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.     

Creep and fracture behaviour of English Electric Mark III transition welds: Part (b) Ex-service weldments

The microstructure and high temperature behaviour of an ex-service weldment have been assessed. Exposure at elevated temperature resulted in some carbide development in the niobium stabilised buttered layer. However, significant decarburisation in the low alloy steel was not detected. Cross-weld uniaxial creep testing invariably led to low ductility failures in the heat affected zone on the 2.25 Cr1Mo steel side of the weld. These failures were associated with the nucleation, growth and link-up of cavities on prior austenite grain boundaries. Assessment of tests interrupted at selected life fractions indicated that the development of cavitation followed a sensible trend with creep exposure. The cavitation behaviour of the transition weld was in general agreement with data obtained from creep tests undertaken on material heat treated to simulate heat affected zone structures. The level of creep damage in these welds can be assessed by evaluation of local strain accumulation or through monitoring the presence of cavities.     

Creep and creep-rupture behaviour of a bainitic 2 14 Cr-1 Mo steel

The creep-rupture behaviour of a bainitic $\text{2}\text{1}\text{4}$ Cr-1 Mo steel was determined at 454, 510 and 566°C. Tests were made on specimens taken from a commercial heat of steel in the normalised-and-tempered condition. In addition to the creep-rupture behaviour, secondary and tertiary creep behaviour were examined. A relationship between the onset of tertiary creep and rupture life was obtained. The results from these tests were compared with available data on annealed, normalised-and-tempered and quenched-and-tempered base metal and with tempered weld metal. The results of these comparisons gave insights into the types of metallurgical change that occur in $\text{2}\text{1}\text{4}$ Cr-1 Mo steel at elevated temperatures.     

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.     

Residual life assessment of steam turbine casing containing crack defect

In steam turbine casing made of cast steel of type 0.6Cr0.5Mo0.25V ( SN 42 2744), which run about 25% over designed life, several crack defects were found. The most serious case was represented by a crack in valve chest, which reached nearly up to one half of wall thickness. In spite of this it was necessary to operate the turbine for some time. The aim of this contribution is to describe procedures used for assessment of safe residual life.Material data necessary for this life assessment were evaluated during research of material properties of another steam turbine casing made of the same type steel whose operating time was finished after 1.23×10⁵ h. The mentioned material data are also presented.     

A creep life assessment method for boiler pipes using small punch creep test

The small punch creep (SPC) test is considered as a highly useful method for creep life assessment for high temperature plant components. SPC uses miniature-sized specimens and does not cause any serious sampling damages, and its assessment accuracy is at a high level. However, in applying the SPC test to the residual creep life assessment of the boiler in service, there are some issues to be studied. In order to apply SPC test to the residual creep life assessment of the 2.25Cr–1Mo steel boiler pipe, the relationship between uniaxial creep stress and the SPC test load has been studied.     

Creep and damage analysis of a serviced tee intersection in a boiler header: Comparison between numerical and experimental results

In the last few years modelling of damage phenomena under creep condition has been developed in order to take into account the microstructural material evolution in life predictions of high temperature components. The new analytical methods based on “Continuum Damage mechanics” and experimental creep and creep-rupture data aim at describing both stress-strain and damage field in structures in order to predict crack initiation. These models are implemented in computer finite element programs and should be subjected to rigorous experimental verification for a practical use in power plant assessment activities.

In the present paper the numerical results obtained from some creep and damagement analyses of a header component (10 CrMo 910 steel) are shown and compared with the experimental ones. The creep analyses have been performed by the computer code ABAQUS and the damage evaluation has been carried out by means of proper in-house developed user's subroutine and post processor.     

