The Specifications Dilemma Posed by Ultra High Toughness Line Pipe Steels

Transcription

1 The Specifications Dilemma Posed by Ultra High Toughness Line Pipe Steels B N Leis J M Gray, F J Barbaro Principal Managing Partner Director B N Leis, Consultant, Inc. Microalloyed Steel Institute Inc. Barbaro & Associates, Pty Ltd Worthington, OH Houston, TX Wollongong, NSW bleis@columbus.rr.co. 1

2 Abstract The unplanned release of hydrocarbons from a pipeline can lead to significant pollution and/or the horrific potential of explosion and a very large fire, depending on the transported product. For this reason, the manufacturing procedure specification (MPS) developed to ensure the design requirements are met by the steel and pipe making process is a critical element of the fracture control plan, whose purpose is to protect the environment and ensure public safety, and preserve the operator s investment in the asset. Pipelines transporting compressible hydrocarbons like methane or high vapor pressure liquids under supercritical conditions are uniquely susceptible to long propagating failures. Consequently, and so compliance and control of both MPS and the manufacturing procedure qualification testing (MPQT) are critical particularly for designs that involve higher pressure and larger diameters. This paper considers pipeline steel specification with particular concern for long propagating shear failures in advanced design larger diameter higher pressure pipelines made of thinner wall highergrade steels. Assuming that the arrest requirements can be reliably predicted it remains to specify the steel design, and associated MPS. While standards exist for use in MPQTs to establish that the MPS requirements have been met, very often CVN specimens remain unbroken, while DWTT specimens exhibit features that are inconsistent with the historic response and assumptions that underlie many standards. In addition, sub width specimens are often used, whereas there is no standardized means to scale those results consistent with the full width response required by some standards. Finally, empirical models such as the Battelle two curve model (BTCM) widely used to predict required arrest resistance have their roots in sub width specimens, yet their outcome is widely expressed in a full size context. This paper reviews results for sub width specimens developed for steels in the era that the BTCM was calibrated to establish scaling rules to facilitate prediction in a full size setting. Thereafter, issues associated with the use of sub width specimens are reviewed and criteria are developed to scale results from such testing for use in the MPS, and MPQT, which is presented as a function of toughness. Finally, issues associated with the acceptance of data from unbroken CVN specimens are reviewed, as are known issues in the interpretation of DWTT fracture surfaces. Acronyms AISI American Iron and Steel Institute API American Petroleum Institute BMI Battelle Memorial Institute BTCM Battelle two curve model CSA Canadian Standards Association CVN charpy vee notch CV100 energy at the onset of a RUS CTOA crack tip opening angle DWTT drop weight tear test FSE full size equivalent ITP inspection and test plan MPS manufacturing procedure specification PAT production acceptance testing MPQT mfgr procedure qualification test QA quality assurance QC quality control Q & T quenched and tempered RUS rising upper shelf (occurs at 100% SA) SA shear area SMYS specified minimum yield stress SBD strain based design USE upper shelf energy (at 100% SA) Keywords BTCM, CVN, DWTT, MPQT, MPS, fracture, line pipe, sub width, specification, unbroken specimen 2

3 Introduction The earliest hydrocarbon pipelines first moved natural gas from a local source to and between nearby houses through hollow logs in Fredonia, NY circa 1825 (1), while oil was transported first near Oil City, PA through a short 2 inch diameter cast iron system in the early 1860s (2). Pipelines came into use in Canada in 1862, moving oil from the Petrolia field near the city of Sarnia into the city, with natural gas being transported first by pipeline a year later moving the gas through a castiron system into the city of Trois Rivières. (3) Since those early years, the demand for hydrocarbons has grown almost exponentially, with pipelines increasingly used in raw energy transportation, and also for finished products. Such also has been the case for Australia, wherein pipeline systems continue to expand in number and scope, and also in diameter as coal seam methane export pipelines evolved. As time passed, the local supply basins were exhausted, and easy access to shallow reserves of sweet crude and natural gas diminished. Internationally, supply basins have become increasingly remote to the markets, with many replaced by offshore fields, or arctic scenarios with very low temperatures, and ice offshore or permafrost onshore. Longer shipping distances as occur in Australia lead to increased transportation costs that can be minimized through use of higherstrength pipelines operating at higher pressure (and/or larger diameter). But, the benefits that accrue to thinner wall pipelines can only be realized if concern for factors such as weldability and propagating shear failure can be cost effectively managed. Competition between suppliers is critical to keeping costs manageable, such that access to line pipe that satisfies the design requirements is a critical element of cost management for any operator contemplating construction of a long distance pipeline. Competition can only develop if there is a basis to procure the line pipe in a manner that affects quality delivery of consistently produced pipe steel, and line pipe. Quality has been managed in recent years through the use of a Manufacturing Procedure Specification (MPS), and Manufacturing Procedure Qualification Testing (MPQT). These practices, when coupled with appropriate Quality Assurance (QA) and Quality Control (QC) practices, and an Inspection and Test Plan (ITP) and Production Acceptance Testing (PAT), can provide a basis for competitive procurement whose outcome is matched to the design requirements for the pipeline. This paper considers aspects of pipeline steel and line pipe specifications with which to manage the threat posed by long propagating shear failures, which is a concern for advanced design higher pressure pipelines made of thinner wall higher grade steels. Provided that arrest requirements can be reliably predicted, it remains to specify the steel design, and develop a reproducible manufacturing process. While standards exist for use in MPQT to establish that the MPS requirements have been met, very often Charpy Vee Notch (CVN) specimens remain unbroken, and DWTT specimens lead to features that are inconsistent with the historic response that underlies standards. In addition, sub width specimens are often used, whereas there are no standardized means to scale those results consistent with the full size response required by some standards. Finally, empirical models such as the Battelle two curve model (BTCM) widely used to predict required arrest resistance have their roots in sub width specimens, yet their outcome is widely expressed in a full size context. This paper reviews results for sub width specimens developed for steels from the era when the BTCM was calibrated to establish scaling rules to facilitate prediction of arrest requirements in a full size setting, as required by many standards. Thereafter, issues associated with the use of sub 3

