Zhuojin Pipe Fitting Co., LtdDynamic Stress and Residual Stress Analysis in Hardfacing of Large Frame Structures
Literature Overview
The paper by Zhong Zhiyong, Fu Wei, Gu Yirong, and Bi Gang, published in the Transactions of the China Welding Institution (Vol. 31, No. 6, 2010, pp. 93-96), addresses a critical engineering problem encountered during the repair and restoration of large structural frame components at Baosteel's mechanical workshop. The study investigates the evolution of dynamic stress during large-area laser cladding and MIG hardfacing operations, comparing the mechanical consequences of both processes on the base structure. This work is particularly relevant to engineers working on equipment maintenance, where large-scale overlay welding is applied to restore worn surfaces on structural members such as columns, beams, and frames.
Core Technical Findings
The research establishes a clear distinction between the thermal and mechanical severity of laser cladding versus conventional MIG hardfacing when applied over large areas. Laser cladding, being a highly concentrated energy input process, produces localized thermal gradients that are confined to a narrow zone, resulting in dynamic stress values that remain well below the yield strength of the structural material. In contrast, MIG hardfacing introduces significantly greater thermal input over a broader area, generating dynamic stresses that can exceed the yield strength of the base material.
| Parameter | Laser Cladding | MIG Hardfacing |
|---|---|---|
| Maximum dynamic stress | Well below yield strength | 311.27 MPa (exceeds yield strength) |
| Location of peak stress | Localized near cladding zone | Mid-span of column |
| Post-weld dimensional change | Negligible | +0.4 mm increase in width direction |
| Maximum residual stress (overlay) | ~220 MPa | ~220 MPa |
| Stress measurement method | Blind hole method (before/after) | Blind hole method (before/after) |
Process Analysis and Engineering Implications
The finding that MIG hardfacing generates a maximum dynamic tensile stress of 311.27 MPa at the mid-span of the column is particularly concerning. For typical structural steels such as Q235 or Q345, the yield strength ranges from 235 to 345 MPa. The fact that the dynamic stress exceeds the yield strength implies that plastic deformation occurs during welding, which can lead to permanent distortion and potential loss of dimensional accuracy. The measured 0.4 mm increase in width direction confirms that macroscopic plastic deformation has taken place, which is consistent with the stress analysis results.
From a process engineering perspective, this finding has direct implications for welding procedure specification. When hardfacing large structural members using MIG, the following measures should be considered:
- Preheating and interpass temperature control: Maintaining interpass temperatures between 100-150°C can reduce thermal gradients and mitigate peak dynamic stresses.
- Weld sequence optimization: A symmetric welding sequence from the center outward, or a multi-pass staggered approach, can distribute thermal input more evenly and reduce residual stress concentration.
- Post-weld stress relief: Stress relief heat treatment (typically at 550-650°C for 2-4 hours) should be specified for critical structures where dimensional accuracy is paramount.
- Process selection: For large-area overlay applications where distortion control is critical, laser cladding should be preferred despite higher equipment costs, as the dynamic stress remains sub-yield.
Residual Stress Behavior
The blind hole method results reveal that both laser cladding and MIG hardfacing produce overlay residual stresses of approximately 220 MPa, representing a significant increase from the original stress state. This is consistent with the general understanding that overlay welding introduces compressive stresses in the base metal near the weld and tensile stresses within the weld metal itself. The residual stress of 220 MPa in the overlay layer is substantial and should be considered in fatigue life assessments of the repaired component.
The similarity in residual stress levels between the two processes, despite their vastly different dynamic stress profiles, suggests that residual stress is more strongly governed by the overall thermal cycle and cooling rate rather than the instantaneous dynamic stress state. This observation supports the practice of applying stress relief treatments regardless of the overlay process selected.
Study Insights and Reflections
This paper provides valuable quantitative data that bridges the gap between welding simulation predictions and actual structural response. The comparison between laser cladding and MIG hardfacing on the same structural component is particularly instructive, as it demonstrates that process selection is not merely a matter of productivity or cost but has fundamental implications for structural integrity. For engineers involved in equipment maintenance and structural repair, the key takeaway is that large-area MIG hardfacing should be approached with caution, as the process can induce plastic deformation even in thick structural members. The measured dimensional change of 0.4 mm, while seemingly small, can be critical in applications requiring precise fit-up or alignment. Future work should explore hybrid approaches that combine the precision of laser cladding with the deposition rate of arc welding to achieve optimal results.