The high-temperature ageing of a low-alloy steel in the presence and absence of strain

The effect of high-temperature ageing in the presence and absence of strain was investigated for a high-temperature hot-rolled low-alloy steel. Mechanical tests were conducted on artificially aged material and material removed from operating plant. The steel investigated was developed for less critical elevated-temperature applications, which do not require heat-treated pressure-vessel-quality alloy materials, such as the Cr-Mo steels. Wide scatter in the high-temperature creep properties was observed. Due to a very high phosphorous content, reversible temper embrittlement at temperatures above 400°C results in Chapry V-notch values of less than 4 J cm⁻² after exposure times of only a small fraction of the component's design life. There is a difference in creep mechanism for short-time creep-rupture tests and those executed beyond 10 000 h. Confidence can only be obtained in creep-rupture data if the test field is extended to temperatures and stresses below 480°C and 150 MPa, respectively, and also to yield rupture data for up to 30 000 h. The very low fracture toughness values at room temperature and the high ductile-to-brittle transition temperature of aged material necessitate that special design features be considered. From a utility point of view, the suitability and economics of this steel for large, high-temperature structural applications—where normal design, operating and maintenance philosophies are expected to be followed—are questioned.     

The effect of microstructure on fracture mechanisms of 2·25Cr1Mo low alloy steel, part B: the influence of carbides

The size and distribution of carbides in 2·25Cr1Mo steel after different heat treatments has been studied by undertaking detailed metallographic examination and hardness testing. As expected, prolonged exposure to elevated temperature resulted in significant coarsening of the metallic carbides present. The effect of these changes in carbide size on fracture behaviour was established by undertaking a programme of miniature disk testing. Analysis of these data demonstrated that ageing increased the tendency for brittle fracture. These effects are compatible with the concept that the critical stress for brittle fracture is inversely proportional to the size of the carbides present.     

Using small punch test data to determine creep strain and strength reduction properties for heat affected zones

Abstract

The small punch (SP) test is a miniature technique that can provide information on creep performance of local features in welded components. However, the multiaxial stress and deformation history in SP test means that it can be a challenge to interpret the test results in a way that is comparable to uniaxial standard creep testing. This work aimed to compare SP test results from as new and service exposed P91 (9Cr–1Mo–V–Nb) base material (BM) and heat affected zones (HAZ) to uniaxial creep testing results from welded new and service exposed material. Two methods are proposed for predicting the uniaxial strain response for any zone of the weld: one alternative is to apply the SP rupture data and to accommodate the shape of the uniaxial base material creep curve to that of the zone of interest in the SP test. The other alternative is to use the SP deflection data and an appropriate translation function to the uniaxial creep curve. In both cases, the Wilshire creep equations have been used as the rupture model. The approach will also predict the stress reduction factors of welds and its constituent parts (including the subzones of HAZ). It is proposed that the approach is used to provide the local constitutive creep models for component assessment by finite element analysis (FEA).     

Emerging technology for component life assessment

Operating plant component damage and failure experience is reviewed. Loading conditions such as thermal stratification and striping, turbulent flow and flow-induced vibrations are often found to limit useful life, even though such loadings were typically not considered when the components were designed. High cycle thermal and mechanical fatigue are identified as important damage mechanisms. A new method of correlating fatigue data and extrapolating to the very high cycle regime is described.

The results of environmental degradation testing during the past fifteen years have shown that such effects are much more deleterious than previously assumed. Therefore environmental and aging effects must be taken into account in evaluating the reliability and dependability of components for extended periods of operation. Since most of the available data on environmental effects focus on measured crack growth rates, methods of developing improved fatigue life evaluation methods which include environmental effects on crack growth rates are now being developed.

Fatigue tests on polished specimens are characterized by nominal stress amplitudes over yield, where linear elastic fracture mechanics (da/dn vs. ΔK) methods, such as those used in the ASME Code, are not valid. The small plastic zone corrections used in the Code do not account for the plastic crack-driving energies encountered in low-cycle fatigue testing. J-integral solutions are used herein to evaluate the growth of cracks in these specimens. This approach can be shown to correlate the growth of cracks over the entire range of loading from elastic to grossly plastic conditions in widely different geometries and sizes, including the growth of very short cracks for materials of major interest in pressure vessels and piping. It can be used to correct S-N fatigue life evaluation, curves for known differences in crack growth rates whether they are due to corrosion-assisted fatigue or other variables.