4 width specimens are reviewed and criteria are developed to scale results from such testing for use in the MPS, and PQT/PAT, which are presented as a function of toughness. Finally, issues associated with the acceptance of data from unbroken CVN specimens are reviewed, and issues in the interpretation of DWTT fracture surfaces briefly noted. Concepts in Specifying Modern Line Pipe The foundation for much of the design of today s pipelines had evolved even before the first hydrocarbon pipelines were build, as for much of the world pipelines are sized and specified in terms of working stress design, the basis for which is linear elasticity. The concept of linear elastic response was first stated in the format of an anagram in 1660, whereas the mathematical framework for the theory of elasticity including the solution to problems much more complex than a pressurized pipeline was laid down during the 1800s. (4) This theory led to a simple equation between the pressure contained by the pipe, the diameter and wall thickness of the pipe, and the yield stress of the material from which it was made. Because engineering materials can be prone to variable properties, the steel s yield stress was reduced to provide a margin of safety that pipeline regulations and codes term the specified minimum yield stress (SMYS). This design basis, known to many in the pipeline industry as Barlow s equation, has been in use for pressure piping since it emerged in the first half of the 19 th century. Pipeline design today involves much more than consideration of SMYS. Concern for ground stability and related issues has led to the introduction of strain based design (SBD) concepts and technology, with the integrity of girth welds recognized as being as important in this context as the longitudinal strain capacity of the pipe body. Weldability requirements for the line pipe steel emerge in that context, as does the development of reliable and cost effective welding procedures and consumables. As noted earlier, another concern involves the arrest of propagating brittle and ductile (shear) failures. Historically this has been addressed through a fracture control plan, which included toughness requirements to ensure arrest in terms of a minimum CVN energy, and ductile response established by the steel s behavior in a DWTT. Although this historic view of arrest based on fracture concepts is shifting toward a collapse based assessment, until that transition is completed specifications will continue to be written in terms of CVN energy and DWTT shear area. Other concerns include corrosion resistance, coating and protection, and so on, but as these are often addressed independent of the steel and line pipe, they are noted here simply for the sake of completeness. As indicated earlier, the present paper targets only a portion of these concerns in reference to the MPS and MPQT involved in the procurement process. The concept of the MPS for present purposes emerged in a codified sense through changes made to API 5L (5), in the 3 rd Addendum of its 44 th Edition, circa This Addendum also considered MPQT requirements, while other aspects such as the ITP have been added since. These concepts for such purposes can be defined as follows: the MPS sets forth the main characteristics of the manufacturing process to be used for a specific pipe order the critical parameters presented and quantified therein are subject to audit by a third party; the MPQT identifies the testing methods used during the initial / trial production as the basis to qualify the MPS; and the ITP details the inspection activities, calibration requirements, responsible party or parties, and the acceptance criteria for the production run. 4

5 Since these concepts were introduced, MPQT typically has relied on standardized and so widely accepted test methods, as has PAT. This facilitates the development of a MPS by several potential suppliers, and so creates a basis for competitive procurement through a process that is expedient and cost effective. That being said, some applications that involve demanding performance requirements also have led to issues associated with such standard practices. One long known example of this involves the DWTT, which in some specimens exhibit what has been termed inverse fracture (6). Problems also have been evident with the CVN test, such as the occurrence of unbroken specimens, and the need to resort to sub width specimens. While these issues can significantly affect the utility and certainty of a MPQT or a PAT, they have not been widely discussed in the literature, where at best they get passing consideration (7). Problems also have been traced to the use of technologies that rely on these standardized tests, such as the broadly discussed limitations with the BTCM (8). While much has been written on those issues, a lesser known but equally consequential concern involves the BTCM s calibration based on sub width samples, and its generalization to a full size equivalent (FSE) value neither of which has been properly considered in any detail. Yet, this generalization is essential, as some codes require that full size specimens are used when dealing with pipe geometries that could yield such a sample (e.g.,9). Because issues involving the DWTT and inverse fracture have now been discussed for many years, this paper focuses on the more recent issues that have emerged in MPQT/PAT involving the CVN test, and related concerns that involve the relationship between sub width and full size specimens. The following sections first consider sub width specimens and the derivation of the BTCM relative to 2/3 size specimens in turn, before discussing unbroken specimens and closing with general discussion. Use of Full Width vs Sub Width Specimens Sub Sized versus Width Modified CVN Specimens The pipeline industry often uses the term sub sized specimen in applications where the thickness of the pipe precludes the use of a full size specimen, as was the case when the BTCM was being developed. In contrast, outside the pipeline community the term sub sized refers to a subset of a more general database that includes size modified specimens, where size modified refers to any variation of a standard notched bend bar impact test specimen. The term sub sized in that context embraces all specimens that are physically downsized from the standard so called full size specimen. Sub sized specimens that are less than 10 mm deep are depth modified specimens, while those that are narrower than 10 mm wide are width modified specimens. Much work has also been published on specimens whose span or notch depth is less than in the standard, moving toward so called miniature specimens. Dimensional analysis indicates that some specimen dimensions affect the bending load linearly, as for example the width, while other dimensions such as beam depth in regard to moment of inertia affect load nonlinearly. While solid mechanics makes clear the role of such dimensions, their quantitative effect on aspects such as constraint is less apparent. Given that concern for propagating shear failure involves axial through wall propagation along the pipeline, the limitation on the specimen size when the thickness of the pipe precludes a full size specimen affects the width of the test beam, not its depth, such that the sub sized specimens of interest to the pipeline community are width modified specimens. Because the BTCM was 5

