Abstract
Recent evidence suggests that using hardness as the sole acceptance criterion for Grade 91 steels is inadequate for predicting service performance. Components can achieve acceptable initial hardness values through heat treatment despite suboptimal elemental composition, leading to poor tempering resistance and unexpectedly low creep strength during service. Paradoxically, some components with lower initial hardness may perform better due to slower degradation rates. While the relationship between parent material properties and Type IV cracking susceptibility remains under investigation, heat-affected zones (HAZ) in welds are emerging as primary locations for service failures. This complexity emphasizes the need for comprehensive evaluation criteria incorporating stress, temperature, and material properties when assessing component serviceability.
