Interlayer Stress Analysis and Process Optimization in Metal Overlay Forming

Literature Overview and Research Context

The research by Xu Yan, Li Bingru, Bao Yang, Zhou Jianping, and Xue Ruilei, published in Foundry Technology (2017), addresses a fundamental challenge in overlay welding and cladding: the management of interlayer residual stresses that develop during multi-pass metal deposition. This study was supported by the National Natural Science Foundation of China (51665055), the Xinjiang Uygur Autonomous Region High-Tech Research Project (201113129), and the Xinjiang Youth Science and Technology Innovation Talent Training Program (gn2015yx008), indicating its importance in both academic and industrial contexts.

Interlayer stresses arise from the thermal cycling inherent in multi-pass welding. Each successive pass subjects the previously deposited layer to reheating and cooling, generating complex stress states that can lead to cracking, distortion, and reduced service life. Understanding and controlling these stresses is critical for ensuring the integrity of overlay weldments, particularly in pressure vessel and heavy equipment applications.

Stress Development Mechanisms

The research systematically analyzes the mechanisms by which interlayer stresses develop during overlay welding. The following table categorizes the primary stress sources:

Stress Source	Mechanism	Magnitude (Typical)	Effect
Thermal contraction	Differential cooling rates	200–500 MPa	Tensile residual stress
Phase transformation	Austenite-to-martensite transformation	100–300 MPa	Additional tensile stress
Dilution mismatch	Thermal expansion coefficient difference	50–150 MPa	Interface stress concentration
Constrained cooling	Substrate constraint on overlay	100–250 MPa	Stress buildup in overlay
Hydrogen embrittlement	Hydrogen accumulation in HAZ	Variable	Delayed cracking

Thermal Stress Analysis

The thermal stress component dominates in most overlay welding scenarios. As each weld pass cools from the welding temperature (approximately 1500–1800 °C) to ambient temperature, the deposited metal contracts. However, this contraction is constrained by the previously deposited layers and the base material, generating significant tensile residual stresses. The research demonstrates that peak thermal stresses can approach the yield strength of the overlay material, particularly in the early passes when the substrate provides strong constraint.

The stress distribution varies with pass number. Early passes experience higher stresses due to greater constraint from the base material. Later passes benefit from the thermal mass of previously deposited layers, which moderate cooling rates and reduce peak stresses. However, the cumulative effect of multiple thermal cycles can lead to stress relaxation and redistribution, potentially creating new stress concentrations at pass boundaries.

Phase Transformation Stresses

In steel overlay systems, phase transformation stresses add complexity to the stress analysis. When the overlay material undergoes austenite-to-ferrite or austenite-to-martensite transformation during cooling, the associated volume change generates additional stresses. Martensitic transformation, with its volume expansion of approximately 3–5%, can partially compensate for thermal contraction stresses, but it also introduces its own stress concentrations at the transformation front.

The research identifies that the timing and rate of phase transformation relative to the cooling curve significantly influence the final stress state. Rapid cooling promotes martensitic transformation, which can generate high local stresses. Slower cooling allows diffusional transformations (pearlite, bainite) with lower volume change but may result in softer microstructures with reduced wear resistance.

Process Optimization Strategies

Based on the stress analysis, the research proposes several process optimization strategies:

Strategy	Implementation	Stress Reduction
Preheating	200–400 °C preheat	Reduces thermal gradient by 30–50%
Interpass temperature control	Maintain 150–350 °C	Limits thermal cycling amplitude
Backing heat	Apply heat to back of workpiece	Reduces cooling rate asymmetry
Multi-directional welding	Alternate pass directions	Redistributes stress concentrations
Weld sequence optimization	Center-out or step-back sequences	Minimizes cumulative distortion
Post-weld heat treatment	Stress relief at 550–650 °C	Reduces residual stress by 60–80%

Weld Sequence Optimization

The research emphasizes that weld sequence is a powerful tool for stress management. The following sequence strategies are recommended:

Center-out sequence: Start from the center and weld outward in alternating passes, allowing the center to expand freely and reducing edge constraint stresses
Step-back sequence: Weld in short steps, alternating between forward and backward directions, which distributes thermal input more evenly
Symmetric sequence: For large areas, weld symmetrically from the center to both edges simultaneously, minimizing asymmetric distortion

Microstructural Control for Stress Management

The microstructure of the overlay layer significantly influences stress development and crack resistance. The research recommends:

Retained austenite content of 5–15% to provide transformation toughening and absorb transformation stresses
Fine grain size (ASTM 8–10) to improve ductility and stress relaxation capacity
Uniform carbide distribution to prevent stress concentration at carbide-matrix interfaces
Absence of brittle phases such as sigma phase or Laves phase that could initiate cracking under stress

Numerical Simulation and Validation

The research incorporates finite element analysis (FEA) to predict stress distributions and validate experimental findings. The simulation results show good agreement with measured residual stresses, with deviations typically within 15–20%. The FEA model incorporates:

Thermomechanical coupling to capture temperature-dependent material properties
Phase transformation modeling to account for volume change during solidification and transformation
Plastic deformation modeling to capture yield behavior under high-temperature loading
Constraint conditions representing the base material and previously deposited layers

The simulation enables prediction of stress states for different process parameter combinations without extensive experimental testing, accelerating the optimization process.

Engineering Applications and Quality Assurance

For pressure vessel and heavy equipment applications, interlayer stress management is critical for ensuring long-term service integrity. The following quality assurance measures are recommended:

Residual stress measurement using X-ray diffraction or magnetic methods after each major welding stage
Stress relief heat treatment following welding completion, typically at 550–650 °C for 2–4 hours per 25 mm thickness
Non-destructive testing (RT, UT, MT) to detect stress-induced cracking before it propagates
Documentation of welding parameters and sequence for traceability and future reference

Key Insights and Technical Reflections

This research provides a comprehensive framework for understanding and managing interlayer stresses in overlay welding. Several key insights emerge:

Interlayer stresses are not merely a result of thermal contraction but involve complex interactions between thermal, transformation, and mechanical effects
Process optimization requires a holistic approach that considers thermal management, microstructural control, and weld sequence simultaneously
Numerical simulation is a powerful tool for stress prediction but must be validated against experimental data for specific material combinations
The economic cost of stress-induced failures (cracking, distortion, reduced fatigue life) far exceeds the cost of proper stress management during fabrication

For engineers working on bimetal pressure vessels and cladding applications, this research underscores the importance of stress management as a design consideration, not merely a fabrication afterthought. The stress state of the overlay layer directly influences its service performance, particularly under cyclic loading, thermal cycling, and corrosive environments.

Summary and Conclusions

The research by Xu Yan and colleagues provides a rigorous analysis of interlayer stress development in metal overlay forming, combining theoretical understanding, numerical simulation, and experimental validation. The proposed process optimization strategies offer practical tools for engineers to reduce residual stresses and improve the integrity of overlay weldments. The emphasis on a holistic approach—integrating thermal management, microstructural control, and sequence optimization—reflects the maturity of the field and the increasing complexity of modern overlay welding applications. For practitioners in pressure vessel fabrication and heavy equipment manufacturing, this research provides both theoretical foundation and practical guidance for achieving high-quality, stress-controlled overlay weldments.