Search Results for ferrite

Effects of Ni content and heat treatment condition on impact toughness and creep strength of precipitation strengthened 15Cr ferritic steels were investigated in order to discuss a possibility of improvement in both mechanical properties. Both creep strength and impact toughness of the developing steels were improved drastically by solid solution treatment with water quenching. However, an addition of Ni reduced the long-term creep strength of the steels, though Ni was effective in improvement in impact toughness. It was found that water quenching suppressed formation of coarse block type particles and precipitate free zones around them, and precipitation of plate type fine particles and thermal stability of them within ferrite phase were promoted by solid solution treatment with water quenching. However, martensite phase with sparsely distributed coarse block type particles were formed in the Ni added steels, and such microstructure reduced the precipitation strengthening effect slightly. On the other hand, increase in impact values of the steel indicated no relation to volume fraction of martensite phase. It was supposed that the impact toughness of ferrite phase itself was improved by solid solution treatment and addition of Ni.

In order to evaluate long term creep strength of modified 9Cr ferritic steels, the system free energy of creep ruptured specimens at both 650 and 700 °C is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. Change ratio of the system free energy and creep stress showed the relationship with one master curve irrespective of creep conditions, indicating that the steel ruptures when the applied stress exceeds a limited stress depending on the microstructural state expressed by the change ratio of system free energy. Furthermore, it was found that dominant factor of the change ratio was the chemical free energy change. On the basis of these results, long term creep strength of the steel was evaluated at 700 °C, for example, 19MPa at 700 °C after 10 5 h. It is concluded that long term creep strength of modified 9Cr ferritic steels can be predicted by the system free energy concept using the ruptured specimens with various creep conditions.

Because of the problems experienced with steam-side oxidation in commercial power plants, there has been continuing interest in better understanding the steam oxidation behavior of creep strength enhanced ferritic steels such as grades 23, 24 and 91 as well as 300-series stainless steels such as 347H and 304H. Analysis of field-exposed tubes has provided information on the oxidation reaction products but relatively few specimens are available and there is limited information about the kinetics. Specimens have included tube sections with a shot peened surface, a treatment that is now widely used for austenitic boiler tubes. To complement this information, additional laboratory studies have been conducted in 1bar steam at 600°-650°C on coupons cut from conventional and shot-peened tubing. Exposures of 1-15 kh provide some information on the steam oxidation kinetics for the various alloys classes. While shot-peened type 304H retained its beneficial effect on oxidation resistance past 10,000 h at 600° and 625°C, the benefit appeared to decline after similar exposures at 650°C.

A new martensitic steel for power generation applications was developed: Tenaris High Oxidation Resistance (Thor) is an evolution of the popular ASTM grade 91, offering improved steam oxidation resistance and better long-term microstructural stability, with equal or better creep strength. Thanks to its design philosophy, based on consolidated metallurgical knowledge of microstructural evolution mechanisms, and an extensive development performed in the last decade, Thor was engineered to overcome limitations in the use of ASTM grade 91, above 600 °C, particularly related to scale growth and liftoff. After laboratory development, Thor was successfully validated at the industrial level. Several heats up to 80 metric tons were cast at the steel shop, hot rolled to tubes of various dimensions, and heat treated. Trial heats underwent extensive characterization, including deep microstructural examination, mechanical testing in the as-received condition and after ageing, long-term creep and steam oxidation testing. This paper presents an overview of metallurgical characterization performed on laboratory and industrial Thor material, including microstructural examination and mechanical testing in time-independent and time-dependent regimes. Data relevant to the behavior and the performance of Thor steel are also included.

High chromium HiperFer (High performance ferritic) materials present a promising concept for the development of high temperature creep and corrosion resistant steels. The institute for Microstructure and Properties of Materials (IEK-2) at Forschungszentrum Jülich GmbH, Germany develops high strength, Laves phase forming, fully ferritic steels which feature excellent resistance to steam oxidation and better creep life than state of the art 9-12 Cr steels. Mechanical strength properties of these steels depend not only on chemical composition, but can be adapted to various applications by specialized thermo(mechanical) treatment. The paper will outline the sensitivity of tensile, creep, stress relaxation and impact properties on processing and heat treatment. Furthermore an outlook on future development potentials will be derived.

