Abstract
The article describes the cracking of argon-welded TIG-welded circumferential butt joints of light-wall tubes made of steel X10CrMoVNb9-1 (T91), using filler metal C9 MV-IG. Fragments of welded joints and weld deposits underwent macro- and microscopic metallographic examination involving light microscopy. The investigation also involved tests of mechanical properties, impact energy and hardness, as well as chemical composition analyses. Structural tests of the areas containing cracks revealed the presence of high-temperature ferrite delta (ferrite δ). It was determined that the cracks in the welds of circumferential butt joints were caused by too high yield point and tensile strength of the weld metal not subjected to heat treatment (if compared with the corresponding properties of steel T91). The publication also provides technological recommendations specifying how to prevent the formation of cracks in such types of welded joints.

Key words
martensitic steel, welding, welded joint, microstructure, cracking

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