6 calibrated in reference to width modified specimens, this section considers only width modified data even when they may be referred to as sub sized. Approach and Scope In concept, the effects of full size vs sub width specimens on the failure response in a CVN test could be based on numerical simulation that considered the effects of thickness on the stressstrain response and the evolution of void nucleation, growth, and coalescence leading to failure. However, this is a dynamics problem that involves physical and geometric nonlinearity, and the effects of strain rate, which requires calibration data that often are not available or are tacitly ignored. As an experimental matrix is needed to broadly develop such calibration inputs, this paper evaluates results for CVN studies concerning the effects of size on absorbed energy as a function of temperature, to develop trends as a function of specimen width. This approach ensures that steels relevant to the era the BTCM was calibrated are considered, and avoids uncertainty in the outcome that enters via the complexity of numerical modelling. Interest in width modified specimens becomes evident even before the 1930s, which was a period of much change in this testing practice (9) as it evolved from its roots as a screening tool to avoid brittle failure circa Such changes led to the release of a tentative standard in 1933 (ASTM E23 33T). Refinements continued into the mid 1950s, leading to the first release of this test practice free of its tentative status in (11) Because of inconsistencies in test practices prior to the mid 1950s, data developed prior to this era have been excluded. (e.g.,12). That said, results from replicated testing in 14 studies focused on width modified specimens (13 25) are readily available as the basis to quantify trends in sub width specimen effects. Because the BTCM dealt with propagating shear (ductile) failure, it was formulated in terms of upper shelf energy quantified by the CVN test, with ductile response relative to the full pipe wall thickness and concern for the transition temperature independently assured via results from the DWTT. Accordingly, the focus herein is the so called upper shelf energy (USE), or plateau energy, which tends to limit, but not preclude, the effects of factors such as plastic constraint and strain rate dependence. As steels that show a rising upper shelf (RUS) lead to temperature dependent values of the USE, the comparison between energy as a function of size in such cases was made at the onset of the RUS, which is often termed CV100. The BTCM and Relevant Sub Width Specimen USE Response What is Relevant Regarding the Development of the BTCM? The equations that underlie the BTCM appear in papers presented at conferences as early as 1971 (26). For this reason the data used in their formulation and calibration reflects full scale and laboratory scale experiments carried out by Battelle up through 1970, with the majority of the testing done in the late 1960s. Data potentially useful to evaluate the validity of the BTCM in the format published in 1974 (27) also includes the full scale experiments done for the American Iron and Steel Institute (AISI) by US Steel, all of which were completed and reported in the early 1970s (28). The pipe wall thickness for the Battelle database runs from to inch (7.92 to 9.93 mm) in diameters from 26 to 42 inches (660 to 1067 mm), covering Grades between X52 and X65 (358 and 448 MPa), with one test done on an experimental Q&T X100 (689 MPa). The AISI testing included one Grade X52 (358 MPa) test, but focused on X60 and X65 (413 and 448 MPa), with one test also done for X70 (482 MPa) and for X90 (620 MPa). 6

