CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Hydrogen on Toughness and Ductility of Stainless Steel Electroslag Overlay Layers
Literature Overview
This 1999 study by Liu Hang, Tian Zhiling, and colleagues from the Iron and Steel Research Institute and Jinling Petrochemical Company investigates the detrimental effects of hydrogen on the toughness and ductility of stainless steel electroslag welding (ESW) overlay layers. Hydrogen-induced cracking (HIC) and hydrogen embrittlement are well-recognized failure modes in hydrogen-containing environments, and this research is particularly relevant to engineers in the petrochemical and oil and gas industries where hydrogen service is common. The study provides critical data on how hydrogen content in the overlay affects its mechanical integrity and service reliability.
Electroslag Overlay Process Characteristics
Electroslag welding overlay is a process that uses the high thermal efficiency of a molten slag pool to melt both the electrode and the substrate, producing thick overlay layers with low dilution. The process is particularly suitable for building up thick corrosion-resistant layers on large components such as pressure vessels, heat exchanger tubesheets, and reactor linings. The key process parameters for stainless steel ESW overlay are as follows:
| Parameter | Typical Range | Effect on Hydrogen Content |
|---|---|---|
| Current | 800–1500 A | Higher current increases slag pool temperature, may increase hydrogen pickup |
| Travel speed | 100–200 mm/min | Slower speeds increase residence time in slag, more hydrogen pickup |
| Electrode diameter | 12–20 mm | Larger electrodes require higher currents |
| Slag composition | Rutile-fluorite type | Basic slag reduces hydrogen solubility |
| Preheat temperature | 150–250°C | Higher preheat increases hydrogen pickup |
| Shielding gas | None (slag protection) | No external gas shielding, relies on slag |
The ESW process inherently has a higher risk of hydrogen pickup compared to processes like GTAW or GMAW because the molten metal is exposed to the slag pool for a longer duration, and the high temperatures of the slag (typically 1400–1600°C) increase the solubility of hydrogen in the metal.
Hydrogen Content and Mechanical Properties
The study systematically examines the relationship between hydrogen content in the ESW overlay and the resulting mechanical properties. The key findings are presented below:
| Hydrogen Content (mL/100g) | Tensile Strength (MPa) | Elongation (%) | Impact Energy (J, Charpy V) | Hardness (HV) |
|---|---|---|---|---|
| < 1.0 | 580–620 | 35–42 | 80–120 | 180–200 |
| 1.0–2.0 | 560–600 | 30–38 | 60–90 | 185–205 |
| 2.0–4.0 | 520–570 | 22–30 | 30–60 | 190–210 |
| 4.0–6.0 | 480–540 | 15–22 | 10–30 | 195–215 |
| > 6.0 | 440–500 | 8–15 | < 10 | 200–220 |
The data clearly demonstrate that increasing hydrogen content leads to progressive degradation of ductility and toughness, while having a relatively minor effect on tensile strength and hardness. This pattern is characteristic of hydrogen embrittlement, where the hydrogen atoms reduce the cohesive strength of the metal without significantly altering the bulk mechanical properties.
Hydrogen Embrittlement Mechanisms
The study identifies several mechanisms by which hydrogen degrades the performance of the ESW overlay:
- Hydrogen-induced cracking (HIC): At hydrogen concentrations above approximately 4 mL/100g, stepwise cracking can occur parallel to the rolling direction of the substrate plate. These cracks are particularly dangerous in pressure vessel applications because they can propagate through the overlay layer and compromise containment integrity.
- Delayed cracking: Hydrogen-induced cracking may not occur immediately after welding but can manifest hours or even days later as hydrogen diffuses to trap sites such as inclusions, grain boundaries, or phase boundaries. This delayed nature makes it particularly insidious in service.
- Reduction in fracture toughness: The Charpy impact energy data show a dramatic reduction in fracture toughness with increasing hydrogen content. At hydrogen levels above 4 mL/100g, the overlay transitions from ductile to brittle fracture behavior, which is unacceptable for pressure-containing applications.
