Candidate: Master of Science Juhani Laitila
Place: L10, Linnanmaa, University of Oulu
Remote connection: zoom
Date & time: 01.02.2021 10:00
Topic: Effect of forced weld cooling on high-strength low alloy steels to interpass temperature
Opponents: Adjunct professor Mika Lohtander & Professor Antti Salminen
Custos: Professor Jari Larkiola
One of the challenges in welding ultrahigh- and high-strength steels is the effect of limited heat input on the production times. As the cooling times of these steels have to be controlled tightly to achieve the desired weld qualities, such as impact toughness, the allowed heat input is low, which translates to reduced material deposition rates. This means that multipass welding is often required to achieve good weld quality. Because multipass welding is often used, the time spent waiting for the weld to cool to the desired interpass temperature, which is usually 100 °C, is often multiple minutes for each weld pass. The trend so far has been that more efficient welding processes have been developed to maximize the material deposition rates without increasing the heat input. Even with these efforts the use of multipass welding is still required. The use of ultrahigh- or high-strength steels reduces the material cost, allows for lighter and thinner structures while reducing transport costs as well. However, the production cost can negate some of these cost savings because of the wasted time due to the aforementioned problems with multipass welding.
To establish the feasibility of introducing forced cooling to the welding process, the effect of the cooling on the mechanical properties was studied by conducting tensile, impact, and fatigue strength experiments with Gleeble simulated and with welded specimens. Also, the microstructural differences were studied. The tensile properties were improved when forced cooling was used to cool the weld down to the temperature of 100 °C and the impact toughness was also improved or remained unchanged. The cooling also demonstrated that it may have a positive effect on the fatigue strength of the steel. The grain size was usually reduced due to the forced cooling and larger quantities of lower bainite could be seen in some of the experiments. Overall there were no negative effects caused by the forced cooling on the steel.
The external cooling method used was water-cooled copper heat sinks that were placed on top of the steel being welded. This external cooling had the potential to reduce the time it takes for the steel to cool down to 100 °C by 83.6% when 6 mm thick steel was welded. Overall, taking into consideration other processes, such as Setup times, the potential time savings that can be achieved by applying such cooling methods to multipass welding processes is a significant and worthwhile option to consider.
Last updated: 2.2.2021