Comparison of methods to determine CTOD for SENB specimens in different strain hardening steels

Methods for determining crack tip opening displacement (CTOD) given in national and international standards are compared for steels with a range of strain hardening characteristics. Crack tip opening displacement measurements were made from single‐edge notched bend notches using a silicone rubber casting method. The finite element model produced good agreements with predictions of these CTOD measurements. The versatility of the finite element model enabled CTOD from the original crack tip and the 45° intercept method to be compared. The 45° CTOD generally underestimates the original crack tip CTOD, and is less useful for conditions with stable crack extension. Apart from the high strain hardening material, CTOD calculated using BS 7448‐1, WES 1108 (JWES), and ASTM E1820 was slightly lower than the values determined from silicone measurements and modelling, which is conservative. ASTM E1820 gave the largest underestimation of CTOD, whilst BS 7448‐1 may be unsuitable for higher strain hardening steels, where the standard predicts higher CTOD than measured from the replica. JWES gives the most consistent estimation of CTOD for steels with a wide range of strain hardening values.     

不锈钢硫酸—硫酸铜晶间腐蚀结果评定方法探讨

通过对硫酸-硫酸铜晶间腐蚀后弯曲产生裂纹或断裂的奥氏体不锈钢基体、双相不锈钢基体、不锈钢堆焊复层及不锈钢焊接接头试样进行金相、扫描或透射电镜微观分析,表明用GB4334-2008 E法评定操作性存在问题,参照此法判断试验材料是否有晶间腐蚀倾向,判断结果存在争议;硫酸-硫酸铜晶间腐蚀标准(GB4334-2008 E法)中结果评定方法应在现基础上细化、补充,评定方法中金相法如何操作和如何评定应详细说明,适当可以补充图片指导,结果有争议时,不只限于150～500倍的光学显微镜下观察,根据需要,可在扫描电镜下、甚至是在透射电镜下进行观察分析;在高倍显微镜下准确判断出晶间腐蚀所致裂纹和非晶间腐蚀所致裂纹试样,总结出其低倍形貌特征并补充于标准评定方法中,对于提高标准的操作性具有重要意义。     

J-Resistance curve testing of HY80 steel using SE(B) specimens and normalization method

The normalization method is adopted for standard and nonstandard specimens in this paper to develop J-R curves for HY80 steel directly from load versus load-line displacement records without use of automatic crack length measurement. The standard specimens usually contain high crack-tip constraints, while the nonstandard specimens involve low crack-tip constraints. To obtain J-R curves with different constraints, a series of single edge notched bend (SE(B)) specimens with different crack lengths for an HY80 steel are tested in accordance with ASTM standard E1820. The normalization method is then used for determining crack extension and J-R curves for these SE(B) specimens.To validate the normalization method, the J-R curves determined using the normalization method are compared with those obtained by the elastic unloading compliance method for the SE(B) specimens. The comparison shows that good agreements exist between the two methods, and the normalization method is a viable tool to be used to determine J-R curves of the HY80 steel for the standard as well as nonstandard SE(B) specimens. In the J-integral calculations, the resistance curve test method, the basic test method and the modified basic test method specified in ASTM E1820 are evaluated. The results indicate that the modified basic method can be equivalent to the resistance curve method.     

A CTOD equation based on the rigid rotational factor with the consideration of crack tip blunting due to strain hardening for SEN(B)

Crack tip opening displacement (CTOD) from national and international standards was shown to give different values. This paper investigates the feasibility of CTOD determined based on the concept of rigid rotational factor in single‐edge notched bend (SENB) specimens. Based on validated modelling methods, finite element (FE) models were simulated for crack ratios 0.3 ≤ a₀/W ≤ 0.7 and yield‐to‐tensile ratio 0.44 ≤ σ_ys/σ_uts ≤ 0.98. This covers cases of shallow to deeply cracked specimens and a wide range of strain hardening properties. CTOD obtained from the FE models was used as the basis of a newly implemented strain hardening corrected rotational factor, which considers the effects of crack tip blunting due to strain hardening, r_{p sh}. An improved equation considering strain hardening was implemented based on the r_{p sh}. The equation gives accurate estimation of CTOD from the FE models compared with the equation from BS 7448‐1, ASTM E1820, and WES 1180.     

