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Einfluss des Legierungselements Aluminium auf die Mikrostruktur nichtrostender Stähle mit überwiegend austenitischer Matrix

Fachliche Zuordnung Metallurgische, thermische und thermomechanische Behandlung von Werkstoffen
Förderung Förderung von 2015 bis 2022
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 274385700
 
Erstellungsjahr 2022

Zusammenfassung der Projektergebnisse

Al was added to stainless steels to understand different aspects related to the development of novel lightweight stainless steels. Austenite-stabilizing elements such as Ni, Mn, Co and C were included in the chemical composition to counter the ferrite-stabilizing effects of Cr and Al and to obtain steels with austenitic microstructures. In cases where the Al concentration was too high, duplex or ferritic matrix microstructures were obtained. The high driving force for the formation of B2-(Ni,Fe)Al intermetallic precipitates in ferrite enabled to study the formation and dissolution of B2 precipitates in ferrite and its influence on the mechanical properties. Once the strengthening effect of B2 precipitates was confirmed, a series of alloys with a martensitic matrix microstructure were cast to evaluate the concept of B2-strengthened Al-added stainless steels as alternatives for standard precipitation-hardenable stainless steels. Peak hardness values in excess of 600 HV were achieved in the aged condition. This hardness level is superior to those of standard precipitation-hardenable martensitic stainless steels and comparable to those of the heavier, and less corrosion-resistant 18Ni maraging steels. The influence of Al on the mechanical properties of austenitic stainless steels was studied by tensile tests between -196 °C and 300 °C. Reference Al-free steels were tested as well. When deformation temperature was high enough to enable the diffusion of C, thereby its interaction with dislocations during plastic deformation, serrated flow due to dynamic strain aging was observed. Serrated flow was accentuated by the presence of Al. Accordingly, a higher work hardening rate was obtained with Al-added steels. Consideration of the mechanical properties and microstructure characterization results after tensile tests for steels with and without Al indicated an increase in the mechanical stability of austenite in the presence of Al. This implied an increase in the stacking fault energy of austenite. In the case of Al-added steels, the tensile test temperature associated with maximum elongation was somewhat lower than Mdγ→α' temperature. In other words, the formation of small amounts of deformation-induced α'- martensite just below Mdγ→α' temperature did not cause an immediate loss of ductility. Since the type of deformation-induced micro-mechanisms in both steels were quite similar, with clear formation of deformation twins, the difference in the sensitivity of steels to the deformationinduced α'-martensite formation was interpreted in terms of inhomogeneities in the spatial distribution of alloying elements. The latter would cause local variations in the stacking fault energy and deformation-induced micro-mechanisms. The ferritic solidification of Al-added steels and inverse segregation of Al (enrichment in dendrites) are expected to have played a role. Co addition was also found to reduce the Mdγ→α' temperature and the fraction of deformation-induced α'-martensite. However, the suppressing effect of Co was significantly smaller than that of Al. Electron backscatter diffraction analysis of M23C6 carbides indicated that the orientation relationship between M23C6 carbides and austenite deviated from the cube-on-cube relationship commonly reported for M23C6 carbides in standard low-C austenitic stainless steels. The identified orientation relationship could be approximated by the K-S orientation relationship, which is commonly observed between austenite and α'-martensite. This peculiar observation was explained by the chain reaction austenite → M7C3 → M23C6.

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