An Effective Method for Investigating Hydrogen-Induced Delamination in Weld Overlay Cladding Layers

Literature Overview

This paper, published in the Journal of Shanghai Jiaotong University in 1994 by Xu Ying, Yao Shoushan, Lin Dongliang, Wang Lan, Sun Yongjian, Yu Bairong, Wang Zhengdong, and Wu Dongdi from East China University of Science and Technology, addresses one of the most persistent and insidious problems in cladding technology: hydrogen-induced delamination (also referred to as hydrogen blistering or hydrogen-induced cracking) at the interface and within the weld overlay layer. The study proposes a systematic experimental methodology to evaluate and characterize this failure mode, which has been a critical concern in the fabrication of stainless steel and nickel-based alloy overlay cladding on carbon steel substrates for pressure vessels and heat exchangers.

The research team recognized that hydrogen-induced delamination is not merely a surface defect but a structural integrity issue that can compromise the entire overlay system under service conditions, particularly in environments containing hydrogen sulfide or under cyclic loading. The paper presents a methodological framework that combines controlled hydrogen charging, mechanical loading, and microstructural analysis to reproduce and evaluate delamination behavior in a laboratory setting.

Core Technical Content and Methodology

Experimental Methodology

The authors developed a multi-step experimental protocol to study hydrogen-induced delamination. The core methodology involves:

Sample preparation - Fabricating representative weld overlay specimens using standard cladding processes (typically SAW or ESW overlay of stainless steel such as 304 or 316 on carbon steel substrates), ensuring the specimens are representative of actual production conditions.
Hydrogen charging - Subjecting the specimens to controlled electrochemical hydrogen charging or cathodic hydrogen charging to simulate hydrogen absorption that occurs during welding and in service environments. The hydrogen charging parameters (voltage, duration, electrolyte composition, temperature) are systematically varied.
Mechanical evaluation - Applying tensile, peel, or shear loading to the hydrogen-charged specimens to determine the critical stress or strain at which delamination initiates and propagates.
Microstructural characterization - Conducting metallographic examination, fractographic analysis (SEM), and hydrogen concentration profiling to identify the failure mechanisms and damage initiation sites.

Key Findings

The study identified several critical observations regarding hydrogen-induced delamination:

Hydrogen preferentially accumulates at the weld overlay interface and at grain boundaries within the dilution zone, creating localized embrittlement zones.
The severity of delamination is strongly influenced by the base metal hydrogen diffusivity, the residual hydrogen content from the welding process, and the microstructural characteristics of the overlay layer.
A quantitative relationship was established between hydrogen concentration, applied stress, and the probability of delamination occurrence.
The method demonstrated that even low levels of hydrogen (in the range of 1-5 ppm) can significantly reduce the fracture toughness of the overlay layer interface.

Process Parameters and Their Influence

Parameter	Typical Range	Effect on Delamination Susceptibility
Hydrogen charging voltage	1-3 V	Higher voltage increases hydrogen absorption rate
Charging duration	2-24 hours	Longer duration leads to deeper hydrogen penetration
Electrolyte pH	1-14	Acidic conditions favor hydrogen absorption
Temperature	20-80°C	Higher temperature increases hydrogen diffusivity
Overlay layer thickness	3-15 mm	Thicker layers may trap more hydrogen internally
Welding process	SAW, ESW, GTAW	Different processes produce different hydrogen levels

Engineering Practice Implications

Relevance to Pressure Vessel Fabrication

The findings of this research are directly applicable to the fabrication of bimetal pressure vessels, particularly hydrogenation reactors and heat exchangers that operate in hydrogen-containing environments. In accordance with standards such as GB/T 150, NB/T 47002, and ASME VIII Div. 1, the integrity of the weld overlay layer is a critical design and fabrication requirement. Hydrogen-induced delamination represents a failure mode that is not adequately addressed by conventional NDE methods such as MT or PT alone, as the damage may exist beneath the surface and is difficult to detect without specialized techniques.

Practical Countermeasures

Based on the methodology and findings presented, the following engineering countermeasures can be implemented:

Pre-weld hydrogen control - Using low-hydrogen fluxes and consumables, maintaining strict control over consumable storage and handling to minimize moisture content.
Post-weld heat treatment (PWHT) - Applying appropriate PWHT cycles to allow hydrogen diffusion and recombination, thereby reducing residual hydrogen content below critical levels.
Overlay layer design optimization - Selecting overlay layer compositions and thicknesses that minimize hydrogen trapping at the interface.
NDE enhancement - Incorporating UT or TOFD examinations specifically targeted at the overlay layer interface to detect subsurface delamination.
Material selection - Choosing base metals with lower hydrogen diffusivity and overlay alloys with microstructures that resist hydrogen embrittlement.

Integration with Quality Control Systems

The methodology proposed in this paper can be integrated into a comprehensive quality control system for cladding fabrication. Using an FMEA (Failure Mode and Effects Analysis) approach, hydrogen-induced delamination can be identified as a critical failure mode with the following characteristics:

FMEA Element	Description
Failure Mode	Hydrogen-induced delamination at overlay interface
Severity	High (structural integrity compromise)
Occurrence	Medium (depends on welding and PWHT conditions)
Detection	Low (difficult to detect without specialized methods)
Risk Priority Number	High
Recommended Actions	Control hydrogen sources, optimize PWHT, enhance NDE

Study Insights and Reflections

This 1994 paper remains highly relevant to contemporary cladding technology practice. The systematic approach to studying hydrogen-induced delamination laid important groundwork for later research, including the companion study on PWHT effects (Topic 908, 1995) by the same research group. The methodology presented here emphasizes the importance of understanding the fundamental mechanisms of hydrogen embrittlement in weld overlay systems, rather than relying solely on empirical rules or trial-and-error approaches.

One particularly valuable aspect of this work is the emphasis on reproducibility and standardization of the hydrogen charging procedure. In engineering practice, the lack of standardized test methods for evaluating hydrogen-induced delamination susceptibility has been a persistent challenge. The method proposed here provides a practical framework that can be adapted for qualification testing of cladding processes and materials.

The research also highlights the importance of considering the entire hydrogen lifecycle in cladding systems - from hydrogen generation during welding, through hydrogen trapping during solidification, to hydrogen reactivation under service conditions. This holistic perspective is essential for engineers responsible for the design and fabrication of bimetal pressure vessels intended for hydrogen service.

In conclusion, this paper provides a valuable methodological foundation for investigating hydrogen-induced delamination in weld overlay cladding, with direct practical implications for ensuring the long-term integrity of bimetal pressure vessels in demanding service environments. The systematic approach to hydrogen charging, mechanical testing, and microstructural analysis offers engineers a reliable tool for evaluating and mitigating this critical failure mode in cladding applications.