TY - BOOK

T1 - Mechanical Properties and Deformation Mechanisms of austenitic Nitrogen Steels under different Loading Conditions

AU - Hahn, Carola Elisabeth Katharina

N1 - embargoed until 11-11-2025

PY - 2021

Y1 - 2021

N2 - Hadfield Steel combines high toughness with high ductility and extraordinary strain hardening and is used as material for railroad crossings. The initially low yield strength allows Hadfield Steel to adapt to the local loading situation in the crossing nose during the roll over of wheels. However, the crossing geometry continuously degrades during service, which is among other things assumed to be due to the low yield strength. Therefore, a new castable austenitic Mn-steel concept is desired, allowing a geometry adaption to the real loading in the beginning, but reducing geometry degradation with longer service time. Austenitic steels alloyed with C and N (called Nitrogen Steels) are candidate materials, because they have a higher initial yield strength due to strong solid solution strengthening. The present work focuses on the development of an austenitic Fe-Mn-Cr-C-N alloying concept: (1) materials design based on thermodynamic phase diagram calculations and (2) analysis and evaluation of the microstructure, stacking fault energies, mechanical properties and deformation mechanisms in comparison to Hadfield Steel. N has a negative influence on its own solubility in the melt and pressure melting is not an option, hence additions of Cr are required to increase the N-solubility. Although the production requirements limit the possible N-contents, an increase of the yield strength in comparison to Hadfield Steel is achievable, while maintaining a similar strain hardening during static tensile loading. However, the response to impact loading at different temperatures and the low cycle fatigue behavior is significantly altered by additions of Cr and N. Cr and N are thought to introduce short-range order, leading to an enhanced slip planarity and causing a ductile to brittle transition, which shifts closer to 0℃ with increasing N-content. Higher twinning stress were found for the Fe-Mn-Cr-C-N alloys compared to Hadfield Steel, hence, higher stresses are required for the onset of deformation twinning in the Fe-Mn-Cr-C-N alloys. The combination of short-range order induced slip planarity at lower local stresses and the higher twinning stress of the Fe-Mn-Cr-C-N steel reduce the ability for twin formation during low cycle fatigue, which results in strain softening. The response to cyclic loading is very important for crossings, as every wheel passing the crossing introduces plastic deformation, which successively accumulates. Consequently, a softening behavior during cyclic loading is detrimental, because plastic deformation and geometry changes increase. Therefore, an application of the developed material concept for cast crossing noses is questionable.

AB - Hadfield Steel combines high toughness with high ductility and extraordinary strain hardening and is used as material for railroad crossings. The initially low yield strength allows Hadfield Steel to adapt to the local loading situation in the crossing nose during the roll over of wheels. However, the crossing geometry continuously degrades during service, which is among other things assumed to be due to the low yield strength. Therefore, a new castable austenitic Mn-steel concept is desired, allowing a geometry adaption to the real loading in the beginning, but reducing geometry degradation with longer service time. Austenitic steels alloyed with C and N (called Nitrogen Steels) are candidate materials, because they have a higher initial yield strength due to strong solid solution strengthening. The present work focuses on the development of an austenitic Fe-Mn-Cr-C-N alloying concept: (1) materials design based on thermodynamic phase diagram calculations and (2) analysis and evaluation of the microstructure, stacking fault energies, mechanical properties and deformation mechanisms in comparison to Hadfield Steel. N has a negative influence on its own solubility in the melt and pressure melting is not an option, hence additions of Cr are required to increase the N-solubility. Although the production requirements limit the possible N-contents, an increase of the yield strength in comparison to Hadfield Steel is achievable, while maintaining a similar strain hardening during static tensile loading. However, the response to impact loading at different temperatures and the low cycle fatigue behavior is significantly altered by additions of Cr and N. Cr and N are thought to introduce short-range order, leading to an enhanced slip planarity and causing a ductile to brittle transition, which shifts closer to 0℃ with increasing N-content. Higher twinning stress were found for the Fe-Mn-Cr-C-N alloys compared to Hadfield Steel, hence, higher stresses are required for the onset of deformation twinning in the Fe-Mn-Cr-C-N alloys. The combination of short-range order induced slip planarity at lower local stresses and the higher twinning stress of the Fe-Mn-Cr-C-N steel reduce the ability for twin formation during low cycle fatigue, which results in strain softening. The response to cyclic loading is very important for crossings, as every wheel passing the crossing introduces plastic deformation, which successively accumulates. Consequently, a softening behavior during cyclic loading is detrimental, because plastic deformation and geometry changes increase. Therefore, an application of the developed material concept for cast crossing noses is questionable.

KW - TWIP Steels

KW - austenitic steels

KW - nitrogen steels

KW - twinning

KW - low cycle fatigue

KW - short range order

KW - stacking fault energy

KW - twinning stress

KW - TWIP Stähle

KW - austenitische Stähle

KW - Stickstoffstähle

KW - Zwillingsbildung

KW - niederzyklische Ermüdung

KW - Nahordnung

KW - Stapelfehlerenergie

KW - Zwillingsbildungsspannung

M3 - Doctoral Thesis

ER -