Effects of welding defects on reducing lifetimes of first stage nozzles of a 123 MW gas turbine made of FSX-414 alloy

In this paper, premature failure of one set of first stage nozzles (stationary blades) in a 123 MW gas turbine has been analyzed. Metallurgical and mechanical experiments showed that the main mechanism for such unexpected cracks is thermal fatigue phenomenon accelerated by presence of welding defects in nozzles. Based on the results, it is recommended that repaired nozzles be thoroughly checked before being re-installed in turbines. Moreover, it is recommended; based on the results; to coat nozzles by corrosion/oxidation resistant coatings after their welding by low chromium filler metals such as L-605.     

Failure analysis in after shell section of gas turbine combustion liner under base-load operation

The present study investigates the failure analysis and the lifetime prediction in the after shell section of gas turbine combustion liner with internal cooling passages called C-channel. To calculate distributions of temperature and stresses, 3D-numerical simulations using FVM and FEM commercial codes are performed. As a result, the discrepancy in thermal expansion between hot and coolant side walls induces high thermal stresses in the welding region and above the divider of the C-channel. Thus, these two regions are much weaker than the other regions. The locations match well to those of thermal cracks in actual gas turbine combustors in service.     

Failure Analysis of HAZ Cracking in Low C–CrMoV Steel Weldment

This case study describes the failure analysis of steel nozzle in which cracking was observed after a circumferential welding process. The nozzle assembly was made from low C–CrMoV alloy steel that was subsequently single pass butt welded using gas tungsten arc welding. No cracks were found in visual inspection of the welds; however, X-ray radiography showed small discontinuous cracks on the surface in the area adjacent to weld bead, i.e. heat affected zone. The welding of nozzle parts made of same material was a routine process and this type of cracking did not occur in the past. Therefore, it became essential to determine the root cause of the failure. A detailed investigation including visual examination, non-destructive testing, optical microscopy, microhardness measurements and residual stress measurements were carried out to find out the primary cause of failure and to identify actions required to avoid its reoccurrence in future. Results of the investigation revealed that the principal cause of failure was the presence of coarse untempered martensite in the heat affected zone due to localized heating. The localized heating was caused by high welding heat input or low welding speed and resulted in the high transformation stresses. These transformation stresses combined with the thermal stresses and the constraint conditions to cause intergranular brittle fracture.     

Failure Analysis of AISI-304 Stainless Steel Styrene Storage Tank

This paper presents the failure analysis of AISI-304 stainless steel tank that was fabricated by welding and used for the storage of styrene monomers. After about 13 years of satisfactory operation, significant cracking was observed adjacent to the weld joints and in base plate near tank foundation. Weld repair was by shielded gas arc welding using AISI 308 stainless steel filler wire. The failed base plate was replaced with the new AISI 304 base plate of same thickness. After a short period of time, seepage was observed along the weld bead. Upon nondestructive testing cracks were found in the heat-affected zone and in the base plate. The failure investigation was carried out on welded and base plate samples using spectroscopy, optical and scanning electron microscopy, fractography, SEM–EDS analysis, microhardness measurements, tensile and impact testing. The results revealed transgranular cracks in the HAZ and base plate, and the failure was attributed due to stress corrosion cracking. Cracks initiated as a result of combined action of stresses developed during welding and the presence of a chloride containing environment due to seawater. It was further observed that improper welding parameters were employed for weld repair which resulted in sensitization of the structure and postweld heat treatment to remove weld sensitization and minimize the residual stresses was not done.     

Failure analysis of gas turbine transition pieces,leading to a solution for prevention

In this study the failure analysis of transition pieces of a gas turbine is investigated. Transition piece connects combustion chamber to the turbine and acts as a nozzle which leads hot gases to stationary blades of turbine. The problem of this transition piece in this gas turbine, which was common in other similar units, was cracks developing on the lower wall near the connection to the turbine. Study of gas turbine operation history, cracks apparent form microstructure analysis and fracture surface, revealed that the thermal fatigue was the main reason for the failure and also oxidation facilitated the crack propagation. In order to prevent the failure of transition pieces, it was proposed to create a row of holes, at 1 cm above the impaired region for local cooling and stopping probable cracks. Due to the implemented solutions, the failure of transition pieces and their annual repair are prevented.     

