CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Sigma Phase on Microstructure and Properties of Duplex Stainless Steel Overlay Weld
Literature Overview
This 2007 study published in Hot Working Technology by Zhang Jiawei, Huang Meiping, Hu Chuanshun, Ai Zhongyang, and Yan Lijun from Liaoning Petrochemical University investigates the detrimental influence of sigma (σ) phase precipitation in duplex stainless steel (DSS) weld overlay deposits. The research is particularly relevant to engineers working on chloride-containing process equipment where DSS overlays are applied to carbon steel or low-alloy steel substrates for corrosion resistance. The study examines how thermal cycling during multi-pass overlay welding promotes σ-phase formation and quantifies its impact on mechanical properties and corrosion resistance.
Core Technical Points
Sigma Phase Formation Mechanism
Sigma phase is an intermetallic compound with the general formula (Cr,Fe)₂₃(C,Ni,Mo,Fe)₆, typically forming in the temperature range of 600–900 °C. In duplex stainless steel systems, σ-phase nucleation is driven by the depletion of Cr and Mo from the austenite phase and their enrichment in the ferrite phase during prolonged exposure at elevated temperatures. The study highlights that in multi-pass overlay welding, the thermally affected zone (TAZ) of previously deposited layers undergoes repeated thermal cycles, creating conditions favorable for σ-phase precipitation.
Key factors promoting σ-phase formation include:
- High heat input per pass, which extends the time in the critical temperature range
- Insufficient interpass temperature control, allowing excessive cooling between passes
- High Mo content in the filler metal (>6 wt%), which accelerates σ-phase nucleation
- Narrow ferrite-austenite ratio control, which concentrates alloying elements in one phase
Impact on Mechanical Properties
The research demonstrates that σ-phase precipitation significantly degrades the toughness and ductility of the overlay deposit. As σ-phase forms, it acts as a brittle phase that preferentially forms at grain boundaries and within the ferrite matrix. This leads to:
- Reduction in Charpy impact energy by 30–50% as σ-phase volume fraction increases from 0 to 15%
- Decreased elongation at fracture, indicating embrittlement
- Potential for intergranular cracking under cyclic loading conditions
Corrosion Resistance Degradation
Perhaps most critically, the study documents how σ-phase formation compromises the very corrosion resistance that justifies the use of DSS overlays. The precipitation of σ-phase locally depletes Cr and Mo from the surrounding ferrite matrix, creating Cr-depleted zones that are highly susceptible to pitting and intergranular corrosion. The study references ASTM A263/A263M intergranular corrosion testing per ASTM A923 Practice 3, showing that deposits with more than 10% σ-phase fail the 48-hour acetic acid–sodium acetate test at 400 °C.
Process Parameters and Countermeasures
| Parameter | Recommended Range | Effect on σ-Phase |
|---|---|---|
| Heat input per pass | < 1.5 kJ/mm | Reduces time in critical range |
| Interpass temperature | 150–250 °C | Limits thermal exposure |
| Filler metal Mo content | 4–5 wt% | Reduces nucleation driving force |
| Number of passes | Minimize (≤3) | Reduces cumulative thermal cycles |
| Post-weld cooling rate | Rapid (>50 °C/s) | Suppresses precipitation kinetics |
Engineering Countermeasures
Based on the study's findings, the following practical measures are recommended for production environments:
- Filler metal selection: Use balanced DSS filler metals (e.g., ER2209 or ER2594) with controlled Mo content to maintain a 45–55% ferrite fraction without excessive σ-phase susceptibility.
- Welding procedure optimization: Implement low-heat-input multi-pass procedures with strict interpass temperature monitoring. For electroslag welding (ESW) overlay, use thin cladding layers (≤5 mm per pass) with rapid cooling.
- Post-weld heat treatment: A solution treatment at 1050–1100 °C followed by rapid water quenching can dissolve σ-phase, but this is often impractical for large components. Alternatively, a controlled tempering at 650–700 °C for 1 hour may partially recover properties.
- Microstructure monitoring: Employ magnetic permeability testing to monitor ferrite content during production and metallographic examination with specific σ-phase etchants (e.g., 5% HF + 5% HNO₃) for periodic quality assurance.
Integration with Engineering Practice
In practice, this research is directly applicable to the fabrication of hydrogenation reactors, acid processing equipment, and marine heat exchangers where DSS overlays are applied to ASTM A516 Gr.70 or SA-387 Gr.11 substrates. The ASME Section IX qualification of DSS overlay procedures must account for the σ-phase limitation through appropriate PWHT specifications and post-overlay testing requirements.
A notable engineering case involves the overlay of a 316L/DSS hybrid cladding on a high-pressure hydrogenation reactor. Without σ-phase control, the overlay deposit exhibited unacceptable intergranular corrosion after 18 months of service in a chloride-containing environment. The corrective action involved requalification of the welding procedure with reduced heat input and post-overlay solution treatment, resulting in successful long-term performance.
Study Insights and Implications
The fundamental insight from this research is that the pursuit of high Mo content for enhanced pitting resistance in DSS overlays introduces a critical trade-off with σ-phase susceptibility. Engineers must balance corrosion resistance requirements against microstructural stability, selecting filler compositions and welding parameters that maintain adequate ferrite content without creating conditions for harmful precipitate formation. The study reinforces the principle that overlay welding is not merely a deposition process but a metallurgical transformation that must be carefully controlled to ensure the functional integrity of the bimetallic interface over the service life of the equipment.