Observation of type IV cracking in welded joints of high chromium ferritic heat resistant steels

Type IV cracking in welded joints of high Cr ferritic heat resistant steels at 923 K is investigated in the present study. First, internal pressure creep tests were performed to investigate the features of cracking at 923 K and at stresses of 108-167 MPa. Results showed that at low stresses cracks occurred in the fine grained heat affected zone (FGHAZ), showing low deformation, a vast amount of creep voids and short creep life: these were identified to be type IV cracks. However, as the stress was increased the crack locations moved further away from the fusion line, the creep void area density decreased, and the slope of the main crack ratio curve increased. At high stress, cracks occurred in the base metal and were identified to be normal cracks. Second, creep tests were carried out on one pass welded specimens to clarify the relationship between the creep property deterioration in the FGHAZ and the weld thermal cycle. Type IV cracking was repeated successfully and results showed good correspondence with the position of cracks among the softened region, the highest creep void area density and the fracture location. Both internally pressurised and one pass welded specimens showed that at low stress the FGHAZ was the weak zone and creep voids played an important role in type IV cracking.