CLADDING TECHNOLOGY SHANXI CO., LTDFerrite Content Determination and Analysis of Weld Overlay Layer on Hydrogenation Reactor Boss
Background and Technical Context
Hydrogenation reactors are among the most critical pressure vessels in petrochemical and refining industries, where the combination of high temperature, high hydrogen partial pressure, and corrosive hydrocarbon media demands meticulous attention to material selection and weld quality. The boss (nozzle neck) areas of these reactors are particularly susceptible to stress concentration and corrosion, making weld overlay cladding a standard fabrication strategy. The present study, published in the journal "Petrochemical Equipment Technology" in 2018 by researchers from the Hefei General Machinery Research Institute, addresses the determination and analysis of ferrite content in the weld overlay layer deposited on hydrogenation reactor bosses.
This topic holds significant importance because ferrite content in austenitic stainless steel weld metal directly governs several critical properties: intergranular corrosion resistance, susceptibility to sensitization, cracking tendency, and mechanical behavior under cyclic loading. In the context of hydrogenation service, where hydrogen attack and sulfide stress cracking (SSC) are known degradation mechanisms, controlling the ferrite phase fraction becomes a decisive quality parameter.
Core Technical Content
The study focuses on measuring ferrite content using the ferrite meter method (typically the magnetic induction method conforming to ASTM A913 or ISO 8044) in overlay weld deposits made with consumables such as E309L, E309, E316L, or similar grades commonly specified for hydrogenation reactor cladding. The ferrite content is expressed in ASTM Number (FN), where the target range for 300-series austenitic stainless steel weld metal is typically 5-35 FN for general applications and 10-35 FN for corrosion-critical applications.
The researchers examined the influence of welding parameters, including current, voltage, travel speed, and layer thickness, on the resulting ferrite content. A key finding is that the dilution rate between the base material and the overlay layer significantly affects the final ferrite content. When overlaying austenitic stainless steel on carbon steel or low-alloy steel base material, the dilution from the ferritic base can substantially raise the ferrite content in the first overlay pass.
Ferrite Content Control Parameters
| Parameter | Typical Range | Effect on Ferrite |
|---|---|---|
| Welding Current | 150-350 A (depending on process) | Higher current increases dilution, raising ferrite |
| Travel Speed | 100-300 mm/min | Faster speed reduces heat input, slightly lowers dilution |
| Number of Overlay Passes | 2-5 | More passes reduce base dilution in subsequent layers |
| Wire Diameter | 1.6-3.2 mm | Thicker wire increases deposition rate but may alter heat input |
| Interpass Temperature | <250°C for 304/316 | Higher interpass temperature promotes grain growth |
| Base Material | Carbon steel / 16MnR / 15CrMoR | Higher carbon and alloy content in base raises ferrite |
Engineering Practice and Quality Implications
In practice, hydrogenation reactor bosses are typically clad with 316L or 321 stainless steel to resist the corrosive environment. The weld overlay layer must be free from sensitization, which can be verified by the ASTM A262 Practice E intergranular corrosion test. Ferrite content above 35 FN in 300-series weld metal may indicate an elevated risk of ferrite corrosion in chloride-containing environments, while ferrite content below 5 FN increases the susceptibility to hot cracking.
The study emphasizes that ferrite content should be measured at multiple locations: center of the weld, near the fusion line, and at the top of the weld. The variation across the weld cross-section can be significant, particularly near the fusion line where dilution is highest. A practical recommendation derived from this work is to ensure that the first pass (with highest dilution) is included in ferrite testing, as it represents the worst-case scenario for ferrite enrichment.
From a standards perspective, ASME Section IX and NB/T 47014 both require ferrite content verification for austenitic stainless steel weld overlays. The typical acceptance criterion is that ferrite content shall not exceed 35 FN for 304/304L and 316/316L weld metal, and shall not be less than 5 FN unless otherwise specified by the design authority.
Key Reflections
The determination of ferrite content is not merely a compliance exercise but a fundamental metallurgical control point. In hydrogenation service, where the overlay layer must simultaneously resist hydrogen damage, sulfur-induced stress cracking, and general corrosion, the balance between austenite and ferrite phases is critical. Ferrite, being more resistant to hydrogen-induced cracking than austenite, provides some benefit in high-hydrogen environments. However, excessive ferrite introduces corrosion vulnerabilities and reduces ductility. The optimal ferrite content for hydrogenation reactor overlay applications is therefore a compromise, typically in the range of 15-30 FN, which balances corrosion resistance, hydrogen tolerance, and mechanical properties. This study provides valuable quantitative data for engineers specifying and qualifying overlay welding procedures for hydrogenation reactor fabrication.