Microstructural Evolution of Dissimilar Metal Welds ... ... Microstructural Evolution of Dissimilar

Microstructural Evolution of Dissimilar Metal Welds ... ... Microstructural Evolution of Dissimilar
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Transcript of Microstructural Evolution of Dissimilar Metal Welds ... ... Microstructural Evolution of Dissimilar

  • Michelle Kent, Sean Orzolek, Dr. John N. DuPont

    Lehigh University, Materials Science and Engineering

    Microstructural Evolution of Dissimilar

    Metal Welds Involving Grade 91 Steel

    Objective The objective is to understand the microstructural evolution across the

    weld interface of dissimilar metal welds involving grade 91 steel.

    Background • Dissimilar metal welds (DMWs) join two alloys of different

    compositions, resulting in a high-energy interface prone

    to failure due to differences in chemical potential,

    coefficient of thermal expansion, and carbon diffusivity

    across the interface [1].

    • Power plants use hundreds of DMWs to join low-cost

    ferritic steels used in environments up to 550°C with more

    expensive austenitic metals that withstand the high-

    temperature and high-pressure environment of steam

    vessels [1,2].

    • DMW failure in welds involving grade 91 steel has been

    shown to occur within a 1-3 µm ferritic band that forms

    along the fusion line in the base metal.

    Problem Statement Hundreds of dissimilar metal welds (DMWs) are used throughout every fossil and nuclear power plant. DMWs involving grade 91 steel have been found to fail along the fusion line well before their 30 year expected creep lifespans. DMW failures causing power plant shutdown can cost up to $850,000 per day [2].

    Alloy 625/Grade 91 GTAW Welds

    •As welded condition

    •Post weld heat treated (PWHT), 1 hour at 760 °C

    •1-hr PWHT at 760 °C, aged 6000 hours at 625 °C

    Experimental Methods • DICTRA Diffusion Modeling

    • Microhardness Indentation Testing

    • Light Optical Microscopy (LOM)

    • Scanning Electron Microscopy

    (SEM)

    References 1. William Gordon Clark, John. (2015).

    Investigating chemical and microstructural

    evolution at dissimilar metal welds. EngD

    Thesis, University of Nottingham.

    2. DuPont, J. N. (2012). Microstructural

    evolution and high temperature failure of

    ferritic to austenitic dissimilar welds,

    International Materials Reviews, 57:4,

    208-234,

    DOI:10.1179/1743280412Y.0000000006.

    3. DuPont, John N, et al. Welding Metallurgy

    and Weldability of Nickel-Base Alloys.

    Wiley, 2009.

    4. DuPont, J. N. (2016). Microstructural

    Characterization and Modeling of

    Dissimilar Weld Failures Involving Grade

    91, Whitehall, PA: JND Metallurgical

    Consulting, LLC.

    Conclusions and Remarks •Grade 91 steel softened with PWHT due to martensite tempering, while Alloy 625 hardened with PWHT and again with aging due to carbide and precipitate formation

    •The carbide-free band is associated with failure

    •Nanohardness indentation is being used to analyze the short-range hardness trends near the weld interface

    Figure 4. The high cooling rate of

    welding causes hard martensite to form

    in the base metal. After 1 hour PWHT,

    strain relaxes and the hardness

    decreases as martensite tempers.

    Base Metal

    Figure 3. Microhardness indent traces taken across the DMW

    fusion line with a 5 g load and 30 µm spacing. Welds were

    tested as welded, 1 hour PWHT condition, and 6000 hour

    aged condition. In the base metal, the hardness decreases

    after 1 hour PWHT and remains constant after aging. In the

    weld metal, the hardness increases after 1 hour PWHT and

    again after 6000 hours of aging.

    Partially

    Mixed Zone

    Fusion

    Zone

    Heat

    Affected

    Zone

    Base

    Metal

    Location of

    Carbide-Free

    Region

    500 µm

    Figure 2. DMW creep failure has

    been shown to occur within a narrow

    ferritic band along the fusion line. [4]

    Creep Voids

    Carbide Free

    Region

    FCC/BCC

    Interface

    Type I

    Carbides

    Figure 7. Hardness and simulated carbon concentration

    as a function of distance across the interface. A localized

    increase in hardness correlates with an increase in carbon

    concentration just inside the weld metal.

    Figure 5. M6C carbides are predicted to form after 1 hour PWHT at 760 °C. Ni3Nb precipitates are predicted to form after 6000 hours of aging at 625 °C. [3]

    Figure 6. M6C carbides and Ni3Nb are

    predicted in alloy 625 according to the

    time transformation diagram. [3]

    Weld Metal

    Figure 1. Schematic cross section

    of a DMW. The location of the

    carbide free region is indicated.

    ……………………………..

    Acknowledgements:

    Results