In advanced ultra-supercritical (A-USC) power plants, which operate at steam temperatures of 700 °C or higher, there is a need to replace 9 to 12Cr martensitic steels with high-strength nickel-base superalloys or austenitic steels for components exposed to the highest temperatures. However, due to the high cost of nickel-base superalloys, it is desirable to use 9 to 12% Cr martensitic steels for components exposed to slightly lower temperatures, ideally expanding their use up to 650 °C. Key challenges in developing ferritic steels for 650 °C USC boilers include enhancing oxidation resistance and long-term creep rupture strength, particularly in welded joints where resistance to Type IV cracking is critical for constructing thick-section boiler components. The current research aims to investigate the creep deformation behavior and microstructure evolution during creep for base metals and heat-affected-zone (HAZ) simulated specimens of tempered martensitic 9Cr steels, including 9Cr-boron steel and conventional steels like grade 91 and 92. The study discusses the creep strengthening mechanisms and factors influencing creep life. It proposes an alloy design strategy that combines boron strengthening and MX nitride strengthening, avoiding the formation of boron nitrides during normalizing heat treatment, to improve the creep strength of both base metal and welded joints.

Ferritic 9-12 wt.% chromium steels are commonly used for thick-walled high-temperature components in thermal power plants, but they face two major limitations in high-temperature service. Firstly, a reduction in creep strength occurs after approximately 10,000 hours at service temperatures around 600°C, due to the dissolution of finely dispersed V-rich nitrides and the precipitation of coarse particles of the modified Z-phase, [(Cr,V,Nb)N]. Secondly, welded joints of nearly all ferritic steel grades are prone to premature creep failures in the fine-grained heat-affected zone, known as Type IV cracking, which results from a strength loss of up to 50% compared to the base material. This study describes the development of a 9Cr3W3CoVNb steel with added boron and controlled nitrogen content. Preliminary creep testing results up to 24,000 hours at 650°C show a significant improvement in creep strength compared to established ferritic 9Cr grades like P91 and P92, attributed to a reduced driving force for the precipitation of modified Z-phase particles. Crosswelds of the new 9Cr3W3CoVNbBN steel also demonstrate improved creep behavior at 650°C, with creep rupture strength comparable to the mean base material creep strength of the best commercially available grade P92.

A study of Grade 91 steel's creep rupture behavior at 600°C (up to 90,000 hours) and 650°C (up to 23,000 hours) reveals that static recovery of tempered martensite lath structures leads to decreased stress exponent and breakdown of creep strength. While M 23 C 6 and MX particles initially stabilize lath structures by hindering sub-boundary migration, the progressive aggregation of M 23 C 6 particles reduces their pinning force, triggering static recovery. Although Grade 91 steel shows better M 23 C 6 thermal stability compared to Grade 122 type steels (9-12%Cr-2W-0.4Mo-1Cu-VNb), coarsening of M 23 C 6 particles and subgrain width is expected to occur slightly beyond 100,000 hours at 600°C, potentially leading to creep strength breakdown.

A study of P92 steel's creep-rupture behavior at 625°C revealed distinct relationships between phase chemistry and stress rupture properties across two regions: high-stress/short-term (180-150 MPa for 30-454 h) and low-stress/long-term (140-110 MPa for 2881-10,122 h). Using EPMA-EDS with Multiphase Separation Method (MPSM), researchers analyzed how M 23 C 6 and Laves phase coarsening and chemistry (focusing on Cr, W, and Mo distribution) varied between these regions. This multi-region analysis established a framework for more efficient creep testing and improved extrapolation of short-term results to predict long-term rupture strengths, while providing reference phase chemistry data for future studies.

A detailed examination has been carried out of the microstructural evolution and mechanical properties of samples of T91 and T92 steels which have been subjected to both a ‘normal’ pre­service heat treatment and an extended stress relief heat treatment at 765°C for up to 16 hours. The samples have subsequently been creep tested to failure at different stresses ranging from 66 to 112 MPa. In each case, a reduction in rupture time was observed of 20-30% in the samples which had experienced the additional stress relief heat treatment compared to those which had not. It is shown that these data, when compared with the mean values expected from European Creep Collaborative Committee (ECCC) Datasheets, result in a reduction in stress of approximately 10% of the mean value predicted from the ECCC data, which is within the allowable scatter band.

In this paper, the microstructural evolution of P92 steel were studied in the viewpoint of degradation mechanism based on the creep rupture experiment results obtained at elevated temperature by means of macroscopic, metallographic, electronic microscope, energy spectrum, XRD and TEM examination. The results show that the decrease of mechanical properties of P92 steel is mainly due to the change of microstructure and the transformation of carbides, and there is definite relationship between microstructure evolution, mechanical properties and life loss of P92 steel. The results are beneficial to the further study of mechanism of high temperature creep rupture strength and microstructural evolution of heat-resistant steel. It also has important instructive significance to quantitative identification of scientific selection of materials.

