Numerical Simulation of Cladding Weld Overlay on Dissimilar Steel Flat Plates

Literature Overview and Core Objective

This study investigates the thermal and mechanical behavior during weld overlay cladding of austenitic stainless steel onto low-alloy steel flat plates using finite element analysis. The research addresses a critical engineering challenge: the prediction of residual stresses, deformation, and microstructural evolution at the dissimilar metal interface. In bimetal pressure vessel fabrication, the bond between the base metal and the overlay layer directly determines service reliability under cyclic loading and corrosive environments. The numerical model typically employs coupled thermo-mechanical analysis, where the transient heat transfer problem is solved first to obtain the temperature history, which then serves as input for the mechanical analysis.

Key Technical Parameters and Modeling Approach

The simulation generally involves several critical input parameters that govern the accuracy of predictions. The following table summarizes typical values used in such studies:

The thermal model typically uses a moving heat source, often a Gaussian distribution or a double-ellipsoid model to represent the arc energy input. The mechanical model employs elastic-plastic constitutive relations with temperature-dependent material properties. Thermal expansion coefficient mismatch between austenitic stainless steel (approximately 17.3 × 10⁻⁶ /°C) and low-alloy steel (approximately 12 × 10⁻⁶ /°C) generates significant residual stresses at the interface, which is the primary concern for engineers.

Interpretation of Technical Points

The most significant finding from such numerical studies is the prediction of peak longitudinal residual stresses, which often approach or exceed the yield strength of the base material. The stress distribution shows a characteristic pattern: high tensile stresses in the weld metal and adjacent HAZ, balanced by compressive stresses in the far-field base metal. This stress state has direct implications for fatigue life and stress corrosion cracking susceptibility.

The dilution effect is another critical aspect. When the heat input is excessive or the interpass temperature is poorly controlled, the dilution ratio can exceed design limits. For a 304L overlay on carbon steel, a dilution exceeding 20% may compromise the corrosion resistance of the overlay layer, as measured by intergranular corrosion tests per ASTM A923 or GB/T 4334. The numerical model helps optimize the number of passes, wire feed rate, and travel speed to maintain dilution within acceptable limits.

From a metallurgical perspective, the simulation reveals the cooling rate gradient across the overlay thickness. The first pass, in contact with the cooler base metal, experiences a higher cooling rate and may develop a martensitic or ferritic microstructure at the interface. Subsequent passes benefit from the heat of previously deposited layers, resulting in a more favorable austenitic-ferritic microstructure. This progressive microstructural evolution must be accounted for in quality planning.

Connection with Engineering Practice

In actual fabrication of clad pressure vessels per NB/T 47002 or GB/T 150, the numerical simulation results serve as a design tool for welding procedure qualification. The predicted residual stress distribution informs the selection of stress relief procedures. Post-weld heat treatment (PWHT) at 550-650 °C for carbon steel and 1050-1100 °C for austenitic stainless steel must be carefully sequenced to avoid intergranular sensitization of the overlay layer.

A practical case involves the fabrication of a hydrogenation reactor shell using 15CrMo base plate with a 316L overlay layer of 6 mm thickness. The simulation predicted peak interface stresses of approximately 420 MPa before stress relief. After PWHT at 620 °C for 4 hours, the residual stresses were reduced to below 100 MPa, well within the allowable limit. The simulation also predicted a maximum angular distortion of 0.3% of plate length, which was confirmed by post-fabrication measurement.

Non-destructive testing requirements per JB/T 4730 should be correlated with the predicted stress concentration zones. Areas of predicted high residual stress warrant additional UT or PAUT examination to detect potential lack of bond defects. The simulation thus serves as a complementary tool to physical NDT, helping to prioritize inspection resources.

Key Questions and Reflections

One critical question arises from the numerical results: how accurately do the assumed material properties reflect the actual behavior of the weld metal, which is a complex multiphase microstructure with varying composition due to dilution? The simulation assumes homogeneous material properties for the weld metal, whereas in reality, the composition varies significantly from pass to pass. This simplification may lead to underestimation of local stress concentrations.

Another important consideration is the effect of welding sequence on the final stress state. The simulation typically models a single-direction deposition, but in practice, multi-pass welding with various sequence strategies (such as zig-zag, step-back, or symmetric approaches) can significantly alter the residual stress distribution. Engineers should use simulation results as a baseline and validate with strain gauge measurements or X-ray diffraction stress measurements on actual weldments.

The study also raises questions about the applicability of the simulation to thin-overlay scenarios (less than 3 mm). In such cases, the thermal mass of the overlay is insufficient to moderate the cooling rate, and the interface microstructure may deviate significantly from the predictions. For thin overlays, experimental validation becomes even more critical.

Study Insights and Implications

This literature provides valuable guidance for welding procedure optimization in dissimilar metal cladding applications. The key takeaway is that numerical simulation, when properly calibrated with experimental data, can significantly reduce the number of trial weldments needed for procedure qualification. This translates directly into cost savings and schedule compression for bimetal pressure vessel fabrication projects.

The engineer should treat simulation results as a predictive tool rather than an absolute truth. The model assumptions regarding material properties, boundary conditions, and heat source geometry all introduce uncertainties. A systematic approach combining simulation with limited experimental validation — such as thermocouple measurements for temperature verification and residual stress measurements for mechanical validation — provides the most reliable basis for production welding procedures.

The integration of numerical simulation into the quality planning process represents a paradigm shift from purely empirical welding procedure development to a knowledge-based approach. This shift aligns with modern standards requirements for welding procedure qualification and supports the transition toward digital fabrication and traceable quality documentation in the pressure vessel industry.