Research on Hydrogen-Induced Delamination Cracking at Austenitic Stainless Steel Overlay Weld Sites

Literature Overview

This paper by Zeng Weihua, Han Huaiyue, Yin Fengchun, and Liao Juzhi, published in the Journal of Iron and Steel Research (1992, Vol. 4, Issue 2, pp. 67–76), presents a comprehensive study on hydrogen-induced delamination cracking at austenitic stainless steel overlay weld sites. The research employed high-pressure hydrogen charging tests and investigated the effects of various hydrogen exposure conditions and cooling cycles on delamination susceptibility. The work was conducted on 2½Cr-1Mo thick forged plates with three different strip and welding process overlay configurations. This research is of critical importance for the safe operation of hydrogenation reactors in the petrochemical industry.

Background and Motivation

Hydrogenation reactors in the petrochemical industry operate under high temperatures and pressures in the presence of hydrogen gas. The internal surfaces of these reactors are typically protected by an overlay layer of austenitic stainless steel to resist hydrogen attack and sulfidation. However, the overlay weld site is a critical area of concern because:

1. The interface between the base metal and overlay layer is a potential site for hydrogen accumulation.
2. The heat-affected zone (HAZ) of the base metal may have reduced resistance to hydrogen-induced cracking.
3. The microstructural complexity of the overlay weld site creates multiple potential crack initiation sites.
4. The thermal stresses from welding and operation can interact with hydrogen embrittlement to cause delayed cracking.

Hydrogen-Induced Cracking Mechanisms

Experimental Methodology

1. Test Specimen Preparation

Three types of overlay weld specimens were prepared on 2½Cr-1Mo thick forged plates:

2. Hydrogen Charging Conditions

The high-pressure hydrogen charging test was conducted under the following conditions:

3. Examination Methods

After hydrogen charging and cooling, the specimens were examined using:

• Visual inspection: Surface examination for visible cracks
• Magnetic particle inspection (MT): Detection of surface and near-surface cracks
• Ultrasonic testing (UT): Detection of internal delamination
• Metallographic examination: Cross-sectional examination of crack morphology
• Scanning electron microscopy (SEM): Fractography analysis of crack surfaces
• X-ray diffraction (XRD): Phase identification in the crack zone

Key Findings

1. Nature of Delamination Cracking

The study established that delamination cracking is a delayed phenomenon that occurs during or after cooling from the hydrogen charging temperature, not during the charging itself. This is a critical finding because it means that the cracking may not be detectable during the high-temperature exposure but only after cooling to room temperature.

2. Crack Morphology

The cracks exhibited the following characteristics:

3. Effect of Hydrogen Pressure

Increasing the hydrogen partial pressure significantly increased the delamination susceptibility:

4. Effect of Cooling Rate

The cooling rate after hydrogen charging had a significant effect on delamination susceptibility:

The faster the cooling rate, the higher the delamination susceptibility. This is attributed to the trapping of hydrogen in the microstructure during rapid cooling, which increases the local hydrogen concentration at susceptible sites.

5. Effect of Overlay Material and Process

The different overlay configurations showed different susceptibility levels:

The double strip configuration showed lower susceptibility due to reduced dilution and a more favorable microstructure at the interface.

Engineering Implications

1. Material Selection

Based on the research findings, the following material selection guidelines are recommended:

2. Process Optimization

The following process optimization measures are recommended:

• Low dilution: Use multiple thin passes to minimize base metal dilution.
• Fine grain microstructure: Use interpass temperature control to promote fine grain growth.
• Avoid carbide networks: Use low-carbon overlay materials (309L, 347L) to minimize carbide precipitation.
• Controlled cooling: Implement slow cooling after welding to reduce residual stress.

3. Quality Control

A comprehensive quality control plan is essential for hydrogenation reactor overlay welding:

• 100% UT inspection of all overlay welds for internal defects.
• 100% MT inspection of all weld surfaces for surface cracks.
• Representative coupon testing for hydrogen embrittlement susceptibility.
• Post-weld stress relief at 600–650 °C for 2 hours per 25 mm of thickness.

4. Operational Safety

The research findings have important implications for operational safety:

• Pre-startup inspection: All overlay welds must be inspected before the reactor is brought into service.
• Periodic inspection: Regular UT inspection during operation to detect any developing delamination.
• Cooling rate control: During shutdown, the cooling rate must be controlled to minimize hydrogen-induced cracking.
• Emergency procedures: Emergency shutdown procedures must account for the potential for delayed cracking during rapid cooling.

Key Questions and Reflections

The research raises several important questions for further investigation:

1. Long-term behavior: The study was conducted under accelerated conditions. What is the long-term behavior of overlay welds under actual service conditions?
2. Effect of impurities: The study did not address the effect of impurities such as sulfur, phosphorus, and nitrogen on delamination susceptibility.
3. Mitigation strategies: Are there post-weld treatments or operational procedures that can mitigate the risk of delamination cracking?
4. Modern materials: Have advances in overlay material development since 1992 reduced the susceptibility to hydrogen-induced delamination?

Study Insights and Implications

This research is of critical importance for the safe operation of hydrogenation reactors in the petrochemical industry. The identification of hydrogen-induced delamination as a delayed cracking phenomenon that occurs during cooling has significant implications for inspection and maintenance practices. The findings on the effects of hydrogen pressure, cooling rate, and overlay material selection provide a scientific basis for material and process selection decisions. For contemporary engineers, this research serves as a reminder of the importance of understanding the fundamental mechanisms of hydrogen-induced cracking and the need for comprehensive quality control and operational safety measures. The principles established in this research remain relevant today and continue to guide the design and operation of hydrogenation reactors worldwide.