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Induction Bending of Internally Clad Steel Pipes: Failure Mechanisms & Processing Parameter Optimization in Ni-base Alloy Weld Overlays

Alexandre, Rex T
http://orcid.org/0000-0001-8422-4543
http://rave.ohiolink.edu/etdc/view?acc_num=osu1480422106975311

Abstract Details

2016, Master of Science, Ohio State University, Welding Engineering.
Cracking in corrosion resistant clad overlays on low alloy carbon steel pipes made with Alloy 825 has been experienced in the industry in an effort to reduce production costs by changing the cladding material from the more costly Alloy 625. A detailed metallurgical investigation was carried out to understand the root cause of the cracking phenomenon. Analysis consisting of optical and scanning electron microscopy along with hardness traverses and mapping revealed weld metal heat-affected zone liquation cracks in the second overlay after welding, as well as a region of high hardness in the planar growth region of the weld metal directly adjacent to the dissimilar metal weld interface. Serial sectioning shows that ductility-dip cracks form between the pre-existing weld metal liquation cracks and microcracks forming in the embrittled planar growth region, ultimately leading to through-thickness cracks of the overlay during induction bending. The strain-to-fracture test was modified to replicate the bending procedure, and an optimal parameter window consisting of bending temperature, total strain, and strain rate was identified based on test results. ThermoCalc pseudo-binary phase diagrams were created using both the equilibrium and Scheil models. Neither diagram predicts the formation of any low melting eutectic constituents that could lead to liquation during welding. EDS results show spikes of titanium in the bulk weld metal, presumably due to the presence of titanium carbide particles. A plot of solidus temperature versus weight percent titanium created in Thermocalc reveals severe melting point depression in the Alloy 825 matrix as the titanium content increases. It is hypothesized that the weld metal heat-affected zone liquation cracking occurs via constitutional liquation of titanium carbide particles in close proximity to the fusion zone during welding of the second overlay. The region of high hardness at the DMW interface was observed to correlate with microcracks forming in that region as initial straining began during induction bending. Gleeble testing showed that avoidance of microcrack formation by manipulation of bending parameters is not possible. A study on hardness at the interface during processes steps was performed, revealing that the hardness increase occurs during the normalizing post-weld heat treatment before bending. A DICTRA diffusion model was carried out to further understand the mechanism behind the increased hardness in the planar growth region. Results show a pile-up of carbon extending approximately 100-150 microns into the weld metal at the DMW interface. It is theorized that avoidance of microcracks at the DMW interface is best achieved by elimination of the PWHT. Replication of the induction bending process in the Gleeble thermo-mechanical simulator was achieved by modification of the strain-to-fracture test. Results show that reducing the strain rate opens the safe bending parameter window in terms of temperature and total strain. A bending temperature of 975 ± 25 °C is suggested to successfully induction bend pipes without causing ductility-dip cracking.
Boian Alexandrov, PhD (Advisor)
Antonio Ramirez, PhD (Committee Member)
178 p.
Materials Science; Metallurgy
induction bending; weld overlay; ni-base alloy; alloy 825; dissimilar metal weld;

Recommended Citations

Citations

  • Alexandre, R. T. (2016). Induction Bending of Internally Clad Steel Pipes: Failure Mechanisms & Processing Parameter Optimization in Ni-base Alloy Weld Overlays [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480422106975311

    APA Style (7th edition)

  • Alexandre, Rex. Induction Bending of Internally Clad Steel Pipes: Failure Mechanisms & Processing Parameter Optimization in Ni-base Alloy Weld Overlays. 2016. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1480422106975311.

    MLA Style (8th edition)

  • Alexandre, Rex. "Induction Bending of Internally Clad Steel Pipes: Failure Mechanisms & Processing Parameter Optimization in Ni-base Alloy Weld Overlays." Master's thesis, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1480422106975311

    Chicago Manual of Style (17th edition)

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