Double-Sided Double-Layer Three-Wire Automatic Submerged Arc Welding of 09MnNiDR Low-Temperature Steel Pipe

Literature Overview

The paper by Shang Caizhong and colleagues from Hebei Haiqianwei Steel Pipe Co., Ltd., published in Steel Pipe in 2014 (Vol. 43, No. 4, pp. 23-29), presents a comprehensive welding process study for 09MnNiDR low-temperature steel pipe manufactured using a JCOE (J-C-O-E) production line. The study focuses on a double-sided double-layer three-wire automatic submerged arc welding (SAW) configuration, addressing the critical challenge of achieving reliable low-temperature impact toughness in the weld metal and heat-affected zone (HAZ).

Material and Welding Configuration

09MnNiDR Steel Characteristics

The 09MnNiDR grade is specifically designed for low-temperature applications, with the nickel addition providing microalloying benefits for toughness without significantly increasing carbon equivalent. The DR suffix indicates a requirement for low-temperature impact testing, typically at -46°C or lower depending on the specification.

Welding Configuration Details

The double-sided double-layer three-wire SAW process on the JCOE line involves:

• Front side: Three parallel wires operating simultaneously with flux coverage, producing the root pass and a first fill layer.
• Back side: Three parallel wires operating after pipe rotation, producing the back-side reinforcement and final cap layer.
• Flux: CHF105DR, a low-hydrogen flux specifically formulated for low-temperature applications with appropriate alloy content.
• Wire: CHW-S13, a low-carbon, low-sulfur, low-phosphorus wire with controlled alloy additions for toughness.

The three-wire configuration effectively creates a multi-pass effect within a single traverse, with each wire contributing a partial deposit. This results in a total deposited cross-section equivalent to multiple conventional passes but achieved in a single operation.

Welding Metallurgy and Microstructure Control

Heat Input Management

The paper emphasizes the principle of "heat input priority control" (热输入优先控制原则), which means that the welding heat input is the primary parameter governing weld metal microstructure and properties, taking precedence over other parameters such as travel speed or voltage.

The three-wire configuration inherently provides a moderate heat input distribution because each wire operates at reduced current density compared to a single-wire equivalent. The overlapping heat affected zones from adjacent wires create a natural inter-pass cooling effect, promoting fine grain formation.

Acicular Ferrite Formation Mechanism

The formation of acicular ferrite (AF) is the key metallurgical objective for achieving low-temperature toughness. The multi-wire SAW process promotes AF formation through several mechanisms:

1. Multi-source heating: The three simultaneous wires create a complex thermal field with multiple cooling paths, reducing the effective cooling rate below the critical threshold for AF nucleation (typically 5-10°C/s).
2. Refined grain structure: The overlapping weld passes create grain refinement at the boundaries between adjacent wire deposits.
3. Microalloying effects: The manganese and nickel in both the base metal and filler metal promote AF nucleation by providing favorable nucleation sites at precipitate particles.

Post-Weld Heat Treatment (PWHT)

The study recommends a PWHT cycle of:

• Soaking temperature: 590-620°C
• Soaking time: 60 minutes
• Cooling: Air cooling below 300°C

This PWHT serves multiple purposes:

• Stress relief of welding residual stresses, reducing the risk of delayed cracking.
• Temper embrittlement avoidance by rapid cooling through the 300-500°C range.
• Microstructure stabilization, converting any retained austenite to fine ferrite-pearlite mixtures.
• Improvement of HAZ toughness by allowing carbon diffusion from coarse grains to finer regions.

The temperature range of 590-620°C is carefully selected to be above the Ac1 temperature for the weld metal but below the temperature that would cause significant grain growth in the HAZ. The 60-minute holding time is sufficient for stress relief but short enough to minimize time-dependent embrittlement effects.

Quality Control and Inspection

Non-Destructive Testing Requirements

Common Defects and Countermeasures

• Porosity: Caused by inadequate flux coverage or moisture contamination. Countermeasure: maintain flux coverage ratio >3:1 and control flux moisture <0.1%.
• Undercut: Results from excessive current or travel speed. Countermeasure: optimize wire stick-out and contact tip alignment.
• Lack of fusion: Associated with insufficient heat input or poor joint fit-up. Countermeasure: verify JCOE forming tolerances and maintain minimum current per wire.
• Low-temperature brittle fracture: Indicates inadequate toughness. Countermeasure: reduce heat input and verify PWHT cycle compliance.

Study Insights and Reflections

This study demonstrates a practical approach to welding process development for a specific low-temperature steel grade on a specific production line configuration. The emphasis on heat input control as the primary parameter for toughness optimization is well-supported by welding metallurgy principles and represents sound engineering practice.

The three-wire SAW configuration offers several advantages over conventional multi-pass welding: reduced production time, consistent weld geometry, and inherent grain refinement through multi-source thermal effects. However, the complexity of the configuration also introduces challenges in process monitoring and quality assurance. Each wire must be individually monitored for current, voltage, and wire feed rate, and any imbalance between wires can lead to asymmetric weld profiles and inconsistent microstructure.

The DC reverse polarity (直流反接) configuration mentioned in the paper is selected to maximize arc stability and penetration characteristics for submerged arc welding. The reverse polarity provides deeper penetration with the wire acting as the cathode, which is beneficial for root pass formation. For subsequent passes, the balance between penetration and reinforcement height must be carefully managed.

From a production engineering perspective, the study provides a validated process specification that can be directly implemented on JCOE lines. The combination of appropriate consumables, controlled heat input, and optimized PWHT creates a robust process window that accommodates normal production variations while maintaining the critical low-temperature toughness requirement. This is particularly valuable for large-diameter low-temperature line pipe applications where production efficiency and quality consistency must be simultaneously achieved.