Creep Strength Enhanced Ferritic Steels Research Articles

Phase-field finite element modelling of creep crack growth in martensitic steels

Creep fracture presents a major concern for structural materials and critical components operating at elevated temperatures, thus requiring effective computational models. This study presents a phase-field framework for modelling creep crack growth and fracture behaviour of modern high-temperature materials such as Creep Strength Enhanced Ferritic (CSEF) martensitic steels. The model was formulated using the thermodynamic principles of the variational phase field theory of fracture and considering some physical aspects of creep fracture. Within the modelling framework, a dissipation potential dependent on creep damage is introduced to capture the effect of creep cavitation on the fracture energy and the creep crack growth resistance of the solid in a phenomenological manner. An elastoplastic power-law creep model is coupled to the phase-field formulations to account for the non-linear deformation processes ahead of the crack tip due to inelastic deformations, as well as their contribution to fracture at high temperatures. The capability of the proposed model is assessed against experimental data from compact tension (CT) creep tests conducted on P91 and P92 steels. Good agreement was obtained between the FE-predicted creep crack growth behaviour and the experimental measurements, showcasing the model’s feasibility. Further, numerical experiments were conducted using the proposed model to elucidate some key aspects influencing the fracture behaviour of martensitic steels. The proposed computational framework not only demonstrated good capability but was also able to offer improved mechanistic insights into the influence of material tendency to develop creep cavities on crack growth behaviour. This work contributes valuable insights into understanding the fracture process of CSEF steels at elevated temperatures and further demystifies the role of creep ductility.

Reassessment of Thor® 115 parent metal performance including consideration of notch behavior

In recent years, the waning of creep rupture ductility in the long term, and sensitivity to creep damage tolerance of creep strength-enhanced ferritic (CSEF) steels were put under the spotlight. The loss of ductility and damage accumulation at stress concentrations may result in early failures of boiler components, and potentially result in severe economic impact and/or safety concerns. To address the potentially undesirable materials performance, the ASME Boiler and Pressure Vessel Code has recently introduced rules for identification of damage intolerant behavior by means of testing, and for design of boiler components with creep damage-intolerant CSEF steels, as Code Case 3048. As of today, only Grade 91 Type 2 material has been classified as a damage tolerant steel.The Thor® 115 (T115) CSEF steel has entered the market in recent years and has since been used for the construction of several combined cycle power plants. This CSEF steel was developed as an evolution to Grade 91, with chemical composition modifications to enhance the resistance to steam oxidation while ensuring long-term microstructural stability, and therefore permitting its use for the higher temperature boiler components, as an alternative to stainless steels. While chemical composition and manufacturing precautions prescribed for Grade 91 Type 2 also apply to T115, an assessment of the materials’ damage tolerance following design standards criteria, and including extensive metallurgical characterization, has not yet been published.This work presents up-to-date uniaxial creep testing results of T115 CSEF, and the derived creep-rupture ductility and damage tolerance parameter evaluation. In addition, results from notched bar creep testing are presented, illustrating the material's response to multiaxial stress states. Post-test metallographic examination of ruptured specimens was also performed and findings are discussed.

Large feature cross-weld creep test performance in the novel creep strength enhanced ferritic steel Thor® 115 including consideration of the post-weld heat treatment temperature

No present manuscripts have assessed the cross-weld creep performance of Tenaris high oxidation resistance (Thor®) 115. Furthermore, few studies (if any) have compared the performance of martensitic creep strength enhanced ferritic (CSEF) steels in the as-welded or post-weld heat treatment (PWHT) temperature, and for feature test geometries. As CSEF steels are widely specified in modern thermal fleet applications, it is critically important to assess weldment performance and consider the influence of PWHT for new construction or repair applications. In this study, it is shown that the life and failure location in Thor® 115 is not a function of PWHT temperature and that failures in this steel are identical to those for the mainstay CSEF steel Grades 91 and 92.

