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Effects of Niobium on Weld Metal Microstructure and Properties in Submerged Arc Welded X70 Steel

Patterson, Tate
http://orcid.org/0000-0003-4727-9419
http://rave.ohiolink.edu/etdc/view?acc_num=osu1531125285493652

Abstract Details

2018, Master of Science, Ohio State University, Welding Engineering.
Microalloyed steels are specifically designed to meet mechanical properties and/or corrosion performance for the oil and gas industry. Submerged arc welding (SAW) of American Petroleum Institute (API) grade X70 steel is used to produce line pipe. X70 is thermo-mechanically control processed (TMCP) to achieve desired microstructures and mechanical properties. The X70 of interest is currently alloyed with Nb which is beneficial for strength and toughness within the TMCP base material. Nb is effective at minimizing the austenite grain size during rolling by forming NbC precipitates which pin the grain boundaries and impede grain growth. Minimizing the size of the austenite grains leads to finer ferrite grains upon cooling which give the as processed plate a high yield strength while maintaining or improving toughness. Also, Nb is more effective at grain refinement and/or a cheaper alternative than other microalloy additions such as V or Ti. To manufacture the line pipe, SAW is used in a two-run process that performs a first pass on the pipe inside diameter followed by a second pass on the outside diameter. The inherent high heat input of SAW causes base metal dilutions greater than 50% and allows for large grain growth which is inherently detrimental to toughness. The X70 composition of interest contains Nb upwards of 0.1 weight percent (wt%), and weld metal will contain relatively high levels of Nb. Industry is concerned that Nb-bearing X70 will lead to poor or inconsistent toughness within the fusion zone (FZ). The objective of this research is to understand the effects Nb has within the weld metal based on microstructural changes that will determine the toughness and its influence with other microalloy or weld metal chemical compositions. Initially, weld metal “button” samples using two different SAW wires at various dilutions with Nb-bearing (0.1 wt% Nb) X70 were produced via arc melting and the microstructural transformation behavior was evaluated. Hardness testing was performed at all dilutions for initial microstructural characterization. Pre-diluted weld metal buttons were also melted into countersunk X70 steel attempting to simulate the dilutions and weld metal respective of SAW. SA welds were performed using a tandem arc process with a commercially available electrode as used in the dilution analysis. Welds were deposited in a bead-on-plate fashion on low-alloy steel to minimize effects of other microalloy additions. Three different base plate compositions were analyzed, and Nb was systematically introduced into the weld metal by “doping” a commercially available SA weld flux with ferro-Nb. Similarly, two-run welds were deposited onto a double, V-groove joint configuration using a tandem wire SA process to imitate line pipe manufacturing. Microstructural characterization of the dilution analysis and SA weld simulations using pre-diluted buttons show bainitic morphologies over all ranges of dilutions. Microstructural differences are caused by an increase in the percentage of the Ti-B containing electrode and a decrease in the lathy bainitic morphologies. The as-melted X70 without electrode additions contains the most apparent lath like bainitic structure. Highest hardness is achieved at the highest dilution of the Ti-B electrode which is caused by an increase in the M-A constituents resulting in the granular bainitic appearance. Bead-on-plate weld results indicate an overall decrease in as-deposited weld metal toughness above concentrations of approximately 0.02 wt% Nb. Increasing the amount of Nb appears to decrease the width of the grain boundary ferrite veins while simultaneously promoting an increase in acicular ferrite. However, toughness decreases with the increased acicular ferrite. The two-run weld impact toughness reduces with increasing Nb content from a based dilution of 0.014 wt% up to 0.084 wt% Nb. Increasing Nb to 0.030 wt% has a negligible influence on hardness in the as-deposited second pass and reheated first pass. The highest Nb content of 0.084 wt% increased hardness in the as-deposited weld metal and significantly increased the hardness within the reheated first pass. The higher hardness is apparent in all regions of the first pass caused by the second pass heat affected zone, but the largest hardness increase is within the first pass weld exposed to temperatures below the second pass A1 reheat temperature. Overall, Nb is deleterious to toughness in the analyzed weld metal compositions and Nb contents respective of SAW Nb-bearing X70 plate. The least variability and best impact toughness properties are obtained with weld deposits with the lowest C and higher Mn (˜1.5 wt%) and Si (˜0.3 wt%) with additional alloying elements such as Cr and Mo to guarantee intragranular acicular ferrite transformation. Based on these results, Nb above approximately 0.02 wt% will negatively influence weld metal toughness.
John Lippold, PhD (Advisor)
Alexandrov Boian, PhD (Committee Member)
184 p.
Engineering

Recommended Citations

Citations

  • Patterson, T. (2018). Effects of Niobium on Weld Metal Microstructure and Properties in Submerged Arc Welded X70 Steel [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1531125285493652

    APA Style (7th edition)

  • Patterson, Tate. Effects of Niobium on Weld Metal Microstructure and Properties in Submerged Arc Welded X70 Steel. 2018. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1531125285493652.

    MLA Style (8th edition)

  • Patterson, Tate. "Effects of Niobium on Weld Metal Microstructure and Properties in Submerged Arc Welded X70 Steel." Master's thesis, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1531125285493652

    Chicago Manual of Style (17th edition)

osu1531125285493652
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This open access ETD is published by The Ohio State University and OhioLINK.