The effect of stress relaxation loading cycles on the creep behaviour of 2.25Cr-1Mo pressure vessel steel

This paper investigates the effects of repeated stress relaxation loadings and post stress relaxation creep to assess the stress relaxation-creep interaction and microstructural evolution of 2.25Cr-1Mo steel. Prior to creep testing, the microstructure of the material subjected to stress relaxation exhibited a structure which was non-conservative in predicting the remaining creep life of the material. The results obtained in the test program showed that the damage due to the effects of stress relaxation was crucial and had a significant effect on the creep life of this material. The study has also shown that the extent of metallurgical degradation, due to stress relaxation, may not be evident through microstructural assessment. Consequently, established life assessment procedures may not represent conservative estimates of remaining life because the microstructural indicators of damage, due to stress relaxation, are not evident.The effects of stress relaxation on the creep properties of ferritic pressure vessel steel are life limiting for critical high temperature power generation plant. In this study a comprehensive test program has been undertaken to assess the interaction of stress relaxation with creep and microstructural evolution in 2.25Cr-1Mo steel.     

Loss due to transverse thermoelastic currents in microscale resonators

In order to understand the loss mechanisms associated with microscale resonators we have studied the importance of thermoelastic (TE) damping due to transverse thermal currents. In the work presented here, we study this damping mechanism as it applies to structures involving torsional vibration, or in general possessing a non-trivial mode shape such as those associated with microelectromechanical systems (MEMS) devices. A model of thermoelastic dissipation is presented that is based on the observation that the resonant modes of elastic structures almost always contain some flexural component. We determine a flexural energy participation factor and apply this to Zener’s model for damping of a simple reed in pure bending. Predictions agree well with internal friction measurements for a macroscale single-crystal silicon double paddle oscillator (300 μm thick) at temperatures from 130 to 300 K. The approach has also been successfully applied to predict microscale oscillator (1.5 μm thick) internal friction measurements at room temperature. Our results indicate that the internal friction arising from this mechanism is strong and can be quite significant for silicon-based MEMS (Q<10⁴) and persists down to 50 nm scale structures even for nominally torsional or even slightly asymmetric compressional devices which one might conclude have no loss. The importance of the thermoelastic mechanism is examined as a function of material properties. From this perspective, diamond possesses desirable thermal expansivity and diffusivity that is examined with our model.     

X-ray investigation of solid solution partitioning in 2.25Cr-1Mo steel after extended elevated temperature service in power station

Abstract

Carbide phase transformation and alloying element partitioning in service exposed (about 45 000-160 000 h) 2.25Cr-1Mo boiler tubes from a thermal power station were studied using X-ray diffraction and X-ray fluorescence techniques on electrochemically extracted carbide precipitates. The objective of the present investigation was to develop a faster method of health assessment of this steel at elevated temperatures on a routine basis. Various carbide phases were detected with different lengths of service. The molybdenum content in extracted carbide specimens showed a systematic decrease with aging. Replenishment of molybdenum in the matrix may be a useful indicator in life assessment of this steel. When the dissolved molybdenum content in the matrix was less than ~0.8 wt-%, the hardness of service exposed specimens was found to decrease. However, carbide precipitates extracted from new specimens subjected to accelerated heat treatment at 600 and 700 ° C showed an increase in molybdenum content with increase in temperature. The X-ray data were supplemented by microscopy, EDAX and hardness measurements.     

Beschreibung des Zeitdehnverhaltens warmfester Stähle Teil 2: Kriechgleichungen für die Stahlsorten 10 CrMo 9 10 und X 20(22) CrMoV 12 1

Creep Behaviour of Heat Resistant Steels Part II: Creep Equations for Steels 2.25 Cr-1 Mo and 12 Cr-1 Mo-0.3 V Creep data scatter bands of steels 2.25 Cr-1 Mo and 12 Cr-1 Mo-0.3 V were evaluated with the aid of model functions based on time temperature parameters. From the times to reach given strain values, mean isostrain curves in the stress time diagramme were calculated and therefrom, mean creep curves were derived. On this basis, creep equations were established, which include primary-, secondary- and tertiary-creep and are valid in the main range of application of each steel. Further, mean stress strain curves from hot tensile tests were used to describe the initial plastic strain in the creep equations. The values calculated with the established creep equations agreed relatively well with the correspondent original scatter band values from the creep tests.     