Residual stress analysis, microstructural characterization, and mechanical properties of tungsten inert gas-welded P92/AISI 304L dissimilar steel joints

In the present work, the creep strength-enhanced ferritic martensitic P92 steel was welded with 304L austenitic stainless steel. The dissimilar joining of these two different grades of material was performed by using tungsten inert gas welding process. The nickel-based ERNiCr-3 welding consumable was used. The influence of post-weld heat treatment (760 °C, 2h), called as tempering, was also investigated. The secondary electron and optical image confirmed that ERNiCr-3 weld metal is accompanied by an equiaxed austenitic microstructure with Ni weight percentage of 68.29%. The energy-dispersive X-ray spectroscopy and electron probe microanalyzer results of the weld fusion zone revealed the Nb, Cr, and Ti segregation in the inter-dendritic region. The elemental mapping of the carbon-depleted zone, and the interface was performed using an electron probe microanalyzer, which revealed the diffusion of Ni, Cr, and Fe across the interface of P92 steel and ERNiCr-3 filler weld. The tensile test result indicated that part of the dissimilar weld joints (DWJs) with relatively weak ultimate tensile strength was ERNiCr-3 weld metal. The ultimate tensile strength value of the DWJs was observed as 610 MPa and 580 MPa in the as-weld and post-weld heat treatment situations, respectively. The Charpy V-notch impact energy of the ERNiCr-3 weld fusion zone was achieved as 135 ± 2 J and 140 ± 2 J in as-weld and post-weld heat treatment situations, respectively. The low impact toughness of the ERNiCr-3 weld metal was attributed to the presence of niobium carbide (NbC) and titanium carbide (TiC) particles. The peak longitudinal residual stress of 490 MPa and transverse residual stress of 442 MPa were noted in the ERNiCr-3 weld fusion zone at a depth of 3 mm from the top surface. The tempering heat treatment exhibited a significant drop in the residual stresses value for the weld fusion zone and heat affected zone (HAZ).

Bending and Welding of New High Oxidation Material - Thor™ 115

Development of materials used in the power industry for the production of USC boilers poses new challenges. The introduction of new alloying agents intended at obtaining the best possible mechanical properties, including creep resistance, affects the fabricability of new steel grades. All new materials have to undergo a lot of tests, particularly as regards bending and welding processes, with the aim of enabling the development of technologies ensuring failure-free production and assembly of boiler components. Martensitic steels containing 9% Cr, used in the production of steam superheaters shall have good creep resistance and, at the same time, low oxidation resistance at a temperature above 600°C. In turn, steels with a 12% Cr content, for example, VM12-SHC or X20CrMoV12-1 are characterized by significantly higher oxidation resistance but have lower strength at higher temperatures, which translates to their limited application in the production of modern USC and A-USC boilers.X20CrMoV12-1 was withdrawn from most of the power plants across Europe and VM12-SHC was supposed to replace it, but unfortunately, it failed in regards of creep properties. To fulfill the gap a new creep strength-enhanced ferritic steel for service in supercritical and ultra-supercritical boiler applications was developed by Tenaris and named Thor™115 (Tenaris High Oxidation Resistance). This publication covers the experience obtained during first steps of fabrication which includes cold bending and TIG welding of homogenous joints.

Effect of post-weld heat treatment on microstructure and mechanical properties of deep penetration autogenous TIG-welded dissimilar joint between creep strength enhanced ferritic steel and austenitic stainless steel

The present study centres on the effect of post-weld heat treatment (PWHT) on microstructure and mechanical properties of the deep penetration keyhole Tungsten Inert Gas (K-TIG) welded dissimilar joint between creep strength enhanced ferritic (CSEF) steel and austenitic stainless steel (ASS). The as-received normalized and tempered CSEF steel was joined with ASS in a single pass without using any filler materials and edge preparation. Detailed characterization across the welded joint was conducted using stereomicroscope, electron microscopy, energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), hardness test, tensile test and Charpy impact test. Results showed that PWHT had significant effect on the microstructure and mechanical properties of both the weld metal and CSEF steel heat-affected zone (HAZ), while it had little influence on the ASS side. By using proper PWHT, the hardness gradient across the welded joint could be mitigated and toughness in both the weld metal and the CSEF steel HAZ could be restored. 760 °C was considered the most appropriate PWHT temperature for such dissimilar joint in terms of the overall mechanical properties. The tensile properties of K-TIG welded joint after PWHT were comparable to both friction stir welded joint and laser and/or electron beam welded joint, indicating that deep penetration TIG welding technology may provide a good alternative for the nuclear industry. The correlation among welding thermal cycle, various heat treatment, microstructure evolution and mechanical properties was also analysed in detail.

