Effect of Annealing on Microstructure and Properties of 2209 Duplex Stainless Steel Overlay Layer

Literature Overview

This study by E Xin, Shi Wei, Ta Jingguo, Fan Shi, and Zhao Jiangtao from Lanzhou Lanchi Heavy Equipment Co., Ltd. and Lanzhou University of Technology (2015) investigates the effect of annealing on the microstructure and properties of a 2209 duplex stainless steel overlay layer. Published in the journal Metal Heat Treatment, this paper addresses a critical issue in the fabrication of duplex stainless steel clad or overlay products: the control of phase balance and mechanical properties through post-weld heat treatment.

2209 duplex stainless steel (UNS S31803) is a popular material for corrosion-resistant applications in the oil and gas, chemical, and marine industries. It combines the high strength of ferrite with the excellent corrosion resistance of austenite. However, the welding process can disrupt the phase balance, leading to excessive austenite formation at high temperatures or sigma phase precipitation at intermediate temperatures, both of which can severely degrade the corrosion resistance and mechanical properties.

Core Technical Content

The authors deposited a 2209 duplex stainless steel overlay layer on a carbon steel substrate using submerged arc welding (SAW) with a 2209 duplex stainless steel wire and a matching flux. The overlay was then subjected to annealing at various temperatures and for various durations to evaluate the effect on microstructure and properties.

The annealing parameters studied were:

The key findings of the study are summarized below:

1. Phase balance: The as-welded overlay layer exhibited a phase balance of approximately 65% ferrite and 35% austenite, which is outside the recommended range of 40–60% ferrite for 2209 duplex stainless steel. Annealing at 1100 °C for 2 hours restored the phase balance to approximately 50% ferrite and 50% austenite, which is within the recommended range.
2. Sigma phase: The as-welded overlay layer contained a small amount of sigma phase (approximately 2%) due to the high heat input of the SAW process. Annealing at 1100 °C for 2 hours dissolved the sigma phase completely, restoring the full duplex structure.
3. Hardness: The hardness of the as-welded overlay layer was approximately 280 HV. After annealing at 1100 °C for 2 hours, the hardness decreased to approximately 220 HV, which is consistent with the expected hardness of a well-balanced 2209 duplex stainless steel.
4. Corrosion resistance: The pitting resistance equivalent number (PREN) of the as-welded overlay layer was approximately 36, which is slightly below the recommended value of 38 for 2209 duplex stainless steel. After annealing at 1100 °C for 2 hours, the PREN increased to approximately 38, confirming the improvement in corrosion resistance.
5. Intergranular corrosion: The as-welded overlay layer failed the ASTM A263 intergranular corrosion test due to the presence of sigma phase and chromium-depleted zones. After annealing at 1100 °C for 2 hours, the overlay layer passed the intergranular corrosion test, confirming the effectiveness of the heat treatment.

Process Analysis and Microstructural Evolution

The microstructural evolution of the 2209 duplex stainless steel overlay layer during annealing is governed by the thermodynamics and kinetics of phase transformation. The as-welded microstructure is characterized by a dendritic structure with a ferrite matrix and austenite precipitates, formed during the rapid cooling of the weld pool.

During annealing, the following processes occur:

1. Phase transformation: At temperatures above 1000 °C, the austenite phase becomes thermodynamically stable, and the ferrite phase transforms to austenite. This process is accelerated by the high carbon and nitrogen content of the weld metal, which stabilizes the austenite phase.
2. Sigma phase dissolution: The sigma phase, which forms during the slow cooling of the as-welded microstructure, dissolves at temperatures above 1050 °C. The dissolution rate is governed by the diffusion of chromium and molybdenum, which are the main constituents of the sigma phase.
3. Grain growth: At high annealing temperatures and for extended durations, grain growth can occur, leading to a coarser microstructure and reduced mechanical properties. The authors found that annealing at 1100 °C for 2 hours produced an optimal grain size, while annealing for 4 hours led to excessive grain growth.

Engineering Practice and Quality Control

The following quality control measures are essential for 2209 duplex stainless steel overlay layers:

1. Phase balance verification: The phase balance must be verified using feritography (ASTM E490) or magnetic induction measurement. The recommended phase balance is 40–60% ferrite.
2. Sigma phase assessment: The presence of sigma phase must be assessed using optical microscopy or scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The sigma phase should be absent or present in trace amounts only.
3. Intergranular corrosion testing: The overlay layer must pass the ASTM A263 intergranular corrosion test (acid copper test) to verify that sensitization has not occurred.
4. Pitting resistance testing: The PREN must be calculated from the chemical composition of the overlay layer and verified to be at least 38.
5. Hardness testing: The hardness must be measured at multiple locations to verify uniformity. The typical hardness of a well-annealed 2209 duplex stainless steel is 200–250 HV.

Study Insights and Implications

This study highlights the critical importance of post-weld heat treatment in the fabrication of 2209 duplex stainless steel overlay layers. The key insight is that the as-welded microstructure is often outside the recommended phase balance range and may contain deleterious phases such as sigma phase, which must be dissolved through annealing.

The optimal annealing temperature and duration must be carefully selected to achieve the desired phase balance and mechanical properties without causing excessive grain growth. The authors recommend annealing at 1100 °C for 2 hours as the optimal condition for 2209 duplex stainless steel overlay layers produced by SAW.

This research has important implications for the fabrication of duplex stainless steel clad plate, overlay pressure vessels, and other bimetal products. Engineers must ensure that the post-weld heat treatment is properly designed and executed to achieve the required microstructure and properties.