Search Results for welding investigations

The mechanisms of recent cracking failures of HR3C super heater pipes of a fossil power plant in the Netherlands were investigated. Initial failure investigations showed that pitting corrosion of the sensitized HR3C initiated subsequent stress corrosion cracking (SCC). It was concluded that magnesium chloride hydrates from condensed seawater had initiated pitting corrosion as well as SCC similar to the standard ASTM G36 SCC test. By experimental application of the ASTM G36 procedure, this tentative mechanism is reproduced and confirmed by a series of laboratory tests with pure magnesium chloride as well as with synthetic seawater. It included the effects of temperature, magnesium chloride concentrations of the evaporating water and applied bending moments on cracking. As a result for the 175h testing period in MgCl2*6H 2 O cracking increases significantly above 100°C up to 120°C but is reduced slightly at temperatures up to 155°C. With increasing bending moments, the U-shaped test pieces revealed increasing crack depths up to total fracture of the 5mm thick sections. Lower magnesium chloride concentrations as in concentrated seawater provided identical cracking, however, to a lower extent. It is therefore concluded that the operational failure of the sensitized HR3C super heater pipes was initiated in presence of condensed seawater and followed the same mechanism as found in the experimental investigation. As a conclusion, the presence of seawater saturated air at temperatures between 100° and 155°C should be avoided.

This paper reports the results of a collaborative investigation of an ex-service grade 91 bend carried out by the UK generating companies Centrica, SSE, Engie and RWE. As part of the handover exercise for Centrica’s Langage power station in 2009 a number of routine checks were carried out on the main steam and hot reheat grade 91 steam pipework. In some cases low hardness readings were found with subsequent metallurgical replication showing the presence of an aberrant non martensitic microstructure. This led to a more extensive inspection programme on the steam lines and the discovery of other areas of suspect material. A review of the operating capability of the plant, including detailed pipework stress analysis and a pipework peaking assessment, along with the assumption that lower strength grade 91 material was present, led to the steam lines being down rated and returning to service under these revised conditions. At the first C inspection in December 2012, after the HRSG and associated pipework had operated for 18720 hours, a bend with a soft weld, along with a section of the straight pipe on either side, was removed from service. An investigation was undertaken to establish how long this component would have survived, had it been left in service, and to consider the implications for the future operation of the plant.

There is a constant need for improved knowledge of the influence of non-standard processing on the expected performance of creep strength enhanced ferritic (CSEF) materials as the total installed tonnage of these materials is rapidly increasing across the power generation industry. Cr-Mo-V steel grades micro-alloyed with niobium and titanium designed for pressurized equipment operating in the supercritical steam range proved to be very sensitive to relative minor variations in the principal heat treatment parameters time and temperature, when compared to the traditional Cr-Mo-V grades. A key component for successful welds is optimised post weld heat treatment (PWHT). Under certain conditions premature failures of welds can occur when incorrect weld and heat treatment performance result in a reduction of specified mechanical properties and high temperature creep performance, it is therefore of significant importance to have a good understanding of actual material properties for effective operation and plant life studies. This study investigated the effect and impact variations of post weld heat treatment time and temperature on mechanical properties of tungsten inert gas (TIG) and manual metal arc (MMA) welds on Grade 91 pipes from a set of reference samples. This is in preparation of establishing a benchmark set of tests to determine the integrity and expected long-term performance of butt-welds from limited site sample volumes, providing a non-intrusive methodology to identify welds suspected to have received non-standard PWHT cycles on Grade 91 pipework systems.

In several material qualification programs tubes and thick-walled components mainly from Alloy 617 and Alloy 263 were investigated. Results as low cycle fatigue and long term creep behavior of base materials and welds are presented. Numerical models to describe the material behavior have been developed and verified by multiaxial tests. In order to ensure the feasibility of A-USC plants two test loops have been installed in GKM Mannheim – one for tube materials and a new one for thick-walled piping and components. The latter consists of a part with static loading and a part subjected to thermal cycles and is in operation since November 2012. First results of measurements and numerical calculations for a pipe bend (static loading) as well as pipes and a header (thermal cycles) are presented.