- Sulfide stress cracking (SSC) susceptibility: In the presence of hydrogen sulfide (H2S), the combination of hydrogen and sulfide ions creates a highly aggressive environment for stress cracking. The study notes that the ESW overlay, with its potentially higher hydrogen content compared to other overlay processes, is particularly susceptible to SSC in sour service.
Countermeasures and Process Controls
Based on the findings of this research, the following countermeasures are recommended to minimize hydrogen content in ESW overlay layers:
| Countermeasure | Implementation Method | Expected Hydrogen Reduction |
|---|---|---|
| Electrode drying | Bake at 300–400°C for 2–4 hours before use | 30–50% |
| Slag conditioning | Dry flux at 250–300°C for 2 hours | 20–40% |
| Post-weld baking | 200–300°C for 2–4 hours after welding | 40–60% |
| Slag composition optimization | Increase basicity (CaO/SiO2 ratio > 2.5) | 15–25% |
| Current density control | Maintain within 20–40 A/mm² | 10–20% |
| Travel speed optimization | Avoid excessively slow travel speeds | 10–15% |
| Preheat reduction | Limit preheat to minimum required (≤ 150°C) | 10–20% |
The most effective single countermeasure is post-weld baking, which can reduce hydrogen content by 40–60% through diffusion and outgassing. However, this must be balanced against the need to avoid sensitization of the stainless steel overlay, which can occur at temperatures above 450°C. A bake temperature of 200–300°C is optimal for hydrogen removal without sensitization risk.
Quality Control and Inspection
For ESW overlay applications in hydrogen-containing service, the following quality control measures are essential:
- Hydrogen content testing: Gas chromatography or electrolytic extraction methods should be used to measure hydrogen content in production overlays. The acceptance criterion for hydrogen service should be ≤ 2.0 mL/100g, and preferably ≤ 1.0 mL/100g for critical applications.
- Non-destructive testing (NDT): Ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) should be performed to detect hydrogen-induced cracks, particularly stepwise cracks that may be difficult to detect by conventional methods. Magnetic particle testing (MT) or penetrant testing (PT) should be used for surface crack detection.
- Mechanical property verification: Charpy impact testing at service temperature should be performed on production welds to verify that the toughness meets the minimum requirements. For hydrogen service, a minimum impact energy of 47 J at -20°C is recommended.
- Post-weld inspection timing: Because hydrogen-induced cracking can be delayed, NDT should be performed both immediately after welding and again after a 24–48 hour delay to detect any delayed cracking.
Engineering Practice and Industry Relevance
This research is of direct relevance to the fabrication of pressure vessels and heat exchangers for hydrogenation reactors, hydrotreater units, and other sour service applications in the petrochemical industry. The ESW overlay process, while capable of producing thick, high-quality overlay layers, requires careful hydrogen control to ensure long-term service reliability. Engineers should incorporate hydrogen content monitoring and post-weld baking into their welding procedure specifications (WPS) and quality assurance plans for any ESW overlay application in hydrogen-containing environments.
Key Reflections and Study Insights
The study by Liu Hang et al. provides a sobering reminder of the hidden dangers of hydrogen in weld overlay deposits. While the ESW process offers significant advantages in terms of deposition rate and overlay thickness, its inherent susceptibility to hydrogen pickup makes it a process that demands rigorous process control and quality assurance. The progressive degradation of toughness with increasing hydrogen content, as documented in this research, highlights the importance of hydrogen content monitoring as a critical quality parameter in overlay welding for hydrogen service. For engineers involved in bimetal pressure vessel fabrication, this research reinforces the principle that process selection must always be evaluated in the context of the service environment, and that the most efficient process may not always be the most appropriate one. The integration of hydrogen control measures into the overlay welding procedure, combined with thorough NDT and mechanical property verification, is essential for ensuring the long-term integrity and safety of pressure-containing equipment in hydrogen-containing service.