7 Figure 1a trends the FSE CVN energy for Battelle s testing, while Figure 1b combines that data with the AISI database, with both shown as a histogram and a cumulative distribution. The BTCM emerged shortly after the CVN test practice became a standard free of its initial tentative designation in 1966 (9). As this came after stabilization in the refinements that led to this action, the CVN results that underlie calibration of the BTCM have been developed according to a known standard, and so are comparable to results of other such testing according to that standard considering the effects of sub width versus full width specimens in that timeframe. a) BMI database b) BMI AISI combined database Figure 1. Range of toughness that underlies the BTCM Figure 1a indicates that Battelle s full scale testing involved rather low toughness levels as compared to today s production. All such results fall below ~68 J (50 ft lb), one half of which are at levels less than 47 J (35 ft lb). The range of toughness for the AISI testing largely fall below 108 J (~80 ft lb). It follows that width scaled USE results that fall within the scope evident in Figure 1 are appropriate in assessing the best approach to scale the original 2/3 size predictions to a FSE value, as required in Codes like CSA Z245.1 (10) (for larger diameter line pipes). Zeno (13) was first to consider the effects of sub width specimens after the test practices stabilized in the mid 1950s (9). He evaluated two lower toughness steels: a normalized SAE 1020; and a quenched and tempered (Q&T) SAE 9130 steel. In comparison to the toughness of line pipe steels common today, these steels had relatively low USE, with both at about 47 J (35 ft lb). The results developed by Zeno are shown as the six open diamond symbols in Figure 2, with each data pair representing the average of triplicate testing. The y axis in this figure is the ratio of the USE to fail a width modified specimen divided by the USE for the corresponding full size specimen, while the x axis is the measured width of the specimen. From this figure it is apparent that Zeno s data fall along the dashed trend that represents a linear scaling with specimen width, for both 1/4 size and 1/2 size specimens, which are all benchmarked against full size data. USE Data Relevant to the BTCM Curll (14) presented two extensive reports on sub sized specimens shortly after Zeno s work was published. His work involved nine Q&T steels that covered a broad scope of specimen widths, from a two to one (2/1) upsized width, down through 1/8 subwidth specimens, as well as other size modified variants. Excluding data for other than width modified specimens reduces the scope to five steels, including AISI 3140M, AISI 4042M, AISI 4140, Class 90, and a Ni Steel, in widths down to 1/4 size. These data cover a range of USE from 35 to 107 J (26 to 78 ft lb). The 12 data pairs that each quantify the average of triplicate results are also included in Figure 2, being shown as open triangle symbols. As for Zeno s work, the data pairs for sub width tests scatter either side of 7

8 a linear scaling on thickness across the range of sizes tested, whereas the result for the 2/1 upsized specimen falls somewhat below this linear trend. Figure 2. Effect of specimen width for lower toughness steels for tests done in the 1950s Results for width modified specimens relevant to the era the BTCM was calibrated also can be derived from testing done in the 1960s. Results for three structural steels and a very high strength Q&T steel were reported in 1965 by McNichol (15). Excluding the Q & T steel, their USE values run from 234 to 280 J (~173 to ~207 ft lb) which are very high for 1960s steels. Results were also reported in 1968 (16) for a quite tough semi killed carbon steel with a USE of 203 J (~150 ft lb). Finally, Gross alluded to data covering a broad range of grades in a 1969 paper, but only a small part of this dataset was actually presented (17). The USE for that steel was more representative of the 1960s, but still quite high at 129 J (~95 ft lb). Figure 3 illustrates the full range CVN response for the A283 steel evaluated by McNichol, with data presented for a 3/2 upsize width, the reference full size width, and three sub widths: 1/2, 1/4; and 1/8 size. Inspection of these curves indicates that the values of the USE for the upsize and full size specimens are roughly the same, whereas the sub width values of the USE differ relative to the full size value more or less in proportion to the specimen width. It follows that these results will deviate for the pattern evident for the lower toughness 1950s steels shown in Figure 2, at least for the upsized specimen. Figure 3. CVN energy and %SA versus temperature for A283 steel (digitized from (15)) The USE results from Figure 3 are presented along with the other 1960s data in Figure 4 using the same format as in Figure 2. It is apparent from this figure that the results for the moderately 8

9 tough A302 B steel tested by Gross follow the same linear scaling trend evident tracked by the work done in the 1950s. In contrast, the results for the high toughness steels evaluated by McNichol fall well above and are offset to the linear scaling trend. Results for the five steels alluded to by Gross (17) were subsequently reported in 1970 (29), which cursory trending indicates tendencies similar to that for the A302 B results shown in Figure 4. Based on the trends in Figures 2 and 4 it follows that for lower toughness steels tested through the end of the 1960s there is no clear effect of specimen width among the 15 width scaled data pairs. A similar trend is evident for the 14 width scaled data pairs representing the 1960s. These 29 data pairs reflect steels whose toughness ranges up to just beyond 190 J (140 ft lb). Because this trending reflects the average of triplicate or greater testing, concern for the effects of scatter is mitigated. As these data reflect a robust scope of steels and toughness levels one can conclude that the FSE energy in the era the BTCM was calibrated can be reliably determined by simply scaling the energy per unit width in direct proportion to that for a full size specimen. Figure 4. Effect of specimen width for tests done in the 1960s across a range of toughness Summary Regarding Width Scaling BTCM Predictions and Related Discussion Close examination of the results in Figures 2 and 4 shows no trends regarding the class of steel considered, which is also the case for width scaled data among that trended by Wallin (18). For this reason, the results in Figures 2 and 4 can be considered to be independent of the class of steel, and it s processing. As these data are directly relevant to the range of toughness involved in calibrating the BTCM and free of influence of class of steel, it can also be concluded that linear scaling can be applied in the context of the BTCM. Thus, the equations that underlie the original BTCM that were developed and expressed in reference to a 2/3 size specimen (27), can be simply scaled in proportion to width to obtain the full size equivalent value of required arrest resistance. The observation that the FSE USE can be determined by simply scaling the energy per unit width is not a surprise when dealing with lower toughness steels, for several reasons. First, USE data for such steels involves relatively small shear lips, and the overall distortion of the specimen is limited. Second, very little crack tip blunting occurs, such that high excess energy can be developed by most test systems, and the hammer velocity at impact is not slowed appreciably during the test. As the hammer velocity is sustained, and the crack tip remains sharp, the strain rate at the cracktip remains high, which keeps the yield stress as its high rate level that in turn keeps the crack tip plastic zone very confined. Third, and most important, the energy dissipated in creating new crack surface under the conditions just noted is the dominant fraction of the total energy dissipated, 9

