CLADDING TECHNOLOGY SHANXI CO., LTDToughness and Hydrogen-Induced Delamination in the Fusion Zone of Austenitic Stainless Steel Overlay on 1Cr-1/2Mo Steel
Literature Overview
Published in 1990 by Chen Zigang and colleagues from Dalian Institute of Railway Technology, this paper investigates the toughness and hydrogen-induced delamination behavior in the fusion zone when overlaying austenitic stainless steel on 1Cr-1/2Mo low-alloy steel. The work is particularly relevant to the fabrication of pressure vessels and piping systems in the petrochemical and power generation industries, where 1Cr-1/2Mo steel is commonly used for high-temperature service and austenitic stainless steel overlays are applied for corrosion resistance. The combination of these materials is widely used in hydrogenation reactors, heat exchangers, and other high-pressure equipment, making the understanding of the fusion zone behavior critical for safe and reliable operation.
Material System and Welding Process
The base material, 1Cr-1/2Mo steel (equivalent to ASTM A217 Grade CA6NM or similar), is a low-alloy steel with good high-temperature strength and moderate corrosion resistance. The overlay material, austenitic stainless steel (typically 309 or 310 type), is selected for its excellent corrosion resistance and resistance to thermal cycling. The welding process used in the study was gas tungsten arc welding (GTAW), which provides precise heat input control and minimal dilution.
| Parameter | Base Metal (1Cr-1/2Mo) | Overlay Metal (Austenitic SS) | Welding Process |
|---|---|---|---|
| Carbon content | 0.15 to 0.25 wt% | 0.08 to 0.20 wt% | GTAW |
| Chromium content | 0.8 to 1.2 wt% | 19 to 27 wt% | Argon shielding |
| Molybdenum content | 0.4 to 0.6 wt% | 0 to 1 wt% | Wire feed rate: 3 to 6 g/min |
| Nickel content | 0 to 0.5 wt% | 9 to 14 wt% | Travel speed: 2 to 5 mm/min |
| Equivalent carbon | 0.20 to 0.35 | 0.10 to 0.20 | Current: 100 to 180 A |
The fusion zone in this material system is characterized by a gradient of composition from the ferritic base metal to the austenitic overlay, with a transition region that contains a mixture of ferrite and austenite phases. This microstructural gradient is the source of the mechanical property variations and the susceptibility to hydrogen-induced cracking.
Fusion Zone Microstructure and Toughness
The fusion zone microstructure was examined using optical and electron microscopy. The microstructure evolved from coarse ferrite and pearlite in the base metal, through a mixed ferrite-austenite region, to fine-grained austenite with some delta ferrite in the overlay. The transition region, which is most critical for toughness, contained a lamellar structure of ferrite and austenite with varying proportions.
The Charpy V-notch impact test results showed that the toughness of the fusion zone was significantly lower than that of both the base metal and the overlay material. The minimum impact energy occurred at a specific location within the fusion zone, typically at the position where the ferrite-austenite ratio was approximately 50:50. This is attributed to the brittle ferrite phase, which is susceptible to cleavage fracture, and the microsegregation of carbon and alloying elements at the ferrite-austenite boundaries.
The authors also examined the effect of welding parameters on the fusion zone toughness. Higher heat input resulted in coarser microstructures and lower toughness, while lower heat input produced finer structures but increased the risk of cracking. The optimal heat input was found to be in the range of 1.5 to 2.5 kJ/mm, which produced a fine-grained microstructure with acceptable toughness.
Hydrogen-Induced Delamination
The most significant finding of this paper is the observation of hydrogen-induced delamination in the fusion zone. The delamination occurred along the ferrite-austenite boundaries within the fusion zone, where hydrogen atoms accumulated at the phase boundaries due to the trapping effect of the microstructural features. The hydrogen was introduced during welding from the moisture in the welding environment and from the decomposition of hydrogen-containing compounds in the base metal and filler metal.
The mechanism of hydrogen-induced delamination can be described as follows:
- Hydrogen atoms are generated during welding from moisture in the shielding gas, from hydrogen in the base metal, and from the decomposition of hydrocarbons in the welding arc.
- The hydrogen atoms diffuse into the molten weld pool and solidify with the weld metal.
- During solidification, the hydrogen is trapped at the ferrite-austenite boundaries due to the difference in hydrogen solubility between the two phases.
- Under subsequent thermal or mechanical stress, the trapped hydrogen reduces the cohesive strength of the phase boundaries.
- Cracks initiate at the weakest phase boundaries and propagate along the ferrite-austenite interfaces, resulting in delamination.
The authors conducted hydrogen embrittlement tests by exposing the weld samples to hydrogen charging and then measuring the reduction in impact energy. The results confirmed that the fusion zone was highly susceptible to hydrogen embrittlement, with the impact energy dropping to near zero at relatively low hydrogen concentrations.
Countermeasures and Process Optimization
Based on the findings, the authors proposed several countermeasures to mitigate hydrogen-induced delamination:
- Preheating the base metal to 150°C to 250°C to reduce hydrogen diffusion into the fusion zone.
- Using low-hydrogen welding consumables and dry shielding gas.
- Applying post-weld heat treatment at 600°C to 650°C for 2 to 4 hours to allow hydrogen to diffuse out of the fusion zone.
- Using a multi-pass welding technique with interpass temperature control to minimize residual hydrogen.
- Selecting a filler metal with lower carbon content to reduce the formation of brittle carbides at the phase boundaries.
The effectiveness of these countermeasures was verified through additional welding and testing. The combination of preheating, low-hydrogen consumables, and post-weld heat treatment resulted in a significant improvement in the fusion zone toughness and a complete elimination of hydrogen-induced delamination.
Engineering Practice and Standards Considerations
In engineering practice, the fabrication of austenitic stainless steel overlays on 1Cr-1/2Mo steel is governed by standards such as ASME IX and NB/T 47014, which require weld procedure qualification and performance qualification. The findings of this paper highlight the importance of including hydrogen embrittlement testing in the qualification procedure, particularly for applications involving hydrogen service or high-temperature operation.
The paper also raises the question of inspection requirements for the fusion zone. Conventional radiographic testing may not detect hydrogen-induced delamination, as the cracks are often too fine to be visible on radiographs. Ultrasonic testing with high-frequency probes may be more effective, but the complex microstructure of the fusion zone can produce signal noise. The authors recommend complementary inspection methods, including magnetic particle testing and penetrant testing, to detect surface and near-surface delamination.
Summary
This paper provides a detailed investigation of the toughness and hydrogen-induced delamination behavior in the fusion zone of austenitic stainless steel overlays on 1Cr-1/2Mo steel. The key findings are that the fusion zone is susceptible to hydrogen embrittlement due to the trapping of hydrogen at ferrite-austenite phase boundaries, and that this susceptibility can be mitigated through careful process control including preheating, low-hydrogen consumables, and post-weld heat treatment. For engineers designing and fabricating clad pressure vessels and piping systems, this research underscores the importance of understanding the metallurgical behavior of the fusion zone and implementing appropriate countermeasures to ensure long-term reliability in hydrogen-containing service environments.