Microstructural Evolution of Dissimilar Metal Welds Involving Grade 91

Dissimilar metal weld failures between low alloy Cr-Mo ferritic steels and austenitic stainless steels made with Ni-base filler metals are typically observed along the fusion line. Such DMW failures often exhibit the onset of damage well before their expected service life. Failure is typically associated with a carbon-depleted region in the ferritic steel and formation of creep voids along a row of so-called Type I carbides. More recently, the formation of Type I carbides adjacent to a carbide-free ferrite band has also been observed in DMWs where the ferritic steel was a 9 pct Cr creep strength-enhanced ferritic steel. However, it has not been completely clarified whether these microstructural features formed during welding, post-weld heat treatment or service. In this study, single pass bead-on-plate welds were prepared on a Grade 91 steel substrate using commonly specified Ni-base weld metals; ENiCrCoMo-1, ENiCrMo-3, ENiCrFe-3, ENiCrFe-2 and EPRI P87 (ENiFeCr-4) and were characterized in the as-welded, post-weld heat treatment (PWHT) and aged conditions. The evolution of the ferrite band was found to form by a diffusion-controlled process and was observed after PWHT and aging associated with regions exhibiting steep concentration gradients in the partially mixed zone (PMZ). In this study, steep concentration gradients were observed along the toe of the weld whereas relatively shallow gradients were observed at the weld bottom-center, resulting in a discontinuous ferrite band along the fusion line during PWHT and aging. The variation in chemical gradients in the PMZ corresponded to different chemical potential gradients, which lead to differences in carbon diffusion during high-temperature PWHT and aging. The results of this study provide insight into potential solutions for minimizing the risk of failure in future applications.

Comparative study on SAW welding induced distortion and residual stresses of CSEF steel considering solid state phase transformation and preheating

In the present study, experimental and 3-D Finite element analysis of single pass single sided square butt submerged arc welding of 10 mm thick Creep Strength Enhanced Ferritic (CSEF) steel plate was carried out to investigate the residual stresses and deformations with and without the effect of preheating. In Finite Element (FE) model, the bead geometry was considered similar to weld joint for more accuracy. The solid-state phase transformation or austenitic-martensitic transformation characteristic of CSEF steel was also considered in modelling. Thermal history, distortion and residual stresses were predicted for as welded and preheated weld models and validated with experimentally measured results. Deep hole drilling techniques was carried out for residual stress measurement. The residual deformation was measured on coordinate measuring machine. It was observed that preheating effectively reduced the edge deflection and angular deformation values by 41.7 % and 54.1 % respectively. Preheating exhibited a favourable effect on longitudinal residual stress distribution by reducing the values by the maximum difference of 90 %. It was found that FE modelling with solid state phase transformation effect effectively predicted the residual stresses and deformation results with an error of 16.67 % & 26.57 % (maximum) and 3.15 % & 4.35 % (minimum) respectively.

Thermo-mechanical study of single and multi-pass welding of CSEF steel for residual stresses and deformations considering solid state phase transformation

The present study involves thermo-mechanical modelling of submerged arc welding of CSEF (Creep Strength Enhanced Ferritic) steel plate of thickness 10 mm as single pass and multi-pass welding. A square butt joint with no edge preparation and a V-butt joint with 15° included angle are considered for SAW welding. The martensitic-austenitic transformation phenomenon in P91 steel was also implemented with specific changes in the properties of the weld and base material. It is observed that SAW process with single pass demonstrates stable residual stress distribution, whereas in multi-pass weld, the residual stresses show uneven distribution across the weld. The longitudinal residual stresses are higher in magnitude comparing to transverse residual stresses for both the welding cases. The single pass SAW weld fusion zone has shown compressive stress throughout the thickness of the plate with maximum value at the top side. However, in multi pass weld, the compressive residual stresses have multiple peak values along the weld thickness. The deflection pattern is not effectively altered with multi pass with slight difference from single pass. Therefore, it is suggested that if possible, single pass weld can be preferred over multi pass weld.