The creep rupture properties of welded joints of advanced 9%Cr-Mo-Co-B steel used for 620°C USC steam turbine have been studied. The welded joints were prepared by means of shielded metal arc welding (SMAW). A lot of creep tests have been conducted and the results indicate that fracture usually occurs in the intercritical heat affected zone (ICHAZ) of the welded joint and is typical of Type IV cracking. The microstructure of the HAZ has been investigated by using optical microscopy, SEM and TEM. The degradation mechanism of welded joint of the 9%Cr-Mo-Co-B steel has been explored by analysing the phases of precipitates. Creep voids were observed in the vicinity of the coarse Laves phase particles, resulting in the degradation of the creep rupture properties.

In a European ultra-supercritical (USC) power station repaired reheater bundle tubes made out of 25% Chromium stainless steels developed stress relief damages at the tube-to-tube butt welds, leading to leakages after only 8.500 hours of operation. Laboratory investigations of the leakages revealed common features of stress relief cracking (SRC) such as highly localized intergranular cracking in the heat affected zone (HAZ) near the fusion line, creep void formation at the crack tip and around the crack. At that time no other SRC damages were known for the employed 25% Chromium stainless steel boiler tubes. This article briefly describes the SRC damage found on the repaired reheater bundle tubes. It further provides insight on the several laboratory tests employed to assess the SRC behavior of welded joints of different creep resistant stainless steels. Among the selected test methods were Slow-Strain-Rate-Tests (SSRT), static 3-point bending tests derived from the Van Wortel approach and component tests. The results provided by the described tests methods have shown that the SRC behavior of a given material combination must be assessed by different techniques. This is especially the case for the evaluation of potential countermeasures and for the determination of the service conditions leading to the highest susceptibility.

Metallurgical factors affecting the fusion boundary failure and damage mechanism of DMWs (Dissimilar Metal Welds) between the CSEF (Creep Strength Enhanced Ferritic) steels and austenitic steels were experimentally and theoretically investigated and discussed. Long-term exservice DMWs up to 123,000 hours were investigated; the precipitates near the fusion boundary were identified and quantitatively evaluated. Comparing with the other generic Ni-based weld material, MHPS original filler metal HIG370 (Ni bal.-16Cr-8Fe-2Nb-1Mo) showed superior suppression effect on fusion boundary damage of DMWs, which was verified by both of the microstructure observation and thermodynamic calculation. Based on the microstructure observation of crept specimen and ex-service samples of DMWs, temperature, time and stress dependence of fusion boundary damage of DMWs were clarified. Furthermore, fusion boundary damage morphology and mechanism due to precipitation and local constituent depletion was discussed and proposed from metallurgical viewpoints.

To improve the economics of critical components, such as receivers and heat exchangers, for Generation 3 (Gen 3) concentrating solar power (CSP) plants, research was conducted to understand how manufacturing impacts the high-temperature performance of various tube production routes. Gen 3 CSP components are expected to require the use of heat-resistant nickel- based alloys due to the elevated operating temperatures in designs carrying molten salt or supercritical CO 2 . INCONEL alloy 740H (alloy 740H) was investigated as an alternative to UNS N06230 (alloy 230) as it possesses superior high-temperature creep strength which can lead to overall reductions in material cost. A key challenge is understanding how autogenous seam welding with and without re-drawing can be used to manufacture thin-wall tubing for CSP receivers and heat-exchangers to further reduce costs over traditional seamless production routes. Alloy 740H welded tube was successfully fabricated and re-drawn to several relevant tube sizes. Since traditional mechanical testing samples could not be removed from the thin-wall tubing, full-sized tubes were used for tensile, fatigue, and vessel testing (internally pressurized creep- rupture) which was critical to understanding the weld performance of the manufactured product forms. The generated vessel test data exhibited a creep strength reduction when compared to wrought product with no clear trend with temperature or test duration. It was found that redrawing the welded tubes improved the creep strength to approximately 82% of the wrought material performance and elevated temperature tensile and fatigue behavior exceeded 85% of the design minimums. Detailed, post-test characterization found that nano-sized carbides formed during the laser seam-welding process remained stable after multiple solution-annealing steps, which restricted grain growth, and impacted the time-dependent performance. This paper will focus on the time-dependent behavior of the examined welded and redrawn tubes, supporting metallographic evidence, and give perspective on future considerations for using alloy 740H in CSP components.