10 which has been found to remain more or less constant at toughness levels up to ~100 J (~75 ftlb) (30). In such cases the creation of new crack surface over a limited distance approaches the circumstances characterized by Griffith s concept of surface energy density (31). Thus, the energy to fail geometrically comparable beam depths (as occurs in width scaled CVN specimens) is anticipated to scale directly with width. Modern Tougher Steels and Sub Width Energy Scaling Correlations Figure 4 indicates that steels with high FSE energies also tend to show a linear scaling, but this trend is offset to higher energy ratios. Such a difference in scaling is not unexpected as the USE increases, as mechanics theory indicates that the failure behavior would change with the shift from fracture controlled to collapse controlled failure as the toughness increases. High USE levels also promote continued blunting and the spread of plasticity as the notch root advances into the CVN specimen, whereas at lower values of USE the crack tip plasticity is confined leading simply to the formation of new crack surface. This shift to collapse control is influenced by differences in the flow properties of the steel, such that strain rate, strain hardening exponent, and distribution of strains and related dissipation are important factors. The excess energy of the test system, tup geometry, and other factors also can affect differences in constraint also couple to complicate the response and attempts to predict this shift and the extent of its effects. Post 1960s Results for Width Modified Specimens Figure 5 presents results generated since the BTCM was calibrated (18 25,29,30), adding 39 data pairs that are representative of replicated testing of width modified specimens done from the 1970s until This additional width modified data includes only a small portion of the sub size data reported over this time period. This is because the focus of sub size testing in this era has been mini specimens that incorporate other dimensional downsizing designed to maximize the data extracted from dwindling stocks of irradiated surveillance material. As depth is often among the scaled dimensions in such mini specimens, this testing very often leads to a nonlinear correlation with specimen size. Figure 5. Additional data representing more recent steels and a scaling rule As evident in the symbol key in Figure 5, the additional data include steels whose toughness runs slightly higher than for the prior data, but otherwise falls within the scope discussed in the context of the pre 1970s data. Inspection of this figure indicates that the results from this more recent 10

11 testing track the response established during the 1950s and 1960s. It is evident that the results for the higher toughness steels tend to show a linear scaling, with this trend being offset to higher energy ratios. In contrast, the results for the lower USE steels scatter about a linear trend for subwidth specimens, but fall below this linear trend for upsized specimens. In addition to the linear scaling rule discussed above, Figure 5 also presents the only sub size scaling rule developed specifically in regard to line pipe steels (25). Developed in Australia circa 1996 based on CVN testing of thin X70 and X80 skelp, this correlation expresses the resistance of a sub width specimen as a power law function of the width or area ratio, with the value of the exponent n taken at 1.5. It is apparent from Figure 5 that the trend generated at n = 1.5 lies along the lower bound of the sub width data, but falls well above the limited data for upsized specimens. As many modern steels including those produced in as well as sourced for use in Australia show a RUS, it is instructive to more broadly illustrate and quantify the potential effects of RUS behavior for typical modern higher strength steels, as discussed in the following section. Potential Effects of RUS Behavior Figure 6 presents sub width CVN results for modern steels produced in Australia or sourced for use in Australia. The 3/4 sub width results presented in Figure 6a represent 2011 Australian production for a 457 by 9.1 mm Gr 482 (18 by inch X70) pipeline (32). It is apparent from the composition of this steel and insight as to its processing that it is not unusual relative to typical domestic production. As usual, the x axis in Figure 6a is the test temperature, with energy plotted on the left margin and a second y axis value on the right hand margin to present percent SA. The SA data are shown as diamond symbols, while energy data are shown as + symbols, with the energy values being linearly scaled in this figure to a FSE value. a) 2011 X70 line pipe b) post millennial Ultra Pipe Figure 6. CVN results for recent Australian production of line pipe and Ultra Pipe steels It is apparent that this steel shows fully ductile response to below 73 C ( 100 F), leading to CV100 of about ~135 J (100 ft lb) at about 93 C ( 135 F). It can be seen from Figure 6a that a significant RUS develops above what is a quite low CV100 temperature, and that the USE increases in this RUS by just more than 135 J (100 ft lb). The results in this figure indicate that comparison of a shelf energy measured on this 3/4 width specimen to results from another CVN test at any 11

