CrMoV Steel Research Articles

Development of a new methodology for estimating the CTOD of structural steels using the small punch test

The small punch test (SPT) is very convenient for estimating tensile mechanical properties, being the estimation of fracture toughness still a controversial subject of debate. One of the new strategies developed is the use of notched specimens. In this paper, two different grades of CrMoV steels were employed to analyse the evolution of the notch mouth opening displacement of the small punch sample (δSPT). Complete and interrupted tests were performed on specimens with longitudinal non-through notches with a notch length to thickness ratio of 0.3. A numerical model was also developed for corroborating the experimental results. A material-independent relationship between δSPT and the punch displacement (d) was found: δSPT=0.217d. Since crack length measurement is not possible on SPT samples, the value of δSPT at crack growth onset (δSPT_ini) was used for comparison with the CTOD values for crack initiation in the standard tests (δini). Crack growth onset in the SPT specimens was verified by observation after splitting them in two halves, as well as comparing the numerical curves (without damage model) and the experimental ones. Larger values have been obtained by means of the SPT, due to the lower constraint of the test. However, the developed methodology seems to be suitable when dealing with ductile steels, although other different materials are needed to be tested.

Surface Layer Modification of 12Cr1MoV and 30CrMnSiNi2 Steels by Zr+ Ion Beam to Improve the Fatigue Durability

Abstract With the use of optical/scanning electron microscopy and microindentation some peculiarities of structure modification over cross-section as well as mechanical properties of 12Cr1MoV and 30CrMnSiNi2 steel specimens subjected to ion-arc treatment by Zr ion beam were investigated. It is shown that changing of the microstructure takes place over entire cross-section that is not the case of material softening due to annealing. Within the revealed results on structure modification the data on changing mechanical properties under cyclic and static tension of the irradiated specimens are interpreted. Of particular interest are results on strain measurement by digital image correlation technique.

Effect of Pipe Bend Shape Imperfections on Creep-fatigue Crack Growth

This study is concerned with assessing the structural integrity of power plant pipes which operate under creep and fatigue conditions. In the manufacturing process of pipe bends it is difficult to avoid thickening on the inner radius of the pipe bend and thinning on the outer radius of the pipe bend. The cross section of the bend also becomes non circular due to bending process; this tends to cause ovality in pipe bends. The acceptability of pipe bends is based on the induced level of shape imperfections considered. The influence of internal pressure, operation time and shape imperfections of pipe bend on stress-strain redistributions and creep-fatigue crack growth rate was investigated using FE-analysis and experiments at elevated temperature. Full-size stress-strain state analysis of pipe bend was performed for different stage of lifetime under considering loading conditions. As a result it was found that the effect of ovality and thinning cross sections interaction is significant and is a function of creep time. The tensile tests were performed for determination the main mechanical properties of pipe bend's material after loading history. The smooth and compact specimens were cut out from four tension and compression critical zones of the same pipe bend with given ovality and thinning cross sections as well as operating time. The creep-fatigue crack growth rate tests were performed on special designed program test-cycle. Comparison between new and old materials shows tensile properties of 12Cr1MoV steel have degraded by long-term high-temperature service; while comparison between the creep-fatigue crack growth rate characteristics indicates that they are a function of the critical zone positions in the considered pipe bend. A new parameter for characterization of the crack growth resistance for power plant materials and structures under elevated temperature is used in the form of creep stress intensity factor.

Development of a methodology to study the hydrogen embrittlement of steels by means of the small punch test

Two different methodologies for analysing the deterioration of mechanical properties due to hydrogen embrittlement by means of the small punch test (SPT) have been studied. In the first, specimens were electrochemically pre-charged before testing, while in the second, they were charged at the same time as testing. A novel, simple, easy-to-manage SPT device was developed for the latter purpose. Two different CrMoV steel grades, a base and a weld metal, tempered at different temperatures, were tested. Tensile tests of hydrogen pre-charged specimens as well as hydrogen content measurements were also performed. Greater hydrogen absorption was observed in the higher strength CrMoV weld metal due to its microstructure composed of low tempered bainite. This steel was fully embrittled in both tensile and small punch tests in the presence of hydrogen, and no significant difference between the two SPT methodologies were found in this case. The CrMoV base metal was only embrittled, however, when hydrogen charging was performed at the same time as testing, showing the greater suitability of this small punch test methodology. The fracture pattern of SPT specimens changed completely from ductile to brittle when testing in hydrogen. Typical SPT parameters also exhibited a marked decrease in ductility and fracture toughness, the CrMoV weld metal being more susceptible to hydrogen embrittlement. Finally, the feasibility of the small punch test for ranking the hydrogen embrittlement susceptibility in steels was demonstrated, and the most suitable SPT parameters for analysing the reduction in mechanical properties were defined.

