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Über dieses Buch

This is the fourth volume in the well-established series of compendiums devoted to the subject of weld hot cracking. It contains the papers presented at the 4th International Cracking Workshop held in Berlin in April 2014. In the context of this workshop, the term “cracking” refers to hot cracking in the classical and previous sense, but also to cold cracking, stress-corrosion cracking and elevated temp. solid-state cracking. A variety of different cracking subjects are discussed, including test standards, crack prediction, weldability determination, crack mitigation, stress states, numerical modelling, and cracking mechanisms. Likewise, many different alloys were investigated such as aluminum alloys, copper-aluminum dissimilar metal, austenitic stainless steel, nickel base alloys, duplex stainless steel, creep resistant steel, and high strength steel.



Hot Cracking Phenomena I—Testing


A Historical Perspective on Varestraint Testing and the Importance of Testing Parameters

This paper provides a historical perspective on the development of the Varestraint testing method as well as the design of a new Varestraint testing machine, utilized on a daily basis in the aerospace industry to solve hot cracking issues in production. The paper also discloses the importance in choosing the right testing parameters in order to minimize scatter in test results and to be able to make a reliable judgment in susceptibility towards hot cracking. Weld current, weld speed, bending rate and strain level as well as the evaluation procedure with i.e. measurement errors are included in the analyses.
Joel Andersson, Jonny Jacobsson, Carl Lundin

Improved Understanding of Varestraint Testing—Nickel-Based Superalloys

By Varestraint testing, the susceptibility of an alloy to hot cracking during welding can be evaluated on test plates when they are bent at the same time as welding takes place. The strains imposed by welding can thus be augmented by the strains imposed by the bending action to find the strain limits when hot cracks appear and also the sensitivity to hot cracking by counting the number and measuring the length of the individual cracks as a means to differentiate between the weldability of different alloys. Supports are usually recommended to avoid hinging and to use test plates thicker than 10 mm in order to minimize the influence of the compression strains (lower part of the bent specimen) on the weld cracking at the bending. The cracking response of two precipitation hardening Nickel-based superalloys—ATI 718Plus® and Haynes® 282®—was analysed in the context of the actual tensile/compression ratio imposed and measured by strain gauges attached to the upper and lower surface of the test plates. It was found that no influence of the compressive strains on the cracking response in Varestraint testing takes place. It was also seen that the hot cracking susceptibility of Haynes® 282® is lower compared to that of ATI 718Plus®.
Joel Andersson, Jonny Jacobsson, Anssi Brederholm, Hannu Hänninen

Towards Establishment of Weldability Testing Standards for Solidification Cracking

The establishment of standards for weldability testing would be desirable to both industrial and academic research laboratories. This would have the obvious advantage of allowing data to be reliably compared between different research labs. But making decisions regarding standards requires some careful thought and agreement on (i) how test parameters affect test results, (ii) what exactly needs to be measured, and (iii) how test results should be interpreted and reported. Our depth of understanding on these points has matured significantly over time and while there is not always universal agreement, it is at least possible to identify factors important to standards. This paper examines these factors, including:
Welding Parameters. When comparing different alloys having different thermal characteristics, the use of constant welding parameters (common practice) will result in variable weld penetration and weld pool shape. This can influence grain shape and mushy zone size, which can result in inequitable weldability comparisons.
Restraint. Welding on test coupons having different dimensions can affect restraint, which will influence the strain fields around a moving weld pool. Variation in test fixtures may also affect restraint. High restraint does not always result in higher crack susceptibility.
Travel Speed. Use of high travel speed gives tear-drop shaped weld pools that are more susceptible to cracking, and it reduces the size of the mushy zone. However, high speed can also result in grain refinement in certain alloys, giving improved weldability. Speed can also shift the location of compression/tension regions behind the weld pool.
Rate of Loading. Extrinsic tests, such as the Varestraint Test, involve the controlled application of a strain during welding. The rate of loading, relative to the weld travel speed, can influence crack length due to crack extension during loading.
Cracking Index. Selection of an appropriate cracking index is required for data analysis. Quantification of crack length and brittle temperature range are common indexes used for comparison. Threshold strain and critical strain rate are additional indexes. How well these indexes actually represent weldability remains unclear.
This paper will examine and quantify these issues in detail, thus providing the reader with a comprehensive appreciation of all things that must be considered when preparing a standardized procedure for weldability testing.
N. Coniglio, C. E. Cross

