Dynamic Stress and Residual Stress Analysis During Hardfacing of Large Frame Structures

Literature Overview

This paper by Zhong Zhiyong and colleagues from Baosteel Machinery Plant investigates the dynamic stress evolution and residual stress behavior during two distinct overlay processes applied to large frame structures: laser cladding and MIG (Gas Metal Arc) hardfacing. Published in the Chinese Journal of Welding in 2010, the study addresses a practical engineering concern that has significant implications for large-scale equipment repair and rehabilitation, particularly in the steel and heavy machinery industries. The authors employed both numerical simulation and experimental measurement techniques, including the blind hole method for residual stress determination, to provide a comprehensive picture of the stress state induced by each process.

Core Technical Findings

The most striking result from this investigation is the fundamental difference in stress induction between laser cladding and conventional MIG hardfacing. Laser cladding, due to its highly localized and rapid energy input, generates dynamic stresses that remain well below the material's yield strength throughout the entire process. This is consistent with the physical understanding that laser cladding involves a very small heat-affected zone and rapid solidification, which limits the extent of thermal plastic deformation. In contrast, MIG hardfacing produces significantly higher dynamic stresses, with the maximum tensile stress reaching 311.27 MPa at the mid-span of the column section, exceeding the yield strength of the base material.

The post-weld dimensional measurement reveals a 0.4 mm increase in the width of the structure after MIG hardfacing, which is a direct consequence of the plastic deformation occurring in the heat-affected zone. This expansion, though seemingly small, can have cumulative effects when multiple weld passes are applied to large structures, potentially leading to geometric deviations that compromise assembly tolerances.

Process Mechanism Interpretation

The difference in dynamic stress behavior between the two processes can be attributed to their fundamentally different thermal cycles. Laser cladding delivers energy through a focused beam with high power density, resulting in rapid heating and cooling rates that confine plastic deformation to a very thin layer. The rapid solidification also produces a fine microstructure with high strength, which can accommodate thermal strains without extensive plastic flow in the substrate. MIG hardfacing, on the other hand, involves a much larger molten pool and slower cooling rates, leading to more extensive thermal plastic deformation in the surrounding material. The large thermal gradient generates significant thermal stresses that, when combined with restraint from the base structure, can drive the material into the plastic regime.

The blind hole method results showing approximately 220 MPa residual stress in both cases are particularly noteworthy. Despite the vastly different dynamic stress levels during welding, the final residual stress states converge to similar magnitudes. This suggests that the residual stress is governed more by the overall thermal cycle and the final equilibrium of elastic-plastic deformation than by the peak dynamic stress during welding. The base material's yield strength acts as a natural cap on residual stress levels, as the material cannot sustain elastic stresses beyond this threshold.

Engineering Practice Implications

For engineers involved in large structure repair, this study provides clear guidance on process selection. When dimensional accuracy is critical, such as in precision machinery frames or structural components subject to strict tolerance requirements, laser cladding offers a significant advantage due to its minimal thermal distortion. However, laser cladding equipment is substantially more expensive and has limitations in terms of deposition rate and achievable layer thickness.

The 0.4 mm dimensional change observed in MIG hardfacing must be accounted for in repair planning. For structures with multiple hardfacing areas, the cumulative distortion can be estimated by considering the interaction between adjacent weld zones. Engineers should implement pre-compensation strategies, such as designing the repair geometry to account for expected distortion, or using post-weld stress relief treatments to minimize dimensional deviations.

The residual stress levels of approximately 220 MPa are significant enough to warrant consideration in fatigue-critical applications. If the repaired structure will be subjected to cyclic loading, the residual stress should be incorporated into fatigue life calculations. Post-weld stress relief through controlled heating and slow cooling, or through mechanical methods such as shot peening, can be employed to reduce residual stresses to acceptable levels.

Key Questions and Reflections

A question that arises from this study is why the residual stresses converge despite the different dynamic stress paths. One possible explanation is that the base material's yield strength determines the maximum sustainable residual stress, regardless of the welding process used. Another consideration is that the blind hole method measures stresses at a specific depth, and the stress distribution through the thickness may differ significantly between the two processes. Future work should investigate the full three-dimensional residual stress field using techniques such as neutron diffraction or X-ray synchrotron methods.

The practical significance of this work extends beyond frame repair to any application involving overlay welding on large structures, including pipe spools, heat exchanger shells, and pressure vessel components. The findings reinforce the principle that process selection should be based not only on metallurgical compatibility and deposition quality but also on the mechanical impact on the base structure.

Study Insights and Outlook

This paper demonstrates the value of combining numerical simulation with experimental validation in understanding welding-induced stress states. The authors' approach of measuring both dynamic stresses during welding and residual stresses after welding provides a more complete picture than either method alone. For engineers, the key takeaway is that laser cladding represents a promising technology for minimizing thermal distortion in large structure repair, while MIG hardfacing requires careful planning to manage dimensional changes and residual stresses. As laser cladding technology continues to advance in terms of deposition rate and equipment cost, it is likely to become the preferred process for precision overlay applications in the future.