A study to estimate crack length using the separability parameter Spb in steels

The separability property with S_pb parameter was used in this work to estimate the instantaneous crack length in pre-cracked specimens. A test matrix, pre-cracked ASTM C(T), SE(B) specimens and non-standard A(B) arc-shaped geometry was prepared. Materials were ASTM 387-Gr.22 2.25Cr-1.0Mo steel, an API Gr.N80 and HSLA welded joints. Initial and final crack lengths were measured on the crack surface and instantaneous crack length was determined by the compliance method to compare against the values of crack length estimated using the S_pb parameter. The difference between the resulting values was less than 15%Δa suggested as reference in ASTM E1820-96.     

Creep crack growth on AISI 316 base material,weld metal and “heat affected zone”

In the main frame of the PEC joint venture the austenitic stainless steel ASTM A240 Tp 316 was tested over a wide range of characteristics. Particular emphasis was given to the temperature range between 550°C and 650°C, in which creep testing up to 20 kh and creep crack growth up to 18 kh including influences due to specimen geometry, crack starter notch machining and ageing heat treatment. Finally a GTAW+SMAW weld was tested in creep crack growth and revealed crack nucleation times shorter than all other tested conditions but a crack growth behaviour similar to the aged condition.     

Experimentally determined key curves for fracture specimens

Crack extension during fracture toughness tests of ferritic structural steels cannot be determined from measurements of unloading compliance or electric potential change when the specimen is dynamically tested. Measurements of crack extension in fracture toughness tests are also very difficult when the test temperature is high or the test environment is aggressive. To circumvent this limitation, researchers for years have been developing key curve and normalization function methods to estimate crack extension in standard elastic-plastic fracture toughness test geometries. In the key curve method (Ernst et al., 1979; Joyce et al., 1980) a load-displacement curve is measured for a so-called `source' specimen that is sub size or has a blunt notch so that the crack will not initiate during elastic-plastic loading. The load and displacement are then converted to normalized stress-strain units to obtain a key curve that can be used to predict crack extension in geometrically similar `target' specimens of same material loaded at similar loading rates and tested under similar environmental conditions. More recently Landes and coworkers (Herrera and Landes, 1990; Landes et al., 1991) proposed the normalization data reduction technique – Annex A15 of ASTM 1820 specification – that presents an alternative to the standard E1820 unloading compliance procedure. Although the normalization method works well in many cases, it has serious drawbacks: the load, displacement and crack length at the end of the test must be measured; the prescribed functional form that is fitted to the initial and final data may not be accurate for all materials; and the iterative method of inferring crack length from the combination of the data and the normalization function is complex. The compliance ratio (CR) method developed in this paper determines key curves for predicting crack extension as follows. First, a statically loaded source specimen with the unloading compliance procedure specified in ASTM 1820. Second, the so-called CR load-displacement curve is calculated for the source specimen, which is the load-displacement record that would have been obtained if the crack had not extended. Third, non-dimensionalizing the CR load by the maximum load and the displacement by the elastic displacement at the maximum load, P ^* _i/P _max and v _i/v _{el max} from the source specimen yields the adjusted key curve. Analysis of extensive data shows that the key curve is independent of notch type, initial crack length and temperature. But it is dependent on specimen size and steel type. Assuming that the key curves of the source and target specimens are one and the same, the compliance of the target specimens are calculated with a reverse application of the compliance ratio method, and the crack length is obtained using the equations in ASTM E1820. The CR Method is found to be much simpler than the normalization method described in the Annex A15 of ASTM 1820. With the compliance ratio method, Joyce et al. (2001) successfully predicted crack extension in dynamically loaded specimens using a key curve of a statically loaded specimen.     

Interpretation of creep crack initiation and growth data for weldments

Creep crack growth testing of macroscopically homogeneous materials is well established and standardised test procedures are detailed in ASTM E1457. In ASTM E1457 the use of the compact tension C(T) specimen is specified and crack growth data are interpreted using the fracture mechanics parameter C^∗. The resulting benchmark crack growth data are used in assessment procedures, together with estimates of the value of C^∗ in the component, to predict creep crack growth behaviour. In this work, the results of a series of creep crack growth tests performed on a Type 316 stainless steel weldment at a temperature of 550 °C are presented. The initial crack is located within the heat affected zone (HAZ) of the weldment. Since there are currently no agreed methods for determining C^∗ in inhomogeneous laboratory specimens, this paper examines the application of procedures in ASTM E1457 for the characterisation of crack growth in weldments. In addition, the creep toughness parameter is evaluated for the material. It is shown that the creep crack growth rates in the weldment may be described by the C^∗ values obtained following ASTM E1457 in conjunction with parent material properties. Comparison of the results with similar data for Type 316H stainless steel parent material shows that the crack growth rates for the crack located within the HAZ are higher and the initiation times lower than the parent values, for the range of test conditions examined. Simple analytical models based on ductility exhaustion arguments have been shown to bound the crack initiation and growth data for the weldment.     

