Cracking of the mixing chamber caused by sigma phase precipitation in austenitic steel welded joints

In the air mixing chamber of the chipboard production line, made of austenitic X15CrNiSi20-12 steel, cracks have been found, which were located in the fillet welds connecting the suspension ring with the chamber shell and also in the butt welds of the suspension ring. The failure investigation covered temperature measurement of the suspension ring, chemical analysis of welds and parent material, metallographic examination and Vickers hardness measurements of welded joints as well as Charpy V impact testing of the as received samples and after solution heat treatment. The examination results revealed a low impact strength (28–32 J) of the butt weld and parent material, caused by precipitation of sigma phase during prolonged heating at temperatures exceeding 650 °C. After solution heat treatment of the parent material the impact strength was restored to values over 300 J. It was found that the main reason of cracking is the sigma phase formation in welded joints, with its effect of embrittlement, with the crack initiation on stress concentrating welding defects.