Investigation of hydrogen embrittlement failure in a steam separator by field metallography

In this work the failure by cracking of a steam separator vessel constructed from ASTM A-516 Gr.60 steel was investigated by field metallography. The application of this non-destructive method allowed the determination of failure mechanism, which was useful to give recommendations for repair by welding and prevent future failures. The cracks identifications were made by penetrant liquid and after this; the field metallography test was performed in the diverse regions with cracks. Finally, the replicas were analyzed in laboratory by optical microscopy. The failure was caused by hydrogen embrittlement process principally in the heat affected zone (HAZ) because of the high reinforcement welding joint, residual stresses values and elevated concentrations of compounds controlling the parameters of the boiler feed water.     

Hot corrosion failure in the first stage nozzle of a gas turbine engine

First-stage nozzles of gas turbines, which are the first elements after the combustion chamber, encounter hot gases from the combustion process and have the task of directing the fluid path and increasing the velocity of combustion products. This paper reports on the incidence and failure of the first-stage nozzles of a gas turbine in September 2013 at a seaside pump-house located in the South-West of Iran. The nozzle was made of nickel-based superalloy Nimonic105. Due to nozzle failure, the turbine was damaged severely. The cause of nozzle failure was investigated. The results of visual inspection, XRD analysis of deposits on the blade airfoil, SEM images and EDAX analysis showed the characteristics of hot corrosion. Finite-element analysis (FEM) revealed that the cause of blade trailing edge failure was thermal stress leading to thermal fatigue, which accelerated nozzle blade failure in addition to the hot corrosion.     

Effects of residual stresses on the fatigue behaviour of welded steel structures

Residual stresses due to the welding process in steel structures can significantly affect the fatigue behaviour. Usually, high tensile residual stresses up to the yield strength are conservatively assumed at the weld toes. This conservative assumption can result in misleading fatigue assessments. Areas with compressive residual stresses may be present in complex structures, where the details are less critical than predicted. This is shown in the paper by the example of fillet‐welded stiffener ends, where beneficial compressive residual stresses cause the initiation of fatigue cracks at other locations in less‐strained areas. Another example for the effects of residual stresses concerns the stress initiation and propagation at a structural detail under fully compressive load cycles. Fatigue cracks are possible here due to high tensile residual stress fields. The conclusion is that the welding‐induced residual stresses should be known in advance for a reliable fatigue assessment, which becomes possible to an increasing extent by numerical welding simulation.     

Residual stress field determination in welds by means of X‐ray,synchrotron and neutron diffraction

To what extent the welding residual stresses influence fatigue is still unclear and matter of debate. An important reason for this lack of clarity is that the exact determination of residual stress fields in welds is complicated which leads to conservative assumptions about these stresses in the fatigue design codes. The advances in the diffraction analysis of materials offer the opportunity for the full‐field residual strain mapping in welds albeit at the cost of time and technical complexity. In this work residual stress field determination in welded S1100QL specimens by means of the x‐ray, synchrotron and neutron diffraction techniques was undertaken. The results revealed that the maximum values of surface residual stresses are not as frequently assumed, as high as the yield strength in small scale specimens. At the weld toe which could serve as a fatigue crack initiation site, even lower residual stresses than the weld centreline could be present. The in‐depth measurements revealed that the effective part of the residual stress field which could be decisive for the fatigue failure initiation is concentrated at the surface of the weld.     

Finite element analysis of the effect of welding heat input and layer number on residual stress in repair welds for a stainless steel clad plate

Stainless steel clad plate is widely used in petroleum, chemical and medicine industries due to its good corrosion resistance and high strength. But cracks are often formed in clad layer during the manufacture or service, which are often repaired by repair welding. In order to ensure the structure integrity, the effects of residual stress need to be considered. The objective of this paper is to estimate the residual stress and deformation in the repair weld of a stainless steel clad plate by finite element method. The effects of heat input and welding layer number on residual stresses and deformation have been studied. The results show that large residual stresses have been generated in the repair weld. The heat input and layer number have great effects on residual stress distribution. With the heat input and welding layer number increasing, the residual stresses are decreased. Using multiple-layer welding and higher heat input can be useful to decrease the residual stress, which provides a reference for optimizing the repair welding technology of this stainless steel clad plate.     

