Search Results for creep

In order to enable a compact design for boiler superheaters in modern thermal power plants, cold-worked tube bending is an economical option. For service metal temperatures of 700 °C and above, nickel-based alloys are typically employed. To ensure a safe operation of such cold-worked alloys, their long-term mechanical behavior has to be investigated. In general, superheater tube materials in a cold-worked state are prone to a degradation of their long-term creep behavior. To predict this degradation, sensitive experiments have to be conducted. In this publication, the effects of cold working on the long-term creep behavior of three currently used nickel-based alloys are examined. Creep and creep rupture experiments have been conducted at typical service temperature levels on nickel-based alloys, which have been cold worked to various degrees. As a result, Alloy 263 exhibits no significant influence of cold working on the creep rupture strength. For Alloy 617, an increase of creep strength due to cold working was measured. In contrast, Alloy 740 showed a severe degradation of the creep strength due to cold working. The mechanism causing the sensitivity to cold working is not yet fully understood. Various formations of carbide precipitates at the grain boundaries are believed to have a major influence. Nevertheless, the experimentally observed sensitivity should always be considered in material selection for boiler tube design.

A methodology is developed for evaluating its creep rupture life from analysis of an on-going creep curve with the aid of an Ω creep curve equation. The method is applied to on-going creep curves of grade 91 steel for evaluating their rupture lives. Quick decrease in creep rupture strength has been reported recently in long-term creep of grade 91 steel. The quick decrease of the steel is discussed by using the rupture lives evaluated. The quick decrease is confirmed in the present study in the time range longer than 3 x 10 4 h at 600°C.

Fossil fuels continue to be the primary source of energy in the U.S and worldwide. In order to improve the efficiency of fossil power plants, advanced structural materials need to be developed and deployed to meet the need of high temperature creep resistance and corrosion resistance. Examples include creep strength enhanced ferritic (CSEF) steels, austenitic stainless steels, nickel-based superalloys, and oxide dispersion strengthened alloys. Welding is extensively used in construction of fossil power plants. The performance of the weld region can be critical to the safe and economical operation of fossil power plants. Degradations in performance such as reduced creep strength and premature failure in the weld region (e.g. Type IV failure in ferritic steels) are examples of longstanding welding and weldability problems for boiler and other components. In the past, extensive studies have been carried out to characterize the different microstructures in different regions of a weld, and to a certain extent, to establish the correlations between the microstructure and the creep strength. However, the metallurgical or microstructural induced local stress/strain variations have been seldom quantified. In addition, it has been long recognized that, due to the sharp microstructure and property gradients in the weld and HAZ, the standard creep testing procedure for the base metal can produce erroneous results when used for weld testing.

This paper presents the creep and creep-fatigue crack growth behaviors of 30Cr1Mo1V turbine rotor steel which had been in service for 16 years. Two typical sections of the rotor, i.e. high and low temperature sections, are examined at 538°C, with crack initiation and propagation monitored by D.C. potential drop method in a compact tension (CT) specimen. The material of the high temperature section has the lower resistance to creep and creep-fatigue crack growths than the low temperature section. The creep crack initiation (CCI) time decreases with the increase of initial stress intensity factor. The creep-fatigue crack growth (CFCG) is dominated by the cycle-dependent fatigue process when the hold time at the maximum load is shorter, but it becomes dominated by the time-dependent creep process when the hold time becomes longer. The high temperature section shows a larger influence of time-dependent creep behavior on CFCG than the low temperature section.

Boron and nitride additions are emerging as a promising design concept for stabilizing the microstructure of creep-resistant martensitic high-chromium steels. This approach, known as MarBN steel (martensitic steel strengthened by boron and nitrogen), combines the benefits of solid solution strengthening from boron with precipitation strengthening from nitrides. However, initial welding trials revealed challenges in achieving a uniform fine-grained region in the heat-affected zone (HAZ), which is crucial for mitigating Type IV cracking and ensuring creep strength. Despite these initial hurdles, preliminary creep test results for welded joints have been encouraging. This study presents an improved MarBN steel formulation and its investigation through uniaxial creep tests. Base material and welded joints were subjected to creep tests at 650°C for up to 25,000 hours under varying stress levels. The analysis focused not only on the creep strength of both the base material and welded joints but also on the evolution of damage. Advanced techniques like synchrotron micro-tomography and electron backscatter diffraction were employed to understand the underlying creep damage mechanisms. By combining long-term creep testing data with 3D damage investigation using synchrotron micro-tomography, this work offers a novel perspective on the fundamental failure mechanisms occurring at elevated temperatures within the HAZ of welded joints in these advanced steels.

