Two galvanizable high-Al, low-Si transformation-induced plasticity (TRIP)-assisted steels were subjected to isothermal bainitic transformation (IBT) temperatures compatible with the continuous galvanizing (CGL) process and the kinetics of the retained austenite (RA) to martensite transformation during room temperature deformation studied as a function of heat treatment parameters. It was determined that there was a direct relationship between the rate of straininduced transformation and optimal mechanical properties, with more gradual transformation rates being favored. The RA to martensite transformation kinetics were successfully modeled using two methodologies: (1) the strain-based model of Olsen and Cohen and (2) a simple relationship with the normalized flow stress, r flow Àr YS r YS . For the strain-based model, it was determined that the model parameters were a strong function of strain and alloy thermal processing history and a weak function of alloy chemistry. It was verified that the strain-based model in the present work agrees well with those derived by previous workers using TRIPassisted steels of similar composition. It was further determined that the RA to martensite transformation kinetics for all alloys and heat treatments could be described using a simple model vs the normalized flow stress, indicating that the RA to martensite transformation is stress-induced rather than strain-induced for temperatures above the M r s .

Strain-based designs for Arctic pipelines place stringent demands on properties of the pipe body as well as the girth weld and associated heat affected zone. The pipe body must demonstrate good work hardening behavior in addition to satisfactory strength and toughness properties. Girth welds are required to overmatch the strength of the pipe body; both the weld and heat affected zone must also provide good toughness. In this study, X80 line pipe produced using the UOE and spiral welding processes were compared. The UOE process provides some degree of work hardening resulting from cold expansion. This extra hardening renders the UOE pipe more responsive than the spiral pipe to aging effects associated with pipe coating. However, the UOE pipe has an advantage in balancing LPA (longitudinal to pipe axis) and TPA (transverse to pipe axis) strengths. Greater strengths in the TPA orientation afford the capacity to meet specified minimum requirements of the pipe grade and lower strengths in the LPA orientation facilitate overmatching by girth welds. The two types of line pipe offer both advantages and disadvantages for strain-based designs. It must be emphasized that good work hardening characteristics can be maintained in the UOE pipe when the coating process involves a low temperature, which is an objective of modern coating technologies. It was also observed that aging effects did not affect toughness properties significantly.

The production of large diameter pipe by the helical welding process requires that consecutive coils be welded together to facilitate the advance of each subsequent coil through the forming section of the pipe mill. Traditionally, the skelp ends have been joined by a single-pass submerged arc weld simply designed to join the skelp ends together and provide sufficient strength and ductility to survive the pipe forming operation. Subsequent to pipe forming, the length of pipe containing the skelp end weld (SEW) has been cut off and discarded. This process results in both a substantial yield loss as well as additional processing costs as the shortened pipes are later double jointed to produce full size lengths. To overcome these inefficiencies, a process has been developed for making high quality skelp end welds which meet API and CSA requirements. In this paper, the welding process will be described and evaluation of the integrity of the skelp end weld is discussed. Of particular interest are the properties of both the weld and associated heat affected zone in the vicinity of the “T” where the skelp end weld merges with the helical weld. This paper demonstrates that skelp end welds meeting rigorous integrity specifications can be successfully produced. Incorporation of suitable skelp end welding and inspection procedures in the pipe production process significantly enhances the efficiency of helical pipe production.

Tensile testing is a key part of the qualification process of Line Pipe. When qualifying pipe products various items are considered when tensile testing; Yield Strength (YS), Ultimate Tensile Strength (UTS), Percent Elongation (%EL), and the Yield Strength to Tensile Strength Ratio (Y/T) are all important. Of these, the YS is the most critical and yet the most sensitive to both preparation and measurement techniques. During the pipe forming process, the base material is plastically formed into a curved shape, and then welded into the final product. The Transverse to Pipe Axis (TPA) tensile specimen removed for testing is curved and must be flattened prior to testing. The flattening process is varied in many facilities and the standards to which testing is conducted are not specific enough to ensure uniformity of procedures. ASTM acknowledges flattening processes and the degree of flatness “may affect test results”, though no guidance is given. This paper will provide an overview of ongoing research efforts, concerning the measurement of the Yield Strength of TPA tensile specimens and its relationship to curvature and flattening methods, prior to testing. By comparing flattened strap tests, to round bar and ring expansion tests, it is shown that the flattened strap test provides a conservative estimate of the actual YS of the pipe.