充氢Cr-Mo钢变形过程的声发射特征

对电化学充氢后的2.25Cr-1Mo钢进行拉伸实验,并在实时拉伸过程中采集声发射信号。结果表明:充氢后2.25Cr-1Mo钢抗拉强度为536.30MPa,下降约57MPa;断面收缩率为43.62%,下降约7%。拉伸断口上出现由氢脆引起的"白点"特征与准解理断裂形貌。充氢后试样拉伸过程弹性阶段的声发射信号活动增强,而屈服阶段的声发射信号活动减弱,变形过程的声发射信号累积绝对能量值要比未充氢试样低约1个数量级。充氢试样拉伸产生的声发射信号比未充氢试样的信号幅值降低约0.33mV,频宽降低0.06MHz。通过对声发射信号的分析发现,充氢试样变形的微观机制为氢促进位错发射与运动,而交叉滑移受到抑制。     

Supersmall punch test to estimate fracture toughness JIc and its application to radiation embrittlement of 2.25Cr-1Mo steel

A supersmall punch test has been used to extract fracture strain information on irradiated 2.25Cr-1Mo steel from transmission electron microscopy disc specimens as small as 3 mm in diameter and 0.25 mm in thickness. The test is based on driving a steel ball punch through a clamped specimen. The size effect of biaxial equivalent fracture strain of various kinds of materials and irradiated steel has been demonstrated. The results of fracture strain obtained from specimens 3 mm in diameter has been related almost linearly to the fracture toughness J_Ic for elastic and plastic behaviour. The relationship between fracture strain and fracture toughness J_Ic has been verified for the irradiated nuclear pressure vessel steel 2.25Cr-1Mo so that large amounts of irradiation space in nuclear reactor could be saved.     

Creep rupture ductility related to creep fracture mechanisms in 2.25Cr-1Mo steel

Abstract

The long term creep rupture properties of 2.25Cr-1Mo ferritic steel have been studied at eight temperatures between 723 and 923 K and the rupture lives varied in the range of 100 to 113 000 h. Three fracture mechanisms have been identified on the basis of microstructural investigations, namely transcrystalline, cavitation, and recrystallisation. A transcrystalline fracture mode was observed at most of the test temperatures and often up to medium term rupture lives. A transition from transcrystalline fracture mode to cavitation mode was observed at very long rupture lives, whereas a transition to recrystallisation fracture mode was observed at the highest test temperatures. The variation of reduction in cross-sectional area of the sample at fracture, with the Larson-Miller parameter, has been found to reflect the transitions in fracture behaviour. Changes in the density of the material due to creep deformation were measured and found to exhibit a close correlation with reduction in cross-sectional area.     

Microstructure-fracture toughness correlation in weld joints of Cr-Mo steel

The strength-toughness-microstructure relationship in relation to the micromechanics of a fracture process has been investigated in the weld joints of two alloys: 0.5 Mo and 2.25 Cr-1 Mo steels. These alloys are extensively used to fabricate super-heater tubes, boilers, piping, gas lines, etc., by welding. The applications require high temperature and pressure to be maintained during service. The crack initiation toughness and tearing resistance were evaluated using crack tip opening displacement/J-integral parameters at different temperatures. Quantitative analysis of micro-structure and fracture surfaces was used to study the micromechanics of fracture process in the heat-affected zone (HAZ) of the alloys. Molybdenum steel exhibited a higher percentage of ferrite and lower martensite content, while the other steel showed aligned carbide as the major constituent. The higher hardness and strength values in the HAZ and welding zone (WZ) of Cr-Mo steel, compared to molybdenum steel, may be attributed to the higher amount of martensite phase in the alloy. The higher initiation toughness at 200° C in both the alloys was reflected in the larger dimple size, compared to the size observed at room temperature. A tendency for void sheet formation was noticed in both alloys. Acicular ferrite and martensite appeared to be the most influential constituents affecting tearing resistance and initiation toughness.     

Microstructural evolution of simulated heat-affected zone in modified 2.25Cr-1Mo steel during high temperature exposure

The effect of heat input with 20, 50 and 80 kJ/cm on the microstructural evolution of the simulated heat affected zone (HAZ) has been studied in a modified 2.25Cr-1Mo steel. The microstructures of simulated coarse-grained HAZ has been examined by optical metallography and transmission electron microscopy. It was found that large amounts of martensite with small quantities of bainite exist in the specimen with 20 kJ/cm. However, significant amount of bainite with a few amounts of allotriomorphic ferrite can be detected in the specimen with 50 kJ/cm heat input. In the case of heat input with 80 kJ/cm, the simulated HAZ microstructures were composed chiefly of bainite with a few amounts of martensite and allotriomorphic ferrite. In order to study the sequences of carbide transformation, the HAZ specimens were tempered at 700°C for different intervals (1, 2, 5, 10, 20, 50, 100, 200 and 500 h). The sequence of carbide transformation in the HAZ zone of this modified 2.25Cr-1Mo steel has been proposed.     