This paper summarizes recent efforts to improve creep performance in Grade 91 (Mod. 9Cr-1Mo, ASTM A387) steel weldments via non-standard heat treatments prior to welding. Such heat treatments offer a potential solution for minimizing Type IV failures in creep strength enhanced ferritic (CSEF) steels. A lower temperature tempering (LTT, 650°C) of the 9Cr steels prior to gas tungsten arc welding (GTAW) resulted in improved creep-rupture life at 650°C compared to the samples tempered at a standard condition (HTT, 760°C) before welding. From detailed characterization of precipitation kinetics in the heat affected zone, it was hypothesized that M 23 C 6 carbides in the fine-grain heat-affected zone (FGHAZ) in the LTT sample were fully dissolved, resulting in re-precipitation of strengthening carbides during post weld heat treatment (PWHT). This was not the case in the HTT sample since M 23 C 6 in the FGHAZ was only partially dissolved prior to welding, which caused coarsening of existing M 23 C 6 after PWHT and premature creep failure in the FGHAZ. However, it was also found that the LTT raised the ductile-brittle transition temperature above room temperature (RT). Two different thermo-mechanical treatments (TMTs); two-step tempering and aus-forging/aus-aging, of the modified 9Cr-1Mo steels were attempted, in order to control the balance between creep properties and RT ductility, through control of precipitation kinetics of the M 23 C 6 carbides and/or MX carbo-nitrides. The hardness map of the TMT samples after GTAW and PWHT were evaluated.

Inflection is observed at 50% of 0.2% offset yield stress, that is HALF YIELD, on the relation between stress and creep rupture life of creep strength enhanced ferritic steels with tempered martensitic microstructure. Similar shape is generally recognized on the ferritic steels with martensitic or bainitic microstructure, in contrast to ferritic steels with ferrite and pearlite microstructure, as well as austenitic steels and superalloys except for several alloys. Ferritic steel with martensitic or bainitic microstructure indicates softening during creep exposure, however, hardening due to precipitation takes place in the ferritic steels with ferrite and pearlite microstructure and austenitic steels. This difference in microstructural evolution is associated with indication of inflection at half yield. Stress range of half yield in the stress vs. creep life diagram of creep strength enhanced ferritic steels is wider than that of conventional ferritic creep resistant steels with martensitic or bainitic microstructure. As a result of wide stress range of boundary condition, risk of overestimation of long-term creep rupture strength by extrapolating the data in the high-stress regime to the low-stress regime is considered to be high for creep strength enhanced ferritic steels.

The current study proposed a new method that utilizes digital image correlation (DIC) techniques to measure in-situ full field strain maps of creep resistant material welds. The stress-rupture test is performed in a Gleeble thermal mechanical simulator. This technique successfully captured a significant difference in the local creep deformation between two Grade 91 steel welds with different pre-welding conditions (standard and non-standard). Strain contour plots exhibited inhomogeneous deformation in the weldments, especially at the heat-affected zone (HAZ). Standard heat-treated specimens had significant creep deformation in the HAZ. On the other hand, non-standard heat treated specimens showed HAZ local strains to be 4.5 times less than that of the standard condition, after a 90-hour creep test at 650°C and 70 MPa. The present study measured the full field strain evolution in the weldments during creep deformation for the first time. The proposed method demonstrated a potential advantage to evaluate local creep deformation in the weldments of any creep resistant material within relatively short periods of time.

Grade 92 steel, a creep strength-enhanced ferritic (CSEF) steel, is used in supercritical steam fossil power plants for boilers and piping systems. While its creep strength is crucial, understanding the interaction between creep and fatigue damage is also vital for assessing component integrity under cyclic loading. Despite existing studies on its creep-fatigue behavior, additional data under creep-dominant conditions relevant to plant evaluations are needed. Girth welds, critical to piping system integrity, are particularly important in this context. EPRI and CRIEPI initiated a project to develop a comprehensive database on the creep-fatigue behavior of Grade 92 steel's base metal and welded joints and to establish a suitable life estimation procedure. Key findings include: (i) a thick pipe with submerged arc welding (SAW) was manufactured for testing; (ii) base metal and cross-weld specimens showed similar behavior under short-term creep and cyclic loading; (iii) these specimens had lower creep strengths than average literature values for this steel class in the short time regime, with differences decreasing as stress decreased; and (iv) the fatigue and creep-fatigue behavior of these specimens were similar to those of Grade 91 and 122 steels, with common characteristics in creep-fatigue failure prediction models across the three CSEF steels.