Use of Computational and Experimental Techniques to Predict Susceptibility to Weld Cracking

A range of computational and experimental techniques has been employed to determine relative susceptibility to various types of weld metal cracking. These techniques can be valuable tools that facilitate both filler metal selection and filler metal development. A combination of thermodynamic and kinetic modeling is employed alongside experimental validation. In some cases, a design of experiment can be used to reduce the number of experiments and optimize a filler metal composition. Two examples will be given; one regarding solidification cracking in Ni-base alloys, and the other on local brittle zone formation in dissimilar metal overlay of a Ni-base alloy on carbon steel. High-chromium, Ni-base filler metals are used for construction and repair applications of nuclear power plants based on their exceptional corrosion resistance. Niobium is added to these alloys to improve resistance to ductility dip cracking by formation of NbC. Additions of Nb also cause low melting point eutectics to form at the end of solidification, which increases susceptibility to solidification cracking. Alternative carbide formers have been investigated using a design of experiment methodology along with ThermoCalc-Scheil simulations to determine the potential for solidification cracking based on the magnitude of the solidification temperature range. Compositions were optimized and then verified using a combination of button melting and small scale weldability testing. Dissimilar metal welds are widely used in the petrochemical industry to improve corrosion resistance and facilitate field fabrication of welded structures. For some metal combinations, such as Ni-base alloys to steels, service failure can occur at the fusion boundary. This failure is related to a brittle zone that forms due to carbon diffusion from the steel towards the interface during postweld heat treatment (PWHT). Carbon diffusion during PWHT has been modeled using DICTRA®. Hypothetical alloy combinations have been simulated with this model in order to demonstrate the influence of carbon content and PWHT conditions. Examples will be given of dissimilar combinations that reduce the potential for carbon migration during PWHT that avoid brittle zone formation at the interface.
A. T. Hope, J. C. Lippold

Hot Cracking Phenomena II—Design Considerations


Considerations for Sound Parameter Studies in Electron Beam Welding of Thick Walled Components

Joining new materials, material combinations or geometries by means of electron beam welding requires some preliminary parameter studies to assess the proper welding range. Determining parameters for thick walled applications is usually done by performing bead on plate welds. Due to resource efficiency, several welds are placed in a single block. After welding, the block is cut perpendicularly to the welding direction to judge the geometry of the molten zone and the influence of the used parameter combination on the joint quality. In different bead on plate welding experiments for several steels, the cross-sections revealed cracks, located in the centerline of the weld seam. In further experiments it turned out that these failures were not fully repeatable and therefore could not be attributed to the welding parameters or on the material solely. The experimental setup as a whole is crucial. Therefore, experiments were planned and performed to investigate the issues of these centerline cracks in EBW bead on plate welding studies. This contribution documents the performed failure case analysis. It shows that these cracks where not caused by miscalculated welding parameters or due to the metallurgic sensitivity of the material. Cracks appearance occurs however by reasons of a combination of shrinkage strain and dendritic solidification. Furthermore this document provides some recommendations to avoid this defect and beware of misleading interpretation of welding trials.
C. Wiednig, N. Enzinger, C. Beal

Consideration of Welding-Specific Component Design on Solidification Crack Initiation

A commonly used way of minimizing the occurrence of hot cracks, especially solidification cracks during component welding, is mainly to analyse and vary process parameters such as welding speed and consequently the heat input. Metallurgy and component design are however hardly ever considered due to special production requirements and therefore, restricted flexibility in material selection and design. Such conditions, especially crack-critical welding positions are given by slot-welds or welds near pre-deformed areas, for instance bending edges. Hence, it follows that increased local and global residual component stress caused cracking on reaching a solidification crack critical level, which is characterised by solidification crack initiation.
Christian Gollnow, Thomas Kannengießer

Prediction of Ductility-Dip Cracking in Narrow Groove Welds Using Computer Simulation of Strain Accumulation