Environmental effects on the crack initiation phase of fatigue failure can be directly incorporated into S-N life evaluation curves. Once the crack propagation effects are included, the resulting improved S-N curves provide a means for plant operators to evaluate the current condition of these components and systems, taking into account the cumulative damage from operating transients and cycles which the plant has experienced. The safe residual life can then be evaluated using the S-N curves to include cumulative damage for the anticipated future period of operation. This plant life evaluation approach is applicable even where in-service inspections are not feasible. It provides a sound quantitative basis for making repair/replacement decisions.     

Study on fracture strain of Cr–Mo steel in high pressure hydrogen

Cr–Mo steel is often used as the material of the hydrogen storage vessel, but its ductility can be deteriorated by high pressure hydrogen, which makes it possible that the local area of strain concentration on the hydrogen storage vessel made of Cr–Mo steel may fail due to excessive plastic deformation. The limit criterion of local strain established according to the study of the fracture strain is the basis for local failure assessment of the vessel. However, the correlation between the fracture strain and the stress state of Cr–Mo steel in high pressure hydrogen is still unclear, so the limit criterion of local strain for hydrogen storage vessel made of Cr–Mo steel has not been established. In this paper, the slow strain rate tensile test (SSRT) of notched specimens with different notch sizes was carried out in air, 45 MPa hydrogen and 100 MPa hydrogen, respectively. Based on the test results, the whole process from tensile to fracture of the specimens was simulated by finite element method. The distribution of stress triaxiality and plastic strain during the tensile process was analyzed, and the correlations between the stress triaxiality and the fracture strain in different environments were obtained. Finally, the limit criterion of local strain for local failure assessment of 4130X hydrogen storage vessel was established.     

Creep crack growth in service-exposed weld metal of 2.25Cr1Mo

Creep crack growth (CCG) has been studied for ex-service weld metal of 2.25Cr1Mo (P22). The testing was conducted at a temperature of 550°C and prior to testing, the material had been exposed to high temperature service for 110 000 h at 530°C. The results show a marked effect of the service exposure on the CCG properties of the material when compared to similar testing performed on a new material. The CCG rate was higher by a factor 3.1 in the service-exposed material, which should be compared to the model value of 3.2 based on the relations between the elongation values. The consumed deformation capacity was also estimated with the omega model for tertiary creep. In this case, an enhanced growth rate of 2.4 was obtained. Within a distance of about 10 mm in front of the propagating cracks, the number of creep cavities was significantly higher than in the surrounding material. The variation of the density of cavities as a function of distance from the crack tip was successfully modelled.     

A new high-temperature creep-fatigue damage rule based on the strain range partitioning concept

A new life prediction model based on the strain range partitioned crack growth rate equations is presented which can evaluate the material damage and the residual life under variable creep-fatigue strainings. Two-step sequential creep-fatigue straining tests such as High-Low and Low-High tests were conducted on Mod. 9Cr-1Mo steel and type 316LC stainless steel in order to examine the validity of the proposed model. It was found that the proposed model can be more successfully applied to the creep-fatigue life prediction under variable strainings than the conventional life prediction methods.     

Ductile tearing in austenitic stainless steel weldments

The UK Technical Advisory Group on the Structural Integrity of Nuclear Plant (TAGSI) was asked to respond to a Question that was submitted by the Ministry of Defence (MoD) on the use of tearing resistance data and the interaction with fatigue crack growth, including the effects of thermal or residual stresses, in austenitic stainless steel weldments. The TAGSI response was initially prepared in 2000, but the basic response is still considered relevant and timely. Additional comments on more recent developments are included.

TAGSI concluded that for single loading conditions, material resistance curve data can be used to produce an integrity justification with up to Δa_t=2 mm of tearing. Further work would be required to justify an increase in this limit, but for applications in thick sections it would be prudent to test 50-mm thick specimens as well as standard 25-mm thick specimens. For combined tearing/fatigue conditions, TAGSI considers that the results it has reviewed are encouraging in supporting linear summation of tearing and fatigue components to assess margins. TAGSI considers that it would be prudent to include secondary loading due to welding residual stresses as well as that due to thermal loading in any prediction of stress conditions up to the crack growth limit of 2 mm for applications of the type considered.