Soft zone formation by carbon migration and its effect on the high-cycle fatigue in 9% Cr–CrMoV dissimilar welded joint

This paper studies carbon migration in the welds between 9% Cr steel and CrMoV steel during high-cycle fatigue (HCF) test at various temperatures. The dissimilar joints were fabricated using a narrow gap submerged arc welding (NG-SAW) technique. The HCF tests were carried at room temperature (RT), 400°C and 470°C. It was observed that the specimens tested at RT and 400°C fractured at the CrMoV side while the specimens tested at 470°C fractured at the 9% Cr fusion line. The microstructure of the 9% Cr fusion zone showed that the fracture occurred in a carbon-depleted zone. In addition, the micro-hardness test revealed that the hardness in the carbon-depleted zone was lower than at other locations of the welds while the hardness in the carbon-enriched zone was the highest in the whole welded joint. It is concluded that the carbon migration at elevated temperatures is considered as a very important factor in high stress HCF performance.

Fatigue Life of CrMoV Steel after Long Term Service

The paper presents the results of research on the microstructure and mechanical properties of 12HMF steel after long-term service. The investigated material was taken from a pipe after service at the temperature of 490°C, steam pressure of 8 MPa and service time of 419 988 hours. Performed research has shown that the 12HMF steel after service was characterized by ferritic-bainitic microstructure without any visible advanced processes of its degradation, a typical microstructure for this grade of steel. The tests of mechanical properties have proved that the examined steel after service was characterized by very low impact strength KV, and yield strength lower than the required minimum. Performed fatigue tests of constant amplitude, as well as the programmed ones, have shown that the investigated steel after service is characterized by cyclic softening without a clear period of stabilization of the parameters of hysteresis loop. Moreover, it has been proved that slight changes in the hysteresis loop parameters, as a function of the number of stress cycles, have a significant influence on the obtained material data used when calculating the life.

Fatigue life improvement of 12Cr1МoV steel by irradiation with Zr+ ion beam

Cyclic tension and bend tests were performed on heat-resistant 12Cr1MoV steel specimens in as-supplied condition as well as after Zr+ ion beam surface irradiation. Distinct differences in strain induced relief, as well in cracking pattern of modified surface layer were observed by optical microscopy and interference profilometry. Changes in subsurface layer are characterized by means of nano- and microindentation and fractography of fracture surfaces (with the help of scanning electron microscopy). It is shown that the main influence on mechanical properties is mostly induced by thermal treatment during irradiation rather than formation of a 2μm thick layer doped with Zr. The differences in deformation behavior may be explained by physical mesomechanics concepts.

Temperature and stress–regime dependent primary–secondary–tertiary creep constitutive model

High temperature deformation analysis of components such as steam turbine rotors requires a knowledge of the material deformation response for a wide range of stresses and temperatures. Deformation analysis of steam turbine rotors deals with stresses ranging above the material proof strength (shortly after plant start-up) down to those responsible for very long rupture durations (for the steady running phase of operation) at various temperatures. This study describes the construction of a temperature and stress–regime dependent (primary–secondary–tertiary) creep constitutive model to provide a more reliable representation for the material deformation response over wide ranges of stresses and temperatures. The adopted equation set is a refinement of the ‘Characteristic Strain’ model and depends in its formulation mainly upon creep rupture data. Successful application of the model for a 1CrMoV steel for a wide range of stresses over the temperature range of 450–675°C is demonstrated.

Effect of start/stop position distribution on residual stresses in the multi-pass welded 12Cr1MoV/P91 dissimilar pipe

Welding residual stresses between 12Cr1MoV and P91 steel pipes were analyzed by experiment and finite element method (FEM). Both measured hoop and axial stresses are in general agree with the simulation results. Meanwhile, to examine the effect of start/stop welding position on residual stresses, a simulation model whose welding passes all started at the same location during multi-pass welding was compared with a model whose start/stop positions were uniformly distributed along circumferential direction of the dissimilar pipe. The results show that higher residual stresses occurred near the superimposed start/stop position than the steady region when calculated by the model whose welding passes all started at the same location. Whereas, start/stop welding positions which are distributed uniformly along the circumferential direction of the pipe can significantly relieve this effect, correspondingly leading to a more uniform residual stress distribution around the multi-pass welded dissimilar pipe.

