Analysis of factors affecting type IV cracking in welded joints of high chromium ferritic heat resistant steels

In the present work, creep tests on single pass welded joints were carried out to investigate the factors affecting the deterioration of the creep strength of weldments of high Cr heat resistant steels. In creep tests at a temperature of 923 K and stresses of 90 and 120 MPa, creep fracture occurred in the fine grained heat affected zone (FGHAZ) and was identified as type IV cracking. It was found that a peak temperature between Ac-1 and Ac-3 in the FGHAZ caused the observed creep property deterioration. The factors identified as leading to the deterioration are a fine grain structure, low hardness, and large precipitates. Further investigations confirmed the influence of the fine grain structure as a critical factor decreasing the creep rupture time. The hardness difference between the FGHAZ and the base metal is insignificant, especially at 923 K However, SEM investigations revealed that many large precipitates are distributed on the boundaries of the martensite laths and prior austenite grain boundaries. It is considered that the large precipitates also have a significant influence on the creep strength deterioration.