No abstract

Most published SCC results for the near-neutral pH condition were produced under cyclic loading. However, the presence of stress corrosion cracks in pipeline systems involving very small pressure fluctuations suggests the cracking should initiate and grow without large dynamic loads. This study was designed to investigate this issue. A Grade 448 (X-65) line pipe steel and a prototype Grade 550 (X-80) steel were evaluated in near-neutral pH solutions. The maximum stress applied was at 95% of the respective yield strengths and the R values applied were between 0.98 and 1.0. Two solutions were used for each steel: NS4 and NS4/clay mixture. The solutions were purged with a gas mixture of 95%N2 and 5%CO2. Recognizing that the crack propagation rate can be very slow under such near-static conditions, relatively long-term tests were carried out. The durations of the three tests using the prototype Grade 550 (X-80) steel were 110 days, 54 days and 26 days, and the duration for the X-65 steel was 110 days. After 110 days, the majority of the cracks in the Grade 550 (X-80) steel were in the range of 5 to 30 micrometers (μm) deep, giving an average crack propagation rate of 2*10−9 mm/s. Tests at short durations revealed that only a few cracks were detectable after 26 days and that several more cracks were produced after 54 days. So majority of the cracks in the 110-day were likely produced after 54 days of testing. The NS4/clay mixture was found to be less aggressive than the NS4 solution for both steels studied. The cracks in the prototype Grade 550 (X-80) steel were deeper and more numerous in comparison with the X-65 steel. Possible reasons for this observation are also explored in terms of the presence of martensite-austenite (MA) phase in the Grade 550 (X-80) steel.

Stress fields and constraint parameters (Q and A2) of circumferentially-cracked high strength pipe in displacement-controlled tension are compared with those of small-scale single-edge notched samples tested in tension (SE(T)) and bending (SE(B)). The factors affecting transferability of fracture toughness (J-resistance) data from small-scale laboratory tests to cracked high strength pipe are discussed. The crack-tip stress field is of similar form for a circumferential crack in a pipe and a SE(T) test specimen, while for a SE(B) specimen there is a significant gradient in the crack-tip stress field. Hence, the fracture toughness can be characterized by only two parameters (J and Q or J and A2) for tension-loaded pipe and SE(T) tests, but for SE(B) tests one more parameter is needed to describe the bending term. It is concluded that the constraint in a SE(T) test with ratio of span between load points to width H/W = 10 provides a reasonable match to that for a circumferential crack in a pipe subjected to tensile loading.

The objective of this investigation was to provide a detailed evaluation of the heat-affected zone (HAZ) toughness of a high-strength TMCP steel designed for low-temperature applications. The results from both Charpy-vee notch (CVN) and cracktip-opening displacement (CTOD) tests conducted on two straight-walled narrow groove welds, produced at energy inputs of 1.5 and 3.0 kJ/mm, show that significantly lower toughness was exhibited by the grain-coarsened HAZ (GCHAZ) compared with the intercritical HAZ (ICHAZ) region. This is explained based on the overall GCHAZ microstructure, and the initiation mechanism which caused failure. For the particular TMCP steel investigated in this study very good ICHAZ toughness properties were recorded using both HAZ Charpy and CTOD tests. In general, this was attributable to the low hardness, relatively fine ferrite microstructure, and the formation of secondary microphases that were not overly detrimental to the toughness. The lower-bound GCHAZ CTOD results obtained for both welds (KA W-L and KA W-H) did not meet the targeted requirement of δ = 0.07 mm at −50°C. It was found in both welds that low CTOD toughness was associated with the initiation of fracture from nonmetallic inclusions, which were complex oxides containing Ce, La, and S. The sites were located in the subcritical GCHAZ (SCGCHAZ) region in the case of the 1.5 kJ/mm weld and in the GCHAZ for the 3.0 kJ/mm weld. Some variation in CVN toughness was observed at different through-thickness locations. Toughness was lowest for the GCHAZ of the weld deposited at 3.0 kJ/mm and was related to the proportion of GCHAZ being sampled, which was ~55 percent for the bottom compared to 25–30 percent for that of the top location. Recommendations are proposed on the preferred practices and criteria that should be used in establishing guidelines and specifications for evaluating the HAZ toughness of candidate steels for construction of Arctic class ships.

The guidelines and recommendations for fracture toughness testing of pipeline girth welds outlined in CSA Z662-03, Annex K are reviewed in this work. In Annex K of CSA Z662-03, the specimen type and notch location have been grouped into four categories and the CTOD tests are to be carried out in accordance with either BSI Standard 7448 or ASTM Standard E 1290. In the present study, CTOD tests have been conducted on a manual shielded-metal-arc weld (SMAW) that was prepared in a high strength X80 pipeline steel. The experimental results obtained by applying the two testing standards are compared. The focus was to identify the differences between these two standards that may significantly affect the test results, such as the requirements for straightness of the fatigue crack, and the equations and parameters used for evaluation of CTOD. Some additional factors affecting the testing, such as selection of test specimen location and procedures for targeting specific weldment microstructures as well as the application of local compression, are also discussed. The variation of strength and toughness with clock position around the circumference of the girth welds has also been studied.