Out-of-Plane Deformation Mechanism in Single-Pass Weld Overlay Joints of Q345 Steel under External Constraint

1. Definition and Technical Context

Out-of-plane welding deformation refers to the three-dimensional distortion that occurs in the direction perpendicular to the base plate surface during the welding process. Unlike in-plane deformation (longitudinal shrinkage and transverse shrinkage), out-of-plane distortion manifests as angular distortion, buckling, and warping that deviate the weld joint geometry from its intended planar configuration. This phenomenon is particularly pronounced in thin-to-medium thickness plates and becomes increasingly significant when external mechanical constraints—such as fixture clamping, backing plates, or structural boundaries—are imposed on the workpiece during welding.

Q345 steel (equivalent to ASTM A572 Gr. 50 or EN 10025 S355) is a low-alloy high-strength structural steel widely used in pressure vessels, pipelines, heavy machinery, and structural fabrication. Its yield strength of 345 MPa and ultimate tensile strength of 470–630 MPa make it a preferred base material for overlay welding applications requiring enhanced corrosion resistance, wear resistance, or high-temperature performance. When single-pass weld overlay is applied to Q345 steel under external constraint conditions, the interplay between thermal residual stresses, mechanical restraint forces, and the metallurgical transformation behavior creates a complex deformation field that directly impacts dimensional accuracy, joint integrity, and downstream qualification.

This technical study represents a systematic investigation into the out-of-plane deformation mechanism, establishing a theoretical and empirical foundation for process optimization in weld overlay manufacturing. The findings are directly applicable to the company's qualification of welding procedures (WPS) for clad plate and clad pipe fabrication across all three technology routes.

2. Fundamental Principles of Out-of-Plane Deformation

2.1 Thermal Stress Development

During single-pass weld overlay, the localized heat input creates a steep thermal gradient between the weld pool and the surrounding base metal. The weld zone experiences rapid heating to melting temperatures (approximately 1400–1600°C for typical overlay alloys) followed by rapid cooling. This thermal cycle generates the following stress states:

• Transient thermal stresses: Compressive stresses develop in the hot zone (weld pool and immediate HAZ) while tensile stresses develop in the cooler surrounding regions. The magnitude of these stresses is governed by the coefficient of thermal expansion (α ≈ 12×10⁻⁶ /°C for Q345), the elastic modulus (E ≈ 206 GPa), and the temperature differential (ΔT).
• Residual stresses: Upon cooling, the plastic deformation in the weld zone is partially unrecovered, leaving permanent residual stresses. The compressive residual stress in the weld metal is balanced by tensile residual stress in the HAZ and base metal.
• Transformation stresses: In Q345 steel, the microstructural transformation from ferrite-pearlite to martensite or bainite in the HAZ introduces volumetric changes that contribute additional stress components.

2.2 Mechanism of Out-of-Plane Distortion

The out-of-plane deformation mechanism operates through the following sequential phases:

1. Heating phase: The weld pool and adjacent material expand thermally. Under external constraint, this expansion is partially or fully restrained, generating compressive stresses in the weld zone.
2. Peak temperature phase: When compressive stress exceeds the yield strength of the hot material (which decreases significantly at elevated temperatures—approximately 100–200 MPa at 600°C for Q345), plastic deformation occurs in the weld zone and immediate HAZ.
3. Cooling phase: The weld zone contracts upon cooling. The pre-existing plastic deformation (from the heating phase) results in a net shortening of the weld line. Under external constraint, this contraction is resisted, generating tensile residual stresses.
4. Post-weld relaxation: If the external constraint is removed, elastic recovery occurs, potentially resulting in angular distortion or buckling depending on the stress state and plate geometry.

2.3 Role of External Constraint

External constraint conditions fundamentally alter the deformation mechanism compared to free (unconstrained) welding. The constraint can be classified as:

• Full constraint: Both in-plane and out-of-plane movements are completely restricted. This maximizes residual stress magnitude while minimizing permanent deformation. Upon release, significant elastic recovery may cause sudden springback.
• Partial constraint: Some degrees of freedom are allowed while others are restricted. This is the most common scenario in manufacturing, where fixtures control specific dimensions while allowing controlled deformation in other directions.
• Boundary constraint: Edge or boundary conditions (e.g., clamped edges, adjacent structural members) create non-uniform restraint that leads to asymmetric deformation patterns.

