatures, this phase can be excluded (Figure 3a). In previous works the influence of Laves phase in ferritic stainless steels during long-term annealing was investigated [9]. An analysis of the microstructure in different states annealed at 900 °C showed that the Laves phase precipitates grow and rapidly coarsen.…”

Evolution of the Laves Phase in Ferritic Heat-Resistant Steels During Long-term Annealing and its Influence on the High-Temperature Strength

“…From the literature [19] and our previous investigations, Nb is known to be the most potent Laves phase former and its strong effect can be enhanced by combining it with Si. [20] However, low concentrations of C and N, which are unavoidable during steel production, decrease the solvus temperature of the Laves phase due to the high affinity of Nb for C and N, thus leading to the formation of carbides/nitrides instead of the Laves phase. [21] Microalloying with Ti minimizes primary precipitation of niobium nitrides and niobium carbides, so that this element is still available for Laves phase formation.…”

Influence of Formation and Coarsening of the Laves Phase on the Mechanical Properties of Heat‐Resistant Ferritic Steels

“…This leads to the conclusion that a stable Laves phase content of about 1 vol% at service temperature could effectively retard grain coarsening . Thus, systematic formation of the Laves phase in addition to solid‐solution hardening may lead to a new way of enhancing the high‐temperature strength of heat‐resistant ferritic steels …”

“…This work investigates the influence of the Laves phase and grain size on strength, toughness, and ductility at room temperature. Three previously developed steels of the system Fe‐Cr‐Nb(‐Al) (designated Fe18CrW, Fe18CrMoW, and Fe19CrWAl) are the basis of this investigation. These steels are designed to contain about 1 vol% Laves phase at 900°C, and their high‐temperature strength is enhanced by alloying with Nb, W, and/or Al .…”