In the present study a creep resistant, ferritic steel, based on the chemical composition of Crofer 22 H, was analysed regarding microstructure and particle evolution. Because of the preceding hot-rolling process formation of sub-grain structures was observed, which disappears over time. Additionally formation of particle-free zones close to high angle grain boundaries was observed. These zones are considered to be responsible for long-term material failure by lacking particle hardening and thus a concentration of deformation. Therefore in-depth analyses by transmission and scanning electron microscopy were performed to investigate dislocation behaviour in these areas

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.

In order to study the effect of precipitation strengthening by MX precipitates on the restriction of microstructure degradation in 9 mass% Cr ferritic heat-resistant steels, V, Nb additioned model steels were evaluated by microstructure analysis through TEM and EBSD with reference to the creep test and creep interrupting test. VN precipitation increased the creep strength if the content was higher than 0.02%. Simultaneous addition of Nb and V in the specimen resulted in the complex NbC-VN precipitates even in the as-heat-treated specimens. The coherent and fine-needle-type VN was also detected in the steel. These precipitates are expected to increase the creep strength according to the creep strain curves. V variation up to 0.02% did not affect the crystallographic character of the grain boundary in the as-heat-treated specimens. Nb variation affected the crystallographic character of the grain boundary significantly because of the grain refinement effect of NbC. VN precipitation during the creep test restricted the crystallographic misorientation-angle-profile degradation. Integrating all intragranular precipitates, VN, restricts the crystallographic degradation significantly. The long-term creep test results and the precise precipitation analysis will be disclosed by the presentation.

The effects of Cr and W on the creep rupture life of 8.5-11.5Cr steels at 650°C were evaluated. Throughout this paper the specimen composition is expressed in mass percent. The creep rupture life of 8.5Cr steel is the longest in 8.5-11.5Cr steels at 650°C under the stress of 78MPa. The creep rupture life of 9Cr steel at 650°C was extended with increasing W content. The creep strength of the modified steel, 9Cr-4W-3Co-0.2V-NbBN steel, at 650°C did not decrease sharply up to 32000h. The 105h creep rupture temperature of this steel under the stress of 100MPa was estimated to be approximately 635°C using Larson-Miller parameter. M 23 C 6 type carbides and VX type carbonitrides were observed on the lath boundary of the modified steel. The stability of these precipitates in the modified steel is likely to suppress the degradation of the long term creep strength at 650°C.

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.

The change in hydrogen desorption characteristic due to creep was investigated to examine the possibility of hydrogen as tracer for detecting and evaluating the creep damage accumulated in high Cr ferritic boiler steel, Gr.91. Hydrogen charging into the creep specimen was conducted by means of cathodic electrolysis. Next, the thermal desorption analyses (TDA) were carried out at temperature range from room temperature to 270°C for measuring the hydrogen evolution curve. The experimental results revealed that the amount of hydrogen desorbed during analysis, C H , increased with increasing creep life fraction, although the trend of increase in C H was strongly dependent on the stress level. Moreover, there was an almost linear correlation between the logarithm of C H measured on the creep ruptured specimen and the Larson-Miller parameter (LMP), which was approximated by “log C H = 0.39 LMP – 13.4”. This can be a criterion for creep rupture and means that as far as the C H does not reach the line, the rupture never occurs.

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.

High nitrogen steel was manufactured by solid state nitriding and Laminate- rolling at laboratory to study the nitride morphology and creep properties through the TEM, EPMA and creep strain test. Nitriding made the nitride dispersing steels possible. Solid state nitriding of thin plates and those laminate rolling enabled the high nitrogen containing thick plate steel. Precipitated coarse nitrides during the nitriding resolved by normalizing and re-precipitated by tempering finely. Needle type VN was detected in V containing high nitrogen steels. Its coherency seems to affect the creep strength significantly. V precipitated steels indicated the higher creep strength than the steels without VN precipitation. Thermodynamically stable precipitates like VN increases the creep rupture strength. Ti and Zr containing high nitrogen steels also will be evaluated and discussed by the presentation.

