Ferritic Steel Pipe Research Articles

Low-cycle fatigue behavior and microstructure evolution of ODS steel pipes at high temperatures

Oxide-dispersion-strengthened (ODS) steels are candidate materials for application in advanced nuclear reactors. In this study, the low-cycle fatigue performances of 13Cr-ODS ferritic steel pipes were investigated at 600, 700, and 800 °C. Cyclic softening was observed at high strain amplitudes with an increase in the number of fatigue cycles. However, cyclic hardening appeared first, and then cyclic softening occurred at a low strain amplitude with the increase in the number of fatigue cycles. By comparing the cyclic stress–strain curves and the monotonic stress–strain curves, it was found that cyclic softening occurred regardless of the strain amplitude. The Coffin–Manson and Basquin equations were used to predict the fatigue of the pipes. Microstructure analysis indicated that cyclic softening was induced by the dynamic recovery and recrystallization, which reduced the number of low-angle grain boundaries in the deformed grains by promoting dislocation annihilation and reorganization. A complex multi-layer core–shell structure with a large size (∼500 nm) was observed.

High sensitivity ultrasonic NDT technique for detecting creep damage at the early stage in power plant steels

Failures of welded main steam pipes in power station plants are related to cracks forming and propagating in the fine-grained zone, so called heat-affected zone (HAZ). Experience shows that Type IV cracks are indeed some of the most common causes of creep failure for low-alloy ferritic steel pipes and headers in power stations. Typically, crack damage will be characterised by high strain rates in the HAZ and local grain boundary separation, which leads to crack growth. In this paper, the high sensitivity technique for detecting creep damage (Type IV cracking), based on focused ultrasound was implemented. Signal processing algorithms were proposed that enable associating the properties of backscattered signals to the internal characteristics of the metal structure. As the result, the boundaries of the parent metal, HAZ and weld metal in an ultrasonic image can be indicated. The approach was validated on the samples exposed to different levels of creep damage and verified with metallographic analysis. It demonstrated that technique allowed detecting clouds of early stage creep damage.

Study of early P91 dual corrosion in steam and simulated combustion gases from a gas-fired boiler

Abstract P91 ferritic steel pipes face dual environments during boilers operation: steam-side and fire-side. This P91 steel assessment differs from the dual studies performed to simulate coal-fired boilers -oxyfuel/steam atmospheres- since the fuel source is replaced by natural gas. This research work includes designing a device to reproduce dual corrosion studies at 650 °C and testing times up to 200 h. One coupon face was exposed to combustion gases while the other to steam. As a main result, the duplex's inner layer allowed to state that combustion gases overcome the steam oxidation rate by a factor of 1.6. Besides, we supplied physical-chemistry information about the surface and bulk of oxide layers by atomic force microscopy, scanning electron microscopy, x-ray photoelectron spectroscopy, and x-ray diffraction analysis. Thus, our experiments aimed to obtain data about the P91 early degradation under the simultaneous 72.73N2/8.30CO2/3.37O2/15.60H2O %mol and steam influence. We last for a future work the isolated evaluation of both environments to determine their role on the corrosion rate obtained in the current study.

Analysis of Forming Parameters Involved in Plastic Deformation of 441 Ferritic Stainless Steel Tubes

A welded stainless steel tube is a component used in several industrial applications. Its manufacturing process needs to follow specific requirements based on reference standards. This calls for a predictive analysis able to face some critical issues affecting the forming process. In this paper, a model was adopted taking into account the tube geometrical parameters that was able to describe the deformation process and define the best industrial practices. In this paper, the effect of different process parameters and geometric constraints on ferritic stainless steel pipe deformation is studied by finite element method (FEM) simulations. The model sensitivity to the input parameters is reported in terms of stress and tube thinning. The feasibility of the simulated process is assessed through the comparison of Forming Limit Diagrams. The comparison between the calculated and experimental results proved this approach to be a useful tool in order to predict and properly design industrial deformation processes.

