Performance Analysis of Overlay Layers Using Ultra-Low Carbon Nitrogen-Strengthened Submerged Arc and Self-Shielded Flux-Cored Wires

Literature Overview

This 2006 study published in the "Transactions of the China Welding Institute" by researchers from the Welding Institute of CSMC Institute of Building Research investigates the mechanical and metallurgical properties of overlay weld layers produced using ultra-low carbon, nitrogen-strengthened consumables in both submerged arc welding (SAW) and self-shielded flux-cored arc welding (FCAW-S) processes. The research addresses the ongoing challenge of developing high-performance overlay consumables that combine excellent wear resistance with adequate toughness.

Technical Background

Nitrogen strengthening represents a significant advancement in overlay welding consumable design. Nitrogen dissolves interstitially in austenitic and austenitic-ferritic matrices, providing solid solution strengthening without the embrittlement associated with carbon. However, the challenge lies in controlling nitrogen content to achieve the desired strength-hardness combination while maintaining adequate ductility and weldability.

Consumable Design Philosophy

Property	Ultra-Low Carbon Wire	Conventional High-Carbon Wire
Carbon content	≤ 0.03%	0.5–1.5%
Nitrogen content	0.3–0.6%	0.02–0.05%
Hardness (as-welded)	250–350 HV	400–600 HV
Toughness (CVN)	50–80 J @ -40°C	5–15 J @ -40°C
Cracking susceptibility	Low	Moderate to High
Dilution sensitivity	Moderate	High

Process-Specific Analysis

Submerged Arc Welding (SAW) Overlay

SAW provides the highest deposition rate and most consistent quality for thick overlay layers:

Current: 500–700 A
Voltage: 30–38 V
Travel speed: 180–300 mm/min
Electrode diameter: 1.6–2.4 mm
Flux: Low-alloy type matched to wire composition
Typical dilution: 35–50% (single layer on carbon steel)
Layer thickness per pass: 4–8 mm

Self-Shielded Flux-Cored Welding (FCAW-S) Overlay

FCAW-S offers portability and atmospheric flexibility for field applications:

Current: 200–350 A
Voltage: 22–30 V
Travel speed: 150–250 mm/min
Wire diameter: 1.2–1.6 mm
Shielding: Self-shielded flux (no external gas)
Typical dilution: 45–60% (single layer on carbon steel)
Layer thickness per pass: 2–4 mm

Microstructural Analysis

Metallographic Observations

The overlay microstructure is characterized by:

Matrix: Predominantly austenitic (γ) with 5–15% retained ferrite (δ), stabilized by nitrogen
Carbides: M(C,N) type carbides (Cr, Mo, W, V) dispersed in the matrix
Carbide morphology: Fine, spherical to semi-spherical distribution (0.5–3 μm)
Grain size: Refinement due to nitrogen partitioning, typically 5–15 μm
Bond interface: Diffusion bonding with minimal intermetallic formation when proper transition layers are used

Hardness Distribution

Position	SAW Overlay Hardness	FCAW-S Overlay Hardness
Surface of overlay	320–380 HV	280–350 HV
Mid-overlay	280–340 HV	250–310 HV
Near bond interface	220–280 HV	200–260 HV
HAZ (base metal)	200–250 HV	220–280 HV
Base metal (far)	180–220 HV	180–220 HV

Mechanical Properties

Tensile and Bending Properties

Tensile strength: 550–650 MPa (overlay), exceeding base metal
Yield strength: 400–500 MPa (overlay)
Elongation: 20–35% (overlay), demonstrating adequate ductility
Bond strength (shear): 180–250 MPa
Bond strength (tensile): Fracture typically occurs in base metal, indicating bond strength exceeds base metal strength

Impact Toughness

The nitrogen-strengthened overlay exhibits superior toughness compared to conventional high-carbon hardfacing:

CVN at 20°C: 60–90 J (2 mm × 10 mm × 55 mm)
CVN at -40°C: 30–60 J
CVN at -60°C: 15–40 J

This represents a 3–5× improvement over conventional high-carbon overlay consumables at sub-zero temperatures, making the material suitable for cryogenic and low-temperature service.

Performance Comparison and Application Selection

Application	Recommended Process	Wire Type	Key Requirement
Thick overlay (≥10 mm)	SAW	Ultra-low C, N-strengthened	High productivity, low dilution
Field repair	FCAW-S	Ultra-low C, N-strengthened	Portability, no external gas
Cryogenic service	SAW	Ultra-low C, N-strengthened	Low-temperature toughness
Abrasive wear	SAW + tempering	Ultra-low C, N-strengthened	Hardness > 350 HV
Combined wear + corrosion	SAW	Ultra-low C, N-strengthened	Balanced properties

Engineering Insights

The research demonstrates that nitrogen strengthening enables a fundamentally different approach to overlay consumable design. By shifting from carbon-based to nitrogen-based strengthening, the consumable achieves:

Reduced cracking susceptibility: Ultra-low carbon eliminates carbide precipitation at grain boundaries during solidification, reducing hot cracking risk
Improved weldability: Lower carbon equivalent (CE) values improve hot cracking resistance in the HAZ
Maintained hardness after dilution: Nitrogen's lower atomic weight means less mass transfer to base metal during dilution, preserving overlay properties even at higher dilution rates
Superior toughness: The absence of coarse carbide networks preserves ductility at low temperatures

The practical implication is that nitrogen-strengthened overlay consumables can replace conventional high-carbon hardfacing in many applications where toughness and weldability are critical, without sacrificing wear resistance. This opens up new application possibilities in the fabrication of wear parts for mining equipment, cement mill liners, and marine propeller shafts where sub-zero temperatures are encountered.