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

In the petrochemical industry, some subsea F22 steel components are exposed to an H2S-containing fluid environment; the corrosion resistant alloy (CRA) 625 is overlaid on the steel components for protection. For some components, the area exposed to the H2S-containing environment is not fully overlaid with 625, but only partially overlaid for sealing purposes. In the latter case, the F22/625 interface is exposed to the H2S environment and faces increased sulfide stress cracking susceptibility. Postweld heat treatment (PWHT) is applied to stress relieve the F22/625 dissimilar metal weld (DMW); the coarse-grained heat affected zone (CGHAZ) hardness is kept lower than 250HV as required by NACE MR0175/ISO 15156. However, the PWHT promotes the build-up of carbon at the F22/625 interface and forms a new crack susceptible region. An optimal PWHT should be designed for F22/625 DMW to achieve the overall lowest cracking susceptibility.
Delayed hydrogen cracking tests (DHCT) are carried out on Gleeble simulated F22 and 8630 CGHAZs under different PWHT conditions. DHCT are also carried out on F22/625 and 8630/625 DMWs under different PWHT conditions. SSC testing will be performed on the F22/625 and 8630/625 DMWs under different PWHT conditions. These testing procedures will analyze HIC susceptibility of F22 and 8630 base metals, and reveal the effect of PWHT on the susceptibility to HIC and SSC. Additionally, the optimal PWHT conditions which minimize crack susceptibility will be determined. Metallographic work and hardness testing have been performed on the as-welded F22/625 and 8630/625 DMWs. They could be used to explain the fracture phenomenon in DHCT and SSC testing. 

Industry Sponsor: OneSubsea, Acute Technological Services

Faculty: John Lippold (OSU)

Graduate Student: Tao Dai

Industry Contact: Dean Hannam, Timothy Thompson