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Weldability of High Strength Line Pipe Steels

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Weldability of High Strength Line Pipe Steels Weldability of High Strength Line Pipe Steels 90 to 100° C preheat eliminates cracking at moderate levels of applied stress when Grade 483 line pipe is misaligned during welding BY T. H. NORTH, A. B. ROTHWELL, A. G. GLOVER AND R. J. PICK ABSTRACT. Full-scale weldability tests why such a situation still exists, and cracking arises (and may be prevented) in showed that Grade 483 line pipe material whether, despite the vastly increased the real world. Weldability tests that are was resistant to cracking up to very high fund of knowledge related to hydrogen- able to simulate, in full-scale, the exact levels of general stress, in the absence of assisted cold cracking, we are still sequence of welding and manipulation misalignment. When pipe misalignment approaching practical problems in the which a pipe may experience in the field was introduced a preheat of 90 to 100°C wrong way. are the only ones in which this relation­ (194 to 212°F) was necessary to eliminate One of the difficulties which arises in ship is direct. Such tests are expensive cracking at moderate levels of applied addressing the specification of suitably and cumbersome, however, and only stress. crack-resistant materials is the plethora of one pipeline company is known to use Full-scale weldability and laboratory formulae which are proposed for the them on a production basis (Ref. 3, 4). weldability test results correlated well assessment of a material's cracking ten­ Lastly, it is important to realize that the when using slot testing and WIC restraint dency. While many authorities agree that one "full-scale" test which is habitually cracking tests. The implant test can give the IIW carbon equivalent formula is carried out prior to production weld­ good indications of the susceptibility of a inadequate for modern, low-carbon ing—the procedure qualification test — material to HAZ cracking, but its rele­ steels (Ref. 1, 2), most codes and stan­ provides no protection against any but vance must be questioned in situations dards and —indeed —most company the grossest of inadequacies relative to where cracking occurs mainly in the weld specifications still depend primarily on this cold cracking behavior. It is quite unrea­ metal. expression. The additional consideration sonable to assume that the welding of of different ranges of carbon content in two 500 mm (19.7 in.) long pups, under some of the more progressive standards carefully-controlled shop (or at worst, Introduction and specifications represents a major yard) conditions will give any indication The field welding of high strength line improvement. of the likely behavior of 24 m (78.7 ft) pipe steels has been studied for many Again, a vast array of laboratory tests long double joints weighing, perhaps, years, and numerous recommendations have been advanced as providing an over 10 tons, when welded and manipu­ have been advanced concerning the assessment of the "weldability" of a lated on a roller-coaster right-of-way at determination of weldability and the material. While some such tests do com­ subzero temperatures. specification of materials to ensure ade­ bine the virtues of simplicity, economy It was with some of these perplexities quate field performance. Much of the and reproducibility, they are of very little in mind that the present program of work recent steel development activity in the use in the present context if it is not was begun. Initially, a theoretical stress line pipe area has been specifically direct­ possible to relate the experimental vari­ analysis was undertaken in order to ed towards the improvement of hydro­ ables and outcome to the way in which determine the sources and relative gen-assisted cold cracking resistance, a importance of stresses which could lead property upon which heavy demands are to cracking. In particular, a finite element made by the still-prevalent stovepipe analysis was used to relate local stresses welding technique. Paper presented on the 63rd Annual AWS in different regions of the root pass to Convention in Kansas City, Missouri, during After nearly two decades of develop­ general bending stress arising from lifting April 26-30, 1982. ment in this area, it may appear unlikely and to assess the effects of weld geome­ that anything new can be contributed to T. H. NORTH is Senior Research Associate, try. Stelco Inc.; A. B. ROTHWELL is Manager- the subject. Those close to the pipeline The second phase of the program Welding Technology, NOVA, An Alberta Cor­ industry, however, will be aware that the poration; A. G GLOVER is Manager-Industrial involved cold cracking tests on full-size problem of cold cracking has far from Applications, Welding Institute of Canada; and samples of line pipe, using a purpose- disappeared; a number of recent, major R. J. PICK is Professor, Mechanical Engineeringbuilt , four-point bending frame. Finally, a projects have experienced serious diffi­ Department. University of Waterloo, Cana­ number of laboratory-scale tests were culties of this kind. It is legitimate to ask da. examined to determine whether any log- WELDING RESEARCH SUPPLEMENT | 243-s 1 to cool into the cracking range also con­ 1 ST ] 3 RD 1 4 TH 5 TH ROOT-BEAD tributes to the diffusion of hydrogen out ROOT- ROOT- 13 1 ROOT- 0 ROOT- ! Z I RCOT- I z z of the root-pass region; other results are BEAD z BEAD BEAD | 5 ! BEAD ! o | BEAD the softening of hardened microstruc­ COMPLETED 3 COMPLETED] 5 tCMPLETED y I ! - JCCMPL£~Q ~c COMPLETE! 