Prediction of ductile crack growth in a narrow gap Inconel dissimilar weld

Predicting the behavior of large ductile crack propagation in a pipe is a major concern for many industries, especially in the nuclear industry. Leaning on the STYLE project, a European experimental program, tests are predicted using the local approach model of Gurson.The mock-up presents a narrow gap INCONEL (nickel base alloy) dissimilar metal weld joining a low alloy ferritic steel pipe and a 316L type stainless austenitic steel pipe. It was performed in the frame of the STYLE project funded by the European Commission (EC) (2010–2013). During the four points bending test on the mockup, the behavior of the crack growth was highly unexpected, and could not be deeply analyzed. The initial crack, located at the interface between the weld and the ferritic Heat-Affected Zone (HAZ), grew in the weld and deviated from it to finally reach the austenitic HAZ. A local approach modeling in order to explain the crack behavior has thus been performed, using the Gurson-Tvergaard-Needlemann (GTN) model.The characterization of the materials composing the weld was made through the STYLE project. Then, GTN parameters are identified using experimental results from the laboratory tests. The behavior of Compact Tension (CT) specimens and the pipe are then numerically predicted using finite element simulations. It is shown that the GTN model is able to catch the mechanical differences between the CT specimens and the pipe, providing a good prediction of the crack propagation during the four points bending test performed for the STYLE project.

Microstructure-dependent fracture toughness (JIC) variations in dissimilar pipe welds for pressure vessel system of nuclear plants

In present study, dissimilar metal weld (DMW) joints between SA508Gr.3cl.1 ferritic steel and SS304LN pipes were prepared using Inconel 82/182, and Inconel 52/152 consumables. Metallurgical properties and their influence on fracture toughness of weldment regions and interfacial regions could play a significant role in integrity assessment of these joints. Ni-based consumables exhibit complex metallurgical properties at interfacial regions. The metallurgical characterization and fracture toughness studies of Inconel 82/182 and Inconel 52/152 joints have been carried out for determining the optimum consumable for DMW joint requirements and the effect of microstructure on fracture toughness in weldment regions. The present codes and procedures for integrity assessment of DMW joints have not given due considerations of metallurgical properties. The requirements for metallurgical properties by considering their effect on fracture toughness properties in integrity assessment have been discussed for reliable analysis. Inconel 82/182 is preferred over Inconel 52/152 joints owing to favorable metallurgical and fracture toughness properties across the interfacial and weldment regions.

Development of 9Cr Ferritic Steel Tube and Pipe SAVE12AD for Advanced Power Boilers

Development of 9Cr Ferritic Steel Tube and Pipe SAVE12AD for Advanced Power Boilers

Fracture behavior evaluations for ferritic steel piping with circumferential double flaws on the inner surface

Methods for assessing the structural integrity of nuclear components having some flaws are provided in the Rules on Fitness-for-Service for Nuclear Power Plants of the JSME code (JSME FFS code). Although the JSME FFS code provides such methods for piping with a single flaw, it does not describe any method for fracture assessment of piping with multiple flaws including flaw coalescence criteria. Some investigations on the fracture behavior of mainly austenitic stainless steel piping with multiple flaws, whose fracture mode is plastic collapse, have recently been reported and fracture assessment methods have been proposed. In the present study, fracture tests and analyses of carbon steel piping with a single and two circumferential flaw(s) on the inner surface were conducted to investigate a method for fracture assessment of ferritic steel piping with multiple flaws. It was found that fracture assessment based on the twice elastic slope method and the plastic collapse mechanism gave inadequate results for a large single flaw. Including this case, two kinds of elastic–plastic fracture assessment method, one using the Z-factor in the JSME FFS code and the other by ductile instability analysis, gave conservative estimates of fracture strength even when the structural factor SF was not considered (i.e. SF = 1).

Numerical evaluation of the effects of a mitigation treatment on the residual stresses of a tubular dissimilar metal weld

Dissimilar metal welds (DMW) between ferritic steel pipes and austenitic steel nuclear components are located in the primary coolant circuit of pressurized water reactors. Those tubular joints, made using an Inconel weld filler metal which is known to be susceptible to stress corrosion cracking, showed in the past some weaknesses, as encountered in particular in the USA and Japan, where stress corrosion cracks have been observed. In this context, the goal of this work is to optimize the manufacturing of these dissimilar joints in order to decrease the negative effects of welding residual stresses regarding these integrity defects. We present in this paper a numerical evaluation of the residual stresses of a specific tubular dissimilar weld, and the numerical setup and evaluation of a mitigation treatment which allows redistributing more favorably the residual stresses. The present work focuses on identifying mitigation process key parameters affecting the residual stresses field. The feasibility of this mitigation treatment is quite encouraging for the studied geometry and materials which could justify an experimental evaluation in the future.

