TY - THES

T1 - CCT diagrams of high-strength weld metal

AU - Marin Morales, David

N1 - embargoed until 01-09-2028

PY - 2023

Y1 - 2023

N2 - In this thesis, two Continuous Cooling Transformation (CCT) diagrams have been created for a metal cored wire with a yield strength higher than 1100 MPa classified as TZ2T15 1M21A N4C1M2-H5 according to EN ISO 18276-B. The characterisation was carried out mainly by dilatometry, with high temperature laser scanning confocal microscopy (HTLSCM) as an auxiliary technique, to determine the transformation temperatures in the last three beads of a multipass weldment for different cooling rates and peak temperatures of 1000 °C and 1300 °C, respectively. From the dilatometric results, an unexpectedly significant effect of position on the microstructure of the samples was discovered, which is most likely due to the reheating of previous beads and the presence of strong carbide formers in the material. To minimise this effect, only the last bead of the weld metal was used to develop the CCT diagrams. Microstructural characterisation of the dilatometer samples for the last bead was carried out using light optical microscopy and scanning electron microscopy for phase identification and quantification, with electron backscatter diffraction and energy dispersive X ray spectroscopy for more precise phase identification. Vickers hardness measurements were also carried out. The microstructure of the samples was found to change from fully martensitic to mixed martensitic-bainitic to fully bainitic. The hardness was found to decrease in the same way as the cooling rate was reduced from rapid to moderate to slow. A small amount of ferrite formation was detected by HTLSCM at the slowest cooling rates, but ferrite could not be identified or quantified by any other technique. The microstructure was found to be finer for the samples with 1000 °C peak temperature than for the samples with 1300 °C, which also results in an increased hardness for the same cooling rates. Finally, two welding CCT diagrams were obtained for peak temperatures of 1000 °C and 1300 °C, where the present phases for each corresponding cooling rate were the same for both diagrams. The curves for the diagram with 1300 °C peak temperature are slightly shifted towards lower temperatures and longer times compared to the diagram with a peak temperature of 1000 °C. This is caused by the former having a higher temperature and thus more austenite grain growth.

AB - In this thesis, two Continuous Cooling Transformation (CCT) diagrams have been created for a metal cored wire with a yield strength higher than 1100 MPa classified as TZ2T15 1M21A N4C1M2-H5 according to EN ISO 18276-B. The characterisation was carried out mainly by dilatometry, with high temperature laser scanning confocal microscopy (HTLSCM) as an auxiliary technique, to determine the transformation temperatures in the last three beads of a multipass weldment for different cooling rates and peak temperatures of 1000 °C and 1300 °C, respectively. From the dilatometric results, an unexpectedly significant effect of position on the microstructure of the samples was discovered, which is most likely due to the reheating of previous beads and the presence of strong carbide formers in the material. To minimise this effect, only the last bead of the weld metal was used to develop the CCT diagrams. Microstructural characterisation of the dilatometer samples for the last bead was carried out using light optical microscopy and scanning electron microscopy for phase identification and quantification, with electron backscatter diffraction and energy dispersive X ray spectroscopy for more precise phase identification. Vickers hardness measurements were also carried out. The microstructure of the samples was found to change from fully martensitic to mixed martensitic-bainitic to fully bainitic. The hardness was found to decrease in the same way as the cooling rate was reduced from rapid to moderate to slow. A small amount of ferrite formation was detected by HTLSCM at the slowest cooling rates, but ferrite could not be identified or quantified by any other technique. The microstructure was found to be finer for the samples with 1000 °C peak temperature than for the samples with 1300 °C, which also results in an increased hardness for the same cooling rates. Finally, two welding CCT diagrams were obtained for peak temperatures of 1000 °C and 1300 °C, where the present phases for each corresponding cooling rate were the same for both diagrams. The curves for the diagram with 1300 °C peak temperature are slightly shifted towards lower temperatures and longer times compared to the diagram with a peak temperature of 1000 °C. This is caused by the former having a higher temperature and thus more austenite grain growth.

KW - CCT-diagram

KW - high strength weld metal

KW - multipass weldment

KW - metal-cored wire

KW - ZTU-Diagramm

KW - hochfestes Schweißgut

KW - Mehrlagenschweißung

KW - Metallpulver Fülldraht

U2 - 10.34901/mul.pub.2023.181

DO - 10.34901/mul.pub.2023.181

M3 - Master's Thesis

ER -