In response to the strong needs for the life assessment of various components in fossil power plants, studies on Grade 91 and Grade 92 steels have been jointly performed by EPRI and CRIEPI for a last decade. These studies have been covering the effects of load variation (creep- fatigue) and stress multiaxiality as well as the behavior under uniaxial creep conditions. Based on abundant test data accumulated in this period and associated analytical evaluation, approaches based on inelastic strain energy have been developed for accurately assessing creep damage and failure lives under various conditions. The essence of these efforts is presented in this paper.

Creep brittle behaviour in tempered martensitic, creep strength enhanced ferritic (CSEF) steels is linked to the formation of micro voids. Details of the number of voids formed, and the tendency for reductions in creep strain to fracture are different for the different CSEF steels. However, it appears that the susceptibility for void nucleation is related to the presence of trace elements and hard non-metallic inclusions in the base steel. A key factor in determining whether the inclusions present will nucleate voids is the particle size. Thus, only inclusions of a sufficient size (the critical inclusion size is directly linked to the creep stress) will act directly as nucleation sites. This paper compares results from traditional uniaxial laboratory creep testing with data obtained under multiaxial conditions. The need to understand and quantify how metallurgical and structural factors interact to influence creep damage and cracking is discussed and the significant benefits available through the use of high quality steel making and fabrication procedures are highlighted. Details of component behaviour are considered as part of well-engineered, Damage Tolerant, design methods.

Creep rupture strength is the principal material property prioritized in designing power generation plants against the steady-state stress due to internal pressure. Increasingly plants must cycle so there is a possibility of life reduction due to creep-fatigue interaction. Grade 92 steel is one of the creep strength enhanced ferritic (CSEF) steels which has superior creep strength compared to other CSEFs. It is expected to be widely used in coal-fired ultra-super critical plants as well as in LNG-fired combined cycle plants. However, at present there is insufficient information regarding the creep-fatigue behavior of this material. A joint study has been conducted to understand the behavior of this steel under creep-fatigue condition and see how accurate the failure life can be estimated. Three kinds of base materials as well as two kinds of welded joints have been tested under strain-controlled cyclic loading with or without hold times as well as under constant load creep condition. Continued decrease in the number of cycles to failure was observed with the extension of hold time in all the base metals and cross-weld specimens. It was found that the modified ductility exhaustion approach based on inelastic strain, as well as its extension employing the inelastic strain energy density, made reasonably accurate predictions of failure lives under a wide range of test conditions. Temperature- and rate-dependencies of fracture limits in terms of inelastic strain and energy density were able to be uniquely expressed using simple thermal activation energy parameters.

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.

With the desire for higher operating temperatures and pressures to improve the thermal efficiency of new power generating plant there have been significant changes in the materials used. For operation up to 620°C, a new range of ferritic steels with 9-13%Cr has been developed. With proper control of composition and heat treatment these materials, including Grades 91 and 92,exhibit predominantly martensitic microstructures and a good balance between strength and ductility. However, fabrication processes such as welding and bending, normally combined with extreme operating conditions have resulted in in-service damage. Examples of factors leading to accelerated creep, creep fatigue and oxidation damage are described.