Comparison of CTOD standards: BS 7448-Part 1 and revised ASTM E1290

Crack tip opening displacement (CTOD) has been calculated using the plastic hinge model with an assumed rotational center since the British Standards Institution (BS) standardized BS5762 in 1979. The American Society for Testing and Materials (ASTM) accepted the plastic hinge model and standardized E1290 in 1989. However, ASTM revised E1290 in 2002, and has proposed a conversion from J to CTOD. CTOD-based fracture toughness evaluation has been widely used for the defect assessment of many welded structural components, and two different CTOD calculations could lead to confusion for Fitness-for-Service. In this study, the effects of CTOD testing methodologies on CTOD values were investigated according to round robin tests conducted by the Japan Welding Engineering Society (WES), and the concept of CTOD as a fracture parameter is discussed.     

Crack tip displacements of microstructurally small cracks in 316L steel and their dependence on crystallographic orientations of grains

The paper presents an analysis of the effect of the grain orientations on a short Stage I surface crack in a 316L stainless steel. The analysis is based on a plane‐strain finite element crystal plasticity model. The model consists of 212 randomly shaped, sized and oriented grains that is loaded monotonically in uniaxial tension to a maximum load of 1.12R_p0.2 (280 MPa). The influence of random grain structure on a crack is assessed by calculating the crack tip opening (CTOD) and sliding displacements (CTSD) for single crystal and polycrystal models, considering also different crystallographic orientations. In the single crystal case the CTOD and CTSD may differ by more than one order of magnitude. Near the crack tip slip is activated on all the slip planes whereby only two are active in the rest of the model. The maximum CTOD is directly related to the largest Schmid factors. For the more complex polycrystal cases it is shown that certain crystallographic orientations result in a cluster of soft grains around the crack‐containing grain. In these cases the crack tip can become a part of the localized strain, resulting in a large CTOD value. This effect, resulting from the overall grain orientations and sizes, can have a greater impact on the CTOD than the local grain orientation. On the other hand, when a localized soft response is formed away from the crack, the localized strain does not affect the crack tip directly, resulting in a small CTOD value. The resulting difference in CTOD can be up to a factor of 4, depending upon the crystallographic set. Grains as far as 6xCracklength significantly influence the crack tip parameters. It was also found that among grains with favourable orientation the CTOD increased with the size of such a grain. Finally, a significant change in CTOD and CTSD was observed when extending the crack into the second grain and placing it in the primary or the conjugate slip plane.     

Failure of a premix feeder line on a land-based gas turbine

A premix feeder line on a land-based gas turbine was found to be leaking, and the source of the leakage was traced back to a crack in a pipe-to-fitting weld. The weld joined a coupling made from the X6CrNiMoTi17-12-2 austenitic stainless steel to a type 316L austenitic stainless steel reducer. A root-cause assessment was conducted to determine the cause of the crack. Evidence of fatigue and corrosive attack was found. Various factors that influence the susceptibility to corrosion and cracking, such as heat tints, geometry of the weld, and surface finish of the component, are discussed. Recommendations are made for corrective actions and failure prevention.     

The Effect of Hydrogen on Fatigue Properties of Metals used for Fuel Cell System

The effect of hydrogen on the fatigue properties of alloys which are used in fuel cell (FC) systems has been investigated. In a typical FC system, various alloys are used in hydrogen environments and are subjected to cyclic loading due to pressurization, mechanical vibrations, etc. The materials investigated were three austenitic stainless steels (SUS304, SUS316 and SUS316L), one ferritic stainless steel (SUS405), one martensitic stainless steel (0.7C-13Cr), a Cr-Mo martensitic steel (SCM435) and two annealed medium-carbon steels (0.47 and 0.45%C). In order to simulate the pick-up of hydrogen in service, the specimens were charged with hydrogen. The fatigue crack growth behaviour of charged specimens of SUS304, SUS316, SUS316L and SUS405 was compared with that of specimens which had not been hydrogen-charged. The comparison showed that there was a degradation in fatigue crack growth resistance due to hydrogen in the case of SUS304 and SUS316 austenitic stainless steels. However, SUS316L and SUS405 showed little degradation due to hydrogen. A marked increase in the amount of martensitic transformation occurred in the hydrogen-charged SUS304 specimens compared to specimens without hydrogen charge. In case of SUS316L, little martensitic transformation occurred in either specimens with and without hydrogen charge. The results of S-N testing showed that in the case of the 0.7C–13Cr stainless steel and the Cr–Mo steel a marked decrease in fatigue resistance due to hydrogen occurred. In the case of the medium carbon steels hydrogen did not cause a reduction in fatigue behaviour. Examination of the slip band characteristics of a number of the alloys showed that slip was more localized in the case of hydrogen-charged specimens. Thus, it is presumed that a synergetic effect of hydrogen and martensitic structure enhances degradation of fatigue crack resistance.     

