Nuri Akdut

High-temperature deformation properties of austenitic Fe-Mn alloys

The influence of the Mn content on the hot deformation properties of austenitic binary Fe-Mn alloys containing 1 to 20 mass pct Mn has been investigated for the first time. The influence of the Mn content on the constitutive equations was determined for the temperature range of 950 °C to 1250 °C and the strain rate range of 0.1 to 2 s−1. The activation energy for hot working increased with increasing Mn content, and dynamic recrystallization was observed for all the Fe-Mn binary alloys. The Mn was found to delay dynamic recrystallization. An increase in the Mn content resulted in an increase of both the peak stress σp and the corresponding peak strain εp, thus revealing the pronounced influence of Mn on the hot deformation process.

High-temperature stress and strain partitioning in duplex stainless steel

Abstract The high-temperature strain partitioning in duplex stainless steel was studied by means of single-pass hot-rolling tests at 1040 and 1245°C. By varying the temperature and the amount of reduction during the hot-rolling pass the influence of the austenite volume fraction and the strain could be investigated. By means of optical microscopy and application of the law of mixtures the partitioning of strain between the ferrite matrix and the austenite islands was obtained. A distinct strain partitioning was observed. At 1245°C, the ferrite-austenite strain ratio increased from 2.8 at 30% rolling reduction to 4.2 at 80% reduction. In addition, single-hit hot-torsion tests were performed on single-phase ferritic and austenitic materials, as well as on the duplex stainless steel in order to study the stress partitioning between the phases. The austenite-ferrite stress ratio was found to increase up to 3.5 at 1200°C. The torsion tests confirmed that the deformation behaviour of the duplex stainless steel ...

Room-temperature aging of manganese-alloyed high nitrogen duplex stainless steels

The room-temperature aging behavior of two duplex stainless alloys with different austenite stability was investigated. Both alloys readily aged at room temperature. Even for aging times as short as 30 seconds, the originally continuous yielding behavior becomes discontinuous upon reloading after prestraining. The magnitude of the stress increase due to aging was higher in the presence of strain-induced martensite, even though it was shown that aging also occurred in the austenite phase. The aging response was shown to be thermally activated, with increasing age hardening associated with increasing aging times. The results could be explained by the combination of aging phenomena in the bcc phases by interstitials and the aging by interstitial-vacancy complexes in the fcc phase, where the interstitials are thought to be immobile during the short aging times used and aging would occur due to short-range migration of vacancies instead.

Static Strain Aging in Cold Rolled Metastable Austenitic Stainless Steels

Transformation of austenite to martensite during cold rolling operations is widely used to strengthen metastable austenitic stainless steel grades. Static strain aging (SSA) phenomena at low temperature, typically between 200°C and 400°C, can be used for additional increase in yield strength due to the presence of α’-martensite in the cold rolled metastable austenitic stainless steels. Indeed, SSA in austenitic stainless steel affects mainly in α’-martensite. The SSA response of three industrial stainless steel grades was investigated in order to understand the aspects of the aging phenomena at low temperature in metastable austenitic stainless steels. In this study, the optimization of, both, deformation and time-temperature parameters of the static aging treatment permitted an increase in yield strength up to 300 MPa while maintaining an acceptable total elongation in a commercial 301LN steel grade. Deformed metastable austenitic steels containing the “body-centered” α’-martensite are strengthened by the diffusion of interstitial solute atoms during aging at low temperature. Therefore, the carbon redistribution during aging at low temperature is explained in terms of the microstructural changes in austenite and martensite.

Nitrogen Content and Orientation and Their Relation to Slip Activity, Twinning and Crack Formation during Cold Rolling of Austenitic Single Crystals

This study investigates how nitrogen content and initial orientation affect the deformation mechanisms of austenites. To eliminate the overlapping influence of grain boundaries and to reduce the observed effects on the presence of nitrogen, N-alloyed and N-free austenitic single crystals were chosen from which two initial orientations were cut out and cold rolled up to 75% thickness reduction, the corresponding (111) pole figures were measured and the slip traces were analysed from the longitudinal section. In one of the N-alloyed austenitic single crystals massive crack formation was observed. This observation was discussed in terms of the planarity of slip caused by the interstitially solved alloying element nitrogen. Nitrogen has a strong affinity to dislocations and thus alters the slip character from homogeneous to planar and the slip distribution from single slip to coarse slip. The present results prove that the coarse planar slip caused by the alloying element nitrogen in dependency of the initial orientation is responsible for crack formation during rolling. The planarity of slip is also reflected in the measured pole figures, The {111} poles of the N-alloyed single crystals show less scattering and higher maximum intensities than the poles of the N-free single crystals. The differences were attributed to the influence of the nitrogen content on deformation mechanisms.