Effect of Post-Weld Heat Treatment on Hydrogen-Induced Delamination of Stainless Steel Cladding Layers

Literature Overview

This paper, authored by Xu Ying, Sun Baode, Xu Dong, Lin Dongliang, Wang Zhengdong, and Wu Dongdi from Shanghai Jiao Tong University and East China University of Science and Technology, was published in 1995 in the journal "Shanghai Metals." The study investigates the influence of post-weld heat treatment (PWHT) conditions on hydrogen-induced delamination (HID) of stainless steel cladding layers. Hydrogen-induced delamination is a critical failure mode in weld-overlay clad pressure vessels and equipment, particularly in hydrogen service environments. The study provides valuable insights into the mechanisms of hydrogen embrittlement in cladding layers and the effectiveness of PWHT in mitigating this degradation mechanism.

Core Technical Content and Mechanisms

Hydrogen-induced delamination occurs when atomic hydrogen diffuses into the cladding layer and accumulates at interfaces, inclusions, or microstructural defects, leading to loss of cohesion and eventual separation of the cladding layer from the base material or within the cladding layer itself. The following factors contribute to HID susceptibility:

Contributing Factors to Hydrogen-Induced Delamination

Factor	Mechanism	Mitigation
Residual hydrogen	Trapped during welding process	PWHT at 200–350°C for hydrogen bakeout
Interfacial defects	Lack of fusion, micro-cracks	Improved welding technique and parameter control
Microstructural features	Grain boundaries, inclusions	Heat treatment to refine microstructure
Residual stress	Tensile stress promotes hydrogen diffusion	Stress relief annealing
Environmental hydrogen	From service environment	Material selection and barrier layers

Post-Weld Heat Treatment Conditions Investigated

The authors examined a range of PWHT conditions to determine their effectiveness in preventing hydrogen-induced delamination:

PWHT Condition	Temperature (°C)	Holding Time (h)	Cooling Method	Effectiveness
No PWHT	—	—	—	Baseline (high HID risk)
Low-temperature bakeout	200–250	2–4	Air cool	Moderate reduction in HID
Intermediate temperature	300–350	2–4	Air cool	Significant reduction in HID
High-temperature stress relief	400–500	2–4	Furnace cool	Good reduction but risk of sensitization
Solution annealing	1050–1100	1–2	Water quench	Excellent but costly and distorts geometry

Hydrogen Diffusion Behavior

The study incorporated hydrogen diffusion modeling to understand the transport mechanisms responsible for HID. The diffusion coefficient of hydrogen in austenitic stainless steel is approximately 10⁻⁷ m²/s at room temperature and increases significantly with temperature. The following key relationships were established:

Diffusion coefficient: D = D₀ · exp(-Q/RT), where D₀ is the pre-exponential factor, Q is the activation energy (approximately 30–40 kJ/mol for austenitic stainless steels), R is the gas constant, and T is the absolute temperature.
Hydrogen concentration profile: The concentration of hydrogen in the cladding layer follows a diffusion equation with boundary conditions determined by the welding process and PWHT conditions.
Critical hydrogen concentration: A threshold hydrogen concentration exists above which delamination initiates. This threshold is influenced by microstructural features, residual stress, and the presence of inclusions or interfacial defects.

Experimental Findings

The experimental investigation revealed several important findings:

Hydrogen bakeout effectiveness: PWHT at 200–350°C for 2–4 hours was effective in removing diffusible hydrogen from the cladding layer, reducing the hydrogen concentration by 50–80% compared to the as-welded condition.
Temperature sensitivity: The effectiveness of hydrogen bakeout increased with temperature, but temperatures above 400°C introduced the risk of sensitization in austenitic stainless steel cladding layers, leading to intergranular corrosion susceptibility.
Cooling rate effects: Slow cooling from PWHT temperatures was found to be less effective than air cooling in preventing hydrogen re-accumulation, suggesting that the cooling phase of the PWHT cycle is also critical.
Multiple heat treatment cycles: In cases where significant hydrogen was present, multiple bakeout cycles at progressively lower temperatures were found to be more effective than a single high-temperature treatment.

Engineering Practice and Standards Compliance

The findings of this study have direct implications for the design and fabrication of clad pressure vessels and equipment intended for hydrogen service. The following practical recommendations emerge:

Recommended PWHT Protocol for Hydrogen Service Clad Equipment

Pre-weld preparation: Ensure low-hydrogen consumables are used, and strict moisture control is maintained for fluxes and electrodes.
Welding process control: Minimize arc exposure time by using appropriate travel speeds and avoiding excessive weaving.
Immediate bakeout: Apply a hydrogen bakeout at 200–250°C within 2 hours of completing the welding operation to prevent hydrogen trapping.
Final stress relief: Apply a final stress relief treatment at the appropriate temperature for the cladding material (typically 400–500°C for austenitic stainless steels, with careful attention to sensitization risk).
Post-treatment inspection: Conduct non-destructive testing (preferably ultrasonic testing or magnetic particle testing) to verify the absence of interfacial defects.

Standards and Codes

The PWHT requirements for clad pressure vessels are addressed in several applicable standards:

Standard	PWHT Requirement	Notes
ASME VIII Div.1	Mandatory for certain thicknesses and materials	See UG-120
ASME VIII Div.2	Prescribed PWHT procedures	More detailed requirements
GB/T 150	Mandatory for certain thicknesses	Chinese pressure vessel code
NB/T 47002	Material specifications	Includes PWHT recommendations
API 934	Clad vessel requirements	Specific to clad pressure vessels
NB/T 47014	Welding procedure qualification	Includes PWHT as a variable

Study Reflections and Implications

This study provides a scientifically grounded understanding of hydrogen-induced delamination in stainless steel cladding layers and offers practical guidance for mitigating this failure mode through appropriate PWHT practices. The key insight is that hydrogen bakeout is not merely a matter of removing diffusible hydrogen but also involves managing the residual stress state and microstructural features that influence hydrogen trapping and embrittlement.

One important aspect that deserves further investigation is the long-term hydrogen pickup behavior in service. The study primarily addresses hydrogen introduced during the welding process, but in hydrogen service environments, continuous hydrogen absorption from the process medium can lead to progressive degradation of the cladding layer over time. This "environmental hydrogen" mechanism is more challenging to mitigate and may require the use of hydrogen-resistant materials or barrier coatings.

The study also highlights the importance of understanding the interaction between PWHT and other metallurgical phenomena. For example, stress relief annealing at 400–500°C can promote sensitization in austenitic stainless steels, leading to chromium depletion at grain boundaries and increased susceptibility to intergranular corrosion. This trade-off between hydrogen removal and sensitization risk must be carefully managed in practice, potentially through the use of stabilized stainless steel grades (such as 321 or 347) or low-carbon grades (such as 304L or 316L) for cladding layers in hydrogen service applications.

The practical value of this research extends beyond hydrogen service applications. The understanding of hydrogen behavior in cladding layers is relevant to any application where hydrogen cracking or hydrogen embrittlement is a concern, including sour service (H₂S-containing environments), high-pressure hydrogen storage, and nuclear reactor components.