Microstructure and Properties of Explosive-Rolled TA1 Q345R Clad Plate

Background and Technical Significance

The combination of titanium (TA1) and low-alloy steel (Q345R) in a clad plate configuration addresses a fundamental engineering challenge: titanium offers exceptional corrosion resistance and specific strength but is prohibitively expensive for structural applications, while Q345R provides economical structural integrity but lacks corrosion resistance in aggressive environments. Explosive-rolled clad plates combine these advantages through a two-step process: explosive welding creates the initial metallurgical bond, followed by hot rolling to reduce thickness and improve properties.

Process Description and Parameters

The explosive-rolled TA1/Q345R clad plate manufacturing process involves two critical stages:

Explosive Welding Stage

Hot Rolling Stage

Microstructural Analysis

Titanium Layer Microstructure

The TA1 titanium layer undergoes significant microstructural evolution during explosive rolling:

• Alpha grain refinement: The original coarse alpha grains (50-200 μm) are refined to 5-20 μm through the combined effects of explosive jetting deformation and subsequent hot rolling.
• Grain elongation: Rolling creates elongated grain structures aligned with the rolling direction, improving transverse mechanical properties.
• Deformation bands: Alpha-beta transformation during rolling introduces deformation bands that enhance dislocation density and solid solution strengthening.
• Oxygen pickup: Surface oxidation during hot rolling introduces a thin oxide layer (1-5 μm) that may slightly affect surface properties.

Steel Layer Microstructure

The Q345R steel layer experiences the following changes:

• Pearlite-ferrite refinement: The original lamellar pearlite is refined through deformation, improving toughness.
• Grain boundary migration: Dynamic recovery during hot rolling promotes grain boundary migration and subgrain formation.
• Phase stability: The microstructure remains predominantly ferrite-pearlite without phase transformation in the rolling temperature range.

Interface Microstructure

The explosive welding interface in the rolled plate exhibits distinctive features:

• Wavy bond line: The characteristic wavy interface from explosive welding is preserved but attenuated by rolling deformation.
• Jetting products: Titanium-rich jetting products are entrained at the interface, providing mechanical interlocking.
• Diffusion zone: Limited interdiffusion occurs during hot rolling, creating a narrow transition zone (5-20 μm) with gradient composition.
• Bond quality: The interface maintains metallurgical bonding throughout the rolling process when parameters are properly controlled.

Mechanical Properties

Bond Strength Evaluation

Bond strength testing is critical for explosive-rolled clad plates. The following methods are employed:

Corrosion Performance

The primary motivation for TA1/Q345R clad plates is corrosion protection. Key findings from corrosion testing include:

• Potentiodynamic polarization: The TA1 overlay exhibits passive behavior in 3.5% NaCl solution with a corrosion current density of 10^-8 to 10^-9 A/cm².
• Intergranular corrosion: TA1 does not exhibit sensitization-related intergranular corrosion, unlike austenitic stainless steels.
• Galvanic coupling: The potential difference between TA1 and Q345R in seawater is approximately 0.3-0.5 V, with TA1 acting as the cathode. This provides cathodic protection to exposed Q345R areas at the interface.
• Edge corrosion: The primary corrosion concern is at the plate edge where the base metal is exposed; proper edge treatment (grinding flush or additional cladding) is required.

Engineering Applications and Considerations

Explosive-rolled TA1/Q345R clad plates find application in:

• Marine heat exchanger tubesheets and channels
• Chemical processing equipment handling hydrochloric acid and chlorinated solutions
• Desalination plant components
• Nuclear reactor coolant system components
• Mining equipment in aggressive leaching environments

Key engineering considerations include:

• Fabrication compatibility: TA1 is highly reactive above 400°C; welding procedures must use argon shielding and preheating restrictions.
• Forming limits: The TA1 layer has limited formability in the rolled condition; deep drawing requires solution treatment.
• Thermal expansion mismatch: The coefficient of thermal expansion difference between TA1 (8.6×10^-6/°C) and Q345R (12.0×10^-6/°C) must be accounted for in design.
• Hydrogen embrittlement: TA1 is susceptible to hydrogen embrittlement from acidic environments; design must avoid hydrogen generation conditions.

Study Insights and Technical Reflections

The explosive-rolled process offers distinct advantages over alternative cladding methods for TA1/Q345R combinations. Unlike explosion welding alone, the subsequent rolling step enables significant thickness reduction (from 30-50 mm to 3-10 mm total) while maintaining bond integrity, making the product economically viable for thin-wall applications. The microstructural refinement achieved through rolling improves the mechanical properties of both layers simultaneously, creating a synergistic effect that neither material achieves in its as-welded condition.

However, the process introduces unique quality challenges. The rolling temperature window is narrow: temperatures above 1050°C risk excessive grain growth in TA1, while temperatures below 900°C may not achieve sufficient deformation to improve properties. Additionally, the differential thermal contraction between titanium and steel during cooling from rolling temperatures creates residual stresses at the interface that must be relieved through stress-relief annealing.

The most significant finding from studying this technology is that explosive-rolled TA1/Q345R clad plate represents an optimal balance between manufacturing cost, material performance, and service capability for applications requiring titanium-level corrosion resistance with structural steel economics. The technology maturity has advanced to the point where it is now specified in international standards for nuclear and petrochemical applications.