Manufacture and Performance of Welds in Creep Strength Enhanced Ferritic Steels.

Welding is a vital process required in the fabrication of ‘fracture critical’ components which operate under creep conditions. However, often the procedures used are based on ‘least initial cost’. Thus, it is not surprising that in many high energy applications, welds are the weakest link, i.e., damage is first found at welds. In the worst case, weld cracks reported have had catastrophic consequences. Comprehensive Electric Power Research Institute (EPRI) research has identified and quantified the factors affecting the high temperature performance of advanced steels working under creep conditions. This knowledge has then been used to underpin recommendations for improved fabrication and control of creep strength enhanced ferritic steel components. This review paper reports background from this work. The main body of the review summarizes the evidence used to establish a ‘well engineered’ practice for the manufacture of welds in tempered martensitic steels. Many of these alternative methods can be applied in repair applications without the need for post-weld heat treatment. This seminal work thus offers major benefits to all stakeholders in the global energy sector.

Metallurgical and Stress State Factors Which Affect the Creep and Fracture Behavior of 9% Cr Steels

EPRI‐supported research has identified critical material information regarding the factors affecting the performance of creep strength‐enhanced ferritic steels, in general, and Grade 91 steel, in particular. EPRI recommendations emphasize that a five‐point, integrated strategy should be used for the effective life management of components fabricated from tempered martensitic steels. This integration promotes a balanced use of resources which, when properly focused, reduces uncertainty regarding creep and fracture behavior. Tighter control of processes from steel making, steel processing, and heat treatment ensures that alloys with deficient properties never enter service. One cornerstone of this proactive approach is the definition of ‘Metallurgical Risk’ which links the presence of inclusions and trace elements to the susceptibility for creep damage. The improved confidence in the high temperature performance of CSEF steel components promotes reliability, increases efficiency, and minimizes the risk of component fracture.

Effect of Normalizing Temperature on Fracture Characteristic of Tensile and Impact Tested Creep Strength-Enhanced Ferritic P92 Steel

The high-temperature Cr-Mo creep strength-enhanced ferritic (CSEF) steels are mainly used in nuclear and thermal power plants. In the present investigation, a systematic study on fracture surface morphologies of tensile and impact tested specimens and mechanical properties of cast and forged (C&F) P92 steel was performed for various heat treatment conditions. The heat treatment was carried out in normalizing temperature range of 950-1150 °C and then tempered to a fixed tempering temperature of 760 °C. The effect of varying normalizing temperatures before and after tempering on microstructure evolution, tensile properties, Vicker’s hardness and Charpy toughness was studied. The normalizing temperature before and after tempering was having a noticeable effect on mechanical properties of as-received P92 steel. The fracture surface of impact and tensile tested samples was also studied for various normalizing temperatures with or without tempering. Fracture surface morphology was affected by the presence of secondary phase carbide particles. The fraction area of cleavage facets on the tensile fracture surface was found to be increased with an increase in the normalizing temperature. The fractured tensile specimens were characterized by transgranular ductile dimples, tear ridges and transgranular cleavage facets for various heat treatments. The fracture mode of impact tested samples was more complex. It showed both quasi-cleavage facets and ductile dimple tearing for various normalizing temperatures.

Creep ductility considerations for high energy components manufactured from creep strength enhanced steels

It is well established that the tendency for low ductility ‘creep brittle’ fracture behaviour in tempered martensitic steels is linked to the formation and growth of micro voids or ‘cavities’. Details of the contributions of all factors affecting damage development are still under investigation. However, it is known that for tempered martensitic steels voids often initiate over most of the creep life. Nucleation has been recorded on both prior austenite grain boundaries and at other micro structural features such as lath boundaries. The number of voids formed, and the fracture behaviour observed, depend on the type of creep strength enhanced ferritic (CSEF) steel and specific details of fabrication and heat treatment. In Grade 91 steel, void nucleation is sensitive to metallurgical factors such as composition and steel making practices. Key indicators of susceptibility to creep cavitation also include the levels of trace elements present and the presence of hard non-metallic inclusions. In Grade 92 steel, creep void formation has been linked to boron nitrides and other inclusions. These inclusions are present when there has been insufficient control of composition and heat treatment. Metallurgical factors linked to whether a particle will nucleate a void include the nature of the inclusion/matrix interface, the shape and size and the location of the inclusions within the microstructure. This paper describes the results of critical uniaxial and multiaxial testing for CSEF steels and compares data from nominally the same steels which have different metallurgical susceptibilities to void formation.

