InSitu Analysis of Pipeline Metallurgy - dnV

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While the mechanical properties of the cap can be similar in both types of seam the properties beneaththe cap can be quite different.Chemical CompositionChemical analysis in the field is used by a variety of industries to verify the composition of incomingcorrosion resistant, heat resistant, or high strength alloys. However, in the pipeline industry, chemicalanalysis is most frequently performed on in-service pipelines to determine carbon equivalent of thesteel to support selection of optimized welding procedures. Therefore, for pipeline applications thechemical analysis methods must be capable of accurately measuring the elements that are included inthe applicable carbon equivalent equations. The most frequently used equations are the CE IIW andPcm equations shown below in Equations 1 and 2.:CE IIW : %C + %Mn/6 + (%Cr+%Mo+%V)/5 + (%Ni+%Cu)/15 (1)Pcm: %C + (%Cr+%Cu+%Cr)/20 + %Ni/60 + %Mo/15 +%V/10 + 5x%B (2)However, of the available field portable PMI (positive metal identification) techniques, only the opticalemission spectrography (OES) method is capable of measuring carbon (atomic number 6) (Figure 4).The x-ray fluorescence (XRF) method will not typically detect and measure elements lighter than atomicnumber 12 (magnesium). Even some portable OES devices are not capable of measuring carbon withsufficient accuracy. Those that use an argon flushed sensor normally provide the best determination oflight elements.In our experience, preparation of steel pipe surfaces for PMI includes first removing at least 0.25 mm(0.01 in.) of the pipe surface to minimize or eliminate the influence of the decarburization that is oftenpresent on the ID and OD surfaces (Figures 5 and 6). Inadvertent inclusion of the decarburized surfacein the analysis results in measurement of an anomalously low carbon content. In view of the ease ofsurface preparation and performance of the measurements, three measurements are normally made,including at least one measurement on either side of the longitudinal seam (for example, about 50-75mm (2-3 in.) from the seam). After the completing the measurements, the metallurgical effects of the“burned” spot caused by the arcing can easily be removed by using a powered sanding disc.Metallographic cross sections through typical spectrographic burn marks show that the related heataffected zone is only about 30 microns (1.2 mils) deep (Figure 7).Note that the results of field portable spectrographs are sensitive to maintenance and calibration. Aninstrument can appear to be functioning properly but produce results that are significantly different fromthose obtained from standard laboratory chemical analysis methods. Sometimes the results for onlyone or a few elements are inaccurate and technicians are not always adequately trained to recognizethe questionable results. Verification of proper instrument maintenance and on-site verification ofcalibration is important.The alternative method of determining the chemical composition of the steel involves removing steelfilings from the steel surface and then analyzing the filings using traditional chemical analysis methodsin a laboratory (Figure 8). As in the case of the PMI measurements, filings should not be collected untilat least 0.25 mm (0.01 in.) of the pipe surface have been removed to minimize the effect ofdecarburization. The procedure for collecting the filings is detailed in Attachment A). While theprocedure references a specific suggested volume of filings, the required amount of filings should beverified with the specific laboratory that is performing the analysis.While removing steel from the pipe surface would seem to be destructive in nature, it is rarelynecessary to remove a total of more than about 0.4-0.5 mm (16-20 mils) of the pipe surface (includingthe initial removal of 0.25 mm (10 mils) to minimize the effects of decarburization) from an area about 6inches in diameter to obtain enough material for analysis. A thickness reduction of 0.5 mm (20 mils)
from a pipe having a nominal original wall thickness of 4.78 mm (0.188) inches represents a localizedthickness reduction of only 10.6% of the wall thickness. The example below illustrates the effect of themetal removal on the integrity of a hypothetical pipeline.Pipe specification: API 5L X42O.D.: 219 mm (8.625 in.)Wall thickness: 4.78 mm (0.188 in.)Diameter of metal loss: 152 mm (6 in.)Depth of metal removal: 0.5 mm (0.02 in.)Calculated failure pressure using the effective area method of RSTRENG: 114% SMYSFor the hypothetical example of removing filings from a thin wall pipe, no repair would be required otherthan recoating the exposed steel since the failure pressure exceeds the SMYS. The same metal losson thicker wall pipe would be even less significant.The surface area and depth of metal removal required to collect the required mass of steel filings foranalysis can be approximated from Equation 3.M req / (16.39 cm 3 /in 3 x 7.86 gm/cm 3 ) = L x W x d (3)Where: M req = milligrams of filings required for analysisL = length of area from which filings are removed (in.)W = Width of area from which filings are removed (in.)d = depth of removal (after removal of 10 mils) (mils)Hardness TestingHardness testing on pipelines can be performed for a variety of purposes, including:Evaluating the acceptability of “hard spots” on pipe surfacesChecking weld heat affected zone hardnessChecking ERW seams and other seams for evidence of effective postweld heat treatmentEstimating tensile strength and lower bound yield strengthHard heat affected zones can be subject to cracking from exposure to fluids containing H 2 S (i.e., “sourservice”) or from the combined presence of sufficient amounts of stress, hydrogen from welding, andhard microstructures (i.e., “hydrogen cracking” or “underbead cracking”).Several types of field-portable hardness testers are available for determining the acceptability of hardspots, weld heat affected zones, or for determining the lower bound expected yield strength. However,each hardness tester has its own strengths and limitations. For example, results from some hardnesstest methods are significantly influenced by technique, material thickness, surface curvature andsurface preparation and may even be affected by residual magnetism, vibration, or temperature. 7One of the most obvious differences among different hardness test methods pertains to the volume ofthe metal that is sampled by the indentation. The indentations produced by a telebrineller have aspherical cap profile that may be a few millimeters across, thus making them suitable for measuring thegeneral hardness of an area, but unsuitable for measuring the maximum hardness of a weld heataffected zone. In contrast, the pyramid shaped indentations associated with the ultrasonic compact(UCI) method may be barely visible to the unaided eye (Figures 9 and 10).
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InSitu Analysis of Pipeline Metallurgy - dnV

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