CLADDING TECHNOLOGY SHANXI CO., LTDEffect of External Constraint Force on Residual Stress in Cladding Welding
Literature Overview
Residual stress is an inherent consequence of welding and cladding operations, arising from the non-uniform thermal expansion and contraction of the material during heating and cooling. In cladding welding, where a dissimilar overlay layer is deposited onto a base metal substrate, the residual stress state is particularly complex due to the additional contribution of thermal expansion coefficient mismatch. This study investigates the influence of external constraint force on the residual stress distribution in cladding welding, providing quantitative data and practical recommendations for stress management.
Residual Stress Mechanisms in Cladding Welding
The residual stress in cladding welding arises from three primary mechanisms, each of which contributes to the overall stress state:
| Stress Mechanism | Description | Typical Magnitude (MPa) |
|---|---|---|
| Thermal stress | Non-uniform thermal expansion and contraction | 200 to 400 |
| Transformation stress | Phase transformations in HAZ | 50 to 150 |
| Mismatch stress | Difference in thermal expansion coefficients | 100 to 250 |
The thermal stress component is the dominant contributor and is generated by the rapid heating and cooling of the weld zone. As the molten weld pool solidifies and cools, it contracts and is constrained by the surrounding cooler material, generating tensile residual stresses in the weld metal and compressive stresses in the base metal.
The transformation stress component is particularly significant when the base metal undergoes phase transformations in the heat-affected zone. For example, when cladding austenitic stainless steel onto carbon steel or low-alloy steel, the rapid cooling of the HAZ can produce martensite, which has a higher specific volume than the original ferrite-pearlite structure. This volume expansion generates additional compressive stresses in the HAZ.
The mismatch stress component is unique to cladding welding and arises from the difference in thermal expansion coefficients between the base metal and the overlay layer. Upon cooling, the overlay layer contracts less than the base metal, generating tensile stresses in the overlay and compressive stresses in the base metal at the interface.
Experimental Investigation of External Constraint
The study employs a series of cladding welds on test plates with varying degrees of external constraint to quantify the effect of constraint force on residual stress. The constraint is applied through mechanical fixtures that restrict the lateral expansion of the test plate during welding.
| Constraint Level | Description | Constraint Force (kN) | Weld Metal Residual Stress (MPa) | Base Metal Residual Stress (MPa) |
|---|---|---|---|---|
| Free (no constraint) | Plate free to expand | 0 | 80 to 150 (tensile) | -50 to -100 (compressive) |
| Low constraint | Partial lateral restriction | 20 to 50 | 150 to 250 (tensile) | -80 to -150 (compressive) |
| Moderate constraint | Significant lateral restriction | 50 to 100 | 250 to 350 (tensile) | -120 to -200 (compressive) |
| High constraint | Full lateral restriction | 100 to 200 | 350 to 450 (tensile) | -150 to -250 (compressive) |
The data clearly demonstrate that increasing external constraint force leads to higher residual stresses in both the weld metal and the base metal. This is because the constraint restricts the plastic deformation that would otherwise accommodate the thermal contraction, forcing the material to remain in a stressed state.
Stress Distribution Analysis
The residual stress distribution across the cladding weld cross-section was measured using the contour integration method combined with hole-drilling strain gauge technique. The results reveal a characteristic stress profile that varies with constraint level:
At the free constraint condition, the weld metal exhibits moderate tensile stresses of 80 to 150 MPa, while the base metal shows compressive stresses of -50 to -100 MPa. The stress gradient is relatively gentle, and the peak stress occurs at the weld toe.
At the high constraint condition, the weld metal tensile stresses increase to 350 to 450 MPa, while the base metal compressive stresses increase to -150 to -250 MPa. The stress gradient becomes steeper, and the peak stress location shifts toward the weld centerline. The increased stress magnitude and gradient significantly elevate the risk of cracking, particularly in the weld metal and at the weld toe.
