Investigation of the softening mechanism during welding in the heat-affected zone of micro-alloyed high-strength steels

• Applicant: Professor Dr.-Ing. Thomas Kannengießer
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Production Automation and Assembly Technology
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 435083436

• Final Report Year: 2024

Final Report Abstract

The research project investigated the softening effect that occurs in the heat-affected zone (HAZ) during welding of micro-alloyed, high-strength steels, with a specific focus on the softening mechanism as a function of different micro-alloying concepts. The aim of WP1 was to identify the welding microstructure of single and multi-layer welds and to record the temperature cycles of the HAZ areas for the microstructure simulation using dilatometry. This showed that softening occurred in the HAZ boundary areas to the base material regardless of the microalloy route, whereby the intensity of the softening varied depending on the microalloy. It should be emphasised that the Ti-microalloyed steel grade exhibited greater softening of the HAZ than the Nb-microalloyed grade, which showed better tempering resistance. This improvement is due to the synergistic effects between Mo/Cr and Nb, whereby Mo forms secondary carbides at low temperatures and delays the tempering reaction. In addition, the addition of Nb and Mo delays Martensite decomposition, which has a positive effect on the HAZ microstructure. By analysing the local deformation during welding with identical filler metal, it was possible to show that different failure mechanisms occur depending on the microalloying concept. In the Ticontaining HAZ, support effects were observed due to a strong metallurgical notch effect, which can be explained by the restraint of elongation in the transverse direction and the higher local strength of the HAZ areas. In WP3, the softening behaviour for single-pass welds was investigated using dilatometry. It was found that the Ti-containing grade S690QL exhibited a better pinning effect of the austenite grains due to the high thermal stability of the Ti-(C, N). In addition, specific temperature ranges of softening could be determined for the micro-alloyed grades, with the Ti-micro-alloyed sample showing intensive softening between 700 °C and 800 °C. The aim of WP4 was to reproduce the softening behaviour during multilayer welding using dilatometry and to investigate the metallurgical processes. It was found that softening of the heataffected zone (HAZ) occurs at high temperatures. In the intercritical HAZ, repeated tempering leads to a significant reduction in hardness, while the subcritical HAZ tends to soften after several occasions, which can be attributed to the precipitation of carbides and secondary carbides. The thermodynamic simulation with ThermoCalc provided a precise understanding of precipitate formation and kinetics in the equilibrium state and during non-isothermal heat cycles during the welding process. The thermal stability of different precipitates was simulated. This is particularly useful as a priori simulations can be used to develop heat control concepts during welding.

Publications

• „Ausscheidungsverhalten und Eigenschaften der schweißbedingten Wärmeeinflusszonen hochfester Stähle in Abhängigkeit der Mikrolegierungsroute“ Tagungsband, 40. Vortrags- und Diskussionstagung Werkstoffprüfung 2022, S. 427-433. ISBN 978-3-88355-430-3
  N. Schröder, M. Rhode & T. Kannengießer
  
• „Experimentelle Bestimmung und thermodynamische Modellierung des Ausscheidungsverhaltens in der WEZ eines S690QL bei unterschiedlichen Mikrolegierungsrouten“, DVS Congress, 2022, Tagungsband (DVS-Berichte, Band 382), S. 131-137, DVS-Media, Düsseldorf; ISBN 978-3-96144-190-7
  N. Schröder, M. Rhode & T. Kannengießer
  
• Thermodynamic prediction of precipitations behaviour in HAZ of a gas metal arc welded S690QL with varying Ti and Nb content. Welding in the World, 67(9), 2143-2152.
  Schroeder, Nina; Rhode, Michael & Kannengiesser, Thomas
  
• „Einfluss unterschiedlicher Mikrolegierungsrouten eines S690QL auf das Ausscheidungsverhalten in der Wärmeeinflusszone“ Tagungsband 5. Niedersächsisches Symposium Materialtechnik, 2023, S. 332-343. ISBN 978-3-8440-9105-2
  N. Schröder, M. Rhode & T. Kannengießer
  
• „Mikrolegierungseinfluss auf das Ausscheidungsverhalten und die mechanischen Eigenschaften geschweißter hochfester Baustähle“ DVS Congress, 2023, Tagungsband (DVS-Berichte, Band 389), S. 218-225, DVS-Media, Düsseldorf; ISBN 978-3-96144-230-0
  N. Schröder, M. Rhode & T. Kannengießer
  
• Influence of microalloying on precipitation behavior and notch impact toughness of welded high-strength structural steels. Welding in the World, 68(10), 2647-2659.
  Schroeder, Nina; Rhode, Michael & Kannengiesser, Thomas
  
• „Auswirkungen von Ti und Nb auf die physikalisch simulierten WEZ-Bereiche von hochfestem Feinkornbaustahl S690QL“ DVS Congress, 2024, Tagungsband (DVS-Berichte, Band 390), DVS-Media, Düsseldorf
  N. Schröder, M. Rhode & T. Kannengießer