Constricted steam oxidation resistance and finite microstructural stability limits the use of 9 - 12 wt.-% chromium ferritic-martensitic steels to steam temperatures of about 620 °C. Newly developed 12 wt.-% Cr steels are prone to Z-phase precipitation, which occurs at the expense of the strengthening precipitates, and therefore suffer an accelerated decline in strength during longterm operation. While the concept of ferritic-martensitic chromium steels thus seems to hit technological limitations, further improvement in steam power plant efficiency necessitates a further increase of steam pressure and temperature. Furthermore increasing integration of intermitting renewable energy technologies in electrical power generation poses a great challenge for supply security, which has to be ensured on the basis of conventional power plant processes. Besides improved efficiency for resource preservation, load flexibility, thermal cycling capability and downtime corrosion resistance will play key roles in the design of tailored materials for future energy technology. Under these preconditions a paradigm shift in alloy development towards improvement of cyclic steam oxidation and downtime corrosion resistance in combination with adequate creep and thermomechanical fatigue strength seems to be mandatory. The steam oxidation, mechanical and thermomechanical properties of fully ferritic 18 - 24 wt.-% chromium model alloys, strengthened by the precipitation of intermetallic (Fe,Cr,Si)2(Nb,W) Laves phase particles, indicate the potential of this type of alloys as candidate materials for application in highly efficient and highly flexible future supercritical steam power plants.

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 Cr and W effect on the creep strength of ferritic steels were studied using the new strengthening hypothesis, precipitation strengthening mechanism, by examining the residual aligned precipitates consisting of W and Cr. In 2 mass% W-containing steel, the increase in Cr content up to 10 mass% resulted in the creep life extension. However, the Cr content higher than 11 mass% decreased the creep life. In 9 mass% Cr-containing steel, the increase in W content decreased the creep deformation rate with creep time. However, it also shortened the time to reach the minimum creep rate. Therefore, optimum Cr and W contents possibly resulted in the optimum alloy design. To understand the effect of W and Cr contents on creep strength, the precipitation strengthening hypothesis by the precipitates at the block boundary must be introduced. The residual aligned precipitation line is supposedly an effective obstacle for the dislocation motion at the interparticle space of the aligned precipitates. The new hypothesis will be activated after block boundary migration. It occurs during the acceleration creep period. On the basis of the hypothesis, creep strength was expressed as the summation of threshold creep stress and effective internal creep stress. According to the experimental data of microstructure recovery, the effective internal stress decreased with creep deformation and consequently vanished. In such cases, creep strength is decided only by the threshold stress of creep. Integrating all, we concluded that the creep deformation mechanism of ferritic creep-resistant steel possibly transits from the viscous dislocation gliding mode to the microstructure recovery driven type mode during the acceleration creep.

Prediction of long-term creep strength is an important issue for industrial plants operated at elevated temperatures, although the creep strength of high Cr ferritic steels depends on their microstructural evolution during creep. The state of microstructure in metallic materials can be expressed as numerical values based on a concept of system free energy. In this study, in order to evaluate long term creep strength of 9Cr ferritic steel containing B, change in the system free energy during creep of the steel is evaluated as the sum of chemical free energy, strain energy and surface energy, which are obtained by a series of experiments, i.e., chemical analysis using extracted residues, X-ray diffraction, and scanning transmission electron microscopy. The system free energy decreases with creep time. Change in the energy is expressed quantitatively as a numerical formula using the rate constants which depend on applied stress. On the basis of these facts, long term creep strength of the steel can be evaluated at both 948K(675°C) and 973K(700 °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.

Microstructural analyses by FE-SEM and TEM were performed on a ferritic heat-resisting steel that contained 12mass% chromium and 2mass% tungsten to characterize its multi-scale structure, consisting of prior austenite grains, packets, blocks, subgrains and precipitates. The size distributions of the block, subgrains and precipitates were quantitatively evaluated before and after a creep-fatigue test to relate them to their creep-fatigue property. Our results showed that the occupancy of precipitates on prior austenite grain boundaries increased markedly and subgrains became coarse during the creep-fatigue test, while block size did not change. It is suggested that the growth of grain boundary precipitates and coarse subgrains plays an important role in the intergranular fracture mechanism caused by creep-fatigue.

New Monte Carlo models have recently been developed to predict microstructural evolution in steels and aluminum alloys during heat treatment and high-temperature service. These models can control precipitate type and size distribution, distinguishing between pure lattice and grain boundaries. Consequently, they can forecast the precipitate size distribution within grains and on grain boundaries as a function of time. This paper describes the model validation for ferritic Fe-9Cr P92 steels. The model provides new information over a range of time intervals adding up to the total plant lifetime in an ultra-supercritical plant. This information can be incorporated into continuum damage mechanics models for predicting creep rate and stress rupture life. The paper discusses how this technique is used as a materials development tool to forecast necessary compositional modifications for improving creep properties in ferritic steels.