Microstructural and Mechanical Characterization of the Transition Zone of 9%Ni Steel Cladded with Ni-Based Superalloy 625 by GTAW-HW

• Published in: Metals (Basel ) Vol. 8; no. 12; p. 1007
• Main Authors: Cipriano Farias, Francisco Werley, Payão Filho, João da Cruz, Barreto de Azevedo, Luiz Maurício
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.12.2018
• Subjects: Allotropic transformation; Bend tests; Chemical elements; cladding; CO2 injection unit; Corrosion; corrosion resistant alloy; Crack propagation; cryogenic steel; Crystallography; Diamond pyramid hardness; dissimilar transition zone; Electron backscatter diffraction; electron backscatter diffraction (EBSD); Gas industry; Gas tungsten arc welding; Gases; Hydrogen embrittlement; Hydrogen sulfide; interface of dissimilar metals; Mechanical properties; Microstructure; Nickel base alloys; Nickel steels; Peninsulas; Solidification; Steel pipes; Stress cracking; Substrates; Superalloys; type II boundary; weld overlay; weldments; United States > US
• ISSN: 2075-4701; 2075-4701
• DOI: 10.3390/met8121007

9%Ni steel was recently used for the first time in the field of injection unit (IU) for the injection of CO2 into oil wells. Because such steels are operated in H2S medium and are susceptible to sulfide stress cracking, their pipes are cladded with Ni-based superalloy 625 by using gas tungsten arc welding with a hot wire to prevent this phenomenon from occurring. The transition zone of substrate/clad can have high hardness and low toughness, and promote failure of the component during service; therefore, it is very important to know its characteristics. In this work, this transition zone was analyzed through optical and scanning electron microscopy with energy dispersive X-ray spectrometry and electron backscatter diffraction, as well as Vickers microhardness, shear and bend tests. Metallographic analysis identified type I and II boundaries with distinct chemical gradients, MC-type carbides, Laves/γ eutectics, peninsulas macrosegregation, crystallographic texture close to in the clads, residual strain, and drop of microhardness across the transition zone. The clads were approved in the shear and bend tests. This work proposes a new type II boundary formation mechanism in dissimilar welds of steels that do not exhibit the allotropic transformation δ → γ during the welding thermal cycle and do not experience a change in the solidification mode.