Separability property and ηpl factor in ASTM A387-Gr22 steel plate

In this work the possibility of obtaining the function G(b/W) from the variable separability property to calculate η_pl factor following the methodology proposed by Sharobeam and Landes using a deformation function H with total displacement was studied. The material employed was an ASTM A387-Gr22 steel plate and standard ASTM SE(B) and C(T) geometries were used for the test pieces with different side grooving ratios.For SE(B) specimens, the differences observed between η_pl factor values determined with total displacement, plastic displacement and that recommended in ASTM 1820-96 were not significant. For C(T) specimens, the differences observed were lower than 2% for all side grooving ratios studied. The η_pl factor values resulting from the use of Sharobeam and Landes methodology using total and plastic displacement did not seem to follow the dependence with crack length suggested by ASTM E1820-96. It was not possible within the scope of this work to find a side groove influence on the η_pl factor value for this geometry.     

Creep crack growth behaviour in air and sodium for an unstabilized austenitic stainless steel and assessment of evaluation concepts

The austenitic stainless steels used for permanent structures in the heat transfer systems of liquid metal fast breeder reactors (LMFBR's) are subjected to flowing sodium at elevated temperatures. During this exposure to sodium environment the structural materials can undergo compositional and microstructural changes which could alter the mechanical behaviour as compared to air. An investigation of creep crack growth in air and flowing liquid sodium was performed on the austenitic stainless steel X6 CrNi 18 11 (similar to AISI 304 ss) under constant load at 823 K using compact tension specimens. The sodium conditions are characterized by an upstream position in a non-isothermal loop with a temperature gradient of 170 K. The experimental data are evaluated correlating the crack growth rate with the net section stress σ_net, the stress intensity factor K_Q, and the energy rate integral C^*, respectively.     

The influence of crystallographic orientation on crack tip displacements of microstructurally small, kinked crack crossing the grain boundary

The paper presents an analysis of the effects of grain orientations on a short, kinked surface crack in a 316L stainless steel. The kinking of the crack is assumed to take place at the boundary between two neighbouring grains. The analysis is based on a plane-strain finite element crystal plasticity model. The model consists of 212 randomly shaped, sized and oriented grains, loaded monotonically in uniaxial tension to a maximum load of 0.96R_p0.2 (240 MPa). The influence that a random grain structure imposes on a Stage I crack is assessed by calculating the crack tip opening (CTOD) displacements for bicrystal as well as for polycrystal models, considering different crystallographic orientations. Since a Stage I crack is assumed, the crack is always placed in a slip plane. Results from a bicrystal case show that the maximal CTODs are directly related to the stiffness of the grain containing the crack extension. Anisotropic elasticity and crystal plasticity both contribute to this grain stiffness, resulting in maximal CTOD when Schmid factors are the highest on two slip planes. Such crystallographic orientation results in a soft elasto-plastic response. Anisotropic elasticity can additionally increase the softness of a grain at certain crystallographic orientations. Minimal anisotropic elasticity at the crystallographic orientations with the highest Schmid factors causes the CTOD to be maximized. Presuming that the crack will preferably follow the slip plane where the crack tip opening displacement is highest, we show that the crystallographic orientation can affect the CTOD values by a factor of up to 7.7. For a given grain orientation the maximum CTOD is attained when the crack extension deflection into a second grain is between −75.141° and 34°. For the polycrystal case we show that grains beyond the first two crack-containing grains change the CTOD by a factor of up to 3.3 and that the largest CTODs are obtained when placing the crack into a slip plane with crack extension that results in a crack extension being more perpendicular to the external load.     

