Clarifying creep-susceptible zone in 9Cr steel weldments via interrupted testing

Abstract

9Cr steel welded joints are prone to Type IV premature failure during high-temperature service. Understanding the creep cracking mechanism and elucidating the microstructural evolution within the creep-susceptible zone constitute critical countermeasures preventing Type IV creep cracking. However, the exact location of the creep-susceptible zone remains ambiguous. Our findings reveal a notable migration of the soft zone in actual weldments during the secondary creep stage. Ensuing microstructural analysis further corroborates that microstructures within the creep-susceptible zone consist of tempered martensites with small prior austenite grain sizes and a high proportion of fine-grain ferrites (80.8 %). Fine-grained ferrites degrade rapidly during creep, accompanied by a swift decrease in dislocation density, leading to a significant reduction in hardness. Compared to tempered martensites, ferrite grain boundaries are more susceptible to creep cavity formation and coalescence, ultimately resulting in creep cracking along grain boundaries. These findings may offer valuable insights into the mechanisms of Type IV creep cracking and will facilitate tackling challenges pertinent to circumventing this failure mode.