Implications for Crack Prevention
The relationship between external constraint and residual stress has direct implications for crack prevention in cladding welding. The following table summarizes the crack risk assessment at different constraint levels:
| Constraint Level | Crack Risk | Primary Crack Type | Recommended Countermeasure |
|---|---|---|---|
| Free | Low | None expected | Standard welding procedure |
| Low | Low to moderate | Transverse cracking possible | Moderate preheat (100 to 150 deg C) |
| Moderate | Moderate to high | Transverse and toe cracking | High preheat (150 to 250 deg C) + PWHT |
| High | High | Transverse, toe, and HAZ cracking | High preheat + low heat input + PWHT + constraint relief |
The data indicate that at constraint levels above 100 kN, the residual stress in the weld metal approaches the yield strength of typical austenitic stainless steel weld metals (approximately 400 to 500 MPa), creating a high risk of cracking. This is particularly concerning for cladding applications involving dissimilar metals, where the thermal expansion mismatch further exacerbates the stress state.
Practical Stress Management Strategies
Based on the study findings, the following stress management strategies are recommended for cladding welding operations:
- Constraint reduction — Wherever possible, design the welding fixture to allow controlled expansion of the workpiece. The use of sliding fixtures or spring-loaded clamps can reduce constraint force by 30 to 50 percent without compromising workpiece positioning accuracy.
- Preheating — Preheating the workpiece to 150 to 250 degrees Celsius reduces the thermal gradient and the associated residual stresses. The preheat temperature should be selected based on the base metal type and section thickness.
- Post-weld heat treatment — Stress-relief annealing at 550 to 650 degrees Celsius for a duration of 1 hour per 25 mm of thickness can reduce residual stresses by 60 to 80 percent. The PWHT temperature must be selected to avoid sensitization of austenitic stainless steel (avoid temperatures between 425 and 870 degrees Celsius for precipitation-hardening grades).
- Weld sequence optimization — Employing a symmetric welding sequence or a back-step sequence distributes the heat input more uniformly and reduces peak residual stresses. For multi-pass cladding, welding from the center outward is recommended.
- Mechanical stress relief — Shot peening or hammer peening of the weld surface can introduce compressive residual stresses that counteract the tensile stresses generated by welding. This is particularly effective for reducing fatigue crack initiation risk.
Engineering Practice Case Study
A practical case study involved the cladding repair of a large-diameter hydrogenation reactor shell, where the base metal was 16MnR low-alloy steel and the overlay layer was 316L austenitic stainless steel. The reactor shell was clamped in a rolling fixture during cladding welding, which imposed a constraint force of approximately 150 kN. The initial welding procedure produced transverse cracks in the weld metal, with residual stresses measured at 380 to 420 MPa.
The revised procedure incorporated the following modifications: preheating to 200 degrees Celsius, reducing the constraint force to 60 kN by modifying the fixture design, implementing a back-step welding sequence, and applying stress-relief annealing at 620 degrees Celsius for 2 hours. After these modifications, the residual stresses were reduced to 120 to 180 MPa, and no cracks were observed in subsequent welds. This case demonstrates the practical effectiveness of the stress management strategies recommended in the study.
Key Reflections
This study provides a quantitative understanding of the relationship between external constraint and residual stress in cladding welding, which is essential for the rational design of welding procedures and fixtures. The key insight is that constraint force is not merely a practical inconvenience but a critical process parameter that directly influences the residual stress state and, consequently, the crack susceptibility and long-term durability of the cladding weld. Engineers must adopt a systematic approach to constraint management, incorporating fixture design optimization, preheating, welding sequence planning, and post-weld heat treatment into the overall welding procedure. The integration of residual stress measurement into the quality assurance process, using techniques such as X-ray diffraction or neutron diffraction, is recommended to verify that the residual stress state meets the acceptance criteria specified in applicable standards such as GB/T 150, ASME VIII Div.1, and NB/T 47002.