Ductility-dip cracking (DDC) in high chromium nickel-base weld metals has been an issue during fabrication and repair of nuclear power plant components for many years. DDC is a solid-state cracking phenomenon, and several theories [110] have been proposed for the mechanism. Research conducted to develop these theories has primarily been performed using test methods involving small-scale specimens that may not replicate actual welding conditions (e.g., strain-to-fracture, hot-ductility, and varestraint). Due to the complexities of welding, there are potentially significant differences in the strain, strain-rates, stresses, and thermal cycles that can occur between these small-scale test methods and actual welding conditions. To eliminate this uncertainty, a high-restraint, narrow groove weld mockup was developed to assess DDC in this work. Filler metals 52 and 52M (AWS specifications ERNiCrFe-7 and ERNiCrFe-7A, respectively), compositions considered susceptible to DDC, are deposited with cold wire GTAW in a narrow groove with precise heat input and bead placement controls to isolate the occurrence of DDC to a known region of the weld deposit. Computer modeling using SysWeld with validated weld parameter inputs was also performed to simulate the narrow groove weld. Comparison of test specimens to computer simulations shows that the highest occurrence of DDC is in weld regions with multiple reheat cycles and high strain accumulation. This and future work is intended to develop a method to predict DDC susceptibility in multi-pass welds and to develop procedures and techniques that minimize the occurrence of DDC.
Steven L. McCracken, Jonathan K. Tatman

Hot Cracking Phenomena—Materials


Ductility-Dip Cracking Susceptibility of Commercially Pure Ni and Ni-Base Alloys Utilizing the Strain-to-Fracture Test

The ductility dip cracking (DDC) response of commercially pure Ni and two solid-solution strengthened Ni-base alloys was conducted using the strain-to-fracture (STF) test method that was developed at Ohio State University by Nissley and Lippold (Welding J 82(12):355–364, 2003 [1]). Alloys 200, 600, and 625 were tested over a range of strain at 950 °C to determine the threshold strain for fracture (εmin) and the cracking response as a function of strain. Using established procedures, samples were prepared with an autogenous gas-tungsten arc (GTA) spot weld in order to provide consistent grain boundary character among samples and then tested using a Gleeble® thermo-mechanical simulator. In order to evaluate the effect of oxygen on DDC susceptibility, spot welds were made using Ar-2 % O2 shielding gas. It was found that Ni-200 was most susceptible to DDC due to the large grain size and non-tortuous migrated grain boundaries (MGB’s). Alloy 625 was extremely resistant to DDC, as reported by Zhang et al. (Trans JWRI 14(2):325–334, 1985 [2]), exhibiting a strain threshold greater than 15 %. The addition of oxygen reduced DDC resistance in alloys 600 and 625. Metallographic analysis showed that DDC initiates at grain boundary triple points and then propagates along straight migrated grain boundaries. SEM fractography revealed that crack surfaces exhibited smooth intergranular features with little evidence of ductile rupture.
Vern C. Kreuter V, John C. Lippold

Evaluation of Solidification Cracking Susceptibility in Austenitic Stainless Steel Welds Using Laser Beam Welding Transverse-Varestraint Test

In order to quantitatively evaluate the solidification cracking susceptibility in laser welds of type 310S and type 316L stainless steels, the Varestraint testing system for laser beam welding (LBW transverse-Varestraint test) was newly constructed. The timing-synchronisation among the laser oscillator, welding robot and hydraulic pressure devices was established by employing high-speed camera observation together with electrical signal control among the three components. Moreover, the yoke-drop time measured by high-speed camera was compensated to prevent underestimation of the crack length. The LBW transverse-Varestraint test was conducted varying the welding speed from 10.0 to 40.0 mm/s, and the transverse-Varestraint test with gas tungsten arc welding was also performed varying the welding speed from 1.67 to 5.00 mm/s. As the welding speed increased from 1.67 to 40.0 mm/s, the solidification brittle temperature range (BTR) of type 310S stainless steel welds was reduced from 146 to 120 K, while the BTR enlarged from 36 to 49 K in type 316L stainless steel welds. A numerical simulation of the solid/liquid coexistence temperature range, using solidification segregation model combined with the Kurz–Giovanola–Trivedi model, explained the mechanism of the BTR shrinkage in type 310S stainless steel welds by reduction of the solid/liquid coexistence temperature range of the weld metal due to the inhibited solidification segregation of solute elements and promoted dendrite tip supercooling attributed to rapid solidification in the LBW process. The reason why the BTR enlarged in type 316L stainless steel welds could be clarified by the enhanced solidification segregation of solute elements (mainly P and S), corresponding to the decrement in δ-ferrite content at the solidification completion in the weld metal. It follows that the opposite tendencies on solidification cracking susceptibility with increasing the welding speed in LBW could be explained by the different solidification segregation behaviour of solute elements, closely related with the δ-ferrite content.
Eun-Joon Chun, Hayato Baba, Kazutoshi Nishimoto, Kazuyoshi Saida

