Search Results for optical metallography

The recently developed 12%Cr steel VM12-SHC is characterized by very good creep properties at temperatures up to 620°C. This new material development exhibits an excellent oxidation resistance in steam atmospheres at the typical application temperature but also at temperatures up to 650°C. In comparison to the existing 9% Cr grades T/P91 and T/P92, VM12-SHC steel opens due to its excellent oxidation behavior, new possibilities for its application as a heat exchanger boiler component. It was found that outside its application temperature range VM12-SHC also shows, as all 9-12%Cr steels, the appearance of the so called Z-phase. This effect was investigated to understand its influence on creep properties of this class of ferritic/martensitic steels aiming at controlling the microstructure stabilities for future grade developments. Creep testing has been carried out in the temperature range between 525°C and 700°C. Selected crept specimens have been investigated using light optical microscopy, SEM with EDX and TEM. In this study, the oxidation behavior of a number of typical martensitic 9-12%Cr steels was compared with the newly developed 12% Cr steel VM12-SHC. The compositions and morphologies of oxide scales formed after 5000 h exposure steels in simulated steam environments as function of temperature were characterized by light optical metallography and scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX).

The newly developed 12%Cr steel Super VM12 is characterized by excellent creep rupture strength properties (better than Grade 92) and enhanced steam oxidation resistance of 12%Cr steels such as VM12-SHC. Balanced properties profile of the new steel development in comparison to the existing well-established steels such as Grade 91 and Grade 92, opens opportunities for its application as construction material for components in existing or future high-efficiency power plants. In this study the oxidation behavior of typical 9%Cr steels was compared with the new steel development. The oxide scale morphologies and compositions of different oxide layers as function of temperature and exposure time in steam-containing atmospheres were characterized using light optical metallography, Scanning Electron Microscopy (SEM). Creep testing has been carried out in the temperature range between 525°C and 700°C. Selected creep specimens were investigated using the Transmission Electron Microscopy and the Atom Probe Tomography techniques.

The steam oxidation behaviour of boiler tubes and steam piping components is a limiting factor for improving the efficiency of the current power plants. Spallation of the oxide scales formed during service can cause serious damage to the turbine blades. Vallourec has implemented an innovative solution based on an aluminum diffusion coating applied on the inner surface of the T/P92 steel. The functionality of this coating is to protect the tubular components against spallation and increase the actual operating temperature of the metallic components. In the present study, the newly developed VALIORTM T/P92 product was tested at the EDF La Maxe power plant (France) under 167b and 545°C (steam temperature). After 3500h operation, the tubes were removed and characterized by Light Optical Metallography (LOM), Scanning Electron Microscopy (SEM), with Energy Dispersive X-ray spectrometry (EDX) and X-Ray Diffraction (XRD). The results highlight the excellent oxidation resistance of VALIORTM T/P92 product by the formation of a protective aluminum oxide scale. In addition, no enhanced oxidation was observed on the areas close to the welds. These results are compared with the results obtained from laboratory steam oxidation testing performed on a 9%Cr T/P92 steel with and without VALIORTM coating exposed in Ar-50%H 2 O at 650°C.

The oxidation resistance of 9-12% chromium steels in steam-containing environments simulating the service conditions of steam power plant has been investigated for exposure times ranging from 1 h up to 10 000 h. For characterizing the oxidation behavior, the results of gravimetric studies were combined with data obtained from a number of analysis techniques, such as optical metallography, SEM/EDX and LRS. Different mechanisms of oxidation were observed for the various steels in different temperature regimes, exposure times and exposure conditions. The cracking and spallation of scales was correlated with the type, morphology and growth of pores and voids in the scale and could be influenced by the steel microstructure. For some steels, the steam oxidation resistance increased with increasing exposure temperature. The oxidation rates only slightly depend on the exact water vapor content in the test gas but can be strongly affected by the gas flow rates.

