CLADDING TECHNOLOGY SHANXI CO., LTDFerrite Content Control in Automatic TIG Weld Overlay Cladding
Literature Overview
The 2019 publication by Wang Yinli, affiliated with the Training Center of PetroChina Daqing Petrochemical Company, addresses a critical quality control parameter in automatic tungsten inert gas (GTAW) cladding operations: the ferrite content in austenitic stainless steel weld overlay deposits. This topic is particularly significant in the context of refinery and petrochemical equipment fabrication, where duplex and austenitic overlay layers are routinely applied to carbon steel pressure vessels, heat exchangers, and piping systems to provide corrosion resistance. The study originates from a practical training environment within China's largest petrochemical complex, which suggests strong alignment with industrial production requirements rather than purely academic investigation.
Core Technical Content
The ferrite content in austenitic stainless steel weld metal is governed primarily by the dilution ratio between the base metal and the filler material, the chemical composition of the consumable, and the welding process parameters. In automatic TIG cladding, the arc stability and heat input are inherently more controlled than in manual operations, which provides a favorable platform for achieving consistent ferrite levels. However, the narrow weld bead geometry characteristic of GTAW means that even small variations in travel speed, torch oscillation amplitude, or wire feed rate can produce significant fluctuations in dilution and consequently in the ferrite phase fraction.
The delta ferrite content is typically expressed in ferrite number (FN) units, as defined by ASTM E490 or ISO 8044. For austenitic stainless steel weld overlays used in refinery service, the acceptable ferrite range is generally between 5 FN and 35 FN, depending on the specific application and the governing code. Excessive ferrite promotes intergranular corrosion susceptibility and reduces ductility, while insufficient ferrite increases the risk of solidification cracking, particularly in high-dilution conditions where carbon and sulfur from the carbon steel base metal are absorbed into the weld metal.
Process Parameter Analysis
The following table summarizes the key process parameters that influence ferrite content in automatic TIG cladding and their typical control ranges:
| Parameter | Typical Range | Effect on Ferrite Content |
|---|---|---|
| Travel speed | 50–150 mm/min | Higher speed reduces heat input and dilution, potentially increasing ferrite |
| Wire feed rate | 3.0–8.0 m/h | Higher feed rate increases filler metal deposition ratio, reducing dilution |
| Shielding gas flow rate | 8–15 L/min | Insufficient flow leads to nitrogen pickup, increasing ferrite formation |
| Arc length | 3–6 mm | Longer arc increases spatter and nitrogen absorption, raising ferrite |
| Torch oscillation amplitude | 0–15 mm | Wider oscillation increases dilution, reducing ferrite |
| Preheat temperature | 50–150°C | Higher preheat reduces cooling rate, affecting phase transformation |
| Interpass temperature | 50–150°C | Higher interpass temperature promotes ferrite dissolution in subsequent passes |
The relationship between dilution and ferrite content is nonlinear. At low dilution levels (below 10 percent), the weld metal composition remains close to the filler metal, and ferrite content is low. As dilution increases beyond 20 percent, the carbon and manganese absorbed from the base metal promote austenite stabilization, and ferrite content decreases. However, at very high dilution levels (above 40 percent), the weld metal may become fully austenitic or even martensitic, depending on the nickel equivalent and chromium equivalent values.
Engineering Practice and Quality Control
In refinery and petrochemical applications, the control of ferrite content is not merely a metallurgical exercise but a direct requirement of fitness-for-service assessment. According to NB/T 47014 and the relevant ASME Section IX provisions, weld overlay procedures must be qualified with documented ferrite measurement results. The standard method for ferrite determination is the magnetic induction method using a portable ferriteoscope, as specified in ASTM E490. However, portable ferriteoscopes are susceptible to interference from surface roughness, residual magnetism, and stress states, which can lead to measurement errors of plus or minus 5 FN.
A more reliable approach involves metallographic examination combined with the Schaeffler diagram or the DeLong diagram for predicting weld metal composition and phase balance. The Schaeffler diagram plots chromium equivalent against nickel equivalent to predict the microstructural regions (ferrite, austenite, martensite, etc.). The DeLong diagram incorporates the effect of carbon, which is particularly important when carbon steel base metals are involved, as carbon significantly reduces the nickel equivalent required to maintain a fully austenitic structure.
The following table compares the predicted ferrite content using different calculation methods for a typical 309L overlay on carbon steel base metal:
| Dilution (%) | Cr-Eq | Ni-Eq | Schaeffler FN | DeLong FN | Measured FN |
|---|---|---|---|---|---|
| 10 | 25.5 | 10.5 | 18 | 22 | 20 |
| 20 | 24.0 | 9.0 | 25 | 30 | 28 |
| 30 | 22.5 | 7.5 | 35 | 42 | 38 |
| 40 | 21.0 | 6.0 | 45 | 55 | 48 |
It is evident from this comparison that the DeLong diagram consistently predicts higher ferrite values than the Schaeffler diagram, which is attributed to the inclusion of carbon in the calculation. In practice, the DeLong diagram should be preferred when the dilution from carbon steel base metals is significant, as it provides a more conservative and realistic estimate of the ferrite content.
Common Defects and Countermeasures
The following table summarizes common defects associated with ferrite content control failures and their countermeasures:
| Defect | Cause | Countermeasure |
|---|---|---|
| Solidification cracking | Ferrite content below 5 FN, high sulfur and phosphor in base metal | Increase ferrite content to 10–15 FN by adjusting filler metal composition or reducing dilution |
| Intergranular corrosion | Ferrite content above 35 FN, sensitization due to overheating | Reduce ferrite content to below 30 FN by increasing nickel equivalent or reducing dilution |
| Sigma phase formation | Prolonged exposure to 600–870°C with high ferrite content | Limit ferrite content to below 25 FN and avoid prolonged exposure to sensitizing temperatures |
| Sigma phase formation | Prolonged exposure to 600–870°C with high ferrite content | Limit ferrite content to below 25 FN and avoid prolonged exposure to sensitizing temperatures |
Study Insights and Implications
The study by Wang Yinli underscores a fundamental principle in weld overlay engineering: the ferrite content is not an independent variable but a dependent variable that is determined by the interplay of base metal composition, filler metal composition, and process parameters. This means that effective ferrite control requires a systems-level approach rather than isolated adjustments to individual parameters. In the automatic TIG cladding process, the high degree of process stability provides an opportunity to achieve tight ferrite control, but only if the operator and the engineering team fully understand the underlying metallurgical relationships.
The practical implication for refinery and petrochemical fabrication is that procedure qualification should include ferrite measurement as a mandatory acceptance criterion, not merely as a supplementary test. Furthermore, the ferrite measurement should be performed at multiple locations along the weld length and at multiple depths through the overlay layer, as ferrite content can vary significantly from the top of the weld to the fusion line. The top of the weld typically exhibits lower ferrite content due to the higher nickel equivalent at the surface, while the fusion line may exhibit higher ferrite content due to the higher dilution from the base metal.
In conclusion, the control of ferrite content in automatic TIG weld overlay cladding is a technically demanding but achievable objective that requires a deep understanding of weld metal metallurgy, a disciplined approach to process parameter control, and a rigorous quality assurance program that includes both in-process monitoring and post-weld verification. The experience documented in this literature provides valuable guidance for engineers and technicians involved in the fabrication of corrosion-resistant cladding for petrochemical equipment, and the principles discussed are directly applicable to other overlay processes including submerged arc welding, gas metal arc welding, and plasma transferred arc cladding.