Comparative Evaluation of Mesoscale Sensitivity to Crack Formation in Multi-pass Welds Using Wires IN52 and IN52MSS

Assessment of susceptibility of multipass welds, produced by using wires ERNiCrFe-7 (IN52) and ERNiCrFe-13(IN52MSS), to crack formation was carried out. The PVR test was used to determine susceptibility to cracking. This test uses forced deformation of specimens being welded at a variable rate where the criterion of sensitivity to crack formation is the presence of cracks and their number at the specimen surface as a function of the rate of forced deformation. Welds made using IN52 have high sensitivity to the formation of ductility-dip cracks (DDC) in the weld metal heat-affected zone (HAZ) as compared with IN52MSS, which were not sensitive to the formation of cracks of this type at the same range of deformation rates. The characteristics of plastic deformation of both the weld metals were determined using transmission microscopy and electron backscattered diffraction (EBSD).
K. A. Yushchenko, V. S. Savchenko, A. V. Zvyagintseva, N. O. Chervyakov, L. I. Markashova

Welding Optimization of Dissimilar Copper-Aluminum Thin Sheets with High Brightness Lasers

Dissimilar welding of Al and Cu represents a big challenge due to the formation of brittle intermetallic phases. The present study is focused on the laser beam welding optimization of Al-Cu thin sheets using high brightness lasers. Joints were produced with welding speeds as fast as 12 m/min, characterized both mechanically and electrically, with microstructure analysis. The results exhibit that when Al is on top of Cu, the weld volume is large and several microcracks show up due to the formation of CuAl2 (θ), CuAl (η2) and Cu9Al4 (α) phases. When Cu is on top of Al instead, the weld volume is small and just CuAl2 (θ) phase is formed; no cracks are observed. This behavior is explained through the correlation between the weld volume, the different interaction of Al and Cu with the laser beam and the intermetallic phases generated in the weld.
Fidel Zubiri, María del Mar Petite, Jaime Ochoa, María San Sebastian

Elevated Temperature, Solid-State Cracking in Welds

A number of elevated-temperature, solid-state cracking phenomena are associated with welded fabrication. These include ductility dip cracking which generally occurs during multipass welding, and reheat cracking which is usually associated with postweld heat treatment. Reheat cracking includes stress relief cracking of steels, strain-age cracking of Ni-base alloys, and relaxation cracking of stainless steels and Ni-base alloys. This paper describes the mechanism associated with each of these forms of cracking and methods to avoid such cracking. Weldability tests that can be used to quantify susceptibility to the various forms of cracking will also be described.
John C. Lippold

Hot Cracking Susceptibility of Ni-Base Alloys


Weldability Evaluation of High Chromium, Ni-Base Filler Metals Using the Cast Pin Tear Test

High chromium, nickel-base filler metals have been commonly used throughout the nuclear power industry for the weld overlay repair of dissimilar metal welds. These alloys provide optimum resistance to primary water stress corrosion cracking in nuclear power plant cooling systems. However some of these nickel alloys present weldability challenges including susceptibility to solidification cracking and ductility dip cracking. ERNiCrFe-7A (52M) and ERNiCrFe-13 (52MSS) filler metals, including two heats of 52M and one heat of both 52MSS and 690Nb, have been evaluated in this study. The susceptibility to solidification cracking was evaluated using the cast pin tear test (CPTT). The CPTT was also used to evaluate the effect of dilution between two heats of ERNiCr-3 (FM82) on the solidification cracking behavior. Metallurgical characterization using light optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) in the SEM has been performed in order to identify solidification cracking mechanisms, and to study the effect of liquid film formation and backfilling on cracking susceptibility.
Eric Przybylowicz, Boian Alexandrov, John Lippold, Steven McCracken

Multi-scale Modeling of the Stress-Strain State During Welding of Ni-Based Alloys

Numerical modeling of changes of stress-strain state in welding of alloys of Ni-Cr–Fe alloying system at macro- and mesoscales was carried out. The kinetics of changes of stresses and deformations in the weld and heat-affected zone at sites of probable formation of hot cracks was considered. Calculations data of the stress-strain state at the macroscale were used in modeling of thermal-deformation processes at the mesoscale. During modeling the experimental data on anisotropy of physical properties were used depending on crystallographic orientation of grains in the heat affected zone of a real welded joint. The modeling was performed considering the changes of properties of the material depending on temperature. It was shown that depending on anisotropy of physical and mechanical properties of the metal in the limits of neighboring grains, the non-uniform distribution of plastic deformation becomes apparent. The change of deformation exhibits a gradient with localization of deformation near the grain boundaries.
K. A. Yushchenko, V. S. Savchenko, N. O. Chervyakov, A. V. Zvyagintseva, E. A. Velikoivanenko

