Overlay Welding of 2205 Duplex Stainless Steel on L485Q Pipeline Internal Wall

Literature Overview

This study investigates the overlay welding of 2205 duplex stainless steel on the internal wall of L485Q carbon steel pipeline, a critical application for oil and gas pipelines transporting sour crude oil containing hydrogen sulfide. The L485Q steel is a high-strength low-alloy (HSLA) steel with a minimum yield strength of 485 MPa, commonly used in pipeline applications. The 2205 duplex stainless steel overlay provides resistance to sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC), which are major failure mechanisms in sour service. The study examines the weldability of the 2205 overlay on the L485Q substrate, including microstructure evolution, mechanical properties, and corrosion resistance in simulated sour environments.

Core Technical Findings

Microstructure of the Overlay and HAZ

The 2205 overlay layer exhibits a balanced duplex microstructure of approximately 50% austenite and 50% ferrite in the as-welded condition. However, the ferrite content increases to 60-70% in the heat-affected zone near the fusion boundary due to the dilution effect of the L485Q carbon steel. This shift in phase balance can affect the mechanical properties and corrosion resistance of the overlay. The weld metal microstructure shows columnar austenite and ferrite dendrites with some delta ferrite retained from the solidification process. The presence of secondary phases such as sigma phase and chi phase was not observed in the as-welded condition, but the study warns that prolonged exposure to temperatures above 400°C can promote sigma phase formation, degrading toughness and corrosion resistance.

Mechanical Properties

The hardness of the 2205 overlay layer ranged from 280-350 HV, which is higher than the base metal hardness of 200-250 HV due to the duplex microstructure and solid solution strengthening. The tensile strength of the overlay layer was 620-700 MPa with elongation of 25-35%. The impact energy of the overlay layer at room temperature was 45-65 J, indicating good toughness. The hardness gradient at the overlay-base metal interface was significant, which can create stress concentration under cyclic loading conditions.

Corrosion Resistance in Sour Service

The overlay layer demonstrated excellent resistance to SSC and HIC in NACE MR0175/ISO 15156 simulated sour environments. The study conducted tests per NACE TM0177 (SSC) and NACE TM0284 (HIC) and reported no cracking or blistering after 720 hours of exposure. The pitting resistance equivalent number (PREN) of the 2205 overlay was calculated at 35-38, indicating excellent resistance to chloride-induced pitting. However, the study notes that the overlay thickness must be maintained at a minimum of 3 mm to ensure complete protection against SSC and HIC, as thinner overlays may allow hydrogen to penetrate to the base metal.

Engineering Practice Integration

For pipeline applications, the 2205 overlay on L485Q pipe is typically applied using submerged arc welding (SAW) or gas metal arc welding (GMAW) with a transition layer of ER309L or ER347 to buffer the dilution effect. The overlay thickness is typically 4-6 mm with 2-3 passes, and the internal surface requires post-weld machining or grinding to achieve the required surface finish for fluid transport. The study recommends UT thickness mapping of the overlay to verify uniform coverage and minimum thickness requirements per applicable code (ASME B31.3 or API 5L). For sour service qualification, the study recommends full compliance with NACE MR0175/ISO 15156 requirements, including material certification, welding procedure qualification per NACE SP0102, and operator certification.

Study Insights and Implications

The study confirms that 2205 duplex stainless steel overlay is an effective and economical solution for protecting L485Q pipelines in sour service. The key engineering consideration is maintaining adequate overlay thickness and uniform coverage, as localized thin spots can lead to premature failure. The ferrite content shift in the HAZ is a potential concern for long-term performance, and engineers should consider post-weld heat treatment to rebalance the phase composition if the service temperature exceeds 300°C. The study provides valuable data for welding procedure qualification and quality control protocols, and engineers should incorporate the recommended testing requirements (SSC, HIC, PREN, hardness mapping) into their inspection and testing plans for sour service pipeline overlays.