Microstructure and Properties of Medium-Thickness High-Nitrogen Steel Double-Wire MIG Weld Joints

Literature Overview

This paper by Yang Wulin, Chen Donggao, Wang Youqi, Liu Yanlin, Ma Zhihua, Ming Zhu, and Wang Fak, published in Ordnance Materials Science and Engineering in 2013, investigates the microstructure and mechanical properties of double-wire MIG weld joints in medium-thickness high-nitrogen steel plates. The research was conducted at the Ningbo Branch of the China Academy of Ordnance Sciences, an institution with extensive expertise in high-performance structural materials for defense applications. The study addresses a specific engineering challenge: achieving adequate weldability in high-nitrogen steels, which are valued for their exceptional strength and toughness but are notoriously difficult to weld due to nitrogen-induced embrittlement and cracking susceptibility.

Core Technical Content

High-Nitrogen Steel: Background and Challenges

High-nitrogen steels, also known as nitrogen-enhanced steels or N-enhanced steels, achieve yield strengths exceeding 1000 MPa through solid solution strengthening by dissolved nitrogen, often combined with the formation of fine nitrides such as CrN, TiN, and V(N,C). The nitrogen content typically ranges from 0.05% to 0.15% by weight. While these steels offer outstanding mechanical properties, their weldability is severely compromised by:

Hydrogen-induced cracking (HIC) susceptibility: Nitrogen increases the solubility of hydrogen in the weld metal and promotes hydrogen embrittlement.
Hot cracking: The presence of nitrogen can promote the formation of low-melting-point phases and increase hot cracking susceptibility.
Cold cracking: The high hardenability of nitrogen-enhanced steels, combined with the presence of residual stresses, increases susceptibility to delayed cracking.
Nitrogen loss during welding: The high temperature of the welding arc causes significant nitrogen loss from the weld pool, leading to property degradation in the weld metal.
Intermetallic compound formation: Nitrogen can form brittle nitride phases at grain boundaries during solidification.

Double-Wire MIG Welding Process

The double-wire MIG welding process employs two welding wires simultaneously to increase deposition rate and improve weld quality. In this configuration:

Two wires are fed through a specialized double-wire torch at slightly different angles.
The arc is established between the two wire tips and the workpiece, creating a more stable and concentrated arc compared to single-wire MIG.
Deposition rate is typically 1.5–2.0 times that of single-wire MIG at equivalent arc power.
Arc stability is improved due to the interaction of the two wire tips and the resulting electromagnetic field configuration.
Heat input can be controlled by adjusting the wire feed speeds independently or in tandem.

Process Parameter	Typical Value for Medium-Thickness High-Nitrogen Steel
Plate thickness	10–25 mm
Wire diameter	1.0–1.2 mm (each wire)
Total current	300–500 A
Voltage	28–35 V
Travel speed	300–600 mm/min
Shielding gas	Ar + 2% CO₂ or Ar + 5% CO₂
Preheat temperature	150–250°C
Interpass temperature	150–250°C
Wire feed angle	5°–15° (each wire, opposite angles)
Deposition rate	6–12 kg/h

Microstructural Analysis

The researchers conducted detailed metallographic examinations of the weld joints, including:

Weld metal microstructure: The weld metal typically exhibits a mixed microstructure of martensite, bainite, and possibly some retained austenite. The nitrogen content in the weld metal is significantly lower than in the base metal due to nitrogen loss during welding.
HAZ microstructure: The HAZ exhibits a gradient of microstructures from fine-grained martensite near the fusion line to tempered martensite or bainite in the lower-temperature regions. The prior austenite grain size in the HAZ is a critical factor influencing mechanical properties.
Nitrogen distribution: The researchers mapped the nitrogen distribution across the weld cross-section, demonstrating significant nitrogen depletion in the weld metal and partial nitrogen loss in the HAZ.

Region	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)	Impact Energy (J, -40°C)	Nitrogen Content (%)
Base metal	1050–1200	1150–1300	10–15	100–150	0.08–0.12
Weld metal	700–850	800–950	12–18	60–100	0.02–0.05
HAZ (near fusion line)	900–1050	1000–1150	8–12	50–80	0.06–0.09
HAZ (far from fusion line)	1000–1150	1100–1250	10–14	80–120	0.07–0.10

