CLADDING TECHNOLOGY SHANXI CO., LTDResidual Stress and Microstructure Analysis of Large-Area Overlay Welding on Shaft-Type Parts
Literature Overview
The research by Song Shouxu, Zhao Jiru, and Liu Tao from Hefei University of Technology, published in 2011 in the journal "China Mechanical Engineering" and supported by multiple national funding sources including the National Natural Science Foundation of China (50735006, 50905052), the National Science and Technology Support Program (2008BAC46B01), and the Anhui Provincial Science and Technology Program (08010202010), addresses a critical engineering challenge: the residual stress distribution and microstructural evolution in large-area overlay welding on shaft-type components. This work is particularly significant for industries such as power generation, marine engineering, and heavy machinery where shaft-type components require extensive overlay welding to achieve corrosion resistance, wear resistance, or thermal protection.
The study represents a systematic investigation into the complex interplay between welding process parameters, residual stress development, and microstructural transformation in large-area overlay deposits, providing valuable insights for optimizing welding procedures and predicting component performance.
Residual Stress Analysis
Stress Distribution Characteristics
Large-area overlay welding on shaft-type parts generates a complex residual stress field that differs significantly from that of conventional butt or fillet welds. The key characteristics include:
| Stress Component | Typical Magnitude (MPa) | Distribution | Effect |
|---|---|---|---|
| Longitudinal tensile | 150–450 | Maximum at weld center, decreasing toward edges | Risk of fatigue cracking |
| Transverse tensile | 80–250 | Higher near edges due to constraint | Distortion, warping |
| Radial compressive | -50 to -150 | Uniform through thickness | Generally beneficial |
| Circumferential | 50–200 | Complex, depends on welding sequence | May contribute to distortion |
The residual stress distribution is governed by several factors: the geometry of the shaft (diameter, length, and the ratio of overlay area to base metal area), the welding process parameters (heat input, travel speed, number of passes), and the welding sequence employed.
Influence of Welding Parameters on Residual Stress
| Parameter | Effect on Peak Residual Stress | Mechanism |
|---|---|---|
| Heat input (J/mm) | Higher heat input → lower peak stress | Larger affected zone, more uniform temperature field |
| Travel speed (mm/s) | Higher speed → higher peak stress | Smaller affected zone, steeper temperature gradient |
| Number of passes | More passes → lower per-pass stress | Stress relief by subsequent passes |
| Interpass temperature | Higher → lower peak stress | Stress relief by thermal cycling |
| Preheat temperature | Higher → lower peak stress | Reduced thermal gradient |
| Welding sequence | Balanced sequence → lower stress | Symmetric heat input reduces distortion |
The research demonstrates that the peak residual stress in large-area overlay deposits can reach values approaching the yield strength of the overlay material, particularly in the early passes where the base metal provides significant constraint. As additional passes are deposited, the residual stress distribution evolves through a process of thermal stress relief, where the heat from subsequent passes partially relieves the stresses from previous passes.
Stress Relief Methods
| Method | Stress Reduction | Effectiveness | Side Effects |
|---|---|---|---|
| Post-weld heat treatment (PWHT) | 70–90% | Excellent | May alter microstructure, time-consuming |
| Vibration stress relief (VSR) | 50–70% | Good | May affect dimensional accuracy |
| Shot peening | 30–50% (surface) | Moderate | Compressive surface stress may be beneficial |
| Mechanical working | 40–60% | Moderate | May introduce work hardening |
| Controlled cooling | 20–40% | Limited | Requires careful temperature control |
For shaft-type components where dimensional accuracy is critical, vibration stress relief is often preferred over PWHT because it does not alter the microstructure or cause dimensional changes. However, for applications where fatigue resistance is paramount, PWHT may be necessary despite its time and cost implications.
Microstructural Analysis
Microstructural Zones
The large-area overlay deposit on a shaft-type component exhibits several distinct microstructural zones:
- Weld metal zone: The solidified overlay material, typically consisting of columnar dendrites in the first pass and equiaxed grains in subsequent passes, with composition-dependent phase distribution (austenite, martensite, ferrite, carbides).
- Heat-affected zone (HAZ): The base metal region that has been heated above the recrystallization temperature but not melted. In low-alloy steels, this zone may exhibit grain growth, martensitic transformation, or tempering of pre-existing microstructures.
- Bond line: The interface between the overlay and base metal, which may exhibit dilution, carbide precipitation, or brittle phase formation depending on the composition mismatch.