Hydrogen-Assisted Cracking in Boiler Steam Drums

A statutory inspection of a boiler steam drum that had seen 10 years of service revealed a few scattered cracks on the inner surface of the drum. The drum was used in the refinery and petrochemical industry. Remnant life assessment, including inspection of other drums, showed presence of several cracks in two out of seven boiler inspected. In situ microstructural analysis revealed a variation in the microstructure in the cracked regions compared to the microstructure in uncracked regions. Additionally, a wide variation in hardness was associated with the microstructural variation. Detailed study on a sample extracted from a cracked region demonstrated that the cracking occurred in a crescent zone similar in appearance to that of a heat affected zone (HAZ) generally associated with a spot weld or other sources of hot spots in the material. Subsequent examination of more samples confirmed that repair welding was carried out at several places on the inner surface of the drums before installation and commissioning and all the cracks were around the repair welds. The failure occurred by crack initiation in the HAZ of the repair welds and the cracks then propagated progressively across the inner surface of the drums. The failure mechanism was identified to be cold cracking and the failure analysis showed that all the factors required for cold cracking, namely, vulnerable microstructure, residual stresses and hydrogen atmosphere, during welding had been present in the drum material.     

Using FEM to determine the thermo-mechanical stress in tube to tube–sheet joint for the SCC failure analysis

The stress corrosion cracking in a weld of the tube to tube–sheet region of heat exchangers is a common problem. Thermo-mechanical stress in tube to tube–sheet joints including welding effect should be determined in this situation for failure analysis. In this paper, the Finite Element Method (FEM) is used to predict the thermo-mechanical stresses including welding residual stress in a tube to tube–sheet weld. Both the thermo-mechanical stress distribution with and without the welding residual stress have also been investigated by numerical simulation. The welding, operating temperature, and operating pressure have effect on total stresses. Especially, the welding residual stresses play an important role in total stress state in tube to tube–sheet joints. Geometric discontinuities of the vicinity of gap cause the welding joint to experience a local stress concentration. A high tensile stress in the tube to tube–sheet region has been demonstrated by FEM, which is the stress aspect for the SCC phenomenon of austenitic stainless steel in chloride environment.     

FEM prediction of welding residual stresses in a SUS304 girth-welded pipe with emphasis on stress distribution near weld start/end location

A finite element approach based on Quick Welder software is developed to simulate welding temperature field and welding residual stress distribution in a 3D multi-pass girth-welded pipe model. The characteristics of welding residual stress distributions in a SUS304 stainless steel pipe induced by heating with a tungsten inert gas arc welding torch are investigated numerically. Meanwhile, an emphasis is focused on examining the welding residual stress distributions in and near the weld start/end location. Moreover, the residual stresses predicted by the present computational approach are compared with the measured data; and the comparison suggests that the numerical simulation method has basically captured the feature of welding residual stress distribution near the weld start/end region. The numerical simulation results show that both the hoop and the axial residual stresses near the weld start/end region have sharp gradients and are significantly different from those in the steady range.     

Numerical simulation of three-dimension stress field in double-sided double arc multipass welding process

A new high-efficiency method for welding thick plates of low-alloy high-tensile steel which does not require back chipping – double-sided arc welding (DSAW) – is provided in this paper. Backing run adopts double-sided pulse gas tungsten arc welding, and other passes adopt double-sided gas metal arc welding. Three-dimensional numerical models of DSAW with 50 mm plates are developed to predict the stress distribution by using finite-element analysis, computer parallel processing technology and multiple jobs design, and are compared with single arc welding (SAW). The analysis of the interpass stresses indicates that the stresses of the back and cover pass are to be regarded as the key point in multipass welding. To verify the calculated results, the residual stresses and transient temperature of back run weld measured individually agree approximately with the calculated results, which illustrates that the backing run and residual stresses of DSAW are lower than those of SAW.     

