Strength Overmatch Research Articles

Tensile strength of girth weld joint of linepipe with softened HAZ

In seismic-active area, the buried pipeline is subjected to the axial tensile load exceeding the yield stress of pipe material due to the lateral flow induced by soil liquefaction. Designing a girth weld joint that does not fracture at girth weld but base metal which has high elongation capacity is feasible to ensure adequate ground displacement absorbing capacity of the girth weld joint. However, as for fine grain steel, the heataffected zone (HAZ) is softened by welding heat input, and then, the zone of softened HAZ can be the origin of fracture under tensile load. The apparent strength of softened HAZ is affected by the plastic constraint from the surrounding weld metal and base metal during tensile loading, and the fracture location depends on the apparent strength of HAZ. This study investigates the tensile strength and fracture location of girth weld joints with softened HAZ in the static tensile test. The effect of geometric and mechanical heterogeneity of HAZ, strength overmatching of weld metal, and the width-thickness ratio of tensile test specimen on tensile strength are elucidated using parametric finite element analysis. Subsequently, an equation is proposed to predict the tensile strength and fracture location of the welded joint. Using the proposed equation, the condition of geometric and mechanical heterogeneity of the weld to confirm the girth weld joints that fracture at base metal under tensile load is clarified. The proposed equation can be used to design a girth weld joint with liquefaction earthquake resistance.

Investigation into microstructures and properties of AISI 430 ferritic steel butt joints fabricated by GMAW

Heat input is extremely important for achieving joints with high mechanical properties in ferritic stainless steels. Thus, one of the objectives of this study is to show the effect of heat input on microstructural evolution and thus on the mechanical behavior. For this aim, 5 mm thick AISI 430 plates were butt welded using two heat input levels. Extensive optical microscopy and SEM investigations, microhardness measurements, mechanical tests (i.e., tensile and bending) were conducted to investigate the microstructures evolving in the weld area and to determine the mechanical properties of the joints produced. Strength overmatching was obtained in the weld region of both joints fabricated with both heat inputs employed. In addition, the HAZ region is much wider in the joint fabricated with higher heat input. Although the higher heat input joint exhibited a wider HAZ region the hardness values were similar in the HAZ regions of both joints. As a result, both joints exhibited similar tensile properties, i.e., a strength performance of about 99%. In contrast, the root bend specimen of the higher heat input joint did crack in the bending test while both surface and root bend specimens of the lower heat input joint did not fail.

Evaluation of strain based ECA method in DNVGL-RP-F108

Pipelines may be subject to strain-controlled external loads exceeding yield strength of materials due to many causes, e.g., soil settlements and ground movement. There are several strain based fracture mechanics assessment methods to determine the maximum tolerable defect sizes in girth welds in pipelines. One of these methods is the DNVGL-RP-F108 procedure which was developed from a joint industry project for assessing reeled pipes and has been used for both offshore and on-shore pipelines. However, as with other methods, it requires the weld strength either even- or over-matching the parent steel which may be difficult to achieve for high strength steels. There are also uncertainties about the adequacy of the DNVGL-RP-F108 guidance addressing the effects of internal pressure and welding-induced residual stresses. In this study, the maximum tolerable defect sizes in the girth welds of an X70 spiral welded pipeline were determined using the DNVGL-RP-F108 approach. The effects of several variables on the assessment results, including weld strength mismatch, stress-strain curve magnitude, internal pressure level, fracture toughness, residual stresses and misalignment, were also investigated. A comprehensive finite element analyses were performed to verify the crack driving force determined by the DNVGL-RP-F108 approach. It was found that the DNVGL-RP-F108 approach can be conservatively applied to girth welds with a weld strength under-match by 5% in all cases, and even by 10% for most cases, for the conditions investigated. It was also revealed that use of this guidance could be unduly conservative for strength over-matching welds.

