Zhuojin Pipe Fitting Co., LtdFerrite Content Control in Automatic TIG Overlay Welding of Dissimilar Steel Joints
Literature Overview
This paper by Wang Yinli (2019, Refining and Chemical Engineering, Vol. 30, No. 3) investigates the control of ferrite content in the overlay layer produced by automatic TIG (GTAW) welding on a 12Cr2Mo1R steel base. The study examines overlay welding using ER309L and ER347 consumables, analyzing how welding parameters affect the ferrite content of the deposited metal. The context is the fabrication of hydrogenation reactors and threaded-lock-ring heat exchangers, where the ferrite content of the overlay directly influences susceptibility to hot cracking and the overall corrosion resistance of the weld.
Core Technical Content
Significance of Ferrite Content in Austenitic Overlay Welds
In austenitic stainless steel welds, the microstructure is typically a mixture of austenite and delta ferrite. The ferrite content is measured in ferrite numbers (FN) using the ferrite gauge method (ASTM E490). The acceptable range depends on the application:
| Ferrite Number (FN) | Microstructure | Application Suitability |
|---|---|---|
| 0–3 FN | Fully austenitic | High-temperature creep applications; risk of hot cracking |
| 3–10 FN | Predominantly austenitic with some ferrite | General corrosion resistance; good balance |
| 10–20 FN | Significant ferrite | Good resistance to SCC; moderate corrosion resistance |
| > 20 FN | Excessive ferrite | Reduced corrosion resistance; potential for intergranular corrosion |
For overlay welding on carbon steel or low-alloy steel bases, the dilution from the base material introduces alloying elements (C, Mn, Cr, Mo) that affect the weld metal composition and, consequently, the ferrite content.
Welding Parameters and Their Effect on Ferrite Content
The paper systematically varies the following parameters in automatic GTAW overlay welding:
| Parameter | Range Studied | Effect on Ferrite Content |
|---|---|---|
| Welding current | 120–200 A | Higher current → higher dilution → higher FN |
| Travel speed | 50–150 mm/min | Higher speed → lower heat input → lower dilution → lower FN |
| Arc length | 2–5 mm | Longer arc → more oxidation → affects composition |
| Shielding gas flow | 8–20 L/min | Insufficient flow → oxidation → increased ferrite |
| Preheating temperature | 0–200°C | Higher preheat → lower cooling rate → affects grain size and ferrite morphology |
Key Findings
- Dilution is the primary driver of ferrite content: The ferrite number increases with increasing dilution from the 12Cr2Mo1R base material. At low dilution (< 15%), the weld metal remains predominantly austenitic (FN < 5). At high dilution (> 30%), the ferrite content can exceed 15 FN.
- Welding current has the most significant effect: Increasing the current from 120 A to 200 A increases the heat input and the depth of penetration, leading to higher dilution and higher ferrite content.
- Travel speed inversely affects ferrite content: Higher travel speeds reduce the heat input per unit length, limiting penetration and dilution, thereby reducing ferrite content.
- ER347 vs. ER309L: ER347 contains niobium, which stabilizes carbon and reduces the tendency for delta ferrite formation. However, the difference in ferrite content between ER309L and ER347 overlays is modest when dilution is high, because the base material composition dominates.
- Optimal parameters: The paper identifies a parameter window that achieves a ferrite content in the range of 5–10 FN, which provides a good balance between hot cracking resistance and corrosion resistance.
Hot Cracking Susceptibility
Hot cracking in austenitic welds is primarily a solidification cracking phenomenon, associated with the formation of low-melting-point intergranular films (e.g., MnS) in the final stages of solidification. A fully austenitic microstructure (FN < 3) is most susceptible to hot cracking because there is no ferrite to act as a crack-bridging phase. Conversely, excessive ferrite (FN > 20) can reduce corrosion resistance and may promote intergranular corrosion. The optimal ferrite content for hot cracking resistance is typically 5–15 FN.
Integration with Engineering Practice
In hydrogenation reactor fabrication, the overlay welds serve as a barrier between the high-temperature hydrogen environment and the base steel. The overlay must be free of cracks, pores, and lack of fusion, and must have adequate corrosion resistance. The ferrite content is a critical quality attribute because it directly relates to the weld's resistance to hot cracking during fabrication and its long-term corrosion performance in service.
The paper's findings are directly applicable to the qualification of welding procedures for hydrogenation reactors. The identified optimal parameter window should be incorporated into the WPS (Welding Procedure Specification) and verified through procedure qualification tests (PQR) in accordance with ASME Section IX or ISO 15614-1.
Key Questions and Reflections
- How does the ferrite content distribution vary across the cross-section of a multi-pass overlay—does it differ between the first pass (highest dilution) and subsequent passes (lower dilution)?
- What is the effect of post-weld heat treatment on the ferrite morphology and distribution?
- Are there service data correlating ferrite content with overlay corrosion performance in hydrogenation reactors?
The paper provides valuable parametric data but would benefit from metallographic characterization of the overlay microstructure and hardness profiles at different parameter settings.
Study Insights and Implications
This literature underscores the importance of understanding the metallurgical consequences of welding parameters in dissimilar metal overlay welding. The ferrite content is not merely a number on a test report—it is a direct indicator of the weld's resistance to cracking, its corrosion performance, and its long-term reliability. Engineers involved in the fabrication of critical equipment such as hydrogenation reactors and heat exchangers should pay close attention to dilution control and ferrite content monitoring. The systematic approach of varying one parameter at a time and measuring the response is a classic experimental design methodology that remains highly relevant in welding procedure development.