Failure of the bucket wheel excavator buckets

Buckets are a vital substructure of all digging machines, and are intended for the realisation of the fundamental machine function - soil excavation. This paper presents the results of the experimental-numerical investigation of the cause of the bucket wheel excavator SRs 470 buckets failure. The chemical composition and mechanical properties, the impact toughness, hardness, tendency to cracks and the microstructure were determined using appropriate tests. Experimental examinations of working and residual stresses were performed using strain gauges. The superposition of the experimentally determined working and residual stresses and the calculation of the total principal stresses were conducted using the originally developed procedure presented in this paper. The bucket working stress state was calculated by applying the linear finite element method. Conclusions based on the investigation results show that the main reasons for the buckets failure were the ‘design-in defects’ - oversights made during the procedures of geometrical shaping and material selection. Furthermore, high values of residual stresses, as well as the cold cracking observed on the welded joint of the knife and the bucket body, suggest that the ‘manufacturing-in defects’ also played a significant role in the failure. The superposition of influences of the ‘design-in defects’ and the ‘manufacturing-in defects’ has conditioned the appearance and propagation of long-term fatigue cracks, leading to the total destruction of the buckets. The fact that buckets' failure appeared due to oversights made during geometrical shaping, material selection and manufacturing further points to the importance of the critical approach implementation during the design phase of the earthmoving machines working devices.     

Observations of the effect of varying Hoop stress on fatigue failure and the formation of white etching areas in hydrogen infused 100Cr6 steel rings

White etching cracks (WECs) in wind turbine gearbox bearings have been studied previously. Rolling contact fatigue (RCF) tests are conducted on 100Cr6 bearing steel rings, in this study, to generate WECs like those found in wind turbine bearings. This research studies the effect of two different levels of tensile Hoop stresses on the failure life and formation of WECs in the rings. The rings are pre-charged with hydrogen before RCF tests are conducted. It is found that these rings experience incremental fatigue failure, followed by a sudden rapid failure. The fractography, Reflecting Light Microscopy (RLM) and Scanning Electron Microscopy (SEM) results are presented in this paper.     

Effects of material non-linearity on the residual stresses in a dendritic silicon crystal ribbon

Thermal stresses developed in a dendritic silicon crystal ribbon have been shown to cause plastic deformation and residual stresses in the ribbon. This paper presents an implementation of a numerical model proposed for thermo-elasto-plastic behaviour of a material. The model has been used to study the effects of plasticity of silicon on the residual stresses. The material properties required to implement this model are all assumed, and the response of the material to the variations in these assumed parameters of the constitutive law and in the finite element mesh is investigated. The steady state growth process is observed to be periodic with non-zero residual stresses. Numerical difficulties are also encountered in the computer solution process, resulting in sharp jumps and large oscillations in the stress responses.     

Evaluation of Stress Relaxation Cracking on 800HT Pigtails of the Steam Reformer

Stress relief cracking occurs when susceptible alloys are subjected to thermal stress after welding to reduce residual stresses and improve toughness. Cracks were observed on the external surface of the outlet pigtails of a steam reformer. The failures (cracks) occurred at the toe of the socket weld connecting the pigtail to the catalyst tube. Design parameters are 900 °C and 22 barg; the actual operating conditions are 875 °C and 20 barg. The material of construction is Alloy 800HT, which is an iron–nickel–chromium alloy. The outlet pigtail was used to transfer the process fluid of the steam reformer which is connected to a manifold header. It was requested to conduct a failure analysis on damage in the outlet pigtail tubes that occurred in the form of cracks. The failure analysis study is described in detail in this paper, and the mechanism of the crack is identified and proper recommendation is given to avoid such issues in future operations.     

The indentation of hard metals: the role of residual stresses

Experimental evidence to support Johnson's [6] analysis of the residual surface stress distribution produced in a flat surface by a spherical indenter is presented. The theory suggests that high residual stresses develop just outside the contact area as a result of the superposition of elastic unloading stresses onto the stresses at maximum load when the specimen has deformed plastically. The experiments involved the use of a semi-brittle steel, sufficiently hardened so that, while tensile stresses in the surface produced cracks, the substrate deformed plastically under the triaxial stress system beneath the indenter. Radial cracks produced by the indenter frequently extended after load removal, implying the presence of the high tensile circumferential residual stresses predicted by the theory. This work and recent studies of indentation loading of glasses show that there are important situations where residual stresses can contribute to their failure and wear.