Zhuojin Pipe Fitting Co., LtdAustenitic Stainless Steel Overlay on Q345 Low Alloy Steel via MAG Welding
Literature Overview
Wang Zhiling and Yu Genxi published this study in Foundry Technology (2016, Vol. 37, Issue 4, pp. 771-773), investigating the overlay welding of austenitic stainless steel onto Q345 low alloy steel using MIG/MAG welding. The study employs a two-layer design with ER309L as the transition layer and ER347L as the corrosion-resistant layer, examining bending and shear performance, microstructure, and fracture morphology.
Core Technical Points
Welding Design Philosophy
The two-layer overlay design addresses a fundamental challenge in dissimilar metal welding: the large difference in composition and properties between the carbon steel base (Q345, ~0.2% C, ~1.0% Mn) and the austenitic stainless steel overlay (ER347L, ~0.03% C, ~19% Cr, ~10% Ni). Direct welding of austenitic stainless steel onto carbon steel creates a high-dilution weld metal with excessive carbon pickup, leading to intergranular corrosion susceptibility and reduced corrosion resistance.
The ER309L transition layer (25% Cr, 13% Ni, 0.03% C) serves as a buffer zone that:
- Absorbs carbon from the base metal dilution
- Provides adequate ductility to accommodate thermal expansion mismatch
- Reduces the dilution effect on the final ER347L corrosion-resistant layer
Microstructural Analysis
The overlay layer microstructure is predominantly austenite with 15-25% ferrite (delta ferrite), which is the target range for stainless steel welds to prevent hot cracking. The ferrite content is controlled by the Cr and Ni content of the filler metal and the dilution from the base metal.
| Layer | Filler Metal | Primary Phase | Ferrite Content | Carbon Equivalent |
|---|---|---|---|---|
| Base | Q345 | Ferrite + pearlite | ~100% | 0.35-0.45% |
| Transition | ER309L | Austenite + ferrite | 20-30% | 0.03% (low C) |
| Overlay | ER347L | Austenite + ferrite | 15-25% | 0.03% (low C) |
The presence of trace chromium carbide brittle phases (M₂₃C₆ or Cr₇C₃) was observed in the overlay layer. These carbides form at grain boundaries and can reduce intergranular corrosion resistance if they coarsen during service exposure to 450-850 °C (sensitization temperature range). The use of low-carbon (L-grade) filler metals minimizes this risk.
Mechanical Performance
Bending and shear tests confirmed that the transition layer design significantly improves the mechanical integrity of the overlay. Without the transition layer, the direct weld between Q345 and ER347L would exhibit high residual stresses and potential cracking due to the thermal expansion mismatch (Q345: 12×10⁻⁶/°C; austenitic SS: 17×10⁻⁶/°C).
The shear fracture morphology showed dimple-dominated ductile fracture, indicating good toughness and resistance to delamination. This is critical for overlay layers that must withstand thermal cycling and mechanical loading without spalling.
Process Parameters and Quality Control
| Parameter | ER309L Transition Layer | ER347L Overlay Layer |
|---|---|---|
| Wire diameter | 1.2 mm | 1.2 mm |
| Shielding gas | Ar/CO₂ (80/20) | Ar/CO₂ (80/20) |
| Current | 180-220 A | 160-200 A |
| Travel speed | 400-500 mm/min | 450-550 mm/min |
| Voltage | 22-26 V | 20-24 V |
| Preheat | 50-100 °C | 50-100 °C |
| Interpass temperature | ≤150 °C | ≤150 °C |
The lower current and voltage for the overlay layer reduce dilution from the transition layer, preserving the corrosion resistance of the final layer.
Engineering Applications
This overlay technology is directly applicable to pipeline and piping applications where carbon steel piping must be upgraded for corrosion resistance without full replacement. Common applications include:
- Overlay of carbon steel pipe interiors for sour service (H₂S-containing environments)
- Repair of corroded carbon steel elbows and tees with stainless steel overlay
- Upgrade of carbon steel heat exchanger tubesheets for service in corrosive process fluids
For pipeline applications, the overlay thickness should be at least 3 mm to provide adequate corrosion protection, with a minimum of 1.5 mm of overlay remaining after machining. The overlay must be qualified through corrosion testing (e.g., ASTM G48 for pitting resistance, NACE TM0169 for HIC resistance).
Key Reflections
The two-layer design philosophy is elegant in its simplicity but requires careful execution. The transition layer must be thick enough to buffer dilution (minimum 2-3 mm) but not so thick as to compromise the corrosion resistance of the final layer. In practice, the transition layer is typically deposited in 2-3 passes, while the overlay layer is deposited in 1-2 passes.
The observation of trace chromium carbide brittle phases raises a question about long-term service performance. If the overlay is exposed to temperatures in the sensitization range (450-850 °C), grain boundary carbide precipitation could reduce intergranular corrosion resistance. For high-temperature service, stabilized filler metals (e.g., ER347 with Nb stabilization) should be specified, and post-weld solution heat treatment at 1050-1100 °C may be necessary.
Summary and Conclusions
This study demonstrates that the two-layer ER309L/ER347L overlay design provides an effective solution for upgrading carbon steel surfaces to austenitic stainless steel corrosion resistance. The transition layer significantly improves bending and shear performance, while the low-carbon filler metals minimize intergranular corrosion susceptibility. For pipeline and piping engineers, this technology offers a practical and economical alternative to full stainless steel replacement for corrosion protection of carbon steel infrastructure.