TY - THES

T1 - Segregation analysis of a temper embrittled 51CrV4 with elevated trace element content for green steel production

AU - Teuschl, Hanna

N1 - embargoed until 12-06-2029

PY - 2024

Y1 - 2024

N2 - In the steel industry, blast furnaces are increasingly being replaced by electric arc furnaces, which allows for greater utilization of scrap metal as a secondary raw material, thereby reducing CO2 emissions. However, this shift causes an increase in trace elements (Cu, Sn, Ni, Mo), potentially leading to a significant reduction in the toughness of quenched and tempered steels. The objective of this work is to find the cause of the embrittlement in a 51CrV4 quenched and tempered steel, with increased amounts of these trace elements, through microstructural and grain boundary analysis. Five different heat treatment conditions are investigated, including hardening, and tempering at 450°C and 550°C, and slow and fast cooling after tempering. Scanning electron microscopy and synchrotron X-ray diffraction revealed no differences in microstructure between a 51CrV4 steel containing trace elements and the standard material. Clear segregation of carbide-forming elements (Cr, Mn, V, Mo) to prior austenite grain boundaries (PAGBs) was measured by the means of atom probe tomography (APT). However, in the vicinity of a carbide precipitate the PAGB segregation of these elements was less pronounced as they tend to partition into the carbide. This effect makes it difficult to determine the influence of the carbide-forming elements on the segregation behavior. APT studies of the PAGBs and ferrite/carbide interfaces show P segregation to the interfaces as well as to the PAGBs, presumably resulting in a reduction in toughness in both batches. A closer examination of the Sn segregation in different heat treatment conditions, suggests a correlation between the degree of Sn segregation to the PAGBs and the observed difference in temper embrittlement between both batches. Higher levels of Sn segregation measured by APT, correspond to a more pronounced embrittlement revealed by Charpy impact toughness tests. Meanwhile, no significant Cu or Ni segregation was found in the course of this research.

AB - In the steel industry, blast furnaces are increasingly being replaced by electric arc furnaces, which allows for greater utilization of scrap metal as a secondary raw material, thereby reducing CO2 emissions. However, this shift causes an increase in trace elements (Cu, Sn, Ni, Mo), potentially leading to a significant reduction in the toughness of quenched and tempered steels. The objective of this work is to find the cause of the embrittlement in a 51CrV4 quenched and tempered steel, with increased amounts of these trace elements, through microstructural and grain boundary analysis. Five different heat treatment conditions are investigated, including hardening, and tempering at 450°C and 550°C, and slow and fast cooling after tempering. Scanning electron microscopy and synchrotron X-ray diffraction revealed no differences in microstructure between a 51CrV4 steel containing trace elements and the standard material. Clear segregation of carbide-forming elements (Cr, Mn, V, Mo) to prior austenite grain boundaries (PAGBs) was measured by the means of atom probe tomography (APT). However, in the vicinity of a carbide precipitate the PAGB segregation of these elements was less pronounced as they tend to partition into the carbide. This effect makes it difficult to determine the influence of the carbide-forming elements on the segregation behavior. APT studies of the PAGBs and ferrite/carbide interfaces show P segregation to the interfaces as well as to the PAGBs, presumably resulting in a reduction in toughness in both batches. A closer examination of the Sn segregation in different heat treatment conditions, suggests a correlation between the degree of Sn segregation to the PAGBs and the observed difference in temper embrittlement between both batches. Higher levels of Sn segregation measured by APT, correspond to a more pronounced embrittlement revealed by Charpy impact toughness tests. Meanwhile, no significant Cu or Ni segregation was found in the course of this research.

KW - 51CrV4

KW - Spurenelemente

KW - grüne Stahlproduktion

KW - Segregation

KW - Atomsondentomographie

KW - APT

KW - Anlassversprödung

KW - Vergütungsstahl

KW - Sn

KW - P

KW - 51CrV4

KW - trace element

KW - green steel

KW - segregation

KW - atom probe tomography

KW - APT

KW - temper embrittlement

KW - quenched and tempered

KW - Sn

KW - P

M3 - Master's Thesis

ER -