Single-Layer Submerged Arc Overlay Welding for Hydrogenation Equipment Shell Inner Walls

Literature Overview

This 2023 study by Li Ling, Guo Xiaoxiao, Xin Hongbo, and Yu Shijun from Zhenhai Petrochemical Construction Engineering Co., Ltd., published in Welding (Vol. 2023, No. 9, pp. 59-64), presents a significant advancement in overlay welding technology for hydrogenation equipment shells. The study investigates single-layer submerged arc overlay welding (SAW) on commonly used hydrogenation equipment shell materials — 14Cr1MoR(H) and 12Cr2Mo1R(H) — and demonstrates that the technique can achieve overlay thicknesses of 5-7 mm, exceeding the domestic standard requirement of 4.0-4.5 mm. This represents a meaningful improvement in the state of the art for hydrogenation equipment overlay welding.

Technical Background and Significance

Hydrogenation equipment shells in petrochemical refineries require internal overlay weld protection to resist high-temperature hydrogen attack (HTHA) and corrosion. The overlay layer must meet stringent requirements:

The traditional approach to achieving these requirements involves multi-layer multi-pass SAW welding, which is time-consuming and costly. The study investigates whether a single-layer SAW process can achieve the same results, which would offer significant economic and schedule benefits.

Experimental Approach and Results

The authors conducted overlay welding trials on both 14Cr1MoR(H) and 12Cr2Mo1R(H) base materials, which are the most commonly used materials for hydrogenation equipment shells in China. The following table summarizes the key results:

The achievement of 5-7 mm overlay thickness in a single layer is particularly noteworthy. This exceeds the conventional multi-layer approach and suggests that the single-layer SAW process can produce a thicker, more uniform overlay layer with equivalent or superior quality. The ferrite number of 5-10 FN indicates a balanced austenite-ferrite microstructure that provides good resistance to cracking and adequate ductility.

Metallurgical Analysis

The absence of untempered martensite in the overlay layer, HAZ, and base metal is a critical finding. Untempered martensite is a major concern in overlay welding of low-alloy steels because it is associated with:

• High hardness and brittleness
• Susceptibility to hydrogen-induced cracking
• Poor low-temperature toughness
• Potential for delayed cracking

The absence of untempered martensite indicates that the welding parameters — particularly the heat input and cooling rate — were well-controlled to ensure adequate tempering of the martensite that forms during the welding thermal cycle. This is consistent with the use of appropriate preheat temperatures and controlled interpass temperatures.

The hardness of ≤ 235 HV10 across all zones (overlay, HAZ, and base metal) is also significant. This indicates that the overlay layer has adequate toughness for the service conditions, and that the HAZ has not been excessively hardened by the welding thermal cycle.

Process Parameters and Optimization

The study's emphasis on the combination of experimental and theoretical approaches is methodologically sound. The optimization of welding parameters for single-layer SAW overlay welding involves balancing several competing requirements:

1. Heat input: Sufficient heat input is needed to achieve good fusion and adequate penetration, but excessive heat input can cause excessive grain growth and reduced hardness in the overlay layer.
2. Travel speed: Affects heat input and deposition rate; must be balanced against the need for uniform overlay thickness.
3. Wire feed speed: Directly affects deposition rate and heat input; must be coordinated with travel speed.
4. Shielding gas composition: Affects arc stability, penetration profile, and weld metal composition.
5. Preheat temperature: Controls cooling rate and prevents cracking; typically 150-250°C for low-alloy steel substrates.

The achievement of 5-7 mm overlay thickness in a single layer suggests that the process parameters were optimized to maximize deposition rate while maintaining weld quality. This is a significant technical achievement that has direct implications for the cost and schedule of hydrogenation equipment manufacturing and maintenance.

Engineering Practice Implications

The study's findings have several important implications for hydrogenation equipment manufacturing and maintenance:

1. Cost reduction: Single-layer SAW overlay welding can significantly reduce welding time and labor costs compared to multi-layer approaches, while achieving superior overlay thickness.
2. Schedule improvement: Reduced welding time translates to shorter manufacturing and maintenance schedules, which is critical for refinery turnaround planning.
3. Quality consistency: The single-layer approach may provide better uniformity of overlay thickness and composition compared to multi-layer approaches, which can introduce layer-to-layer variation.
4. Standard development: The demonstrated capability of 5-7 mm single-layer overlay exceeds current domestic standards, suggesting that standards should be updated to reflect the improved technology.

Key Insights and Reflections

The study represents a meaningful advancement in overlay welding technology for hydrogenation equipment. The achievement of 5-7 mm overlay thickness in a single layer, with all quality criteria met, demonstrates that the traditional multi-layer approach is not always necessary and that process optimization can yield significant improvements.

However, several considerations deserve attention:

• Scale-up challenges: The study was conducted on test specimens, and scaling up to full-size equipment shells may introduce additional challenges related to welding access, thermal distortion, and process consistency.
• Long-term performance: The study reports excellent immediate results, but long-term performance under actual service conditions (high temperature, high pressure, hydrogen environment) should be validated through field trials.
• Process robustness: The sensitivity of the single-layer process to parameter variation should be evaluated to ensure that the technique can be reliably reproduced in industrial settings.

The work also highlights the importance of combining experimental validation with theoretical analysis in process development. The use of both experimental trials and theoretical calculations to optimize process parameters is a best practice that should be adopted in similar process development efforts.

Summary

This study demonstrates that single-layer submerged arc overlay welding can achieve overlay thicknesses of 5-7 mm on hydrogenation equipment shell materials (14Cr1MoR(H) and 12Cr2Mo1R(H)), exceeding the domestic standard requirement of 4.0-4.5 mm. All quality criteria — ferrite number, chemical composition, absence of untempered martensite, hardness, and absence of linear defects — were met. The technique offers significant potential for cost reduction and schedule improvement in hydrogenation equipment manufacturing and maintenance. For engineers involved in hydrogenation equipment design and fabrication, the work provides a validated alternative to traditional multi-layer overlay welding that should be considered for future projects, subject to appropriate scale-up validation and field performance monitoring.