On the Corrosion Behavior of a ferritic 18 Cr-2 Mo-steel

The investigations carried out with 18 Cr-2 Mo steel were aimed at its behavior under pitting corrosion, crevice corrosion and stress corrosion cracking conditions. This was done in autoclave laboratory experiments and under experimental heat exchanger conditions in Rhine river water with a chloride content of max. 400 ppm. The test temperatures were 80, 100 and 130°C. Model heat exchangers were fabricated and operated to investigate the influence of filler materials and weld joints between the ferritic 18 Cr-2 Mo steel and a standard austenitic steel. The possibilities of fabricating tube sheets by applying a weld overlay and using explosive bonding were explored. 18 Cr-2 Mo steel has been shown to be suited for applications in cooling water systems which have a chloride content of 400 ppm. No stress corrosion cracking occurs under such conditions. Tubes with a wall thichness up to 3 mm have sufficient toughness. Tube sheets can be made of boiler plate protected by an explosive cladding or a weld overlay of 18 Cr-2 Mo. A combination of Type 321 or 304 L and 18 Cr-2 Mo is possible. Provided 18 Cr-2 Mo is sufficiently resistant to the product to be cooled, it is an alternative to austenitic CrNi-(Mo) steels (e.g. AISI 304) when stress corrosion cracking is likely to occur.     

Characterization of Hydrogen Embrittlement in 2.25cr-1mo-0.25v Steel by Eddy Current Method

Eddy current method has been recently developed to characterize mechanical properties of materials and assess internal hydrogen content of high strength low alloy steels. The application of eddy current testing in evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V ferritic steel has been investigated using tensile test and electrochemical hydrogen charging test. The relationship between the embrittlement index and eddy current signal is well established. It found that there is a good linear relation between the hydrogen-induced plasticity loss and the eddy current signal. This shows eddy current testing is an effective method for evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V steel.     

Thermal and electrical conductivities of an improved 9 Cr-1 Mo steel from 360 to 1000 K

The electrical and thermal conductivities and Seebeck coefficients of three 9 Cr-1 Mo samples were measured over the temperature range 360–1000 K. All of the samples were in the normalized and tempered condition and two of the samples were from different heats of a new, modified alloy. The thermal conductivity of the third sample, which was from a commercial heat, was found to agree well with the ASME code values for this steel. The two heats of modified 9 Cr-1 Mo were found to have significantly higher thermal conductivities and this difference appears to be due to the lower Si content of the modified alloy. The results were compared with the predictions of standard transport theory and data on bcc Fe. These comparisons show that phonon energy transport is important and quite dependent upon the Si content.     

Modifying Cr-Al Coatings by Reactive Element Additions: Eiperimental and Theoretical Considerations

Cr-Al diffusion coatings have been modified by Hf addition using a one step halide activated pack cementation process. The substrate was a low alloy steel 2.25Cr-1 Mo. Cyclic Oxidation properties of the coatings were evaluated, and a Hf modified coating was the most resistant coating. Codeposition pack thermodynamics were computed and analyzed using available thermodynamic tools in order to compare experimentally obtained results with theoretical predictions.     

On the impact of post weld heat treatment on the microstructure and mechanical properties of creep resistant 2.25Cr–1Mo–0.25V weld metal

Creep resistant low-alloyed 2.25Cr-1Mo-0.25V steel is typically applied in hydrogen bearing heavy wall pressure vessels in the chemical and petrochemical industry. For this purpose, the steel is often joined via submerged-arc welding. In order to increase the reactors efficiency via higher operating temperatures and pressures, the industry demands for improved strength and toughness of the steel plates and weldments at elevated temperatures. This study investigates the influence of the post weld heat treatment (PWHT) on the microstructure and mechanical properties of 2.25Cr-1Mo-0.25V multi-layer weld metal aiming to describe the underlying microstructure-property relationships. Apart from tensile, Charpy impact and stress rupture testing, micro-hardness mappings were performed and changes in the dislocation structure as well as alterations of the MX carbonitrides were analysed by means of high resolution methods. A longer PWHT-time was found to decrease the stress rupture time of the weld metal and increase the impact energy at the same time. In addition, a longer duration of PWHT causes a reduction of strength and an increase of the weld metals ductility. Though the overall hardness of the weld metal is decreased with longer duration of PWHT, PWHT-times of more than 12 h lead to an enhanced temper resistance of the heat-affected zones (HAZs) in-between the weld beads of the multi-layer weld metal. This is linked to several influencing factors such as reaustenitization and stress relief in the course of multi-layer welding, a higher fraction of larger carbides and a smaller grain size in the HAZs within the multi-layer weld metal.

