Continuum damage mechanics analyses of type IV creep failure in ferritic steel crossweld specimens

A major high temperature failure mechanism for weldments in ferritic steel steam pipework is circumferential creep cracking within the region of the heat affected zone, adjacent to the parent material, that experiences the lowest temperatures during the welding process. This is commonly known as type IV cracking. In recent years a number of experimental studies have investigated the occurrence of type IV failure in laboratory test pieces, however, there have been few attempts at theoretical modelling of type IV failure to assist in the formulation of design and assessment procedures. This report discusses the use of the creep continuum damage mechanics method for the analysis of the deformation and failure of weldments that are known to fail within the type IV region.The creep behaviour of each of the material regions of a weldment is described with a set of physically based constitutive equations, which incorporate a number of state variables. The finite element creep continuum damage mechanics method is used, with the physically based constitutive equations, to analyse the deformation and failure of the welded testpieces. The computations are shown to be in good agreement with the experimental results. The implications of the analyses are discussed with reference to the assessment of weldments that are susceptible to type IV failure.     

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A_c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.     

Effect of welding process parameters on embrittlement of Grade P92 steel using Granjon implant testing of welded joints

Precipitation of Cr-rich carbides, diffusible hydrogen content and heterogeneous microstructure formation across the weldments makes heat-affected zone (HAZ) susceptible to intergranular cracking and makes weldability of creep strength enhanced ferritic (CSEF) Grade P92 steel a critical issue. In the present research work, the Granjon implant test and mercury method (for diffusible hydrogen measurement) have been performed on Grade P92 steel welded specimens to study the effect of welding parameters on diffusible hydrogen levels and their subsequent effect on hydrogen-assisted cracking (HAC). The weld metal was deposited by a shielded metal arc welding process on Grade P92 steel samples using P92 matching filler. The three different welding conditions are used to measure the diffusible hydrogen level in the deposited metal. Granjon implant test was performed to evaluate HAZ HAC susceptibility with similar welding conditions which were used in the mercury method. Lower critical stress (LCS) was also evaluated using the Granjon implant test. The higher susceptibility of CSEF Grade P92 steel welded plate towards HAZ HAC was noticed in case of lower heat input or higher diffusible hydrogen content. However, by considering LCS, fracture mode and diffusible hydrogen content, the weld deposited using the highest heat input (condition III) offers great resistance to HAZ HAC.     

Analysis of cross-weld creep rupture data on the ½Cr½Mo¼V type IV zone

For the ferritic steel ½Cr½Mo¼V, the operating experience of medium and long term creep failures in power station welds has been of cavitation and cracking in the heat affected zone (HAZ) adjacent to the parent material, in what is referred to as the Type IV zone. This experience has led to the generation of uniaxial cross-weld creep rupture data on ½CrMoV weldments and to the development of life assessment procedures such as R5 Volume 7, which uses the rupture strength of the Type IV zone to calculate the life of power station components. Recently, the ECCC Working Group 3A has conducted a collation of the UK and German cross-weld creep rupture data on ½CrMoV weldments. Creep failure in ½CrMoV cross-weld specimens occurs in a variety of weldment zones; typically in the parent, the Type IV zone or the weld metal. Post test examination of the specimens has enabled those tests that failed in the Type IV zone to be identified and a creep rupture data assessment has been performed to derive a new model for the rupture strength of the Type IV zone.     

Structural integrity assessment of weldments at high temperature: A proposed new approach for R5

The R5 procedures provide a comprehensive methodology for the assessment of structures operating within the high temperature creep regime. This includes advice on the modifications required to the basic procedure to account for weldments in creep–fatigue crack initiation assessments. The current approach is based on the use of a Fatigue Strength Reduction Factor (FSRF) which has a value according to the particular class of welded joint. The FSRF affects the calculation of creep and fatigue damage. However, the current approach can be excessively conservative for as-welded weldments which are the main type of weldments in plant.     

HR3C不锈钢采用镍基焊材的焊接性能试验

选用ERNiCrCoMo-1镍基焊材为HR3C不锈钢的焊接接头填充金属,通过焊接接头的常温力学性能、高温瞬时拉伸性能及高温持久性能试验,结果表明焊接接头的常温、高温力学性能均满足标准的规定,完全可以替代进口焊接材料YT-HR3C.     