The paper discusses a number of restrictions and limitations to the general conclusions outlined above.     

Creep life assessment by low strain rate tensile testing

A testing method, originally proposed by Rajakovics,⁵ has been investigated for its ability to extrapolate creep rates from the results of short-term tests. Low constant strain rate tensile tests have been performed on a number of high temperature materials in the virgin, service-aged and welded conditions in the temperature range from 525°C to 1050°C. A technique has been developed that permits one to determine the steady-state creep rate under a known stress or the creep stress for a given creep rate. A step towards a quantitative residual life assessment of service-aged materials has been made. The results of the proposed extrapolation technique are compared with results from creep tests.     

Difficulties in interpreting data from creep crack growth tests on type 316H weldments

Abstract

Experimental creep crack growth data are generally obtained by following standard methods such as ASTM E1457-07 and subsequently characterised using the C* parameter. These data are then used in assessment procedures, such as R5, together with reference stress estimates of C* in the component, to predict creep crack growth behaviour. Some modifications to the ASTM E1457 creep crack growth testing and analysis methods have already been proposed following a previous analysis of data from long term creep crack growth tests on type 316H parent material. This paper reports the results of creep crack growth tests on type 316H heat affected zone material at 550°C using compact tension (CT) specimens manufactured from non-stress relieved thick section butt welds. It is shown that interpretation of the data from these weldment tests is complicated by both the discontinuous nature of the cracking process and the presence of significant residual stresses in the CT specimens. Further modifications to creep crack growth testing and analysis methods are proposed to address difficulties arising from the discontinuous nature of the cracking process, and further work is identified to investigate the influence of the residual stresses present in the specimens on the observed crack growth behaviour.     

Creep constitutive equations for parent,Type IV,R-HAZ,CG-HAZ and weld material in the range 565–640 °C for Cr–Mo–V weldments

Creep deformation and rupture data over the temperature range 565–640 °C for the parent, Type IV, refined-heat affected zone, coarse grained-heat affected zone and weld materials, associated with a 0.5Cr–0.5Mo–0.25V ferritic steel parent material welded with a 2.25Cr–1Mo steel, has been collected and used to determine a constitutive equation parameter set. The constitutive equation parameter set has been generated for use in a future investigation to analyse a welded medium-bore branched vessel tested at 590 °C; and, also to permit extrapolation and interpolation over the temperature range 565–640 °C.     

Assessment methods of material ageing using Sdobyrev''s creep rupture model

The physical aspects of the activation energy, in higher and high temperatures, of the metal creep process were examined. The research results of creep-rupture in a uniaxial stress state and the criterion of creep-rupture in biaxial stress states, at two temperatures, are then presented. For these studies creep-rupture, taking case iron as an example the energy and pseudoenergy activation was determined. For complex stress states the criterion of creep-rupture was taken to be Sdobyrev's, i.e. σ_red = σ₁ β + (1 − β)σ_i, where: σ₁-maximal principal stress, σ_i-stress intensity, β-material constant (at variable temperature β = β(T)). The methods of assessment of the material ageing grade are given in percentages of ageing of new material in the following mechanical properties: 1) creep strength in uniaxial stress state, 2) activation energy in uniaxial stress state, 3) criterion creep strength in complex stress states, 4) activation pseudoenergy in complex stress states. The methods 1) and 3) are the relatively simplest because they result from experimental investigations only at nominal temperature of the structure work, however, for methods 2) and 4) it is necessary to perform the experimental investigations at least at two temperatures.     

Optimisation of post-weld heat treatment of a 1.25Cr–0.5Mo pressure vessel for high temperature hydrogen service

The ASME pressure vessel design code permits a range of post-weld heat treatment temperatures for 1.25Cr–0.5Mo pressure vessels. Studies using analytical transmission electron microscopy, X-ray diffraction, hardness and Charpy toughness testing, were performed on material from a new pressure vessel to understand the effects of various heat treatment schedules allowed within the code. These techniques have enabled an optimum heat treatment temperature and the time to be determined, with a view to minimising the likely susceptibility of the vessel to temper embrittlement and hydrogen attack.