1 Q tures and the reduction of any local i/i I u~ 1 stresses resulting from pipe manipula­ O (min) tion. Stress Considerations 5 Stresses acting on the root pass before MINUTE HOT-PASS| HOT-PASS I z I I i deposition of the hot pass could, in I O I principle, arise from a number of sources, ICOMPLETEDI COMPLETE [COMPLETE) " I and the relative importance of these has ! i ll not been well understood until quite 0 (min) 26 29 35 38 recently. Bending stresses acting on a pipe due to lifting were considered by Fig. 1—Schematic representation of root and hot pass welding operations Mercer and Needham (Ref. 9), by Lumb and Fearnehough (Ref. 10), and, more recently, by Bufalini et al. (Ref. 11). In the first part of the present program, Higdon ical framework existed within which they the maximum hardness which can be et al. (Ref. 12, 13, 15), using simple beam could provide useful indications of field developed in a given material. theory, derived the general bending behavior. It must also be realized that cracking stresses resulting from lifting, in relation to The steels examined, in the experimen­ can take place in either the HAZ or the the lifting geometry. They then calculated tal part of the work, were typical of those weld metal. As a result, pipe materials, the local stress concentrations in the root currently being supplied by Canadian which are themselves extremely resistant area, using a finite element analysis. They pipe makers for use in the construction of to cracking, are not always sufficient to also considered other sources of stress, demanding pipeline projects such as the ensure freedom from field welding prob­ such as thermally-induced residual stress­ Alaska Highway Gas Pipeline System. lems. For this reason, laboratory tests es and the restrained recovery of ovality which do not force cracking to occur in when the line-up clamp is released. one region rather than the other are Background The main features of this work are those which should relate most closely to discussed elsewhere in this paper. The Before turning to the analytical and field experience. important conclusion was that lifting experimental work, it is in order to stresses are likely to make by far the most review the basic factors which affect Hydrogen Diffusion important contribution to the maximum hydrogen-assisted cold cracking and the tensile stress acting on the weld; the way in which they relate to pipeline field Large quantities of hydrogen (>40 sequence of welding and manipulation welding practices. There is a general mL/100 g deposited metal) are intro­ which takes place in a typical field-weld­ understanding of the contributions which duced into the weld region by the cellu- ing operation must be analyzed in this the material susceptibility, hydrogen and losic-coated electrodes used for stove­ light. applied tensile stress make to the proba­ pipe welding. The extent to which this bility of cracking (Ref. 5), although the hydrogen can diffuse away from the incorporation of this knowledge into weld is largely determined by the cooling Stovepipe Welding quantitative formulations (analogous to rate through the low temperature range; those of fracture mechanics) has not yet this, in turn, is mainly determined by The operations associated with the been accomplished. It is also clear that preheat temperature. Duren et ai. have deposition of the first two passes in hydrogen cracking will not normally shown that an increase in preheat from stovepipe welding, and their timing, have occur at temperatures in excess of about 20 to 100°C (68 to 212°F) leads to a been considered in a recent publication 100°C (212°F). decrease in diffusible hydrogen content (Ref. 14). They can be summarized as from over 35 to «10 mL/100 g (Ref. 8). follows: The addition of a second pass, normally Role of Hardness • A pipe joint is supported at its center with a significantly higher heat input than by a side-boom and brought up to the There has been a widespread tenden­ the first, before the weld region has time end of the already-welded string, where cy to consider hardness as a reasonable indicator of susceptibility to cold crack­ ing.
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