The girth welding of steel pipelines creates a substantial heat affected zone (HAZ) within the base pipeline steel. The HAZ can be considered to be a complex graded microstructure. While there is significant concern as to the fracture and mechanical properties of the HAZ as whole, detailed knowledge about the mechanical properties of the individual microstructures is lacking. For this study, X80 is heat treated in a Gleeble simulator to create samples of bulk microstructures with differing amounts and morphologies of bainite, ferrite and martensite-retained austenite (MA) with a total of 8 microstructures being investigated. The heat treatments were selected specifically to control the level of niobium in solid solution; that is to control whether niobium was fully in solution or contained mainly in niobium carbonitride precipitates. From the heat treated samples a matching tensile and fracture specimens were made. The strongest microstructure proved to be the finest bainitic microstructure, while the lowest strength microstructure was the coarsest bainite sample containing a significant amount of martensite-retained austenite connected along grain boundaries. The fracture behaviour at ambient temperature was studied using the Kahn tear test. The Kahn tear test is a machine notched, thin-sheet, slow strain rate fracture test which has the advantage of being a simple test to conduct. All Kahn tests failed in a ductile manner and it showed that the sample with the coarse bainite, with a connected martensite-retained austenite phase had the lowest unit propagation energy and tear strength while the fine, fully bainitic sample had the highest unit propagation energy and tear strength. Further investigation using SEM measurements of the final fracture surface from the tensile test to determine the tensile toughness. A comparison of the tensile toughness and unit propagation energies showed that there was a complex relationship between the two measurements. However, the samples which had the highest content of MA gave the in lowest unit propagation energy.
The effect of tempering conditions on the carbides in P110 casing steels with two different Cr compositions was studied. Local Mo and Cr concentrations varied from the nominal compositions by ≈± 0.05 wt-% Mo and ≈± 0.07 wt-% Cr. The size of the fine (<20 nm) MC carbides remained relatively unaffected by tempering. Fe-based carbides exhibited a large increase in mean log-normal size from 65 to 102 nm when tempered at 650 and 715°C for 45 min, respectively. This rapid carbide coarsening is attributed to the dissolution of M23C6 carbides at 715°C. Associated with this significant increase in carbide size was an increase in the rate of hardness reduction with increasing Hollmon-Jaffe parameter for temperatures above 650°C.
Demand for improved pipeline efficiency has directed designs towards larger diameters and higher operating pressures. High strength steel with increased wall thicknesses of the pipe provide for greater available pressure capacity for the pipeline. However, toughness of the line pipe, particularly at low temperature, is challenged when increasing the strength of the steel and with a greater wall thickness of the pipe. This work discusses the efforts to optimize alloy and thermomechanical controlled processing (TMCP) designs to achieve X80 with good low temperature toughness. Two X80 helical line pipe steel designs are presented; X80-A with low Ni and X80-B with high Ni with gauges of 18.5 mm and 19.1 mm, respectively. This material was cast, rolled, and formed into pipe as part of production trials. Processing data is presented and shows good consistency was achieved in the trials. Results from the tensile and drop weight tear tests (DWTT) are discussed. Both X80-A and X80-B met tensile requirements for an X80 material with an average yield strength of 614 MPa and 586 MPa. In terms of DWTT, X80-A passed −5°C whereas X80-B passed both −20°C and −30°C. In terms of microstructures, nital and LePera etchings along with optical microscopy showed that X80-A and X80-B have fine grain ferrite microstructures with a minimal amount of martensite/retained austenite. A larger data set consisting of X70 and X80 material from production and trials within the gauge range of 17.8 mm to 20.3 mm are introduced. The data set shows some expected general trends such as decreasing DWTT performance with increases to strength. Various TMCP factors such as roughing last pass temperature and mean flow stress as well as microstructure are also discussed with respect to their impact on DWTT results. Low Ni contents are shown on average to perform better than other levels in the current data set, owing mainly to the optimization efforts of developing a −5°C X80 product.
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