The A286 is one of the earliest superalloys developed. It has been used for manufacturing different components of turbo machineries because of its balanced high temperature properties. These components include shafts, discs, spacers, blades and fasteners. This paper reviews some of the issues and recent field experiences related to metallurgy, fabrication, in-service evaluation and failure of some of these components. The fabrication aspects including the effects of alloy melting processes, forging parameters and different types of heat treatments on the processed parts are discussed. The importance of these factors on the microstructure and properties of A286 are highlighted. The effects of service exposure on some of these parts are also discussed. In service evaluation involves checking for service induced damage or changes in microstructures and properties so that the suitability of the part for continued service can be determined. The failure analysis section of the paper briefly discusses failures of two expander wheels and an expander disc made out of A286 to pinpoint some of the salient features of damage accumulation and fracture during service.

Grade 92 steel has been widely applied in the power generation industry for use as steam pipes, headers, tubes, etc. owing to a good combination of creep and corrosion resistance. For the welding of thick section pipes, a multi-pass submerged arc welding process is typically used to achieve sufficient toughness in the weld. To relieve the internal stress in the welds and to stabilise their microstructures, a post weld heat treatment (PWHT) is commonly applied. The heat treatment conditions used for the PWHT have a significant effect on both the resulting microstructure and the creep behaviour of the welds. In this study, interrupted creep tests were carried out on two identical Grade 92 welds that had been given PWHTs at two different temperatures: 732°C and 760°C. It was found that the weld with the lower PWHT temperature had a significantly reduced stain rate during the creep test. In addition, microstructural examination of the welds revealed that the primary location of creep damage was in the heat affected zone in the sample with the lower PWHT temperature, whereas it was in the weld metal in the sample with the higher PWHT temperature. To understand the effect of the different PWHT temperatures on the microstructure, initially the microstructures in the head portions of the two creep test bars were compared. This comparison was performed quantitatively using a range of electron/ion microscopy based techniques. It was apparent that in the sample subjected to the higher PWHT temperature, larger Laves phase particles occurred and increased matrix recovery was observed compared with the sample subjected to the lower PWHT temperature.

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.

Creep-fatigue crack formation (endurance) and crack growth rate data are necessary inputs for assessing the structural integrity and for estimating the design life of high temperature components in power generation and aircraft engine industries. Ensuring consistency in the reported test data, as well as an understanding of the inherent scatter and its source in the data, are both necessary for assuring quality and limitations of the analyses that rely on the data. In 2008, the American Society for Testing and Materials (ASTM) under the umbrella of its subcommittees E08.05 on Cyclic Deformation and Crack Formation and E08.06 on Crack Growth, and the sponsorship of Electric Power Research Institute (EPRI) through its international experts’ working group on creep-fatigue embarked on the task of developing separate standard test methods for creep-fatigue crack formation and creep-fatigue crack growth. The first standard entitled, “E-2714-09: Standard Test Method for Creep-fatigue Testing” was developed in 2009 and was followed up with a round-robin consisting of 13 laboratories around the world for testing the newly developed standard. This paper discusses the results of this round-robin concluded in 2012 using the widely used P91 steel that led to the formulation of the Precision and Bias statement contained in the version of the ASTM standard E2714 that was successfully balloted in the year 2013.

In this paper, the dissimilar metal welds (DMWs) between 617B nickel-based alloy and 10%Cr martensitic heat-resistant steel filled by 617 filler metal was studied, focused on the high temperature creep rupture properties. The high temperature creep rupture properties of welded joints with different welding processes were tested, and the microstructure of welded joints before and after the creep rupture test was observed by OM and SEM. The results showed that, there were three failure modes: base metal failure, type W failure and interface failure, among which interface failure caused the most serious life reduction. The welded joints using ER NiCr-3 filler metal reduced the strain concentration at the interface, so the fracture location shifted from the interface to HAZ of 10%Cr martensitic heat-resistant steel under high temperature and low stress conditions, and creep rupture life was improved. Similarly, weld cap shifted the creep crack propagation path by changing the groove form, so as to altered the stress state of joint and prolong the creep rupture life.

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.