Microstructure characteristics and temperature-dependent high cycle fatigue behavior of advanced 9% Cr/CrMoV dissimilarly welded joint

Advanced 9% Cr and CrMoV steels chosen as candidate materials are first welded by narrow-gap submerged arc welding (NG-SAW) to fabricate the heavy section rotor. The present work focuses on studying the high-cycle fatigue (HCF) behavior of advanced 9% Cr/CrMoV dissimilarly welded joint at different temperatures. Conditional fatigue strength of this dissimilarly welded joint was obtained by HCF tests at room temperature (RT), 400°C and 470°C. It was observed that the failure occurred at the side of CrMoV base metal (BM), weld metal (WM) and heat affected zone (HAZ) of CrMoV side over 5×107 cycles for the specimens tested at RT, 400°C and 470°C. The detailed microstructures of BMs, WMs and HAZs as well as fracture appearance were observed by optical microscopy (OM) and scanning electron microscopy (SEM). Precipitation and aggregation of carbides along the grain boundaries were clearly detected with the increase of temperature, which brought a negative effect on the fatigue properties. It is interesting to note that the inclusion size leading to crack initiation became smaller for the HCF test at higher temperature. Therefore, reduction in the inclusion size in a welded joint helps to improve the HCF performance at high temperature.

German and Russian Irradiated Reactor Pressure Vessel Steels from PAS Point of View

This paper presents a comparison of commercially used German and Russian reactor pressure vessel steels from the positron annihilation spectroscopy point of view, having in mind knowledge obtained also from other techniques from the last decades. The second generation of Russian reactor pressure vessel steels seems to be fully comparable with German steels and their quality allows prolongation of NPP operating lifetime over projected 40 years. The embrittlement of CrMoV steels is very low due to the dynamic recovery of radiation-induced defects at reactor operating temperatures. Positron annihilation spectroscopy techniques can be e ectively applied for evaluation of microstructural changes caused by extreme external loads by proton implantation, with aim to simulate irradiation and for the evaluation of the e ectiveness of post-irradiation thermal treatments. We used our actual and previous results, collected during last 20 years from measurements of di erent reactor pressure vessel steels in as received , irradiated and post-irradiation annealed state and compare them with the aim to contribute to general knowledge based on experimental positron annihilation spectroscopy data.

Role of butter layer in low-cycle fatigue behavior of modified 9Cr and CrMoV dissimilar rotor welded joint

The present work aims at studying the role of butter layer (BL) in low-cycle fatigue (LCF) behavior of modified 9Cr steel and CrMoV steel dissimilar welded joint. The significant difference of the chemical composition of base metals (BMs) makes it a challenge to achieve sound welded joint. Therefore, buttering was considered to obtain a transition layer between the dissimilar steels. The LCF tests of two kinds of specimens without and with butter layer were performed applying strain-controlled cyclic load with different axial strain amplitudes. The test results indicated that the number of cycles at higher strain amplitudes of welded joint without butter layer was greatly higher than that of the joint with butter layer, while the fatigue lifetime to crack initiation (2Nf) became closer to each other at low and middle strain amplitudes. The failure was in the tempered heat affected zone (HAZ) at the CrMoV side for specimens without BL, while the fracture occurred at the tempered HAZ in the BL for specimens with BL. The microstructure details of BM, BL, HAZ and weld metals (WMs) were revealed by optical microscopy (OM). It was found that the tempered martensite was major microstructure for welded joint and much more carbides were observed in tempered HAZ than other parts due to the repeated tempering. Microhardness test indicated a softest zone existing tempered HAZ of BL and also there was a softer zone in tempered HAZ at the CrMoV side due to repeated tempering during welding and post weld heat treatment (PWHT). And scanning electron microscopy (SEM) was applied to observe the fractography. It was indicated that the fatigue crack initiation occurred from the specimen surface and all specimens were ductile–brittle mixed fractures. It is deemed that the softening behavior in BL caused by twice tempering correspondingly decreased the LCF lifetime at higher strain amplitudes. So suitable welding parameters and heat treatment processes became a key measure to ensure LCF property without losing other properties for welded joint with BL.

