Upper bainite
A bainite variant forming at higher temperatures consisting of ferrite laths with coarse parallel carbides in between. It exhibits lower impact toughness compared to lower bainite.
Upper bainite forms in the heat-affected zone when the material cools too slowly through a certain temperature range after welding. The structure consists of ferrite needles with carbide lines in between, which makes the material harder and more brittle than normal ferrite-pearlite structure.
If you want to avoid upper bainite, you have to pass through the critical range quickly during cooling. You can achieve this with higher heat input or by preheating, so that the cooling rate doesn't fall into the bainite range. With thick plates or cold material, this often happens unnoticed.
If you want to improve the structure afterwards, stress-relief annealing helps. This transforms the upper bainite into a softer structure and removes the hardness.