The microstructural evolution has been investigated for an 18Cr-12Ni stainless steel (347HFG) that has been subject to a thermo-mechanical treatment to obtain a fine grain size (ASTM 7-10). In particular, sigma phase precipitation and growth has been evaluated. Samples of 347HFG stainless steel have been isothermally heat treated to reproduce and accelerate the ageing conditions experienced in-service at temperatures between 600 and 750 °C for up to 10,000 hours. Results have shown that sigma phase is precipitated at triple points and along grain boundaries after as little as 1000 hours which is contrary to thermodynamic predictions. In addition X-ray diffraction (XRD) and image analysis has been carried out to semi-quantitatively measure the amount of sigma phase present. The area fraction of sigma has been found to be 2.77 and 2.23 percent at 700 and 750 °C respectively. This is a higher volume fraction of sigma phase than has been previously observed in regular 347H at these conditions. It is thought that this is due to the reduced grain size that has provided an increase in nucleation sites and diffusion paths that can enhance the precipitation and growth of sigma phase. The results from this study are discussed with regards to the effect of precipitation on the service life of a 347HFG stainless steel tube operating in advanced supercritical boilers.

Nitridation is a high-temperature material degradation issue that can occur in air and in environments containing nitrogen, ammonia, etc., and in a variety of industrial processes. The nitridation behavior of several commercial nickel- and cobalt-based alloys is reviewed in this paper. The alloys include Haynes 230, Haynes 188, Haynes 625, Haynes 617, Haynes 214, Hastelloy X, and Haynes 233. The environments discussed are high-purity nitrogen gas between 871°C and 1250°C, 100% ammonia gas at 982°C and 1092°C, and a simulated combustion atmosphere at 982°C. The results showed that nitridation occurred in all the environments containing nitrogen. The nitridation attack was strongly influenced by the alloy compositions and the type of oxide formed (i.e., chromia or alumina), as some degree of oxidation was expected in the environments in which residual oxygen was present. Thermal cycling is briefly discussed because the integrity of protective oxides is also an important factor in resisting high-temperature oxidation and nitridation attack.

Modified 9Cr-1Mo alloy steel has been developed over the last few decades and has since gained wide acceptance in the boiler industry for the production of a variety of pressure-critical components, including tubing, piping and headers. The properties of creep-strength enhanced ferritic steels such as grade 91 are critically dependent on manufacturing parameters such as steelmaking, hot deformation, heat treatment and welding. Since the applications for which this material is used impose strict requirements in terms of resistance, corrosion, and creep behavior, poor process control can severely compromise the service behavior. This work discusses the impact of total deformation during the rolling process, and heat treatment parameters on time-independent and time-dependent properties for grade 91. For this study, two heats with similar chemical composition were produced with different reduction ratios: to which, several normalizing and tempering combinations were applied. For each combination, the microstructure was characterized, including evaluation of segregation by metallographic examination, and analysis of secondary phase precipitates by means of X-ray powder diffraction. Mechanical testing and creep testing were performed. A comparison of results is presented, and recommendations on the optimal process parameters are provided to ensure reliable performance of grade 91 material.

9-12%Cr martensitic steels can be applied to the next highest temperature components such as boiler tracts, steam pipelines and turbines of advanced ultra-supercritical power plants with steam temperatures of 650°C. New 10%Cr martensitic steels with high B and low N contents can be a worthy candidate for use in production because it has superior creep resistance. At the same time, resistance to cyclic and dynamic loads is very important. In this work, we studied the low cycle fatigue (LCF) properties at room and elevated (500-650°C) temperatures and Charpy impact toughness at temperatures ranging from -196…100°C of advanced 10% Cr martensitic steel with high B and low N contents. The effect of new alloying scheme and corresponding peculiarities of M 23 C 6 carbides on the low cycle fatigue resistance and impact toughness of the 10%Cr martensitic steel is analyzed. It is revealed that fine and densely distributed carbides has no effect on the fatigue resistance except for the slight improvement of fatigue life at small strain amplitudes and shift the ductile-brittle transition temperature (DBTT) to higher but satisfactory value of +10°C as compared to other high-chromium martensitic steels.