Effect of strength mismatch on limit load of dissimilar metal welded joint for connecting pipe-nozzle of nuclear pressure vessel

The interface region cracks in dissimilar metal welded joint (DMWJ) for connecting pipe-nozzle of a reactor pressure vessel (RPV) to safe end are important concerned cracks in fracture assessment. In this work, based on a large amount of finite element analyses, the effect of strength mismatch on the limit loads of two interface cracks in an actual DMWJ has been investigated for wide ranges of strength mismatch factors and crack sizes. The results indicate that for shallower cracks, the limit load is dominated by fully yielding of the safe end pipe and are independent of strength mismatch and crack size. For deeper and longer cracks, the limit load is dominated by fully yielding of the ligament area materials. As the mismatch factor M increases from undermatch (M < 1) to overmatch (M > 1), the limit loads increase. To improve the load-bearing capacity of the DMWJ and prevent plastic collapse failure, a high degree of strength overmatch (increasing strength of buttering and weld metals) should be designed and adopted. The normalized limit load solutions as functions of mismatch factor and crack size have been obtained for the two interface cracks.

Investigation on material characteristics and fatigue crack behavior of thermite welded rail joint

This study investigates the material characteristics across the thermite welded joint and discusses the inherent correlation between the material properties and the mechanism of the fatigue cracking occurring at the fusion line. Various material regions across the welded joint are identified and the characterization of the mechanical and fatigue properties are conducted. The weld defects are investigated, and it is found that the defects lead to a reduction of fatigue limit and the local severe stress concentration which acts as potential crack initiation sites at the fusion line. The fatigue crack propagation behaviors were studied and the result reveals that the moderate material strength overmatch is detrimental for interfacial cracks propagation. The interfacial cracks, once initiated, grow towards the base rail metal and away from fusion line resulting in a shorter fatigue propagation life. It is concluded that the fatigue cracks occurred at the fusion line of the thermite welds are caused by the high density of weld defects and the mismatch of strength and fatigue properties.

Microstructure and mechanical properties of friction-stir welded St52 steel joints

The aim of this work is to investigate the mechanical properties and microstructures of friction-stir welded (FSWed) St52 structural steel joints. In this study, St52 steel plates with a thickness of 4 mm were butt-welded by friction-stir welding (FSW) using a tungsten carbide tool having a conical pin. The microstructure of the welded zone consists of equiaxed fine ferrite, grain boundary ferrite, Widmanstatten ferrite, and aggregates of ferrite + cementite. The microhardness measurements showed that the hardness of the welded zone was significantly higher than that of the base metal. The FSWed St52 joint exhibited a significant strength overmatching in the weld region and a strength performance similar to or slightly higher than that of the base plate.

Proposal of the Penalty Factor Equations Considering Weld Strength Over-Match

Abstract This paper proposes penalty factor equations that take into consideration the weld strength over-match given in the classified form similar to the revised equations presented in the Code Case N-779 via cyclic elastic-plastic finite element analysis. It was found that the K e analysis data reflecting elastic follow-up can be consolidated by normalizing the primary-plus-secondary stress intensity ranges excluding the nonlinear thermal stress intensity component, S n to over-match degree of yield strength, M F . For the effect of over-match on K n × Κ ν , dispersion of the K n × Κ ν analysis data can be sharply reduced by dividing total stress intensity range, excluding local thermal stresses, S p-lt by M F . Finally, the proposed equations were applied to the weld between the safe end and the piping of a pressurizer surge nozzle in pressurized water reactors in order to calculate a cumulative usage factor. The cumulative usage factor was then compared with those derived by the previous K e factor equations. The result shows that application of the proposed equations can significantly reduce conservatism of fatigue assessment using the previous K e factor equations.

Effects of specimen geometry and anisotropic material response on the tensile strain capacity of flawed spiral welded pipes

Spiral welded pipes have gained interest for application in strain-based design related projects. However, there is a lack of fundamental knowledge about their performance in terms of tensile strain capacity. By means of parametric finite element analyses, the influence of pipe forming angle, weld strength overmatch and material strength anisotropy on the tensile strain capacity was investigated for (un-)pressurized pipe sections and wide plates. It is concluded that larger forming angles, when compared to lower angles, result in significant higher strain capacity and less sensitivity to anisotropy. Neglecting anisotropy in finite element analysis may overestimate strain capacity.