Weldability of Cast and Wrought Nickel Base Alloys 59, 617 and 625

In this study, the hot cracking susceptibility of centrifugal and sand cast nickel base alloys was evaluated compared to the wrought products. Three solid-solution strengthened nickel base alloys—alloy 59, alloy 625 and alloy 617—were studied with respect to the formation and propagation of hot cracking in the cast and wrought microstructure. Hot cracking tests were performed by PVR (programmable deformation crack) test to rank the cracking susceptibility of the cast base metals against the wrought equivalents. Further investigation on the crack susceptible region in the base metal heat-affected zone (HAZ) of cast and wrought alloy 625 was conducted by hot ductility testing using a Gleeble® system. PVR testing indicates a considerably higher hot cracking susceptibility of the cast nickel base alloys compared to the wrought forms of the same composition. The much more severe HAZ liquation cracking of the cast alloys is attributed to the presence of low melting constituents or constitutional liquation of constituents in the large grained and very segregated cast microstructure. Results of hot ductility testing showed that the brittle temperature range (BTR) of the cast alloy 625 is almost three times as wide as for the wrought equivalent. Indicating a much higher hot cracking susceptibility of the cast alloy, this is in good correlation to the PVR test results. Metallurgical and fractographic evaluation of hot ductility samples revealed that in the cast microstructure severe liquation initiates in the vicinity of MC (NbC) carbides. This carbide constitutional liquation reaction leads to the formation of grain boundary liquid films providing a strong driving force for HAZ liquation cracking in cast alloy 625.
C. Fink, M. Zinke, S. Jüttner

Cold Cracking Phenomena


Numerical Investigations on Hydrogen-Assisted Cracking in Duplex Stainless Steel Microstructures

Duplex stainless steels (DSS) are used in various industrial applications, e.g. in offshore constructions as well as in chemical industry. DSS reach higher strength than commercial austenitic stainless steels at still acceptable ductility. Additionally, they exhibit an improved corrosion resistance against pitting corrosion and corrosion cracking in harsh environments. Nevertheless, at specific conditions, as for instance arc welding, cathodic protection or exposure to sour service environments, such materials can take up hydrogen which may cause significant property degradation particularly in terms of ductility losses which, in turn, may entail hydrogen-assisted cracking (HAC). The cracking mechanism in DSS is different from steels having only a single phase, because hydrogen diffusion, stress-strain distribution and crack propagation are different in the austenite or ferrite phase. Therefore, the mechanism of HAC initiation and propagation as well as hydrogen trapping in DSS have not been fully clarified up to the present, as for most of the two-phase microstructures. At this point the numerical simulation can bridge the gap to a better insight in the cracking mechanism regarding the stress-strain distribution as well as hydrogen distribution between the phases, both austenite and ferrite, of the DSS. For that purpose, a two dimensional numerical mesoscale model was created representing the microstructure of the duplex stainless steel 1.4462, consisting of approximately equal portions of austenite and ferrite. Hydrogen assisted cracking was simulated considering stresses and strains as well as hydrogen concentration in both phases. Regarding the mechanical properties of austenite and ferrite different statements can be found in the literature, dependent on chemical composition and thermal treatment. Thus, various stress-strain curves were applied for austenite and ferrite simulating the HAC process in the DSS microstructure. By using the element elimination technique crack critical areas can be identified in both phases of the DSS regarding the local hydrogen concentration and the local mechanical load. The results clearly show different cracking behavior with varying mechanical properties of austenite and ferrite. Comparison of the results of the numerical simulation to those of experimental investigations on DSS will improve understanding of the HAC process in two phase microstructures.
T. Mente, Th. Boellinghaus

