Abstract

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 <100> 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.

Highlights

  • The exploration of oil and gas in the Brazilian pre-salt presents a great technological challenge because of the unique operating conditions

  • OM analyses on the transition zone of substrate/clad (TZ-S/C) of clads I and II indicated heterogeneous epitaxial nucleation (Figure 2a), with boundarytype type II (BT I) growing approximately perpendicular to the TZ-S/C interface, as well as a planar growth solidification (PGS) region that develops due to intense heat extraction by the substrate at the beginning of the solidification, which stabilizes the liquid/solid interface

  • Since the chemical composition of the PGS region varies gradually, as indicated by Silva et al [12], only a portion of this region was revealed by chemical attack

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Summary

Introduction

The exploration of oil and gas in the Brazilian pre-salt presents a great technological challenge because of the unique operating conditions. The great sea depths (1000 m to 2000 m) and the distances between the reservoirs and the seabed (4000 m to 6000 m) require the manufacture of risers up to 8000 m in length that can withstand pressures of about 550 bar. The oil produced in floating production storage and offloading (FPSO) contains high amounts of water, CO2 , and contaminants (H2 S, among other fluids) [1]. CO2 is separated from other fluids and injected into the well to reduce the viscosity of the oil and maintain the well pressure (to gain productivity) [2]. 9%Ni steel (ASTM A333 Gr. 8) had only been used in the oil and gas industry once before the.

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