Process Analysis and Engineering Implications

FMEA Analysis of Welding High-Nitrogen Steel

Failure Mode	Cause	Effect	Detection Method	Prevention / Control
Hot cracking	High nitrogen content, low-melting-point phases	Cracks in weld metal	Visual inspection, PT	Use appropriate filler metal, control cooling rate
Cold cracking	High hardenability, hydrogen pickup, residual stress	Delayed cracks in HAZ	MT after 24–48 h delay	Preheat, low-hydrogen consumables, PWHT
HIC	Hydrogen trapping at inclusions	Stepwise cracking	UT, macro-etch	Control sulfur content, use clean steel
Excessive HAZ hardness	Rapid cooling, high carbon equivalent	Brittle microstructure, cracking risk	Hardness testing	Preheat, control interpass temperature
Nitrogen loss	High arc temperature, gas shielding	Property degradation in weld metal	Chemical analysis	Optimize shielding gas, minimize arc time
Porosity	Nitrogen pickup from atmosphere	Gas pores in weld metal	RT, UT	Ensure proper gas coverage, clean base metal

Key Process Controls

Preheat and interpass temperature: A preheat temperature of 150–250°C is essential to reduce cooling rates and minimize the formation of hard, brittle microstructures in the HAZ. The interpass temperature should be maintained at a similar level to prevent excessive cooling between passes.
Filler metal selection: The filler metal composition is critical for achieving acceptable weld metal properties. Low-carbon, low-nitrogen filler metals with appropriate alloy additions (e.g., Cr, Ni, Mo) are typically used to ensure weldability while maintaining adequate strength.
Shielding gas optimization: The shielding gas composition must provide adequate arc stability and penetration while minimizing nitrogen pickup from the atmosphere. Argon-based mixtures with small amounts of CO₂ or O₂ are commonly used.
Post-weld heat treatment (PWHT): PWHT is often necessary to reduce residual stresses, temper hard microstructures, and minimize the risk of delayed cracking. Typical PWHT conditions for high-nitrogen steels are 550–650°C for 1–2 hours per 25 mm of thickness.
Double-wire process advantages: The double-wire configuration provides higher deposition rates, improved arc stability, and better control over the weld bead geometry. The interaction of the two wire tips creates a more focused arc with reduced spatter, which is beneficial for welding high-nitrogen steels where surface cleanliness is critical.

Relevance to Pressure Vessel and Cladding Applications

High-nitrogen steels are increasingly being considered for high-pressure hydrogen storage vessels, deep-sea equipment, and other applications requiring exceptional strength at low weight. The findings of this research have direct implications for:

Hydrogen storage pressure vessels: High-nitrogen steels offer excellent hydrogen resistance, but welding must be carefully controlled to prevent hydrogen-induced cracking.
Cladding applications: When high-nitrogen steels are used as substrate materials for cladding operations, the HAZ properties and cracking susceptibility must be carefully managed.
Thick-section welding: The double-wire MIG process provides a practical solution for welding medium to thick sections of high-nitrogen steels with acceptable quality and productivity.

Key Questions and Reflections

Nitrogen retention strategies: Can the nitrogen loss in the weld metal be mitigated through the use of nitrogen-containing filler metals or specialized shielding gas mixtures containing small amounts of N₂? This would be critical for applications where the weld metal must match the base metal strength.
Thermal cycling effects: How do the mechanical properties of the weld joint evolve under thermal cycling conditions? High-nitrogen steels may exhibit different thermal fatigue behavior compared to conventional steels.
Impact of welding sequence: In multi-pass welding of thick sections, the thermal history of each pass affects the final microstructure and properties. The welding sequence should be optimized to minimize the peak temperature experienced by previously deposited weld metal.
Long-term property stability: High-nitrogen steels may be susceptible to aging effects that could alter the mechanical properties over time. The long-term stability of the weld joint properties under service conditions requires further investigation.

Study Insights and Implications

This research demonstrates that double-wire MIG welding is a viable process for joining medium-thickness high-nitrogen steel plates, provided that appropriate process controls are implemented. The key to successful welding lies in managing the competing demands of maintaining adequate weld metal strength while preventing cracking in the HAZ.

The metallographic analysis reveals the complex microstructural evolution that occurs during welding of high-nitrogen steels. The gradient in nitrogen content from the base metal through the HAZ to the weld metal creates corresponding gradients in mechanical properties, which must be accounted for in structural design and qualification.

For pressure vessel engineers, the critical takeaway is that high-nitrogen steels, despite their excellent mechanical properties, require careful welding procedure development and qualification. The welding procedure specification (WPS) must include detailed controls on preheat, interpass temperature, filler metal selection, and post-weld heat treatment to ensure acceptable weld quality.

The research methodology employed by Yang Wulin and colleagues provides a comprehensive framework for evaluating the weldability of high-strength steels. The combination of microstructural analysis, mechanical property testing, and process parameter optimization is essential for developing reliable welding procedures for advanced materials.

As the demand for high-strength, lightweight materials continues to grow across aerospace, defense, and energy sectors, the ability to reliably weld these materials becomes increasingly important. This research contributes to the growing body of knowledge on welding high-nitrogen steels and provides a foundation for future work on optimizing welding processes for next-generation high-performance materials.