- Thermally affected zone: The region beyond the HAZ where temperatures exceed the lower transformation temperature but remain below recrystallization. This zone may exhibit tempering, precipitation hardening, or phase transformation.
Dilution and Its Effects
Dilution from the base metal is a critical concern in large-area overlay welding, as it affects the composition, microstructure, and properties of the overlay:
| Dilution Level | Effect on Microstructure | Effect on Properties | Acceptable Range |
|---|---|---|---|
| <10% | Minimal change | Properties close to nominal | Acceptable for most applications |
| 10–20% | Reduced austenite content | Increased hardness, reduced toughness | Acceptable with monitoring |
| 20–30% | Significant martensite formation | High hardness, high brittleness | Generally unacceptable |
| >30% | Predominantly martensitic | Excessive hardness, cracking risk | Must be avoided |
The research demonstrates that dilution increases with each successive pass in a multi-pass overlay, as the base metal is progressively incorporated into the deposit. This is particularly problematic for stainless steel overlays on carbon steel shafts, where dilution can reduce the chromium content below the threshold required for corrosion resistance (typically 12 wt% Cr minimum).
Process Optimization and Engineering Recommendations
Welding Sequence Optimization
For large-area overlay welding on shaft-type parts, the welding sequence is a critical process parameter that affects residual stress, distortion, and microstructure. The research evaluates several welding sequences:
| Sequence Type | Description | Stress Level | Distortion | Recommended Use |
|---|---|---|---|---|
| Continuous circumferential | Weld around circumference continuously | High | Significant warping | Not recommended |
| Alternating diametrical | Weld opposite segments alternately | Moderate | Minimal warping | Recommended |
| Spiral | Weld in spiral pattern | Low | Very low | Recommended for large areas |
| Block | Weld in rectangular blocks | Variable | Moderate | Acceptable with care |
The spiral welding sequence produces the most uniform heat input distribution and the lowest residual stresses, making it the preferred approach for large-area overlays on shaft-type components. However, it requires more sophisticated equipment and operator skill compared to simpler block or alternating sequences.
FMEA Analysis of Common Failure Modes
| Failure Mode | Severity | Occurrence | Detection | RPN | Countermeasure |
|---|---|---|---|---|---|
| Transverse cracking | 10 | 4 | 6 | 240 | Preheat, reduce heat input, PWHT |
| Bond-line cracking | 9 | 5 | 5 | 225 | Control dilution, use transition layer |
| Excessive distortion | 7 | 6 | 3 | 126 | Optimize sequence, use fixtures |
| Incomplete fusion | 8 | 3 | 4 | 96 | Increase heat input, clean surface |
| Hardness exceedance | 6 | 5 | 3 | 90 | Monitor dilution, PWHT if needed |
| Porosity | 5 | 4 | 6 | 120 | Dry consumables, adequate shielding |
Key Reflections and Practical Implications
The research by Song, Zhao, and Liu provides a comprehensive understanding of the residual stress and microstructural challenges associated with large-area overlay welding on shaft-type components. Several key insights emerge from this work:
First, the residual stress field in large-area overlays is not static but evolves dynamically during the welding process. The stress state at any given point depends on the entire welding history, not just the local heat input. This has important implications for process planning, as the welding sequence must be designed with awareness of the cumulative stress effects.
Second, the interaction between residual stress and microstructure is bidirectional. High residual stresses can promote phase transformations (such as martensite formation in austenitic overlays), while microstructural transformations can generate additional stresses (such as those from martensitic transformation). This coupled behavior makes process optimization a complex, multi-variable problem.
Third, the research underscores the importance of post-weld heat treatment for critical applications. While PWHT is time-consuming and may alter the overlay microstructure, it is often the only reliable method for achieving acceptable residual stress levels in large-area deposits. The PWHT parameters must be carefully selected to relieve stresses without degrading the overlay properties.
For engineering practice, I recommend adopting a systematic approach to large-area overlay welding that includes: detailed process simulation to predict residual stress and distortion; careful selection of welding sequence and parameters based on the predicted stress field; implementation of in-process monitoring (such as thermal imaging) to verify process stability; and comprehensive post-weld inspection including residual stress measurement (by X-ray diffraction or hole-drilling method) and NDT.
This research contributes significantly to the body of knowledge on overlay welding technology and provides practical guidance for engineers working on the repair and enhancement of shaft-type components in power generation, marine, and heavy industry applications. The systematic approach to residual stress analysis and process optimization presented in this study should serve as a model for similar investigations in other welding applications.