Effects of orientation,stress and exposure time on short intergranular stress corrosion crack behaviour in sensitised type 304 austenitic stainless steel

Intergranular stress corrosion cracking (IGSCC) in austenitic stainless steels occurs at susceptible grain boundaries after sensitisation. In this study, the effects of test duration, static stress (applied and residual) and microstructure orientation on the developed populations of short crack nuclei are reported for a sensitised type 304 austenitic stainless steel in an acidified potassium tetrathionate (K₂S₄O₆) solution. The crack populations were analysed using the Gumbel distribution method, showing an increase in the characteristic crack lengths with increasing time and grain size. There is a weak, but measurable effect of stress on crack length. Tensile stress increases crack growth and compressive residual stresses introduced by surface machining are shown to be beneficial. A significant dependence on sample orientation is observed and this cannot be explained in terms of the bulk microstructure properties or characteristics, which showed no significant variations.     

Characterization of Weld Solidification Cracking in a Duplex Stainless Steel

Weld solidification cracking in the duplex stainless steel SAF 2205 has been investigated and compared with that of alternate duplex and austenitic stainless steels. Varestraint weld-ability testing showed SAF 2205 to exhibit a lower cracking susceptibility than that of the duplex stainless steel Ferralium 255 but greater than that of a Type 304 austenitic stainless steel which solidified as ferrite and exhibited Ferrite Number 8 (FN 8) in the weld fusion zone. The high augmented strain levels required to induce cracking in these three alloys during Varestraint testing indicated a high resistance to solidification cracking at strain levels normally encountered in structural weldments. Cracking susceptibilities of the duplex and Type 304/FN-8 stainless steels were appreciably lower than that of a Type 304L stainless steel which solidified entirely to austenite and exhibited less than FN 1 in the weld fusion zone.

Microstructural characterization of SAF 2205 using conventional black-and-white and two different color metallography techniques showed solidification cracks to be associated with ferrite grain boundaries. Color metallography was also effective in revealing the fusion zone solidification structure and delineating second phases, including inter- and intragranular austenite and fine Cr₂N precipitates. Fractographic analysis of solidification crack surfaces from SAF 2205 Varestraint samples revealed dendritic and flat topographies, and confirmed a solidification versus solid-state cracking mechanism.     

Effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel

The effect of grain boundary microstructure on fatigue crack propagation in austenitic stainless steel was investigated in order to control fatigue crack propagation. The fraction of low-Σ coincidence boundaries in specimens was controlled by thermomechanical processing. The specimen with the higher fraction of low-Σ boundaries (73%) showed the lower propagation rate of fatigue crack than the specimen with the lower fraction of low-Σ boundaries (53%). The ratio of intergranular fracture segments to the total crack length was lower for the specimen with the higher fraction of low-Σ boundaries. Moreover, the roles of grain boundaries in the fatigue crack propagation were investigated in connection with grain boundary microstructure, i.e., the character distribution and geometrical configuration of grain boundaries. It is evidenced that the approach to grain boundary engineering is applicable to controlling fatigue crack propagation in austenitic stainless steel.     

MECHANISMS AND FACTORS THAT INFLUENCE POSTWELD INTERGRANULAR UNDERCLAD CRACKING

Abstract— Forged components of ferritic steel can be protected by a welded austenitic stainless steel clad. Intergranular cracking can take place in the ferritic phase close to the ferritic austenitic interface. After developing a technique for fabrication of these cracks, the formation conditions are studied. Auger electron spectroscopy investigations of specimens containing a real crack opened inside the vacuum chamber are used for interpretation. Sulphur segregations embrittle the grain boundaries which are cracked by residual and thermal stresses during the postweld heat treatment.     

Tensile and Impact Properties of Shielded Metal Arc Welded AISI 409M Ferritic Stainless Steel Joints

The present study is concerned with the effect of filler metals such as austenitic stainless steel, ferritic stainless steel and duplex stainless steel on tensile and impact properties of the ferritic stainless steel conforming to AISI 409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass butt welded joints. Tensile and impact properties, microhardness, microstructure and fracture surface morphology of the joints fabricated by austenitic stainless steel, ferritic stainless steel and duplex stainless steel filler metals were evaluated and the results were reported. From this investigation, it is found that the joints fabricated by duplex stainless steel filler metal showed higher tensile strength and hardness compared to the joints fabricated by austenitic and ferritic stainless steel filler metals. Joints fabricated by austenitic stainless steel filler metal exhibited higher ductility and impact toughness compared with the joints fabricated by ferritic stainless steel and duplex stainless steel filler metals.