Long-term Overheating Cracking Analysis of 12Cr1MoV Boiler Superheater Tube

After running 39 840 h, a boiler superheater tube made by 12Cr1MoV steel in a plant cracked. The tube is blocked and the thick-lip-shape fracture is covered with oxides. By utilizing optical microscope(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM), energy dispersive spectroscopy(EDS), transmission electron microscopy(TEM) and other methods to detect the tube, it is found that the compositions of blocking substances are Na3PO4, Fe2O3 and NaFeO2. In the unblocking position of the inner wall oxide is Fe3O4, however, in the blocking position, it is Fe2O3. In terms of the shape of the fracture morphology, it is inter granular, along with secondary cracks and pores in the grain boundary. The research results illustrate that the metallographic microstructures in samples of near crack and away from the crack are the same. With the lamellar pearlite of the initial state dissolves, the carbides gather and grow at the grain boundaries. The organization spherifies seriously, and the carbide is M23C6. According to the electric power industry standard, the spherical degree is identified as level 5. By utilizing spheroidization level and run-time to estimate the wall temperature, it can be know the tube run in over-temperature environment. To be specific, the hardness in all the samples is about 135 HV, which is relatively low. In all, the failure mode of superheater tube is high temperature creep cracking which is caused by long-term overheating resulted from the oxide layer and the blocking substances.

Hydrogen Crack Growth Resistance of Thermal Power Plant Material Collector

The influence of electrolytical hydrogenation on fracture toughness, corrosion crack-growth resistance and fracture micromechanisms of operated 12Cr1MoV steel of thermal power plant superheater collector has been studied. Compact tension specimens were cut from perforated surface of thermal power plant superheater collector dismounted after 178,500hours of operation.Corrosion crack-growth resistance under tension of previously hydrogenated compact specimens with fatigue cracks was studied. Due to the increased concentration of hydrogen in solution an additional buffer was being created that prevents hydrogen leakage from the specimen through the fracture surface during the experiment.The hydrogenation causes the significant decrease of critical stress intensity factor Kc, during the experiment in 0.1N NaOH solution as compared with critical stress intensity factor K of non-hydrogenation 12Cr1MoV steel obtained by the 5% secant line method and in comparison with critical stress intensity factor Kc, determined through the J-integral.The areas of ductile crack growth in hydrogenated and non-hydrogenated specimens were found to have similar material fracture micromechanisms with dimples creation of different shape and size. But on the ductile crack growth area in hydrogenated specimens material intergranular fracture mechanisms were found caused by the hydrogen embrittlement which are similar to areas without ridges with the products corrosion traces.

Structural Integrity Assessment of Thermal Power Plant Superheater Collector

Thermal power plants (TPPs) belong to the important objects, the failure of which can cause accidents with severe consequences. A large portion of Ukrainian TPP has exceeded its design life, which is determined by the main elements of steam power systems. Therefore, the main task is to assess correctly the superheaters collectors limit state using modern approaches and taking into account the service degradation of material.TPP superheater collectors operate in the steamy environment under the pressure of 15.5MPa at temperature 545°C. As a result of combined action of prolonged thermo-mechanical stresses, corrosive and hydrogen environment and slow deformation, fatigue cracks emerge in these components on the inner collector surface.The limit state of boiler steam superheater collector model was assessed using failure assessment diagram (FAD). The FADs were built for Kmat = KIc and Kmat = Kfc taking into account safety factors for plastic collapse and for brittle fracture mechanisms for the collector model with part circumferential internal surface crack for various ratio of crack length l to depth a.The critical sizes of the inner surface defect, that is perpendicular to the axis of the cylinder, considering fracture toughness of 12Cr1MoV steel for static and cyclic loading, were assessed.It was found out, that the increase of l/a ratio decreases the minimum crack depth that is critical for structural element.

