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

Literature Overview

This study addresses the critical issue of hydrogen-induced delamination (HID) in stainless steel cladding layers after post-weld heat treatment (PWHT). The research is particularly relevant to hydrogenation reactors, hydrogen storage vessels, and pressure vessels operating in hydrogen service, where PWHT is mandatory to relieve residual stresses but can inadvertently promote hydrogen embrittlement and delamination of the overlay layer. The work systematically investigates how different PWHT conditions—temperature, duration, and atmosphere—affect the susceptibility of stainless steel cladding layers to hydrogen-induced damage.

Core Technical Findings

The study used 304L and 316L stainless steel cladding layers applied to carbon steel substrates by submerged arc welding (SAW) and gas tungsten arc welding (GTAW). The cladding layers were subjected to various PWHT conditions, followed by hydrogen charging and examination for delamination. The hydrogen charging was performed using electrochemical charging in a 5% NaOH solution at room temperature, simulating the hydrogen exposure conditions in hydrogenation reactors.

PWHT Conditions Evaluated

Condition	Temperature (°C)	Duration (h)	Atmosphere	Purpose
A	550	2	Air	Standard stress relief
B	600	2	Air	Higher temperature stress relief
C	650	2	Air	Elevated stress relief
D	600	4	Air	Extended duration
E	600	2	Vacuum (10⁻³ Pa)	Vacuum stress relief
F	600	2	Nitrogen	Inert gas stress relief
G	600	2	Hydrogen (10% H₂)	Hydrogen atmosphere stress relief

The results showed that PWHT at 550 °C (Condition A) produced minimal hydrogen-induced delamination, while PWHT at 650 °C (Condition C) resulted in significant delamination along the cladding-substrate interface and within the overlay layer. The threshold temperature for significant HID susceptibility was identified as approximately 600 °C, above which the risk increases sharply.

Microstructural Changes During PWHT

The key finding is that PWHT above 600 °C promotes chromium carbide precipitation (Cr₂₃C₆) at grain boundaries in the stainless steel overlay, leading to chromium depletion in the adjacent regions. This chromium depletion reduces the local corrosion resistance and creates preferential pathways for hydrogen ingress. Additionally, the PWHT causes changes in the retained hydrogen content within the overlay, as hydrogen solubility in austenitic stainless steel increases with temperature and then decreases upon cooling, potentially trapping hydrogen at microstructural features.

The study also revealed that the welding process itself introduces residual hydrogen into the overlay. SAW produces higher hydrogen levels than GTAW due to the flux chemistry and the larger weld pool. The residual hydrogen from welding can be partially relieved during PWHT, but if the PWHT conditions promote carbide precipitation, the net effect may be detrimental.

Hydrogen-Induced Delamination Mechanism

The delamination mechanism was identified as a combination of hydrogen embrittlement and stress corrosion cracking. The sequence is as follows: hydrogen atoms diffuse into the overlay during PWHT and subsequent service; they accumulate at grain boundaries, carbide-matrix interfaces, and the cladding-substrate interface; the accumulated hydrogen reduces the cohesive strength of these interfaces; under the combined action of residual stresses and external loads, cracking initiates and propagates along these weakened interfaces.

Quantitative Results

PWHT Condition	Delamination Area (%)	Maximum Crack Depth (μm)	Interface Cracks (count)
No PWHT	2–5	50–100	0–2
A (550 °C/2h)	3–8	80–150	1–3
B (600 °C/2h)	8–15	150–300	3–8
C (650 °C/2h)	20–35	300–600	8–15
D (600 °C/4h)	12–22	200–400	5–12
E (600 °C/2h, vacuum)	5–10	100–200	2–5
F (600 °C/2h, N₂)	6–12	120–250	2–6
G (600 °C/2h, 10% H₂)	25–40	400–700	10–20

The vacuum atmosphere (Condition E) showed improved results compared to air atmosphere at the same temperature and duration, likely because vacuum removes oxygen and moisture that can promote surface oxidation and hydrogen recombination.

Engineering Practice Implications

For hydrogenation reactors and hydrogen service pressure vessels, the PWHT specification must be carefully controlled to avoid temperatures above 600 °C for stainless steel overlay layers. The ASME Section VIII Div. 1 code allows PWHT up to 850 °C for certain materials, but this is clearly inappropriate for stainless steel cladding layers in hydrogen service.

Recommended PWHT Protocols

Application	Recommended PWHT	Rationale
H₂ reactor with 304L overlay	520–550 °C, 2h, N₂ atmosphere	Below carbide precipitation threshold
H₂ reactor with 316L overlay	520–550 °C, 2h, vacuum	Mo improves resistance but still sensitive
H₂ storage vessel with Inconel 625 overlay	550–580 °C, 2h, N₂	Ni-based alloys more resistant to HID
Low-pressure H₂ service	500–530 °C, 1.5h, N₂	Lower stress, lower temperature acceptable

Defect Analysis and Countermeasures

Defect	Cause	Detection Method	Countermeasure
Interface delamination	High PWHT temp, H₂ exposure	UT (tapping method), MT	Limit PWHT temp, use low-H₂ welding
Intergranular cracking in overlay	Carbide precipitation, H₂ embrittlement	PT, MT	Avoid 600–850 °C PWHT, use L-type grades
Subsurface porosity	Trapped H₂ during cooling	UT, RT	Slow cool after PWHT, bake-out before welding
Cracking at weld toe	Stress concentration + H₂	MT, PT	Optimize weld geometry, reduce residual stress

Key Questions and Reflections

A critical question is how to balance the need for residual stress relief with the risk of hydrogen-induced damage. Residual stresses from welding can be as high as 300–400 MPa, which is significant for fatigue life and dimensional stability. However, aggressive PWHT to relieve these stresses can promote HID. The solution may lie in alternative stress relief methods such as vibration stress relief, thermal stress relief at lower temperatures, or design modifications to reduce inherent residual stresses.

The study also raises the question of whether the PWHT conditions should be different for the substrate and the overlay. In practice, the entire vessel is heat-treated as a unit, but the substrate and overlay have different sensitivities. This mismatch is a fundamental challenge in bimetallic vessel fabrication that requires careful process planning.

Study Insights and Reference Value

This literature provides essential guidance for engineers designing and fabricating hydrogen service pressure vessels with stainless steel cladding. The clear identification of 600 °C as the threshold temperature for increased HID susceptibility is a practical and actionable finding that can be directly incorporated into fabrication specifications and quality procedures. The demonstration that vacuum atmosphere PWHT reduces HID compared to air atmosphere offers a practical solution for situations where higher temperatures are unavoidable. The comprehensive defect analysis and countermeasure recommendations provide a complete toolkit for engineers to manage this critical failure mode in hydrogen service applications.