Characterization of cracks of welded joint of ferritic stainless steel pipe AISI 430

This paper presented the main objective characterization of cracks in welded joints of ferritic steels, since these are considered one of the most serious types of discontinuities in welds of these steels. These discontinuities are formed when tensile stresses are developed in a fragile material, unable to plastically deform the absorbing it. High tensile stresses are created and developed in the weld region as a result of thermal expansion and contractions located ( associated with non-uniform heating of the weld ), the volume changes due to phase transformations and as a result of links between pieces being welded and the rest of the structure. To induce cracking of the sample Varestraint test was used, as well as to measure the fracture toughness testing we performed Charpy Impact. However, in this study we observed that microfissures solidification in HAZ (heat affected zone), cracks in the center of the weld cracking due to thermal expansion and overhead imposed by the test were prevalent in these steels AISI/ASTM 430 specification. There was also the toughness of the weld proved always above to the base material.

In situ neutron diffraction measurement of residual stress relaxation in a welded steel pipe during heat treatment

Many previous studies have presented results on the relaxation of residual stress in a welded component as a result of postweld heat treatment. Techniques such as neutron diffraction and deep hole drilling have been used to measure the residual stress after the heat treatment and compare this with the residual stress for the component in the as-welded condition. The work described in this paper is novel: neutron diffraction is used to measure the relaxation of residual stress continuously as the heat treatment is being carried out. Residual stresses are measured in a butt-welded ferritic steel pipe as the pipe is heat treated to 650°C and then cooled to room temperature. The results identify those parts of the heat treatment that lead to significant stress relaxation and the mechanisms responsible for this relaxation. The techniques developed during this work allow future heat treatments to be optimised to achieve the low levels of residual stress in welded components.

Development of Load Multipliers (Z-Factors) in Elastic-Plastic Fracture Mechanics Evaluation in the JSME Rules on Fitness-for-Service for Nuclear Power Plants

Appendix E-9, “Elastic-Plastic Fracture Mechanics Evaluation,” in the JSME Rules on Fitness-for-Service for Nuclear Power Plants (Codes for Nuclear Power Generation Facilities) uses the load multiplier Z-factor that is applied to elastic-plastic fracture mechanics evaluation for a pipe circumferential flaw of austenitic stainless steel piping and ferritic steel piping. The Z-factor is defined as the ratio of the limit load to the load at tearing instability. Here, the Z-factor of the current JSME Rules on Fitness-for-Service for Nuclear Power Plants was developed for a through-wall pipe circumferential flaw with the angle of 60°. In Code Case JSME S NA-CC-002, ”Alternative Rule of Criteria on Allowable Flaw Angle for Circumferential Crack” published in 2005, the Z-factor equations are applicable to a circumferential surface flaw including the angle of greater than 60°, but the Z-factor equations of the Code Case were conservatively formulated as upper-bounded those Z-factors for through-wall pipe circumferential cracks including the angle of greater than 60°. However, the original purpose of the Code Case was to eliminate the limitation of flaw angle and evaluate non-through-wall flaws. Accordingly, Z-factors for non-through-wall flaws should be developed to have the consistency. Therefore, Z-factor equations of austenitic stainless steel piping and ferritic steel piping have been developed for non-through-wall flaws.

Maximum utilization of negative wall-thickness tolerances in pipe rolling on KhPT mills

The cold rolling of pipe on KhPT mills with the maximum possible use of negative wall-thickness tolerance is considered. Special formulas permit the calculation of the minimum permissible pipe-wall thickness for maximum metal savings. Results for the rolling of large batches of austenitic and ferritic corrosion-resistant steel pipe show that metal savings of up to 8.7% relative to the rated mass are possible in controlled pipe rolling on KhPT mills.

A Comparison Between Measured and Modeled Residual Stresses in a Circumferentially Butt-Welded P91 Steel Pipe

Residual macrostresses in a multipass circumferentially butt-welded P91 ferritic steel pipe have been determined numerically and experimentally. The welded joint in a pipe with an outer diameter of 290 mm and a wall thickness of 55 mm is typical of power generation plant components. An axisymmetric thermomechanical finite element model has been used to predict the resulting residual hoop and axial stresses in the welded pipe. The effects of the austenite to martensite phase transformation have been incorporated into the simulation. Residual stresses have been measured using the X-ray diffraction technique along the outer surface of the pipe and using the deep-hole drilling technique through the wall thickness at the center of the weld. Good correlation has been demonstrated between the residual hoop and the axial stresses obtained numerically and experimentally. The paper demonstrates the importance of using a mixed experimental and numerical approach to determine accurately the residual macrostress distribution in welded components.

信頼性評価に基づいた配管減肉管理に関する検討(<特集>第14回動力・エネルギー技術シンポジウム)

Pipe wall thinning is sporadically detected in ferritic steel piping in Japanese BWR plants. The suitability for continued service of wall thinning pipe is basically evaluated by using the "Rules on pipe wall thinning management for BWR power plants." The probabilistic fracture mechanics (PFM) analysis was performed in order to confirm the failure probability applied the rule. Based on the result, the issues of the rule which should be solved were clarified.