Fracture Toughness Characterization of High-Pressure Pipe Girth Welds Using Single-Edge Notched Tension [SE(T)] Specimens

Abstract The safety and reliability of large-diameter pipelines for the transport of fluid hydrocarbons is being improved by the development of high-strength steels, advanced weld technologies, and strain-based design (SBD) methodologies. In SBD, a limit is imposed on the applied strains rather than the applied stresses. For high-pressure pipelines, SBD requires an assured strength overmatch for the weld metal as compared to the base material, in order to avoid strain localization in the weldment during service. Achieving the required level of strength overmatch, as well as acceptable ductility and low-temperature fracture toughness, is a challenge as the pipe strength increases. Published studies show that low constraint geometries such as single-edge tension [SE(T)] or shallow-notched single-edge bend [SE(B)] specimens represent a better match to the constraint conditions of surface-breaking circumferential cracks in large-diameter pipelines during service (Shen, G., Bouchard, R., Gianetto, J. A., and Tyson, W. R., “Fracture Toughness Evaluation of High Strength Steel Pipe,” Proceedings of PVP2008, ASME Pressure Vessel and Piping Division Conference, Chicago, IL, July 27–31, ASME, New York, 2008). However, the SE(T) geometry is not included in any of the most widely used elastic-plastic fracture mechanics (EPFM) test standards. A procedure has been developed for performing and analyzing SE(T) toughness tests using a single-specimen technique that includes formulas for calculating the J-integral and crack-tip opening displacement, as well as for estimating crack size using rotation-corrected elastic unloading compliance. Here, crack-resistance curves and critical toughness values obtained from shallow-crack SE(T) specimens (a0/W ≈ 0.25) are compared to shallow-crack (a0/W ≈ 0.25) SE(B) specimens. We believe that the SE(T) methodology is mature enough to be considered for inclusion in future revisions of EPFM standards such as ASTM E1820 and ISO 12135, although additional work is needed to establish validity limits for SE(T) specimens.

The Strain Capacity of the Girth Weld Joint Cracked at “Near-seam Zone” in Tensile Test

The specimens of girth weld joint of Φ813mm×14.7 mm pipeline cracked at “near-seam zone” in tensile test. It was considered unacceptable for strain-based design pipelines in some current standards. The girth weld joint for X70 pipelines with “near-seam zone” crack were researched by CWP test and single-edge notch test (SENT). The high strain capacity is demonstrated by resistance curve tangency approach and CWP test results. The results imply the considerable tensile fracture resistance and consequently strain capacity when the strength overmatch.

Mechanical properties of Ti6.5Al2Zr1Mo1V titanium alloy with EBW under different temperatures

With the development of welding technology, electron beam welding (EBW) is manufacturing the aero parts with heavy thickness. To improve welding quality and mechanical property, we employed EBW with beam oscillation on Ti6.5Al2Zr1Mo1V alloy with 60mm thickness, and observed the weld microstructures after X-ray NDT, and conducted the tensile and impact tests under different temperatures. The results showed the crystals of the welds were fringe, and the microstructures of the welds were acicular α′ martensite. The acicular and isometric crystals existed in HAZ, which were α′ martensite mixed with α+β structure. With the increase of the temperatures, tensile properties of the top and bottom joints gradually lowered, and the strengths were almost equal to base metal, and the percentage elongations after fracture were lower than those of base metals. The strength overmatching of the joints resulted in the inhomogeneous elastic–plastic deformations, and tensile specimens of the joints ruptured in base metal. The impact toughnesses of top welds were equal to the bottoms under different temperatures, which were lower than those of base metals. Electron beam oscillation improved the uniformity of the microstructures, which contributed to the homogeneity of mechanical properties from the top to bottom joints.