A comprehensive EPRI initiative launched in 2006 has addressed the critical need to better understand creep-fatigue interactions in power plants experiencing cyclic operation. This international collaboration of industry experts has focused on evaluating current test methods, analyzing crack initiation and growth methodologies, examining life prediction approaches for various applications, identifying deficiencies in creep-fatigue damage assessment, and determining future research requirements. This paper presents key findings from the project, with particular attention to the performance of creep-strengthened ferritic steels, specifically Grade 91 and 92 steels, providing essential insights for power plants facing increasingly demanding operational conditions.

The Institute for Materials Science, Welding and Forming (IWS) conducts extensive research on modern martensitic 9-12% Cr steels intended for use in environmentally friendly power plants. Their comprehensive research program encompasses mechanical testing of base and weld metals, analysis of creep and damage mechanisms, weldability studies, microstructural evolution during creep, mathematical modeling of precipitation and coarsening kinetics, and simulation of complex heat treatments and creep deformation behavior. Through these interconnected projects, which are briefly described, IWS develops a thorough understanding of these materials while working toward a quantitative model of their creep behavior.

Creep-resistant austenitic stainless steels are known to be the potential candidate materials for use as super- and re-heater tubes in ultra-super critical (USC) power plants. Among them, ASTM A213/A213M S30432, a novel 18-8 stainless steel (18Cr- 9Ni-3Cu-Nb-N), has attracted considerable attention from electric industry due to its combined lower cost and more excellent performance in contrast to traditional TP347H steel. More than 10 years of service in Japan laid a solid foundation for the steel being selectable USC boiler materials. Steels of S30432 have been recently developed in China during the past few years. This paper presents the evaluation results of S30432 tubes manufactured by four steel plants in China as well as Sumitomo super304H tubes for comparison. A detailed microstructural analysis of the tubes has been performed by using optical and electron microscope, and mechanical properties of the tubes have been evaluated using hardness testing as well as tensile testing up to 700°C. It was found that the impurity elements, nonmetallic inclusions and grain size of the S30432 tubes were well controlled. TEM observation revealed the microstructural changes for a selected batch of S30432 specimens in condition of hot rolled material, as-extruded tube, solution treated tube and 650°C/1000h aged tube. Most attention was paid to the morphology and distribution of precipitates in the microstructure which should be responsible for the enhanced performance of the steel. Although the hardness of all the evaluated tubes was measured to be similar, they showed more or less differences in tensile properties between each other. Creep rupture testing is still in progress, and the steel might exhibit excellent long-term creep rupture strength at 650°C as was predicted from the currently available testing results.

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.

Metallurgical factors affecting the fusion boundary failure and damage mechanism of DMWs (Dissimilar Metal Welds) between the CSEF (Creep Strength Enhanced Ferritic) steels and austenitic steels were experimentally and theoretically investigated and discussed. Long-term exservice DMWs up to 123,000 hours were investigated; the precipitates near the fusion boundary were identified and quantitatively evaluated. Comparing with the other generic Ni-based weld material, MHPS original filler metal HIG370 (Ni bal.-16Cr-8Fe-2Nb-1Mo) showed superior suppression effect on fusion boundary damage of DMWs, which was verified by both of the microstructure observation and thermodynamic calculation. Based on the microstructure observation of crept specimen and ex-service samples of DMWs, temperature, time and stress dependence of fusion boundary damage of DMWs were clarified. Furthermore, fusion boundary damage morphology and mechanism due to precipitation and local constituent depletion was discussed and proposed from metallurgical viewpoints.

Microstructure in the gage sections of ruptured GX12CrMoWVNbN10-1-1 cast steel specimens was examined after creep tests under applied stresses ranging from 120 to 160 MPa at T=893 K. The microstructure after tempering consisted of laths with an average thickness of 332 nm. The tempered martensite lath structure was characterized by M 23 C 6 -type carbide particles with an average size of about 105 nm, and MX carbonitrides with an average size of about 45 nm. Precipitation of Laves phase occurred during creep test. The structural changes in the gauge section of the samples were characterized by the evolution of relatively large subgrains with remarkably lowered density of interior dislocations within former martensite laths. MX carbonitrides and M 23 C 6 -type carbide particles increase in size slightly under long-term creep. Microstructural degradation mechanisms during creep in GX12CrMoWVNbN10-1-1 cast steel are discussed.