The behaviour of ferritic weldments in thick section pipe at elevated temperature

A major collaborative research programme is being carried out within the CEGB to examine the correlation between data, produced from a range of test methods, which are currently used in the design of welded steam pipes. In the part of the programme reported here, the elastic and creep deformation occurring in low alloy ferritic steel pipe-to-pipe weldments has been studied in pressure vessel experiments conducted at 565°C and 455 bar internal steam pressure. The welds were made in parent pipe using mild steel and low alloy 1CrMo, 2CrMo and weld metals. All the weldments were post-weld heat treated for 3 h at 700°C prior to testing. In addition, the weldments, represented as parent material, heat affected zone and weld metal, have been analysed to determine stresses and strains using a finite element three-material model.The main features of the macro- and micro-structures of the four weldments are briefly described. Results are then presented for the elastic and creep deformations observed in both the hoop and axial directions in the weldments. The experimental creep strain data are then used as a basis for calculating the stationary state stresses present on the surface of the weldments. The surface stationary state stress distribution and corresponding steady state strain rates, determined using the finite element model, are then presented.The pressure vessel experimental results and the data from the finite element analysis are discussed in terms of the hoop and axial deformation in the weldments. An assessment is then made of the correlation between the results from the experimental and analytical approaches. Finally, the practical implications of the present results are considered with respect to the design of operating plant.     

Modelling of creep in friction stir welded copper

Abstract

Copper canisters for storage of nuclear waste will be exposed to creep. The canisters will be closed with friction stir welding (FSW). To describe the creep behaviour of the welds, uniaxial creep tests have been performed. A previously developed fundamental creep model for parent metal is applied to the different weld zones. The differences in microstructure and yield strength between the weld zones are taken into account. Creep strain versus time curves for the weld zones have successfully been predicted without the use of any adjustable parameters. It should be noted that the temperature range of interest of 50–100°C is deep down in the power law break down regime with Norton exponents between 25 and 100. The constitutive equations are used in FEM computations of creep in the canister weldments.     

Assessment of creep behaviour of a narrow gap weld

Finite element creep analyses have been performed using Norton's secondary creep law and continuum damage equations for a thick-walled narrow gap pipe weld. The creep stress and failure life were obtained using combinations of material properties which allowed different strengths for the parent material, heat-affected zone and weld metal for 1/2Cr1/2Mo1/4V: 2 1/4Cr1Mo welds. The stationary-state failure prediction was performed based on a steady-state rupture stress. The failure times obtained from continuum damage modelling were used for assessing the results and the accuracy of the steady-state approach. The results show that the creep stress and failure life of the narrow gap weldments of both materials data options were similar to those of the V-shaped weldments over a range of narrow gap weld widths from 8 to 12 mm. The conservatism of the steady-state analysis method is illustrated, from the life estimates. The data also allow estimation of a possible effect of the presence of a weld in a plain pressurised pipe.     

Structural behaviour of a welded superalloy cylinder with internal pressure in a high temperature environment

Steady and cyclic creep tests with internal pressure were performed at temperatures of 800 to 1000°C on Hastelloy X cylinders with and without a circumferential Tungsten Inert Gas (TIG) welding technique. The creep rupture strength of the TIG welded cylinders was much lower than that of the non-welded cylinders whilst creep rupture strength reduction by the TIG technique was not observed in uniaxial creep tests. The reason for the low creep strength of welded cylinders is discussed and it is noted that the creep ductility of weld metal plays an essentially important role.In order to improve the creep strength of the TIG welded cylinder, various welding procedures with assorted weld metals were investigated. Some improvements were obtained by using welding techniques which had either Incoloy 800 or a modified Hastelloy X material as the filler metal.     

Creep fatigue crack development in dissimilar metal welded joints between steels and nickel based alloy

Abstract

Creep and strain controlled cyclic/hold creep fatigue tests have been performed at temperatures in the range of 550–575°C on specimens extracted from dissimilar metal welded (DMW) joints between two classes of steel and a nickel based alloy. The details and results of the tests are described. While crack development in the cyclic/hold creep fatigue test specimens tends to be creep dominated, the microstructural paths followed in the steels in the vicinity of their heat affected zones are not identical to those observed in creep rupture testpieces taken from the same DMW joint. In pure creep tests, cracking may occur adjacent to the fusion line and/or in the fine grain heat affected zone (FGHAZ), with rupture location being dependent on temperature stress and microstructural condition. In contrast, creep dominated creep fatigue cracking typically occurs in the intercritical heat affected zone/FGHAZ or the overtempered parent material on the steel side of such weldments, depending on the composition of the joint.     