Steel castings of creep-resistant steels are critical components in the high and intermediate pressure turbine sections of fossil fuel-fired power plants. As plant efficiencies improve and emission standards tighten, steam parameters become more stringent, necessitating constant enhancement of material creep resistance. Steel foundries alone cannot conduct necessary material development at an appropriate scale, so all power plant component suppliers cooperate to define optimal chemical compositions, perform test melts, creep tests, microstructure investigations, and test pilot components, such as through the COST program developing new 9-12%Cr cast steel grades. This paper illustrates a steel foundry's role in COST, describing the transfer of these new cast steel grades from research into commercial production of heavy cast components, outlining incurred problems, process development cycles, comparisons with low-alloy steels, welding tests, base material/weld investigations, heat treatment optimization, and casting of pilot components/weldability test plates to verify castability of larger parts and make necessary adjustments. Parallel to ongoing COST creep tests, the steel grades were introduced into commercial large component production, involving solutions to process-related issues, with over 180 components successfully manufactured to date, while further COST program developments present ongoing challenges.

The voestalpine foundry group, operating at locations in Linz and Traisen, Austria, specializes in heavy steel casting components ranging from 1 to 200 tons for power generation, oil and gas, chemical processing, and offshore applications. Their manufacturing expertise encompasses high-alloyed martensitic 9-12% Cr-steels and nickel-based Alloy 625, particularly for ultra-supercritical (USC) and advanced USC power generation systems operating at temperatures from 600°C to over 700°C. The production of these complex, thick-walled components relies on advanced thermodynamic calculation and simulation for all thermal processes, from material development through final casting. The foundries’ comprehensive capabilities include specialized melting, molding, heat treatment, non-destructive testing, and fabrication welding, with particular emphasis on joining dissimilar cast, forged, and rolled materials. Looking toward future innovations, the group is exploring additive manufacturing for mold production and robotic welding systems to enhance shaping and surface finishing capabilities.

Advanced martensitic 9% chromium steels have been identified as the most favored group of materials for high temperature applications in thermal power plants. To extend the temperature range of martensitic steels up to 650°C large effort was put on the development of new alloy concepts. The so-called MARBN concept (Martensitic steel with defined Boron/Nitrogen relation) provides increased creep rupture strength due to higher solid solution strengthening and improved microstructural stability. The major improvement is the reduction of type IV cracking in welded joints, which shifts the focus to the creep rupture strength of the weld metal. This paper illustrates the process experience of the steel foundry for production of heavy cast components in latest state of the art 9-12%Cr-MoCoVNbNB-alloyed cast steel grades and the newest state of development and prototype components in MARBN cast steel grades. Metallurgy, solidification, heat treatment and welding are main items to be considered for development of new, complex steel grades for foundry processing with the help of empiric processing in test programs and thermo-physical simulation. As welding is an essential processing step in the production of heavy steel cast components a good out-of-position weldability is required. Moreover a stress-relieve heat-treatment takes place subsequently after welding for several hours. This contribution also deals with the development of matching welding consumables for the production of heavy cast components and discusses the challenges of defining appropriate welding and heat treatment parameters to meet the requirements of sufficient strength and toughness at ambient temperature. Additionally, first results of creep rupture tests are presented.

The presented work summarizes the results of more than 10 years of research at TU Graz and TU Chemnitz and partners on a martensitic boron and nitrogen stabilized 9Cr3W3Co (MARBN) steel grade. The design philosophy of MARBN steels is presented and critical issues regarding boron and nitrogen balance are discussed. Microstructural characterization of two different laboratory heats, is presented and efforts in European projects towards an upscaling of melts are presented. Base material creep testing data at 650 °C up to 50.000 hours is presented and assessed to commercial alloys such as ASTM grades P91 and P92. An increase of creep rupture stress of more than +20% was recorded. Oxidation tests in steam at 650°C revealed an anomalous response of the material. Several specimens exhibited excellent oxidation resistance commonly only seen for grades of higher chromium content. The anomalous oxidation behaviour is identified and discussed, although the causes remain yet unclear. Results of manufacturing, characterization and testing of different MARBN welds, including gas-tungsten-arc-, gas-metal-arc-, friction stir and electron beam welds reveal a microstructure memory effect in the heat affected zone, so that no uniform fine-grained zone is present. The behaviour of crosswelds during long-term creep testing at 650 °C up to more than 32.000 hours is assessed and the susceptibility to Type IV cracking is discussed. The manuscript summarizes research of more than 10 years, presents current research activities on MARBN and describes open questions for an alloy identified as a promising martensitic steel grade for elevated temperature components.

