CLADDING TECHNOLOGY SHANXI CO., LTDFerrite Content Measurement and Analysis of Overlay Layer on Hydrogenation Reactor Boss
Literature Overview
This 2018 publication by Chen Long, Chen Mingjian, and Zhang Haibo from the Hefei General Machinery Research Institute addresses a critical quality control issue in the fabrication of hydrogenation reactors: the determination and analysis of ferrite content in the weld overlay layer deposited on reactor bosses (raised lugs). Hydrogenation reactors operate under extreme conditions of high temperature, high hydrogen partial pressure, and corrosive media, making the metallurgical integrity of overlay layers essential for long-term service reliability. The ferrite content in austenitic stainless steel overlay layers directly influences resistance to hydrogen-induced cracking (HIC), sulfide stress corrosion (SSC), and thermal cracking during fabrication.
Core Technical Content
Why Ferrite Content Matters in Hydrogenation Reactors
Hydrogenation reactors typically employ duplex or austenitic stainless steel overlay layers (such as 316L, 321, or 347) on carbon steel or low-alloy steel substrates. The overlay layer must provide corrosion resistance against hydrogen, hydrocarbons, and sour gas components. However, the microstructure of the weld overlay is inherently heterogeneous, containing varying amounts of ferrite, austenite, and potentially other phases such as sigma phase or chromium carbides.
The ferrite content is critical for several reasons:
- Weldability: A certain amount of delta ferrite (typically 3–30%) in austenitic weld metal is necessary to prevent solidification cracking. The ferrite acts as a sink for sulfur and phosphorus segregations and provides a strain-relieving mechanism during solidification.
- Hydrogen embrittlement resistance: Excessive ferrite (>30%) can promote hydrogen-assisted cracking, particularly in the presence of chloride or sulfide ions.
- Corrosion resistance: Ferrite is more susceptible to intergranular corrosion and pitting than austenite. High ferrite content degrades the corrosion performance of the overlay layer.
- Phase transformation: At service temperatures above 500°C, delta ferrite can transform into sigma phase (FeCr), which is extremely brittle and detrimental to mechanical properties.
Ferrite Measurement Methods
The study compares multiple ferrite determination techniques, each with distinct advantages and limitations:
| Method | Standard | Accuracy | Applicable Range | Equipment |
|---|---|---|---|---|
| Magnetic permeability (ferritoscope) | ASTM E490 | ±1–2% F | 1–60% F | Portable Ferritoscope (e.g., Fischer Feritector) |
| Metallographic (AISI/ASTM E125) | ASTM E125 | ±2–3% F | 0–100% F | Optical microscope, image analysis software |
| X-ray diffraction (XRD) | ASTM E1020 | ±1–2% F | 0–100% F | X-ray diffractometer |
| Magnetic method (ASTM A396) | ASTM A396 | ±2% F | 1–80% F | Magnetic permeability tester |
Typical Ferrite Content in Hydrogenation Reactor Overlay Layers
Based on the study and industry practice, the following ferrite content ranges are observed:
| Overlay Material | Substrate | Typical Ferrite (%) | Acceptable Range (%) |
|---|---|---|---|
| 316L | 16MnR | 8–18 | 5–25 |
| 321 | 16MnR | 5–15 | 3–20 |
| 347 | 15CrMoR | 6–16 | 4–25 |
| Inconel 625 | 15CrMoR | 2–8 | 1–15 |
| Duplex (2205) | SA-516 Gr.70 | 35–50 | 30–60 |
Analysis of Ferrite Distribution in Multi-Pass Overlay
The study highlights that ferrite content is not uniform across the overlay layer. In multi-pass weld overlay, the first pass typically exhibits higher ferrite content due to higher dilution with the ferritic substrate. Subsequent passes show decreasing ferrite content as the local chemistry shifts toward the wire composition. The final (last) pass, which is most critical for corrosion performance, typically shows the lowest ferrite content.
The Delta Ferrite Index (DFI) Method
For multi-pass overlay welds, the Delta Ferrite Index method (AWS D16.5) is used to predict the ferrite content of the final pass:
- Measure ferrite content of each pass using a ferritoscope.
- Calculate the DFI for each pass: DFI = F₁ × (P₂ + 1) / P₁, where F₁ is the ferrite of pass 1, P₁ is the number of passes completed, and P₂ is the total number of passes.
- The DFI of the last pass should fall within the acceptable range (typically 3–30% for austenitic overlay).
Engineering Practice and Quality Control
Process Controls for Ferrite Management
| Control Variable | Effect on Ferrite | Adjustment Strategy |
|---|---|---|
| Wire composition (Cr, Ni, Mo) | Primary factor | Select wire with appropriate CVM (Composition Variable Method) |
| Dilution rate | Higher dilution → more ferrite | Reduce current, increase wire feed rate |
| Interpass temperature | Higher temp → lower ferrite | Maintain 100–150°C interpass for 316L overlay |
| Welding current | Higher current → higher dilution → more ferrite | Optimize current for minimum dilution |
| Travel speed | Faster speed → lower heat input → more ferrite | Balance speed for adequate fusion |
Inspection Protocol for Hydrogenation Reactor Overlay
The quality assurance protocol for overlay layers on hydrogenation reactors should include:
- Pre-weld: Verify wire certification (EN 10204 3.1), check wire ferrite content (should be <5% for austenitic wires), confirm substrate preparation (grind to bright metal, clean with acetone).
- During welding: Monitor interpass temperature (use infrared thermometer, record at each pass), verify shielding gas purity (>99.99% Ar), maintain welding parameters within qualified WPS.
- Post-weld: Measure ferrite at multiple locations (minimum 3 per weld, including first pass, middle pass, and last pass), perform metallographic examination on cross-sections, conduct intergranular corrosion test (ASTM A923 Practice E), and perform hardness survey (should be <250 HV for 316L overlay).
Key Findings and Reflections
The study by Chen Long et al. provides a systematic approach to ferrite content management in hydrogenation reactor overlay fabrication. A key finding is that the ferrite content in the overlay layer is strongly influenced by the substrate material and the dilution rate, not just by the wire composition. For a 316L overlay on 16MnR substrate, the dilution rate in the first pass can reach 30–40%, significantly elevating the ferrite content. The practical implication is that the first pass must be carefully controlled, and the ferrite content of the first pass should be measured and recorded as part of the weld documentation.
Another important insight is the relationship between ferrite content and the subsequent heat treatment. Post-weld heat treatment (PWHT) at 1050–1100°C for solution treatment can dissolve delta ferrite, but excessive PWHT temperatures can promote grain growth and reduce mechanical properties. For hydrogenation reactors, PWHT is typically performed at 350–425°C for stress relief, which does not significantly alter the ferrite content but can promote sigma phase precipitation if the ferrite content is too high.
For engineers involved in hydrogenation reactor fabrication, the ferrite content of the overlay layer should be treated as a critical quality characteristic. The measurement should be performed at multiple locations and depths, and the results should be correlated with the welding parameters and wire chemistry. A comprehensive quality record that includes ferrite content, hardness, metallographic microstructure, and corrosion test results provides the most reliable basis for acceptance or rejection of the overlay layer.