Investigation of Bulging Behavior of Coke Drum: Feasible Study on Causes of Bulging

Coke drums undergo cyclic operations typically in the temperature range from ambient temperature to about 500 °C (930 °F). During quenching, the coke drum is inevitably subjected to a rapid drop in temperature because cooling water is injected directly into the coke drum through the bottom inlet nozzle. The temperature profile on the shell surface is no more uneven in quenching, and can vary in each cycle of quenching operation. Such complicate thermal profile induces large strains in the shell portion of the coke drum, and eventually causes damage like bulging or cracking. This study makes investigations into the bulging behavior of the coke drum by the thermal elastic-plastic FE-analysis. In this work, a feasible study is conducted on potential causes of bulging. As factors inducing a heterogeneous plasticity in the shell structure of the coke drum, the strength overmatch of welds and the uneven temperature field in quenching as well as quenching conditions are focused. The analytical result shows that strength overmatch in girth seam welds can be one of the causes of plastic distortion under one operating cycle. The lower rising rate of cooling source can induce plastic straining over the whole shell wall, which tends to induce more remarkable plastic distortion.

Effects of Initial Temper Condition and Postweld Heat Treatment on the Properties of Dissimilar Friction-Stir-Welded Joints between AA7075 and AA6061 Aluminum Alloys

In this study, dissimilar AA7075-O/6061-O and AA7075-T6/6061-T6 butt joints were produced by friction stir welding (FSW), and postweld heat treatment (PWHT) was applied to the joints obtained. The effects of initial temper condition and PWHT on the microstructure and mechanical properties of the dissimilar joints were thus investigated. It was demonstrated that sound dissimilar joints can be produced for both temper conditions. A hardness increase in the joint area (i.e., strength overmatching) was obtained in the joints produced in the O-temper condition, whereas a hardness loss was observed in the joint area of the joints obtained in the T6 temper condition. It was also well demonstrated that PWHT could be used in order to improve the joint properties for both O and T6 joints provided that the joint is defect-free prior to subsequent heat treatment.

CTOD Toughness Correction for Laser Welded Joints with Narrow Hardened Zone

The influences of very narrow weld bead and highly strength overmatching on the fracture toughness of laser beam welded components are investigated. This study focuses on the difference of plastic constraint in fracture toughness specimen (3-point bend specimen, 3PB) and structural components with crack in the weld metal. The equivalent crack tip opening displacement (CTOD) ratio, β, for the correction of CTOD toughness for constraint loss in structural components is numerically analyzed. The narrowness of weld bead induces constraint loss by plastic deformation in the base metal, both the 3PB and structural components. Nevertheless, in the case of the wide plate with shallow crack, the highly strength overmatching elevates the opening stress. Therefore, it is found that the β increase with increasing strength mismatch ratio. The influence of highly strength overmatching on β is important for assessment of fracture performance of laser beam welded component. The applicability of β to correction of critical CTOD in laser beam welded components is discussed. The fracture toughness tests with the 3PB and tension panel (edge through-crack panel) of the laser beam welded joints were conducted at a low temperature in the brittle fracture range. It is indicated that β can be applied to the CTOD toughness correction in laser beam welded component.

Weld Strength Mismatch in Strain Based Flaw Assessment: Which Definition to Use?

Weld strength mismatch is a key factor in the strain based assessment of flawed girth welds under tension. A strength overmatching weld shields potential flaws within the weld itself from remotely applied deformations and consequently reduces crack driving force. Although this effect has been recognized for decades, different weld strength overmatch definitions exist, and it is not yet fully established which of those is most relevant to a strain based flaw assessment. In an effort to clarify this unsolved question, the authors have performed a large series of parametric finite element analyses of curved wide plate tests. This paper provides an experimental validation of the model and subsequently discusses representative results. It is found that crack driving force is influenced by the shape of the pipe metals' stress–strain curves, which influences the representativeness of two common mismatch definitions (based on yield strength and on ultimate tensile strength). Effects of strength mismatch on strain capacity of a flawed girth weld are best described on the basis of a flow stress, defined as the average of yield and ultimate tensile strength. Based on the observations, a framework for a new strain capacity equation is proposed.