12 temperature above 93 C ( 135 F) will lead to a result that is temperature dependent unless the RUS trends with temperature for the two tests are identical. In turn this means that the exponent in the power law scaling rule is potentially a strong function of temperature for RUS steels. Figure 6b presents a broad range of data for post millennial Australian production (33) wherein the y axis is CVN energy normalized to a FSE value by a power law consistent with Reference 25. As evident in this figure, this normalization of PAT done at 0 C (32 F) leads to an exemplary fit using n = 1.6, as there is virtually no bias in the USE with CVN specimen thickness. While this appears to be a compelling argument that the scaling rule for these steels is nonlinear, what is not clear for these results is whether this domestic production shows a RUS such as that in Figure 6a. If these steels showed a RUS, then testing at any temperature above CV100 leads to exponent values that vary with PAT temperature, which limits the utility of such trending. While this remains an open question, because similar steels produced using similar processing schedules can be anticipated to behave similarly in such properties tests, one can anticipate that the steels that underlie Figure 6b would emulate to some extent the trend evident in Figure 6a. Of note in this context, recently obtained (34,35) CVN results for domestic production and/or pipe sourced for use in Australia presented in form of Figure 6a both show the traits evident in this figure. On this basis, before it can be concluded that width scaling is nonlinear for domestic applications, it is suggested that CVN data be developed as a function of temperature, to quantify the response of these steels relative to CV100, after which a basis for scaling energy could be confirmed. While normalizing the data in Figure 6b using n = 1.6 minimizes the bias in CVN specimen width, it also leads to significant scatter. For example, the spread of normalized CVN values for the 3/4 size specimens ranges from 204 J up through 295 J. Correctly matching these energies relative to what was observed in the testing leads to a swing in values of the exponent from 1.05 up to As becomes clear in later discussion, such trends can be rationalized by the effects of a RUS, which reinforces the merits of more broadly understanding the response of such steels particularly if such data are associated with designs where propagating shear failure is a concern. Trending RUS Behavior for Modern Steels Figures 7a, 7b, and 7c present CVN resistance curves for sub width CVN testing of a modern steel that is useful to more broadly illustrate the effects of RUS behavior. This data has been developed in regard to a recent MPQT for line pipe being produced on the Pacific Rim. All data represent fully ductile response as evident in the figures by the notation USE. These results reflect valid CVN testing according to ASTM E23 (36) and were developed in reference to a MPS that targeted high all heat average (AHA) CVN. MPQT of full size specimens made from the same heats of steel at 5 C (23 F) led to an AHA ~358 J for what is noted in the figures as the heavier wall plate (20.6 mm or inch), and ~449 J for the lighter wall plate (22.9 mm or inch). Note that all results in these figures have been linearly scaled to a FSE energy to facilitate comparison with this full size data. In view of the rising energy evident in Figure 7a and in reference to the full size data noted above it is clear that this steel develops a strong RUS. Comparing the averaged trends for triplicate or better testing of two heats of the lighter plates (labeled X and Y), it is apparent that the RUS can vary significantly for nearly sequential heats of nominally the same steel. The observation that nominally the same steel that has been made and processed to the same MPS can develop strongly variable RUS behavior suggests this behavior is quite sensitive to what is nominally 12

13 identical steel composition and rolling schedule. This same pattern also developed among the several heats that underlies the AHA for the heavier plate. a) 3/4 width b) 1/2 width c) 1/3 width Figure 7. CVN results for line pipe from recent Pacific Rim production (all at same scale) Back extrapolating the CVN trends in Figure 7 to a common point of convergence at a lower temperature indicative of CV100 infers that the onset of this RUS behavior develops near 73 C ( 100 F), which is similar to that for Figure 6a. Referenced to this CV100 value, this steel tested using a 3/4 size specimen indicates a RUS as large as 200 J (148 ft lb), which is roughly 50 percent larger than observed for the Australian production discussed in reference to Figure 6a. The fact that the RUS can vary greatly within the restricted processing window controlled by a MPS typical of that in current use, as evident in Figure 7a, implies a significant swing will occur in the value of the exponent in a power law scaling rule in data developed for the same steel. The extent of the variation in this exponent increases when the results apparent in Figures 7b and 7c are considered. This reflects the observation that for this steel the extent of the RUS diminishes sharply with decreasing specimen thickness. This is clearly evident by comparing of the trends for the 1/2 size specimens in Figure 7b, which show only a modest influence of a RUS, and the trends for the 1/3 size specimens in Figure 7b, which show no clear evidence of a RUS. Consistent with prior practice (25) in using one sub size result as the benchmark to quantify the response of another, data for 1/3 size specimens can be taken as the benchmark for comparison to results of the 3/4 size specimens. If this is done at 0 C (32 F), as is usual in Australia to compare PAT results, then the value of the exponent needed to match the observed energy results ranges from 1.04 (to fit the X heat data) down to 0.40 (for the Y heat data). In contrast, inspection of the trends in Figure 7 indicates that adopting a temperature for such a comparison that lies above 0 C (32 F) has little effect on the just noted values of the exponent, whereas choosing a lower temperature would lead to smaller values of the exponent. Finally, if the trends for each part of Figure 7 are extrapolated back to a common value of CV100, and the comparison is made at that inferred value of CV100, then the value of the exponent is found to be unity, or very close to it. The point to be made in light of Figure 6a is that this Australian production shows clear evidence of a strong RUS, while the point relative to Figure 7 is that the extent of the RUS can vary strongly with 1) the processing of the steel within the scope of the MPS, 2) the width of CVN specimen used, and 3) the temperature used for the comparison. On this basis it is appropriate to either 1) broadly establish the absence of a RUS and/or 2) establish scaling rules at CV100 at which point the RUS is not a factor and the comparison is independent of the temperature adopted. 13

