Zhuojin Pipe Fitting Co., LtdWeld Gun Oscillation Amplitude Effects on 304L Stainless Steel Reinforcement Plate TIG Weld Joint Microstructure and Properties
Literature Overview
This paper by Zhu Jialei, Zhu Wenlei, Jiao Xiangdong, Li Shougen, Guo Fangtao, and Zhang Hongtao, published in Materials in Mechanical Engineering (2025, Vol. 49, No. 8, pp. 15–18), investigates the effects of TIG welding gun oscillation amplitude on the microstructure, phase composition, microhardness, and corrosion resistance of 304L stainless steel reinforcement plate weld joints. The study uses ER316L filler wire and examines oscillation amplitudes of 2, 3, and 4 mm. Funded by multiple sources including the National Natural Science Foundation (U22B20127) and Beijing Science and Technology Program (KZ202210017023), this work addresses a practical engineering need in pressure vessel and pipeline repair.
Technical Analysis
Oscillation Amplitude and Microstructure Evolution
The weld metal microstructure consists primarily of gamma-austenite and delta-ferrite phases, as expected for austenitic stainless steel welds. The oscillation amplitude directly influences the phase composition and microstructure:
| Oscillation Amplitude | Gamma (111) Peak Intensity | Delta (110) Peak Intensity | Delta Ferrite Distribution |
|---|---|---|---|
| 2 mm | Higher | Lower | Dispersed |
| 3 mm | Lower | Higher | Skeletal along grain boundaries |
| 4 mm | Lowest | Highest | Skeletal along grain boundaries |
As the oscillation amplitude increases, the gamma-phase (111) crystal plane diffraction peak intensity decreases while the delta-phase (110) peak intensity increases. At larger oscillation amplitudes (3 and 4 mm), delta-ferrite forms a skeletal network along the grain boundaries of gamma-austenite columnar and equiaxed grains. This skeletal distribution is particularly detrimental to corrosion resistance and mechanical properties.
Phase Composition and Ferrite Content
The increase in delta-ferrite content with oscillation amplitude is attributed to several factors:
- Increased heat input — Larger oscillation amplitudes increase the effective heat input, which affects the solidification path and promotes ferrite formation.
- Dilution ratio changes — The oscillation pattern alters the weld pool geometry, which affects the dilution ratio and the effective composition of the weld metal.
- Solidification rate — The oscillation-induced fluid flow affects the solidification rate and the competitive growth between austenite and ferrite.
The use of ER316L wire (with lower carbon and higher Ni content than ER304L) is intended to minimize delta-ferrite formation, but the oscillation-induced changes in thermal conditions can still promote significant ferrite content.
Mechanical Property Effects
| Property | 2 mm Amplitude | 3 mm Amplitude | 4 mm Amplitude | Trend |
|---|---|---|---|---|
| Hardness (HV) | Baseline | Increased | Highest | Increases with amplitude |
| Self-corrosion Potential (mV) | Higher | Lower | Lowest | Decreases with amplitude |
| Self-corrosion Current Density (μA/cm²) | Lower | Higher | Highest | Increases with amplitude |
| Corrosion Resistance | Best | Moderate | Worst | Decreases with amplitude |
The hardness increases with oscillation amplitude due to the combined effects of increased delta-ferrite content (which is harder than austenite), reduced austenite content, and grain refinement. However, this hardness increase comes at the expense of corrosion resistance, as evidenced by the decrease in self-corrosion potential and increase in self-corrosion current density.
Corrosion Mechanism Analysis
The degradation of corrosion resistance with increasing oscillation amplitude is primarily attributed to:
- Increased delta-ferrite content — Delta-ferrite is less corrosion-resistant than austenite in chloride-containing environments and acts as a preferential site for pitting initiation.
- Skeletal ferrite distribution — The skeletal network of ferrite along grain boundaries provides continuous paths for intergranular corrosion propagation.
- Grain boundary sensitization — The oscillation-induced thermal cycling may promote chromium depletion at grain boundaries, increasing susceptibility to intergranular corrosion.
Engineering Practice Implications
Reinforcement Plate Welding Considerations
Reinforcement plate welding is a critical repair and modification technique used in pressure vessels, pipelines, and storage tanks. The weld joint must maintain the integrity of the pressure boundary while accommodating the stress concentrations at the plate-to-base metal junction. The selection of oscillation amplitude must balance:
| Factor | Low Amplitude (2 mm) | High Amplitude (4 mm) |
|---|---|---|
| Penetration | Narrower, shallower | Wider, deeper |
| Corrosion Resistance | Better | Worse |
| Hardness | Lower | Higher |
| Weld Width | Narrower | Wider |
| Productivity | Lower | Higher |
Recommended Practices
- For corrosion-critical applications — Use the minimum oscillation amplitude that achieves adequate penetration (typically 2 mm). This minimizes delta-ferrite content and maintains superior corrosion resistance.
- For structural applications — A moderate oscillation amplitude (3 mm) may be acceptable if the service environment is non-corrosive and the primary concern is joint strength.
- Post-weld heat treatment — Solution heat treatment at 1050–1100°C followed by rapid quenching can reduce delta-ferrite content and restore corrosion resistance, but this is often impractical for in-service repairs.
- Filler wire selection — Consider using ER309L or ER347 wire for reinforcement plate welding to further suppress delta-ferrite formation and improve corrosion resistance.
Study Insights and Reflections
This paper provides valuable insights into the trade-offs inherent in oscillation welding of austenitic stainless steel. The finding that increased oscillation amplitude improves weld geometry but degrades corrosion resistance is a critical consideration for engineering design. In pipeline and pressure vessel applications, where corrosion resistance is often the governing design criterion, the use of oscillation welding must be carefully evaluated.
The skeletal distribution of delta-ferrite at higher oscillation amplitudes is particularly concerning, as it creates a continuous network of corrosion-prone phases along grain boundaries. This is analogous to the sensitization phenomenon observed in welds of stabilized stainless steels, where chromium carbide precipitation at grain boundaries leads to intergranular corrosion. For 304L stainless steel reinforcement plate welds, the delta-ferrite network may serve as initiation sites for stress corrosion cracking (SCC) in chloride-containing environments, which is a major concern in chemical processing and marine applications.
Future work should investigate the effects of oscillation frequency and pattern (circular vs. linear) on phase composition and corrosion behavior. Additionally, the interaction between oscillation welding and post-weld heat treatment should be studied to develop comprehensive repair procedures that combine the benefits of oscillation welding (improved penetration, wider weld) with adequate corrosion resistance. For engineers involved in pressure vessel and pipeline repair, this study underscores the importance of considering not just mechanical properties but also corrosion resistance when selecting welding parameters for austenitic stainless steel applications.