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.

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.

The long-term creep strength of creep strength-enhanced ferritic steels has been overestimated due to changes in the stress dependence of creep rupture life at lower stress levels. To address this, creep rupture strength has been reassessed using a region-splitting analysis method, leading to reductions in the allowable tensile stress of these steels as per Japan’s METI Thermal Power Standard Code in December 2005 and July 2007. This method evaluates creep rupture strength separately in high and low stress regimes, divided at 50% of the 0.2% offset yield stress, which corresponds approximately to the 0% offset yield stress in ASME Grade 122-type steels. In the high-stress regime, the minimum creep rate follows the stress dependence of flow stress in tensile tests, with the stress exponent (n) decreasing from 20 at 550°C to 10 at 700°C. In contrast, the low-stress regime exhibits an n value of 4 to 6 for tempered martensitic single-phase steels, while dual-phase steels containing delta ferrite show an even lower n value of 2 to 4. The significant stress dependence of creep rupture life and minimum creep rate in the high-stress regime is attributed to plastic deformation at stresses exceeding the proportional limit. Meanwhile, creep deformation in the low-stress regime is governed by diffusion-controlled mechanisms and dislocation climb as the rate-controlling process.

High temperature components with notches, defects and flaws may be subject to crack initiation and crack propagation under long-term service conditions. To study these problems and to support an advanced remnant life evaluation, fracture mechanics procedures are required. Since a more flexible service mode of power plants causes more start up and shut down events as well as variable loading conditions, creep-fatigue crack behavior becomes more and more decisive for life assessment and integrity of such components. For steam power plant forged and cast components, the crack initiation time and crack growth rate of heat resistant steels were determined in long-term regime up to 600 °C. Component-like double edge notched tension specimens have been examined. The results are compared to those obtained using the standard compact tension specimen. Crack initiation time and crack growth rate have been correlated using the fracture mechanics parameter C*. The applicability of the stress intensity factor K I to describe the creep crack behavior is also being assessed. A modified Two-Criteria-Diagram was applied and adapted in order to recalculate crack initiation times under creep-fatigue conditions. Recommendations are given to support the use of different fracture mechanics parameters in order to describe the long-term crack behavior under creep and/or creep-fatigue conditions.

The strength of welded joints in high-chromium steels is a critical concern for operators of ultra-supercritical thermal power plants. To investigate this, a series of creep-fatigue tests with tensile strain holds were conducted on welded joints of two widely used high-chromium steels: Grade 91 and Grade 122. The tests revealed that failure consistently occurred in the fine-grain heat-affected zone, even at relatively low temperatures and short durations, whereas in simple creep tests, failure occurred in the plain base metal region. Four different procedures were used to predict failure life, and their results were compared with experimental data. Among them, a newly proposed energy-based approach provided the most accurate failure life estimations, independent of material type and temperature.

This paper investigates the effect of high temperature tensile strain on subsequent creep strength in grade 91 steel. Failed hot tensile specimens have been sectioned at various positions along the specimen axis, and therefore at different levels of hot tensile strain, to obtain material for creep strength evaluation. Because of the limited amount of material available for creep testing obtained in this way, creep testing has been carried out using the specialised small-scale impression creep testing technique. The grade 91 material has been tested in both the normal martensitic condition and in an aberrant mis-heat treated condition in which the microstructure is 100% Ferrite. The latter condition is of interest because of its widespread occurrence on operating power plant with grade 91 pipework systems.