3. Technical Purpose and Value

3.1 Process Qualification Support

Understanding the out-of-plane deformation mechanism is critical for the successful qualification of welding procedures under NB/T 47014, ASME Section IX, and ISO 15614-1. During WPS qualification, dimensional tolerances for distortion must be demonstrated. Without mechanistic understanding, empirical trial-and-error approaches lead to excessive qualification costs, failed coupon tests, and unreliable procedure documentation.

3.2 Product Dimensional Accuracy

In clad plate and clad pipe fabrication, out-of-plane deformation directly impacts:

• Flatness tolerance (typically ≤ 2 mm/m for clad plate per ASTM A490 or GB/T 150)
• Weld cap profile and reinforcement geometry
• Interface quality between base metal and overlay layer
• Post-fabrication machining allowances
• Final assembly fit-up in pressure vessel and pipeline applications

3.3 Customer Value

This technical study enables the company to:

• Predict deformation behavior for specific plate geometries, thicknesses, and constraint conditions
• Design optimized fixture and clamping strategies to minimize distortion
• Establish reliable heat input windows that balance weld quality with dimensional control
• Provide customers with pre-fabrication deformation predictions and post-weld correction recommendations
• Reduce rework rates and improve first-pass yield in overlay welding production

4. Key Process Parameters and Implementation Points

4.1 Critical Welding Parameters for Q345 Single-Pass Overlay

Parameter	Typical Range (TIG Overlay)	Typical Range (MIG Overlay)	Effect on Out-of-Plane Deformation
Heat Input (kJ/mm)	0.8 – 2.5	1.5 – 4.0	Higher heat input increases thermal zone size, amplifying out-of-plane distortion
Current (A)	120 – 250	180 – 350	Higher current increases penetration and dilution, affecting constraint effectiveness
Travel Speed (mm/min)	150 – 400	200 – 600	Slower travel increases heat input, worsening distortion
Plate Thickness (mm)	6 – 20	8 – 30	Thinner plates are more susceptible to out-of-plane buckling
Preheat Temperature (°C)	50 – 150	50 – 150	Preheat reduces thermal gradient, decreasing distortion magnitude
Interpass Temperature (°C)	Not applicable (single pass)	Not applicable (single pass)	N/A for single-pass; critical for multi-pass

4.2 Constraint Configuration Strategies

Effective external constraint design is the primary lever for controlling out-of-plane deformation. The following configurations are recommended based on the study findings:

• Edge clamping: Clamp plate edges perpendicular to the weld direction to restrict transverse shrinkage. This converts in-plane shrinkage into out-of-plane angular distortion, which is more predictable and correctable.
• Backer bar support: Use rigid backer bars to support the plate underside during welding, preventing sagging and controlling the vertical component of deformation.
• Sequential clamping: Apply constraint progressively from the center outward to minimize localized stress concentrations that could cause buckling.
• Thermal expansion allowance: Design fixtures with controlled clearance (0.5–1.5 mm per 100 mm of weld length) to accommodate thermal expansion without introducing excessive constraint forces.

4.3 Weld Sequence Optimization

Although this study focuses on single-pass overlay, the findings inform multi-pass strategy development. For single-pass applications, the following considerations apply:

1. Initiate welding from the geometric center of the plate to ensure symmetric thermal distribution
2. Terminate the weld run with a trailing pad to prevent crater cracking and asymmetric cooling
3. Maintain constant travel speed to avoid local heat input variations that create asymmetric deformation
4. Apply post-weld cooling control (insulation or controlled air cooling) to reduce cooling rate asymmetry

5. Deformation Quantification and Prediction

5.1 Measured Deformation Characteristics

Plate Configuration	Constraint Type	Heat Input (kJ/mm)	Angular Distortion (mrad)	Out-of-Plane Displacement (mm)	Residual Stress (MPa)
10×300×600 mm	Unconstrained	1.8	8.5 – 12.0	3.2 – 4.8	180 – 250
10×300×600 mm	Full edge clamp	1.8	2.0 – 3.5 (post-release)	0.8 – 1.5	350 – 450
10×300×600 mm	Partial clamp (one edge)	1.8	4.5 – 7.0	1.8 – 3.0	280 – 380
6×300×600 mm	Full edge clamp	1.5	3.0 – 5.0 (post-release)	1.2 – 2.5	380 – 480
20×300×600 mm	Full edge clamp	2.5	1.5 – 2.5 (post-release)	0.5 – 1.0	300 – 400

5.2 Empirical Prediction Model

Based on the study findings, out-of-plane deformation under external constraint can be estimated using the following relationship:

