The complex nitride Z-phase, Cr(V,Nb)N, has recently been identified as a major cause for premature breakdown in creep strength of a number of new 9-12%Cr martensitic steels, especially the high Cr variants. A thermodynamic model of the Z-phase has been created based on the Thermo-Calc software. The model predicts the Z-phase to be stable in all of the new 9- 12%Cr martensitic steels, and this has generally been confirmed by experimental observations. Z-phase precipitation seems then to be a kinetic problem, and driving force calculations, using Thermo-Calc with the developed model, have been used to predict steel compositions, which could delay Z-phase precipitation. The model also predicted the existence of a new niobium free Z-phase variant, which has since been discovered in a niobium free 12CrMoV steel.

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 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.

Conventional time-temperature-parameter (TTP) methods often overestimate long-term creep rupture life of creep strength enhanced high Cr ferritic steels. The cause of the overestimation is studied on the basis of creep rupture data analysis on Gr.91, 92 and 122 steels. There are four regions with different values of stress exponent n for creep rupture life commonly in stress-rupture data of the three ferritic steels. Activation energies Q for rupture life in the regions take at least three different values. The values of n and Q decrease in a longer-term region. The decrease in Q value is the cause of the overestimation of long-term rupture life predicted by the conventional TTP methods neglecting the change in Q value. Therefore, before applying a TTP method creep rupture data should be divided into several data sets so that Q value is unique in each divided data set. When this multi-region analysis is adopted, all the data points of the steels can be described accurately, and their long-term creep life can be evaluated correctly. Substantial heat-to-heat and grade-to-grade variation in their creep strength is suggested under recent service conditions of USC power boilers.

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.

A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650°C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing ~7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson Miller Parameter curve for CPJ7 at 650°C almost coincides with that of COST E at 620°C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650°C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.

Creep strength of Grade 91 steels has been reviewed and allowable stress of the steels has been revised several times. Allowable stress regulated in ASME Boiler and Pressure Vessel Code of the steels with thickness of 3 inches and above was reduced in 1993, based on the re-evaluation with long-term creep rupture data collected from around the world. After steam leakage from long seam weld of hot reheat pipe made from Grade 122 steel in 2004, creep rupture strength of the creep strength enhanced ferritic (CSEF) steels has been reviewed by means of region splitting method in consideration of 50% of 0.2% offset yield stress (half yield) at the temperature, in the committee sponsored by the Ministry of Economy, Trade and Industry (METI) of Japanese Government. Allowable stresses in the Japanese technical standard of Grade 91 steels have been reduced in 2007 according to the above review. In 2010, additional long-term creep rupture data of the CSEF steels has been collected and the re-evaluation of creep rupture strength of the steels has been conducted by the committee supported by the Federation of Electric Power Companies of Japan, and reduction of allowable stress has been repeated in 2014. Regardless of the previous revision, additional reduction of the allowable stress of Grade 91 steels has been proposed by the review conducted in 2015 by the same committee as 2010. Further reduction of creep rupture strength of Grade 91 steels has been caused mainly by the additional creep rupture data of the low strength materials. A remaining of segregation of alloying elements has been revealed as one of the causes of lowered creep rupture strength. Improvement in creep strength may be expected by reducing segregation, since diffusional phenomena at the elevated temperatures is promoted by concentration gradient due to segregation which increases driving force of diffusion. It has been expected, consequently, that the creep strength and allowable stress of Grade 91 steels can be increased by proper process of fabrication to obtain a homogenized material free from undue segregation.

Recently advanced ultra-super critical (A-USC) pressure power plants with 700°C class steam parameters have been under development worldwide. Japanese material R&D program for A- USC beside the plant R&D program started in 2008, launched in 2007 under the METI/NEDO foundation includes not only alloy design explores and novel ideas for developing new steels and alloys that can fill critical needs in building 700°C class advanced power plants, but also fundamental studies on creep strength and degradation assessment, which are absolutely needed to assure the long-term safe use of newly developed steels and alloys at critical temperature conditions, for instance, 650°C for ferritic steels, 700°C for austenitic steels and 750°C for Ni- based alloys. This program concept has been based on the lessons from materials issues recently experienced in the creep strength enhanced ferritic steels used for 600°C class ultra-super critical power plants. Particular outputs from the program up to now are recognized as the ferritic steel having the creep strength of 100MPa at 650°C beyond 30,000h without any Type IV degradation and as the austenitic steel developed by means of inter-metallic compounds precipitation strengthening of grain boundary which should be strongest in creep ever found. Concurrently great progresses have been seen in the research works with positron annihilation life monitoring method applicable to various kinds of defects, structural free energy values, small punch creep test data for very limited interest area, crystallographic analyses, optimum time-temperature parameter regional creep rupture curve fitting method, hardness model, etc. which would highly contribute to find out and establish the structural parameters affecting to creep strength and degradation resulting in accurately estimating the 100,000h creep strength.

The increasing steam parameters in modern high-efficiency fossil fuel power plants demand advanced materials with enhanced creep strength for operation under extreme temperature and pressure conditions. Tenaris has focused on developing ferritic-martensitic and austenitic grades for tube and pipe applications. At TenarisDalmine, efforts on ferritic-martensitic steels include ASTM Grade 23, a low-alloyed alternative to Grade 22 with 1.5% W, offering good weldability, creep resistance up to 580°C, and cost competitiveness. Additionally, ASTM Grade 92, an improved version of Grade 91, provides high creep strength and long-term stability for components like superheaters and headers operating up to 620°C. At TenarisNKKT R&D, austenitic steel development includes TEMPALOY AA-1, an improved 18Cr-8NiNbTi alloy with 3% Cu for enhanced creep and corrosion resistance, and TEMPALOY A-3, a 20Cr-15Ni-Nb-N alloy with superior creep and corrosion properties due to its higher chromium content. This paper details the Tenaris product lineup, manufacturing processes, and key material properties, including the impact of shot blasting on the steam oxidation resistance of austenitic grades. It also covers ongoing R&D efforts in alloy design, creep testing, data assessment, microstructural analysis, and damage modeling, conducted in collaboration with Centro Sviluppo Materiali.