Microstructural evolution and mechanical properties of Grades 23 and 24 creep strength enhanced ferritic steels

Grades 23 and 24 creep strength enhanced ferritic (CSEF) steels are relatively new materials that have been designed with improved high-temperature strength compared to conventional 2.25Cr–1Mo alloys. These new steels share alloying strategies that were used in the past for CrMo derivatives such as the CrMoV steels. Grades 23 and 24 are often grouped with similar CSEF steels in terms of mechanical properties and fabrication characteristics. However, recent experience indicates that these alloys require more unique care to ensure adequate properties for the intended service in both the base metal and welds. The differences in behaviour between these new steels and more conventional CrMo steels can be attributed to relatively minor variations in composition that lead to rather significant differences in microstructure. This review summarises technical information and practical experience on the manufacture and use of Grades 23 and 24 CSEF steels for power generation applications. Factors that influence the microstructure and strength of base metal are discussed for each alloy along with influences from common fabrication processes such as welding and bending. The results of this review provides manufacturers, fabricators and end users with information that can be applied to help ensure optimal performance of these steels in high-temperature applications associated with power generation.

Optimisation of laser welding parameters for welding of P92 material using Taguchi based grey relational analysis

Abstract Creep strength enhanced ferritic (CSEF) steels are used in advanced power plant systems for high temperature applications. P92 (Cr–W–Mo–V) steel, classified under CSEF steels, is a candidate material for piping, tubing, etc., in ultra-super critical and advanced ultra-super critical boiler applications. In the present work, laser welding process has been optimised for P92 material by using Taguchi based grey relational analysis (GRA). Bead on plate (BOP) trials were carried out using a 3.5 kW diffusion cooled slab CO2 laser by varying laser power, welding speed and focal position. The optimum parameters have been derived by considering the responses such as depth of penetration, weld width and heat affected zone (HAZ) width. Analysis of variance (ANOVA) has been used to analyse the effect of different parameters on the responses. Based on ANOVA, laser power of 3 kW, welding speed of 1 m/min and focal plane at −4 mm have evolved as optimised set of parameters. The responses of the optimised parameters obtained using the GRA have been verified experimentally and found to closely correlate with the predicted value.

Effect of Heat Treatment on Microstructure and Hot Impact Toughness of Various Zones of P91 Welded Pipes

The new generation super critical thermal power plants are required to operate at enhanced thermal efficiency of over 50% to reduce the fuel consumption and environmental pollution. Creep strength-enhanced ferritic steels, commonly known as Cr-Mo alloys such as P91 (X10CrMoVNb 9-1) are such material of choice for the next generation power plants. The operating requirement of these next generation power plants is that steam temperature of around 650 °C is maintained. For such high-temperature application, creep strength of material is the primary consideration together with adequate weld heat-affected zone (HAZ) toughness. Present work deals with the effect of high service temperature on impact toughness of P91 (X10CrMoVNb 9-1) base material, weld fusion zone, and HAZ. The impact toughness of HAZ for conventional weld groove design and narrow weld groove design has been evaluated experimentally in as-welded and at different post-weld heat treatment conditions. Fractography of the impact toughness specimens of base metal, weld fusion zone, and HAZ was carried out using scanning electron microscope. The effects of heat treatment schemes on the percentage of element present at the fracture surface were also studied.

