Numerical Simulation and Residual Stress Analysis of Multi-Pass Multi-Layer Cladding Forming

Literature Overview and Technical Context

This study employs finite element analysis (FEA) to investigate the thermal-mechanical behavior and residual stress distribution during multi-pass multi-layer weld overlay cladding. In engineering practice, cladding is a critical process for extending the service life of pressure vessels, heat exchangers, and structural components by depositing a corrosion- or wear-resistant surface layer. However, the residual stresses generated during the cladding process can be detrimental, leading to cracking, distortion, and premature fatigue failure. Understanding the residual stress field through numerical simulation enables process optimization and provides a basis for post-weld stress relief (PWSR) procedure development.

The study models a typical multi-pass multi-layer cladding sequence on a cylindrical substrate, considering the thermal-mechanical coupling effects of sequential weld passes. The simulation accounts for material nonlinearity, phase transformations, plastic deformation, and the sequential heating and cooling of each pass.

Core Technical Points

Numerical Model and Simulation Approach

The study uses a sequential coupled thermal-mechanical FEA model implemented in ABAQUS. The thermal analysis employs a moving heat source (Goldak double-elliptical model) to simulate the welding arc, while the mechanical analysis uses the "birth and death" element technique to represent the sequential addition of material. The material properties are temperature-dependent, incorporating the effects of phase transformation in the HAZ through the Q990 model (extended Johnson-Cook model).

Residual Stress Distribution

The simulation results reveal a characteristic residual stress distribution pattern. The longitudinal residual stresses (along the welding direction) are tensile in the weld metal and compressive in the adjacent HAZ, with a peak tensile stress of approximately 380 MPa in the center of the final pass. The transverse residual stresses (perpendicular to the welding direction) are predominantly compressive in the weld metal and tensile in the HAZ, with a peak compressive stress of approximately -250 MPa.

The circumferential residual stresses (for the cylindrical geometry) are complex, with alternating tensile and compressive zones depending on the pass location relative to the cylinder axis. The simulation shows that the maximum von Mises stress at the fusion line reaches 420 MPa, which is below the yield strength of the 304L overlay at room temperature (approximately 215 MPa) but exceeds it in the HAZ where the material is tempered to a lower strength level.

Effect of Pass Sequence on Residual Stress

A key finding of the study is the significant influence of pass sequence on the residual stress distribution. Three different pass sequences were compared:

The symmetric sequence (welding from the outside passes inward) produces the lowest residual stresses and minimum distortion, consistent with the well-known principle that welding should proceed symmetrically about the neutral axis to minimize bending moments. The alternating sequence provides a moderate improvement over the sequential sequence, while the sequential sequence produces the highest stresses and distortion.

Residual Stress Relaxation Through Post-Weld Stress Relief

The study also simulates the effect of post-weld stress relief (PWSR) on the residual stress distribution. A PWSR cycle of 650°C for 2 hours with a controlled cooling rate of 100°C/hour is modeled. The simulation shows that the PWSR reduces the peak longitudinal stress from 380 MPa to approximately 80 MPa, a relaxation of 79%. The residual stress distribution after PWSR is predominantly compressive in the weld metal, which is beneficial for fatigue resistance but may be detrimental if the compressive stresses are insufficient to prevent stress corrosion cracking (SCC) in aggressive environments.

Process Optimization Recommendations

Based on the simulation results, the study recommends the following process parameters and practices for minimizing residual stresses in multi-pass multi-layer cladding:

1. Pass sequence: Use a symmetric welding sequence (outside-in) to minimize bending moments and distortion.
2. Heat input control: Limit the heat input to below 12 kJ/mm to reduce the thermal gradient and associated residual stresses.
3. Preheat: Apply a preheat of 150–200°C to reduce the thermal gradient between the weld pool and the base metal.
4. Interpass temperature: Maintain the interpass temperature at 200–250°C to avoid excessive cooling rates that promote martensite formation in the HAZ.
5. Post-weld stress relief: Perform PWSR at 600–650°C for a minimum of 2 hours, with a controlled cooling rate below 100°C/hour to prevent the formation of new residual stresses during cooling.
6. Welding direction: Use a multi-directional welding strategy (e.g., alternating left-to-right and right-to-left) to partially cancel the transverse residual stresses.

Engineering Practice Implications

Validation of Simulation Against Experimental Data

The study validates the FEA model against experimental measurements obtained through X-ray diffraction (XRD) stress measurement and neutron diffraction (ND) stress measurement. The XRD measurements are performed on the surface of the cladded component and provide the surface residual stress profile. The ND measurements penetrate to a depth of 30 mm and provide the through-thickness residual stress distribution. The comparison shows good agreement between the simulation and experimental data, with deviations within ±30 MPa for the longitudinal stress and ±20 MPa for the transverse stress.

Application to Pressure Vessel Cladding

The residual stress analysis has direct implications for the design and fabrication of clad pressure vessels. According to ASME VIII Div. 1 and GB/T 150, the residual stresses in clad pressure vessels must be evaluated as part of the fitness-for-service assessment. The simulation results provide a basis for predicting the residual stress field in clad pressure vessels of various geometries and cladding configurations, which can be used to determine the necessity and parameters of PWSR.

The study also highlights the importance of considering residual stresses in the assessment of clad pressure vessel integrity under cyclic loading. The combination of residual stresses and cyclic service stresses can lead to ratcheting (progressive plastic deformation) at the fusion line, which can result in fatigue failure at stress levels below the material's fatigue limit. The simulation provides a framework for evaluating the ratcheting risk under various loading conditions.

Study Insights and Reflections

The most valuable aspect of this study is the systematic investigation of pass sequence effects on residual stress distribution. In engineering practice, the pass sequence is often determined by the welder's preference or the shop's standard practice, without consideration of its influence on the residual stress field. The simulation results clearly demonstrate that a symmetric pass sequence can reduce peak residual stresses by up to 26% and distortion by up to 46% compared to a sequential sequence. This is a compelling argument for incorporating pass sequence optimization into the welding procedure specification (WPS).

The study also highlights the importance of the "birth and death" element technique in accurately modeling the sequential material addition during multi-pass welding. In my experience, many engineering organizations use simplified models that apply all weld passes simultaneously or use a single heat source to represent the entire cladding process. These simplifications can lead to significant errors in the predicted residual stress field, particularly in the through-thickness direction. The study's use of the sequential birth-and-death technique provides a more accurate representation of the physical process and should be adopted as the standard approach for residual stress analysis of multi-pass cladding.

The validation of the FEA model against XRD and ND measurements is another strength of the study. In engineering practice, residual stress measurements are often limited to surface techniques (XRD, hole drilling) due to cost and accessibility constraints. The study's use of ND measurements provides a valuable benchmark for through-thickness stress prediction, which is critical for assessing the integrity of clad components under cyclic loading.

In conclusion, this literature provides a rigorous and well-validated numerical framework for predicting and optimizing the residual stress field in multi-pass multi-layer cladding. The key takeaways for engineers are: (1) pass sequence is a critical process parameter that should be optimized for residual stress control, (2) the sequential birth-and-death FEA technique is essential for accurate residual stress prediction, and (3) experimental validation through multi-technique stress measurement (XRD + ND) is necessary to establish confidence in the simulation predictions before applying them to engineering design decisions.