Increasing demand on efficiency and power output of steam generators leads to new designs of welded rotors. The reason for rotor welding is the large size of rotors, which are difficult to produce in a single piece. Secondly, as there are varying operation conditions along the rotor length. In a heterogeneous rotor, several materials appropriate for local service conditions can be used. At the rotor service temperatures, creep properties are crucial for successful design. The weakest point of every welded component is the heat affected zone. Therefore, the creep properties of a heterogeneous weld are subject of the investigation herein the current study, a heterogeneous weld of COST F and COST FB2 materials is investigated. The welding was performed by multi pass technique with overlaying welding beads that applied several heating cycles to heat affected zone. Metallographic investigation of the weld was performed and the weakest microstructure spots were detected. With the use of FEM simulation, appropriate heating/cooling cycles were obtained for the detected weak points. The temperature cycles obtained were subsequently applied to both base materials under laboratory conditions by induction heating. Creep properties of these materials were investigated. The influence of the initial base material’s grain size was also considered in the investigation. Two heating/cooling schedules were applied to both base materials with two grain sizes. Altogether, 8 different microstructures were examined in short term creep tests and the results were summarized.

Welded joints of Ni-base alloys are often the critical part of components operated under high temperature service conditions. Especially welds in thick-walled structures are susceptible to various crack phenomena. Creep rupture and deformation behavior of different similar welds of Alloy 617B, both circumferential and longitudinal, were determined in many research German projects with the aim to qualify the nickel alloys and its welded joints for the use in highly efficient Advanced Ultra Supercritical (AUSC) power plants. Damage mechanisms and failure behavior have also been investigated within these projects. In order to reduce the welding residual stresses in thick-walled components a post weld heat treatment (PWHT) for Alloy 617B is recommended after welding. This PHWT reduces not only residual stresses but causes changes in the damage mechanisms and failure behavior of welded joints of Alloy 617B. Improving effects of PWHT have been investigated in this study and results of microstructural investigations were correlated with the material behavior.

Research on low-alloyed, heat-resistant 12Cr2MoWVTiB steel, implemented in China to power plants in 50’s last century, was performed to investigate a possibility of its application for pressure elements of boilers, in particular for membrane walls. The microstructure of the as-received 12Cr2MoWVTiB tube, investigated by light microscopy, scanning- and transmission electron microscopy, consists of ferritic grains with some bainite areas between them as well as primary carbides (MC) and secondary carbides (VC, M 23 C 6 , M 6 C) formed during tempering of the steel. Results of mechanical tests of 12Cr2MoWVTiB welded joints (butt- and fillet welded joints) as well as microstructure analyses of are satisfactory.

According to ASME Case N-888-3, Similar and Dissimilar Metal Welding Using Ambient Temperature SMAW or Machine GTAW Temper Bead Technique, a 48 hr waiting period before conducting the final nondestructive examination (NDE) is required when ferritic filler weld metal is used. The purpose of the 48 hr hold is to confirm the absence of hydrogen-induced cracking in the temper bead heat-affected zone. In previous research, the effect of post-weld heat treatment (PWHT) and temper bead welding (TBW) on the hydrogen-induced cracking (HIC) susceptibility in the coarse-grained heat-affected zone (CGHAZ) in welds of SA-508, P-No. 3 Group 3, pressure vessel steel was investigated using the Delayed Hydrogen Cracking Test (DHCT). In that previous study, the Gleeble thermomechanical simulator was used to generate six CGHAZ microstructural conditions: as-welded (AW), PWHT, and AW with single a TBW reheat at 675, 700, 725, and 735°C. Hydrogen was introduced to the specimen through cathodic charging under in situ constant tensile stress. The HIC susceptibility for these microstructures was ranked by the DHCT at a diffusible hydrogen level significantly exceeding typical GTAW and SMAW processes. The work described in this paper investigates the susceptibility to HIC of these same CGHAZ microstructures with DHCT at variable current densities, further ranking each condition. Test results were analyzed by fracture surface examination of failed tests, and cross-section microstructural analysis under a scanning electron microscope (SEM). Future steps include evaluating critical hydrogen content levels using gas chromatography for each condition. The results from this study will be used to consider potential elimination of the NDE hold time requirement in Case N-888-3 when ferritic weld metal is used.