High Temperature Performance of Creep Strength Enhanced Ferritic Steels Under Steady and Cyclic Operating Conditions

High energy components must exhibit reliable long-term performance under cyclic operating conditions. Historically assessment of the performance of these components focused on defining the transients related to hot, warm and cold starts and stops. The complexity and range of cycles in many plants now includes rapid changes in generating output to operating levels of 30 % of rated capacity. In many cases, the desirable levels of low load operation are below the values considered in the original design. EPRI programs have thus been working with utilities to implement monitoring campaigns which record the changes in local pressure, temperature and flow with time at different locations within a system and to undertake analysis to assess the influence of these effects on performance. The present review summarizes EPRI achievements linked to transient effects in modern generating plant, with particular emphasis on the behavior of creep strength enhanced steels. These steels, typically based on 9–12 % Cr, offer significant benefits to the design and fabrication of components in high efficiency fossil fueled plants because, when properly processed, tempered martensitic steels offer an excellent combination of strength and toughness. However, assessment of in-service experience demonstrates that cracking in creep strength enhanced ferritic steel components has occurred relatively early in life. Solutions to prevent cracking are presented.

Weld repair of Grade 91 piping and components in power generation applications, creep performance of repair welds

Creep strength-enhanced ferritic steels, such as Grade 91, are the preferred material for much of the high-energy boiler tubing and piping components used in modern power generating plants. Weld repair techniques that achieve the necessary performance without the need for high-temperature post weld heat treatment (PWHT) offer particular benefits for Grade 91 steel. These benefits arise since there are many examples of poor heat treatment control which have resulted in component microstructures with below the minimum properties expected by design codes. Furthermore, even a controlled PWHT at temperatures at around 760 °C will further temper the base material. This is significant because excessive base metal tempering is one suggested criterion requiring component replacement. Successful demonstration of controlled welding techniques linked to minimal or no PWHT would alleviate these problems. This article presents results from a major project which is aimed at considering options for designing a ‘well-engineered’ repair. In this project, the creep performance of candidate repair methods was evaluated using large, feature test-type specimens containing the entire weldment including both fusion lines and heat-affected zones. The results show that the cross-weld life in Grade 91 steels does not appear to be a function of whether or not the welding procedures include PWHT. The results offer the potential to qualify ‘cold’ weld repairs in these steels.

Serviceability Assessment of Creep Strength Enhanced Ferritic Steels

The metallurgical advancements exhibited in creep strength enhanced ferritic (CSEF) steels have changed the dynamics of boiler design and operation by improving the fatigue life of thick-walled components and improving cycle efficiency by increasing operating temperatures. These advancements have a similar effect on the assessment of these materials. No longer can a serviceability evaluation be based on a general assumption of metallurgical properties. Instead, condition assessment protocols, examination methods, and flaw assessment approaches must reflect the metallurgical nuances of these high temperature alloy steels. This paper will provide guidance on new issues to be considered when developing a serviceability program.

Weld Hardness Study of P91 Welded Pipes with Post Weld Heat Treatment for Elevated Temperature Application in Power Plants

The creep strength-enhanced ferritic (CSEF) steels are undergoing an encouraged use around the world especially in power plant construction. On construction sites, it has always been the target to have no problems in welded joints but premature failures are being encountered. The primary reason of these premature failures is found to be the improper heat treatment that is mandatorily carried out to achieve the required weld hardness. Weld hardness has close relationship with creep strength and ductility of the welded structures. Hence it is important for any weld to achieve certain level of weld hardness. This study aims at ascertaining the importance of Post Welding Heat Treatment (PWHT) in achieving the required hardness in creep-strength enhanced ferritic (CSEF) materials.The study was carried out on the welding of alloy steel ASTM A335 Gr. P-91 with the same base material (ASTM A335 Gr. P-91) by Gas Tungsten Arc Welding (GTAW) process using ER90S-B9 filler wire with pre-heat of 200oC (min) and inter-pass temperature of 300oC (max). After welding, the joints were tested for soundness with Radiography testing. Induction heating was used for heat treatment of P91 pipes during welding and post weld heat treatment. The effect of Post Weld Heat Treatment (PWHT) was investigated on the Weld metal and the Heat Affected Zones (HAZ) by hardness testing. It is perceived that the scattered and higher hardness values, more than 250HB in 2” P91 pipes in the weld metal and in the heat affected zones, can be brought into the lower required level, less than 250HB, with an effective post weld heat treatment at 760°C for 2hrs.It is concluded that PWHT is the most effective way of relieving the welding stresses that are produced due to high heat input in the welding process and to achieve the required level of hardness in the weld as well as in the heat affected zones (HAZ) in thermal power plant main steam piping.