Hydrogen Assisted Cracking of a Subsea-Flowline

Since the mid-nineties, supermartensitic stainless steels (SMSS) have increasingly been applied to welded subsea-pipeline systems in the North Sea oil and gas fields, especially to flowlines at mild sour service conditions. However, in 2001 cracking and leaks occurred during installation and service start-up of two SMSS flowlines in the Norwegian Tune gas condensate field, welded with a new developed matching filler wire. Brittle transgranular cracking started especially at inter-run lack of fusion and propagated brittle, predominantly through the weld metal. The present paper provides a brief overview of the original failure case and respective sequence of events leading to complete replacement of the SMSS by carbon steel flowlines in 2002. Then, detailed investigations of a circumferential weld sample of the failed Tune flowline are highlighted, targeted at comparison of the failure appearance to previous investigations of this filler material type and to search for possible explanations for the brittle fracture at the crack initiation area. SEM investigations of the fracture surface revealed brittle areas only in the direction towards the top side of the weld while the major part of the investigated surface exhibited ductile fracture. As an approach to clarify, if the fracture was a consequence of hydrogen assisted cracking, five small sized specimens have been cut out of the original sample. Cracking has been introduced parallel to the original fracture surface in these specimens at respective saw cuts and bending. The results show that brittle transgranular cracking appeared only in the specimen cooled down to very low temperatures by liquid nitrogen and in the sample charged with hydrogen to an average concentration of about 15 ml/100 g. However, a fracture similar to the original surface was observed only in the hydrogenized specimen. As a further result, very similar fracture surfaces of supermartensitic stainless steel weld metals had been observed on specimens subjected to hydrogen assisted cold cracking (HACC) as well as to hydrogen assisted stress corrosion cracking (HASCC). In total, the results indicate that brittle fracture starting at the inter-run lack of fusion were not initiated by high notch tip deformation rates, but rather influenced by hydrogen, probably taken up during welding.
Th. Boellinghaus, E. Steppan, T. Mente

Stress Corrosion and Cold Cracking Phenomena


Numerical Modelling of Hydrogen Assisted Cracking in Steel Welds

Hydrogen assisted stress corrosion and cold cracking represent still a major topic regarding the safety of welded steel components against failure in many industrial branches. Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or at cathodic protection. Additionally, understanding and avoidance of hydrogen entry into weld microstructures from gaseous pressurized environments becomes increasingly important for renewable energy components. There are two types of metallurgical mechanisms associated with hydrogen assisted cracking, i.e. the cracking as well as hydrogen transport and trapping mechanisms. For numerical modelling, it has to be considered that both types are not independent of each other, that the mechanisms are not yet completely clarified and that validation of such models strongly depends on implementation of the correct hydrogen related materials properties. However, quite significant achievements have been made in modelling of hydrogen assisted cracking by indirect coupling of thermal, stress-strain as well as hydrogen uptake and diffusion analyses. After a brief introduction into the subject and by revisiting various proposed cracking mechanisms, the present contribution focuses on recent developments of a numerical model based on a comparison of actual hydrogen concentrations and mechanical loads with respective hydrogen dependent material properties as crack initiation and propagation criteria. The basic procedure for numerical simulation of crack initiation and propagation is outlined and it is shown how such numerical simulations can be validated experimentally. Furthermore, it is highlighted how such a procedure has been extended to a comprehensive model for life time prediction of welded steel pipeline components and experimentally verified. Finally, it is outlined how the model can be extended to simulate cracking in heterogeneous steel microstructures on the different scales.
Th. Boellinghaus, T. Mente, P. Wongpanya, E. Viyanit, E. Steppan

Metallurgical Factors Influencing the Susceptibility of Hydrogen Assisted Cracking in Dissimilar Metal Welds for Application Under Cathodic Protection

Dissimilar metal weld (DMW) overlays of Nickel-based filler metals on low-alloy steel pipes are used in the oil and gas industry in order to eliminate the need for field post weld heat treatment (PWHT) of adjacent closure welds. Brittle failures have been reported along some of these DMW interfaces, especially in AISI 8630-IN625 weld combinations during subsea service under cathodic protection (CP). These failures have been attributed to hydrogen assisted cracking (HAC) due to local hydrogen embrittlement of susceptible microstructures that form at the fusion boundary during welding and PWHT. Testing at The Ohio State University using the delayed hydrogen cracking test (DHCT) has concluded that this type of HAC is strongly affected by the base metal/filler metal combination, and by the welding and PWHT procedures. These controlling factors determine the microstructures that form at the fusion boundary. Thermo-CalcTM and DictraTM software was used to simulate the above-mentioned controlling factors. Based on the simulations conducted, carbon concentration near the fusion boundary was monitored closely. The results of the simulations yielded results that correlate well with DHCT experiments. Also, the controlling factors of HAC can be optimized to reduce the accumulation of carbon at the fusion boundary.
D. Bourgeois, B. Alexandrov, J. Lippold, J. Fenske