14 Summary in Regard to High Toughness Steels It follows that for steels such as those that fall into the upper data cluster in Figure 5 the energy recorded in testing 1/2 size to full size specimens requires no correction for the effects of specimen size. However, narrower specimens do require linear scaling following the trend evident in this figure. Unfortunately, at present too little is known to establish with any certainty what discriminates between this upper data cluster and that below it that follows the usual linear scaling. As noted at the outset of this section, many factors complicate the failure response of a notched impact bar, with a systematic study of these needed to understand the cause of this double clustered response, and other related issues. The width scaling rule for such specimens made of tough steels will remain uncertain until such work is completed. Unbroken Specimens ASTM E23 (36) indicates that an unbroken specimen result is acceptable for use in calculating the energy to fail a specimen in cases where the measured energy is less than 80 percent of the rated capacity of the test system. As this 20 percent excess energy requirement must be met to conclude that the result for a broken specimen is valid, this requirement does not uniquely qualify acceptance for unbroken specimens. Accordingly, results for unbroken specimens could be made acceptable by simply purchasing and qualifying a larger capacity CVN test machine. For this reason testing that led to unbroken specimens has not posed an issue for the steel or pipe mills as the producers simply upsized their facilities and thereby generated valid MPQT or PAT data. This fact is clear in that ASTM E23 has remained unchanged regarding unbroken specimens through several reaffirmation cycles. The acceptance of results for unbroken specimens as noted above is viable provided that 1) the inclusion of results for unbroken specimens does not affect a non conservative energy bias and 2) the appearance of an unbroken specimen differs little from those that break, in profile or in reference to the appearance of the fracture surface. Consider the images in Figure 8 in this context. a) unbroken with thin remaining ligament b) unbroken with thick remaining ligament Figure 8. Contrast in appearance of characteristically different unbroken CVN specimens Figure 8a presents a profile view that is characteristic of unbroken specimens that involve a narrow unbroken ligament, which also show limited evidence of plastic deformation local to contact with the anvils and point of loading. The profile view in Figure 8b, which is typified by a quite thick unbroken ligament, is characteristically much different from the unbroken specimen shown in Figure 8a. The wide unbroken ligament evident for such specimens is often one half of the initial net depth of the CVN specimen or more. In addition, such specimens show extensive 14

15 evidence of plastic deformation local to the contact with the anvils and point of loading, which in some cases show evidence of material scraped from the specimen as it passed through the anvils. Figure 9 provides further perspective for the contrasting appearances evident in Figure 8. The image shown in Figure 9a is a typical view of the fracture appearance for specimens like that shown in Figure 7a. This image has been captured from a location directly above the fracture surface with this positioning facilitated by breaking the thin remaining ligament in a full size specimen by the application of a single cycle of reversed bending, as outlined in ASTM E23. It shows features that are typical of a CVN fracture surface, except that for this steel they led to an unbroken specimen. Cracking initiates at the base of the V notch that is evident running across the bottom of this image. That cracking forms in combination with relatively small shear lips, and runs almost the full depth of this specimen, where examination of these features indicates the change in fracture morphology over the area traversed by the single cycle of reversed bending that failed the unbroken ligament. a) unbroken with thin remaining ligament b) unbroken with thick remaining ligament Figure 9. Fracture features for characteristically different unbroken CVN specimens Such features are in strong contrast to those typical of unbroken specimens that involve a thick remaining ligament, as shown in Figure 9b wherein the fracture surface has been outlined by a dotted rectangular box. In contrast to the image in Figure 9a, which is square as it shows the full 10x10 mm cross section of that specimen, the tearing evident in Figure 9b develops only through a portion of the depth thus its rectangular shape. Given the relative lengths evident for the sides of the dotted rectangular box in this perspective view, this tearing runs across ~40 percent of the initial depth of this specimen. As that depth includes the V notch evident across the topside of this figure, the features in this image indicate that this tearing runs into just ~20 percent the specimen s depth. Analysis using typical properties indicates that this very thick remaining ligament cannot be broken by a single cycle of reversed bending, but rather would require many such cycles, and require significant bending force. Because the results of such tests are used to quantify the resistance of a line pipe steel for comparison with the specified value of require arrest toughness, conceptually there should be similitude between the features from such testing and that used to calibrate predictive models, 15

16 like the BTCM. However, the contrast between historically typical response in Figure 9a versus that evident in Figure 9b makes clear such is not the case at least in terms of fracture features. The significant differences in the characteristic appearance between the two types of unbroken specimens suggests that the total energy dissipated likely differs systematically. Trending the results of instrumented CVN testing confirms this, as evident in Figure 10 (37). It is apparent from Figure 10 that a strong nonlinear shift from energy dissipated in propagation to initiation and deformation occurs for the CVN test beginning at about ~100 J (75 ft lb). Likewise, it is clear that the ratio of the initiation (plus deformation) energy to the propagation energy in a CVN specimen is inherently different for high toughness steels as compared to that for low toughness steels, and that the energy dissipated in initiation increased with toughness, as did that for plastic deformation. Changes such as those evident in Figures 9 and 10 pose no problems in a MPQT or a PAT using the CVN practice provided that the results remain useful in demonstrating that the steel satisfies the desired arrest requirements as quantified by the measured values of USE. It is prudent in this context to contrast the measured values for populations of broken versus unbroken specimens from matched sets of data, to ensure that differences like that evident in Figure 9 do not adversely affect the outcome. Figure 10. Shift in CVN dissipation behavior as total plateau energy increases Such results are shown in histogram format in Figure 11a and cumulative Cunnanes (38) probability in Figure 11b. These results represents pre production testing for an order targeting FSE CVN energy in excess of 325 J (240 ft lb). Note in comparison to Figure 10 that this target toughness approaches the situation where the dissipation in the CVN test is dominated by the deformation and initiation energy components. The CVN testing was done in a machine with a capacity for which any test under ~640 J (472 ft lb) satisfied the excess energy requirements of ASTM E23 (36), or ISO (39). As it is a histogram, the y axis in Figure 11a is the frequency of occurrence, while that for Figure 11 is cumulative Cunnanes probability. The x axis for both parts of this figure is the average energy for unbroken specimens or broken specimens, representing identically oriented often adjacent samples cut from within the same test panel, removed either from plate or pipe. While sampling 16