The behaviour of ferritic weldments in thick section 12Cr12Mo14V pipe at elevated temperature

A major collaborative research programme is being carried out within the CEGB to examine the correlation between data, produced from a range of test methods, which are currently used in the design of welded steam pipes. In the part of the programme reported here, the elastic and creep deformation occurring in low alloy ferritic steel pipe-to-pipe weldments has been studied in pressure vessel experiments conducted at 565°C and 455 bar internal steam pressure. The welds were made in $\text{1}\text{2}\text{Cr}\text{1}\text{2}\text{Mo}\text{1}\text{4}\text{V}$ parent pipe using mild steel and low alloy 1CrMo, 2CrMo and $\text{1}\text{2}\text{CrMoV}$ weld metals. All the weldments were post-weld heat treated for 3 h at 700°C prior to testing. In addition, the weldments, represented as parent material, heat affected zone and weld metal, have been analysed to determine stresses and strains using a finite element three-material model.The main features of the macro- and micro-structures of the four weldments are briefly described. Results are then presented for the elastic and creep deformations observed in both the hoop and axial directions in the weldments. The experimental creep strain data are then used as a basis for calculating the stationary state stresses present on the surface of the weldments. The surface stationary state stress distribution and corresponding steady state strain rates, determined using the finite element model, are then presented.The pressure vessel experimental results and the data from the finite element analysis are discussed in terms of the hoop and axial deformation in the weldments. An assessment is then made of the correlation between the results from the experimental and analytical approaches. Finally, the practical implications of the present results are considered with respect to the design of operating plant.     

Creep damage zone development in advanced 9%Cr steel weldments

Details are given of a new technique for the mapping of creep damage zone development ahead of stress concentrations in laboratory testpieces and structures which have been loaded for long times at high temperatures. The technique has been developed as part of a project concerned with the characterisation of creep damage generated under multi-axial loading conditions in advanced martensitic 9%Cr steel weldments, but is applicable to other engineering materials.The approach may be effectively used to demonstrate how the creep damage tolerances of individual microstructural constituents determine the fracture path in welded structures operating at high temperatures. Damage zone sizes determined by this technique compare favourably with the results of analytical predictions when a consistent boundary criterion is adopted.     

Performance of repair welds on aged 2.25Cr–1Mo boiler header welds

In order to clarify the performance of repair welds on power boiler, thick parts such as header and steam piping, an ex-service aged 2.25Cr–1Mo header was repaired using SMAW with postweld heat treatment and the mechanical properties of the repair welded joints were experimentally evaluated.Creep rupture life of the repair welded joint was almost same as that of service-degraded base metal and heat-affected zones. It was proved that the life reduction would not be caused by repair welding. In creep–fatigue tests with strain holding, some type of repair welded joints was fractured at the heat affected zone caused by repair welding. This may be caused by strain concentration at the heat-affected zone under strain holding. Charpy impact toughness of the simulated heat affected zone due to repair welding was much higher than that of service-degraded base metal. It was proved that the toughness would be restored by repair welding.     

Creep crack growth of high temperature weldments

The experimental programme of the EC supported project (SMT 2070) SOTA aimed at addressing a technical and industrial need to provide guidelines for creep crack growth (CCG) testing and data analysis of weldments. Mechanical and creep properties were determined on two pressure vessel steels of P22 (2.25Cr1Mo) and P91 (9Cr1MoVNb). The specimens were taken from pipe welds for weld metal (WM) tests, and simulated heat affected zone (HAZ) material for the HAZ tests.The CCG tests were carried out on cross-weld compact tension (CT) specimens machined out from weldment of pipes. The tests were done at 550 and 600°C on P22 and P91 materials, respectively. The CT specimens were notched using electrical discharge method, for a sharp starter crack. This method of initiating sharp starter crack was chosen to make sure that all partners will test specimens with starter crack location as specified in the work programme to study crack initiation and growth in WM and HAZ (both in the centre and type IV region). The CCG tests were carried out following the ASTM E1457-92 [ASTM E1457-92, Standard test method for measurement of creep crack growth rates in metals, ASTM, Philadelphia, PA 19103, USA]. The partners assessed their data and sent both assessed and their raw data to be further assessed centrally. All the data from partners were analysed and compared with those of partners' own assessed data.The present paper reports on the analyses of CCG data obtained in the programme including six laboratories from six European countries. The programme addresses the differences and difficulties in testing and the assessment of weldments, and provides guidelines for harmonisation of testing procedures for reliable data production for remanent life assessment of plants with welded components.     

