Welded hulls and turrets were introduced in the early 1930s and, as opposed to riveted ones, were made as a monocoque.

How to Get Welded
Rolled + Rolled

Welded Doodle Tanks Combo
+MS-1 Turret = Sloped
+Endurance = Nut Turret
+Machine Gun Turret = T-35
+Dukhov = Pike