Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


PhD dissertation_Tao Dai.pdf (26.15 MB)

ETD Abstract Container

Abstract Header

Effect of Postweld Heat Treatment on the Properties of Steel Clad with Alloy 625 for Petrochemical Applications

Dai, Tao, Dai
http://rave.ohiolink.edu/etdc/view?acc_num=osu1523572474171801

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Welding Engineering.
Subsea high pressure equipment used in production of oil and gas is routinely clad with nickel base alloys for corrosion protection. In the equipment with partial clad for sealing purpose, dissimilar metal interfaces are possibly exposed to the production fluids containing H2S. After cladding, a high hardness heat affected zone (HAZ) is produced in the base metal adjacent to the fusion boundary and is possibly susceptible to hydrogen assisted cracking (HAC) and sulfide stress cracking (SSC). National Association of Corrosion Engineers (NACE) standard MR0175/International Standard Organization (ISO) 15156 requires that HAZ hardness should be less than 22 HRC or 250 VHN. Postweld heat treatment (PWHT) is applied to reduce the HAZ hardness to meet this requirement. However, PWHT causes the carbon to diffuse from the base metal to the weld metal and pile up in a narrow region adjacent to the fusion boundary, possibly causing interface embrittlement. Also, prolonged PWHT can overtemper the base metal and impair its strength. Therefore, the optimal PWHT conditions need to be determined, which reduce the HAZ hardness to meet the industry standard, do not harm base metal strength, and do not increase the HAC and SSC susceptibility near or at the fusion boundary. In this work, nickel base Alloy 625 overlays on F22 (2.25Cr-1Mo) steel and AISI 8630 steel, or F22/625 and 8630/625 dissimilar metal welds (DMWs), were studied. A wide range of PWHT conditions indicated by Hollomon-Jaffe Parameter (HJP) was investigated to determine an optimal balance between HAZ softening and interface embrittlement. Vickers hardness testing revealed that the CGHAZ hardness decreases with the HJP increase due to martensite decomposition. There is a secondary hardening effect in F22 CGHAZ. The hardness of the planar growth zone (PGZ) of the interface and the weld metal increases with HJP, and the PGZ hardness increases at a higher rate than the weld metal. Nanoindentation and optical microscopy revealed the partially mixed zone containing high hardness virgin martensite and clusters of carbides causing precipitation hardening in the PGZ after extensive PWHT. The modified delayed hydrogen cracking tests were conducted to simulate the HAC behavior of the DMWs. The optimal HJP ranges with high HAC resistance for F22/625 and 8630/625 samples are respectively 19,500-20,200 and 18,500-19,500. Compared to 8630/625 samples, the HAC resistance of F22/625 samples is higher and also a stronger function of HJP. Fractography work using SEM and EDS found six typical fracture surface morphologies. Four-point bend tests in H2S solutions found that these DMWs are very susceptible to SSC cracking and the fusion boundary is the most susceptible location for cracking rather than the HAZ. Reducing HAZ hardness below 250 VHN according to NACE standard MR0175/ISO 15156 does not give assurance of satisfactory performance in sour service. With Abaqus simulation, the SSC mechanism of the DMWs was proposed. The PWHT can generally decrease the SSC susceptibility for F22/625 and 8630/625 DMWs. For F22/625 samples, different PWHT conditions showed similar improvement on the SSC resistance. For 8630/625 samples, the SSC susceptibility decreases with the increase of HJP. There is little correlation between the HAC and SSC behavior of these DMWs due to different cracking mechanisms.
John Lippold (Advisor)
Boian Alexandrov (Committee Member)
David Phillips (Committee Member)
261 p.
Engineering; Materials Science; Metallurgy
Alloy 625, F22 steel, 8630 steel, dissimilar metal weld, postweld heat treatment, Hollomon-Jaffe parameter, Vickers hardness, nanoindentation, modified delayed hydrogen cracking test, hydrogen assisted cracking, sulfide stress cracking, fractography

Recommended Citations

Citations

  • Dai, Dai, T. (2018). Effect of Postweld Heat Treatment on the Properties of Steel Clad with Alloy 625 for Petrochemical Applications [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1523572474171801

    APA Style (7th edition)

  • Dai, Dai, Tao. Effect of Postweld Heat Treatment on the Properties of Steel Clad with Alloy 625 for Petrochemical Applications . 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1523572474171801.

    MLA Style (8th edition)

  • Dai, Dai, Tao. "Effect of Postweld Heat Treatment on the Properties of Steel Clad with Alloy 625 for Petrochemical Applications ." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1523572474171801

    Chicago Manual of Style (17th edition)

osu1523572474171801
582
© 2018, some rights reserved.Creative Commons License Effect of Postweld Heat Treatment on the Properties of Steel Clad with Alloy 625 for Petrochemical Applications by Tao Dai Dai is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by The Ohio State University and OhioLINK.