     

Acoustic emission as a technique to study breakaway oxidation and spalling behaviour of 2.25Cr-1 Mo steel

The onset of breakaway oxidation and cracking of the oxide scale formed on 2.25Cr-1Mo steel in air at 1173 K have been studied by the acoustic emission (AE) technique. AE parameters, i.e. events, ring-down counts, rise time, event duration and root mean square voltage show negligible increase during isothermal heating at this temperature, until a point where a sudden increase in AE activity is found. This point corresponds to the onset of breakaway oxidation. An enormous increase in AE activity after the start of cooling from the oxidation temperature has been attributed to separation of the oxide scale from the matrix. The peak amplitude distribution is measured and a derived b parameter is calculated. This has helped in distinguishing the phenomena of isothermal oxidation at 1173 K and internal cracking of oxide scale during cooling from 1173 K.     

Rare-earth modified chromium-aluminide coatings applied by pack cementation method on low-alloy steels

In metallurgical practice the presence of a minor amount of certain elements can induce significant changes in a material's physical, mechanical and chemical properties. In this paper we present the nature, morphology and properties of protective coatings containing Cr, Al and minor amounts of a rare earth element, applied on 2.25 Cr-1 Mo low-alloy steel. The coating technology, pack cementation, was used to achieve the simultaneous deposition of the coating elements in a one step process. Cyclic oxidation, conducted in air at 800 °C, induced thermal stresses on the alloy samples, promoting the tendency of the oxide scales formed to spall. The rare earth modified coatings, under these conditions showed the beneficial effects of the rare earth addition. These coatings, upon exposure, developed an adherent oxide scale which had better properties, and retarded the alloy's degradation. This lead to a significant reduction in the overall oxidation rate.     

Application of Electron Energy Loss Spectroscopy and Energy Filtering Transmission Electron Microscopy for Microchemical Studies in 2.25Cr-1Mo Steel

Electron enregy loss spectroscopy (EELS) and energy filtering transmission electron microscopy (EFTEM) investigation on 2.25Cr-1Mo steel was carried out to understand the nature of evolution of secondary carbides. The filtered images obtained from two different ageing treatments indicate that the steel evolves to a more stable carbide namely M23C6 in comparison to M2C. Microchemical information was generated from EELS spectra. Suitable choice for estimating the microchemical state was discussed. To evaluate the behaviour of ageing an elemental ratio of Fe to Cr is employed.     

Microstructural Evolution of 2.25Cr-1.6W-V-Nb Heat Resistant Steel during Creep

2.25Cr-i.6W-V-Nb developed in Japan, is a low alloy heat resistant steel with good comprehensive properties. Influence of long term creep at elevated temperature on the structure of 2.25Cr-I.6W-V-Nb steel was studied in this paper, and the micromechanism of creep strength degradation was elucidated, too. Both TEM observation and thermodynamic calculation reveal that during creep the transformation occurs from M7C3 and M23C6 to M6C, which can be cavity nucleation sites. Besides, creep at 600℃ also leads to the decrease of dislocation density, the coarsening and coalescence of M23C6, the nucleation of cavities and development of cracks. The strength decrease of 2.25Cr-1.6W-V-Nb steel after long term creep is related to the decrease of dislocation hardening,precipitation hardening,solution hardening,the nucleation of cavities and development of cracks.     

CYCLIC STRESS-STRAIN RESPONSE OF 2 1/4Cr-1 Mo STEEL AT ELEVATED TEMPERATURES

The temperature dependence of the cyclic stress-strain response of 2 1/4 Cr-1 Mo steel has been studied in the temperature range 20–550°C. Fatigue hardening-softening curves and cyclic stress-strain curves were obtained. A more detailed analysis was based on the statistical theory of the hysteresis loop and the concept of internal and effective stresses. A stress-dip procedure allowed the separation of the contribution of internal and effective stresses both in dependence on temperature and during fatigue life in high temperature cyclic straining. The high effective stress contributes to the fatigue resistance of the steel. In high temperature fatigue both the internal and the effective stress decrease with cyclic straining.     

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.