Laser arc hybrid weld microstructure in nickel based IN738 superalloy

Abstract

A study of the microstructural characteristics of laser arc hybrid welded nickel based IN738 superalloy was performed. Laser arc hybrid welding produced a desirable weld profile in the alloy, similar to what is usually obtained during laser beam welding, and no cracking occurred exclusively in the fusion zone. Elemental partitioning pattern in the fusion zone was studied by electron probe microanalysis and calculating the volume fraction of the weld metal that resulted from the consumption of the filler wire. The result showed that Ti, Ta, Nb, Mo, Al and Zr partitioned into the interdendritic regions of the fusion zone. SEM and TEM examination of the fusion zone showed the presence of secondary solidification reaction constituents, which consists of MC type carbides. The study further revealed that non-equilibrium liquation of various second phases that were present in the alloy prior to welding contributed to intergranular liquation in the heat affected zone (HAZ), which consequently resulted in extensive HAZ intergranular cracking during welding. Although laser arc hybrid welding appears promising for improving the weldability of nickel based IN738 superalloy, a suitable weldability improvement procedure is required in order to minimise HAZ intergranular cracking and thereby enhance the applicability of this technology to the joining of the superalloy.     

The tempering performance of low-alloy steel weldments

High-energy piping systems are frequently fabricated from steel components joined by multipass steel weldments. The creep behaviour of these weldments is then related both to the range of microstructures developed during manufacture and the effect of long-term, high-temperature exposure on microstructural changes. In the present work, laboratory aging experiments which simulate in-service tempering have been performed. Results indicate that for conditions equivalent to those encountered up to the nominal weldment design life, tempering effects are consistent with diffusion controlled coarsening of carbides. However, for more extreme situations, coarse grained ferrite of low hardness is developed within the weld heat-affected zone. The implications of these microstructural changes on high-temperature performance are discussed.     

A review of factors affecting toughness in welded steels

This review paper discusses the local toughness at various positions within a weldment in plain carbon, carbon-manganese, microalloy and low-alloy ferrite steels. Single pass weldments in such steels, which are austenitic where temperatures during welding exceed the critical values for that steel but which transform to other phases at lower temperatures, can be divided into six zones—subcritical heat-affected zone (HAZ), inter-critical HAZ, grain-refined HAZ, grain-coarsened HAZ, partially molten zone and weld metal.Microstructural and compositional factors which affect toughness are qualitatively reviewed for each of these zones, in both as-welded and heat-treated conditions. Some areas which require further research are pointed out.     

Assessing heat-affected zone properties of pressure vessel weldments using simulation techniques

This paper describes simulation methods for mechanisms occurring in the coarse grain heat affected zone (HAZ) of thick-walled weldments. The simulation process is divided into overheating simulation and simulation of stress-relief heat treatment. Overheating simulation is mainly practised by inductive or conductive heating and produces an HAZ-representative microstructure. For the stress-relief heat treatment simulation, annealing tests and constant load creep tests at stress-relief temperatures are carried out.Classifications in order of crack sensitivity and service-suitability are proposed. The order of crack sensitivity is derived from the magnitude of deformability and creep resistance in constant load creep tests. The order of service-suitability is based on the results of notch-impact tests (ISO-V) and tensile tests subsequent to both simulation treatments. The techniques are demonstrated on an example of an ASTM A 508 CI 2 (22 NiMoCr 37) plate material.     

Numerical investigation on the creep damage induced by void growth in heat affected zone of weldments

In structural welded joints after long-term service at high temperature, fracture occurs mainly in the fine-grained heat affected zone (HAZ). Recently, the nucleation and growth of creep voids in the fine-grained HAZ of weldments, recognized as Type IV fracture, have become an important problem for low alloy ferritic heat resisting steels. In this paper, a new constitutive model was introduced to analyze the creep damage development in HAZ induced by void growth. This model is based on the equations of continuum damage mechanics (CDM) and combines a micromechanism-based method to account for the void growth process, which is different from the previous studies of creep damage. By coding a user-defined material subroutine (UMAT) in the FEA software ABAQUS, the proposed model was used to investigate the creep damage development in HAZ of a multi-material cross-weld specimen and a medium bore welded branched pipe where four different material properties: base material, coarse-grained HAZ, fine-grained HAZ, and weld material, were taken into account.