Effects of temper condition and post weld heat treatment on the microstructure and mechanical properties of friction stir butt-welded AA7075 Al alloy plates

In this study, AA7075-O and AA7075-T6 Al alloy plates were friction stir butt welded using two sets of weld parameters in order to investigate the effects of temper condition prior to joining and post weld heat treatment on microstructure and mechanical properties of the joints. Another goal of the work is to determine the possibility of restoring the loss of strength in the joint area experienced in welding of this alloy in the age-hardened condition by subsequent heat treatment. The study revealed that the friction stir welding resulted in a strength undermatching when the alloy joined in T6 condition while a significant strength overmatching was obtained in the joints produced in O temper condition. The post weld heat treatment led to a significant recovery in the strength of the joints produced in T6 condition, thus a significant increase in the joint performance. On the other hand, it led to a decrease in the joint performance value of the joints produced in O temper condition although the strength was increased. Furthermore, the post weld heat treatment resulted in abnormal grain growth in the joint area the degree of which depends on weld parameters used and the prior temper condition.

J and CTOD estimation formulas for C(T) fracture specimens including effects of weld strength overmatch

This work focuses on an evaluation procedure to determine the elastic–plastic J-integral and Crack Tip Opening Displacement (CTOD) fracture toughness based upon the η-method for C(T) fracture specimens made of homogeneous and welded steels. The primary objective of this investigation is to enlarge on previous developments of J and CTOD estimation procedures for this crack configuration while, at the same time, addressing effects of strength mismatch on the plastic η-factors. The present analyses enable the introduction of a larger set of factors η for a wide range of crack sizes (as measured by the a/W-ratio) and material properties, including different levels of weld strength mismatch, applicable to pipeline and pressure vessel steels. Very detailed non-linear finite element analyses for plane-strain and 3-D models of C(T) fracture specimens with centerline-cracked welds provide the evolution of load with increased load-line and crack mouth opening displacement required for the estimation procedure. Overall, the present study, when taken together with previous investigations, provides a fairly extensive body of results to determine parameters J and CTOD for different materials using C(T) specimens with varying overmatch conditions.

Effect of electron beam welding on fracture behaviour of thick TC4-DT alloy

Fracture toughness of a damage tolerant Ti6Al4V (TC4-DT) alloy and its electron beam welded joint was investigated using compact tension specimens. Moreover, tensile test, microhardness test and microstructure examination were carried out. The result indicates that the weld metal is strength overmatching. The tensile strength and microhardness of the weld metal are higher than those of the base metal, but the ductility and the energy absorbed before fracture are lower. In addition, the fracture toughness K1c of the base metal is relatively high. The fracture toughness of the weld metal decreases compared with the base metal. It is expected that the toughness reduction of the weld metal is mainly related to the as solidified coarse β columnar grain structure containing martensite in comparison with the bimodal structure of the base metal. Moreover, fractographic observation shows that the fracture mode of both weld and base metals is found to be ductile.

Design of a (mini) wide plate specimen for strain-based weld integrity assessment

Wide plate tension tests are commonly executed to investigate the integrity of defective weldsunder a uniaxial load. The specimen can be flat or curved, depending on the geometry from which it hasbeen extracted (plate or pipe). Despite its usefulness, the design of the (curved) wide plate test is still notstandardized up-to-date. This paper compares two specimen designs with a different length-to-width ratiothrough finite element analysis, using a design-of-experiments approach to account for different influentialfactors. The results reveal significant differences between the interpretation of tests with net sectioncollapse and gross section collapse, promoted by weld strength overmatch. Further, both investigateddesigns tend to provide similar estimates of failure mode, strain capacity and crack driving force. Hence,the shorter specimen is considered an acceptable alternative to the slightly more representative longerspecimen.

Microstructure and Hardness of Friction Stir Welds in Pure Copper

The aim of present research was to study the effect of the position of the tool relative to the support backing plate of the FSW machine on the formation of defects and on alterations of the microstructure and mechanical properties of friction stir welds in phosphorus-deoxidised copper (Cu-DHP) thin sheets of 1 mm thick. The welds were carried out using position control conditions; distances between the tool and the backing plate of 0.1 mm, 0.075 mm and 0.05 mm were used. The formation of defects like continuous voids along the weld is very influenced by the tool position, though the heat-input plays an important role in the process. Large grain refinement was observed in the nugget of the welds; the change of the relative tool position has little effect on this grain refinement. Substantial hardening was observed in the thermomechanically affected zone (TMAZ) of the welds. The welds exempt of defects, such as continuous voids, attained a little tensile strength overmatch condition.