17 occasionally represents both ends of one pipe, no attempt has been made to track the data in that context, nor to trend differences between plate and pipe. a) histogram b) cumulative Cunnanes probability Figure 11. Comparison of CVN energy for broken and acceptable unbroken specimens It is clear from the trends in Figure 11, all of which satisfy current standards regarding acceptable measured energy, that unbroken specimens comprise a distinctly different population as compared to the results for broken specimens. This is apparent from three perspectives. First, both parts of Figure 11 make clear that the average energy for broken specimens falls well below that for the unbroken specimens, by ~55 percent. Second, the coefficient of variation for the unbroken specimens is about 7 percent, which is comparable to that for mechanical properties testing, while that for the broken specimens at 18 percent is more typical of that found for CVN testing. Finally, the population results for the unbroken specimens reasonable fits a Normal distribution, except in the lower tail, whereas the distribution for the broken specimens is poorly characterized in terms of a Normal distribution. Discussion The results in Figure 11 make clear that the energies for the population of unbroken specimens should not be comingled with those for broken specimens. This figure also indicates that the currently accepted practice of comingling data for unbroken specimens leads to an artificial overestimate of fracture resistance in the context of models based on traditional CVN testing. While one can question whether the CVN specimen in its traditional use as a metric of fracture resistance is relevant at the higher energy levels considered in this paper, or even in the era it was introduced for use with the BTCM (e.g., see 8), the fact remains that it is the only currently accepted basis to quantify fracture resistance. Unfortunately, this is the case for all fracture metrics based on 3 point bend bar testing, regardless of whether that specimen is measuring CVN energy, DWTT energy, or any of the other notch treatments or parameters, including metrics such as crack tip opening angle (CTOA). Concern for the utility of such fracture specimens traces to the realization that propagating shear failure develops under collapse controlled conditions in which case fracture is a consequence rather than controlling (8). The real quandary lies in how steel and pipe producers can develop an MPS and supporting MPQTs that are relevant to collapse controlled metrics, as occurs at higher toughness levels. In the early 17

18 years, at toughness levels like those cited in Figure 1, the equivalent to a MPS was developed in regard to established practices, with MPQTs relying on existing standards. However, as steels have become tougher, the trends and images in Figure 5 and Figures 9 through 11 indicate that the results of such fracture testing are either difficult to interpret or are no longer viable. Until the technology develops to characterize the collapse controlled resistance of tough steels the key question is how do I specify pipe and test to ensure its adequacy relative to the design requirements? Because data such as those in Figure 5 and Figures 9 through 11 indicate the results depend on both the chemical composition of the steel and its processing, the outcome of a MPQT is steel supplier specific, and also specific to the pipe making practices. It follows that one important step in bridging the technology gap can be taken by the steel suppliers working in collaboration with the pipe producers, wherein each generates typical trends for the effects of sub width specimens and contrasts broken and unbroken specimens, so prospective procurement departments have some understanding of typical trends. In this context the steel and pipe producers can develop a MPS that is generic to the extent it can be while also addressing unique supplier specific trends. At the same time, technology should be developing to address the transition away from fracturebased metrics to those that quantify collapse controlled propagating shear failure, such as the work developing under the auspices of CBMM s Shear Propagation and Arrest Consortium. Summary and Conclusions This paper has considered pipeline steel specification in reference to long propagating shear failures that can occur in pipelines transporting compressible hydrocarbons such as methane or high vapor pressure liquids at supercritical conditions. Managing this threat opens up complications in developing the MPS and in establishing the MPQT strategy and parameters for a pipeline project. For example, very often CVN specimens remain unbroken, or sub width specimens are used absent a standardized basis to scale the results to the full size value required by some standards. This paper has trended results for sub width specimens developed for steels in the era that the BTCM was calibrated as the basis to establish scaling rules to facilitate prediction in a full size setting. Issues in using sub width specimens have been reviewed and criteria developed to scale results from such testing for use in the MPS, and MPQT, which is developed as a function of toughness. Finally, issues associated with the acceptance of data from unbroken CVN specimens have been assessed. While conclusions have been drawn throughout the paper, a few are considered key and so worthy of repetition, including: the BTCM formulated using sub width specimens and initially expressed relative to a 2/3 size specimen can be generalized to a FSE value by linearly scaling that result in proportion to the size of the specimen; linear scaling in proportion to the size of the specimens is viable at toughness values up through about 190 J (140 ft lb), above which the scaling rule should be qualified by a testing program that is specific to the steel composition and its processing; comparison of the upper shelf resistance of CVN samples is complicated for steels that show a rising upper shelf, which can lead to apparent scaling rules that are not relevant in applications involving arrest toughness requirements; a comprehensive evaluation of CVN results under systematic variation in shear area and specimen width is needed across a range of grades and production practices to better 18