Life Prediction of Two Rotor Steels under Creep-Fatigue Interaction at Elevated Temperature

The fatigue tests with 16 hours hold time have been conducted for two rotor steels, 1CrMoV and 2CrMoNiVW at 550°C to investigate their creep-fatigue interaction behaviors, as well as those without hold time for contrast. And two life prediction methods are used to correlate the present creep-fatigue life data. It is shown that a significant reduction in fatigue life is observed with hold time in tension, and it is also observed that 1CrMoV steel exhibits a higher creep-fatigue life than 2CrMoNiVW when strain hold time is 16 hours, and 2CrMoNiVW seems to be more sensitive to hold time influence. In addition, both methods could give satisfactory creep-fatigue life predictions, but with their own limitations.

Correlation between grade of pearlite spheroidization and laser induced spectra

Laser induced breakdown spectroscopy (LIBS) which is used traditionally as a spectrochemical analytical technique was employed to analyze the grade of pearlite spheroidization. Three 12Cr1MoV steel specimens with different grades of pearlite spheroidization were ablated to produce plasma by pulse laser at 266 nm. In order to determine the optimal temporal condition and plasma parameters for correlating the grade of pearlite spheroidization and laser induced spectra, a set of spectra at different delays were analyzed by the principal component analysis method. Then, the relationship between plasma temperature, intensity ratios of ionic to atomic lines and grade of pearlite spheroidization was studied. The analysis results show that the laser induced spectra of different grades of pearlite spheroidization can be readily identifiable by principal component analysis in the range of 271.941–289.672 nm with 1000 ns delay time. It is also found that a good agreement exists between the Fe ionic to atomic line ratios and the tensile strength, whereas there is no obvious difference in the plasma temperature. Therefore, LIBS may be applied not only as a spectrochemical analytical technique but also as a new way to estimate the grade of pearlite spheroidization.

Grain Boundary Fracture Behavior and Segregation of P under Low Tensile Stresses in Steel 2.25Cr1Mo and 12Cr1MoV

Experimental investigation about the grain boundary fracture behavior and segregation behavior of phosphorus under low tensile stresses in steel 2.25Cr1Mo and 12Cr1MoV was carried out in this paper. AES (Auger electron spectroscopy) experiments and dynamic analyses on the non-equilibrium grain-boundary segregation (NGS) of phosphorus and the SEM photos of grain boundary fracture in Auger specimens of both steels were obtained. The variation of phosphorus segregation level in grain boundary at different aging time was studied. Results show that the non-equilibrium segregation of phosphorus occurred at the grain boundaries in the two steels while subjected to a tensile stress of 30MPa and held at 500°C. The corresponding critical time was about 1 hour for steel 2.25Cr1Mo and 3.5hour for steel 12Cr1MoV respectively. SEM photos of grain boundary fracture in Auger specimens of the test steels show that the grain boundary fracture rate increased with increasing concentration of phosphorus, and that the fracture toughness of steel 12Cr1MoV is much lower than the one of steel 2.25Cr1Mo.

The importance of creep strain in linking together the Wilshire equations for minimum creep rates and times to various strains (including the rupture strain): an illustration using 1CrMoV rotor steel

This paper highlights the observation that the Wilshire equations for failure times and times to various strains, as reported in the original literature, may not be the most appropriate ones for all materials—including the one selected in this study. Further, such appropriateness can be determined by looking at the consistencies between the parameter estimates obtained using minimum creep rates in comparison to using failure times. It is shown, using 1CrMoV steel as an illustration, that the parameter consistency can be achieved by generalising the Monkman–Grant relation so that it contains a temperature correction. Indeed, the ability of the Wilshire equations to produce meaningful physical parameters, such as the activation energy, is shown to be highly dependent upon a valid specification for the Monkman–Grant relation. It is shown that variations in the measured values for some of the Wilshire parameters (w and k 3) with strain indicate that the causes of deformation are different at different strains and different stresses. Finally, the measured variations in the parameters of the Monkman–Grant relation with strain enable accurate interpolated and extrapolated creep curves to be calculated for any test condition.

Failure Analysis and Welding Repair for Reheater Tube Crack of 12Cr1MoV Steel

Through methods, such as microscope analysis, chemical analysis, hardness analysis, metallography examination etc, the causes resulting in reheater tube cracking of 300MW station boiler have been analyzed. The result indicates this reheater tube cracking is reheat crack. The higher sensitive temperature in operating, excessive welding residual stress are the primarily inducement of cracking. In welding repair, the measures , such as replacing the material ,lower welding heat input, preheat treatment and so on, were taken. The welding repair succeeded greatly.