Microstructure and Properties of CMT-TIG Cladding on X80-2205 Bimetallic Composite Pipe Ends

Technical Background and Motivation

Bimetallic composite pipes combining an X80 carbon steel substrate with a 2205 duplex stainless steel overlay are increasingly used in sour gas and offshore oil and gas applications where both high mechanical strength and excellent chloride stress corrosion cracking resistance are required. The pipe ends of such composite pipes present a unique challenge: the butt weld joint must join two dissimilar materials while maintaining the corrosion protection of the 2205 overlay. Conventional welding of pipe ends often results in dilution of the overlay alloy by the carbon steel substrate, leading to a loss of duplex phase balance and reduced corrosion resistance in the heat-affected zone (HAZ). The CMT-TIG (Cold Metal Transfer combined with Gas Tungsten Arc) cladding technique offers a novel approach to restore and maintain the 2205 overlay integrity at the pipe end weld joints.

CMT-TIG Cladding Process Description

CMT-TIG is a hybrid welding process that combines the wire-feeding mechanism of Cold Metal Transfer (CMT) with the arc stability and deep penetration of Gas Tungsten Arc (TIG) welding. In CMT mode, the welding wire is fed and retracted cyclically, creating short-circuit arcs with very low heat input and minimal spatter. When combined with TIG, the process achieves a balance between low thermal input and sufficient dilution control. For the X80-2205 pipe end cladding application, the process parameters were carefully optimized to minimize dilution of the 2205 overlay while ensuring complete fusion with the underlying X80 substrate.

Typical Process Parameters

Parameter	Value	Purpose
TIG arc current	80–120 A	Provides arc stability and deep penetration
CMT wire feed rate	2.0–4.0 m/min	Controls deposition rate
CMT wire diameter	1.2 mm	Balances wire stiffness and feeding accuracy
Shielding gas	98% Ar + 2% O₂	Stabilizes arc, improves wetting
Travel speed	150–250 mm/min	Controls heat input and dilution
Wire composition	2205 equivalent duplex SS	Maintains overlay corrosion resistance
Heat input	0.5–1.2 kJ/mm	Minimizes thermal damage to substrate

Microstructural Analysis

Metallographic examination of the CMT-TIG cladding layers revealed a well-defined duplex microstructure consisting of austenite (γ) and ferrite (α′) phases. The phase fraction ratio was maintained within the optimal range of 40–60% ferrite, which is critical for achieving the desired balance of mechanical strength and chloride SCC resistance in 2205 duplex stainless steel. The dilution rate at the interface between the cladding layer and the X80 substrate was measured to be approximately 15–25%, which is significantly lower than the 40–60% dilution typically observed with conventional GTAW or GMAW cladding methods.

Phase Distribution and Microstructure

Zone	Ferrite (%)	Austenite (%)	Grain Size (μm)	Remarks
Cladding Layer 1 (first pass)	45–55	45–55	25–35	Near-optimal duplex ratio
Cladding Layer 2 (second pass)	40–50	50–60	30–40	Slight austenite enrichment
Cladding Layer 3 (final pass)	42–52	48–58	35–45	Well-balanced duplex
HAZ (substrate side)	0–5	95–100	50–80	Martensitic transformation in X80

The low dilution achieved by CMT-TIG is attributed to the unique short-circuit arc cycle, which limits the time each wire segment spends in the arc and reduces the amount of substrate metal melted and mixed into the weld pool. The multi-pass cladding strategy further dilutes the residual substrate content in subsequent passes, progressively restoring the duplex phase balance toward the nominal 2205 composition.

Mechanical and Corrosion Properties

Tensile testing of the cladded pipe end specimens demonstrated yield strengths of 520–580 MPa and ultimate tensile strengths of 620–680 MPa, which are comparable to or slightly exceed the base metal properties of 2205 duplex stainless steel. Hardness measurements across the cladding layers showed consistent values of 280–320 HV, with a gradual transition to the lower hardness of the X80 substrate (approximately 220–250 HV) at the interface.

Corrosion resistance was evaluated through potentiodynamic polarization testing in 3.5% NaCl solution and ferric chloride (FeCl₃) intergranular corrosion testing per ASTM A262 Practice E. The CMT-TIG cladded specimens exhibited pitting corrosion potential (E_pit) values of -100 to -50 mV vs. SCE, which is comparable to the wrought 2205 base material. Intergranular corrosion testing confirmed that the cladding layers did not exhibit sensitization-induced intergranular cracking, indicating that the low heat input of the CMT-TIG process effectively avoided chromium carbide precipitation at grain boundaries.

Engineering Implications and Reflections

The CMT-TIG cladding technique represents a significant advancement for bimetallic pipe end fabrication in sour service applications. The ability to maintain the duplex phase balance at dilution levels below 25% is a game-changing capability that was previously unattainable with conventional welding methods. From a standards perspective, this technology opens the door to qualifying welding procedures under ASME IX Section IX or NB/T 47014 for critical sour service piping systems governed by NACE MR0175/ISO 15156.

However, several challenges remain for widespread industrial adoption. The equipment complexity and cost of CMT-TIG systems are higher than conventional GMAW or GTAW machines, and the process requires careful parameter optimization for each specific pipe geometry and wall thickness combination. Additionally, the welding procedure qualification (WPQ) documentation must address the hybrid process nature of CMT-TIG, which may require additional technical justification during regulatory review. Engineers should also consider the long-term creep and fatigue behavior of the cladded joint under cyclic sour gas loading conditions, as the heterogeneous microstructure at the cladding-substrate interface may serve as a preferential crack initiation site under prolonged stress.