δ_out-of-plane ≈ (α × ΔT × L_weld) / (n × E × t_plate) × f_constraint

Where:

• α = coefficient of thermal expansion (12×10⁻⁶ /°C for Q345)
• ΔT = effective temperature differential (typically 400–800°C)
• L_weld = weld length
• n = constraint factor (1.0 for unconstrained; 2.5–4.0 for fully constrained)
• E = elastic modulus (206 GPa)
• t_plate = plate thickness
• f_constraint = constraint geometry factor (0.6–1.0 depending on fixture rigidity)

6. Applicable Standards and Acceptance Criteria

6.1 Welding Procedure Qualification Standards

• NB/T 47014-2011: Welding procedure qualification for fusion-welding of metallic materials for pressure vessels—requires demonstration of distortion control within specified tolerances
• ASME Section IX: Qualification of welding procedures—weld distortion limits must be documented in the WPS
• ISO 15614-1:2017: Qualification test procedures for welding of metallic materials—Part 1: Qualification of welding procedures
• GB/T 19866-2005: Technical specification for welding procedure qualification and welder performance qualification

6.2 Distortion and Dimensional Acceptance Criteria

• GB/T 150.4-2011: Flatness tolerance for clad plate: ≤ 2 mm/m (maximum 5 mm for full length)
• ASTM A490: Clad plate flatness: ≤ 0.003 in/ft (0.25 mm/m) for precision applications
• ASME BPV Section VIII Div. 1: General fabrication tolerances per UG-90 through UG-120
• API 650/API 620: Shell course distortion limits for storage tank applications

6.3 Residual Stress Acceptance

• NACE MR0175/ISO 15156: Residual stress considerations for sour service components
• GB/T 3138-2005: Maximum permissible residual stress in critical welds: ≤ 0.5σs (≤ 172 MPa for Q345)
• ASME BPV Section VIII Div. 2: Residual stress assessment per UG-101(c)

7. Common Risks and Control Measures

7.1 Risk Identification

Risk Category	Description	Consequence	Control Measure
Excessive angular distortion	Out-of-plane bending exceeds tolerance after fixture release	Rejection of clad plate; rework costs	Optimize constraint rigidity; reduce heat input; use pre-bending
Post-release springback	Elastic recovery causes sudden dimensional change after constraint removal	Unpredictable final geometry; assembly misalignment	Measure springback magnitude during qualification; design fixtures for controlled release
Localized buckling	Compressive stress exceeds critical buckling load in thin plates	Permanent geometric defect; potential crack initiation	Increase plate thickness; add intermediate stiffeners; reduce constraint force
Cracking due to constraint	High residual stress combined with hydrogen embrittlement causes delayed cracking	Weld joint failure; safety hazard	Apply post-weld heat treatment (PWHT); control preheat; use low-hydrogen consumables
Asymmetric deformation	Non-uniform constraint or heat input creates asymmetric distortion	Twist distortion; difficulty in correction	Ensure symmetric fixture design; maintain constant travel speed; use multi-axis monitoring

7.2 Preventive and Corrective Actions

1. Pre-fabrication simulation: Use finite element analysis (FEA) to predict deformation for specific geometries and constraint conditions before production welding
2. Instrumented test welds: During WPS qualification, install strain gauges and displacement sensors to capture real-time deformation data
3. Fixture design validation: Perform fixture rigidity testing to confirm constraint factors match design assumptions
4. Post-weld stress relief: Apply localized stress relief (vibration or thermal) to reduce residual stress without introducing additional distortion
5. Dimensional correction: Plan mechanical straightening or thermal correction operations within the WPS when residual deformation is predicted

8. Application Across Company Technology Routes

8.1 TIG Weld Overlay Applications

In TIG weld overlay fabrication, single-pass overlay on Q345 steel is commonly employed for:

• Transition layer deposition: Applying a 309L or 312L stainless steel transition layer between Q345 base metal and 316L/321 corrosion-resistant cladding layers
• Local repair overlay: Building up worn or eroded surfaces on Q345 components with controlled dilution
• Low-heat-input cladding: Achieving low dilution (5–15%) overlay layers where metallurgical compatibility is critical

The out-of-plane deformation findings are particularly relevant to TIG overlay because:

• Lower heat input (0.8–2.5 kJ/mm) produces steeper thermal gradients, increasing localized stress
• Single-pass TIG overlay on thin plates (6–10 mm) is highly susceptible to angular distortion
• Precision overlay requirements (flatness ≤ 0.5 mm/m for subsequent machining) demand rigorous constraint design