Detailed knowledge of the creep and creep crack behavior is essential for a safe operation of thick-walled components in thermal power plants. High mechanical loads and temperatures of more than 700 °C often require the application of nickel-based alloys, e.g. alloy C-263. Unfortunately, manufacturing and non-destructive evaluation (NDE) of thick-walled components (> 50 mm) made of nickel-based alloys are quite challenging. Tolerable critical flaw sizes, experimentally validated for long service durations, play an important role in the quality assurance of such components. It is commonly accepted that manufacturing parameters, e.g. heat treatment procedures, have a significant influence on creep ductility and time-dependent crack behavior. By means of adjusting the process parameters, the ductility and the creep life of notched specimen can be significantly improved in the case of alloy C-263. Essential root cause is the decoration of grain boundaries with carbides which drastically influences creep crack initiation and growth. This results in significant differences for allowable critical flaw sizes and thus, the potential use of the candidate material. On a first generation of alloy C-263 “G1”, a dense population of carbides on the grain boundaries was found, which resulted in an inadmissible creep crack behavior. The resulting critical flaw sizes were only a few tenths of a millimeter. On a second generation “G2”, the grain boundary occupation was positively influenced, so that a satisfactory creep crack behavior could be found. The critical flaw sizes are in the order of one millimeter or more. A critical or impermissible material behavior under creep conditions can be demonstrated by testing smooth and notched round specimens. For example, the first generation “G1” notched round specimens fails earlier than the smooth round specimens, indicating notch sensitivity. On the second generation “G2”, however, a notch insensitivity was found. The critical defect sizes can be determined by a method that takes into account a simultaneous examination of the crack tip situation and the ligament situation.

This research compares creep crack growth behavior of two heats of creep strength enhanced ferritic (CSEF) steel, grade 91. These heats represent extremes of creep damage susceptibility, one heat exhibiting low creep ductility and the other high creep ductility. Creep crack growth tests were performed with compact tension specimens and were monitored with direct current potential drop and optical surface measurements. Load line displacement was measured throughout the duration of the tests. Specimens were sectioned, mounted, and analyzed using optical and scanning electron microscopy to assess the presence of oxidation, micro-cracking, creep damage, and void density. Tests were performed over a range of initial stress intensities on the low ductility material to investigate the impact of creep ductility. Metallurgical evidence and test data for each crack growth test was assessed to evaluate crack growth behavior linked to creep crack growth parameter (C*) and stress/creep damage distribution in the vicinity of the crack.

There is an increased interest in miniature testing to determine material properties. The small punch test is one miniaturized test method that has received much interest and is now being applied to support the design and life assessment of components. This paper presents the results of a test program for a small punch creep test at 650°C of 316L stainless steel produced from additive manufacturing. A major finding is that the deflection rate curve versus time may have multiple minima as opposed to forged 316L with only one minimum. This is believed to be due to microcracking and has direct consequences on the determination of the creep properties that that are based on a single minimum value in the CEN Small Punch Standard. In the paper, aged and nonaged materials are compared, and small punch creep results are also compared with standard uniaxial creep tests. The multiple minima feature means that the approach to determine equivalent stress and strain rate from the minimum deflection rate needs to be modified. Some approaches for this are discussed in the paper. Under the assumption that the multiple minima represent cracking, it opens up opportunities to quantify reduced creep ductility by the small punch test.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of simple Design by Formula rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. The metallurgical findings from the selected tests are the focus of the Part II paper. The association of performance with notch geometry, weld strength, and other potential contributing factors will be highlighted with a primary objective of informing the reader of the variability, and heat-specific behavior that is observed among this class of alloys widely used in modern thermal fleet components and systems.

The time-dependent behavior of 9Cr creep strength enhanced ferritic (CSEF) steels has long fixated on the creep life recorded in uniaxial constant load creep tests. This focus is a consequence of the need to develop stress allowable values for use in the design by formulae approach of rules for new construction. The use of these simple rules is justified in part by the assumption that the alloys used will invariably demonstrate high creep ductility. There appears to be little awareness regarding the implication(s) that creep ductility has on structural performance when mechanical or metallurgical notches (e.g., welds) are present in the component design or fabricated component. This reduced awareness regarding the role of ductility is largely because low alloy CrMo steels used for very many years typically were creep ductile. This paper focuses on the structural response from selected tests that have been commissioned or executed by EPRI over the last decade. The results of these tests demonstrate unambiguously the importance that creep ductility has on long-term, time-dependent behavior. This is the second part of a two-part paper; Part I reviewed the selected tests and discussed them from a mechanical perspective. The association of performance with specific microstructural features is briefly reviewed in this paper and the remaining gaps are highlighted for consideration among the international community.