TY - THES

T1 - Phase fraction and morphology of retained austenite in high-strength weld metal

AU - Rinnhofer, Nicole

N1 - embargoed until 01-10-2029

PY - 2024

Y1 - 2024

N2 - High-strength steels play a decisive role in the realisation of lightweight constructions with low material input. Multipass welding is commonly used for such steels by utilizing high-strength filler metals to create joints from thicker plates and to enhance the weld metals properties by regraining the microstructure through the heat treatment of the subsequent welding beads. This results in the formation of a heat-affected zone (HAZ) in the base metal and within the weld metal, termed the weld metal HAZ. Retained austenite (RA) and martensite-austenite constituents can be present in the last bead (LB) as well as in various regions of these weld metal HAZ, which can drastically influence the mechanical properties. To address this, in this thesis, the microstructure of two high-strength low alloy multipass all-weld metals with different strength levels, 700 MPa and 1100 MPa, were investigated by light optical microscopy, scanning electron microscopy and electron backscatter diffraction (EBSD). The morphology and content of RA were investigated by EBSD. The weld metal with 1100 MPa yield strength was scanned using high-energy X-ray diffraction and the RA content was determined by Rietveld refinement and the method of intensity ratio. In addition, the lattice constants of RA and the base centered cubic phase were calculated from the diffraction data. Furthermore, a hardness mapping was carried out for each weld metal. The onset of the HAZ as well as the subcritical HAZ (SCHAZ) and the intercritical HAZ could be determined by means of the microstructural investigation. Coarse martensite-austenite constituents along prior austenite grain boundaries could not be observed in any of the investigated zones. Furthermore, identifying martensite-austenite constituents with EBSD measurements is not possible, as the Kikuchi patterns were too weak to obtain accurate indexing. The comparison of the RA content measured by EBSD and high-energy X-ray diffraction revealed that the EBSD investigation is not feasible to determine the RA content. The RA content obtained by Rietveld refinement and the method of intensity ratio agree well, whereas the method of intensity ratio provides a bit lower values. The RA content mapping shows that the highest RA content is present in the intercritical HAZ in the LB, and that the RA content decreases with the subsequent welding beads. The lowest RA contents were observed within the SCHAZ for each welding bead. The RA lattice constant showed that compressive stresses are present, especially in the LB. Tempering reduces the stresses and lead to an increased lattice constant, especially in the SCHAZ. The weld metal with higher yield strength shows a continuous decrease in hardness starting from the LB and no softening in any specific zone was observed. The weld metal with the lower yield strength, on the other hand, showed a drop in hardness in the SCHAZ. ¿

AB - High-strength steels play a decisive role in the realisation of lightweight constructions with low material input. Multipass welding is commonly used for such steels by utilizing high-strength filler metals to create joints from thicker plates and to enhance the weld metals properties by regraining the microstructure through the heat treatment of the subsequent welding beads. This results in the formation of a heat-affected zone (HAZ) in the base metal and within the weld metal, termed the weld metal HAZ. Retained austenite (RA) and martensite-austenite constituents can be present in the last bead (LB) as well as in various regions of these weld metal HAZ, which can drastically influence the mechanical properties. To address this, in this thesis, the microstructure of two high-strength low alloy multipass all-weld metals with different strength levels, 700 MPa and 1100 MPa, were investigated by light optical microscopy, scanning electron microscopy and electron backscatter diffraction (EBSD). The morphology and content of RA were investigated by EBSD. The weld metal with 1100 MPa yield strength was scanned using high-energy X-ray diffraction and the RA content was determined by Rietveld refinement and the method of intensity ratio. In addition, the lattice constants of RA and the base centered cubic phase were calculated from the diffraction data. Furthermore, a hardness mapping was carried out for each weld metal. The onset of the HAZ as well as the subcritical HAZ (SCHAZ) and the intercritical HAZ could be determined by means of the microstructural investigation. Coarse martensite-austenite constituents along prior austenite grain boundaries could not be observed in any of the investigated zones. Furthermore, identifying martensite-austenite constituents with EBSD measurements is not possible, as the Kikuchi patterns were too weak to obtain accurate indexing. The comparison of the RA content measured by EBSD and high-energy X-ray diffraction revealed that the EBSD investigation is not feasible to determine the RA content. The RA content obtained by Rietveld refinement and the method of intensity ratio agree well, whereas the method of intensity ratio provides a bit lower values. The RA content mapping shows that the highest RA content is present in the intercritical HAZ in the LB, and that the RA content decreases with the subsequent welding beads. The lowest RA contents were observed within the SCHAZ for each welding bead. The RA lattice constant showed that compressive stresses are present, especially in the LB. Tempering reduces the stresses and lead to an increased lattice constant, especially in the SCHAZ. The weld metal with higher yield strength shows a continuous decrease in hardness starting from the LB and no softening in any specific zone was observed. The weld metal with the lower yield strength, on the other hand, showed a drop in hardness in the SCHAZ. ¿

KW - HSLA Schweißgut

KW - Restaustenit

KW - Martensit-Austenit Insel

KW - Elektronen-Rückstreubeugung

KW - EBSD

KW - hochenergetische Röntgenbeugung

KW - HEXRD

KW - high-strength low alloy weld metal

KW - HSLA

KW - retained austenite

KW - martensite-austenite constituent

KW - electron backscatter diffraction

KW - EBSD

KW - high-energy X-ray diffraction

KW - HEXRD

U2 - 10.34901/mul.pub.2024.249

DO - 10.34901/mul.pub.2024.249

M3 - Master's Thesis

ER -