Hydrogen Trapping in Supermartensitic Stainless Steel TIG Welds

A number of common defects in stainless steel welding are the result of the presence of hydrogen in the weld. In addition, the service life of the stainless steel joints is significantly dependent on the presence of hydrogen in the respective environment and the susceptibility of the various weld microstructures to hydrogen degradation. Hydrogen’s effects on various Tungsten Inert Gas (TIG) welded SMSS microstructures are investigated by means of X-ray diffraction (XRD) and optic (OM) and electron microscopy (SEM). A number of methods for estimating the amount of absorbed hydrogen have been employed. Hydrogen interaction with structural defects and the characteristics of hydrogen desorption have been studied by means of thermal desorption spectroscopy (TDS). The effects of the respective microstructure on hydrogen absorption and desorption behavior is discussed in detail. The powerful abilities of TDS for studying the absorption/desorption behavior and trapping effects in supermartensitic stainless steel TIG welds is also examined.
Th. Boellinghaus, D. Eliezer

Elevated Temperature Solid-State Cracking


Stress-Relief Cracking in Simulated-Coarse-Grained Heat Affected Zone of a Creep-Resistant Steel

Cracking has been reported in newly constructed water wall panels of fossil power plants during startup testing. Both high hardness (exceeding 350 HV) and high level of welding residual stress have been reported in welds of waterwall panels made of T23 and T24 steels. Stress-relief cracking (SRC) is being considered as a possible failure mechanism during high temperature exposure such as PWHT. High temperature exposure of non PWHT-ed welds of Grade T23 and T24 steels leads to hardening in the weld and coarse-grained heat-affected zone (CGHAZ). It has been suggested that such a hardening mechanism can lead to stress-relief cracking (SRC). The objective of this study is to evaluate the susceptibility to SRC in the coarse grained heat affected zone (CGHAZ) of Grade T24 steel utilizing a Gleeble-based SRC test developed at The Ohio State University. The strain-age cracking test developed at The Ohio State University was modified in order to better replicate the conditions of PWHT in highly restrained welds and quantify the stress-relief cracking susceptibility in creep resistant steels. In addition to reduction in area and time to failure, the modified test allows quantification of the stress and strain that cause failure during SRC testing. This test utilizes the Gleeble® 3800 thermo-mechanical simulator. SRC testing of simulated-CGHAZ in Grade T24 Steel has revealed ductile failure for the sample tested at 600 °C, predominantly intergranular with ductile features for the sample tested at 650 °C, and brittle intergranular failures for the samples tested at temperatures of 675 °C and above. For PWHT above 600 °C at residual stress levels close to the yield stress, the CGHAZ in Grade T24 steel welds may be susceptible to SRC.
Katherine Strader, Boian T. Alexandrov, John C. Lippold

Testing Approaches for Stress Relaxation Cracking in Gamma-Prime Strengthened Ni-Base Alloys

Increasing the efficiency and reducing the emissions of coal-fired power plants is most economically accomplished by increasing maximum steam temperatures and pressures. Designs have progressed beyond the temperatures where typical power plant steels are useful and Ni-based superalloys are being investigated for use in the highest temperature areas. Construction with superalloys is common in the aerospace industry where solution annealing heat treatments following welding may be done in a furnace. In power plant construction, solution annealing is difficult with standard postweld heat treatment techniques and directly aging weldments without solution annealing has been studied. The practice of directly aging weldments does not allow for residual stress relaxation to occur before gamma prime precipitation. Simultaneous stress relaxation and precipitation can lead to stress relaxation cracking via a creep mechanism. There is very limited experience with superalloys regarding power plant construction and there is some evidence that stress relaxation cracking could be a problem. Stress relaxation cracking testing approaches, which are not standardized, can be categorized as self-restrained or externally-loaded. Tests typical of each approach are currently underway. Externally-loaded tests typically achieve failure in a short duration by applying uniaxial loads (or strains) and this practice allows some level of quantification. Self-restrained tests require much longer times but can be used to study triaxial stress states. Ongoing testing of a number of Ni-base alloys including 740H, 282, 617, 718 and Waspaloy is reviewed.
David C. Tung, John C. Lippold
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