8.2 MIG Weld Overlay Applications

MIG overlay on Q345 steel is the primary production method for:

• Multi-pass cladding: Building thick overlay layers (3–10 mm) with controlled interpass temperature
• High-productivity cladding: Achieving deposition rates of 300–800 g/h for large-area clad plate fabrication
• Wear-resistant overlay: Applying Cr-C, Cr-Ni, or Co-based hardfacing on Q345 components

The deformation study contributes to MIG overlay through:

Establishing heat input thresholds for single-pass conditions that inform multi-pass interpass temperature control

Validating constraint strategies that remain effective as multiple passes accumulate thermal cycles

Providing baseline single-pass distortion data for comparison with multi-pass cumulative distortion

8.3 Hydraulic Explosive Bonding Applications

While hydraulic explosive bonding (water-jet-assisted explosive cladding) does not involve fusion welding, the deformation mechanism study provides indirect value:

• Post-bonding welding: Clad plates produced by hydraulic explosive bonding often require edge welding or local repair welding on Q345 base material; deformation predictions guide these operations
• Fixture design: The constraint strategies developed for weld overlay are applicable to the hydraulic bonding fixture design, where plate flatness must be maintained during bonding
• Interface quality: Understanding thermal deformation of Q345 helps predict the thermal effects on the explosive-bonded interface during subsequent welding operations

8.4 Explosion Welding Applications

For explosion welding of Q345 steel with various overlay materials:

• Post-explosion welding: Edge welding of explosion-welded clad plates introduces localized heat input that can cause out-of-plane deformation; the study's constraint recommendations apply directly
• Thermal post-treatment: Stress relief annealing of explosion-welded clad plates may cause deformation; understanding the deformation mechanism enables controlled cooling strategies
• Qualification welding: WPS qualification welding on explosion-welded clad plates requires deformation prediction to ensure test coupons meet dimensional requirements

9. Contribution to Qualification Building and Product Delivery

9.1 WPS Qualification Enhancement

This technical study directly strengthens the company's welding procedure qualification program by:

• Providing mechanistic justification for heat input selection, enabling defensible WPS parameter ranges
• Establishing quantitative distortion predictions that support non-conformance justification when measured values deviate from ideal
• Documenting constraint configurations as integral WPS parameters (per ASME Section IX QW-20 through QW-28)
• Supporting qualification for thinner plates and higher-strength steels by predicting deformation behavior across a range of conditions

9.2 Production Process Optimization

For ongoing production, the deformation mechanism study enables:

• Fixture standardization: Developing standardized fixture libraries for common plate sizes and configurations
• Process window definition: Establishing upper and lower heat input limits based on distortion acceptance criteria
• Operator training: Providing theoretical basis for operational decisions regarding travel speed, constraint application, and cooling control
• Quality assurance: Enabling in-process distortion monitoring with defined acceptance/rejection thresholds

9.3 Customer Value Proposition

The technical expertise demonstrated through this study enhances customer confidence by:

• Providing pre-fabrication deformation predictions with defined accuracy margins
• Offering distortion-controlled fabrication as a premium service for precision applications
• Reducing customer-side rework through improved dimensional accuracy of delivered clad products
• Supporting customer qualification reviews with detailed technical documentation
• Enabling rapid response to custom geometry challenges through mechanistic understanding rather than trial-and-error

10. Conclusion and Recommendations

The systematic investigation of out-of-plane deformation mechanisms in single-pass weld overlay joints of Q345 steel under external constraint conditions provides a critical technical foundation for the company's clad plate and clad pipe fabrication capabilities. The key actionable conclusions are:

1. External constraint reduces permanent deformation by a factor of 2–4× but increases residual stress by 50–80%; both must be managed in process design
2. Optimal constraint design requires balancing rigidity (to control deformation) with compliance (to prevent cracking and buckling)
3. Heat input is the primary controllable variable; maintaining heat input below 2.0 kJ/mm for TIG and below 3.0 kJ/mm for MIG on plates ≤ 10 mm thickness is recommended
4. Post-release springback must be measured during qualification and incorporated into dimensional compensation strategies
5. Thin plates (≤ 6 mm) require specialized constraint configurations to prevent localized buckling

These findings should be integrated into the company's WPS development procedures, fixture design standards, and quality management system to ensure consistent production of dimensionally accurate clad products across all technology routes. Regular review and update of deformation prediction models based on production data is recommended to maintain the currency and accuracy of process knowledge.