A new methodology challenges the conventional use of a constant C-value in the Larson-Miller Parameter (LMP) for 9-12% Cr ferritic steels, proposing instead a multi-C region analysis to address creep strength breakdown issues. Using NIMS data and other publications, the study demonstrates that C-values vary both between steel types and across stress regions. The new approach enables prediction of long-term (10 5 hours) creep rupture properties using only short-term (5×10 3 hours) test data, while d[g(σ)]/d[P(t r ,T)] versus P(t r ,T) analysis provides insight into property stability. This methodology offers a more cost-effective and accurate approach to acquiring and assessing long-term creep rupture data for these heat-resistant steels.

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.

Long-term creep strength property of creep strength enhanced ferritic steels was investigated. Stress dependence of minimum creep rate was divided into two regimes with a boundary condition of macroscopic elastic limit which corresponds to 50% of 0.2% offset yield stress (Half Yield). High rupture ductility was observed in the high stress regime above Half Yield, and it was considered to be caused by relatively easy creep deformation throughout grain interior with the assistance of external stress. Grades T23, T/P92 and T/P122 steels represented marked drop in rupture ductility at half yield with decrease in stress. It was considered to be caused by inhomogeneous recovery at the vicinity of prior austenite grain boundary, because creep deformation was concentrated in a tiny recovered area. High creep rupture ductility of Grade P23 steel should be associated with its lower creep strength. It was supposed that recovery of tempered martensitic microstructure of T91 steel was faster than those of the other steels and as a result of that it indicated significant drop in long-term creep rupture strength and relatively high creep rupture ductility. The long-term creep rupture strength at 600°C of Grade 91 steel decreased with increase in nickel content and nickel was considered to be one of the detrimental factors reducing microstructural stability and long-term creep strength. The causes affecting recovery of microstructure should be elucidated in order to obtain a good combination of creep strength and rupture ductility for long-term.

Grade 91 steel, while increasingly popular in high-temperature power plants for both retrofit and new construction applications, faces significant challenges with Type IV cracking at the outer parent side edge of the weld heat affected zone. This structural integrity issue has led to extensive weld inspection requirements and, in severe cases, the premature replacement of grade 91 retrofit headers before their intended design life. This paper presents a method for estimating Type IV cracking timelines in operating grade 91 components by analyzing crossweld Type IV data to determine when Type IV life deviates from parent life. By combining test results from various temperatures, the method generates a generalized prediction of Type IV life that can be extrapolated to any temperature of interest, providing a practical lower bound estimate for service life of the weakest grade 91 material. This approach, which can be applied to service operating conditions to establish realistic inspection timelines for plant components, has already successfully identified early-stage Type IV cracking in two retrofit headers and is being expanded to additional grade 91 components.

Advanced chromium-molybdenum-vanadium steels 9CrMoV [P(T)91] have seen extensive global adoption across power, petrochemical, and other industrial sectors over the past decade, driven by the demand for materials with superior high-temperature properties to improve efficiency. Experience with P(T)91 base metals and weldments has revealed that these steels require substantially more attention than the commonly used P(T)22 grade and similar materials. This presentation examines Grade 91's various design code requirements across power, petroleum, and nuclear industries, focusing on fabrication and welding considerations. The discussion covers critical material properties and heat treatment parameters, including the significance of maintaining proper preheat and interpass temperatures, while highlighting the risks associated with interrupted heating cycles and improper postweld heat treatment. The paper also addresses factors influencing the use, development, and procurement of Modified Grade 91 welding consumables for heavy wall applications, and explores the subtle technical differences between North American and European approaches to production and utilization, ultimately emphasizing the considerable care required during joining processes to ensure acceptable long-term properties.

Creep strength enhanced ferritic (CSEF) steels, particularly modified 9Cr steels Grade 91 and 92, are increasingly used in advanced coal-fired power plants for header and steam piping construction. While these materials typically enter service after receiving a standard high-temperature normalizing treatment followed by lower temperature tempering to achieve optimal microstructure, practical situations like welding operations may expose components to additional heat treatment exceeding the Ac 1 , and potentially the Ac 3 , temperature before returning to tempering temperature. This research examines the effects of simulated post weld heat treatments (PWHT) on Grade 91 and 92 materials using dilatometer-controlled heating and cooling rates, with peak temperatures below Ac 1 , between Ac 1 and Ac 3 , and above Ac 3 , followed by heat treatment at 750°C for 2 hours. Hardness measurements revealed significant reduction when exceeding the Ac 1 temperature, while advanced electron microscopy, including electron back scatter diffraction, was employed to analyze changes in martensite laths and grain structure, along with detailed carbide size distribution analysis using both scanning and transmission electron microscopy. The findings are discussed in terms of how such PWHT overshoots might affect mechanical properties during high-temperature service.