ASME Grade 91 steel seam-welded elbow pipe, which has been used in a USC plant (A-Plant) for about 6 × 10 4 h, was investigated to clarify the microstructure and remaining creep life of the material at long-term region. SEM and TEM observations were conducted on specimens cut from the welded portions of the intrados and extrados of the elbow, and the number density of creep voids in fine-grained HAZ was measured in the wall-thickness direction. Then, creep rupture tests were performed to examine the remaining life of each portion of the base metal and welded joint. On the basis of the results, it was suggested that the microstructural changes were small and that the cumulative creep damage was also small for the elbow pipe during its use at the USC plant for about 6 × 10 4 h. The present result was compared with the result of an investigation on Grade 91 steel elbow used in another USC plant (B-Plant) for about 5 × 10 4 h. The A-Plant material had a creep life about ten times longer than that of the B-Plant material for not only the base metals but also the welded joint. Through the comparison of the investigation results, it was suggested that the difference in the creep deformation property between the base metals of the elbows was the main reason for the difference in their creep lives.

Starting in 2010 a new generation of coal fired power plants in Europe operating at a steam temperature of up 620°C was commissioned. During that commissioning process many cracks occurred in welds of T24 material which was extensively used as membrane wall material in nearly all of the new boilers. The cracks were caused by stress corrosion cracking (SCC) only occurring in the areas of the wall being in contact to high temperature water during operation. The question which step of the commissioning process really caused the cracking was not answered completely even several years after the damage occurred. To answer this question and to define parameters which will lead to cracking in high temperature water many tests were conducted. Generally it was found that slow tensile tests in controlled environment are well suited to get information about materials SCC sensitivity in the laboratory. In the present paper, first the influence of the cracking of welded T24 material in acidic environment containing well-defined amounts of H2S is investigated to address the question if a chemical cleaning process prior to the testing might lead to hydrogen induced SCC. As a second step, cracking behaviour in high temperature water is being investigated. Here the influence of the temperature, the oxygen concentration of the water, the deformation speed of the sample, the heat treatment and the condition of the material on the SCC is analysed.

Boron and nitride additions are emerging as a promising design concept for stabilizing the microstructure of creep-resistant martensitic high-chromium steels. This approach, known as MarBN steel (martensitic steel strengthened by boron and nitrogen), combines the benefits of solid solution strengthening from boron with precipitation strengthening from nitrides. However, initial welding trials revealed challenges in achieving a uniform fine-grained region in the heat-affected zone (HAZ), which is crucial for mitigating Type IV cracking and ensuring creep strength. Despite these initial hurdles, preliminary creep test results for welded joints have been encouraging. This study presents an improved MarBN steel formulation and its investigation through uniaxial creep tests. Base material and welded joints were subjected to creep tests at 650°C for up to 25,000 hours under varying stress levels. The analysis focused not only on the creep strength of both the base material and welded joints but also on the evolution of damage. Advanced techniques like synchrotron micro-tomography and electron backscatter diffraction were employed to understand the underlying creep damage mechanisms. By combining long-term creep testing data with 3D damage investigation using synchrotron micro-tomography, this work offers a novel perspective on the fundamental failure mechanisms occurring at elevated temperatures within the HAZ of welded joints in these advanced steels.

The present study presents a detailed investigation on the evolution of the microstructure during welding on virgin and long-term service exposed (creep aged 1 = 535°C; 16.1 MPa; 156 kh and creep aged 2 = 555°C; 17.0 MPa; 130 kh) 12% Cr (X20CrMoV11-1) martensitic steel. This study was carried out in order to understand the impact of welding on prior creep exposed Tempered martensite ferritic (TMF) steel and to explain the preferential failure of weldments in the fine grained heat affected zone (FGHAZ) of the creep aged material side instead of the new material side. Gleeble simulation (Tp = 980°C; heating rate = 200 °C/s; holding time = 4 seconds) of the FGHAZ was performed on the materials to create homogeneous microstructures for the investigation. Quantitative microstructural investigations were conducted on the parent plate and simulated FGHAZ materials using advanced electron microscopy to quantify: a) voids, b) dislocation density, c) sub-grains, and d) precipitates (M 23 C 6 , MX, Laves, Z-phase) in the materials. Semi-automated image analysis was performed using the image analysis software MIPARTM. The pre-existing creep voids in the creep aged parent material and the large M 23 C 6 carbides (Ø > 300 nm) in the FGHAZ after welding are proposed as the main microstructural contributions that could accelerate Type IV failure on the creep aged side of TMF steel weldments.