Spatial variation of residual stresses in a welded pipe for high temperature applications

Measurements of residual macro-stresses have been undertaken in a feature multipass circumferential single V butt-weld made from a P91 ferritic steel pipe over different spatial depths: (i) ≤10 μm by X-ray diffraction, (ii) ≤1 mm by incremental centre-hole drilling and (iii) through wall section using deep-hole drilling. The ability to make near-surface X-ray residual stress measurements on as-oxidised surfaces has been demonstrated and the implications for use in the evaluation of overall integrity are discussed. Each of the three measurement techniques provides complementary and consistent measurement of induced residual stresses for weld metal, heat affected zone and parent metal for the as-welded and the post-weld heat-treated conditions over the complete spatial range. The results are discussed with respect to the importance of the weld capping run in introducing near-surface compressive residual stresses, the through-wall profiles of the residual stresses measured at the weld metal position in hoop and axial directions and the presence of existing surface oxide.

CURRENT CAPABILITIES OF THE THERMO-MECHANICAL MODELLING OF WELDING PROCESSES

Some fundamental principles and advanced applications of thermo-mechanical modelling techniques used for industrial welding processes are described. The paper covers a range of modelling procedures comprising welding simulation and residual stress analysis of multi-pass, thick-walled ferritic steel pipes; welding simulation and distortion analysis of nickel-based superalloy thin plates; and inertia friction welding of dual alloy drive shafts. A number of pertinent mechanical and metallurgical concepts are discussed, including material behaviour models and material properties, microstructural evolution, solid state phase transformation and post-weld heat treatment. The paper emphasises the general methodologies of thermal modelling procedures and their role in the holistic process of the life assessment and design improvement of power plant piping systems and aeroengine casings and shaft components, operating under high temperature creep and fatigue service conditions.

B109 信頼性評価に基づいた配管減肉管理に関する検討(保全管理・補修/材料強度・破壊評価,OS6 保全・設備診断技術)

Pipe wall thinning is sporadically detected in ferritic steel piping in Japanese BWR plants. The suitability for continued service of wall thinning pipe is basically evaluated by using the "Rules on pipe wall thinning management for BWR power plants." The probabilistic fracture mechanics (PFM) analysis was performed in order to confirm the failure probability applied the rule. Based on the result, the issues of the rule which should be solved ware clarified.

High-temperature creep rupture of low alloy ferritic steel butt-welded pipes subjected to combined internal pressure and end loadings

Constitutive equations are reviewed and presented for low alloy ferritic steels which undergo creep deformation and damage at high temperatures; and, a thermodynamic framework is provided for the deformation rate potentials used in the equations. Finite element continuum damage mechanics studies have been carried out using these constitutive equations on butt-welded low alloy ferritic steel pipes subjected to combined internal pressure and axial loads at 590 and 620 degrees C. Two dominant modes of failure have been identified: firstly, fusion boundary failure at high stresses; and, secondly, Type IV failure at low stresses. The stress level at which the switch in failure mechanism takes place has been found to be associated with the relative creep resistance and lifetimes, over a wide range of uniaxial stresses, for parent, heat affected zone, Type IV and weld materials. The equi-biaxial stress loading condition (mean diameter stress equal to the axial stress) has been confirmed to be the worst loading condition. For this condition, simple design formulae are proposed for both 590 and 620 degrees C.

Approximate creep rupture lifetimes for butt welded ferritic steel pressurised pipes

The creep rupture of butt welded ferritic steel pipes composed of a range of weld and heat affected zone materials has been previously analysed in detail (Leckie and Hayhurst, 1994). These analyses required substantial computational resources which cannot be justified during the preliminary phases of the design process. To reduce cost and improve speed an approximate method for the analysis of kinematically determinate structures, known as the modal method (Leckie and Hayhurst, 1974), has been developed to compute creep rupture lifetimes. This paper reports an extension of the method for the analysis of multi-material structures, such as weldments, and its implementation as a post processor to a stationary state finite element creep analysis. Histories of stress and damage have been determined using the modal method for weldments with a range of heat affected zone and weld material combinations. The lifetimes determined in this way are compared with those determined using complete continuum damage mechanics analyses, and are shown to be conservative; in addition the regions of intense damage have been shown to be accurately predicted. The modal method is also shown to be superior to more approximate reference stress methods.